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Troubleshooting mechanical seals at equipment disassembly 3-9

All seals fail for the same reasons:

  • The faces open up and allow dirt or solids to penetrate.
  • One of the seal components has been damaged by either the fluid you are sealing, heat, or a cleaner used to flush the system.

After the failure has occurred you’ll frequently get a chance to analyze the failed components. You’re going to be looking for several things:

  • Evidence of corrosion.
  • Wear patterns on those parts that are in contact.
  • Evidence of rubbing or wear on those components that should not be in contact.
  • Discoloration of any of the seal components, especially the metal parts.
  • Parts that are missing. Springs, set screws and drive lugs are examples.
  • Loose hardware. Either a seal component or a foreign object.
  • Product attaching to a rotating component. Carefully inspect the impeller and rotating part of the seal.

In the following paragraphs we’ll be inspecting the individual components and looking for evidence of the above.

THE CARBON FACE

Chipping on the O.D. of the carbon. Indicating vibration.

  • This can be caused by harmonic vibration, or when the rotating equipment hits a critical speed.
  • Slipstick can occur if you’re pumping a fluid with poor lubricating qualities.
  • Mishandling is a common problem. Look for evidence of drive lug wear to eliminate this as a possibility.
  • Vaporization of the liquid causing the faces to rapidly open and then close as the leaking fluid cools the faces.
  • A discharge recirculation line is aimed at the carbon seal face.
  • The pump is cavitating. Remember there are four + types of cavitation.
  • Water hammer is a another possibility.

Pits in the carbon face. This problem is often associated with poor grades of carbon/ graphite.

  • Exploded carbon. Air trapped in the pores of the carbon expands and expels pieces of the carbon when the seal faces get hot. Prior to being blown out, polished sections will be visible; usually with small cracks visible in the center.
  • If the product solidifies between the faces it will tear out pieces of the carbon at start up. This is a common occurrence with ammonia compressor seals because petroleum oil is mixed with the ammonia and it can “coke” at the elevated temperature.
  • Most petroleum products will “coke” because of the higher face temperature, and pull out small pieces of the carbon as the faces rotate. You’ll see evidence of these small pits if you inspect the carbon face under a magnifying glass.

Chips at the I.D. of the carbon

  • Solids, or a foreign object of some type from outside of the pump, are getting under the gland and are being thrown into the seal faces. This can occur if the seal leaked at some time and the product solidified on the outboard side of the seal. It can also occur if liquid, containing solids, is used in the quench connection of an A.P.I. type gland.
  • If the seal was installed outside of the stuffing box, as is the case with non metallic seals, solid particles in the fluid can be centrifuged into the rotating carbon face.
  • If the stationary face is manufactured from one of the carbon grades, it can be chipped if it comes into contact with the rotating shaft. This is a common problem at pump start up, or if the pump is operating off of its B.E.P.

Phonograph finish on the carbon face.

  • A solid product was blown across the seal face. It rolls from the outside to the inside diameter. This happens in boiler feed water applications.

Chemical attack of the carbon.

  • You’re using the wrong carbon. Something in the product or the flush is attacking the carbon filler. Switch to an unfilled carbon such as Pure grade 658 RC or CTI. grade CNFJ.
  • You’re trying to seal an oxidizing agent. Oxidizers attack all forms of carbon including the unfilled type. The carbon combines with the oxygen to form either carbon monoxide or carbon dioxide.
  • Some forms of de-onized water will pit and corrode carbon faces

Cracked or damaged carbon face.

  • The product is solidifying between the faces. Carbons are strong in compression but weak in tension or shear. This problem is common with intermittent service pumps each time they start up.
  • Excessive vibration can bang the carbon against a metal drive lug.
  • A cryogenic fluid is freezing a lubricant that was put on the face.
  • The elastomer is swelling up under a carbon or hard face.
  • The shaft is hitting the stationary face or the rotating seal face is hitting a stationary object.
  • Mishandling.
  • Poor packaging. The lapped seal faces should be able to survive a 39″ (one meter) drop.r.

A coating is forming on the carbon face:

  • A change in temperature. Many products solidify at temperature extremes.
  • The product is taking a pressure drop across the seal faces and solidifying.
  • Selective leaching is picking up an element from the system and depositing it on the seal face.
  • The stuffing box is running under a vacuum because the impeller was adjusted backwards and the impeller “pump out vanes” are causing the vacuum.
  • The pumping fluid is creating a protective oxide on the piping. This oxide is chipping off and depositing at the faces. In hot water systems we experience this problem with magnetite (Fe3O4) until the system stabilizes.

Coking

  • This is a problem with all types of oils, and petroleum products in particular.
  • Coking is caused by the combination of high temperature and time. Contrary to popular belief the presence of air or oxygen is not necessary.

Shiny spots, cracks and raised portions of carbon.

  • The carbon is not dense enough, causing the expanding gases trapped beneath the surface of the carbon to explode through the face.
  • Product is solidifying between the faces and pulling out pieces of the carbon as the seal revolves.

Excessive carbon wear in a short period of time. Evidence of excessive heat is usually present.

  • Heat checking of the hard face. It shows up as a cracking of the hard face. This is a problem with coated or plated hard faces. Cobalt base tungsten carbide can have this problem..
  • The shaft is moving in an axial direction because of thrust. This can cause an over compression and heating of the seal faces
  • The impeller is being adjusted towards the back plate. This is problem with seals installed in Flowserve pumps or any other pump that adjusts the open impeller against the back plate.
  • The inner face of a “back to back” double seal application is not positively locked in position. A snap ring must be installed to prevent the inboard stationary face from moving towards the rotating face when the high pressure barrier fluid pressure is lost or overcome by system pressure.
  • The seal was installed at the wrong dimension. It’s overcompressed
  • A cartridge double seal was installed by pushing on the gland. Friction, between the shaft and the sleeve O-ring is compressing the inner seal.
  • A vertical pump was not vented.
  • Solids have penetrated between the faces.
    • The faces are not flat.
    • The movable face is sluggish.
    • The product is vaporizing between the faces because of either high temperature or low stuffing box pressure .
  • Non lubricants will cause rapid face wear. A non lubricant is any fluid with a film thickness less than one micron at its load and operating temperature..

The carbon has a concave or convex wear pattern

  • High pressure i causing the distortion.
  • The stationary face is not perpendicular to the shaft.
  • Some companies lap a concave pattern as standard. Check with your manufacturer.
  • The shaft is bending because the pump is running off of its B.E.P.

The carbon is not flat.

  • Mishandling.
  • Poor packaging.
  • The hard face has been installed backwards and you’re running on a non-apped surface.
  • The seal was shipped out of flat.
  • The metal/ carbon composite hasn’t been stress relieved and it’s distorting the carbon.
  • When the carbon was lapped the lapping plate was too hot and as a result, not flat.
  • The carbon was lapped at room temperature and the seal is running at cryogenic temperatures.
  • Solids are imbedded in the carbon. The faces have opened.
    • The seal was set screwed to a hard shaft.
    • The elastomer (rubber part) is spring loaded to the shaft causing the faces to open as the shaft moves due to end play, vibration or carbon wear.
    • The shaft/ sleeve is over sized causing an excessive interference between the elastomer and the shaft/ sleeve.
    • The sleeve finish is too rough.
    • The product has changed from a liquid to a solid.
    • Dirt or solids are interfering with the seal movement.
    • Some one put the wrong compression on the faces.
    • Shaft fretting is hanging up the face.
    • The face has been distorted for some reason allowing solid particles to enter.
    • The sliding elastomer has swollen up causing too much interference on the shaft/ sleeve.
    • Poor centering is causing the rotating face to run off the stationary face. Keep in mind the gland bolts are not always concentric with the shaft.
    • If you seal was designed with a single spring, it is wound in the wrong direction. This happens when the seal is put on the wrong end of a double ended pump. These pumps need right and left handed wound sprins.
    • An “out of balance rotating assembly” or bent shaft is causing the rotating face to “run off” of the stationary face.

THE HARD FACE.

Chemical attack.

  • Some ceramics and silicon carbides are attacked by caustic fluids. Check to see if your seal face contains silica. As an example: both reaction bonded silicon carbide and 85% ceramic have this high silica content.

Cracked or broken.

  • The product is solidifying between the faces. Most hard faces have poor tensile or shear strength.
  • Excessive vibration will cause cracking at the drive lug location..
  • A cryogenic fluid is freezing a lubricant that was put on the face.
  • The elastomer is swelling up under an outside seal face. This problem can also occur if the seal design allows a spring to contact the I.D. of the hard face.
  • The shaft is hitting the stationary face or the rotating seal face is hitting a stationary object.
  • Mishandling.
  • Poor packaging.

Heat check (a common problem with coated or plated faces)

  • Caused by a high heat differential across the face. Most hard coating have only one third the expansion rate of the stainless steel base material.

Hard coating coming off of the face.

  • The base material not compatible with the sealed product. These coating are very porous, so if the product attacks the base material the coating will come off in sheets.
  • The plating process was not applied correctly.

Analysis of the wear track on the hard face.

Deep grooves&emdash;excessive wear. Solids imbedded in the carbon are causing the problem. The solids were trapped between the faces when the seal faces opened.

  • The seal was set-screwed to a hard shaft.
  • The elastomer is spring loaded to the shaft preventing it from flexing as the shaft vibrates..
  • The shaft/ sleeve is over sized causing the dynamic elastomer or bellows vibration damper to hang up..
  • The shaft/ sleeve finish is too rough
  • The product has solidified in the seal components.
  • Dirt or solids are interfering with seal movement.
  • Not enough spring compression on the faces.
  • Fretting of the shaft/ sleeve is hanging up the face.
  • The face has been distorted by either excessive temperature or pressure.
  • The sliding elastomer has swollen up due to chemical attack of the product or a cleaner that was flushed through the lines. The wrong choice of rubber lubricant, at installation, can also cause the problem
  • Poor centering is causing the rotating face to run off of the stationary face..
  • The single spring was wound in the wrong direction for the application.

The wear track is wider than the carbon.

  • Worn bearings.
  • Bent shaft.
  • Unbalanced impeller.
  • Sleeve not concentric with the shaft.
  • Seal not concentric with the sleeve.
  • In a stationary seal, the stationary carbon is often not centered to the shaft, causing a wiping action.

The wear track is narrower than the carbon.

  • The soft face (carbon) was distorted by pressure.
  • The hard face was over tightened against an uneven surface.
  • The hard face clamping forces are not “equal and opposite”.
  • The face never was flat, or it was damaged during shipment.

Non Concentric pattern. The wear track is not in the center of the hard face.

  • The shaft is bending because the pump is running off of its best efficiency point.
  • Poor bearing fit.
  • Pipe strain.
  • Temperature growth is distorting the stuffing box.
  • The stationary face is not centered to the shaft.
  • Misalignment between the pump[ and its driver.

Uneven face wear. The hard face is distorted:

  • High pressure.
  • Excessive temperature.
  • Over tightening of the stationary face against the stuffing box.
  • The clamping forces are not equal and opposite.
  • The hard face is not wide enough.
  • You are using a two bolt gland and the gland is too thin causing it to distort.
  • You are using a pump seal in a motion seal application.

The product is sticking to the seal face. The product is changing state and becoming a solid. Most products solidify for the following reasons:

  • A change in temperature.
  • A change in pressure.
  • Dilatants will solidify with agitation. As an example: cream becomes butter.
  • Some products solidify when two or more chemicals are mixed together.

The hard face is not flat.

  • Mishandling.
  • Poor packaging.
  • The hard face has been installed backwards and you are running on a non lapped surface.
  • It was shipped out of flat.

THE ELASTOMER.

Compression set. The O-ring has changed shape.

  • High heat is almost always the cause unless you are dealing with Kalrez, Chemraz, or a similar material where a certain amount of compression set is normal.

Shrinking, hardening or cracking.

  • High heat.
  • The shelf life was exceeded. This is a big problem with “Buna N” that has a shelf life of only twelve months.
  • Cryogenics will freeze just about any elastomer.
  • Chemical attack normally causes swelling, but in rare cases can harden an elastomer.
  • Oxidizing liquids can attack the carbon that is used to color most elastomers black.

Torn nibbled, or extruded.

  • Mishandling.
  • Sliding over a rough surface.
  • Forced out of the o-ring groove by high pressure.
  • The liquid has penetrated the elastomer, vaporizing inside and blowing out pieces. This is a problem with ethylene oxide.
  • Halogenated fluids can penetrate the Teflon coating on an elastomer and cause the base material to swell up, splitting the Teflon jacket.

Swelling, changing color, weight or size. Almost always caused by:

  • Chemical attack.
  • Be careful of the lubricant used to install the elastomer.
  • Solvents or cleaners used in the system may not be compatible with the elastomer.
  • Some compounds are sensitive to steam. Most Vitons are a good example of this problem.
  • The elastomer is not compatible with something in the fluid you are sealing.

Torn rubber bellows.

  • The bellows did not vulcanize to the shaft because you used the wrong lubricant.
  • The shelf life was exceeded.
  • The seal faces stuck together and the shaft spun inside the bellows.
  • The pump discharge recirculation line was aimed at the rubber bellows. Solids entrained in the high velocity liquid are abrading the bellows.

THE METAL CASE OR BODY OF THE SEAL.

Corrosion.

  • General or overall. This is the easiest to see and predict. The metal has a “sponge like” appearance. The corrossion always increases with temperature.
  • Concentrated cell or crevice corrosion. Caused by a difference in concentration of ions, or oxygen in stagnant areas causing an electric current to flow. Common around gaskets, set screws, threads, and small crevices.
  • Pitting corrosion. Found in other than stagnant areas. Extremely localized. Chlorides are a common cause. Can be recognized by pits and holes in the metal.
  • Stress corrosion cracking. Threshold values are not known. A combination of chloride, tensile stress, and heat are necessary. Chloride stress corrosion is a serious problem with the 300 series of stainless steels used in industry. This is the reason you should never use stainless steel springs or stainless metal bellows in mechanical seals.
  • Inter-granular corrosion. Forms at the grain boundaries. Occurs in stainless steel at 800-1600 F. (412-825 C.), unless it has been stress relieved. A common problem with welded pieces. Stabilizers such as columbium are added to the stainless steel to prevent this. Rapid cooling of the welds, the use of 316L and stress relieving after the welding are the common solutions.
  • Galvanic corrosion. Occurs with dissimilar materials in contact with and connected by an electrical current. Common in brine, caustic, and salt water applications.
  • Erosion / Corrosion. An accelerated attack caused by a combination of corrosion and mechanical wear. Vaporization, liquid turbulence, vane passing syndrome, and suction recirculation are special cases often called cavitation. Solids in the liquid and high velocity increase the problem.
  • Selective leaching. Involves the removal of one or more elements from an alloy. Common with demineralized or de ionized water applications.
  • Micro organisms, that will attack the carbon in active stainless steel.

Rubbing–All around the metal body.

  • A gasket or fitting is protruding into the stuffing box and rubbing against the seal.
  • The pump discharge recirculation line is aimed at the seal body.
  • The shaft is bending due to the pump operating off of its best efficiency point.
  • Pipe strain.
  • Misalignment between the pump and its driver.
  • A bolted on stuffing box has slipped.

Partial rubbing — On the metal body.

  • Bent shaft.
  • An unbalanced impeller or rotating assembly.
  • Excessively worn or damaged by corrosion or solids in the product.
  • The product has attached its self to the impeller.
  • The impeller never was balanced.
  • The impeller was trimmed, and not re balanced.
  • The seal is not concentric with the shaft, and is hitting the stuffing box I.D..

Discoloration. Caused by high heat. Stainless steel changes color at various temperatures.

FAHRENHEITCOLOR OF THE METALCENTIGRADE
700 – 800Straw Yellow370 – 425
900 – 1000Brown480 – 540
1100 – 1200Blue600 -650
> 1200Black> 650

NOTE: To tell the difference between discoloration caused high heat and product attaching to the metal part, try to erase the color with a common pencil eraser. Discoloration will not erase off.

Product sticking to the metal surfaces.

  • Heat is the main cause.
  • The product pressure has dropped.
  • Air or oxygen is getting into the system.
    • Valves above the water line.
    • Through the stuffing box.
  • The product was not deaerated.
  • The pump suction is not completely submerged.
  • The bypass return is too close to the pump suction.
  • The liquid is vortexing in the suction line.
  • A non o-rring elastomer is being used in the seal, allowing air to enter the stuffing box when you are sealing a vacuum application.
  • The system protective oxide coating is depositing on the sliding metal components.

The following applications cause a vacuum to be present in the pump stuffing box.

  • Pumps that lift liquid.
  • Heater drain pumps.
  • Pumping from an evaporator.
  • Pumping from the hot well of a condenser.
  • Pumps that prime other pumps.
  • The open impeller was adjusted in the wrong direction and the impeller pump out vanes are causing the vacuum.

The Teflon coating is coming off some of the metal parts.

  • Coatings are very porous. They do not provide corrosion resistance. The base material is being attacked by the product.

DRIVE LUGS, PINS, SLOTS, etc.

Broken.

  • Chemical attack.
  • Excessive side load.
  • The seal faces are glued together because the product has solidified.
  • A cryogenic fluid is sticking the faces together.

Wear on one side of the drive lug or slot.

  • Vibration.
  • Slipstick.
  • The stationary is not perpendicular to the shaft.

The drive pins are falling out of the holder.

  • Corrosion.
  • Improper fit.
  • Bad part.
  • Excessive vibration.

THE SPRINGS.

Broken or cracked.

  • The stationary face is not perpendicular to the shaft causing excessive spring flexing in the metal “plastic range”. The spring material has “work hardened” and fatigued.
  • Chloride stress corrosion problems with 300 series stainless steel.

Corroded.

  • Stressed material corrodes much faster than unstressed material. The springs are always under severe stress.

Clogged.

  • Be sure to distinguish between “cause and effect”. If the springs are located outside the liquid, it happened after the failure.
  • If the product solidifies or crystallizes it can clog springs exposed to the pumped fluid.
  • Dirt or solids in the fluid can clog exposed springs.

Twisted.

  • Almost always an assembly problem. The lugs were not engaged in the slots. This is a problem with many seal designs. Check to see if your seals can come apart easily, or if the drive lugs can change position when the seal is not compressed.

The drive lugs or slots are worn on both sides.

  • Excessive vibration.
  • The single spring, rubber bellows seal, was not vulcanized to the shaft.
  • The stationary is not perpendicular to the shaft, causing excessive spring movement.

Broken Metal Bellows.

  • Fatigue caused by over flexing in the plastic range of the metal
    • Harmonic vibration.
    • Slipstick.
  • The discharge recirculation line is aimed at the thin bellows plates.
  • Excessive wear from solids in the stuffing box.
  • Faces sticking together as the product solidifies.
  • Chloride stress corrosion with 300 series stainless steel.

Because these seals do not have a dynamic elastomer to provide vibration damping some other means must be provided or vibration will always be a problem.

THE SLEEVE, OR SHAFT.

Grooves or pits at the seal dynamic elastomer location.

  • Fretting.
  • Concentrated cell corrosion.
  • The rubber bellows did not vulcanize to the shaft/ sleeve.
  • The set screws slipped on a hardened shaft or were not tightened properly. The seal faces stuck together causing the shaft to rotate inside the static elastomer.
  • Salt water applications are particularly troublesome when a static elastomer or clamp is attached to the shaft. Pitting caused by the chlorides and the low ph of salt water are the main problems.

Rubbing at the wear ring location.

  • The pump is running off of its best efficiency point.
  • The shaft is bending.
  • Bad bearings.
  • Excessive temperature.
  • Sleeve is not concentric with the shaft, or the seal with the sleeve.
  • Bent shaft.
  • Unbalanced impeller or rotating assembly.
  • Pipe strain.
  • Misalignment between the pump and its driver
  • High temperature applications require a “center line: pump design.

Corrosion. See above description under metal corrosion

.THE SET SCREWS.

  • Stripped from over tightening.
  • Corroded. Check to see if you are using hardened set screws. This type is normally supplied with most cartridge seals and can corrode easily.
  • Rounded Allen Head. Alan wrenches wear rapidly. They are an expendable tool.
  • Loose.
    • Sleeve too hard. They are not biting in.
    • Sleeve too soft. They are vibrating loose.

THE GLAND.

Rubbing at the I.D.

  • Partial rubbing.
    • The gland has slipped.
    • Improper installation. It was not centered to the shaft.
    • The shaft is bending.
    • Pipe strain.
  • Rubbing all around.
    • The shaft is not concentric with the sleeve.
    • The seal is not concentric with the sleeve.
    • Bad bearings.
    • Bent shaft.
    • Unbalanced impeller or rotating assembly.
    • Solids attached to the shaft, or caught between the shaft, and the gland.
    • Cavitation.

Corrosion.

  • If there is evidence of rubbing the corrosion will be accelerated.

Passages clogged or not connected properly.

  • A.P.I Gland.
    • Hooked up wrong.
    • Flushing connection clogged.
    • Quench connection clogged.

BUSHINGS

Rubbing at the I. D.

  • Partial rubbing.
    • The A.P.I. gland has slipped.
    • Improper installation. It was not centered to the shaft.
    • The shaft is bending.
    • The gland bolt holes are often not concentric with the shaft/ sleeve.
    • Misalignment between the pump and its driver.
    • Excessive pipe strain.
  • Rubbing all around.
    • The shaft is not concentric with the sleeve.
    • The seal is not concentric with the sleeve.
    • Bad bearings.
    • Bent shaft.
    • Unbalanced impeller.
    • Cavitation

Erosion.

  • Dirt and solids are present in the discharge or suction recirculating fluid.

Corrossion problems associated with stainless steel 4-1

The rotating equipment business uses a great deal of 300 series stainless steel, and as a result we often experience several types of corrosion:

  • General corrosion
  • Galvanic corrosion
  • Pitting
  • Inter granular corrosion
  • Chloride stress corrosion cracking
  • Erosion- corrosion
  • Fretting
  • Concentrated cell or crevice corrosion
  • Selective leaching
  • Micro organisms

At the end of this aticle is a page titled, “The Galvanic Series Of Metals and alloys”. I’ll be referring to this chart during our discussion.

The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a “passive” film which resists further “oxidation” or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, we say that the metal has become “passivated” and the oxidation or “rusting” rate will slow down to less than 0.002″ per year (0,05 mm. per year).

Unlike aluminum or silver this passive film is invisible in stainless steel. It’s created when oxygen combines with the chrome in the stainless to form chrome oxide which is more commonly called “ceramic”. This protective oxide or ceramic coating is common to most corrosion resistant materials.

Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters “ine”. Listed in order of their activity they are:

  • fluorine
  • chlorine
  • bromine
  • iodine
  • astatine (very unstable.)

These are the same chemicals that will penetrate Teflon and cause trouble with Teflon coated or encapsulated o-rings and/ or similar coated materials. Chlorides are one of the most common elements in nature and if that isn’t bad enough, they’re also soluble, active ions; the basis for good electrolytes, the best conditions for corrosion or chemical attack.

GENERAL OR OVERALL CORROSION.

This type of corrosion occurs when there is an overall breakdown of the passive film formed on the stainless steel. It’s the easiest to recognize as the entire surface of the metal shows a uniform “sponge like” appearance. The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C.) of either the product or the metal part.

If the rotating portion of the seal is rubbing against some stationary component, such as a protruding gasket or fitting, the protective oxide layer will be polished off and the heat generated will increase the corrosion as noted above. This explains why corrosion is often limited to only one portion of the mechanical seal, metal casing.

There are many good publications available to help you select the proper metal for any given mechanical seal application. As a general rule, if the wetted parts of the equipment are manufactured from iron, steel, stainless steel or bronze, and they are showing no signs of corrosion, grade 316 stainless is acceptable as long as you do not use stainless steel springs. (see chloride stress corrosion below)

GALVANIC CORROSION

If you put two dissimilar metals, or alloys in a common electrolyte, and connect them with a voltmeter, it will show an electric current flowing between the two. (This is how the battery in your automobile works). When the current flows, material will be removed from one of the metals or alloys ( the ANODIC one) and dissolve into the electrolyte. The other metal (the CATHODIC one) will be protected.

Move down to the end of this aticle and look at the Galvanic Series chart The further apart the materials are located on this chart, the more likely that the one on the ANODIC end will corrode if they are both immersed in any fluid considered to be an electrolyte.

Salt water, is one of the best!

Example #1.

A ship has lots of bronze fittings and a steel hull. Note that steel is located seven lines from the ANODIC end, and bronze is listed at twenty seven rows from the same end. Sea water is a perfect electrolyte, so the bronze fittings would immediately attack the steel hull unless something could be done to either protect the steel ,or give the bronze something else to attack.

The classic way to solve this problem is to attach sacrificial zinc pieces to the hull and let the bronze go after them. Again, looking at the chart, you’ll note that zinc is found on line three from the top of the chart. In other words the zinc is further away from the bronze than the iron, so the galvanic action takes place between the zinc and the bronze, rather than between the steel and the bronze. Zinc paint is used for the same reason.

Example #2

Nickel base tungsten carbide contains active nickel. When this face material is used in dual seal applications it is common to circulate water or antifreeze between the seals (as mentioned in the beginning of this paper, water can be an excellent electrolyte because of the addition of chlorine and fluorine). You’ll note that active nickel is located twenty one rows from the top of the chart. Passivated 316 stainless steel is positioned nine rows from the bottom. This means that the stainless steel can attack the nickel in the tungsten carbide causing it to corrode.

The rate at which corrosion takes place is determined by :

  • The distance separating the metals on the galvanic series chart
  • The temperature and concentration of the electrolyte. The higher the temperature, the faster it happens. Any stray electrical currents in the electrolyte will increase the corrosion also.
  • The relative size of the metal pieces. A large cross section piece will not be affected as much as a smaller one.
  • Many metal seal components are isolated from each other by the use of rubber o-rings or similar materials and designs. Shaft movement that causes fretting of the 316 stainless steel rubs off the passivated layer and exposes the active stainless to the electrolyte until the metal part becomes passivated once more. This is one of the reasons we see corrosion under o-rings, and Teflon wedges. In the following paragraph I’ll be discussing another cause of corrosion under rubber parts.

PITTING

This is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others. It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. As stated earlier, halogens will penetrate passivated stainless steel. Referring to the galvanic chart you’ll note that passivated 316 stainless steel is located nine lines from the bottom and active 316 stainless steel is located thirteen lines from the top. Pit type corrosion is therefore simple galvanic corrosion, occuring as the small active area is being attacked by the large passivated area. This difference in relative areas accelerates the corrosion, causing the pits to penetrate deeper. The electrolyte fills the pits and prevents the oxygen from passivating the active metal so the problem gets even worse. This type of corrosion is often called “Concentrated cell corrosion”. You’ll also see it under rubber parts that keep oxygen away from the active metal parts, retarding the metal’s ability to form the passivated layer.

INTERGRANULAR CORROSION

All austenitic stainless steels (the 300 series, the types that “work harden”) contain a small amount of carbon in solution in the austenite. Carbon is precipitated out at the grain boundaries, of the steel, in the temperature range of 1050° F. (565° C) to 1600° F. (870° C.). This is a typical temperature range during the welding of stainless steel.

This carbon combines with the chrome in the stainless steel to form chromium carbide, starving the adjacent areas of the chrome they need for corrosion protection. In the presence of some strong corrosives an electrochemical action is initiated between the chrome rich and chrome poor areas with the areas low in chrome becoming attacked. The grain boundaries are then dissolved and become non existent. There are three ways to combat this:

  • Anneal the stainless after it has been heated in this sensitive range. This means bringing it up to the proper annealing temperature and then quickly cooling it down through the sensitive temperature range to prevent the carbides from forming.
  • When possible use low carbon content stainless if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries. 316L is as good example of a low carbon stainless steel.
  • Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be “stabilized”

CHLORIDE STRESS CORROSION.

If the metal piece is under tensile stress, either because of operation or residual stress left during manufacture, the pits mentioned in a previous paragraph will deepen even more. Since the piece is under tensile stress cracking will occur in the stressed piece. Usually there will be more than one crack present causing the pattern to resemble a spider’s web. Chloride stress cracking is a serious problem in industry and not often recognized by the people involved. In the seal business it is a serious problem if you use stainless steel springs or stainless steel bellows in your seals. This is the main reason that Hastelloy C is recommended for spring material. Here are some additional thoughts about chloride stress cracking that you’ll want to consider:

  • Chlorides are the big problem when using the 300 series grades of stainless steel. The 300 series is the one most commonly used in the process industry because of its good corrosion resistant proprieties. Outside of water, chloride is the most common chemical found in nature and remember that the most common water treatment is the addition of chlorine.
  • Beware of insulating, or painting stainless steel pipe. Most insulation contains chlorides and piping is frequently under tensile stress. The worst condition would be insulated, steam traced, stainless steel piping.

If it’s necessary to insulate stainless steel pipe, a special chloride free insulation can be purchased, or the pipe can be coated with a protective film prior to insulating.

  • Stress cracking can be minimized by annealing the metal, after manufacture, to remove residual manufactured stresses.
  • Never replace a carbon steel bolt with a stainless steel one unless you’re sure there are no chlorides present. Bolts can be under severe tensile stress.
  • No one knows the threshold values for stress cracking to occur. We only know that you need tensile stress, chlorides, temperature and the 300 series of stainless steel. We do not know how much chloride, stress or temperature.
  • Until I figured out what was happening I had trouble breaking stainless steel fishing hooks in the warm water where I live in Florida.
  • Many cleaning solutions and solvents contain chlorinated hydrocarbons. Be careful using them on or near stainless steel. Sodium hypochlorite, chlorethene. methylene chloride and trichlorethane are just a few in common use. The most common cleaner used with dye checking material is trichloroethane, explaining the reason we sometimes experience cracks after we weld stainless steel and dye check it to inspect the quality of the weld.

EROSION CORROSION

This is an accelerated attack resulting from the combination of mechanical and chemical wear. The liquid velocities in some pumps prevents the protective oxide passive layer from forming on the metal surface. The suspended solids also remove some of the passivated layer increasing the galvanic action. You see this type of corrosion very frequently at the eye of the pump impeller.

FRETTING CORROSION

This type of corrosion is easily seen on the pump shaft or sleeve. You’ll see the damage on the shaft under:

  • The grease or lip seal that is supposed to protect the bearings.
  • The packing used to seal the fluid.
  • The dynamic Teflon or elastomer used in most original equipment seals.
  • The vibration damper used in rotating metal bellows seals.
  • The rubber boot used in low cost seals, if it did not attach to the shaft properly.

As mentioned earlier, 300 series stainless steel passivates its self by forming a protective chrome oxide layer when ever it is exposed to free oxygen. This oxide layer is very hard and when it imbeds into a soft elastomer it will cut and damage the shaft or sleeve rubbing against it. The mechanism works like this:

  • Oxygen passivates the active stainless steel forming a protective ceramic layer.
  • The seal or packing removes the oxide layer as the shaft or sleeve rubs against it.
  • The ceramic passivated layer sticks into the soft elastomer turning it into a “grinding surface”.
  • The oxide reforms when the active metal is exposed and the process starts all over again.
  • A visible groove is cut into the shaft, or sleeve that will cause seal leakage and “hang up”.

CONCENTRATED CELL OR CREVICE CORROSION

This corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications. Salt water applications are the most severe problem because of the salt water low PH (8.0&endash;9.0) and its high chloride content. Here is the mechanism:

  • Chlorides pit the passivated stainless steel surface.
  • The low PH salt water attacks the active layer that is exposed
  • Because of the lack of fluid flow over the attacked surface, oxygen is not available to passivate the stainless steel.
  • Corrosion continues unhampered under the rubber and tight fitting clamp.
  • The inside of the o-ring groove experiences the same corrosion as the shaft or sleeve.

SELECTIVE LEACHING

The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The mechanism is:

  • Metals are removed from the liquid during a de-ionization or de-mineralizing process.
  • The liquid tries to replace the missing elements as it flows through the system.
  • The un-dissolved metals often coat them selves on the mechanical seal faces or the sliding components and cause a premature seal failure.
  • Heat accelerates the process.

MICRO ORGANISMS

These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these “bugs”, one common one is for them to eat the carbon you find in waste and other hydrocarbons, and convert it to carbon dioxide. The “bugs” fall into three categories:

  • Aerobic, the kind that need oxygen.
  • Anaerobic, the kind that do not need oxygen.
  • Facultative, the type that goes both ways.

If the protective oxide layer is removed from stainless steel because of rubbing or damage, the “bugs” can penetrate through the damaged area and attack the carbon in the metal. Once in, the attack can continue on in a manner similar to that which happens when rust starts to spread under the paint on an automobile.GALVANIC SERIES OF METALS AND ALLOYSCORRODED END ( ANODIC OR LEAST NOBLE)MAGNESIUM
MAGNESIUM ALLOYS
ZINC
ALUMINUM 5052, 3004, 3003, 1100, 6053
CADMIUM
ALUMINUM 2117, 2017, 2024
MILD STEEL (1018), WROUGHT IRON
CAST IRON, LOW ALLOY HIGH STRENGTH STEEL
CHROME IRON (ACTIVE)
STAINLESS STEEL, 430 SERIES (ACTIVE)
302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)
NI – RESIST
316, 317, STAINLESS STEEL (ACTIVE)
CARPENTER 20CB-3 STAINLESS (ACTIVE)
ALUMINUM BRONZE (CA 687)
HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE)
LEAD-TIN SOLDERS
LEAD
TIN
INCONEL 600 (ACTIVE)
NICKEL (ACTIVE)
60 NI-15 CR (ACTIVE)
80 NI-20 CR (ACTIVE)
HASTELLOY B (ACTIVE)
BRASSES
COPPER (CA102)
MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
SILICONE BRONZE
NICKEL SILVER
COPPER – NICKEL ALLOY 90-10
COPPER – NICKEL ALLOY 80-20
430 STAINLESS STEEL
NICKEL, ALUMINUM, BRONZE (CA 630, 632)
MONEL 400, K500
SILVER SOLDER
NICKEL (PASSIVE)
60 NI- 15 CR (PASSIVE)
INCONEL 600 (PASSIVE)
80 NI- 20 CR (PASSIVE)
CHROME IRON (PASSIVE)
302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE)
316, 317, STAINLESS STEEL (PASSIVE)
CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825
NICKEL – MOLYBDEUM – CHROMIUM – IRON ALLOY (PASSIVE)
SILVER
TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.)
GRAPHITE
ZIRCONIUM
GOLD
PLATINUM

Cavitation 1-3

Cavitation means that cavities or bubbles are forming in the liquid that we’re pumping. These cavities form at the low pressure or suction side of the pump, causing several things to happen all at once:

  • The cavities or bubbles will collapse when they pass into the higher regions of pressure, causing noise, vibration, and damage to many of the components.
  • We experience a loss in capacity.
  • The pump can no longer build the same head (pressure)
  • The pump’s efficiency drops.

The cavities form for five basic reasons and it’s common practice to lump all of them into the general classification of cavitation. This is an error because we’ll learn that to correct each of these conditions, we must understand why they occur and how to fix them. Here they are in no particular order :

Vaporization .

A fluid vaporizes when its pressure becomes too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement is supplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degrees Fahrenheit (Twenty degrees Centigrade) is the fluid being pumped.

Since there are losses in the piping leading from the source to the suction of the pump, we must determine the head after these losses are calculated. Another way to say this is that a Net Positive Suction Head is Required (N.P.S.H.R.) to prevent the fluid from vaporizing.

We take the Net Positive Suction Head Available (N.P.S.H.A.) subtract the Vapor Pressure of the product we are pumping, and this number must be equal to or greater than the Net Positive Suction Head Required.

To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H. Required. We shall look at each possibility:

Increase the suction head

  • Raise the liquid level in the tank
  • Raise the tank
  • Pressurize the tank
  • Place the pump in a pit
  • Reduce the piping losses. These losses occur for a variety of reasons that include :
    • The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter.
    • A pipe liner has collapsed.
    • Solids have built up on the inside of the pipe.
    • The suction pipe collapsed when it was run over by a heavy vehicle.
    • A suction strainer is clogged.
    • Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather
    • Something is stuck in the pipe, It either formed there, or was left during the last time the system was opened . Maybe a check valve is broken and the seat is stuck in the pipe.
    • The inside of the pipe, or a fitting has corroded.
    • A bigger pump has been installed and the existing system has too much loss for the increased capacity.
    • A globe valve was used to replace a gate valve.
    • A heating jacket has frozen and collapsed the pipe.
    • A gasket is protruding into the piping.
    • The pump speed has increased.
  • Install a booster pump

Lower the pumping fluid temperature

  • Injecting a small amount of cooler fluid at the suction is often practical.
  • Insulate the piping from the sun’s rays.
  • Be careful of discharge recirculation lines. They can heat the suction fluid.

Reduce the N.P.S.H. Required

  • Use a double suction pump. This can reduce the N.P.S.H.R. by as much as 25%, or in some cases it will allow you to raise the pump speed by 40%
  • Use a slower speed pump.
  • Use a pump with a larger, impeller eye opening.
  • If possible, install an Inducer. These inducers can cut N.P.S.H.R. by almost 50%.
  • Use several smaller pumps. Three half capacity pumps can be cheaper than one large pump plus a spare. This will also conserve energy at lighter loads.

It’s a general rule of thumb that hot water and gas free hydrocarbons can use up to 50% of normal cold water N.P.S.H. requirements, or 10 feet (3 meters), whichever is smaller. I would suggest you use this as a safety margin, rather than design for it.

Air ingestion (Not really cavitation, but acts like it)

A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. Air gets into as system in several ways that include :

  • Through the packing stuffing box. This occurs in any packed pump that lifts liquid, pumps from a condenser, evaporator, or any piece of equipment that runs in vacuum.
  • Valves located above the water line.
  • Through leaking flanges.
  • Pulling air through a vortexing fluid.
  • If a bypass line has been installed too close to the suction, it will increase the temperature of the incoming fluid.
  • Any time the suction inlet pipe looses fluid. This can occur when the level gets too low, or there is a false reading on the gauge because the float is stuck on a corroded rod.

Both vaporization and air ingestion have an adverse affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization can both occur, they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump.

Internal Recirculation

This condition is visible on the leading edge of the impeller, close to the outside diameter, working its way back to the middle of the vane. It can also be found at the suction eye of the pump.

As the name implies, the fluid recirculates increasing its velocity until it vaporizes and then collapses in the surrounding higher pressure. This has always been a problem with low NPSH pumps and the term Suction Specific Speed to guide you in determining how close you have to operate to the B.E.P. of a pump to prevent the problem.

The higher the number the smaller the window in which you can operate. The numbers range between 3,000 and 20,000. Water pumps should stay between 3,000 and 12,000. Here is the formula to determine the suction specific speed number of your pump:

Capacity = Gallons per minute, or liters per second of the largest impeller at its BEP

Head= Net positive suction head required (feet or meters) at that rpm

  • For a double suction pump the flow is divided by 2 since there are 2 impeller eyes
  • Try to buy pumps with a suction specific speed number lower than 8500.(5200 metric ) forget those over 12000 (8000 metric) except for extreme circumstances.
  • Mixed hydrocarbons and hot water at 9000 to 12000 (5500 to 7300 metric) or higher, can probably operate satisfactorily.
  • High specific speed indicates the impeller eye is larger than normal, and efficiency may be compromised to obtain a low NPSH required.
  • Higher values of specific speed may require special designs, and operate with some cavitation.
  • Normally a pump operating 50% below its best efficiency point (B.E.P.) is less reliable.

With an open impeller pump you can usually correct the internal recirculation problem by adjusting the impeller clearance to the manufacturers specifications. Closed impeller pumps present a bigger problem and the most practical solution seems to be to contact the manufacturer for an evaluation of the impeller design and a possible change in the design of the impeller or the wear ring clearances.

Turbulence

We always prefer to have liquid flowing through the piping at a constant velocity. Corrosion or obstructions can change the velocity of this liquid, and any time you change the velocity of a liquid, you change its pressure. Good piping layouts would include :

  • Ten diameters of pipe between the pump suction and the first elbow.
  • In multiple pump arrangements locate the suction bells in separate bays so that one pump suction will not interfere with another. If this is not practical, a number of units can be installed in a single large sump provided that :
    • The pumps should be positioned in a line perpendicular to the approaching flow.
    • There must be a minimum spacing of at least two suction diameters between pump center lines.
    • All pumps are running.
  • The upstream conditions should have a minimum straight run of ten pipe diameters to provide uniform flow to the suction bells.
  • Each pump capacity must be less than 15,000 gpm..
  • Back wall clearance distance to the centerline of the pump must be at least 0.75 of the suction diameter.
  • Bottom clearance should be approximately 0.30 (30%) of the suction diameter
  • The minimum submergence should be as follows:
    FLOWMINIMUM SUBMERGENCE20,000 GPM4 FEET100,000 GPM8 FEET180,000 GPM10 FEET200,000 GPM11 FEET250,000 GPM12 FEET

The metric numbers are :

FLOWMINIMUM SUBMERGENCE
4,500 M3/HR1.2 METERS
22,500 M3/HR2.5 METERS
40,000 M3/HR3.0 METERS
45,000 M3/HR3.4 METERS
55,000 M3/HR3.7 METERS

The Vane Passing Syndrome

This type of cavitation damage is caused when the OD of the impeller passes too close to the pump cutwater. The velocity of the liquid increases as it flows through this small passage, lowering the fluid pressure and causing local vaporization. The bubbles then collapse at the higher pressure just beyond the cutwater. This is where you should look for volute damage. You’ll need a flashlight and mirror to see the damage, unless it has penetrated to the outside of the volute.

The damage is limited to the center of the impeller vane. If it’s a closed impeller, the damage will not extend into the shrouds. You can prevent this problem, if you keep a minimum impeller tip to cutwater clearance of 4% of the impeller diameter in the smaller impeller sizes (less than 14′ or 355 mm.) and a 6% clearance in the larger impeller sizes (greater than 14″ or 355 mm.).

To prevent excessive shaft movement, some manufacturers install bulkhead rings in the suction eye. At the discharge side, rings can be manufactured to extend from the walls to the impeller shrouds.

Troubleshooting the positive displacement rotary pump. 12-04

  • The pump is not primed. Prime it from the outlet side by keeping the outlet air vent open until liquid comes out the vent.
  • The rotating unit is turning in the wrong direction.
  • Valves are closed, or there is an obstruction in the inlet or outlet line. Check that the flange gaskets have their center cut out.
  • The end of the inlet pipe is not submerged. You can either increase the length of the inlet pipe into the liquid level or raise the level in the tank.
  • The foot valve is stuck.
  • A strainer or filter is clogged.
  • The net inlet pressure is too low.
  • A bypass valve is open.
  • There is an air leak some where in the inlet line. Air can come in through gaskets or valves above the fluid line.
  • The stuffing box is under negative pressure. Packing is allowing air to get into the system. You should convert the packing to a mechanical seal
  • The pump is worn. The critical clearances have increased.
  • Something is broken. Check the shaft, coupling, internal parts, etc.
  • There is no power to the pump.

The pump is putting out a low capacity.

  • The pump’s internal clearances have increased. It’s time to change some parts.
  • The net inlet pressure is too low; the pump is cavitating.
  • A strainer or filter is partially clogged.
  • The speed is too low. Check the voltage.
  • The tank vent is partially frozen shut.
  • A bypass line is partially open.
  • A relief valve is stuck partially open.
  • The inlet piping is damaged. Something ran over it
  • A corrosion resistant liner has collapsed in the inlet piping.
  • Air is leaking through the packing. You should go to a mechanical seal.

The pump looses its prime after it has been running for a while.

  • The liquid supply is exhausted. Check the tank level; sometimes the float is stuck, giving an incorrect level reading.
  • The liquid velocity has increased dramatically.
  • The liquid is vaporizing at the pump inlet.
  • A bypass line is heating the incoming fluid.
  • An air leak has developed in the suction piping.

The pump is using too much power

  • The speed is too high.
  • The liquid viscosity is higher than expected.
  • The discharge pressure is higher than calculated
  • The packing has been over tightened. You should convert to a mechanical seal.
  • A rotating element is binding. Misalignment could be the problem or something is stuck in a close clearance and binding the rotating element.

Excessive noise and vibration.

  • Relief valve chatter.
  • Foundation or anchor bolts have come loose.
  • The pump and driver are misaligned.
  • The piping is not supported properly.
  • The liquid viscosity is too high. The pump is starving. Check the temperature of the incoming liquid. Check to see if the supply tank heater has failed.

Excessive noise or a loss of capacity is frequently caused by cavitation. Here is how the NPSH required was determined initially:

With the pump initially operating with a 0 psig. inlet pressure and constant differential pressure, temperature, speed and viscosity; a valve in the inlet line is gradually closed until cavitation noise is clearly audible, there is a sudden drop off in capacity or there is a 5% overall reduction in output flow. Cavitation occurs with:

  • A loss of suction pressure.
  • An increase in fluid velocity.
  • An increase in inlet temperature.

Here are some common causes of cavitation problems:

  • A foot valve or any valve in the suction piping is sticking.
  • Something is occasionally plugging up the suction piping. If the pump suction is coming from a river, pond or the ocean, grass is a strong possibility.
  • A loose rag is another common cause.
  • A collapsed pipe liner.
  • A filter or strainer is gradually clogging up.
  • The tank vent partially freezes in cold weather.
  • The sun is heating the suction piping, raising the product temperature close to its vapor point.
  • The level in the open suction tank decreases causing vortex problems that allow air into the pump suction.
  • Several pumps in the same sump are running, decreasing the level too much.
  • The suction tank float is stuck. It will sometimes show a higher level than you really have.
  • A discharge recirculation line, piped to the pump suction, opens and heats the incoming liquid.
  • Sometimes the suction lift is too high. The increase in pipe friction will reduce the suction head.
  • The vapor pressure of the product is very close to atmospheric pressure. The pump cavitates every time it rains because of a drop in atmospheric pressure.
  • The tank is being heated to de-aerate the fluid. Sometimes it is being heated too much.
  • The process fluid specific gravity is changing. This can happen with a change in product operating temperature or if a cleaner or solvent is being flushed through the lines.
  • The source tank is changing from a positive pressure to a vacuum due to the process.
  • A packed valve in the suction piping is at a negative pressure and air is leaking in through the packing.
  • The tank is being pumped dry.
  • The inlet piping has been moved or altered in some way. Has a foot valve, strainer, elbow, or some other type of hardware been installed in the suction piping?
  • Has a layer of hard water calcium or some other type of solid formed on the inside of the suction piping reducing its inside diameter over some period of time?

You are experiencing rapid pump wear.

  • There are abrasives in the liquid you are pumping that arecausing erosion problems. You may have to go to a larger pump running at a slower speed.
  • There is some corrosion in one or more of the pump elements.
  • There is a lack of lubrication.
  • You have a severe pipe strain problem. It could have been caused by thermal growth of the hardware.
  • Too much misalignment.
  • The pump is running dry.
  • When all else fails the best way to reduce NPSH required is to select a larger pump and run it at a slower speed.

What’s wrong with the modern centrifugal pump? 3-10

Ask for a pump recommendation from your favorite supplier and chances are he’ll recommend one of the standard pump designs that conform to either the A.N.S.I., I.S.O. or D.I.N. specifications. On the surface that might seem like a good recommendation, but the fact is that all of these designs will cause you maintenance problems.

Please refer to the following illustration. I’ve pictured some of the more obvious problems that we find with these designs.

THE IMPELLER IS TOO FAR AWAY FROM THE BEARINGS
The pump was designed for packing and that’s where the problem starts. To produce enough axial space to accommodate at least five rings of packing (Any less would cause sealing problems), a lantern or lubricating ring, a gland to tighten the packing and enough room to get your hands in there, the manufacturer had to move the pump impeller too far away from the bearings. He’s depending upon the packing to act as part of the bearing system, especially at start up when the shaft is subjected to its maximum radial deflection.
Impeller imbalance, vibration, misalignment, pipe strain, cavitation, critical speeds, and other forms of shaft deflection add to the existing problem, causing excessive movement of the mechanical seal components.
If the pump had been designed for a mechanical seal the impeller would have been positioned closer to the bearings saving considerable initial investment cost (short shafts cost less money). This was not done, however, and so the seal is jammed into the small radial space provided for the packing.
IN MOST CASES A SLEEVE WAS INSTALLED ON THE SHAFT
The shaft diameter was reduced to accommodate the sleeve and this compounded the problem. Sleeves are installed for several reasons:
To provide corrosion resistance when building the shaft of corrosion resistant material would be too costly.
To provide a wear surface for packing and those seals that frett or damage shafts.
To position an impeller
Some seal manufacturers use the sleeve as a convenient method of attaching a metal bellows seal to the shaft.
In ninety percent of the cases the second reason is why most manufacturers use shaft sleeves. To evaluate the relationship between shaft diameter and length, familiarize yourself with the concept of L3/D4 that was explained in several of the earlier papers published in this series.
THE STUFFING BOX INSIDE DIAMETER IS TOO SMALL
The stuffing box cross section was narrowed to about 3/8 inch (10 millimeter) to accommodate small cross section packing. In the smaller shaft sizes the cross section is 5/16″ (8 mm.). This narrow space doesn’t give the seal enough room to utilize centrifugal force to throw solids away from the lapped seal faces, or provide enough clearance for adequate cooling of the components and sealing fluid. This has caused many customers to provide expensive and unreliable flushing that could be eliminated in many instances, if there was adequate room between the seal and the inside wall of the stuffing box.
THE STUFFING BOX IS TOO LONG.
The length was added to accommodate all the rings of packing and the lantern ring. Recognizing this length as a problem the manufacturer did not leave enough room between the face of the stuffing box and the first obstruction, to accommodate some of the modern cartridge double seals, or the newest split mechanical seals.
THERE IS NO DOUBLE VOLUTE TO PREVENT RADIAL LOADING OF THE IMPELLER
This has caused customers to install inefficient “by pass” lines to prevent shaft breakage and seal damage at start up or when running too far off of the best efficiency point of the pump.
MOST OF THESE PUMPS ARE OF THE BACK PULL OUT DESIGN.
This means that the wet end is left on the piping and the power end along with the adapter are brought back to the shop for seal replacement and repair. Unless you have a seal cartridge mounted, or you are using a split seal design, you will have trouble making an initial impeller adjustment with most of the open impeller designs in use today. The direction of adjustment varies with manufacturers.
THE IMPELLER ADJUSTMENT IS ALMOST ALWAYS MADE FROM THE BEARING END OF THE PUMP.
This means that to compensate for wear (a very common problem when pumping abrasives) the shaft has to be moved either towards the front of the pump volute or, as in the case of the Flowserve pump, back towards the back plate. This movement can be as much as a total of 0.250 inches (6 millimeter). In either case the seal setting is disturbed and short seal life follows. Most plants have both designs that causes confusion with the mechanics.
THE WRONG MECHANICAL SEALS ARE BEING SUPPLIED BY THE PUMP MANUFACTURER.
Unless you have specified a particular seal brand and model number the seals are always unbalanced designs with unknown grades of materials, having very limited application and causing a profusion of spare parts.
Most original equipment manufacturer (O.E.M.) seals will damage shafts (fretting) causing the use of shaft sleeves that will weaken the shaft and raise the L3/D4 number above 60 (2 mm. the metric system)
“C” OR “D” FRAME ADAPTERS ARE NOT BEING PROVIDED AS STANDARD EQUIPMENT.
Although not available for every pump, these adapters can be used to eliminate the need for time consuming and costly alignment procedures. None of the popular pumps are equipped with jack bolts to facilitate the manual alignment and this just compounds the problem. The result is that we find alignment not being done at all in some cases, and done poorly in others, The excuse is always the same, “There is no time to do it correctly”. The result is poor seal and bearing performance.
LIP OR GREASE SEALS ARE BEING PROVIDED TO PROTECT THE BEARINGS FROM WATER GETTING IN AND DESTROYING THE LUBRICATION.
These lip seals have a design life of less than two thousand hours (three months) and will damage the expensive shaft, as they remove the protective oxide layer. All pump manufacturers recognize the short life problem and they install a small rubber ring outboard of the lip seal to try to deflect the water or chemical away from the bearings.
Water ingestion is a major cause of bearing failure. Liquid enters the bearing through the lip seals from three different sources:
Packing leakage
From the water hose that is used to wash away packing leakage.
From the atmosphere (aspiration) when the pump stops and the bearing case cools down. As much as 16 ounces (0,5 liters) of air is expelled from the pump as its’ temperature increases from ambient to operating . This moisture laden air returns through the vent or lip seals as the bearing case cools down at pump shut off.
The problem with water ingestion can easily be solved by replacing the lip seals with mechanical face seals and providing an expansion chamber on the bearing case. Labyrinth seals are another solution although they’re not as totally effective as face seals. Neither the labyrinth seals nor the face seals should cause fretting problems at the bearing location.
THE BEARING LUBRICATION SYSTEM IS POORLY DESIGNED.
The oil level must be located half way through the lower ball of the bearings when the pump is shut off. You need a good sight glass to see this location. Most pumps don’t have a proper sight glass. An automatic oiler doesn’t make any sense since there’s no place for the oil to go except to leak.
Greased bearings applications have no protection to prevent over greasing. The recommended greasing procedures are seldom followed
An oil mist system would be the bestsolutin if you could solve the problem of leakage of the mist to atmosphere and the resultant fugitive emissions problems.
If you open the bearing case of your spare power ends you’ll find that the inside of the case is often badly rusted. The manufacturer should have provided some type of a protective coating to prevent this problem. If you elect to provide your own coating (and you should) be careful about using synthetic oils for your bearing lubrication. They contain strong detergents and can remove many of these protective coatings.
A RECIRCULATION LINE HAS BEEN INSTALLED FROM THE DISCHARGE SIDE OF THE PUMP BACK TO THE STUFFING BOX..
Many liquids contain solids. Centrifugal force will throw these solids against the inner wall of the volute and out this recirculation line. They will then enter into the stuffing box at high velocity, causing premature seal failure.
In most cases the problem can be solved by eliminating this line and connecting a new line from the bottom of the stuffing box to the suction side of the pump. This will recirculate fluid from behind the impeller, (where it is much cleaner) through the stuffing box, and back to the suction side.
CAUTION do not connect to the suction side
If you are pumping the fluid at or near its vapor point. It could flash in the stuffing box.
aif the solids float. Centrifugal force won’t work for you.
This system is not as effective if you are using an open impeller design that adjust towards the back plate (Flowserve as an example)
Double ended pump stuffing boxes are already at suction pressure
THE THRUST BEARING IS BEING RETAINED BY A SIMPLE SNAP RING.
Up to 65% of its efficiency most centrifugal pumps thrust towards the thrust bearing, but between 65% and 100% of the pumps efficiency (the normal running mode) the thrust is towards the pump volute and this means that the simple snap ring is carrying the whole load. This is the reason we see so many bent and broken snap rings. A more positive retaining system is needed.
THE WET END IS NOT A CENTER LINE DESIGN

The above illustration explains the centerline concept. This design will compensate for metal expansion at the wet end of the pump. It should be specified every time the pumping temperature exceeds 200° F (100° C).
Note that the volute is being supported on its sides. This will allow thermal growth to take place both up and down eliminating a great deal of suction pipe strain, wear ring damage and subsequent seal misalignment at the stuffing box face.

A quick reference guide for mechanical seal failure 4-11hooting

Of all the seal related activities, analyzing mechanical seal failure continues to be the single greatest problem for both the consumer and the seal company representative. I’ve addressed this problem in several of my other technical papers. If you will take a little bit of time to familiarize yourself with the following outline you should feel a lot more comfortable the next time you’re called upon to do some seal troubleshooting.

As you look over the failed seal components, keep in mind that a rebuilt seal may have some marks that occurred during a previous failure, making them especially difficult to analyze, but regardless of the design, mechanical seals fail for only two reasons:

  • Damage to one of the components
  • The seal faces open prematurely.

We’ll start with damage. This damage is almost always visible. Look for :

Corrosion – The elastomer swells or the other seal parts become “sponge like” or pitted.

  • The product you’re sealing is attacking one of the seal components.
    • The attack is coming from the cleaner or solvent used to clean the lines between batches, or at the end of a “run”.
    • The attack is coming from lubricants put on the elastomers or seal faces. Petroleum grease on Ethylene Propylene O-rings will cause them to “swell up”.
  • Galvanic corrosion – Happens with dissimilar materials in physical contact and connected by an electrolyte. As an example: stainless steel can attack the nickel binder in a tungsten carbide face.
  • Oxidizers and Halogens attack all forms of carbon including black o-rings.
  • Corrosion always increases with an increase in temperature.

Physical damage.

  • Wear or rubbing of a flexible component.
  • Thermal shock of some seal face materials. Especially those that are hard coated or plated.
  • Thermal expansion of the shaft or sleeve can break a stationary seal face or interfere with the free movement of a dynamic elastomer.
  • The rotating seal hits something because of shaft deflection.
  • Temperature extremes (both high and cryogenic) will destroy elastomers and some seal face materials.
  • Erosion from solids in the product you are pumping.
  • Fretting caused by the dynamic elastomer removing the passivated layer from the corrosion resistant shaft or sleeve.
  • Fluid abrasion that can weaken materials and destroy critical tolerances.
  • A discharge recirculation line circulates high velocity liquid with entrained solids that can break a metal bellows and injure lapped seal faces, as well as interfere with the free movement of the seal.
  • The elastomer or rubber part can swell and breaks the face.
  • Problems at installation. They includes mishandling, setting at the wrong compression, putting the wrong lubricant on the elastomer etc.
  • Fatigue of the springs caused by misalignment.

The seal faces opening prematurely is the second cause

Scoring or wear of the hard face is the most common symptom of this failure. The scoring occurs because the solids imbed into the softer carbon face after they open. The seal faces must stay in contact, but there are all kinds of conditions that are trying to force or pull them open.

Physical causes

  • Axial shaft movement (end play or thrust). This is normal at start up.
  • Radial shaft movement (run out or misalignment)
  • Operating off of the pump’s best efficiency point.(bep)
  • Hysteresis caused by a viscous (thick) product.
  • Centrifugal force tries to separate the faces in a rotating seal application.
  • Hydrodynamic forces generated between the lapped faces.
  • Pressure distortion caused during pressure peaks such as water hammer and cavitation.
  • Thermal distortion that can cause the seal face to separate from its holder or “go out of flat”.
  • A failure to provide equal and opposite clamping across the stationary seal face will cause distortion.
  • A hardened sleeve can cause the seal set screws to slip.
  • A wrong initial setting of the face load.
  • Springs can clog if they are located in the product.
  • Loose set screws. If the sleeve is too soft they can vibrate out.
  • Shaft tolerance and finish is out of specifications.
  • The rotating shaft or seal hits something.
  • A discharge recirculation line can force open the faces.
  • Outside springs painted by maintenance people.
  • A cartridge seal installation method can compress one set of faces and open the other.
  • Vibration.
  • Fretting hang up.
  • Cartridge mounted stationary seals flex excessively unless they have some type of “built in” self-aligning feature.

Product problems . With a loss of an environmental control the fluid can:

  • Vaporize between the lapped faces forcing them open and causing a “chipping” of the carbon outside diameter as well as leaving solids between the lapped faces.
  • Become viscous preventing the faces from following normal “run out”.
  • Solidify between the lapped faces or around the faces.
  • Crystallize between the faces or around the dynamic portions of the seal.
  • Build a film on the sliding components or between the faces causing them to separate.
  • Be a slurry and/ or abrasive
  • Operate in a vacuum causing the ingestion of air between the faces of some unbalanced seal designs.
  • Swell up the dynamic elastomer, locking up the seal .
  • Cause slipstick between the faces if the sealed fluid is a non, or poor lubricant

The common causes of shaft displacement.

  • Operating off the pump’s best efficiency point (bep).
  • Misalignment between the pump and its driver.
  • The rotating assembly is out of balance.
  • A bent shaft.
  • A non concentric sleeve or seal.
  • Vibration
  • Slip-stick
  • Harmonic vibration
  • Induced
  • Passing through, or operating at a critical speed.
  • Water hammer in the lines.
  • The stuffing box is not square to the shaft, causing misalignment problems.
  • Pipe strain.
  • An impeller adjustment is made to compensate for normal impeller wear.
  • Thermal growth of the shaft in both a radial and axial direction.
  • Bad bearings or a poor bearing fit.
  • Two direction axial thrust at start up is normal.
  • The motor is finding its magnetic center.
  • Cavitation – there are multiple types of damage that can be observed.
  • The sleeve moved when the impeller was tightened.
  • The unit is pulley driven causing excessive side thrust
  • The impeller is positioned too far from the bearings. This is a severe problem in mixer or agitator applications.

How to prevent product problems that cause premature seal failure.

Control the environment in the stuffing box.

  • Control the temperature in the seal area
  • Use the correct spring or bellows compression.
  • Use only hydraullically balanced seals.
  • Select a low friction face combination.
  • Avoid “dead ending” the stuffing box.
  • Jacket the stuffing box
  • Quench behind the seal with the correct temperture steam or fluid
  • Use a gland jacket
  • Utilize two seals with a barrier fluid between them
  • Use heat tape around the stuffing box
  • Use a heat pipe to remove heat from the stuffing box.
  • Vent the stuffing box, especially in a vertical application
  • Flush in a cool compatible liquid.
  • Control the pressure in the seal area
    • Be sure to use only hydraulically balanced seals.
    • Discharge recirculation will raise the pressure if you put a restrictive bushing into the bottom of the stuffing box.
    • Suction recirculation will lower the pressure in the stuffing box.
    • Use two seals and let the barrier fluid control the pressure between the seals.
    • Cross connect the stuffing boxes to equalize the stuffing box pressures in a multi stage pump.
    • Stage the stuffing box pressure with tandem seals.
    • Impeller pump out vanes will lower stuffing box pressure.
  • Give the seal more radial space
    • Bore out the existing stuffing box if it is possible.
    • Make or buy a new back plate with the large stuffing box cast into it.
    • Make or buy a large bore stuffing box and attach it to the back plate after you have machined the old one off.
  • Flush the product away if you’re unable to control it.
    • Suction recirculation will bring fluid into the stuffing box from behind the impeller, where it is usually cleaner. This works on most closed impeller pump applications and those open impeller pump applications where the impeller adjusts to the volute rather than the back plate.
    • Flush with a clean liquid from an outside source.
    • A pressurized barrier fluid between two seals can keep solids from penetrating between the faces, if the faces should open. This application will also work if the solid particles are less than one micron in diameter (Kaoline is such a product).

Build the seal to compensate for operating extremes.

Slurry features that can be part of seal design.

  • Springs out of the fluid
  • Teflon coating the metal parts so particles will not stick to sliding components.
  • The elastomer moves to a clean surface as the face wears.
  • Keep the sealing fluid on the outside diameter of the seal to take advantage of centrifugal force that will throw solids away from the lapped faces.
  • Rotate the fluid with the seal to prevent erosion of the seal components. A simple vane arrangement can accomplish this.
  • Use two hard faces if you find it impossible to keep the lapped seal faces together.
  • Use a pumping ring to keep solids away from the faces.
  • Mount the seal closer to the bearings to diminish the affect of shaft deflection.

Design for higher temperature capability

  • Eliminate elastomers when ever possible.
  • If you cannot eliminate elastomers, the O-ring location becomes important. Try to move the elastomer away from the faces.
  • Hydraulically balanced seals generate less heat.
  • Select low friction faces.
  • Fool proof, correct installation dimensions are necessary. A cartridge design is your best choice.
  • Keep a good product circulation around the components.
  • A good lapping technique will keep the faces flat at high and cryogenic temperatures.
  • Pumping rings will keep fluid circulating between two seals. If you are using balanced seals a simple convection tank is usually more than adequate. An air operated diaphragm pump can be used in the line to increase the circulation. Try to avoid the use of petroleum based fluids as the barrier or buffer fluid between the seals. Petroleum based fluids have a very low specific heat that will increase the temperature between the seals,
  • Gland features such as quenching, recirculation, venting and flushing help.
  • Choose well designed faces that will resist thermal distortion. The closer you get to a “square block” design, the better off you’re going to be.
  • Do not insulate the faces with an elastomer.

Design for pressure resistance

  • Limit the number of diameters in any single seal component
  • Laminated bellows will allow you to keep a low spring rate while maintaining pressure capability, if you are using a welded metal bellows design.
  • Finite element analysis of the seal components will prevent pressure and temperature distortion.
  • Use more mass to resist hoop stresses.
  • Higher modulus materials will resist bending and deformation.
  • Use a tandem seal design for pressure break down between two seals.

Design for corrosion resistance

  • Choose good materials, clearly identified by type and grade.
  • Eliminate elastomers when possible. Elastomers are the most corrosion sensitive part of the seal.
  • Design non stressed parts when ever possible
  • Try not to weld any of the metal components. If it is necessary, monitor the temperature to prevent inter granular corrosion
  • Control the temperature. Corrosion increases with temperature.
  • Use non metallic materials for non metallic equipment.
  • Watch out for galvanic corrosion when using dissimilar materials.
  • Do not use stainless steel springs. Stick with Hastelloy “C” if the metal parts of the seal are manufactured from iron, steel, stainless steel, or bronze. If the seal is manufactured from a different metal, use springs manufactured from that material.
  • Do not depend upon flushing to provide corrosion resistance. Use the correct materials, keeping in mind that solvents and steam are sometimes used to flush the lines. Any materials that you select must be compatible with these flushing or cleaning fluids also.

If you need cryogenic capability

  • Go to a welded metal bellows configuration to eliminate all elastomers.
  • You will need a special carbon/ graphite face that has an organic material impregnated to assist in the release of the graphite.
  • Avoid plated or coated hard faces. Differential expansion will cause them to crack.
  • Always lap the faces at a cryogenic temperature.
  • Do not coat the faces with grease or oil. It will freeze at cryogenic temperatures.
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