Troubleshooting mechanical seals at the pump site 4-2
Leakage can occur at any time throughout the life of the mechanical seal. To troubleshoot leaking seals effectively it is helpful to know just when the leakage starts. This is the advantage of being able to troubleshoot a running pump, or one that is still hooked up to its piping. By noting the type of leakage and when the leakage occurs, we can do a more thorough job of analyzing any seal failure. In addition to leakage we’ll also be looking for other symptoms that are visible to the trained troubleshooter.
We’ll start with the different types of leakage. Please look at the following diagram of a stationary seal design in an API gland and installed on a jacketed pump
The leakage can be detected visually, by odor, or by instrumentation. A strobe light can sometimes be used to determine its location. As you can see in the above diagram, there are several leak paths possible. The seal can leak :
At the lapped faces. Since they are a wearable surface the leak will probably get either better or worse. It should never remain constant. The leak started because:
- The outside springs in a dual cartridge seal were painted during routine maintenance.
- The spring load has been reduced because of thermal growth, axial thrust, or impeller adjustment.
- The seal was set-screwed to a hardened shaft and the set screwa have vibrated loose.
- One or both of the seal faces is not flat. Solid tungsten carbide and silicon carbide faces are often lapped flat on only one side. Check to see if the face has been installed backwards.
- The dynamic elastomer has swollen up and seized the spring loaded face; preventing it from remaining in contact with the stationary face.
- The product has prevented the lapped seal faces from remaining in contact.
- Dirt has gotten into the sliding components.
- The product has crystallized.
- The product solidified or became very viscous.
- The product is vaporizing across the seal faces, expanding and blowing them open.
At the static and dynamic elastomer locations.
- This type of leak tends to remain constant and will often stop when the small opening clogs up with solids. The leak can be caused by a damaged rubber part, or damage on the surface where the elastomer seals. In some instances the elastomer is not seated properly. It twisted because of either poor installation, excessive shaft movement, or high pressure extrusion.
At the gland gasket.
- This is the easiest leak to detect because it’s very visible and does not change with shaft rotation.
Between the shaft sleeve and the shaft.
- This is a common problem with double ended pumps, where the sleeve is used to position the impeller and there is no method of sealing the sleeve against the impeller.
Between the seal face and its metal holder.
- The leakage frequently increases, as the product temperature increases, because the metal face holder has an expansion rate three times that of the carbon face.
Through fretting damage
- The damage is caused by spring loaded dynamic o-rings, Teflon wedges, chevrons, U- cups etc.
- You can’t miss the frett marks. They’ll be located on the pump shaft, pump sleeve, or inner sleeve of the mechanical seal.
- The stationary face has been over tightened against the stuffing box face causing it to go out of flat. Statically the carbon will readjust to the distorted hard face and not leak.
- The clamping is not equal and opposite across the static seal face. Look for different width gaskets at the front and rear of the static face. Again, the carbon will readjust and stop leaking when the shaft is not turning.
- Between the face and holder. The holder heats up and expands faster than the pressed in face. The leak will begin when the metal holder comes up to temperature. Remember that metal expands three times faster than a typical seal face.
- Cryogenic (cold) service will harden the elastomer. Be sure to check the lower temperature limit of the elastomer that you selected.
- Misalignment between the pump and the driver.
- The shaft is bending and not allowing the seal to move freely. This occurs if the pump is operating off of its best efficiency point and the shaft L3/D4 is not small enough to resist the bending.
- The product is vaporizing across the seal faces.
- Cavitation, slip stick, harmonic, or some other type of vibration is bouncing the faces open, check the lugs or drive pins for sign of excessive wear.
- The seal was installed without enough compression, or the impeller was adjusted after the seal was installed and thermal expansion of the shaft is opening the faces.
- A discharge recirculation line is aimed at the seal faces, or some other critical point and the faces are being forced open.
- A non- concentric seal, bad sleeve installation, or an out of balance rotating assembly is causing the rotating portion of the seal to run off the stationary face.
- A bent shaft can cause the rotating portion of the seal to run off the stationary face.
- The rotating portion of the seal is hitting a stationary object. Look for:
- A protruding gasket or fitting.
- A foreign object that has worked its way into the stuffing box area.
- A stationary portion of the rotating equipment, such as a close fitting bushing.
- At elevated temperature the product thins out (the viscosity decreases) and is leaking through an elastomer. It will not leak at the cooler temperature when the product viscosity is higher.
- High temperature is causing the lapped seal face to go out of flat.
- The seal is also leaking while running, but the leak is vaporizing and not visible. Hold a piece of white paper over the seal area and see if the paper becomes damp.
- A meniscus caused by centrifugal force and liquid surface tension had formed at the inside diameter of the seal faces. This prevented a leaking seal from dripping while the shaft was turning.
- You are using high temperature grade Kalrez. It’s too hard at ambient temperature. It will soften and seal when the system comes back to operating temperature.
- The pump is running under vacuum and while it is running, air is being pulled into the system. The fluid leaks out when the shaft is static. This can occur if an open impeller, that was designed to be adjusted against the volute has accidentally been adjusted backwards against the back plate. The impeller “pump out vanes” can then pull a vacuum in the stuffing box. This is a common problem if you use a lot of Flowserve pumps and then bring in a few of another brand.
- Face distortion. Caused by a high pressure surge that was created when the pump was started with the discharge valve shut.
- The shaft is bending, and interfering with the seal movement.
- Occurs because the pump is running with the discharge valve throttled or shut. Operators shut the discharge valve at start up to save electricity and prevent cavitation.
- The same problem can occur if the pump is started with the discharge valve wide open and because of the lack of discharge resistance the pump will run to the right hand side of its curve. In some cases you could also burn out the electric motor.
- The product has changed state, and becomes a liquid again when the pump comes up to operating temperature. The product had :
- Became viscous
- Excessive axial shaft movement at start up. This is a common problem with sleeve bearing equipment.
Look for reoccurring events that initiate the leakage. They can include:
- Flushing the lines at the end of a batch or season.
- Alternating pumps in a multiple pump arrangement.
- An additive is being put into the product.
- Batch operations are beginning or ending.
- The cooling water is passing through temperature cycles.
- The outside ambient temperature has changed dramatically. I ran into a situation where a supplier was oiling the bed of his truck to prevent solids from sticking in the winter and this oil attacked the elastomer in the seal.
- Hard water is being used as a flush and it is gradually restricting the flush lines or cooling jacket.
- A filter or strainer is clogged in a flush line.
- The flushing water pressure drops at certain times of the day because of demand.
- The boiler or cooling tower is being blown down.
- There is a control valve in the pump discharge that is causing the pump to occasionally operate too far off of the B.E.P.
- The stuffing box is cycling between a positive and negative pressure.
- Vortexing can occur if the pump suction falls too low. This also occurs in mixers and agitators..
- You are quenching a high temperature application with water. As the quench water vaporizes it leaves dissolved solids outboard of the seal restricting axial movement as the seal faces wear.
- The pump is cavitating on a regular or intermittent basis. Here are a few possibilities:
- The suction level falls too low
- The tank vent freezes.
- The velocity is too high on the suction side of the pump.
- A suction strainer is plugged up.
- A stuck or broken check valve in the pump suction piping.
- A temporary loss of discharge head.
- A booster pump has shut off.
- A suction eccentric reducer was installed up side down allowing slugs of air into the suction of the pump.
- The fluid is vortexing in the supply tank. The level is too low for the pump capacity.
- The pump is lifting liquid and the foot valve is sticking..
- The impeller is too close to the cutwater.
- Air is entering the system through the pump packing.
- A lower “specific speed” impeller as been substituted.
- The pump was specified with too low a “suction specific speed” number.
- The pump is running at a higher speed or a larger impeller was installed after the system heads were calculated.
- In some parallel pump installations, a stronger pump can throttle the weaker one causing shaft deflection.
- The wrong lubricant was used on the dynamic elastomer, causing it to swell up.
- Reaction bonded type, silicon carbide can crack if the lines were flushed with a caustic solution.
The leak rate is changing, It gets better or worse.
- This type of leak is usually associated with seal face leakage because the seal face is a wearable surface.
- The carbon seal face is not flat.
- The seal face was damaged at the time of assembly.
- Dirt or solids are imbedded into one of the faces
- Coke (over heated oil) or some other solid has formed on the seal faces causing them to separate.
- The rotating face is hung up on the shaft.
- Outside seal springs have been painted during routine maintenance.
The faces spit liquid.
- The product is vaporizing at the faces – check the fluid vapor point. When using balanced seals, the stuffing box pressure must be at least one atmosphere higher than the product vapor point. Unbalanced seals require a much higher differential pressure.
- The rotating face is running off of the stationary face.
- The stationary was not centered to the shaft – a common problem.
- The seal is not concentric with the shaft.
- The rotating assembly is out of balance.
- The shaft is bent.
Fire hose type leakage. The leak is following shaft rotation.
- Product has solidified on the seal face and a piece has broken off. This is usually initiated by a high temperature between the faces.
- The rotating face is cracked.
- The hard surfacing, or coating, is lifting off of the rotating face.
- Temperature changes or pressure surges are altering the face flatness within the elastic range of the material.
- The stuffing box is alternating between vacuum and pressure
- The movable face is sluggish and not able to follow run out.
- The product is viscous.
- The product has started to solidify.
- The shaft/ sleeve is too large in diameter restricting movement of the seal. Spring loaded, dynamic elastomers such as Teflon wedges, U- cups, Chevrons and spring loaded O-ring designs are very sensitive to this problem
- Dirt or solids are clogging the seal and preventing it from following shaft run out.
- In a non O-Ring version, the spring load is too high causing the elastomer or Teflon to stick to the shaft.
- The product is occasionally vaporizing between the faces.
- There is a leak between the face and the holder that becomes visible only when the unit comes up to operating temperature.
- A bending, or bent shaft is causing the seal outside diameter to contact the inside diameter of the stuffing box, or some other stationary object.
- The pump is running with too high or too low a head. Check the pump curve against actual operating conditions.
- The application is cycling between ambient and cryogenic temperatures causing the elastomer to harden on the cold cycle and the faces to go out of flat.
The seal area is damp. There is no visible leakage.
- There is a leaking flange or fitting above the seal that is dripping close to the seal location.
- The product is vaporizing. Hold a clean piece of white paper over the running seal, and check for leakage. The paper will become damp.
- Any condition that could cause intermittent leakage will cause this problem.
Constant dripping. It never changes. This cannot be a damaged seal face leak because seal faces are a wearable surface and the leak rate would have to change.
- The elastomer is cut or nicked.
- The shaft/sleeve is damaged at the elastomer location.
- There is damage in the metal o-ring groove. Maybe the o-ring was removed with a sharp metal instrument and this has caused a scratch in the o-ring groove.
- There is a leak path between the carbon and the holder.
- Leaking at the cartridge sleeve location.
- Leaking between the sleeve and the shaft.
- Leaking between the gland and the stuffing box.
- Leaking between the stationary face and the seal gland.
- The seal faces are stuck open.
- The elastomer has swollen up due to chemical attack by either the product, the flush, what ever is being used to clean the lines, or by the lubricant that was put on the elastomer to help the installation. This attack usually takes place within one week of exposure to the non compatible lubricating fluid.
THE STUFFING BOX AREA IS GETTING HOT.
Heat is being generated at the seal faces. Unbalanced seals generate more heat than hydraulically balanced seals.
- The carbon is being insulated by an elastomer and cannot dissipate the heat.
- You’e using high friction face materials. Two hard faces usually generate more heat than carbon vs. a hard face.
- The seal faces are running dry.
- The stuffing box has not been vented. This is especially important in vertical applications.
- You do not have a barrier fluid between the seals in a dual seal application.
- You have lost an environmental control.
- The cooling jacket is clogged or not functioning for some reason.
- The discharge or suction recirculation line is clogged.
- The barrier fluid has stopped circulating in a dual seal application or you are using oil as a barrier fluid. Oil has a low specific heat and poor conductivity, making it a poor choice as a heat transfer medium. If you must use oil as the barrier fluid you may have to forsake convection and go to a forced circulation system or a pumping ring.
- An A.P.I. type gland has been piped incorrectly
- Poor conductivity of the hard face. Silicon carbide is better than 99.5 ceramic.
- There is too much spring load on the seal faces:
- A wrong installation measurement.
- The impeller was adjusted after the seal was installed. Any pump impeller that adjusts against the back plate has this problem. Durco pumps are a good example.
- Excessive axial movement of the shaft.
- Thermal expansion
A seal component is rubbing the inside diameter of the stuffing box, or against a solid that has attached its self to the inside of the stuffing box.
- The seal is not concentric with the shaft.
- The shaft is out of balance.
- The shaft is bent.
The sleeve, shaft or rotating seal is hitting a stationary component.
- A protruding gasket or fitting.
- A bushing in the bottom of the stuffing box.
A foreign object is loose in the stuffing box.
A suction recirculation line was used to lower stuffing box pressure. The high velocity recirculation is heating up the return line.
- The product is cavitating in the pump. There are four types of cavitation:
- Internal recirculation
- The Vane Passing Syndrome
- Air ingestion (It resembles cavitation)
- A component is rubbing.
- The bearings are bad.
- The seal has come loose from the shaft.
- A foreign object has entered into the stuffing box.
- The sleeve is hitting an A.P.I. disaster bushing.
- The seal faces are running dry. They will make a whistling noise.
- You have hit a critical speed.
- Coupling misalignment.
- The noise is coming from the motor or some near by equipment.
- “Slip stick” at the seal faces.
The convection tank
- It’s running backwards.
- The seal faces are off center causing a pumping action across the faces.
- The seal gland inlet and outlet ports are not drilled properly.
- A cartridge dual seal has not been centered properly
- The pressure or level in the convection tank changes.
- One of the seals is leaking. The pressure or level change should tell you which one.
- Temperature change.
- No air pocket in the tank.
- Not convecting. It was installed incorrectly. The minimum and maximum installation dimensions were ignored.
Flow meter not indicating.
- The meter is broke.
- one of the lines is clogged.
- The flow is not high enough.
- The gage graduations are too large.
No flow through the quench and drain connections.
- You’re piped to the wrong connection. Most glands that have been drilled for a quench connection, have a flush connection also. Make sure you’re not connected to that one.
- Valve not open
- The fitting or line is clogged
Loss of jacket cooling. The incoming and outgoing lines are at the same temperature.
- A layer of calcium has built up on the inside of the cooling jacket.
- A discharge recirculation line is connected to the stuffing box (it may be hidden inside the insulation).
- Some one has shut off the cooling water or steam.
- Cavitation. Remember there are four types.
- The pump is operating off of its best efficiency point.
- Unbalanced impeller or rotating assembly. Look for wear or some type of solid attached.
- Bent shaft.
- Bad bearings.
- Misalignment between the pump and driver.
- Pipe strain. Maybe you need a center line design pump
- Rotating component hitting a stationary component.
- The pump is running at a critical speed
- Harmonic vibration induced by nearby equipment.
- Loose hold down bolts.
- Pipe hangers are spaced improperly.
- The mass of the pump base is not five times the mass of the pump, and motor.
- The base is too narrow. Imaginary lines extended downward thirty degrees to either side of a vertical through the pump shaft must pass through the bottom of the foundation, not the sides.
- Seal “slip stick” that can occur when pumping non lubricants such as hot water and most solvents.
- On February 09, 2018