Narratives

THE TUTORIALS (NARRATIVES) TN001

In the following section we will be looking at overviews of several pump and seal subjects:

In these narratives I am attempting to put each of the subjects into perspective. You will use the narratives for multiple purposes:

  • To learn the terms we use for each of these individual subjects.
  • To see how the various subjects fit together.
  • To find out how much you know about any one of the subjects.
  • And you can use the narratives as an outline to teach the subjects to other people.

I suggest that you read the entire narrative and then go back and look up the details of any unfamiliar words or subjects. Any word or phrase in blue and underlined is a link to a detailed explanation of the subject. Most of the time I have tried to use the link only for the first mention of the word or phrase; otherwise the narrative would be full of links.

CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP FOR YOUR APPLICATION.

We will begin by deciding what operating conditions our pump has to meet and then we will approach pump suppliers to see how closely they can satisfy these needs. Unfortunately no comprehensive theory which would permit the complete hydrodynamic design of a centrifugal pump has evolved in the many years that pumps have been around, so the pump manufacturer will be doing the best he can with the information you supply to him.

To clearly define the capacity and pressure needs of our system we will construct a type of graph called a system curve. This system curve will then be given to the pump suppliers and they will try to match it with a pump curve that satisfies these needs as closely as possible.

To start the construction of the system curve I will assume you want to pump some fluid from point “A” to point “B”. To do that efficiently you must make a couple of decisions:

  • Decide the capacity you will need. This means the gallons per minute or cubic meters per hour. You must also consider if this capacity will change with the operation of your process. A boiler feed pump is an example of an application that needs a constant pressure with varying capacities to meet a changing steam demand. The demand for boiler water is regulated by opening and closing a control valve on the discharge side of the pump with a discharge re-circulation line returning the unneeded portion back to a convenient storage place, or the suction side of the pump. Remember that with a centrifugal pump if you change its capacity you change the pressure also. A rotary or positive displacement pump is different. It puts out a constant capacity regardless of the pressure.
  • For other centrifugal pump applications, you are going to have to calculate how much pressure will be needed to deliver different capacities to the place where you will need them. You will need enough pressure to :
    • Reach the maximum static head or height the fluid will have to attain.
    • Enough discharge pressure to over come any pressure that might be in the vessel where the fluid is discharging, such as the boiler we just discussed. This is called the pressure head.
    • Overcome friction resistance in the lines, fittings and any valves or hardware that might be in the system. As an example: high-pressure nozzles can be tricky, especially if they clog up. This resistance is called the friction head.
  • Will you need any special materials for the pump components?
    • The pump manufacturer will try to choose pump metal components that are chemically compatible with what you are pumping as well as any cleaners or solvents that might be flushed through the lines. If the temperature of the pumpage changes the corrosion rate can change also. His choice of materials could have a serious affect on your spare parts inventory. Will he be selecting universal and easily obtainable materials? Unless you have a great deal of experience with the product you are pumping do not select the metal components by using a compatibility chart. Metal selection is a job for metallurgists or your own experience.
    • If the product you are pumping is explosive or a fire hazard, you should be looking at non-sparking materials for the pump components. Do not depend totally upon the pump manufacturer to make this decision for you. If you are not sure what materials are compatible with your product, how will the pump man know? Also, keep in mind that some of the fluids you will be pumping could be proprietary products known only by their trade name.
    • Dangerous and radioactive materials will dictate special materials.
    • Food products require high-density seal and pump materials that are easy to clean.
    • If there are abrasive solids in the pumpage you will need materials with good wearing capabilities. Hard surfaces and chemically resistant materials are often incompatible. You may have to go to some type of coating on the pump wetted parts or select an expensive duplex metal.
  • Occasionally you will find an application where metal is either not compatible or not practical. There are many monomer and polymer materials available for these applications, but their cost is generally higher than comparable metal parts. Be aware that if you are using a mechanical seal in a non-metallic pump, the seal cannot have metal parts in contact with the fluid for the same reasons the pump was manufactured from non-metallic materials. Use a non-metallic seal in these applications

Since we are just getting into the subject, one of the first things we should learn is that centrifugal pump people do not use the word pressure. As mentioned in an earlier paragraph they substitute the word “head”, so you will have to calculate the three kinds of head that will be combined together to give you the total head of the system required to deliver the needed capacity. Here are the three kinds of head you will be calculating:

  • The static head or maximum height that the liquid will reach. We must also learn how to compensate for the siphon affect from down running pipes on the discharge side of the pump. Remember that if you fill a tank from the bottom instead of the top the static head will continually increase. This is not a good application for a centrifugal pump because the capacity is decreasing with an increasing head. If you must fill from the bottom, or if you will be using the pump as an accumulator, a rotary positive displacement pump will be your best choice as long as it can meet the needed capacities.
  • The pressure heads are next if the container we are pumping to, or from, is pressurized. We will have to learn how to convert pressure units to head units because later on we will need this conversion knowledge to read the manufacturers pump curve. Pump gages are labeled in psi or bar. Pump curves are labeled in feet of head, or meters of head.
  • The friction head is the last one that we will have to calculate. This head tells us how much friction or resistance head there is in both the suction and discharge piping, along with the fittings and valves in the piping system. And to make the job a little tougher this head changes dramatically as the pump capacity changes.

You will be calculating these heads on both the suction and discharge side of the pump. To get the total head you will subtract the suction head from the discharge head and that will be the head that the pump must produce to satisfy the application. It will become obvious in the calculations, but I should mention here, that if the suction head is a negative number, the suction and discharge heads will be added together to get the total head. If you subtract a minus number from a positive number you must add the numbers together. As an example: 4 – (-2) = + 6

The total head of a pump seldom remains static. There are a number of factors that can change the head of a pump while it is operating, and you should become familiar with most of them.

All of this head information is calculated from piping, valve, and fitting, friction graphs you will find in the index.This head data will be plotted on a set of coordinates called a system curve. Since we will not be operating at a single point all of the time we will make the calculations for a range of different capacities and heads that we might expect to encounter. This range is described as the operating window we will need to satisfy the application.

Making these calculations is not an exact science because the piping is seldom new, pipe inside diameters are not exact, and the graphs you will be consulting cannot compensate for corrosion and solids built up on the piping, valve and fitting walls.

Life is never simple. This is the point where most people start adding in safety factors to compensate for some of the unknowns. These safety factors will almost always guarantee the selection of an oversized pump that will run off of its best efficiency point (BEP) most of the time.

The final calculations are then plotted on the system curve that describes what the pump has to do to satisfy the requirements of the application. You can learn to do all of this by referencing the following subjects:

  • Calculating the total head in metric units
  • Calculating the total head in USCS (inch) units
  • Making a system curve, S111

The pump manufacturer requires a certain amount of net positive suction head required (NPSHR) to prevent the pump from cavitating. He shows that number on his pump curve. When you look at the curve you will also note that the net positive suction head required (NPSHR) increases with any increase in the pump’s capacity.

You will also be calculating the net positive suction head available (NPSHA) to be sure that the pump you select will not cavitate. Cavitation is caused by cavities or bubbles in the fluid collapsing on the impeller and volute. In the pump business we recognize several different types of cavitation. :

Pump cavitation is recognized in several different ways

  • We can hear cavitation because it sounds like the pump is pumping rocks or ball bearings.
  • We can see the damage from cavitation on the pump’s impeller and volute.
  • The operator can sometimes tell if the pump is cavitating because of a reduction in the pump’s capacity.
  • The main problem with cavitation is that it shakes and bends the shaft causing both seal and bearing problems. We call all of this shaking and bending shaft deflection.

Remember that the net positive suction head required (NPSHR) number shown on the pump curve is for fresh water at 68° Fahrenheit (20°C) and not the fluid or combinations of fluids you will be pumping.

When you make your calculations for net positive suction head available (NPSHA) the formula you will be using will adjust for the specific gravity of your fluid.

  • In some cases you can reduce the NPSH required. This is especially true if you are pumping hot water or mixed hydrocarbons.
  • You may have to install an inducer on the pump, add a booster pump, or go to a double suction pump design if you do not have enough net positive suction head available (NPSHA)

When the pump supplier has all of this in-exact information in his possession he can then hopefully select the correct size pump and driver for the job. Since he wants to quote a competitive price he is now going to make some critical decisions:

He might begin with the type of pump he will recommend:

  • If the capacity were going to be very low he would recommend a rotary, or positive displacement (PD) pump.
  • Between 25 and 500 gpm (5 m/hr – 115 m3/hr) he will probably select a single stage end suction centrifugal pump. It all depends upon the supplier. At higher capacities he may go to a double suction design with a wide impeller, two pumps in parallel or maybe a high-speed pump.
  • You might need a high head, low capacity pump. The pump supplier has several options you should know about.
  • Will he recommend a self-priming pump? These pumps remove air from the impeller eye and suction side of the pump. Some operating conditions dictate the need for a self-priming design. If you do not have a self-priming pump and you are on intermittent service, will priming become a problem the next time you start the pump?
  • How will the pump be operated?
    • If the pump is going to run twenty-four hours a day, seven days a week and you are not going to open and close valves; you will not need a heavy-duty pump. It is easy to select a pump that will run at its best efficiency point and at the best efficiency point (BEP) there is very little shaft displacement and vibration.
    • Intermittent service is the more difficult application because of changing temperatures, vibration levels, thrust direction, etc. Intermittent pumps require a more robust, heavy-duty design with a low L3/D4 shaft.
  • How important is efficiency in your application? High efficiency is desirable, but you pay a price for efficiency in higher maintenance costs and a limited operating window. You should be looking for performance, reliability, and efficiency in that order. Too often the engineer specifies efficiency and loses the other two. The following designs solve some operation and maintenance problems, but their efficiency is lower than conventional centrifugal pumps.
    • magnetic drive or canned pump may be your best choice if you can live with the several limitations they impose.
    • vortex or slurry pump design may be needed if there are lot of solids or “stringy” material in the pumpage.
    • A double volute centrifugal pump can eliminate many of the seal problems we experience when we operate off the pump’s best efficiency point. The problem is trying to find a supplier that will supply one for your application. Although readily available for impellers larger than 14 inches (355 mm) in diameter they have become very scarce in the smaller diameters because of their less efficient design.
  • The supplier should recommend a centerline design to avoid the problems caused by thermal expansion of the wet end if you are operating at temperatures over 200°F (100°C)?
  • Will you need a volute or circular casing? Volute casings build a higher head; circular casing are used for low head and high capacity.
  • Do you need a pump that meets a standard? ANSI, API, DIN, VDMA or ISO are some of the current standards. You should be aware of pump standards problems that contribute to premature seal and bearing failures. An ANSI (American National Standards Institute) standard back pullout design pump has many advantages but presents problems with mechanical seals when the impeller clearance is adjusted, unless you are purchasing cartridge seals.
  • The decision to use either a single or multistage pump will be determined by the head the pump must produce to meet the capacities you need. Some suppliers like to recommend a high speed small pump to be competitive, other suppliers might recommend a more expensive low speed large pump to lessen NPSH and wear problems.

There are additional decisions that have to be made about the type of pump the supplier will recommend:

  • Will the pump be supplied with a mechanical seal or packing? If the stuffing box is at negative pressure (vacuum) a seal will be necessary to prevent air ingestion.
  • If he is going to supply a mechanical seal will he also supply an oversized stuffing box and any environmental controls that might be needed?
  • Will he specify a jacketed stuffing box so that the temperature of the sealed fluid can be regulated? How does he intend to control the stuffing box temperature? Will he be using water, steam or maybe a combination of both? Electric heating is sometimes an option.
  • How will the open or semi-open impeller be adjusted to the volute casing or back plate? Can the mechanical seal face loading be adjusted at the same time? If not, the seal face load will change and the seal life will be shortened.
  • If the pump is going to be supplied with a closed impeller you should have some means of knowing when the wear rings have to be replaced. If the wear ring clearance becomes too large the pumps efficiency will be lowered causing heat and vibration problems. Most manufacturers require that you disassemble the pump to check the wear ring clearance and replace the rings when this clearance doubles.
  • Will he supply a “C” or “D” frame adapter, or will the pump to motor alignment have to be done manually using dual indicators or a laser aligner to get the readings? A closed-coupled design can eliminate the need for an alignment between the pump and driver.
  • What type of coupling will he select to connect the pump to its driver? Couplings can compensate for axial growth of the shaft and transmit torque to the impeller. They cannot compensate for pump to driver misalignment as much as we would like them to. Universal joints are especially bad because they have to be misaligned to be lubricated.
  • He may decide to run two pumps in parallel operation if he needs a real high capacity, or two pumps in series operation if he needs a high head. Pumps that run in parallel or series require that they are running at the same speed. This can be a problem for some induction motors..
  • An inline pump design can solve many pipe strain and thermal growth problems.
  • The pump supplier must insure that the pump will not be operating at a critical speed or passing through a critical speed at start up. If he has decided to use a variable speed drive or motor this becomes a possibility.
  • We all want pumps with a low net positive suction head required to prevent cavitation problems but sometimes it is not practical. The manufacturer has the option of installing an inducer or altering the pump design to lower the net positive suction head required, but if he goes too far all of the internal clearances will have to be perfect to prevent cavitation problems. This modification of the impeller to get the low net positive suction head required (NPSHR) and its affects will be explained when you learn about suction specific speed.
  • The difference between specific speed and suction specific speed can be confusing but you should know the difference.
  • Shaft speed is an important decision. Speed affects pump component wear and NPSH requirements, along with the head, capacity, and the pump size. High speed pumps cost less initially, but the maintenance costs can be staggering. Speed is especially critical if you are going to be specifying a slurry pump.
  • The ratio of the shaft diameter to its length is called the shaft L3/D4number. This ratio will have a major affect on the operating window of the pump and its inital cost. The lower the number the better, but any thing below 60 (2 in the metric system) is acceptable when you are using mechanical seals. A low L3/D4 can be costly in a standard long shaft pump design because it dictates a large diameter shaft that is usually found only on expensive heavy-duty pumps. A short shaft with a smaller outside diameter would accomplish the same goal, but then the pump would no longer conform to the ANSI or ISO standard. We often run into L3/D4problems when you specify, or the pump supplier sells you a low cost, corrosion resistant sleeve, mounted on a steel shaft rather than a more expensive solid, corrosion resistant shaft.

There are multiple decisions to be made about the impeller selection and not all pump suppliers are qualified to make them:

  • The impeller shape or specific speed number will dictate the shape of the pump curve, the NPSH required and influence the efficiency of the pump.
  • Has the impeller configuration been iterated in recent years? Impeller design is improving with some of the newer computer programs that have become available to the design engineer.
  • The suction specific speed number of the impeller will often predict if you are going to experience a cavitation problem.
  • The impeller material must be chosen for both chemical compatibility and wear resistance. You should consider one of the duplex metals because most corrosion resistant materials are too soft for the demands of a pump impeller.
  • The decision to use a closed impeller, open impeller, semi-open, or vortex design is another decision to be made.
  • Closed impellers require wear rings and these wear rings present another maintenance problem.
  • Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent internal recirculation.
  • Vortex pump impellers are great for solids and “stringy” materials but they are up to 50% less efficient than conventional designs.
  • Investment cast impellers are usually superior to sand cast versions because you can cast compound curves with the investment casting process. The compound curve allows the impeller to pump abrasive fluids with less vane wear.
  • If you are going to pump low specific gravity fluids with an open impeller, a non-sparking type metal may be needed to prevent a fire or explosion. You will be better off choosing a closed impeller design with soft wear rings in these applications.
  • The affinity laws will predict the affect of changing the impeller speed or diameter. You will want to be familiar with these laws for both centrifugal and PD pumps..

Either you or the supplier must select the correct size electric motor, or some other type of driver for the pump. The decision will be dictated by the specific gravity of the liquid you will be pumping along with the specific gravity of any cleaners or solvents that might be flushed through the lines. The selection will also be influenced by how far you will venture off the best efficiency point (BEP) on the capacity side of the pump curve. If this number is under-estimated there is a danger of burning out some electric motors.

  • How are you going to vary the pump’s capacity? Are you going to open and close a valve or maybe you will be using a variable speed drive like a gasoline or diesel engine. Will the regulating valve open and close automatically like a boiler feed valve or will it be operated manually? The variable speed motor might be an alternative if the major part of the system head is friction head rather than static or pressure head.
  • The viscosity of the fluid is another consideration because it will affect the head, capacity, efficiency and power requirement of the pump. You should know about viscosity and how the viscosity of the pumpage will affect the performance of the pump. There are some viscosity correctionsyou should make to the pump curve when you pump viscous fluids.
  • After carefully considering all of the above, the pump supplier will select a pump type and size, present his quote and give you a copy of his pump curve. Hopefully you will be getting his best pump technology. To be sure that is true you should know what the best pumping technology is.
  • At this stage it is important for you to be able to read the pump curve. To do that you must understand:
    • Efficiency
    • Best efficiency point (BEP)
    • Shut off head.
    • How to convert pressure to head so you can reference pump gage readings to the pump curve. When you learn the three formulas you will get the conversion information.
    • Brake horsepower (BHP)
    • Water horsepower (WHP)
    • Capacity
    • Net positive suction head required (NPSHR)
    • How to calculate the net positive suction head available (NPSHA) to the pump to insure you will not have a cavitation problem.

If all of the above decisions were made correctly the pump supplier will place his pump curve on top of your system curve and the required operating window will fall within the pump’s operating window on either side of the best efficiency point (BEP). Additionally, the motor will not overheat and the pump should not cavitate.

If the decisions were made incorrectly the pump will operate where the pump and system curves intersect and that will not be close to, or at the best efficiency point, producing radial impeller loading problems that will cause shaft deflection, resulting in premature seal and bearing failures. Needless to say the motor or driver will be adversely affected also.

With few exceptions pump manufacturers are generally not involved in mechanical sealing. You will probably be contacting separate seal suppliers for their recommendation about the mechanical seal.

Recent mergers between pump and seal companies unfortunately does not produce the instant expertise we would like sales and service people to posses.

CENTRIFUGAL PUMP INSTALLATION

Some one has to install the pump and all of its associated hardware. The quality of this pump and driver installation will have a major affect on the performance and reliability of the pump, especially if it is equipped with a mechanical seal.

The pump will be installed on a baseplate. The baseplate will be attached to a foundation and grout will be placed between the baseplate and the foundation to transmit any vibrations from the pump to the foundation.

Once the pump and driver are firmly on the foundation it will be time to connect the piping. Be sure to pipe from the pump to the pipe rack and not the other way, so as to avoid pipe strain that will interfere with the operation of the mechanical seal and bearings.

There are many piping recommendations that you should be familiar with. The leveling, and pump to driver alignment can be made at this point, but you should check the alignment after the pump has come up to its operating temperature because metal parts expand and contract with a change in temperature.

If this is a new piping system some people like to install packing in the pump and run on packing until the new piping has been cleaned of slag or any junk that might be left in the piping system. If it is not a new installation, and there is a mechanical seal in the stuffing box, then installing the mechanical seal environmental controls will come next.

If the pump has an open or semi-open impeller it is time to make the initial impeller clearance setting. The final clearance can be set when the pump comes up to its operating temperature. It is important to note that if you do not have a cartridge seal installed in the pump the seal face loading will change as you make both the initial and subsequent impeller settings and there is nothing you can do about it.

You will now want to do a proper venting of the pump. If it is a vertical installation you will have to pay particular attention to keeping air vented from the stuffing box while the pump is running and be sure to vent the space between dual seals if they have been installed.

After you have done all of the above, it is time to check out the mechanical seal environmental controls to be sure they are working properly. In most cases the environmental control will continue to run after the pump has stopped. Be sure the operators understand this or they might be tempted to shut the control off when the pump is between batches. Seal quench is always a problem with operators because the steam or water dripping out of the seal gland looks like the seal is leaking.

A constant monitoring of the pump is a good idea. Are you familiar with some of the more popular monitoring methods? Unlike vibration analysis, monitoring can tell you if some part of the pump is getting into trouble before the vibration starts.

CENTRIFUGAL PUMP MODIFICATION

If you find that your present centrifugal pump is not satisfying the application and running as trouble free as you would like, and you have checked:

  • All of the internal tolerances are correct.
  • There is no excessive pipe strain.
  • The open impeller has been adjusted to the volute or backplate after the pump came up to operating temperature.
  • The pump to driver alignment was made.
  • The rotating parts were dynamically balanced.
  • The wear ring clearance is within manufacturers specifications.
  • The pump is running at the correct speed, in the right direction, with the correct size impeller.

Then you may have to purchase a different centrifugal pump or you might want to consider modifying the existing pump to get the performance and reliability you are looking for.

Here are a few modifications and pump upgrades you can consider:

  • Modifying the impeller diameter could get you closer to the best efficiency point. The affinity laws will predict the affect the trimming will have on the pump’s head; capacity, net positive suction head required (NPSHR), and horsepower requirement.
  • Converting to an impeller with a different specific speed number will change the shape of the pump curve, power consumption and the NPSH required.
  • Changing to a heavy-duty power end can stop a lot of shaft deflection, and with some pump manufacturers get you the pilot diameter you need to install a “C or D” frame adapter to eliminate pump alignment.
  • Converting from a sleeved to a solid, corrosion resistant shaft will often reduce or stop shaft deflection problems caused by operating off the best efficiency point (BEP). If you are using mechanical seals be sure that you are using the type that prevents fretting corrosion. Most original equipment manufactured (OEM) seals damage shafts, and that is one of the main reasons they supply a sacrificial sleeve.
  • Reducing the overhung shaft length can solve many shaft deflection problems. You should be able to get the L3/D4 number down to a desirable 15-20 (0,5 &endash; 0,6 metric) by either reducing the shaft length or increasing the shaft diameter.
  • Changing the wet end to a double volute configuration will allow the pump to operate in a larger window without the danger of deflecting the shaft too much.
  • You can drill a hole in the end of the stuffing box, at the top, to increase stuffing box venting.
  • Change the flushing or recirculation connection from the top lantern ring connection to the bottom of the stuffing box to insure a better fluid flow through the stuffing box. Try to get close to the seal faces.
  • Enlarging the inside diameter of the stuffing box or going to an oversize stuffing box can solve some persistent seal problems.
  • Converting the wet end of the pump to a centerline design might solve some pipe strain problems by compensating for radial thermal growth.
  • Increasing the impeller to cutwater clearance could stop a cavitation problem
  • Installing a sight glass in the bearing case can help you maintain the correct oil level and prevent overheating problems in the bearings.
  • Replacing the bearing case grease or lip seals with either labyrinth or positive face seals for bearings will keep moisture out of the bearing case and eliminate a lot of premature bearing failure.
  • Converting the radial bearing retention snap ring to a more rugged holding device will eliminate many of he problems associated with axial movement of the shaft.
  • Converting the packed pump to a good mechanical seal will reduce power consumption and product leakage.
  • Converting solid mechanical seals to split mechanical seals can reduce the time it takes to change seals and eliminate the need for other trades to become involved in the process of disassembling a pump and bringing it into the shop.

MECHANICAL SEAL SELECTION

  • In the following pages I will be using the word “pump” to describe the piece of equipment that you will be sealing. If your equipment is anything other than a single stage centrifugal pump with an over hung impeller, the information still applies with a couple of exceptions:
    • Mixers, agitators and similar pieces of equipment sometimes have severe axial thrust and shaft deflection problems due to their high L3/Dnumbers (The ratio of the shaft length to its diameter).
    • Sleeve or journal bearing equipment allows more axial movement of the shaft than those pieces of equipment provided with precision bearings. Axial movement is a problem for mechanical seals because of the changing face load; especially at start up when the axial thrust reverses in a centrifugal pump.
    • Open impeller pumps require impeller adjustment that could cause excessive axial movement of the shaft that will affect the seal face loading. Depending upon the severity of the abrasives being pumped, this could be a frequent occurrence.
    • Multi-stage pumps are seldom as sensitive to operating off the best efficiency point (BEP) as single stage centrifugal pumps. The opposing cutwaters in these pumps tend to cancel out the radial forces created when the pump is operating off of its best efficiency point (BEP).
    • Centrifugal pumps equipped with double volutes are not too sensitive to operating off the best efficiency point (BEP), but do experience all of the other types of shaft deflection.
    • Specialized equipment such as a refiner in a paper mill will experience a great deal of axial travel as the internal clearances are adjusted.

Whenever I use the word fluid, I am talking about either a liquid or a gas. If I say either liquid or gas, I am limiting my discussion to that one phase of the fluid.

Any discussion of mechanical face seals requires that you have many different types of knowledge. The first is, “should you be converting packed pumps to a mechanical seal?” Seals cost a lot more money than conventional packing and unless you are using split seals, they can be a lot more difficult to install. There is a packing conversion down side.

Assuming you have made the decision that the mechanical seal is your best choice for sealing, you must know how to select the correct design for your application. There are many different kinds of seals to choose from:

  • Rotating seals where the springs or bellows rotate with the shaft.
  • Stationary seals where the springs or bellows do not rotate with the shaft.
  • Metal bellows seals used to eliminate elastomers that can have trouble with temperature extremes or fluid compatibility.
  • Elastomer type seals utilizing O-rings and other shape elastomers.
  • Single seals for most applications.
  • Dual seal designs for dangerous and expensive products or any time back up protection is needed.
  • Inside mounted designs that take advantage of centrifugal force to throw solids away from the lapped seal faces.
  • Outside seals. Usually the non-metallic variety for pumps manufactured from non-metallic materials.
  • Cartridge seals to ease installation and allow you to make impeller adjustments without disturbing the seal face loading.
  • Split seal designs that allow you to install and change seals without taking the pump apart and disturbing the alignment.
  • Hydrodynamic or non-contacting seals used for the sealing of gases.
  • Hydrostatic designs are another version of non-contacting vapor seals.

There are some very desirable design features that you should specify for your mechanical seals:

  • The ability to seal fugitive emissions without the use of dual seals, other than having the dual seal installed as a “back-up” or spare seal.
  • Will the seal dynamic elastomer damage or cause fretting corrosion of the pump shaft? Almost all-original equipment designs do. Spring-loaded Teflon® and graphite are notorious for shaft destruction. There are many seal designs available that will not cause fretting corrosion or damage shafts and sleeves, and that is the kind you should be using.
  • The seal should have built in non-clogging features such as springs out of the fluid.
  • The seal should be able to compensate for a reasonable amount of both radial and axial movement of the shaft. There are special mixer seal designs that can compensate for axial and radial travel in excess of 0.125 inches (3 mm) and you should know about them
  • The seal should be designed to be positioned as close to the bearings as possible to lessen the affects of shaft deflection. Ideally the seal would be located between the stuffing box face and the bearing case with a large diameter seal gland allowing plenty of internal radial clearance for the seal.
  • The seal should generate only a small amount of heat. Seal face heat generation can be a problem with many fluids and there is no advantage in letting the seal faces, or the fluid surrounding them get hot
    • Any heat generation between the seal faces should be efficiently removed by conduction away from the lapped faces and dynamic elastomer. Check to see if your design does it efficiently.
  • Any dynamic elastomer (an O-ring is typical) should have the ability to flex and then roll, or slide to a clean surface as the carbon face wears.
  • The seal face load should be adjustable to compensate for open impeller adjustments and axial growth of the shaft. Cartridge seals do this very well.
  • Can you use universal materials to lower your inventory costs and avoid mix-up problems? All of the seal materials should be clearly identified by type and grade. You will need this information if you have to analyze a premature seal failure. Some seal companies try to make everything a secret, do not tolerate it!
  • Will the seals be hydraulically balanced to prevent the generation of unwanted heat between the lapped faces? What is the percentage of balance? If you are using dual seals will the inner seal be a double balanced seal that is hydraulically balanced in both directions? Pressures can reverse in dual seal applications.
  • You will want to become familiar with the effects of heat on:
    • The seal faces, especially the carbon and plated or coated hard faces
    • The elastomers, especially the dynamic elastomer
    • Excessive corrosion of the seal components.
    • The product. It can change with heat. It can vaporize, solidify, crystallize, coke or build a film with an increase in the product’s temperature.
    • Internal tolerances of the seal especially face flatness and elastomer squeeze. Heat causes thermal growth of these components that will alter their critical tolerances.

We would like to be able to install the seal without having to modify the pump. The seal should be the shortest, thinnest design that will satisfy all of the operating conditions. Once you have the shortest, thinnest design that will satisfy the operating conditions there is seldom a need to modify any seal design.

The specific sealing application will dictate which seal design you should choose. If your seal application falls within the following parameters any stationary or rotating, “off the shelf” balanced O-ring seal should be able to handle the application without any serious problems.

  • Stuffing box pressures from a one Torr vacuum to 400 psi. (28 bar). Note that stuffing box pressure is normally closer to suction than discharge pressure
  • Stuffing box temperature from -40°F to 400°F. (-40°C to 200°C)
  • Shaft speed within electric motor speeds. If the surface speed at the seal faces exceeds 5000 fpm. (25 m/sec) you will have to select the stationary version of the seal.
  • Shaft sizes from 1 inch to 4 inches. (25 mm to 100 mm)

You may have to go to a special seal design if your application falls into any of the following categories:

  • Stuffing box pressures in excess of 400 psi. (28 bar) require heavy duty seals.
  • Excessive shaft movement of the type you find in mixers, agitators, and some types of sleeve or journal bearing equipment.
  • The seal must meet fugitive emission standards.
  • No metal parts are allowed in the system. You need a non-metallic seal.
  • Nothing black is allowed in the system because of a fear of color contamination. You cannot use any form of carbon face; you must use two hard faces.
  • There is not enough room to install a standard seal.
  • You are not allowed to use an environmental control or no environmental control is available.
  • Odd shaft sizes often dictate special seals.
  • If the seal components must be manufactured from an exotic metal.

If any of the following are part of the application, you may need a metal bellows design that eliminates all elastomers.

  • You are sealing a non-petroleum fluid and the stuffing box temperature exceeds 400°F (200°C) Petroleum fluids have coking problems that require cooling in the seal area.
  • Cryogenic temperatures.

You should go to a dual seal application if your product falls into any of the following categories:

  • You need two seals to control the seal environment outside the stuffing box.
  • To control the temperature at a seal face to stop a product from vaporizing, solidifying, crystallizing, or building a film.
  • To prevent a pressure drop across a seal face that can cause a liquid to vaporize.
  • To eliminate atmospheric conditions outboard of a mechanical seal when there is a possibility of freezing water vapor in the air.
  • To break down the pressure in a high-pressure application, by inserting an intermediate pressure between the seals. Two lower pressure seals can then be used to seal a high-pressure fluid that would normally require a very expensive high-pressure mechanical seal.
  • To provide a lubricant if one is needed to prevent slip stick between lapped seal faces. This is always a problem when you are sealing a gas or non-lubricating liquid.

You need dual seals as a protection for personnel in the area if your product is any of the following categories:

  • A toxic liquid or gas.
  • A fire hazard
  • A pollutant
  • A carcinogen
  • A radioactive fluid
  • An explosive fluid
  • Etc.

The other places we use dual seals are:

  • Expensive products that are too valuable to let leak.
  • You cannot afford to be shut down in the middle of a batch operation.
  • You do not have a standby pump and experience shows that the seal failure is your highest probability of an unexpected shut down.

In the Sealing Application section you will learn:

  • How to choose the correct seal materials.
  • How to classify the fluid into specific sealing categories
  • The environmental controls you might need to insure the seal will not fail prematurely.

IMPELLER SELECTION

There are multiple decisions to be made about the impeller selection and not all pump suppliers are qualified to make them:

  • The impeller shape or specific speed number will dictate the shape of the pump curve, the NPSH required and influence the efficiency of the pump.
  • Has the impeller configuration been iterated in recent years? Impeller design is improving with some of the newer computer programs that have become available to the design engineer.
  • The suction specific speed number of the impeller will often predict if you are going to experience a cavitation problem.
  • The impeller material must be chosen for both chemical compatibility and wear resistance. You should consider one of the duplex metals because most corrosion resistant materials are too soft for the demands of a pump impeller.
  • The decision to use a closed impeller, open impeller, semi-open, or vortex design is another decision to be made.
  • Closed impellers require wear rings and these wear rings present another maintenance problem.
  • Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent internal recirculation.
  • Vortex pump impellers are great for solids and “stringy” materials but they are up to 50% less efficient than conventional designs.
  • Investment cast impellers are usually superior to sand cast versions because you can cast compound curves with the investment casting process. The compound curve allows the impeller to pump abrasive fluids with less vane wear.
  • If you are going to pump low specific gravity fluids with an open impeller, a non-sparking type metal may be needed to prevent a fire or explosion. You will be better off choosing a closed impeller design with soft wear rings in these applications.
  • The affinity laws will predict the affect of changing the impeller speed or diameter. You will want to be familiar with these laws for both centrifugal and PD pumps.

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Posted

  • On February 17, 2018