CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP FOR YOUR APPLICATION.
The following paper is an excerpt from my book "Bill Mc Nally's Centrifugal Pump And Mechanical Seal Reference Manual". The words marked in blue reference you to the alphabetical section of the book or directly to the subject on the CD for a detailed explanation of that particular topic. If you do not have a copy of the book or CD you can find a lot of the information in the individual papers I have published on the web.Check out the index for a list of my papers by subject.
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 point 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.
- 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.
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 printed 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 be familiar with most of them. You can find these factors in the alphabetical section of my book labeled "head, the reasons for changes in the discharge head of a centrifugal pump".
All of this head information is calculated from charts and graphs you will find in the appendix and articles in the alphabetical section of the manual and on these web pages. 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 charts and 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 find examples of these calculations in the alphabetical section of my book and within these web pags. Look for:
- Calculating the total head in metric units
- Calculating the total head in USCS (inch) units
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. :
- Vaporization cavitation.
- Air ingestion cavitation.
- Internal recirculation cavitation.
- Flow turbulence cavitation.
- Vane Passing Syndrome cavitation.
Pump cavitation is experienced in several different ways
- We can hear cavitation because it sounds like the pump is pumping 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 your fluid.
- In some cases you can reduce the NPSH required. This is especially true if you are pumping hot water or mixed hydrocarbons. Look in the alphabetical section under NPSHR reductions.
- 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 m3/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.
- A magnetic drive or canned pump may be your best choice if you can live with the several limitations they impose.
- A 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 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 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/D4 number. 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/D4 problems 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.
- The affinity laws will predict the affect of changing the impeller speed or diameter. You will want to be familiar with these laws.
- 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 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 miscalculated 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 corrections you 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:
- 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)
- 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. 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.
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