LEARNING ABOUT CENTRIFUGAL PUMPS
This mini course is all about centrifugal pumps. I ‘ve chosen this type of pump to talk about because, unlike positive displacement pumps (PD pumps) they can be altered and modified to just about satisfy what ever pumping needs you might have.
I’ve decided the best way to handle this subject is to write an outline narrative that will get the pumping terms and concepts into some type of perspective. Once you become familiar with the terms you can turn to the individual subjects for a detailed explanation. The phrases colored in blue are links to a more detailed explanation of that particular subject. Here we go!
You want to pump some liquid from point “A” to point “B”. To do that you must first make a couple of decisions:
- Decide what materials you’ll need that are chemically compatible with the fluid you’re pumping, as well as any cleaners or solvents that will be flushed through the pump and the process lines. The pump manufacturer is not always the best qualified to make these decisions. Many times your fluid is a proprietary product and the knowledge of compatible materials is only available from your own people.
- Decide the capacity you’ll need (the gallons per minute or cubic meters per hour). Will this capacity vary at times? If it does, the head will also change with the capacity and your pump must be ruggard enough to operate at various points on the pump curve.
- You’re going to have to calculate how much pressure (psi. or bar) will be needed to deliver this capacity to the point where you’ll need it. You will need enough pressure to :
- Reach the maximum static height the moving fluid will have to attain.
- Overcome the pressure in a pressurized tank or boiler.
- Overcome resistance to the fluid moving in the lines, fittings and any valves or hardware that might be in the system.
- We’ll begin by learning that centrifugal pump people do not use the word pressure. They substitute the word “head” (measured in feet or meters) instead, so you’ll have to calculate the three kinds of head that will be required to deliver the needed capacity :
- The static head or maximum height that the liquid will reach. We must also learn how to compensate for the siphon affect from any down running pipes on the discharge side of the pump.
- The pressure head is next if the container we are pumping to or from is pressurized. A pressurized boiler would be a good example. You’ll have to know how to convert pressure units to head units. We also need this conversion knowledge to read the manufacturers pump curve.
- The friction head is the one that we have to calculate. It tells us how much friction or resistance head there is in both the suction and discharge piping, along with the fittings, strainers, valves etc. And to make the job a little tougher, this head changes as the pump speed or capacity changes. Since the suction and discharge piping are normally different diameters you’ll be doing a lot of “walking the piping,” counting the fittings and valves, looking at charts and graphs, as well as making a lot of calculations.
Some of this information is calculated from charts and graphs you will find in this web site and various other publications. Since we’ll not be operating at a single pumping point all of the time we’ll make the calculations for a range of different capacities and heads that we might expect to encounter. This data will then be plotted on a set of coordinates that we call a system curve. This pumping range is often described as the “operating window” we’ll need for the application.
Making these calculations is not an exact science because the piping is seldom new, diameters are not exact, and the charts and graphs you’ll be consulting cannot compensate for corrosion and solids built up on the piping and fitting walls. Life is never simple. This is where most people start adding in safety factors to compensate for some of the unknowns. These safety factors will almost guarantee the selection of an oversized pump that will operate off of its best efficiency point (B.E.P.) adding to its operating cost and making the pump vulnerable to premature seal and bearing problems.
When the pump supplier has all of this inexact information in his possession he can then try to select the correct size pump and driver for the job. Since he wants to quote a low price he has to make some critical decisions:
- If the capacity is very low he’ll probably recommend a 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 ended design with a wide impeller.
- How will the open or semi-open impeller be adjusted for efficiency? Can the mechanical seal be adjusted at the same time? Impeller adjustments have to be made for both thermal growth and impeller wear.
- Will he supply a “C” or “D” frame adapter to simplify the alignment process, or will the pump to motor alignment have to be done manually?
- Will he supply a centerline pump design to avoid the problems with thermal expansion of both the pump and the piping at operating temperatures over 200°F (100°C).
- Magnetic drive and canned pumps have become very popular in recent years, maybe you should be using that type and eliminate the need for mechanical seals or packing?
- What type of coupling will he select to connect the pump to its driver?
- How will he handle the head/capacity requirement ?
- He may decide to run two pumps in parallel if he needs a real high capacity.
- Or he might run two pumps in series if he needs a high head.
- The decision to use either a single or multiple stage pump is another decision that has to be made.
- Will you need a volute or circular casing? Volute casings build head, circular casing are used for lower heads and higher capacities.
- Should he specify a pump that meets the ANSI or ISO standard? It is one more decision to be made. These standard pumps lack important features that prevent problems with mechanical seals.
- He 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 motor this becomes a possibility.
- The affinity laws will predict the affect of changing the impeller speed or diameter. You will want to be familiar with these laws because in many cases they will dictate if you should be using a variable speed motor.
- Will he recommend a self priming pump? These pumps remove air from the 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?
- If he is supplying a self priming pump, is it equipped with a mechanical seal that can seal a vacuum? If it is not, you are going to have troubles keeping the prime.
- The ratio of the shaft diameter to its length is called the L3/D4 number. This ratio will have a major affect on the operating window of the pump and its initial cost. The lower the number the wider the window, but any thing below 60 (2 in the metric system) is acceptable. A low L3/D4 can be costly in a standard pump design because it dictates a large diameter shaft that is usually found only on expensive, heavy duty pumps. A short shaft 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 accept corrosion resistant sleeves rather than a solid, corrosion resistant shaft.
- How much net positive suction head (NPSH) will you need to prevent cavitation problems? We all want pumps with a low net positive suction head required, but sometimes it’s not practical. The manufacturer has the option of 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 alteration is explained when you learn about suction specific speed.
- A double suction design, or the installation of an inducer on an end suction centrifugal pump is sometimes a sensible solution to NPSH problems.
- Maybe the suction piping can be altered in some manner to solve the problem.
- He must choose pump materials that are chemically compatible with what you’re pumping as well as any cleaners or solvents that might be flushed through the lines. Be sure to remember that if the temperature of the pumpage increases, the corrosion rate will increase also.
- The supplier must choose the correct size electric motor or some other type of driver. His decision will be affected by the specific gravity and the viscosity of the liquid you’ll be pumping. It will also be influenced by how far you’ll venture out of the operating window on the capacity side of the pump curve. If this number is miscalculated there is a danger of burning out the electric motor.
- 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 incorporate “Duplex Metal” materials. Cartridge seals will probably be necessary to compensate for the frequent impeller adjustments.
- Shaft speed is an important decision. Speed affects pump component wear as well as the pump size. High speed pumps cost less initially, but the maintenance costs can be staggering.
- Will this application be affected by the OSHA 1910 regulation for hazardous chemicals? If it is, you will have to address special “fugitive emission” sealing problems.
- How will the bearings be lubricated? The choices are grease, oil level and oil mist. Which should you use, and how are you going to seal the bearing case to prevent the ingress of moisture and solids? A simple grease or lip seal is not a very good choice and developing fugitive emission laws might dictate your final selection. You may want to look at other options that include labyrinths and positive face seals that will add to the initial cost.
- There are lots of decisions to be made about the impeller selection:
- The impeller design or specific speed number will dictate the shape of the pump curve and influence the efficiency of the pump.
- The suction specific speed number of the impeller predicts if you are going to have a cavitation problem.
- The impeller material must be chosen for both chemical compatibility and wear resistance. You might want to consider duplex metals.
- Investment cast impellers can be designed with compound curves that work better in abrasive service. Sand cast versions lose this option.
- The decision to use a closed impeller, open impeller, semi-open, or vortex design is another decision to be made. Closed impellers need wear rings and that presents another maintenance problem.
After carefully considering all of the above, the pump supplier will present his quote and supply you with a copy of his pump curve. At this stage it is important for you to be able to read the curve and to do that you must be able to understand what is meant by :
- The pump’s best efficiency point (BEP)
- Pump efficiency
- Shut off head.
- How to convert pressure to head so you can reference gage readings on the pump and piping to the pump curve.
- Brake horsepower (BHP)
- Net positive suction head required (NPSHR)
- Net positive suction head available (NPSHA)
- How to calculate the NPSH available to the pump to insure you will not have a cavitation problem.
If all of these decisions were made correctly, the supplier will place his pump curve on top of the system curve you supplied and these curves should intersect at the pump’s best efficiency point. At this point the pump will experience minimum vibration, the motor will not overheat, and the pump will 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 at the best efficiency point , causing shaft deflection. Needless to say the motor or driver will be adversely affected along with the bearings and seal.
There are a number of additional conditions that’ll affect the performance and reliability of the pump. You will need to learn about all of them:
- Alignment between the pump and its driver.
- The piping layout. Especially elbows and fittings at the pump suction.
- The causes and affect of pipe strain.
- The base plate, grouting and the pump pedestal where the pump and driver will be mounted.
- Dynamic balancing of the rotating pump components.
- Is the piping new or has calcium or some other type of solids been building up on the piping walls? The smaller the pipe inside diameter the more the resistance we get in the piping.
- The differences between constant running and intermittent service.
- Many constant running pumps use a bypass line connected from the discharge of the pump back to the suction side that will open when the discharge is throttled. This could cause problems with the heating of the liquid at the pump suction.
- Intermittent service is the most difficult for the mechanical seal. At shut down many fluids become viscous, solidify or crystallize clogging up the seal and causing a failure at the next start up.
- Will the lines be cleaned with some cleaner or solvent?
- If this cleaner has a different specific gravity than the pumpage, the electric motor could be damaged.
- The rubber parts in mechanical seals are very sensitive to cleaners and solvents. You may have to use one of the super compounds.
- If the application requires a varying capacity, remember that the head will change with the capacity. In some applications like a boiler feed pump, that may not be desirable.
- Maybe you should install monitoring systems that will give you the running information you need.
- The proper way to start a centrifugal pump.
- Is the mass of the pump and its driver no more than 25% of the mass of the pedestal the hardware will rest upon?
It would be nice if new pumps were trouble free. They seldom are. When we describe pump problems we usually mean:
- The pump is not developing enough head.
- The pump is not developing enough capacity.
- The pump is using too much amperage.
- The pump loses suction.
- The mechanical seal is leaking prematurely.
- The bearings are not giving adequate life
- The pump is experiencing a corrosion problem.
- The pump is cavitating. There are four types of cavitation to learn about
- There is excessive vibration
- The pump is getting hot.
- The pump is making too much noise.
At some point you’re going to have to become familiar with all of these problems and their solution. Knowing some basic rules of thumb about pumps helps.
When it comes to analyzing these pump failures you have several opportunities that require different troubleshooting techniques:
- You can analyze the problem while the pump is still operating. Here you get to use your senses of sight, smell, feel and hearing
- You can look at the various components after the pump has been disassembled in the shop. You will only be able to see evidence of rubbing and damage, and that is usually enough information to troubleshoot the problem.
- There are some pump problems that have no apparent cause, but you still have to solve them .
If you’re lucky, your company will purchase the correct pump the first time, but that is not going to happen so you will probably have to modify your existing pump to get the life you require from the bearings and seals.