Different types of centrifugal pump systems
There are many types of centrifugal pump systems. There are many variations on this including all kinds of equipment that can be hooked up to these systems that are not shown. A pump after all is only a single component of a process although an important and vital one. The pumps’ role is to provide sufficient pressure to move the fluid through the system at the desired flow rate.
Pressure, friction, and flow are three important characteristics of a pump system.
The pressure is the driving force responsible for the movement of the fluid. Friction is the force that slows down fluid particles. Flow rate is the amount of volume that is displaced per unit of time. The unit of flow in North America, at least in the pump industry, is the US gallon per minute, USgpm. From now on I will just use gallons per minute or gpm. In the metric system, flow is in liters per second (L/s) or meters cube per hour (m3/h).
Pump pressure is often expressed in pounds per square inch (psi) in the Imperial system and kiloPascals (kPa) in the metric system.
In the Imperial system of measurement, the unit psig or pounds per square inch gauge is used, which means that the pressure measurement is relative to the local atmospheric pressure so that 5 psig is 5 psi above the local atmospheric pressure. In the metric system, the kPa unit scale is a scale of absolute pressure measurement and there is no kPag, but many people use the kPa as a relative measurement to the local atmosphere and don’t bother to specify this. This is not the fault of the metric system but the way people use it. The term pressure loss or pressure drop is often used, this refers to the decrease in pressure in the system due to friction. In a pipe or tube that is at the same level, your garden hose, for example, the pressure is high at the tap and zero at the hose outlet, this decrease in pressure is due to friction and is the pressure loss.
As an example of the use of pressure and flow units, the pressure available to domestic water systems varies greatly depending on your location with respect to the water treatment plant. It can vary between 30 and 70 psi or more. The following table gives the expected flow rate that you would obtain for different pipe sizes assuming the pipe or tube is kept at the same level as the connection to the main water pressure supply and has 100 feet of length.
Pump Pressure provides the driving force to overcome friction and elevation differences.
It’s responsible for driving the fluid through the system, the pump provides the pressure. The pressure is increased when fluid particles are forced closer together. For example, in a fire extinguisher work or energy has been spent to pressurize the liquid chemical within, that energy can be stored and used later. Is it possible to pressurize a liquid within a container that is open? Yes. A good example is a syringe, as you push down on the plunger the pressure increases and the harder you have to push. There is enough friction as the fluid moves through the needle to produce a great deal of pressure in the body of the syringe
What is friction in a pump system?
Friction is always present, even in fluids, it is the force that resists the movement of objects.
When you move a solid on a hard surface, there is friction between the object and the surface. If you put wheels on it, there will be less friction. In the case of moving fluids such as water, there is even less friction but it can become significant for long pipes. Friction can also be high for short pipes that have a high flow rate and small diameter as in the syringe example.
In fluids, friction occurs between fluid layers that are traveling at different velocities within the pipe. There is a natural tendency for the fluid velocity to be higher in the center of the pipe than near the wall of the pipe. Friction will also be high for viscous fluids and fluids with suspended particles.
Friction depends on:
– average velocity of the fluid within the pipe
– pipe surface roughness
An increase in any one of these parameters will increase friction.
The amount of energy required to overcome the total friction energy within the system has to be supplied by the pump if you want to achieve the required flow rate. In industrial systems, friction is not normally a large part of a pump’s energy output. For typical systems, it is around 25% of the total. If it becomes much higher then you should examine the system to see if the pipes are too small. However, all pump systems are different, in some systems the friction energy may represent 100% of the pump’s energy. This is what makes pump systems interesting, there is a million and one applications for them. In household systems, friction can be a greater proportion of the pump energy output, maybe up to 50% of the total because small pipes produce higher friction than larger pipes for the same average fluid velocity in the pipe (see the friction chart later in this tutorial).
Another cause of friction is all the fittings (elbows, tees, y’s, etc) required to get the fluid from point A to B. Each one has a particular effect on the fluid streamlines. For example in the case of the elbow, the fluid particles that are closest to the tight inner radius of the elbow lift off from the pipe surface forming small vortices that consume energy. This energy loss is small for one elbow but if you have several elbows and other fittings the total can become significant. Generally speaking, they rarely represent more than 30% of the total friction due to the overall pipe length.
Energy and head in pump systems
Energy and head are two terms that are often used in pump systems.
We use energy to describe the movement of liquids in pump systems because it is easier than any other method. There are four forms of energy in pump systems: pressure, elevation, friction, and velocity.
The pressure is produced at the bottom of the reservoir because the liquid fills up the container completely and its weight produces a force that is distributed over a surface which is pressure. This type of pressure is called static pressure. Pressure energy is the energy that builds up when liquid or gas particles are moved slightly closer to each other and as a result, they push outwards in their environment. A good example is a fire extinguisher, work was done to get the liquid into the container and then to pressurize it. Once the container is closed the pressure energy is available for later use.
Elevation energy is the energy that is available to a liquid when it is at a certain height. If you let it discharge it can drive something useful like a turbine producing electricity.
Friction energy is the energy that is lost to the environment due to the movement of the liquid through pipes and fittings in the system.
Velocity energy is the energy that moving objects have. When a baseball is thrown by a pitcher he gives it velocity energy also called kinetic energy. When water comes out of a garden hose, it has velocity energy.
The three forms of energy: elevation, pressure, and velocity interact with each other in liquids.
For solid objects, there is no pressure energy because they don’t extend outwards like liquids filling up all the available space and therefore they are not subject to the same kind of pressure changes.
The energy that the pump must supply is the friction energy plus the elevation energy.
PUMP ENERGY = FRICTION ENERGY + ELEVATION ENERGY
Now, what about head? Head is actually a way to simplify the use of energy. To use the energy we need to know the weight of the object displaced.
Elevation energy E.E. is the weight of the object W times the distance d:
EE = W x d
Friction energy FE is the force of friction F times the distance the liquid is displaced or the pipe length l:
FE = F x l
Head is defined as energy divided by weight or the amount of energy used to displace an object divided by its weight. For elevation energy, the elevation head EH is:
EH = W x d / W = d
For friction energy, the friction head FH is the friction energy divided by the weight of the liquid displaced:
FH = FE/W = F x l / W (see Figure 9b)
The friction force F is in pounds and W the weight is also in pounds so that the unit of friction head is feet. This represents the amount of energy that the pump has to provide to overcome friction.
If I attach a tube to the discharge side of a pump, the liquid will rise in the tube to a height that exactly balances the pressure at the pump discharge. Part of the height of the liquid in the tube is due to the elevation height required (elevation head) and the other is the friction head and as you can see both are expressed in feet and this is how you can measure them.
What is Pump Static Head
Webster’s dictionary definition of head is: “a body of water kept in reserve at a height”.
It is expressed in terms of feet in the Imperial system and meters in the metric system. Because of its height and weight the fluid produces pressure at the low point. The higher the reservoir, the higher the pressure.
The amount of pressure at the bottom of a reservoir is independent of its shape, for the same liquid level, the pressure at the bottom will be the same. This is important since in complex piping systems it will always be possible to know the pressure at the bottom if we know the height.
When a pump is used to displace a liquid to a higher level it is usually located at the low point or close to it. The head of the reservoir which is called static head will produce pressure on the pump that will have to be overcome once the pump is started.
To distinguish between the pressure energy produced by the discharge tank and suction tank, the head on the discharge side is called the discharge static head, and on the suction side the suction static head.
Usually, the liquid is displaced from a suction tank to a discharge tank. The suction tank fluid provides pressure energy to the pump suction which helps the pump. We want to know how much pressure energy the pump itself must supply so therefore we subtract the pressure energy provided by the suction head. The static head is then the difference in height of the discharge tank fluid surface minus the suction tank fluid surface. Static head is sometimes called total static head to indicate that the pressure energy available on both sides of the pump has been considered.
Since there is a difference in height between the suction and discharge flanges or connections of a pump by convention it was agreed that the static head would be measured with respect to the suction flange elevation.
If the discharge pipe end is open to the atmosphere then the static head is measured with respect to the pipe end.
Sometimes the discharge pipe end is submerged, then the static head will be the difference in elevation between the discharge tank fluid surface and the suction tank fluid surface. Since the fluid in the system is a continuous medium and all fluid particles are connected via pressure, the fluid particles that are located at the surface of the discharge tank will contribute to the pressure built up at the pump discharge. Therefore the discharge surface elevation is the height that must be considered for static head. Avoid the mistake of using the discharge pipe end as the elevation for calculating static head if the pipe end is submerged.
Note: if the discharge pipe end is submerged, then a check valve on the pump discharge is required to avoid backflow when the pump is stopped.
The static head can be changed by raising the surface of the discharge tank (assuming the pipe end is submerged) or suction tank or both. All of these changes will influence the flow rate.
To correctly determine the static head follow the liquid particles from start to finish, the start is almost always at the liquid surface of the suction tank, this is called the inlet elevation. The end will occur where you encounter an environment with a fixed pressure such as the open atmosphere, this point is the discharge elevation end or outlet elevation. The difference between the two elevations is the static head. The static head can be negative because the outlet elevation can be lower than the inlet elevation.
Flow rate depends on elevation difference or static head
For identical systems, the flow rate will vary with the static head. If the pipe end elevation is high, the flow rate will be low (see Figure 10). Compare this to a cyclist on a hill with a slight upward slope, his velocity will be moderate and correspond to the amount of energy he can supply to overcome the friction of the wheels on the road and the change in elevation.
If the liquid surface of the suction tank is at the same elevation as the discharge end of the pipe then the static head will be zero and the flow rate will be limited by the friction in the system. This is equivalent to a cyclist on a flat road, his velocity depends on the amount of friction between the wheels and the road and the air resistance (see Figure 11).
If the discharge pipe end is lower than the liquid surface of the suction tank then the static head will be negative and the flow rate high (see Figure 13). If the negative static head is large then it is possible that a pump is not required since the energy provided by this difference in elevation may be sufficient to move the fluid through the system without the use of a pump as in the case of a siphon (see pump system glossary). By analogy, as the cyclist comes down the hill he loses his stored elevation energy which is transformed progressively into velocity energy. The lower he is on the slope, the faster he goes.
Pumps are most often rated in terms of head and flow.
The discharge pipe end is raised to a height at which the flow stops, this is the head of the pump at zero flow. We measure this difference in height in feet. Head varies depending on flow rate, but in this case, since there is no flow and hence no friction, the head of the pump is THE MAXIMUM HEIGHT THAT THE FLUID CAN BE LIFTED TO WITH RESPECT TO THE SURFACE OF THE SUCTION TANK. Since there is no flow the head (also called total head) that the pump produces is equal to the static head.
In this situation, the pump will deliver its maximum pressure. If the pipe end is lowered as in Figure 10, the pump flow will increase and the head (also known as total head) will decrease to a value that corresponds to the flow. Why? Let’s start from the point of zero flow with the pipe end at its maximum elevation, the pipe end is lowered so that flow begins. If there is flow there must be friction, the friction energy is subtracted (because it is lost) from the maximum total head and the total head is reduced. At the same time, the static head is reduced which further reduces the total head.
When you buy a pump you don’t specify the maximum total head that the pump can deliver since this occurs at zero flow. You instead specify the total head that occurs at your required flow rate. This head will depend on the maximum height you need to reach with respect to the suction tank fluid surface and the friction loss in your system.
For example, if your pump is supplying a bathtub on the 2nd floor, you will need enough head to reach that level, that will be your static head, plus an additional amount to overcome the friction loss through the pipes and fittings. Assuming that you want to fill the bath as quickly as possible, then the taps on the bath will be fully open and will offer very little resistance or friction loss. If you want to supply a showerhead for this bathtub then you will need a pump with more head for the same flow rate because the showerhead is higher and offers more resistance than the bathtub taps.
Luckily, there are many sizes and models of centrifugal pumps and you cannot expect to purchase a pump that matches exactly the head you require at the desired flow. You will probably have to purchase a pump that provides slightly more head and flow than you require and you will adjust the flow with the use of appropriate valves.
Note: you can get more head from a pump by increasing its speed or it’s impeller diameter or both. In practice, homeowners cannot make these changes, and to obtain a higher total head, a new pump must be purchased.
Flow rate depends on friction
For identical systems, the flow rate will vary with the size and diameter of the discharge pipe. A system with a discharge pipe that is generously sized will have a high flow rate. This is what happens when you put a large pipe on a tank to be emptied, it drains very fast.
The smaller the pipe, the less the flow. How does the pump adjust itself to the diameter of the pipe, after all, it does not know what size pipe will be installed? The pump you install is designed to produce a certain average flow for systems that have their pipes sized accordingly. The impeller size and its speed predispose the pump to supply the liquid at a certain flow rate. If you attempt to push that same flow through a small pipe the discharge pressure will increase and the flow will decrease. Similarly, if you try to empty a tank with a small tube, it will take a long time to drain.
How does a centrifugal pump produce pressure?
Fluid particles enter the pump at the suction flange or connection. They then turn 90 degrees into the impeller and fill up the volume between each impeller vane. This animation shows what happens to the fluid particles from that point forward.
A centrifugal pump is a device whose primary purpose is to produce pressure by accelerating fluid particles to a high velocity providing them with velocity energy. What is velocity energy? It’s a way to express how the velocity of objects can affect other objects, you for example. Have you ever been tackled in a football match? The velocity at which the other player comes at you determines how hard you are hit. The mass of the player is also an important factor. The combination of mass and velocity produces velocity (kinetic) energy. Another example is catching a hard baseball pitch, ouch, there can be a lot of velocity in a small fast-moving baseball. Fluid particles that move at high speed have velocity energy, just put your hand on the open end of a garden hose.
The fluid particles in the pump are expelled from the tips of the impeller vanes at high velocity, then they slow down as they get closer to the discharge connection, losing some of their velocity energy. This decrease in velocity energy increases pressure energy. Unlike friction which wastes energy, the decrease in velocity energy serves to increase pressure energy, this is the principle of energy conservation in action. The same thing happens to a cyclist that starts at the top of a hill, his speed gradually increases as he loses elevation. The cyclist’s elevation energy was transformed into velocity energy, in the pump’s case the velocity energy is transformed into pressure energy.
How does the flow rate change when the discharge pipe end elevation is changed or when there is an increase or decrease in pipe friction? These changes cause the pressure at the pump outlet to increase when the flow decreases, sounds backward doesn’t it? Well, it’s not and you will see why. How does the pump adjust to this change in pressure? Or in other words, if the pressure changes due to outside factors, how does the pump respond to this change.
Pressure is produced by the rotational speed of the impeller vanes. The speed is constant. The pump will produce a certain discharge pressure corresponding to the particular conditions of the system (for example, fluid viscosity, pipe size, elevation difference, etc.). If changing something in the system causes the flow to decrease (for example closing a discharge valve), there will be an increase in pressure at the pump discharge because there is no corresponding reduction in the impeller speed. The pump produces excess velocity energy because it operates at a constant speed, the excess velocity energy is transformed into pressure energy and the pressure goes up.
All centrifugal pumps have a performance or characteristic curve that looks similar to the one shown in Figure 21 (assuming that the level in the suction tank remains constant), this shows how the discharge pressure varies with the flow rate through the pump.
So that at 200 gpm, this pump produces 20 psig discharge pressure, and as the flow drops the pressure will reach a maximum of 40 psig.
Note: This applies to centrifugal pumps, many homeowners have positive displacement pumps, often piston pumps. Those pumps produce constant flow no matter what changes are made to the system.
What is total head
Total head and flow are the main criteria that are used to compare one pump with another or to select a centrifugal pump for an application. Total head is related to the discharge pressure of the pump. Why can’t we just use discharge pressure? The pressure is a familiar concept, we are familiar with it in our daily lives. For example, fire extinguishers are pressurized at 60 psig (413 kPa), we put 35 psig (241 kPa) air pressure in our bicycle and car tires.For good reasons, pump manufacturers do not use discharge pressure as a criterion for pump selection. One of the reasons is that they do not know how you will use the pump. They do not know what flow rate you require and the flow rate of a centrifugal pump is not fixed. The discharge pressure depends on the pressure available on the suction side of the pump. If the source of water for the pump is below or above the pump suction, for the same flow rate you will get a different discharge pressure. Therefore to eliminate this problem, it is preferable to use the difference in pressure between the inlet and outlet of the pump.
The manufacturers have taken this a step further, the amount of pressure that a pump can produce will depend on the density of the fluid, for a saltwater solution which is denser than pure water, the pressure will be higher for the same flow rate. Once again, the manufacturer doesn’t know what type of fluid is in your system, so that a criteria that do not depend on density is very useful. There is such a criterion and it is called TOTAL HEAD, and it is defined as the difference in head between the inlet and outlet of the pump.
You can measure the discharge head by attaching a tube to the discharge side of the pump and measuring the height of the liquid in the tube with respect to the suction of the pump. The tube will have to be quite high for a typical domestic pump. If the discharge pressure is 40 psi the tube would have to be 92 feet high. This is not a practical method but it helps explain how head relates to total head and how head relates to pressure. You do the same to measure the suction head. The difference between the two is the total head of the pump.
The fluid in the measuring tube of the discharge or suction side of the pump will rise to the same height for all fluids regardless of the density. This is a rather astonishing statement, here’s why. The pump doesn’t know anything about head, head is a concept we use to make our life easier. The pump produces pressure and the difference in pressure across the pump is the amount of pressure energy available to the system. If the fluid is dense, such as a salt solution, for example, more pressure will be produced at the pump discharge than if the fluid were pure water. Compare two tanks with the same cylindrical shape, the same volume and liquid level, the tank with the denser fluid will have a higher pressure at the bottom. But the static head of the fluid surface with respect to the bottom is the same. Total head behaves the same way as static head, even if the fluid is denser the total head as compared to a less dense fluid such as pure water will be the same.
For these reasons, the pump manufacturers have chosen total head as the main parameter that describes the pump’s available energy.
What is the relationship between head and total head?
Total head is the height that the liquid is raised to at the discharge side of the pump less the height that it is raised to at the suction side. Why less the height at the suction side? Because we want the energy contribution of the pump only and not the energy that is supplied to it.
What is the unit of head? First, let’s deal with the unit of energy. Energy can be expressed in foot-pounds which is the amount of force required to lift an object up multiplied by the vertical distance. A good example is weight lifting. If you lift 100 pounds (445 Newtons) up 6 feet (1.83 m), the energy required is 6 x 100= 600 ft-lbf (814 N-m).
Head is defined as energy divided by the weight of the object displaced. For the weight lifter, the energy divided by the weight displaced is 6 x 100 / 100= 6 feet (1.83 m), so the amount of energy per pound of dumbbell that the weight lifter needs to provide is 6 feet. This is not terribly useful to know for a weight lifter but we will see how very useful it is for displacing fluids.
If we use energy to describe how much work the pump needs to do to displace a volume of liquid we need to know the weight. If we use head, we only need to know the vertical distance of movement. This is very useful for fluids because pumping is a continuous process, usually, when you pump you leave the pump turned on, you don’t start and stop the pump for every pound of fluid displaced. We are mainly interested in establishing a continuous flow rate.
The other very useful aspect of using head is that the elevation difference or static head can be used as one part of the value of total head, the other part being friction head as shown in this next figure.
How much static head is required to pump water up from the ground floor to the second floor, or 15 feet up? Remember that you must also take into consideration the level of the water in the suction tank. If the water level is 10 feet below the pump suction connection then the static head will be 10 + 15 = 25 feet. Therefore the total head will have to be at least 25 feet plus the friction head loss of the fluid moving through the pipes.
How to determine friction head
Friction head is the amount of energy loss due to friction of the fluid moving through pipes and fittings. It takes a force to move the fluid against friction, in the same way, that a force is required to lift a weight. The force is exerted in the same direction as the moving liquid and energy is expended. In the same way that head was calculated to lift a certain weight, the friction head is calculated with the force required to overcome friction times the displacement (pipe length) divided by the weight of fluid displaced. These calculations have been done for us and you can find the values for friction head loss in Table 1 for different pipe sizes and flow rates.
The performance or characteristic curve of the pump
The pump characteristic curve has a similar appearance to the previous curve shown that I also called a characteristic curve that showed the relationship between discharge pressure vs. flow (see Figure 21). As I mentioned this is not a practical way of describing the performance because you would have to know the suction pressure used to generate the curve. Figure 30 shows a typical total head vs. flow rate characteristic curve. This is the type of curve that all pump manufacturers publish for each model pump for a given operating speed.
Not all manufacturers will provide you with the pump characteristic curve. However, the curve does exist, and if you insist you can probably get it. Generally speaking, the more you pay, the more technical information you get.
Friction loss on the discharge side of the pump
According to the calculation or the use of tables which is not presented here the friction loss for a 3/4″ tube is has a friction loss of 0.23 feet per feet of pipe. In this case, the distances are 10 feet of run on the main distributor and another 20 feet off of the main distributor up to the bath, for a total length of 30 feet. The friction loss in feet is then 30 x 0.23 = 6.9 feet. There is some friction loss in the fittings, let’s assume that a conservative estimate is 30% of the pipe friction head loss, the fittings friction head loss is = 0.3 x 6.9 = 2.1 feet. The total friction loss for the discharge side is then 6.9 + 2.1 = 9 feet.
The total friction loss for piping in the system is then 9 + 3.1 = 12.1 feet.
The static head as per Figure 41 is 35 feet. Therefore the total head is 35 + 12.1 = 47 feet. We can now go to the store and purchase a pump with at least 47 feet of total head at 10 gpm. Sometimes total head is called Total Dynamic Head (T.D.H.), it has the same meaning. The pump’s rating should be as close as possible to these two figures without splitting hairs. As a guideline, allow a variation of plus or minus 15% on total head. On the flow, you can also allow a variation but you may wind up paying for more than what you need.
What is the pump rating? The manufacturer will rate the pump at its optimum total head and flow, this point is also known as the best efficiency point or B.E.P.. At that flow rate, the pump is at its most efficient and there will be minimal amount of vibration and noise. Of course, the pump can operate at other flow rates, higher or lower than the rating but the life of the pump will suffer if you operate too far away from its normal rating. Therefore, as a guideline aim for a maximum variation of plus or minus 15% on total head.
Calculate the pump discharge pressure from the pump total head
To calculate the pressure at the bottom of a pool, you need to know the height of the water above you. It doesn’t matter if it’s a pool or a lake, the height is what determines how much fluid weight is above and therefore the pressure.
Pressure is equal to a force divided by a surface. It is often expressed in pounds per square inch or psi. The force is the weight of water. The density of water is 62.3 pounds per cubic foot.
We can calculate the discharge pressure of the pump based on the total head which we get from the characteristic curve of the pump. This calculation is useful if you want to troubleshoot your pump or verify if it is producing the amount of pressure energy that the manufacturer says it will at your operating flow rate.
The pump will have to generate lift to get the water up to its suction connection. This means that the pressure will be negative (relative to atmosphere) at the pump suction.
Why is this pressure less than atmospheric pressure or low? If you take a straw, fill it with water, cover one end with your fingertip and turn it upside down you will notice that the liquid does not come out of the straw, try it!. The liquid is pulled downward by gravity and creates a low pressure under your fingertip. The liquid is maintained in balance because the low pressure and the weight of the liquid is exactly balanced by the force of atmospheric pressure that is directed upwards.
The same phenomenon occurs in the pump suction which is pulling up liquid from a low source. Like in the straw, the pressure close to the pump suction connection must be low for the liquid to be supported.
To calculate the discharge head, we determine the total head from the characteristic curve and subtract that value from the pressure head at the suction, this gives the pressure head at the discharge which we then convert to pressure.
We know that the pump must generate 15 feet of lift at the pump suction, lift is negative static head. It should in fact be slightly more than 15 feet because a higher suction lift will be required due to friction. But let’s assume that the pipe is generously sized and that the friction loss is small.
TOTAL HEAD = 100 = HD – HS
HD = 100 + HS
The total head is equal to the difference between the pressure head at the discharge HD and the pressure head at the suction HS. HS is equal to –15 feet because it is a lift, therefore:
HD = 100 + (-15) = 85 feet