CENTRIFUGAL PUMP SYSTEMS part 1

 What are the important characteristics of a pump system? 

- What is head and how is it used in a pump system to make calculations easier? 

- What is static head and friction head and how do they affect the flow rate in a 

pump system? 

- How does a centrifugal pump produce pressure? 

- Why is total head and flow the two most important characteristics of a centrifugal 

pump? 

 CENTRIFUGAL PUMP SYSTEMS

- What is meant by the pump rating? And what is the optimal operating point of a centrifugal pump? 

- How to do details calculations that will allow you to size and select a centrifugal pump? 

- How to verify that your centrifugal pump is providing the rated pressure or head? 

- What is density and specific gravity and how do they relate to pressure and head?

 1. Different types of pump systems 

There are many types of centrifugal pump systems. Figure 1 shows a typical industrial pump system. 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. 

 

Figure 1 Typical industrial pump system. 

 

 Domestic water systems take their water from various sources at different levels depending on the water table and terrain contours.

The system in Figure 2 is a typical domestic water supply system that takes it's water from a shallow well (25 feet down max.) using an end suction centrifugal pump. A jet pump works well in this application.

 The system in Figure 3 is another typical domestic water supply system that takes it's water from a deep well (200-300 feet) and uses a multi-stage submersible pump often called a turbine pump

2-Three important characteristics of pump systems: pressure, friction and flow

Pressure, friction and flow are three important characteristics of a pump system. 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 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).

 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, it means that the pressure measurement is relative to the local atmospheric pressure, so that 5 psig is 5 psi above the local atmospheric pressure. The kPa unit scale is intended to be 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 a 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 a 100 feet of length (see Figure 4a).

 

 The unit of friction is....Sorry, I think I need to wait 'til we get closer to the end to explain the reasoning behind this unit.

 

The pump provides the energy necessary to drive the fluid through the system and overcome friction and any elevation difference. 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

 

 If we apply this idea to the pump system of Figure 5, even though the discharge pipe end is open, it is possible to have pressure at the pump discharge because there is sufficient friction in the system and elevation difference.

 3. 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 be also be high for short pipes which 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 (see Figure 8). 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.

 

 Another cause of friction is the interaction of the fluid with the pipe wall, the rougher the 

pipe, the higher the friction. 

Friction depends on: 

- average velocity of the fluid within the pipe 

- viscosity 

- pipe surface roughness 

 

An increase in any one of these parameters will increase friction. 

The amount of energy required to overcome the total friction loss 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, this is 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 are 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 (see Figure 9), the fluid streamlines that are closest to the tight inner radius of the elbow lift off from the pipe surface forming small vortexes that consume energy. This energy loss is small for one elbow but if you have several elbows and other fittings it can become significant. Generally speaking they rarely represent more than 30% of the total friction due to the overall pipe length.

4. 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. 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. 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. Any time you have liquid in a container, even one that is not pressurized, you will have pressure at the bottom due to the liquid’s weight, this is known as static pressure. 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 pitcher throws a baseball he gives it velocity energy. When water comes out of a garden hose, it has velocity energy

 

In the figure above we see a tank full of water, a tube full of water and a cyclist at the top of a hill. The tank produces pressure at the bottom and so does the tube. The cyclist has elevation energy that he will be using as soon as he moves.

 As we open the valve at the tank bottom the fluid leaves the tank with a certain velocity, in this case pressure energy is converted to velocity energy. The same thing happens with the tube. In the case of the cyclist, the elevation energy is gradually converted to 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 difference in height that the liquid must be raised to which is the elevation energy. 

PUMP ENERGY = FRICTION ENERGY + ELEVATION ENERGY
 

 You are probably thinking where is the velocity energy in all this. Well if the liquid comes out of the system at high velocity then we would have to consider it but this is not a typical situation and we can neglect this for the systems discussed in this article. The last word on this topic, it is actually the velocity energy difference that we would need to consider. In Figure 11 the velocities at point 1 and point 2 are the result of the position of the fluid particles at points 1 and 2 and the action of the pump. The difference between these two velocity energies is an energy deficiency that the pump must supply but as you can see the velocities of these two points will be quite small. Now what about head? Head is actually a way to simplify the use of energy. To use 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 a 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 liquid displaced: 

FH = FE/W = F x l / W 

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. 

I know you are thinking: “…this doesn’t make sense”, how can feet represent energy? 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 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 can be expressed in feet and this is how you can measure them.

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 and the higher the reservoir the higher the pressure (see Figure 13).

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 (see Figure 15). To find out how to calculate pressure from height go to section 14.

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 (see Figure 16).

 

Usually the liquid is displaced from a suction tank to discharge tank. The suction tank fluid provides pressure energy to the pump that 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 tank. 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 (see Figure16).Since there is a difference in height between the suction and discharge flanges or connections of a pump by convention it was decided that the static head would be measured with respect to the suction flange elevation (see Figure 17).

end is open to atmosphere then the static head is measured with respect to the pipe end 

(see Figure 18).

Sometimes the discharge pipe end is submerged such as in Figure 19, then the static head will be the difference in elevation between the discharge tank fluid surface and 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 (see Figure 20). 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.

You can change the static head 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.

 

6. 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 21). 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 of 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 22).

In Figure 23, the discharge pipe end is raised vertically until the flow stops, the pump 

cannot raise the fluid higher than this point and the discharge pressure is at its 

maximum. Similarly the cyclist applies maximum force to the pedals without getting 

anywhere.

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 24). If the negative static

head is large then it is possible that a pump is not required since the energy provided by 

the 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 the pump glossary). By analogy, as 

the cyclist comes down the hill he looses 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. In Figure 23, 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 (see Figure 25). 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 21, 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 shower head for this bathtub 

then you will need a pump with more head for the same flow rate because the shower 

head 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 impeller diameter 

or both. In practice, homeowners cannot make these changes and to obtain a higher 

total head, a new pump must be purchased.

7. 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 use a large drain 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 (see Figure 28).

when the discharge pipe is long, the friction will be high and the flow rate low (see Figure 

29) and if the pipe is short the friction will be low and the flow rate high (see Figure 30)

Later on in the tutorial, a chart will be presented giving the size of pipes for various flow 

rates.

8. 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 plane of the impeller and fill up the volume between each impeller vane. 

The next image shows what happens to the fluid particles from that point forward for an 

animated version go to http://www.fluidedesign.com/downloads.htm in the middle of the page.

A centrifugal pump is a device whose 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 

energy. Another example is catching a hard baseball pitch, ouch, there can be allot 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, they then hit the inner casing of the pump and are decelerated lowering the 

velocity energy and raising the pressure energy. Unlike friction that wastes energy, the 

decrease in velocity energy serves to increase pressure energy; this is the principal 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 looses elevation. The cyclist’s elevation 

energy is transformed into velocity energy; in the pump’s case the velocity energy is 

transformed into pressure energy. 

Try this experiment, find a plastic cup or other container that you can poke a small 

pinhole in the bottom. Fill it with water and attach a string to it, and now you guessed it, 

start spinning.

The faster you spin, the more water comes out 

the small hole, the water is pressurized inside 

the cup using centrifugal force in a similar 

fashion to a centrifugal pump. In the case of a 

pump, the rotational motion of the impeller 

projects fluid particles at high speed into the 

volume between the casing wall and the 

impeller tips. Prior to leaving the pump, the 

fluid particles slow down to the velocity at the 

inlet of the discharge pipe (see Figures 31 and 

32) which will be the same velocity throughout 

the system if the pipe diameter does not 

change. 

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 backwards 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 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 35 (assuming that the level in the suction tank remains constant), 

this shows how the discharge pressure varies with the flow rate through the pump.

At 200 gpm, this pump produces 20 psig discharge pressure, and as the flow drops the 

pressure increases, and will be 40 psig at zero flow. 

Note: this applies to centrifugal pumps, many home owners have positive displacement 

pumps, often piston pumps. These pumps produce constant flow no matter what 

changes are made to the system. This is why all such pump have an internal relief valve 

that opens to relieve pressure and protect the pump from excessive pressure cause by 

the closing of a water tap for example. Also typically, the pump has a pressure switch 

that shuts the motor down when the pressure gets too high saving energy in the 

process. 

9. 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? Pressure is 

a familiar concept, we are familiar with it in our daily lives. For example, fire 

extinguishers are pressurized at 60 psig (410 kPa), we put 35 psig (240 kPa) air 

pressure in our bicycle and car tires. Pump manufacturers do not use discharge 

pressure as criteria 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 as it is in a positive displacement pump. 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. 

Pump 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 salt water 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 does not depend on density is very useful. There is such a criteria and it is called TOTAL 

HEAD, and this 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.

For these reasons the pump manufacturers have chosen total head as the main 

parameter that describes the pump’s available energy. 

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. This is a surprising fact, see this 

experiment on video on the web that shows this idea in action 

(http://www.fluidedesign.com/video1.htm#vid0.9).

10. 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 the liquid is raised to at the suction side (see Figure 36). 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. 

You may be interested to know that 324 foot-pounds of energy is equivalent to 1 calorie. 

This means that our weight lifter spends 600/324 = 1.8 calories each time he lifts that 

weight up 6 feet, not much is it. 

The following figure shows how much energy is required to displace vertically one gallon 

of water.

This next figure shows how much head is required to do the same job.

If we use energy to describe how much work the pump needs to do to displace a volume 

of liquid, we will 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 on for a long time, you 

don't worry about how many pounds of fluid have been displaced we are mainly 

interested in the flow rate. 

The other very useful aspect of using head is that the elevation difference or static head 

can be used directly as one part of the value of total head, the other part being friction 

head as shown in Figure 40.

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.

11. How to determine friction head 

Friction head is the amount of energy loss due to friction caused by fluid movement 

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 2 for different pipe sizes and flow rates.

Table 2 gives the flow rate and the friction head loss for water being moved through 

pipes of different diameter at a velocity of 10 ft /s. I have chosen 10 ft/s because it is a 

typical value for velocity in pipes, it is not too large which would create allot of friction 

and not too small which would slow things down, it’s just right. Velocity depends on the 

flow rate and the pipe diameter, you will find friction loss charts for flow rates of 5 ft/s and 

15 ft/s in Appendix A, imperial and metric. If you wish to do you own calculation of 

velocity, you can find out how in Appendix D. 

If the velocity you are using is less than 10 ft/s then the friction loss will be less and if the 

velocity is higher the loss will be greater. A velocity of 10 ft/s is normal practice for sizing 

pipes on the discharge side of the pump. For the suction side of the pump, it is desirable 

to be more conservative and size pipes for a lower velocity, for example between 4 and 

7 feet/second. This is why you normally see a bigger pipe size on the suction side of the 

pump than on the discharge. A rule of thumb is to make the suction pipe the same size 

or one size larger than the suction connection of the pump. 

Why bother with velocity, isn’t flow rate enough information to describe fluid movement 

through a system. It depends how complicated your system is, if the discharge pipe has 

a constant diameter then the velocity though out will be the same. Then if you know the 

flow rate, based on the friction loss tables, you can calculate the friction loss with the 

flow rate only. If the discharge pipe diameter changes then the velocity will change for 

the same flow rate and a higher or lower velocity means a higher or lower friction loss in 

that portion of the system. You will then have to use the velocity to calculate the friction 

head loss in this part of the pipe. You can find a velocity calculator at this web site http :// www.fluidedesign.com/applets.htm#applets4.

Those who would like to do pipe friction calculations will find the information in Appendix B and pipe fittings friction loss calculations are in Appendix C. A calculator for pipe friction loss is available here (http://www.fluidedesign.com/applets.htm#applets13) and for fittings friction loss here( http://www.fluidedesign.com/applets.htm#applets15). 

12. The performance or characteristic curve of the pump 

The pump characteristic curve has a similar appearance to the previous curve of 

discharge pressure vs. flow (Figure 35). 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 42 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.

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