Designing a Pump
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. 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). 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. Pressure is often expressed in pounds per square inch (psi) in the Imperial system and kiloPascals (kPa) in the metric system.
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.
Flow rate VS Pipe Dia for PVC pipe
Pressure provides the driving force to overcome friction and elevation difference. It’s responsible for driving the fluid through the system, the pump provides the pressure. Pressure is increased when fluid particles are forced closer together.
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 which have a high flow rate and small diameter as in the syringe example. 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 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.
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 then 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.
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.
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.
PUMP ENERGY = FRICTION ENERGY + ELEVATION ENERGY
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
Webster’s dictionary definition of head is: “a body of water kept in reserve at a height”.
