In our quest to fully understand the workings of pumps, we must grasp the basic concepts of how pumps work. Pumps are used to transfer liquids from low to high pressure. This may be to move the liquid from one place to another and very often from a low elevation to a high elevation. In addition to the simple movement of the liquid from one area to another area, pumps are also used to increase the flow rates of liquid.

Basically, the liquid flows through the suction piping and arrives at the suction nozzle. Note that a pump cannot suck the liquid into the pump, the liquid must have sufficient energy to allow the pump to take the energy and work the liquid’s energy. This concept is called Net Positive Suction Head (NPSH).

Positive displacement pumps have some form of movable enclosure. They’re many types of PD pumps; some consist of rotating gears, others may be as simple as a pulsing diaphragm. The fluid enters the suction nozzle at a low pressure zone, the fluid is then mechanically transported through the pump to the discharge nozzle. This mechanical movement basically contracts and as a fluid can’t be compressed the pressure inside the pump’s housing increases and the fluid is expelled into the discharge piping. The head or pressure that a PD pump can generate is predominately a function of the tolerances and strength of the pump components coupled with the thickness of the pump case.

A centrifugal pump contains an impeller and volute. The impeller is attached to a shaft. The shaft is connected to a driving unit, normally but not exclusively, an electric motor. The fluid enters into the eye of the impeller and is captured between the impeller blades. The blades impart velocity to the fluid as the fluid is transferred from the eye to the outside diameter of the impeller. As the fluid accelerates, a low pressure zone is created in the eye of the impeller. This principle is known as the Bernoulli principle – as velocity increases, pressure decreases. Therefore, the fluid leaves the outside diameter of the impeller with increased velocity and as the fluid hits the internal casing of the volute the fluid stops abruptly and the velocity is converted into pressure – the Bernoulli Principle in reverse. As the impeller is spinning, rotary velocity is also occuring and the fluid is transfered around an ever increasing escape channel within the volute. The pathway is increasing and consequently the rotary velocity is decreasing adding additional pressure to the fluid – Bernoulli’s Principle yet again!

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The pressure and flow that a centrifugal pump can deliver is mostly governed by the diameter of the impeller and the speed of the motor. As we have seen from the David Brown DB37 pump above, several impellers can be used in sequence to build up to the required pressure in several stages.

Force (F) is equal to Pressure (P) multiplied by Area (A). So F= P x A.

Therefore, to work out pressure we divide the Force (F) by the Area (A). Pressure = F/A.

When we apply pressure to the surface of a liquid the pressure is transmitted uniformly across the surface and also through the liquid to the walls of the vessel. The pressure can be expressed by a number of imperial and metric units, the most popular bar (metric) and pounds per square inch (psi) – imperial.

Atmospheric Pressure (ATM)

Atmospheric Pressure (ATM) is the force exerted by the weight of the atmosphere on a unit of area. ATM is equal to 14.7 psia at sea level. As elevation rises above sea level, the atmospheric pressure reduces.

Absolute Pressure (psia)

Absolute pressure is the pressure measured from a zero pressure reference. Compound pressure gauges record absolute pressure and absolute pressure is 14.7 psia at sea level.

Gauge Pressure (psig)

Gauge pressure is the pressure indicated on a simple pressure gauge. psig equals psia – ATM.

Vacuum

Understanding vacuum can lead to confusion very often as most people think that vacuum is expressed as a negative psi. This is true with regards to a simple pressure gauge which will record vacuum as a negative psig. However, vacuum is any pressure less than atmospheric pressure. Therefore, anything less than 14.7 psia is a vacuum. Compound gauges record vacuum as a positive psia. When we listen to the daily forecast we often see areas of low pressure described in millibars (1000 millibars is atmospheric pressure). Vacuum can be expressed in many ways. It is probably easiest to think of vacuum in pumping terms as a positive number less than 14.7 psi.

Pump manufacturers often talk in terms of pump head. The basic formula for pump head is Head (H) equals Pressure (psi) divided by Density (D). To convert pressure into head we can use the following equation:

Head ft = (2.31 x Pressure psi ) divided by (specific gravity). The 2.31 is a conversion factor.

Specific Gravity

When we compare the density of a liquid with the density of water, we are detailing the liquid’s specific gravity. The formula for specific gravity is:

Specific gravity = density liquid / density water (i.e. water 60 degrees Fahrenheit at sea level).

Water has a specific gravity of 1.0. Other liquids are either denser or lighter than water. The specific gravity affects the pressure in relation to the head as illustrated in the formula above.

The above is the reason why pump manufacturers sell pumps that will produce a certain feet of head. The pump manufacturer will not know the final liquid that the pump will be pumping therefore, as the above formula details, the working pressure of the pump will vary depending on the specific gravity of the liquid to be pumped.

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