PUMP AND PUMP SYSTEM GLOSSARY

Absolute pressure: pressure is measured in psi (pounds per square inch) in the imperial system and kPa (kiloPascal or bar) in the metric system. Most pressure measurements are made relative to the local atmospheric pressure. In that case we add a "g" to the pressure measurement unit such as psig or kPag. The value of the local atmospheric pressure varies with elevation (see this pressure vs. elevation chart on this page). It is not the same if you are at sea level (14.7 psia) or at 4000 feet elevation (12.7 psia). In certain cases it is necessary to measure pressure values that are less then the local atmospheric pressure and in those cases we use the absolute unit of pressure, the psia or kPa a. pa(psia) = pr(psig) + patm(psia), patm = 14.7 psia at sea level. where pa is the absolute pressure, pr the relative pressure and patm the absolute pressure value of the local atmospheric pressure. and in the metric system pa(kPa a) = pr(kPag) + patm(kPa a), patm = 100 kPa a at sea level.

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Accumulator: used in domestic water applications to stabilize the pressure in the system and avoid the pump cycling on and off every time a tap is opened somewhere in the house. The flexible bladder is pressurized with air at the pressure desired for acheiving the correct flow rate at the furthest point of the house or system. As water is pulled from the tank the bladder expands to fill the volume and maintain the pressure. When the bladder can no longer expand the water pressure drops, the pressure switch of the pump is activated on low pressure, and the pump starts and fills the water volume of the accumulator. The bladder keeps the air from entering into solution with the water resulting in less frequent re-pressurisation of the accumulator.

Pumps are often sold as a package with an accumulator.

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Affinity laws: the affinity laws are used to predict the change in diameter required to increase the flow or total head of a pump. They can also predict the change in speed required to achieve a different flow and total head. The affinity laws can only be applied in circumstances where the system has a high friction head compared to the static head and this is because the affinity laws can only be applied between performance points that are at the same efficiency. see affinity laws.pdf

The following figure shows a system that has a friction head (curve A) higher than its static head for which the affinity laws apply, as compared to curve B, a system with a high static head as compared to the friction head where the affinity laws do not apply.