Pressure

Pressure

It is erroneous to think of a pump as having a single capacity expressed in gpm. The pump has, rather, a range of capacities determined by the pressure or "head" required in the system. Capacity (or delivery) varies inversely with the head, and both must be taken into consideration when sizing a pump.

In water systems work, pressure is expressed either in terms of pounds per square inch (psi) or feet of water. Either term may be used, so long as it's used consistently. For conversion purposes, 1 psi equals 2.31 ft of water.

Pumps are normally selected in terms of pressure and capacity directly from the manufacturer's catalog. Among various terms used in selection tables, the following are most common:

DISCHARGE PRESSURE: This is the pressure of the water as it leaves the pump or well. In the case of jets, it is the pressure on the discharge side of the pump. In the case of subs, it is the pressure at the wellhead.

TANK PRESSURE: Some catalogs also specify pump capacity in terms of tank pressure or pressure switch setting range. In most cases the "tank pressure" refers to the higher end of the pressure switch setting. However this should be checked out in the individual catalog being used.

TOTAL HEAD: The more elaborate catalogs show performance curves for various pumps. These are plots of pump output (in gpm) vs. "total head," usually expressed in feet of water. The total head represents the sum of all pressure requirements on the system, including the following:

Suction lift, the pressure necessary to get water from the pumping level to the surface.

Elevation, the pressure (if any) needed to get water from the surface to the pump level.

Friction loss, the pressure needed to push water through the piping, valves, fittings, etc. It's possible to calculate the friction loss through a given piping network for any rate of flow. However, the simplest procedure is to take friction loss downstream from the tank into consideration when determining pressure switch settings.

Service pressure, the component of pressure required to operate the water-using outlet. As indicated earlier this ranges from 10 psi for most fixtures to 20 psi or more for lawn sprinklers.

It is not always necessary to compute total head in order to size a pump. Most catalogs are set up to show pump capacity at various depths and discharge or tank pressures. The usual procedure is to read down to the necessary depth, then across to the desired capacity at the discharge or tank pressure needed for adequate system performance.

PEAK DEMAND PERIOD IN NONRESIDENTIAL SYSTEMS (MIN.)

HOSPITAL 60

HOTEL 120

MOTEL 120

OFFICE BUILDING 60

RESTAURANT 120

SCHOOL 15-30

STORE 120

THEATER 15

Procedure is to read down to the necessary depth, then across to the desired capacity at the discharge or tank pressure needed for adequate system performance.