16
Pump Motors
Level 2
A.O.Smith
A check valve or foot valve is normally located
in the plumbing line before the pump. This “one
way” valve is needed so that the pressure in the
system will not bleed off when the pump stops. If
the valve leaks, the pump will start and stop,
even through water is not being used.
Since for all practical purposes, water is non
compressible, a pressure tank is used in the sys-
tem.
A pressure switch is used to start and stop the
motor. The “cut in” setting is the pressure at
which the motor starts. The “cut out” setting is
the pressure at which the motor stops. The
amount of water (in gallons) that is pumped in
each cycle is called draw down. In a specific sys-
tem, the draw down amount may be varied by
changing the cut in and/or cut out settings.
The low and high pressures of the system are
determined by the cut in and cut out settings, not
by the amount of air pressure in the system.
As water is pumped, air in the tank is com-
pressed until system pressure reaches the cut out
level. The most popular tanks have a bladder or
diaphragm which separates the air and water.
Other systems use an air volume control and put
air into the tank each time the pump is started.
If the water and air were not separated, or air
was not added, the air in the tank would all dis-
solve in the water and the tank would become
“waterlogged”.
If the air charge in a system is reduced, the
pump will still start and stop at the same pres-
sures, but less water will be pumped in each cycle
because there is less air to compress. As the
amount of air is decreased, the tank approaches a
waterlog condition and the system operates as if
there were no pressure tank.
AIR
WATER
AIR
WATER
DIAPHRAGM
TO HOUSE
PUMPSIDE
DIAPHRAGM
PRESSURE TANK
30 PSI CUT IN
50 PSI CUT OUT
HOUSE
PUMPSIDE
AIR
WATER
HOUSE
PUMPSIDE
BOTH AIR AND WATER
AT 40 PSI
1. FAUCET OPENED
2. AIR PUSHES ON DIAPHRAGM
3. WATER SENT TO SYSTEM
4. AT 30 PSI, PUMPSTARTS
AT 50 PSI,
PUMPSTOPS
