
IOM 490 SXT 10”x10”_0622
Bypass Lines
Self
‐
priming pumps are not air compressors. During the priming cycle, air from the suction line must
be vented to atmosphere on the discharge side. If the discharge line is open, this air will be vented
through the discharge. However, if a check valve has been installed in the discharge line, the
discharge side of the pump must be opened to atmospheric pressure through a bypass line installed
between the pump discharge and the check valve. A self
‐
priming centrifugal pump will not prime if
there is sufficient static liquid head to hold the discharge check valve closed.
NOTE
The bypass line should be sized so that it does not affect pump discharge capacity; however, the
bypass line should be at least 1 inch in diameter to minimize the chance of plugging.
In low discharge head applications (less than 30 feet or 9 meters), it is recommended that the bypass
line be run back to the wet well and located 6 inches below the water level or cut
‐
off point of the
low level pump. In some installations, this bypass line may be terminated with a six
‐
to eight-foot
length of 1 1/4-inch I.D. smooth
‐
bore hose; air and liquid vented during the priming process will then
agitate the hose and break up any solids, grease, or other substances likely to cause clogging.
A bypass line that is returned to a wet well must be secured against being drawn into
the pump suction inlet. It is also recommended that pipe unions be installed at each
90°elbow in a bypass line to ease disassembly and maintenance.
In high discharge head applications (more than 30 feet), an excessive amount of liquid may be
bypassed and forced back to the wet well under the full working pressure of the pump; this will
reduce overall pumping efficiency.
Therefore, it is recommended that Automatic Air Release Valve
(ARV) be installed in the bypass line
.
Automatic Air Release Valves (ARV’s) are reliable and require minimum maintenance. See
AUTOMATIC AIR RELEASE VALVE in this section for installation and theory of operation of the
Automatic Air Release Valve.
If the installation involves a flooded suction such as a below
‐
ground lift station. A pipe union and
manual shut
‐
off valve may be installed in the bleed line to allow service of the valve without shutting
down the station, and to eliminate the possibility of flooding. If a manual shut
‐
off valve is installed
anywhere in the air release piping, it must be a full
‐
opening ball type valve to prevent plugging by
solids.
If a manual shut
‐
off valve is installed in a bypass line, it must not be left closed during
operation. A closed manual shut
‐
off valve may cause a pump which has lost prime to
continue to operate without reaching prime, causing dangerous overheating and
possible explosive rupture of the pump casing. Personnel could be severely injured.
Allow an over
‐
heated pump to cool before servicing. Do not remove plates, covers, gauges, or
fittings from an over
‐
heated pump. Liquid within the pump can reach boiling temperatures, and
vapor pressure within the pump can cause parts being disengaged to be ejected with great force.
After the pump cools, drain the liquid from the pump by removing the casing drain plug. Use
caution when removing the plug to prevent injury to personnel from hot liquid.
AUTOMATIC AIR RELEASE VALVE
Theory of Operation
When properly installed, an Automatic Air Release Valve (ARV) will permit air to escape through the
bypass line and then close automatically when the pump is fully primed and pumping at full capacity.