DARLEY ZSP, Nstallation, Operation, Maintenance, Repair And Troubleshooting Instructions

Page: 23 / 155

Prepared by: EAP Rev. #3
Approved by: MCR
2
Date: 1/29/07
Revised by: JAF Revised Date: 5/1/13
1200509
Air leakage into pump while operating is usually indicated by a rattling sound in pump casing,
miniature explosions in stream issuing from the nozzle, or by losing of prime when operating at very
low capacities.
The usual cause of leaky suction lines is carelessness in handling of suction hose. Bruising of
hose threads by bumping against hard surfaces or sand in the coupling often prevents tightening of
the joints up against the gaskets. The hose gaskets are often defective and are sometimes lost
without being noticed by the operator.
INSUFFICIENT SUBMERGENCE:
The free end of suction hose must be submerged to a sufficient depth to prevent the entrance of air
that may be sucked down from the surface of the water to a considerable depth when operating at
large capacities.
Entrance of air into suction lines in this manner is indicated by a small whirlpool, or vortex, on the
surface of the water over the end of the hose.
A minimum submergence of 4 times the hose diameter to the upper holes in suction strainer is
recommended where full capacity of pump is required. Where sufficient submergence is not
possible, a board or sheet of metal laid over end of suction line will keep air from entering.
SUCTION LINE ENTRANCE TOO CLOSE TO BOTTOM:
If the end of suction line is laid on the bottom of the source of supply, a part of the suction opening
will be shut off; and if the bottom is soft, the hose will suck itself down into the earth closing more of
the opening and loosening sand and mud to be carried into the pump.
The suction entrance should be suspended a foot or more above the bottom, or if this is not
possible, it should be laid on a board or piece of sheet metal. A rope tied to the suction strainer is a
convenient means of holding it off the bottom.
OBSTRUCTION OF SUCTION STRAINER BY FOREIGN MATTER:
The high velocity of water entering the suction line will carry loose foreign bodies in against the
strainer from a considerable distance. Therefore, all weeds and refuse should be removed from
close proximity of the suction entrance.
SUCTION LINE TOO SMALL OR TOO LONG:
The flow of water into the pump is opposed by the frictional resistance in the suction line. This
friction loss must be added to the height of the pump above the water (static lift) to determine the
“total lift” of the pump. When all of the vacuum in the pump (atmospheric pressure) is consumed in
raising water through this total life, then the limit of capacity has been reached. This capacity can
be increased only by decreasing total lift. If the static lift cannot be reduced, then the friction loss
must be reduced by using a shorter or larger suction hose.
The rated capacity of the pump is guaranteed for a static lift of 10 feet for ratings up to 1500 gpm,
with 20 feet of recommended suction hose at 2000 feet. To increase the capacity without reducing
the static lift, or to increase lift without sacrificing capacity, requires larger suction hose.
An excessively long suction line is a handicap to any pump, for besides reducing capacity through
the added friction lose, it retards priming and it produces a detrimental effect known as “cavitation”.
This means a separation of the water column in the pump suction, or void spaces, produced by the
inertia of the heavy mass of water in the line resisting sudden change in the velocity when the
pump starts to deliver or when discharge valves are opened or closed. This phenomenon reduces