10
manufactured at Milton Roy and should not be dis-
assembled in the field.
2.10 BACK PRESSURE SPRING INSTALLA-
TION IN XW DISCHARGE CHECK VALVE
AFTER XW PUMP HAS BEEN FULLY
PRIMED, REMOVE THE DISCHARGE
CAP FROM THE BODY AND INSERT
SPRING THROUGH THE TOP SEAT TO
REST ON TOP OF THE BALL CHECK.
REPLACE CAP AND TIGHTEN UNTIL
FIRMLY SEATED ON BODY. DO NOT
OVER-TIGHTEN, AS O-RING PROVIDES
THE SEAL.
REMOTE HEAD SYSTEMS: DO NOT
INSTALL A BACK PRESSURE SPRING
IN THE DISCHARGE BALL CHECK
CARTRIDGE OF DIAPHRAGM HEADS
WHICH ARE “REMOTE MOUNTED”
(NOT ATTACHED TO THE MAIN HOUS-
ING). A SEPARATE BACK PRESSURE
VALVE MUST BE INSTALLED IN THE
DISCHARGE LINE FROM THE REMOTE
HEAD CARTRIDGE.
TAKE CARE WHEN INSERTING THE
DISCHARGE PIPE NIPPLE INTO THE
BALL CHECK CARTRIDGE. NIPPLE
SHOULD NOT EXTEND INTO CAVITY,
CAUSING CLAMPING OF THE SPRING
AND RETARDING NORMAL OPERA-
TION.
2.11 OPERATION WITH SUCTION LIFT
It is desirable that the mRoy pump operate with a
flooded suction; however, operation with net posi-
tive suction head (NPSH) less than atmospheric
pressure is possible.
NPSH is the head available, above the vapor pres-
sure of the liquid being pumped, to feed the liquid
into the pump suction port. NPSH minimum is the
head below which the pump cavitates.
Both values are calculated at the suction port of the
pump. In controlled volume pump applications, two
conditions must be considered in the selection of a
pump to meet the NPSH minimum requirements:
1. At the start of the suction stroke, the liquid in
the suction line has no velocity and NPSH
minimum depends on the force necessary to
accelerate the liquid in the suction pipeline.
2. At the peak of the suction stroke there is no
acceleration factor and NPSH minimum
depends on friction losses as calculated
from standard flow equations. With all
viscous fluids and in pilot plants and other
places where unusual numbers of fittings
and valves are used, the second condition
that includes friction losses should be
considered. For water-like fluids, the first
condition will define the limiting
configuration.
For Static NPSH (Condition 1)
Available NPSH = Pa + Ph – Pv (must be equal to
or greater than minimum NPSH as listed under
Installation Instructions.
Required NPSH min =
Sp. Gr. (0.0925) LpD
For Dynamic NPSH (Condition 2)
Available NPSH = Pa ± Ph – Pf Le (must be equal
to or greater than minimum NPSH as listed under
Installation Instructions.
D = Plunger Diameter (inches)
Dp = Pipe Diameter (inches)
Lp = Actual Length of suction pipe (feet)
Le = Equivalent length of suction pipe
including allowance for fittings (feet)
Pa = Ambient pressure above liquid (PSIA)
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