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12.2
System Back Pressure
The system back pressure must exceed the suction pressure by at least five (5) psi (0.35bar) to prevent flow
through, however it must not exceed the rated discharge pressure of the pump. Flow through can be defined
as the process liquid flowing from a higher pressure to a lower pressure, which attributes to pump failure and
undesired flow at pump shutdown. If the system back pressure is not at least five (5) psi (0.35bar) greater
than the suction pressure, a back-pressure valve must be installed in the discharge piping. To calculate the
system’s total backpressure, use Equation 3 or 4.
Equation # 3 – For fluid viscosity below 50 centipoises
Equation # 4 – For fluid viscosity above 50 centipoises
12.3
Nomenclature
Table 3A – Definitions of abbreviations used in NPIP Equations
NPIPR
Net positive inlet pressure required, [psi, bar]
NPIPA
Net positive inlet pressure available, [psi, bar]
PA
Pressure at the surface of the liquid being pumped (atmospheric or
supply tank blanket pressure) [psi(a), bar(a)]
PH
Head pressure above (+) or below (-) the pump centerline, [psi, bar,]
(convert from ft or m)
PV
Absolute vapor pressure at pumping temperature of the process
liquid at pump inlet, [psi(a), bar(a)]
LS
Length of suction piping (actual, not equivalent), [ft, m]
R
Pump stroking rate, strokes/min [spm]
G
Specific gravity of process liquid, [no units]
Q
Pump average flow rate, [gph, lph]
d
Internal pipe diameter, [inches, mm]
C1, C2,
C3
Numeric constants used in Equations 1 - 4 [no units] see table 2 for
values
µ
Viscosity of process liquid at pumping temperature, centipoise [cp]
LD
Length of discharge piping (actual, not equivalent), [ft, m]
PP
System discharge pressure, [psi(g), bar(g)]
PT
Peak pump discharge pressure at the discharge port, [psi(g), bar(g)]
VP
Peak liquid velocity generated by the pump, (suction or discharge)
[ft/s, m/s]
