Pulsafeeder Pulsar Shadow - Manual (Page 48)

42
System Back Pressure
The system backpressure must exceed the suction pressure by at least 5 psi (0.35 bar) in order 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 (downhill pumping),
which attributes to pump failure and undesired flow at pump shutdown. If the system backpressure is
not at least 5 psi (0.35 bar) greater than the suction pressure, a backpressure 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 centipoise.
Equation 4. – For fluid viscosity above 50 centipoise.
Nomenclature
NPSH =
R
Net positive suction head required, [psi, bar]
NPSH =
A
Net positive suction head available, [psi, bar]
P =
A
Pressure at the surface of the liquid being pumped (atmospheric or supply tank
blanket pressure) [psi(a), bar(a)]
P =
H
Head pressure above (+) or below (-) the pump centerline, [psi, bar,] (convert from
ft or m)
P =
V
Absolute vapor pressure at pumping temperature of the process liquid at pump inlet,
[psi(a), bar(a)]
L =
S
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]
µ = Viscosity of process liquid at pumping temperature, centipoise [cp]
L =
D
Length of discharge piping (actual, not equivalent), [ft, m]
P =
P
System discharge pressure, [psi(g), bar(g)]
P =
T
Peak pump discharge pressure at the discharge port, [psi(g), bar(g)]
V =
P
Peak liquid velocity generated by the pump, (suction or discharge) [ft/s, m/s]