Pulsafeeder PulsaPro 900 EN - Manual (Page 57)

53
System Back Pressure
The system back pressure must exceed the suction pressure by at least 25 psi (1.7 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 back pressure is not at least 25 psi (1.7 bar) greater than the suction pressure, a
back pressure valve must be installed in the discharge piping. To calculate the system’s total
back pressure use Equation 3 or 4 below.
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]
C
1
,C
2
,C
3
= Numeric constants used in Equations 1- 5 [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]