Pulsafeeder Pulsar Shadow - Manual (Page 39)

10.2 System Backpressure
The system backpressure must exceed the suction pressure by at least 5 psi (0.35 bar) in order to prevent
flowthrough, however it must not exceed the rated discharge pressure of the pump. Flowthrough can be
defined as the process liquid flowing from a higher pressure to a lower pressure (downhill pumping),
which results in a flow output greater than the pumps calibrated capacity, 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 total system
backpressure use Equation 3 or 4.
Equation 3. For fluid viscosity below 50 centipoise.
P P + )
d C
RGQ L
( = P
H P 2
1
D
T
±
Equation 4. For fluid viscosity at or above 50 centipoise.
P P + )
d C
Q L
( + )
d C
RGQ L
( = P
H P
2
4
2
D 2
2
1
D
T
±
µ
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)
[psia, bar(a)]
P
H
= Head pressure above (+) or below (-) the pump centerline, [psi, bar]
P
V
= Absolute vapor pressure at pumping temperature of the process liquid at pump inlet, [psia, 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, [unitless]
Q = Pump average flow rate, [gph, lph]
d = Internal pipe diameter, [inches, mm]
C
1
,C
2
= Numeric constants used in Equations 1- 4
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, [psig, bar(g)]
P
T
= Pump discharge pressure at the discharge port, [psig, bar(g)]
33