Page 200 - Shale Shakers Drilling Fluid Systems

CENTRIFUGAL PUMPS 183
NPSH requirements shown on performance curves
are based on boiling water. As the water tempera-
ture decreases below the boiling point, less and
less head is required to produce the same NPSH
due to the difference in vapor pressure. With suf-
ficient reduction in required NPSH, it is then pos-
sible to pull a suction lift. This is expressed as:
where P atm = Atmospheric pressure in feet of
liquid, absolute
h sh = Suction head, feet of liquid
p vp = Liquid's vapor pressure, feet of
liquid, absolute
h ss = Static suction head, feet of liquid
n
fsuct = Friction head at suction, feet of
liquid
Total Dynamic Suction Lift
Total dynamic suction lift is the sum of two
parameters:
1. Vertical distance of pump above liquid (static by the pump and is expressed as:
suction lift)
2. Friction loss in suction pipe
For example, if it is determined that friction losses where H = Total head
are equivalent to five feet of head for fluids of
"water-like viscosity," the vertical distance of the
centrifugal r-imp suction above the liquid level
cannot exceed ten feet. When pumping viscous
fluids, the pump suction should be below the "suc-
tion tank." In other words, the head that moves
fluid from the fluid level in the suction tank to the termined by the impeller diameter and its rotating
pump is the "total suction head." The "total dis-
charge head" tends to resist the flow of liquid from
the pump to the discharge tank.
These components can be subdivided into three
smaller segments:
1. Static head
2. Surface pressure
3. Friction head
Static head. Static suction head is the height of
the liquid surface in the suction tank above the
centerline of the pump. If the liquid surface is
below the centerline, the static suction head as-
sumes a negative value, referred to as "suction lift." where Q = Flow rate, gpm
Static discharge head is the height of the high-
est liquid surface in the discharge section above
the centerline of the pump. This can be liquid in
the discharge pipe or in the discharge tank, de-
pending on which is highest.
Surface pressure. The suction and/or discharge
compartments may be under pressure other than
atmospheric. Both suction and discharge surface
pressure is converted to feet of liquid.
Friction head. Friction head is the energy nec-
essary per pound of fluid pumped to overcome
friction and turbulent losses as fluid flows through
the system. The values of suction friction head and
discharge friction head depend on pipe lengths, di-
ameters, flow rate, valves and fittings, their con-
figurations, and pipe materials and interior finish.
Total Dynamic Head
The total dynamic head is the sum of total suc-
tion lift plus total discharge head. The sum of the
gauge pressure head and the velocity head at the
discharge flange, minus the sum of the corre-
sponding heads at the suction flange, equals the
energy (in foot-pounds) added per pound of liquid
pumped. This is called the "total head" developed
h gd = Discharge gauge head
h vd = Discharge velocity head
h gs = Suction gauge head
h vs = Suction velocity head
The discharge head of a centrifugal pump is de-
speed, not by the horsepower of the motor.
If the discharge line of a centrifugal pump is
closed, the horsepower input decreases and the
pressure (head) increases only slightly. As the
valve on the discharge is gradually opened, the
horsepower input increases rapidly and pressure
(head) decreases slightly.
Pump horsepower. Hydraulic horsepower (hhp)
is expressed as follows:
H = Head, ft
P = Pressure drop, psi