signed as fluid movers, they may be less efficient as hydraulic turbines than equipment
designed for that purpose. Steam turbines and electric motors are more economical when
steam or electricity is available.
But using a pump as a turbine can pay off in remote locations where steam or electric
power would require additional wiring or piping, in hazardous locations that require non-
sparking equipment, where energy may be recovered from a stream that otherwise would
be throttled, and when a radial-flow centrifugal pump is immediately available but a hy-
draulic turbine is not.
In the most common situation, there is a liquid stream with fixed head and flow rate
and an application requiring a fixed rpm; these are the turbine design conditions. The ob-
jective is to pick a pump with a turbine bep at these conditions. With performance curves
such as Fig. 4, turbine design conditions can be converted to pump design conditions.
Then you select from a manufacturer's catalog a model that has its pump bep at those val-
ues.
The most common error in pump selection is using the turbine design conditions in
choosing a pump from a catalog. Because catalog performance curves describe pump
duty, not turbine duty, the result is an oversized unit that fails to work properly.
This procedure is the work of Fred Buse, Chief Engineer, Standard Pump Aldrich Di-
vision of Ingersoll-Rand Co., as reported in Chemical Engineering magazine.
SIZING CENTRIFUGAL-PUMP IMPELLERS
FOR SAFETY SERVICE
Determine the impeller size of a centrifugal pump that will provide a safe continuous-re-
circulation flow to prevent the pump from overheating at shutoff. The pump delivers 320
gal/min (20.2 L/s) at an operating head of 450 ft (137.2 m). The inlet water temperature is
22O^0 F (104.4^0 C), and the system has an NPSH of 5 ft (1.5 m). Pump performance curves
and the system-head characteristic curve for the discharge flow (without recirculation) are
shown in Fig. 5, and the piping layout is shown in Fig. 12. The brake horsepower (bhp) of
an 11-in (27.9-cm) and an 11.5-in (29.2-cm) impeller at shutoff is 53 and 60, respectively.
Determine the permissible water temperature rise for this pump.Calculation Procedure:- Compute the actual temperature rise of the water in the pump
Use the relation P 0 = Pv + PNPSH, where P 0 = pressure corresponding to the actual liquid
temperature in the pump during operation, lb/in^2 (abs) (kPa); Pv = vapor pressure in the
pump at the inlet water temperature, lb/in^2 (abs) (kPa); PNPSH = pressure created by the
net positive suction head on the pumps, lb/in^2 (abs) (kPa). The head in feet (meters) must
be converted to lb/in^2 (abs) (kPa) by the relation lb/in^2 (abs) = (NPSH, ft) (liquid density
at the pumping temperature, Ib/ft^3 )/(144 in^2 /ft^2 ). Substituting yields P 0 = 17.2 lb/in^2 (abs)
- 5(59.6)7144 = 19.3 lb/in^2 (abs) (133.1 kPa).
Using the steam tables, find the saturation temperature Ts corresponding to this absolute
pressure as Ts = 226.1^0 F (107.8^0 C). Then the permissible temperature rise is Tp = Ts- Top,
where Top = water temperature in the pump inlet. Or, Tp = 226.1 - 220 = 6.1^0 F (3.4^0 C).
- Compute the recirculation flow rate at the shutoff head
From the pump characteristic curve with recirculation, Fig. 13, the continuous-recircula-
tion flow QB for an 11.5-in (29.2-cm) impeller at an operating head of 450 ft (137.2 m) is