Handbook of Civil Engineering Calculations

Flow rate,gal/min(L/s)

FIGURE 11. System-head curves without recirculation

flow. (Chemical Engineering.)

Related Calculations. Safety-service pumps are those used for standby service in a
variety of industrial plants serving the chemical, petroleum, plastics, aircraft, auto, ma-
rine, manufacturing, and similar businesses. Such pumps may be used for water supply,
fire protection, boiler feed, condenser cooling, and related tasks. In such systems the
pump is usually oversized and has a recirculation loop piped in to prevent overheating by
maintaining a minimum safe flow. Figure 11 shows a schematic of such a system. Recir-
culation is controlled by a properly sized orifice rather than by valves because an orifice
is less expensive and highly reliable.
The general procedure for sizing centrifugal pumps for safety service, using the sym-
bols given earlier, is this: (1) Select a pump that will deliver the desired flow QA, using

FIGURE 12. Pumping system with a con-

tinuous-recirculation line. (Chemical Engi-

neering.)

Tank B

Pump

Orifice

Tank A

Pump

and

system

heads.ft(m)