Handbook of Civil Engineering Calculations

ANALYSIS OF PUMP AND SYSTEM

CHARACTERISTIC CURVES

Analyze a set of pump and system characteristic curves for the following conditions: fric-
tion losses without static head; friction losses with static head; pump without lift; system
with little friction; much static head; system with gravity head; system with different pipe
sizes; system with two discharge heads; system with diverted flow; and effect of pump
wear on characteristic curve.

Calculation Procedure:

1. Plot the system-friction curve
   Without static head, the system-friction curve passes through the origin (0,0), Fig. 13, be-
   cause when no head is developed by the pump, flow through the piping is zero. For most
   piping systems, the friction-head loss varies as the square of the liquid flow rate in the
   system. Hence, a system-friction curve, also called a friction-head curve, is parabolic—
   the friction head increases as the flow rate or capacity of the system increases. Draw the
   curve as shown in Fig. 13.


2. Plot the piping system and system-head curve
   Figure 14a shows a typical piping system with a pump operating against a static discharge
   head. Indicate the total static head, Fig. 146, by a dashed line—in this installation Hts =
   110 ft. Since static head is a physical dimension, it does not vary with flow rate and is a
   constant for all flow rates. Draw the dashed line parallel to the abscissa, Fig. \4b.
   From the point of no flow—zero capacity—plot the friction-head loss at various flow
   rates—100, 200, 300 gal/min (6.3, 12.6, 18.9 L/s), etc. Determine the friction-head loss
   by computing it as shown in an earlier calculation procedure. Draw a curve through the
   points obtained. This is called the system-head curve.
   Plot the pump head-capacity (H-Q) curve of the pump on Fig. \4b. The H-Q curve can
   be obtained from the pump manufacturer or from a tabulation of H and Q values for the
   pump being considered. The point of intersection A between the H-Q and system-head
   curves is the operating point of the pump.
   Changing the resistance of a given piping system by partially closing a valve or mak-
   ing some other change in the friction alters the position of the system-head curve and
   pump operating point. Compute the frictional resistance as before, and plot the artificial
   system-head curve as shown. Where this curve intersects the H-Q curve is the new oper-
   ating point of the pump. System-head curves are valuable for analyzing the suitability of a
   given pump for a particular application.

Copocity

FIGURE 13. Typical system-friction curve.

Friction

losses

System friction

curve