Honeywell AUTOMATIC CONTROL Engineering Manual page 353

9 1/2"
100
90
8 3/4"
80
70
8"
60
7 1/4"
50
40
30
SYSTEM CURVE
20
10
0
0 100
To better understand system curves, pump curves, and flow
control, Figure 58 shows the control valve(s) (the only variable
element of a typical system curve) separately from the rest of
the system elements. Lines are shown for each of three valve
positions; full, 80 percent, and 50 percent flow. These lines,
when added to the curve for all other elements of the system
intersect the pump curve at the corresponding operating point(s).
Figure 58 shows a system with 500 gpm and 70 foot head at
design, 10 ft of which is a full open control valve at the end of
the piping run. Line "A" represents the control valve and
connects the pump curve to the static-element system curve. If
all control valves positioned to 80 percent flow, the pump head
rises, the static-element system resistance drops, and the control
valve, represented by line "B", makes the difference; about 36
ft. Similarly, at 50 percent flow, the valve drop, represented by
line "C", accounts for about 63 ft.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
200 300 400
CAPACITY IN GALLONS PER MINUTE
Fig. 57. Matching Pump to System.
BALANCED OPERATING POINT
WITH 9 1/2" IMPELLER
DESIGN OPERATING POINT
OPERATION WITH 8 3/4" IMPELLER
10 HP
7 1/2 HP
5 HP
500 600 700
OPERATING POINT
@ 50% FLOW
80
LINE "A" CONTROL VALVE*
FULL OPEN @ DESIGN 10'
DROP AT 500 GPM
70
LINE "B" @ 80% FLOW,
CONTROL VALVE TAKES
36' DROP
60
LINE "C" @ 50% FLOW,
CONTROL VALVE TAKES
63' DROP
50
* THE LINE "A" VALVE IS
AT THE END OF THE
PIPING RUN, AND THE
SYSTEM CURVE IS FOR
40
THE REST OF THE
SYSTEM.
IN A BALANCED SYSTEM,
THE SYSTEM CURVE IS
30
THE SAME AT ANY AIR
HANDLING UNIT, WITH
THE AIR HANDLING UNIT
BALANCING VALVES
MAKING UP FOR ANY
20
REDUCED PIPING
LOSSES.
10
0
0 100 200
SYSTEM FLOW IN GPM
Fig. 58. Pump and System Curves and Control Valves
343 ENGINEERING MANUAL OF AUTOMATIC CONTROL
15 HP
12 HP
800 C1055-1
DESIGN
PUMP
OPERATING
CURVE
POINT
LINE C LINE B
PUMP REQUIRED:
500 GPM @ 70' HEAD
SYSTEM CURVE FOR STATIC
ELEMENTS OF SYSTEM (CHILLER,
PIPING, FITTINGS, BALANCING COCK,
COILS, STRAINERS, ETC.)
300 400 500
LINE A
M15056