Matching Pumps To Water Distribution Systems; System Curves; Plotting A System Curve; Combining System And Pump Curves - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
MATCHING PUMPS TO WATER
DISTRIBUTION SYSTEMS

System Curves

The pump curves and affinity laws are used to select a pump
or pumps for a particular application. The first step is to establish
a system head curve. This is calculated from design flow and
head loss tables for all the piping, coils, control valves, and
other components of the system.

Plotting A System Curve

An example is shown in Figure 56. The design point is 65 ft
of head at a 515 gpm flow. A system curve (a simple square
root curve) can be plotted once the flow and head are known at
any particular point, since:
2
 gpm 2  h 2
=
 
gpm 1 h 1
 
Where:
gpm 1 = flow at h 1 in ft of head
gpm 2 = flow at h 2 in ft of head
Plot the points (Fig. 56) for flows of 200, 400, and 600 gpm:
2
( )
200
Point A: x 65 ft = 0.1508 x 65 ft = 9.8 ft of water column
515
2
( )
400
Point B: x 65 ft = 0.6032 x 65 ft = 39.2 ft of water column
515
2
( )
600
Point C: x 65 ft = 1.357 x 65 ft = 88.2 ft of water column
515
The system curve assumes all balancing valves are set for
design conditions, that all controls valves are fully open, and
that flow through all loads is proportional. The system curve is
always the same, even if loading is not proportional, at no load
and full (100 percent) load. If the loads are not proportional
(such as, some loads off or some valves throttling), the curve
rises above that shown for values between full and no load.
The system curve in Figure 56 is used in Figure 57 to select the
single speed pump. Using this system curve to determine the
switching setpoint for dual parallel pumps when the load flow
is not proportional can result in damaging pump cycling.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
100
90
80
70
60
50
40
30
20
A
10
0
0 100 200
CAPACITY IN GALLONS PER MINUTE
Fig. 56. System Curve for Pump Application.

Combining System And Pump Curves

The design system flow is 515 gpm. Piping, control valve,
and equipment losses are calculated at 65 ft. An impeller size
and a motor horsepower are selected by imposing the system
curve on the pump curve (Fig. 57). The designer has the option
of selecting a pump with a 9 1/2 in. impeller (515 gpm at a 76
foot head) or a 8 3/4 in. impeller (500 gpm at a 60 foot head).
The smaller impeller requires a 10 horsepower motor and the
larger impeller requires a 15 horsepower motor. Selection of
the 9 1/2 in. impeller requires system balancing valves to reduce
the system pressure differentials to those matching the design
flow of 515 gpm.
When selecting a pump, it is important to remember that:
— With constant speed pumps (and two-way AHU control
valves), flow rides the pump curve. The system curve is
plotted assuming that the control valves are full open,
which in any system, only occurs at the full load. As
control valves throttle and loads are turned off the system
becomes non-proportional and the system curve rises
between no load and design.
— With variable speed pumps, the system control objective
is to have the pump curve ride the system curve by keeping
at least one control valve open and reducing the pump speed
to reduce flow with the diminishing system drop.
342
C
FLOW AT FULL LOAD
B
300 400 500 600
C1054