(a) Automatic Bypass Control Valve. This valve is installed at the
discharge of the combination pumps, bypassing the distribution system, and is
a c t i v a t e d by the boiler inlet waterflow rate. The purpose of the valve is to
e n s u r e minimum required boiler circulation at all times, regardless of load
The valve is provided with a manual bypass.
(b) Temperature Control Valve. T h i s valve is installed in the blending
line to the suction of the combination pumps. Its purpose is to cool water
entering the pumps, b e l o w the point at which flashing may occur. Also, it
serves to regulate the HTW supply temperature by mixing the hot water from the
expansion tank with a portion of the cooler high temperature water return.
The valve is furnished with a manual bypass.
(c) Water Flowmeters. These meters are provided for measurement of
w a t e r f l o w to each boiler inlet piping so that all boiler flows can be
( d ) Automatic Closing Valves. These valves are provi d e d to isolate the
using end of the system should a major break occur in the supply or return
main. In such a case, the automatic bypass control valve referred to in (a)
above operates to direct the discharge water from the combination pumps to the
b o i l e r i n l e t . This maintains the required waterflow through the HTW
generators and prevents tube burnout.
( e ) Pressure Differential Switch. T h i s switch is installed across the
pump suction and discharge headers to terminate firing when the pressure
d i f f e r e n t i a l falls below a preset minimum. Often, a minimum flow switch in
e a c h flowmeter is used instead, which terminates firing when insufficient
w a t e r flows through the boiler.
( f ) Combustion Control Interlocks. The pump starters are interlocked
with the combustion control to prevent boiler operation without pump operation.
6.2 Separate Pumping System. In these systems, w a t e r is circulated through
t h e boiler by individual boiler recirculation pumps, while separate
c i r c u l a t i n g pumps circulate the water through the distribution system. This
is a more flexible arrangement which assures circulation through the boiler
I n d e p e n d e n t l y of the water circulation through the distribution circuit.
Figure 4-2 illustrates a nitrogen pressured HTW separate pumping system.
Separate pumping systems are used for the following conditions:
Where heat loads vary greatly and adequate flow cannot be obtained
under minimum load conditions
W h e r e it is desirable to operate a distribution system for short
p e r i o d s independently of the boilers
Where several zoned distribution systems are required, each with
its own set of system circulating pumps
Where the heat load is over 31,500,000 Btu per hour and the system
i s economically justified