b) For large flow loss, use manual start and stop for emergency
makeup pumps. If there is no emergency pump, provide an electrical interlock
in the system circulating pump starters which would trip out these pumps when
the low level alarm on an expansion tank is activated.
Automatic Isolation Valves. Equip all systems with motor-operated
valves to isolate the distribution system and assure flow through the
generators in case of a distribution line break. The supply isolation valve
should close in approximately 15 seconds with the pressure sensing element
located on the downstream side of the valve.
The return line isolation valve should close as soon as possible
without allowing excessive pressures to develop due to the change in velocity
head into pressure head. The pressure sensing element should be on the
upstream side of the return isolation valve. The controls shall provide for
an interruption of combustion and stoppage of the secondary (distribution)
system pump operation but allow continuation of primary (generating
circulating) pump operation.
Water Flow Control for Steam Pressurized HTW Systems with Natural
a) In plants with combustion system circulation and generator
circulating pumps, part of the water discharged from combination pumps is fed
to steam boilers through single element feedwater regulators. As with forced
recirculation boilers, no other automatic bypass valves are required.
b) Plants with separate system circulation and generator
circulating pumps use the same type of system circulation pumps as for the
forced-circulation boiler systems (see above). They also use the same type of
feed pumps as for steam heating plants.
c) Provide continuous operation makeup pumps which feed either the
boilers or expansion tanks in response to level controls. In case of large
water flow loss in a system, provide for manual start and stop of emergency
Water Flow Control for Inert Gas Pressurized Systems. Use the same
controls as for steam pressurized systems with forced circulation generators
(see above), except, also use automatic level controls for expansion tank.
Overflow Control on Steam Pressurized Expansion Tanks.
pneumatic float valves for control of overflow levels.
a) Pipe connections shall be welded. Screwed joint connections
except for pressure or temperature sensing devices shall not be used.
Connections to valves and equipment shall be flanged or welded. Use metallic
or spiral wound gaskets.