Page 397 - Boiler_Operators_Handbook,_Second_Edition

382 Boiler Operator’s Handbook

pressure regulator like the one in Figure 11-16 controls tion at the spring and diaphragm to a shorter motion at
the flow of gas to maintain a constant outlet pressure. the valve disc to allow higher pressure drops across the
The position of the valve assembly is determined by the valve. The typical house regulator contains an internal
compression of the spring by the diaphragm. When the lever.
pressure at the outlet drops the force on the diaphragm When self contained diaphragm actuated regula-
is less so the spring pushes the valve further open. When tors are used for natural gas the venting of the spring
the outlet pressure goes up the spring is compressed to chamber requires special attention. If the diaphragm
close the valve. leaks the vent of the spring chamber must bleed off the
It contains an internal sensing tube that points gas or the spring will open the valve fully to raise the
downstream allowing the velocity of the gas to produce outlet pressure to unsafe levels. The gas bleeding out of
a small venturi effect at the end of the tube to effectively that vent must be conveyed to a safe location (outside
reduce the pressure in the diaphragm chamber as the the building) to prevent flammable mixtures forming
flow increases. That helps open the valve at higher flows near the valve or displacing air to asphyxiate someone
and reduce the droop. These valves have a limited oper- in the room (someone dies because there’s no air to
ating range as far as pressure drop is concerned because breath).
the spring has to have a low coefficient so the valve can Occasionally the valve is fitted with an internal
stroke completely; it doesn’t have the strength to open pressure relief valve which will drain gas to that vent in
the valve if, during shutdown when the valve is closed, the event the outlet pressure gets too high (from thermal
a high differential pressure between inlet and outlet expansion or main valve leaking) so the vent piping lo-
develops. If you run into a regulator that locks up after cation and size is very important. It’s also important that
a no flow situation then the differential pressure across vent lines for regulators on other boilers, and especially
the valve is too high. You solve the problem temporarily piping from the intermediate vent valves are not con-
by shutting off the supply to the inlet and bleeding off nected to the regulator vent lines.
the pressure upstream of the regulator. I remember a time when one of our steamfitters
Valves that lock up regularly need a larger dia- was replacing a diaphragm on a regulator while the
phragm or should be replaced with a an internal lever adjacent boiler was running. That was back in the 1960’s
actuated or pilot operated valve. Internal lever actuated when many men had long hair and there were few re-
valves use mechanical linkage to convert a longer mo- strictions on smoking. He suddenly found his long hair
on fire because his cigarette had ignited the gas leaking
back down the vent line; gas fed from the regulator on
the adjacent boiler which was also leaking.
Temperature control valves can use a probe mount-
ed on the valve and penetrating the vessel or piping
where it can sense the temperature used to control
flow through the valve (like the one on the hot water
heater) but that controls the location of the valve which
can require extra piping or create other problems with
installation or maintenance. Using a probe connected
to a bellows by a capillary allows the control valve and
temperature sensor to be located separately.
The capillary is a very small diameter tubing
permanently connected to the bellows and probe as-
semblies. These consist of closed systems which are
made up for a particular temperature range and valve
actuating power. The contents of the system can be a
liquid or a gas. Liquid systems are somewhat restrictive
because the liquid expands and contracts with changes
in temperature and develops high pressures quickly if
the expansion is restricted. Gas filled systems change
pressure with variations in temperature and many of
Figure 11-16. Gas pressure regulator them contain mostly liquid that evaporates when heated