Page 311 - Boiler_Operators_Handbook,_Second_Edition

296 Boiler Operator’s Handbook

water flashes to form more steam resulting in an explo-
sion. Hot water heaters commonly rocket their way up
through as many floors as are above them and have flat-
tened many houses.
The temperature element of a PTV is a small cylin-
drical tube that extends from the inlet of the valve. The
valve must be installed so that element will be immersed
in the hot water. Mounting the valve on connecting pip-
ing will not work because the element isn’t exposed to
the heat. Since the element must be in contact with the
heated water PTVs can be installed horizontally and,
when labeled for it, even upside down. Don’t make the
mistake of one contractor in Oklahoma who decided the
PTVs were installed wrong (the stems weren’t vertical)
so he went out to the local hardware and got some street
ells (piping elbows with male thread at one end and a
female thread at the other) to add and turn the PTVs.
The worker assigned the job of changing the valves had
a problem with the little pencil like things hanging out
of the bottom of the valves (they prevented installation
on the street ell) so he broke them off. Lacking the ther-
mal element the PTVs didn’t work when other controls
failed and the heaters exploded. Six children and one
adult were killed and forty-two others were injured. It
was an 80 gallon water heater.
Figure 10-31. Safety valve seat exposed to pressure Boilers larger than 100 horsepower must have two
safety valves, that’s a code requirement. Also, boilers
About the only time a PTV will operate on pressure is with superheaters have to have a safety valve at the
when the water is trapped by a check valve or backflow outlet of the superheater which is set lower than the
preventer (see service water heating) and the pressure is safety valves on the steam drum. It’s essential that the
increased by the water expanding as it is heated. superheater safety valve opens first to maintain a flow of
If the controls fail to shut down the burner or elec- steam through the superheater to prevent it overheating.
tric element or steam supplying a service water heater In addition to monthly and annual testing of safety
the pressure usually doesn’t increase because the pres- valves (see normal operating procedures) you may be
sure is dependent on the water supply. Expansion of the required to send the safety valves out to be replaced or
heated water simply pushes the cold water back down rebuilt. That’s normally a requirement of the insurance
the line out of the heater. Herein lies the problem, when company that doesn’t want their inspectors to spend
the heat continues the water eventually gets so hot that time observing the pop testing of safety valves. It’s less
it starts to turn to steam. The steam takes up a lot more expensive to simply replace a small valve but valve
room than the water and pushes the hot water back the prices increase with size and set pressure to where you
cold water line until the heating element or the bottom would want to have them rebuilt at a much lower cost.
of the heater is exposed to steam instead of water. Now, A contractor that rebuilds safety valves should have
the steam picks up some heat as it is superheated but it ASME or National Board authorization to do that work.
can’t provide all the cooling that evaporating water does You’ll also want to replace a valve or send the
so the temperature of the heating element or the bottom valve out for rebuilding if it starts weeping or leak-
of the heater rises until they get so hot that they fail. ing. The steam condensing on the spring and stem will
Luckily for those of us that have electric hot water accelerate rusting in the topworks of the safety valve
heaters the element shorts out or burns open to stop which can prevent it operating. Continuously operating
adding heat. If you have a piece of fired equipment a boiler with a leaking safety valve is hazardous.
the outcome is not so pleasant, the weakened surface When I encounter leaking safety valves I always
of the heater ruptures. The steam expands and the hot check the vent piping immediately. In my experience it’s