Page 312 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 297

the most common reason for a safety valve leaking. The around the stub and there’s a 1-1/2-inch gap between
boiler always grows (normally it expands upward) as it the vent pipe and drip pan.
heats up. The conventional high pressure package boiler
will grow at least three eighths of an inch from cold to Water Column
operating pressure and a little more before reaching set In the list of trim the water column and gauge
pressure. Unless the vent piping allows the safety valve glass comes right after the safety valves in order of
to move up with the boiler a considerable amount of importance. The water column is a surge chamber
stress is applied to the valve to spring the vent piping that provides a stable water level independent of the
and that stress can deform the valve so it leaks. To pre- splashing and bubbling inside the boiler so the level in
vent any stress on the safety valve we normally install a the attached gauge glass is a true representation of the
drip pan ell (Figure 10-32) which allows the safety valve water level in the boiler. The water column is usually
to move with the boiler without any restraint. fitted with other trim items like a low water cutoff or
When the boiler is installed pipefitters commonly cutoff and pump controller combination. It can incorpo-
stack nuts or washers under the vent pipe in the drip pan rate probes for remote water level indications. Usually
to provide the required gap between vent pipe and drip the controlling and high steam pressure switches are
pan. One plant I visited had all their safety valves leak- mounted on the piping connecting the water column to
ing and I found washers stacked in the drip pans. When the boiler.
I asked the operators why they were there they replied There was a time when the code required petcocks
that the contractor put them in so they always made sure on the column to provide a means of checking the water
they put them back. After they removed the washers their level if the gauge glass was damaged or its indication
problems with leaking safety valves disappeared. questioned. Many manufacturers still provide them
Buildings do settle as they age and there are times and they’re always a good idea for the original reason.
when the structure (which supports the vent pipe) shifts One problem with petcocks was some operators had the
independently of the boiler and its foundation will attitude that they would check their water level using
change the relative position of the safety valve discharge the petcocks and shut off the gauge glass so it wouldn’t
stub and the vent pipe. The settling can shift the struc- blow. I’m sure you won’t be that stupid.
ture so the vent pipe is not centered around the stub but Some operators will argue that you can’t tell if
pressing against it for another way to stress the safety there’s water or steam there so the petcocks are useless.
valve. Annually, preferably right before doing your That’s not true, you can tell. If there is steam at the level
annual pop tests, check that the vent pipe is centered of the petcock then a second after you open it you will
not be able to see anything between the end of the pet-
cock discharge and the cloud of condensate that forms,
steam is invisible. If you want to argue that statement
then maybe you can explain to me why you don’t see
anything in the top of the gauge glass. If there’s water
there you will see it coming out of the petcock.
A water column is always equipped with a drain
valve. That permits blowing down the column to ensure
the connections between the boiler and water column
are open. Refer to checking the low water cutout in the
chapter on normal operation to learn more about blow-
ing down water columns.
Water columns can be separated from the boiler by
valves, provided they are rising stem gate valves. You’ll
notice that they’re seldom valved off. If they are you
should make it a habit of ensuring the valves are open
(stems are sticking up) and keep in mind that the discs
can come off the stem of a gate valve. The only time
those valves should be closed is when the boiler is shut
down to allow maintenance of gauge glasses and other
Figure 10-32. Drip pan ell water column parts while the boiler is still hot or under