Page 317 - Boiler_Operators_Handbook,_Second_Edition

302 Boiler Operator’s Handbook

and be easily readable.
A pressure gauge is normally selected so at normal
operating pressure the needle on the gauge is pointing
straight up. That makes it easy for the operator to deter-
mine if the controls are operating properly. The normal
hydrostatic test pressure for a boiler is 1-1/2 times the
maximum allowable working pressure so the gauge
must always have a scale range that extends to that
value.
Hot water boilers must also have a thermometer
that indicates the highest temperature in the boiler and
code rules for size and scale should also apply to them.
The piping connecting the pressure gauge to the
boiler can’t have any other connections except a drain
connection that’s open to the atmosphere and an extra
valved connection for the inspector to attach a gauge.
A valve in the piping must be a quarter turn valve with
handle in line with the piping when the valve is open on
low pressure boilers and a rising stem valve locked open
during operation on high pressure boilers.
Code requirements do not include provision of
crosses and tees in the piping to permit cleaning it but
I strongly recommend you have them because I have
Figure 10-38. Packed transverse shaft for level switch encountered several instances where the boiler’s pres-
sure gauge piping was plugged with mud that managed
Pressure Gauge to get into the sensing line over the years. The piping
A pressure gauge is a required piece of trim on a should be opened and inspected at the connection to
boiler. It’s obvious that you need a pressure gauge to en- the boiler every year. The rest of the piping should be
sure that the controls are doing what they’re supposed inspected when there is any reason to believe it may
to but I’ve seen plants where the gauges were missing contain some sludge or mud. The piping should also
and the operator’s didn’t seem to miss them until I be blown down every year right after bringing it up
started asking them questions. A plant without a pres- to pressure and before picking up the load. The piping
sure gauge on the boiler is bound to have a lot of other should include a siphon or pigtail, a curl of pipe or tub-
problems and you always wonder exactly how safe it is ing, to ensure water is trapped between the gauge and
to be there when you run into such a simple deficiency. the boiler to ensure the heat of the steam never gets to
The pressure gauge is required by code and its size the working parts of the gauge. Sometimes the gauge is
and scale are also subject to requirements of the code. A connected to a section of piping that traps water for that
favorite violation in many plants is replacing the gauge purpose. Refer to the section on controls and instrumen-
with a much smaller one. The owner thought to save tation for other important considerations for application
some money but ended up buying two gauges because a of pressure gauges.
smaller one doesn’t meet the code requirements. There’s On many larger boilers the gage can be consider-
no specific size required by code but the interpretation ably lower than the steam drum so it’s visible at the nor-
of the code requirement that the gauge be “easily read- mal operating level. Those gages have to be calibrated
able” is interpreted to mean nothing smaller than what for the installation because they have several feet of
the manufacturer installed originally. For low pressure condensate standing in the connecting piping and the
boilers the size is dictated by the travel of the pointer head of that water adds to the gage pressure. If the gage
which must be at least 3 inches for pressure swings from is twenty-three feet below the drum connection it will
0 to 30 psi. Manufacturers do not put on larger gauges to read 10 psi higher unless it is adjusted to compensate for
make the boiler look better, they put that large gauge on that static head. Don’t be like one boilermaker I had that
because the National Board Inspector monitoring that tried to return a gage because it was reading below zero
boiler’s construction considered it as small as it could be when he took it out of the box.