Page 310 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 295

all you will encounter. A review of any drawings and label or stamping but you think its okay? The only thing
instruction manuals for something different along with I can say to you is that’s not a lot different than driving a
the basics of steam generation in this book should let car without any brakes! The ASME valve is an assurance
you figure out what they’re there for and how they affect that the valve will work when it has to, to operate with-
the boiler’s performance. out it is foolhardy, not the actions of a wise operator.
The valve nameplate should also bear the set
pressure and capacity of the valve. The valve has to be
TRIM large enough to dump all the steam (or heat) the boiler
can generate or the maximum fluid input to a pressure
Just as we hang decorations on our Christmas trees vessel. I recall visiting a church to look at the burner on
our boilers have a multitude of objects hanging on them; their boiler and noticed they had installed piping with
that’s why it’s called “trim.” There is no concrete defini- reducing fittings on the two inch safety valve connection
tion for what is included in boiler trim. I choose to say and reduced it to a little 3/4-inch safety valve. I shud-
it includes all devices normally attached to the boiler dered, then turned to the deacon who was escorting
including anything within the jurisdiction of the ASME me and said “you must want your congregation to go
Boiler and Pressure Vessel Construction Code and any- to heaven all at once.” Never replace a valve with less
thing that isn’t attached to something else. capacity than the valve you have.
Since the code for construction of power boilers You should also never add piping between the
usually extends to the far side of the second steam valve safety valve and the boiler and under no circumstances
from the boiler I consider those valves and connecting should you install a valve or a blind between the safety
piping part of the boiler trim. Others seem to include and the boiler. There are times, when testing the boiler
the blowoff and feedwater piping and valves but not the and for other maintenance activities, that you will install
steam piping and valves. The discussion that follows is a blank or plug in place of the safety valves but never
based on my definition. Some manufacturers provide operate the boiler without them. Safety valves must be
covers or enclosures around all or part of the trim to installed with their stems vertical so adding an elbow to
change the appearance of their product but most of the turn the valve so the boiler will fit under some obstruc-
trim is always there and some of it is essential. tion is unacceptable.
Steam safety valves have a special arrangement in
Safety Valves their construction that makes the valve open completely.
First, I’ll point out that the correct title for safety Sometimes operators call them “pop valves” because
valves is “safety relief valves” not to be confused with they pop open. When the valve is closed the disc of the
“relief valves” or safety shutoff valves. I’ll continue to valve is exposed to the pressure in the boiler over the
use “safety valve” because all us boiler operators know area that’s inside the seat as shown in Figure 10-31. As
that we mean the safety relief valves. soon as the valve starts to open the pressure in the boiler
Safety valves are the most important part of our is exposed to the full surface area of the disc (the larger
boiler trim. They’re the final defense against a real disas- circle) so there’s more force on the valve and it pops
ter, a boiler explosion. A safety valve may look simple open. The pressure has to drop to a value lower than the
but it’s the most refined device in the world. The ASME set pressure of the valve before it will close; we call the
Code contains extensive requirements for construction, difference “blowdown” (which has nothing to do with
testing, certification and labeling them. A safety valve boiler blowdown). When you operate too close to the set
manufacturer has to be qualified to use one or more pressure of the safety valves you’ll have to drop your
of the various stamps ASME issues that authorize the operating pressure to get the valve to reseat.
manufacturer to make those valves. There are also rules Service water heaters (for domestic hot water
and procedures for repairing safety valves. heating) have an added feature on their valves. They’re
Our safety valves have to have a nameplate or called PTVs for (pressure, temperature relief valves)
stamp on them that includes the appropriate ASME and they’re essential for preventing the explosion of a
Code Symbol Stamp for the application. The stamps (see service water heater. The hot water heater in your house
appendix) identify valves that have met all the require- has one. The problem with domestic water heaters is the
ments of the code. Notice that they’re application specif- pressure isn’t provided by the source of heat. A typical
ic, you shouldn’t use a safety valve for a pressure vessel valve setting is 125 psig so it won’t lift to dump water
(UV stamped) for a boiler. Your valve doesn’t have a with the normal variations in the water supply pressure.