Page 110 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 95

a little resulting in higher pressures. We tested a large vessel ruptures could do serious damage. That’s why
number of compressed air storage tanks for an installa- flooding a vessel with water for a hydrostatic test is so
tion in the 1980’s at the request of their inspectors. It’s important, the water pressure will drop instantly with
a good thing we did it hydrostatically. Eleven of them a rupture but any air in the system will expand to push
failed, four at pressures below the safety setting and one the water out with considerable force.
just slightly above normal operating pressure. A sound test requires the source of pressure be
A hydrostatic test, done properly, will not result in disconnected and the pressure observed for a period of
injury if the containment fails; a little water will run out time to ensure there are no leaks. Occasionally the pres-
and the pressure will drop instantly. A boiler in opera- sure will increase or decrease as the testing fluid heats
tion doesn’t fail that pleasantly. Which would you rather or cools. If leaks are found, drain the system for repair
have, a rupture (consisting of a leak of cool water) due and repeat the test when the repairs are complete. Note
to a hydrostatic test and when you’re looking for it or that any air test requires precautions and should only be
an explosion of steam and boiling hot water (or worse) used when there’s no option.
when you least expected it? Testing at anything less than A special test not normally performed is a boiler
the standard test pressure is providing false hope that casing test. It ensures there are no significant leaks of the
the containment won’t fail in operation. products of combustion from the boiler into the boiler
That’s why I said “help ensure” back in that first room. The test requires blocking the stack, preferably at
paragraph. Pressure testing a vessel at 150% of its a point outside the boiler room, and the burner open-
maximum allowable working pressure still doesn’t ing into the boiler. The actual test pressure should not
mean it can’t fail at lower pressures. During operation exceed the manufacturer’s rating for the casing or any
temperatures of boilers and many pressure vessels are ductwork connected to the boiler that is also included in
substantially higher than the maximum hydrostatic test the test. The best way to apply pressure is using the test
temperature. Those higher temperatures introduce ad- setup shown in the Figure 2-7 which, by it’s construc-
ditional stress into the vessel and can contribute to fail- tion, serves as a gauge for the test and a way to prevent
ure of one that just passed a 150% hydro. It’s even more exceeding the test pressure. When some bubbles rise
likely to fail in service if it passed a hydro at normal through the loop the test pressure is achieved. Once the
operating pressures. pressure is reached the air supply is disconnected and
One reason you always have somebody at the the level drop observed. It shouldn’t drop more than
pump or valve controlling the application of pressure one inch per minute after bubbles stop rising through
is to release it immediately if a problem is detected. the column. If leaks are indicated drop the pressure,
Another is to make certain that the pressure doesn’t insert a lit smoke bomb through the capped connection,
exceed the chosen test pressure. If a manufacturer (who and reinstate test pressure to locate the leak. Reduce air
has to test at 150%) exceeds the test pressure by more line to 1/2 inch and other piping to match the size of the
than 6% the engineering of the vessel must be repeated observation port if it is less than two inches.
to ensure it was not subjected to excessive stress during Note that the 25-inch water leg is selected for boil-
the hydro. There’s no excuse for letting the test pressure ers designed for a maximum casing pressure of 25 inch-
run above the 150% so don’t do it. Ensure the pressure in es water column. Many are only capable of 10 inches so
the system never exceeds the specified test pressure by the leg should be shorter. I normally specify a 25-inch
more than 6%. If it does, note it in the log and notify the pressure rating and that’s why the system in Figure 2-7
manufacturer to determine if any damage was done by shows it.
exceeding the test pressure. The application of a smoke bomb is necessary to
Check electrical circuits that are connected to the spot leaks in the casing. It’s normally done for a replace-
systems during hydrostatic tests to ensure the liquid did ment casing job and I have yet to see one done where a
not introduce an undesirable ground. Check them again couple of smoke spurts didn’t point out a spot where the
after all test apparatus is removed and normal connec- boilermaker missed a little stretch of casing weld.
tions reinstated. This test only applies to boilers with casings de-
When testing is performed pneumatically (with signed to operate under pressure. A person should
air) the test pressure should not exceed 125% of maxi- remain, hand on air valve, at the test apparatus when-
mum allowable working pressure. Also, the pressure ever the compressed air connection is open. Be certain
must be increased in steps with inspections for leaks at to remove blanks and any combustible sealing material
each step. The rapid expansion of the air in the event the (caulking) when the test is completed.