Page 106 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 91

the water after flashing and 140°F by the difference be- nection is required by code.
tween 140°F and the average quench water temperature. The boiler is then operated in manual control to
The result times 100 is the percent of quench water and raise the steam pressure until the safety valve lifts or the
it’s not unusual for it to be more than 100%. inspector refuses to let the pressure go higher, or you do.
Sorry, the math isn’t done; we need some values to If the boiler is larger than 100 horsepower it will have
justify blowdown heat recovery or the cost to repair it. two safety valves and the inspector can ask you to break
The cost is determined by calculating the heat losses and the valve seals of the valve with the lower setting and
their value then adding the cost of quench water. Since gag it shut so the higher set valve can be tested. After the
we calculated everything in percent it’s easy to figure higher set valve operates you remove the gag and the
out the cost of the heat in the blowdown, multiply the inspector replaces the valve seals. The code requires the
annual fuel bill (or your estimate of it if management valves open within a certain percentage of the pressure
doesn’t have the sense to keep you informed of it) by the their nameplate indicates. If one of the valves fail the
heat in blowdown multiplied by boiler efficiency. Divide test the inspector will require it be sent out for repair or
by 100 each time you use a percentage. If you have a be replaced.
flash tank that recovers the steam multiply that result by Notice I said “used to be.” To reduce their costs
the percentage of water after flashing divided by 100. To many insurance companies have changed their require-
determine the cost of quench water requires knowledge ments to reduce the amount of time an inspector is on
of how much steam you made. Use meter data or one site. It takes some time to set up the boiler, raise the
of the estimating methods described in the section on pressure, and let it fall. In some cases they’ll accept a lift
instrumentation in the chapter on controls. Multiply the test (see maintenance) of the safety valves. Many insur-
pounds of steam made per year by the percent blow- ance companies are now simply requiring the valves be
down divided by 100, correct for flash steam used as just sent out to an authorized shop for rebuilding at five year
described, then multiply by the percent of quench wa- intervals.
ter to determine the pounds per year of quench water. An authorized shop would be one that has re-
Multiply that value by the cost per pound of water, your ceived authorization from the National Board to use
combined water and sewage charge. the “VR” (for valve repair) symbol stamp issued by the
National Board. However, manufacturers who hold an
ASME Certificate of Authorization “V,” “HV,” or “UV”
ANNUAL INSPECTION (depending on the valve) Code Symbol Stamp can also
rebuild safety valves.
The annual inspection is a standard requirement I’m not suggesting you accept those changes. If
except for some jurisdictions. Either the State or your your insurance company will not let the inspector ob-
insurance company will require you arrange for a Na- serve actual lift tests and reseal the valves then suggest
tional Board Commissioned Inspector to inspect your to your employer he get another insurance company.
boilers. The very limited number of incidents with boil- Rebuilding safety valves isn’t an inexpensive proposi-
ers can be attributed to that one requirement more than tion and an owner typically ends up buying a spare set
any other. Normally inspection is a maintenance activity to switch out because the rebuild takes several days. I
but every year you should also have the inspector stop have one customer that simply buys new valves because
by for an operating inspection. The inspector should they cost less than rebuilding. It’s simply false economy
visit to observe the boiler in operation and require you again, save some time for an inspector and spend much
demonstrate the operation of certain safety devices. more than the inspector’s time on new safety valves and
Used to be the inspector wanted to see those safety rebuilding.
valves operate, some still may. To make it possible to I believe the trend is apparent and indicates that
test the safety valves you will be asked to temporarily the slack in testing of safeties is allowing more incidents.
2
jumper the high pressure safety switch or adjust it to a 2002 data show slightly more than 2% of boiler and
value above the safety valve settings. If other boilers are pressure vessel “incidents” could be attributed to failure
on line to carry the load you may also close the boiler’s of a safety valve. That’s more than twice what it used to
isolating valve(s) so the other boilers and piping sys- be. Pop tests of safety valves should be performed every
tems are not affected. The inspector will also require you year. There’s no guarantee that they will pop when they
connect his, or her, test gage to the connection adjacent should just because you can lift them.
to the boiler’s pressure gage; the inspector’s gauge con- After the safety valves are tested you should re-