Page 440 - Boiler_Operators_Handbook,_Second_Edition

Controls 425

tained properly. First there is the matter of size of the A fuel oil sensing line can produce a hazardous
sensing connection; none should be smaller than 1/2 condition. I encountered this one recently where the
inch NPS. I’ve broken a few 1/4 and 3/8 connections piping from the burner manifold to a pressure gauge in
in my day and had to repair damage to a lot of them. the control room was not properly vented. Since the line
A 1/2-inch schedule 80 pipe nipple and valve is strong was full of air it compressed every time the burner oper-
enough for most people to stand on without damage; ated allowing more than half the line to fill with fuel oil.
anything smaller is simply looking for trouble. I once When the burner shut down the air expanded forcing
spent twenty minutes with my finger pressed over a the contents of the sensing line into the furnace through
broken 1/4-inch nipple while someone else was machin- the burner tip. In most instances the oil simply burns off
ing a plug for it. On the other side was 300 psig heated but keep in mind that a tablespoon of fuel oil properly
Bunker C fuel oil at 220°F. atomized and mixed with air to form an explosive mix-
Sensing connections should be made at the side or ture can blow a boiler casing off.
top of process lines to limit any debris settling into the Always bleed the air out of piping when the ac-
smaller line and blocking it. The connection should be cumulating effect of air is not desirable. Provide vent
isolated with a valve as close as reasonable; only pro- valves at the high points of the piping and keep a piece
vide enough room for a hand to get at the valve handle of the appropriate sized pipe bent with a 180° turn to in-
and make allowances for insulation. After the isolating sert in the outlet of those vent valves so you can cleanly
valve you can install smaller piping or tubing from the and safely bleed the air and catch any liquid spill in a
connection to the gauge. If it gets broken you can quick- bucket.
ly shut the valve. On the other hand, some sensing lines and gauges
I mentioned Bunker C, see the section on fuels, are protected by air trapped in the sensing lines. The air
and the fact it was heated. If it isn’t heated heavy fuel can serve as a cushion to limit the impact of noise on the
oil doesn’t flow well and below a certain temperature gauge. A gauge line for a heavy fuel gear pump can use
it becomes quite solid. To prevent blockages in sensing the air to quiet the effect of the bump each time a gear
lines for heavy fuel we don’t put heavy fuel oil in them. squeezes out its oil. Centrifugal pumps can produce
There are two approaches to the problem of sensing fluctuations in the line that are associated with the vanes
pressure of heavy fuel oil and they are dependent on the passing the cutoff. Some acid and caustic processes pro-
fill liquid. You can use a light fuel oil, like Number 2, or vide for the air to separate a process fluid and a pressure
a heavy mineral oil such as Nujol. One is lighter (floats gauge that would be destroyed by that fluid. When you
on) the heavy fuel oil and one is heavier. have situations where it’s desirable to have the gauge
When using light oil the process connection and sensing piping full of air the sensing lines should be fit-
all pipe and tubing connected to the process line has to ted with vent and drain valves to allow removal of any
be flooded in such a manner that the light oil is trapped liquid that may absorb the air.
above the heavy oil. When using a heavy mineral oil the Note I didn’t mention putting air in the sensing
process connection should be on the side of the piping piping. Why not? If you do you could blow up your
and turn down immediately into a separating chamber. boiler or splash someone with a hazardous liquid.
Thereafter the sensing piping can be routed however There’s also the guy that filled his compressed air stor-
you need it. age tank with lube oil.
With both systems the separating oil must be Pressure and flow transmitters, hell—any trans-
injected into the sensing lines at regular intervals to mitter, should be installed where it’s convenient to get
refresh it because it will gradually mix with the heavy at for checking and calibration. I still don’t understand
fuel oil. Since both burn it is best to inject the separating why contractors insist on putting them ten feet above
oil while the burner is in operation. Most heavy fuels are the floor, down in pits, or inside a maze of piping where
fired with steam atomizing and the atomizing steam dif- you have to be a contortionist to get at them. I know
ferential control valves have a chamber filled with oil to why they do it, to avoid extra cost, because that’s where
sense the burner oil pressure; it’s best to inject the sepa- the engineer showed it, or that’s where the workman in-
rating oil at the valve chamber to flush the piping and stalling it could see the girls going in and out of the next
tubing all the way to the process line; a valve for that building. I never allowed such inconsiderate locations
purpose should be provided at the chamber or at the when I was in charge of their going in because I had to
sensing line connection to the chamber. Pump it slowly operate with many such crappy installations.
so you don’t blow the fire out. I insist every transmitter has to be mounted at an