Page 318 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 303

Pressure and Temperature Limit Switches mounted on a siphon the switch can be tilted as pressure
A boiler that vaporizes a fluid should always have builds in the boiler because the siphon tends to straight-
a high pressure switch to stop the burner or isolate the en just like the Bourdon tube in a pressure gauge. That
source of heat in the event the pressure in the boiler would alter the switch operating point. If you have a
gets too high. If the boiler simply heats a fluid it should mercury bulb switch mounted on a siphon make sure
have a high temperature switch. In some instances fluid the travel of the mercury switch is perpendicular to the
heating boilers are served by a common expansion tank siphon.
which can be isolated from the boiler so a high pressure Temperature limit switches are normally installed
switch is also provided. with a thermal bulb penetrating the boiler and the
A hot water or fluid system boiler can also have a switch assembly right on the end of the bulb. When
low pressure switch to prevent operation when the sys- they’re mounted separately to keep the wiring and
tem pressure is so low that vapor would form out in the switch isolated from the high temperatures in the boiler
system (at the high point) to block liquid flow and pos- it’s common for the assembly to include a capillary be-
sibly cause the equivalent of water hammer to damage tween the bulb and the switch bellows or diaphragm so
the piping or heat utilization equipment. the fluid expands in the bulb as it’s heated and some of
All high limit switches require a manual reset. In the fluid is pressed into the capillary which displaces
some jurisdictions this is interpreted as a switch which fluid in the capillary, pushing it into the bellows or dia-
will not close, once it’s opened by a high pressure or phragm chamber to expand it and actuate the switch.
temperature, until the operating personnel push a but- If the capillary is crimped by bending or by physi-
ton located on the switch. I prefer systems that require cal abuse then the flat ends of the bulb tend to bulge out,
the operator reset it at the control panel or boiler front making more room for the expanding fluid because the
and will argue with anyone that insists they be put on crimping of the capillary restricts the movement of the
the switch. In most cases that switch is above the reach fluid in the assembly. The bulging of the flat ends of the
of a boiler operator and it’s seldom mounted where the bulb can act like a spring, maintaining pressure on the
operator can conveniently get at it to push that reset but- fluid so it eventually bleeds through the small restric-
ton. I always suggest the proponents of reset switches tion and acts on the switch. After the boiler cools the
picture themselves alone in the plant at two in the morn- bulging is restored first and it may even reverse, caving
ing trying to climb up to the switch to push the button. in at the end as the liquid shrinks to produce a pressure
Remember that first priority? differential that forces the fluid back through the small
In addition to the high limit switch a boiler can restriction and the switch resets.
have a pressure or temperature control switch which The liquid slowly bleeding through the restric-
provides for on / off control of the boiler. These switches tion results in the switch operating after a delay. Any
are all directly connected to ensure they sense the actual significant delay in the response of a limit or operating
pressure or temperature in the boiler. Location of tem- temperature control is probably due to damage to the
perature switches is important, see the discussion on capillary and the only solution is to replace the switch.
boiler construction. Pressure switches will not have any If the restriction is due to a repeated situation (like a
valves separating them from the boiler unless they’re plant where the operator’s climbed to reach a valve and
rising stem valves and are locked in the open position repeatedly stepped on a capillary draped over a sup-
when the boiler is operating normally; a provision on a port) the capillary can be shut off entirely and the switch
boiler whose continued operation is critical. won’t work.
Pressure switch sensing piping can plug up just Since the switches are normally mounted on the
like the pressure gauge sensing lines although a pres- boiler or the steam drum it’s not at all unusual for them
sure switch is normally mounted close to the boiler to be located where they don’t get regular attention. The
connection (it doesn’t have to be seen all the time like a heat that radiates from the boiler and leaks through the
gauge). It’s always important that the pressure switch casing or lagging creates swirling air currents around
has a siphon or piping arrangement which traps and the boiler and its trim. The air currents can be warm one
holds some liquid in the switch and immediate connect- minute and cold the next so the air around pressure and
ing piping to protect the switch from the high tempera- temperature switches promotes breathing of the switch
ture of the vapor. housing to suck dust into the housing. Dust settles in-
Another concern with pressure switches that use side the housing and can eventually block its operation.
mercury switches is the mounting of the switch. If That’s assuming the cover is on the housing; I would