Page 319 - Boiler_Operators_Handbook,_Second_Edition

304 Boiler Operator’s Handbook

love to have a nickel for every limit switch I found with any pressure and temperature.
a cover dangling or removed. They’re usually quite full No, the code doesn’t require a non-return valve on
of dust. Yes, you have to clean them. all high pressure boilers. Non-returns are recommended
in multiple boiler plants but are not required. Don’t tell
Valves, Steam your boss that if a new plant or new boiler is under con-
The boiler codes don’t have any requirements for sideration though, they cost more and someone that’s
steam valves for low pressure boilers but you might want only interested in first cost will try to save a few bucks
to follow the discussion on high pressure boiler piping by using regular valves. Non return valves just make
because the reasons for valve arrangements could ap- operation of a multiple boiler plant a lot easier for the
ply to your low pressure plant. When a boiler plant has operator so the investment in a more expensive valve
more than one boiler and they’re connected to a common saves in operating headaches.
header two valves have to be installed on the steam outlet A less tangible reason is they prevent high ther-
of each boiler with a manhole; and, the piping between mal variations in the boilers (when operators don’t
them has to be fitted with a free blow drain valve. isolate the boiler and there’s cold water under the hot
The primary purpose of that arrangement is to upper blanket of steam) and flooding (as the steam
protect anyone that’s inside the boiler by providing a condensate accumulates in a cold boiler) which can
vented section of piping between the two valves to iso- result in early equipment failure. Since a non-return
late them from steam generated by other boilers, or high valve acts as a combination globe valve and lift check
temperature hot water. Despite that strict requirement valve it’s treated by operators as an automatic shutoff
I’ve encountered a few plants without the second stop valve for idle boilers (the check function isolates the
valve and many without a free blow drain. Sometimes boiler) that automatically opens when the boiler starts
the conditions of the requirement result in a failure to making steam. With non-return valves the operator has
provide comparable protection. to make a trip to the top of the boiler only for isolating
What exactly is a “free blow drain”? It’s actually it for annual internal inspection. It is important to use
a combination of piping and a valve or valves that con- the free blow drain to remove any condensate from the
nects a piece of process piping to atmosphere with a piping between the two isolating valves to prevent a
constant decrease in elevation of the piping so it drains. slug of condensate rushing down the piping with the
In normal parlance the label refers to the valve that’s in first flow of steam.
that combination. The purpose is to provide an uninter- Wait a minute, I didn’t say to use a non-return
rupted path from the process piping to atmosphere at a valve on a low pressure boiler. The piston type disc in
low point where any liquid in the process will drain out a non-return valve is heavy and it takes a lot to lift it
and, because it’s open to atmosphere, any fluid in the so you can expect to see a two to ten psi pressure drop
process piping will also flow out until there is no pres- across a non-return valve. Since low pressure boilers
sure (or at least an insignificant pressure) in the process typically operate at around ten psi a non-return could
piping. A free blow drain ensures the process line is not prevent any steam getting to the facility. That doesn’t
at pressure and doesn’t contain any process fluids under mean you can’t get the same performance by installing
pressure. A free blow drain should be provided in all a low pressure drop check valve on the boiler outlet.
piping systems that contain a process fluid that can be Just remember that it has to have a low pressure drop.
safely drained without contaminating the atmosphere If you intend to use the check to prevent steam enter-
specifically to allow personnel to confirm lack of fluid ing and condensing in an idle low pressure boiler then
under pressure in that piping. it should be soft seated. When you add that soft seated
I can recall watching some boilermakers working check valve to the steam outlet also add another one as a
inside a boiler removing tubes with steam blowing out vacuum breaker to a branch off the vent connection so a
the leaking packing gland of the valve mounted on top vacuum won’t crush the boiler.
of the boiler. There wasn’t any blank between the valve When you don’t have a non-return or check valve
and boiler either. Needless to say that was in the days in the steam piping the valves have to be operated with
before lock-out tag-out. If your boilers have manholes each startup and shutdown of the boiler so access to
you should use the double valve and drain provision those valves should be as simple and convenient as pos-
for safely working in them; it doesn’t matter whether sible. Either chainwheel operators or properly located
they’re high pressure or low, steam is deadly at any platforms with safe ladders should be provided so the
pressure above atmospheric and can be dangerous at operator can get at them. Operation of steam valves is