Page 69 - Boiler_Operators_Handbook,_Second_Edition

54 Boiler Operator’s Handbook

one drain or vent was open to ensure a vacuum didn’t The first step in filling a system is opening vents
build up in any steam piping I shut down. and drains. Keep in mind that they’re never empty, usu-
A similar instance created havoc when a steam line ally they’re filled with air and it’s necessary to get it out.
on a bridge flooded due to a vacuum. You see the bridge When shutting down a system you have to open the
was temporarily supported during the original hydro- vents and drains so the liquid can drain out and the air
static test of the piping because it was never designed to can fill the space left by condensing steam. Speaking to
support the flooded steam line. Guess what happened! the latter, it’s always important to open some vents first
When manipulating valves on steam piping it’s a little steam escaping proves to you that the valve is
important to remember that a cold line is either full of open.
air or water, it’s rare for it to contain a vacuum. When Once you’ve closed a main steam valve to a piping
shutting down a steam system the space occupied by the system the pressure will drop quickly and a vacuum
steam has to be filled with something when the steam could be generated before you get a vent or drain valve
condenses, either air or water; unless you’re in a plant open. Open the vents first and let a little steam escape
that injects nitrogen into cooling steam piping. Water because it’s safer. On large systems it may take several
setting in any piping system will descend to the lowest vents and drains to admit air fast enough to prevent
level if allowed. Air can compress in piping to preclude pulling a vacuum. Any system containing large pieces
admission of steam or water. Steam at pressures less of equipment (deaerators, tanks, heat exchangers, etc.)
than 15 psig is lighter than air and steam at 15 psig (actu- should be monitored closely as you shut them down
ally a tad lower than that) and above is heavier than air. to ensure a vacuum doesn’t happen because the equip-
It’s one reason we keep a high pressure boiler vent open ment isn’t necessarily designed for a vacuum and atmo-
until the pressure is above 25 psig and vent low pressure spheric pressure can crush them. Fail to do it and you’ll
boilers until we’re carrying a load, counting on the flow appreciate that the day you suck in a heat exchanger that
of steam to sweep the heavier air out of the boiler. costs several thousand dollars to replace.
Air can be trapped high or low in a steam system Simply draining water without venting a system
depending on the pressure and it can create pockets can also create damaging vacuums. Anytime the column
where piping is suddenly heated as the air is displaced. of water in the piping gets over 35 feet it can create as
Some air is desirable in water systems to serve as a pure a vacuum as steam. Draining a water system with-
cushion to absorb the shock of sudden changes in flow. out venting tanks on upper floors can result in all those
There’s always a standing length of piping at the top of tanks being crushed by atmospheric pressure because
any water system. It’s there to trap air for that purpose. the water draining out left a vacuum.
In your house it’ll be in the wall behind your medicine It boils down to knowing the fluid you’re dealing
cabinet. with, what’s in the piping, and what will happen when
Modern plumbing systems use a special fitting you open or close that valve. Filling any large system,
with a seal so the air can’t be absorbed in the water to whether with water or steam, should be done with a
lose the cushion. Plumbers used to know that the so- valve installed for that purpose. Normally it’s a small
lution to a hammering sound in the customer’s pipes valve mounted on the side of the shut-off valve (Figure
every time a valve closed was to drain, then refill, the 2-1) but it can also be piped as a bypass or even consist
system to restore that air cushion. Of course some of of a simple drain and hose bib where you should con-
them made a pretty elaborate thing of it so they could nect a hose from the supply to fill the water piping.
charge more to perform that simple act. Draining and The problem is that sometimes (Okay, I’ll be honest…
refilling the water piping in your house is usually all frequently) we engineers don’t think about it and put in
you have to do to eliminate pipes banging every time a bypass or fill valve that is so small it will take hours
you close a faucet. to fill the system. On the other hand, I’ve seen systems
Every time you fill or drain a system you should where the fill valve was the largest. Just forgive us dumb
follow a prescribed procedure that’s proved successful engineers and take the time to fill the system or, if you
for your plant. If it’s a new plant you’ll have to develop have to fill it regularly, put in a larger bypass or fill valve
the procedures so you should think about how you’ve (like the additional one in Figure 2-1). Please note that I
done others and apply your experience in producing a don’t encourage you to leave the insulation off the valve
prescribed procedure for each piping system in the new and piping.
plant. There’s no sense in busting it before you even get Some operators choose to crack the main isolating
it started. valve to speed up the filling process. Before I continue I