| When an old steam boiler finally needs to be replaced, you
are presented with the opportunity of using either one large boiler or multiple smaller
boilers to replace the original one. As you might expect, there are advantages and
disadvantages to either way. Unlike hot water applications, multiple steam boilers can be
very tricky, so you cannot take the same approach as you would with multiple water
boilers. The near-boiler piping becomes very critical. In multiple boiler applications,
each boiler should be piped as if it were the only boiler in the system, so each boiler
should have a riser(s) piped into its own header and equalizing line. The header collects
all the steam and water that has come out of the boiler through its riser(s), and the
steam, being "lighter," will run along the top of the header and enter the
vertical tee which leads to a common header; the heavier "wet" molecules are
carried along the header and into the equalizing line back to the boiler. When piped this
way, with the pipe sizes recommended by the manufacturer, each boiler will do a good job
of delivering dry steam to the common header. If you pipe each boiler's riser into the
common header without the benefit of its own header and equalizing line, you'll create wet
steam, water hammer, water line problems with each boiler, and a very unhappy customer who
has to push the reset button on each boiler's manual reset low water cut-off control.
It is very difficult to return condensate to a multiple boiler system without the use
of a boiler feed pumping system. Each boiler is under different pressures even though they
are all piped to a common header. That's because steam is dynamic, and is always moving,
condensing, and dropping in pressure. But some people, confusing steam with other gases
like air and propane gas, believe that after you fill the system and pressurize it, the
pressure will be the same throughout. It can't, so multiple boiler systems should employ a
boiler feed system, including an M&M make-up water feeder, to return water to the
boilers. Once you decide to use a boiler feed system, don't try to pipe one pump
controller (such as the M&M 150MD) onto a common equalizing line. The reason: the pump
controller doesn't know the level of the water in each boiler, only the level where it is
located.
Besides, when the pump moves water, where does the water want to go?
Naturally, the water takes the path of least resistance so it enters the boiler with the
lowest pressure. But this is typically the "off" boiler which doesn't need any
water, and the "on" boiler, which has the greatest demand for water, gets little
or no water. The "on" boiler will then shut off on low water, and as the steam
condenses, the boilers will equalize. This raises the water line in all the boilers,
reducing the steam separating chamber within each boiler, and affecting the quality of
steam being supplied to the system.
The best way to return condensate in a multiple boiler
system is for each boiler to have its own pump controller piped onto an equalizing column.
Each controller is wired back to either its own feed pump or its own motorized feed valve.
If motorized valves are used, once the valve opens, its end switch makes contact,
activating a common feed pump. Now the water can only enter the boiler that really needs
more water. This maintains the proper water line in each boiler, improving the quality of
steam and the efficiency of your multiple boiler system.
Finally, when each boiler is piped into a common header,
there will be times when one or more will be "off" because of a light load on
the system. Unfortunately, the steam doesn't know where it is supposed to go, so some of
it enters the "off" boilers and condenses. Condensate accumulates and floods the
"off" boilers. The easiest and most cost effective method for addressing this
situation is to pipe "overflow" traps (usually 3/4" F&T traps) into
each boiler's equalizing line a couple of inches above the operating water level. Then,
when the condensate starts to build up in an "off" boiler, it enters into the
trap and drains into the boiler feed receiver.
If you have any questions regarding this or any other steam
issue, give your local Hoffman/McDonnell & Miller Steam Team
representative a call.
Reprinted from CounterPoint October 1997, Vol. 4, Issue 2 |
Article
Morton Grove
Tel 847 966-3700
Multiple Issues With Multiple Steam Boilers