Multiple Issues With Multiple Steam Boilers
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.