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 larger 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. 

Adopt Individual Header and Equalizing line for Each Boiler to Minimize Wet Steam and Related Issues

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 (also often referred as equalizer). 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” flows 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 boiler manufacturer, each boiler will do a good job of delivering dry steam to the common header. If you piped 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.

Consider a Boiler Feed System to Return Condensate to Boilers under Different Pressures

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 (such as a Domestic Boiler Feed Unit, including a standard Domestic water make-up control, or a McDonnell & Miller 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 Low Water Cut-Off/Pump Controller (such as the McDonnell & Miller 150S) 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.

Apply a Pump Controller Dedicated to Each Boiler to Assure That Condensate Returns to the Right Boiler

Besides, when the pump moves water, where does the water want to go? Naturally, the water takes the path of least resistance.  So if condensate was returned to a common equalizing line, 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 stream 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 its own equalizing line. 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. 

Consider “Overflow” Traps to Avoid Flooding Idle Boilers

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, or down for repairs, or lead/lag rotations scheduled by days not hours. Unfortunately, in such scenarios the stream 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  ¾” F&T traps) into each boiler’s equalizing line a couple of inches above the operating water level, a solution also referred as High Level Spill. Then, when the condensate starts to build up in an “off” boiler, it enters into the trap and drains into the boiler feed receiver. 

Another way to avoid steam entering the “off” boilers is to isolate boilers by installing an automatic stop valve (not shown in Figure 1) on each boiler’s own header. Each boiler would also need a vacuum breaker so on shut down, after the remaining steam in the “off” boiler condensed, the boiler wouldn’t be under vacuum and flood itself by drawing condensate from the return pipe. Whether to choose the overflow traps or the stop valves, or use both varies by case and by boiler manufacturer. 

In this article, we discussed several common issues with multiple steam boilers and solutions. As customer, you should always follow boiler manufacturer’s instructions.   

If you have any questions regarding this or any other stream issue, give your local Domestic®/Hoffman Specialty®/McDonnell & Miller® Stream Team representative a call.

Lu Bai
Product Specialist – Steam Products
Domestic