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A gravity-return steam system uses a combination of two forces to put the returning condensate back into the boiler. First, there’s the steam pressure that's leftover at the end of the mains. Now, this pressure will never be as great as the pressure that’s inside the boiler because as steam travels through the pipes it loses some of its energy to friction. What you wind up with at the end of the main depends on the size of the pipes and the boiler’s load. This has all been figured out years before you were born by the Dead Men.
But now, someone shows up and adds their motorized zone valves to piping that's leaving the steam boiler. They figure that this will be a neat way to save fuel dollars. They're wrong, but you've met people who are wrong before, haven't you? Some of them can be very convincing!
And when Mr. Valve does reopen, Mr. Steam comes raging through, meeting Mr. Condensate and sending him rocketing toward the end of the main where he lands with enough force to knock the building off its foundation. Perhaps you've heard this noise? But this isn't the only problem. Keep in mind you have a burner on that boiler that’s sized to provide steam for the entire building. When any motorized zone valve shuts, the firing rate doesn’t change, does it? Suddenly, you have more steam volume than the pipes can handle. Too much steam volume means the steam will move at a higher velocity, and at the higher the velocity, the faster the steam will suck water out of the boiler. That, of course, leads to water hammer in the zones that are calling for heat.
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The other force that works to put the returning condensate back into the boiler is gravity. Gravity combines with the leftover steam pressure at the end of the main to create a force that’s greater than the pressure inside the boiler. The Dead Men allowed enough vertical space between the end of the lowest steam main and the boiler’s water line to give the returning condensate a place to stack up. That's what most folks in the business call, "Dimension A." Gravity takes over and the condensate slides back into the boiler. It’s all very simple, and it has been for years.
Anyway, these valves will work beautifully - as long as they never close. As soon as one or more does close, however, that "leftover" steam pressure that you were depending on to put the condensate back into the boiler disappears. Now, all you have going for you is gravity, and that’s not enough to put the returning condensate back into the boiler. So the condensate backs up into the main and lies there, waiting for that evil motorized valve to reopen.| Dan Holohan - [Intro] | [Email] | [Website] |
| The views expressed in this article are those of the individual author and do not necessarily reflect the views of the management or staff of MasterPlumbers.com |
