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You suspect that the piping is the wrong size

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Published
July 10, 2009 Subscribe to RSS feed


You are now reading a part of Dan Holohan's book, A Pocketful of SteamProblems (with solutions!). You can order the full text from our store. That way, you'll have HeatingHelp when you're on that nextproblem job!

There are too many radiators attached to the pipes.
A steam pipe can carry just about any load if you getthe pressure high enough, and if you don't care about velocity noise.The challenge with steam heating, however, is to deliver the right loadto the radiators using low pressure, usually not more than two psi atthe boiler. The Dead Men sized the radiators to heat the space on thecoldest day of the year with about one-psi pressure at the radiator.They used low-pressure steam so the radiators wouldn't overheat and thefuel bills wouldn't be too high. They worked with pipe sizing chartsthat showed them the load limits for steam heating (See The GoldenRules of Hydronic Heating). If you connect too much radiation to asteam main or riser you won't be able to heat it all, unless you raisethe pressure to an abnormally high level. And when you raise thepressure, you create other problems: high fuel bills, water levelproblems at the boiler, and noise.
Check the radiation against the carrying capacity of the pipe. Tryclosing some radiators. If that doesn't help, you may have to repipe.

The steam's not leaving the boiler at the right pressure.
If you size the pipes for a certain pressure, you haveto run the system at that pressure. Steam is a compressible gas. Thehigher you raise the pressure of the steam, the tighter you'll squeezeit. Suppose, for instance, you size your pipes to carry, say, 100-psisteam pressure. If you try to run the system at less than 100-psipressure, the steam will expand and the velocity through the pipe willincrease dramatically. That's because, at the lower pressure, the steamtakes up more space. And since it takes up more space, it has to movefaster. The increase in velocity will pull water out of the boiler.You'll wind up with water level problems at the boiler, water hammer inthe system, and very wet steam at the end of the line. This is why incommercial steam systems the operator will always bring the boiler upto full pressure before opening the valves to the piping.
Check the steam pressure and make sure it's right for your application.

You need to crank the pressure down.
In a steam heating system it pays to run on very lowpressure (usually less than 2 psi) this is what the Dead Men intended.Typically, they sized one-pipe steam systems to have a pressure drop ofabout one ounce per 100 feet of equivalent run. With two-pipe steam,they figured about two ounces per 100 feet. Even in a large building,it doesn't take much pressure to get the steam to the furthestradiator. By cranking the pressure down you'll save fuel and allow thecondensate to return more quickly from the system.
When in doubt, crank it down. If it's a space-heating system and itwon't heat on low pressure, the system is probably air locked. Dealwith the air and crank the pressure down.

It's a commercial steam system, and you're not delivering the right pressure to the heating equipment.
Commercial steam systems and steam heating systems arenot the same. With commercial steam, your goal is to make the surfaceof something really hot. The higher you raise the steam pressure, thehotter that surface will get. When engineers size pipes for commercialsteam systems, they have to know how much pressure they'll need at theend of the line (at their process equipment or heat exchanger). Theyalready know how much pressure they'll have at their boiler or pressurereducing valve station. They'll size their pipes to allow for a certaindrop in pressure from one end to the other. In doing this, they'll alsoconsider the velocity of the steam. If the steam moves too quickly, itwill make noise. It will also erode the pipes.
Check the pressure drop and the velocity of the steam against the pipesize using a Moody Friction Flow chart (available from steam trapmanufacturers). Make adjustments, if necessary.

Steam and condensate are flowing in opposite directions.
If they are, the pipes have to be larger. Usuallythey're one size larger than they would be if the steam and condensatewere flowing in the same direction. The pitch also has to be doubledwhen you have counterflow. On steam mains, the proper pitch forcounterflow is one inch in ten feet. Check the pitch with a line level,and check the pipe size against the connected load.