The Humble Steam Trap and How It Makes Your Radiators Work

Thermostatic steam valve

There are numerous little radiator valves and fittings you’ll need to study up on if you’re looking to maintain a radiator system, but one thing you absolutely can’t overlook is steam traps; in fact, it’s nearly impossible to understand how radiators work without understand steam traps. You’ll find steam traps on radiators in both residential and commercial applications, and they’re equally important in both. But what are they, and why do radiators need them? Here’s a brief overview.

What Steam Traps Do

Steam traps, in general, act as both air vents and as valves in steam systems. They work to make sure there’s good steam volume in the system without there being too much condensate. How? They have a valve that opens and closes automatically in order to let non-steam condensate (or excess steam, in some cases) escape the system while keeping the wanted steam in. These valves may operate based on temperature differences, gravitational differences or pressure differences.

Steam Traps on Radiators

In steam radiators, the type of steam trap used is called a thermostatic trap, or one that operates based on temperature. They are also referred to as Hoffman steam traps, and they work like this: A thermostatic trap on a radiator has a small bellows that is filled with an alcohol-water mixture that is set to boil at 180 degrees Fahrenheit. That means that as the steam approaches the trap, the alcohol-water mixture expands, drives the bellows out and closes the trap. But as the steam condenses, the water-alcohol mixture goes down in volume, the bellows compresses and the trap opens so condensate can flow back into the boiler. There are some slight differences in how one-pipe and two-pipe radiator systems work (some two-pipe systems don’t use steam vents), but that’s the basic setup.

Steam Trap Maintenance

As you can imagine, the way steam traps on radiators work means that the valve can be opening and closing quite frequently. Take a New York City building in which steam lines are filled with steam about 900 hours each heating season, and assume the trap closes three times a minute or so. That adds up to 162,000 operations in a single winter, and more than a million in just six years. Mechanical wear and tear alone, therefore, mean that it’s important to regularly inspect and maintain traps if they are to keep working efficiently. You shouldn’t be surprised if steam traps need to be replaced every three to five years, depending on the exact application.

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