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Steam trap types, applications, & locations: here we explain the purpose and function of steam traps use on steam piping systems at the radiator, in the steam piping system, and on the steam mains. If your steam heat radiator is not getting hot the problem could be the steam trap. This article series answers most questions about all types of heating systems and gives important inspection, safety, and repair advice. Here we include a discussion of Hoffman traps on steam radiators and on steam piping, thermostatic steam traps, ball & float steam traps, and mechanical steam traps: inverted bucket type steam traps.
We include identification photographs of different types of steam traps and we list steam trap sources & manufacturers where more technical specifications and literature are available. Our page top photo shows an inverted bucket steam trap incorporating a cooling coil connected to the steam piping system in a private home.
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As we introduced at STEAM HEATING SYSTEMS, steam traps such as the Hoffman-style thermostatic-type steam trap shown at the top of this page are installed on residential steam heating systems (usually at the bottom of the radiator at the opposite end from the steam input side) in order to allow air and condensate out of the radiator while at the same time, stopping the escape of steam (or slowing it) until the steam can condense to water (thus transferring its heat to the radiator itself).
Our photo (page top) shows a typical Hoffman steam trap at the radiator on a two-pipe steam heating system.
The functions of the steam trap, also often referred to as the Hoffman Trap in the field, include:
Steam traps may be found on steam heating systems both on radiators (at the radiator bottom opposite end from the entering inlet valve), and on older steam heating systems a steam trap may also be found on some steam piping where the trap handles condensate produced inside the steam riser piping.
Shown at above left is a steam trap at a steam radiator outlet and at above right is a thermostatic steam trap schematic (adapted from ITT's Steam Book ). In this thermostatic steam trap design, used at radiators and Steam rising in the two pipe steam heating system piping enters the radiators, usually through a Hoffman-type supply valve near the top of the radiator, and a mixture of steam and condensate is separated by the steam trap at the radiator outlet.
Air escapes: he incoming steam pushes air ahead of itself and out through the steam trap at the radiator bottom..
Steam is trapped: as steam begins to pass out through the steam trap it warms the thermal element inside the trap.
A "bellows" [(c) in the sketch above] inside the steam trap is actually a flexible sealed container filled with alcohol and water - a mixture that boils at a lower temperature than the temperature of the steam itself. Thus as the bellows boils (warms up) it expands, closing the steam trap.
The steam trap stays closed until sufficient condensate has been produced inside the steam radiator to enter and cool the steam trap. The condensate then causes the bellows to cool, shrink, open the steam trap.
Condensate escapes: As the incoming steam cools inside the radiator, returning to its water state as condensate, condensate falls to the bottom of the radiator and also needs to exit through the steam trap. Exiting condensate follows return piping back to the boiler.
A ball float type trap sense the difference in density between steam and condensate. This type of steam trap is very responsive to conditions in the steam system and provided the ball float trap includes an automatic air vent, the trap will discharge condensate quickly "as soon as it is formed" independent of changes in steam pressure within the system. These are the only suitable steam trap on a system where steam locking occurs, and these traps in their modern version are resistant to water hammer in the steam system.
Condensate that enters the trap will cause a ball float to rise inside the trap, lifting the valve off its seat and releasing condensate. In this design the ball float steam trap valve is always flooded so neither steam nor air will pass through it. Although they are generally larger and more robust, ball float steam trap mechanical operation is not unlike that of some radiator STEAM VENTS except that the latter are operated by temperature rather than the level of condensate within the trap.
Early ball float steam traps were vented using a manually operated cock at the top of the body. Modern traps use a thermostatic air vent.
Spirax Sarco notes that previously if a steam system was suffering from water hammer the ball float stem trap could become damaged but comments that modern ball float steam traps are robust and can endure water hammer.
Bucket type steam traps like those shown in our steam trap identification table above and listed in our manufacturers list below are a robust mechanical steam trap that reduces banging pipes and clanging steam radiators by resisting water hammer.
Bucket type steam traps are resistant to water hammer problems that occur in some steam systems and unlike the ball float steam traps described just above, bucket traps tolerate high steam pressures (not found on residential steam heat systems) and also can be used on superheated steam systems.
The image at left adapted from Armstrong Industries illustrates the components and operation of an inverted bucket steam trap. - Armstrong Industries (2014) & Daniel Friedman
How Bucket Steam Traps Work:
The name of this steam trap describes the bucket steam trap operation. Inside of this steam trap an inverted bucket-container - formed like an upside-down bucket - is attached to a mechanical lever that is in turn connected to a valve controlling condensate outlet from the device. There is a small air vent in the top of the bucket.
As steam enters the bucket trap at its bottom the bucket floats "up" in the condensate-filled trap, lifting the lever that closes the condensate outlet valve on the trap. Condensate inside the trap flows down (and air moves up) inside the trap. As the condensate level rises (and as air and some stem escapes out of the interior of the bucket through the air vent) condensate eventually accumulates enough inside the inverted bucket to allow the bucket to drop, opening the condensate valve outlet from the trap.
This mechanism has in effect "trapped" condensate, separating it from the steam and allowing it to flow out of the trap.
As Armstrong International explains the operation of their bucket steam traps and the effects of wear as follows:
The bucket trap does have its own limits and vulnerabilities: it can be damaged by freezing, it cannot vent air at high rates, it needs to be protected from fluctuating steam pressures and from water loss. If the trap runs dry the result will be wasted steam.
Main steam piping drip traps should be installed where the steam piping changes elevation (at the lower end of that change of course), in front of expansion loops, and at steam risers
Watts points out that because the condensate quantity (load) at these main drip trap locations is small, usually the main drip steam trap will not be larger than 2/4" (20mm). The schematic below is adapted from Watts Water Technologies. Watts (and other manufacturers) will provide a chart of recommended steam trap products depending on the psi operating range of the steam system.
Two types of mechanical steam traps are found on the mains of commercial and some residential steam systems: Ball Float steam traps and Inverted Bucket steam traps. These explanations are adapted from Spirax Sarco's excellent free online tutorial found at the citation we provide below. Quoting from Spirax Sarco, a producer of all types of steam traps,
The photographs of mechanical steam traps shown below can aid in recognition of these features on steam heat systems.
Is the Steam Trap Working?
ITT reports that steam traps have about a three-year life expectancy, and that on an older steam system chances are the steam traps are not working. An external visual inspection doesn't tell if the steam trap is functional or not, but in addition to the "tests" we discuss below, observation that radiators are noisy, banging, pounding, or not heating, or too hot, can all point to a steam trap problem - if a steam trap is installed. Steam traps may be found at the bottom of a steam radiator on its outlet end, or on steam piping in the condensate return system where they are serving the piping itself.
Tests for Steam Traps
Figuring out if a steam trap is working properly - that is, opening and closing at the proper temperatures - has been described as complicated enough that books have been written on the problem. But ITT recommends a simple practical approach that can make a rough test. 
Rough steam trap test: A special temperature-sensitive colored crayon is used to test steam traps. The user makes a crayon mark on a steam pipe, radiator, or right on a steam trap. The crayon melts if the temperature of the tested surface is hotter than it should be. Tempilstiks are sold by Tempil Division and probably other distributors for this purpose.
A float-and-thermostat steam trap, if present, is more likely to be found on newer, modern steam heating systems in which the boiler and piping heat up rapidly. The trap is operated (opened or closed) by a float assembly that will drain condensate through the trap without depending on temperature or heat-up time.
Our sketch (left, adapted from ITT's Steam Book ) illustrates a float and steam trap. That round ball is the float ball. You can see the red thermostat at the top of the image, and the green color indicates where condensate can flow out of the trap.
You can see that condensate can exit the trap either through the thermostat port or through the interior of the trap body, depending on the float position. A common float and thermostat steam trap found on residential buildings is the Hoffman 53-FT.
Thermostatic steam traps (left) include only a temperature sensing thermostat - no float assembly. The steam trap shown at the top of this page is a thermostatic type trap - as you can recognize from its size and body shape.
Thermostatic steam traps are devices that operate more slowly than "float & thermostat steam traps" described above, and they are more likely to be found on older, slow-heating steam systems.
Rough steam trap test: colored crayon method
A special temperature-sensitive colored crayon is used to test steam traps. The user makes a crayon mark on a steam pipe, radiator, or right on a steam trap. The crayon melts if the temperature of the tested surface is hotter than it should be. Tempilstiks are sold by Tempil Division and probably other distributors for this purpose.
When a boiler is first installed or has just been replaced, the steam trap (and other steam components) should be cleaned to flush out debris stirred by the mechanical activity on pipes and equipment during installation or repair. If you see a plumbing valve between the Hartford Loop and the system piping and a tee and cap on the piping on the boiler side, these fittings were probably installed to permit the Hartford Loop to be opened and cleaned without sending crud into the steam boiler itself.
ITT provided this procedure [adapted and paraphrased] for cleaning the steam trap once the fittings above are installed:
Instead the installer places float and thermostat traps.
The F&T trap as these devices are called in the trade, achieve the same function as the traditional steam trap, but the float switch will open to permit condensate to drain regardless of the (presumably high) temperature inside the trap. Hot condensate will drain easily and more rapidly out of an F&T trap on a modern steam system.
Hoffman also manufactured float and thermostatic traps such as the Hoffman 53-FT used on steam risers and at the end of main condensate drips.
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Frequently Asked Questions (FAQs) about Steam Traps
Questions & answers or comments about radiator steam trap function, installation, repair or replacement: Hoffman steam traps and similar devices .
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