A steam trap is an automatic valve that allows condensate, air and other non-condensable gases (CO2) to be discharged from the steam system while holding or trapping the steam in the system.

  • Condensate: Forms when steam releases its heat energy for any reason.
  • Air: exists in all steam pipes prior to system start-up when the system is cold. Air can enter the system through ex. boiler water make-up systems and vacuum breakers.
  • Non-Condensable gases: Gases other than air such as carbon dioxide exist inside steam systems.



  • Drip Legs are used for removing entrained moisture from steam transmission and distribution lines to ensure high quality steam for use in various plant applications, while also preventing damaging and dangerous water hammer.
  • As steam travels at high velocity through piping, moisture forms as the result of piping heat losses and/or improper boiler control resulting in condensate carryover.
  • Drip legs are therefore located at points where condensate may accumulate to allow for drainage by gravity down to a steam trap for proper discharge from the system. Since condensate drains by gravity, drip legs must be located on the bottom of piping and designed with diameters large enough to promote collection.

Installation guidelines:

  • Drip legs should be located at Vertical Lifts or Drops, end of line.
  • In straight run of piping every 30 to 50 meters.
  • Installed directly ahead of the regulating or control valve, Manual Valves Closed for a Long Time.
  • Provide proper supports (no sagging)
  • Provide slope towards Drip legs.


  • DRIP Applications: drip traps
  • PROCESS Applications: process traps
  • TRACING Applications: tracer traps. Steam tracing refers to using steam to indirectly elevate the temperature of a product using jacketed pipes or tubing filled with steam

Drip Leg Configuration (Fig. 1):

Because condensate drainage from steam systems is dependent upon gravity, drip leg diameter is critical for optimum removal – larger is better.

Figure of a properly configured drip leg.
Fig. 1: Figure of a properly configured drip leg.

Trap Types (Fig. 2):

Different types of steam traps
Fig. 2: Different types of steam traps

Thermodynamic DISC TRAPS (Fig. 3):

  • THERMODYNAMIC traps sense the velocity difference of entering fluids.
  • When condensate enters the trap body, it moves slowly relative to steam – and is freely discharged. When flash or live steam moves across the underside of the disc, its velocity is much higher than water, and the high speed creates a pressure drop which closes the valve head. The valve stays shut until the control chamber steam pressure above the valve head drops, thereby allowing the valve to open.
  • Types: Thermodynamic Disc and Thermodynamic Piston.
  • Since air moves much faster than condensate; thermodynamic disc traps tend to close in the presence of air and are generally not suited for venting large amounts of air.
  • Thermodynamic Disc & Thermostatic. To handle air combination of thermodynamic disc traps and thermostatic air vent can be used.
  • DISC TRAPS: Disc traps operate as a function of velocity. Under normal operating conditions, condensate and air enter the trap and pass through an inlet orifice, a control chamber , an insulating chamber (to isolate the trap against the effects of environment).
  • Rated to operate 10 to 600 psig,
  • Small and light weight therefore easy to install,
  • Frequently inspection required, not energy efficient because of short service life,
  • Not suitable when back pressure high.
Thermodynamic DISC TRAPS
Fig. 3: Thermodynamic DISC TRAPS


  • THERMOSTATIC traps sense the temperature difference of entering fluids.
  • The closure occurs when the fluid, typically hot condensate, has a temperature greater than or equal to a certain threshold value. The hot temperature causes a thermostatic element to move in such a manner that closes a valve. This temperature threshold value is below that of saturated steam.
  • Since air has a temperature significantly lower than steam, thermostatic traps are generally very good at venting large amounts of air
  • Thermostatic traps rated to operate 0 to 300 psig.
  • Fabricated with SS, CS and cast iron housings.
  • Not effective when dirt and scale are present
  • Basic types: Expansion, Balanced Pressure, and Bi-Metal.
  • Expansion type: Expansion trap elements have an internal filling that expands and contracts with temperature change to actuate the valve, but the filling does not vaporize.
  • Wax elements are in a congealed state when cool, and expand when heated
  • Petroleum-based elements are in a contracted liquid state when cool, and expand when heated
  • Balanced Pressure type: Balanced Pressure trap elements have filling which is a mixture of water and mineral spirits that generally vaporizes or condenses at near-to-steam temperature to actuate the valve.
  • Bi-Metal: Bi-Metal trap elements are composed of two dissimilar metal strips bonded together so that temperature change causes deflection in one direction or its opposite to actuate the valve
  • Bellows balanced pressure (Fig. 4)-High capacity
  • Wafer/Diaphragm balanced pressure-Low capacity
  • Bi-metallic-High and low capacity
Bellows Balanced pressure type steam trap
Fig. 4: Bellows Balanced pressure type steam trap


  • Mechanical traps are designed to open for more-dense fluids and close for less-dense fluids.
  • There are two basic categories of mechanical traps that operate on the density principle: Float, and Bucket.
  • Within these categories, there are two types each of density traps: Lever Float, Free Float, Inverted Bucket, and Open Bucket.
  • Air is less dense than water. Hence, density traps tend to close in the presence of air and are generally not suited for venting large amounts of air. For this reason, density traps may contain a separate thermostatic air vent mechanism to handle significant amounts of air.


  • Float & Thermostatic,
  • Bucket & Thermostatic.


  • Float & Thermostatic steam traps combine the action of two principles: thermostatic and density. Each trap has its own discharge orifice. A valve with a ball float actuator drains condensate when the liquid reaches a predetermined level in the trap. When the flow of condensate diminishes’ the float drops, partially closing the valve to accommodate the flow rate.
  • At the top of the trap is a thermostatic element that opens to discharge all air and non-condensable gases as soon as they cause a small temp drop within trap.
  • Operate in between 0 to 250 psig pressure,
  • The condensate valve is located at the bottom and subject to plugging when dirt and scale are present.
  • If the dirt particles prevent the valve from closing, steam energy will be wasted until the condition is detected and corrected.


Inverted bucket traps (Fig. 5) use an inverted bucket that is normally submerged and floats only when steam is present. The bucket sinks when the volume of condensate exceeds a predetermined liquid level. When the bucket sinks, the valve at the top opens.

Inverted Bucket type Steam Trap
Fig. 5: Inverted Bucket type Steam Trap


Selection of steam traps shall be in accordance with the following:

  • Steam traps in low pressure steam drip service shall be inverted bucket style, mechanical traps or bimetallic thermostatic style traps.
  • Steam traps in medium pressure steam drip service shall preferably be inverted bucket style mechanical traps; alternatively disc type thermodynamic traps may be used.
  • Steam traps in high pressure steam drip service shall preferably be inverted bucket style mechanical traps.
  • Steam traps provided for steam turbine inlet drip service shall be a thermodynamic piston type trap.

A commonly accepted practice is to use float & thermostatic (F&T) steam traps for low pressure steam systems up to 30 PSIG, and thermodynamic steam traps for steam pressures over 30 PSIG.

International and European Standards:

  • ISO 6552: 1980/ (BS 6023: 1981): Glossary of technical terms for automatic steam traps.
  • ISO 6553: 1980/CEN 26553: 1991 (Replaces BS 6024: 1981) Marking of automatic steam traps.
  • ISO 6554 1980/CEN 26554: 1991 (Replaces BS 6026: 1981) Face-to-face dimensions for flanged automatic steam trap.
  • ISO 6704: 1982/CEN 26704: 1991 (Replaces BS 6022 : 1983) Classification of automatic steam traps
  • ISO 6948:1981/ CEN 26948: 1991 (Replaces BS 6025: 1982) Production and performance characteristic tests for automatic steam traps.
  • ISO 7841: 1988/CEN 27841: 1991 (Replaces BS 6027: 1990) Methods for determination of steam loss of automatic steam traps.
  • ISO 7842: 1988/CEN 27842: 1991 (Replaces BS 6028: 1990) Methods for determination of discharge capacity of automatic steam traps.

Causes of failure:

Common Causes of failure of steam traps are:

  • Corrosion, due to the condition of the condensate. This can be countered by using particular materials of construction, a good      feed-water conditioning.
  • Water hammer, often due to a lift after the steam trap, traps.
  • Dirt, accumulating from a system where water treatment compound is carried over from the boiler, or where pipe debris is allowed to interfere with trap operation.



Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

5 thoughts on “INTRODUCTION TO STEAM TRAPS AND DRIP LEG: A Brief Presentation

  1. Dear Sir/Madam,
    I am a Piping Designer,
    it’s very nice topic to know all about sta drip leg etc.

    Thanks for good info


  2. This is a very good overview on how and when to use different steam trap. Personnaly, for drip leg applications, I prefer the bi-metalic design. I found them more robust and some manufacturer offer an easy feature to adjust the temperature discharge. Remember, drip leg application have very low load of condensate. So if the drip leg is well done, there will be no worries regarding the amount of condensate that accumulate in the pipe.
    Thanks for your excellent post!

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