Types of Steam Traps

Thermostatic Steam Traps

Thermostatic traps operate in response to the surrounding steam temperature. The operation and benefits of 3 different types are considered on this page - liquid expansion traps, balanced pressure thermostatic and bimetallic traps. Each operates in a different way and is suited to specific types of application.

Liquid expansion steam trap

This is one of the simplest thermostatic traps and is shown in the image below. An oil filled element expands when heated to close the Valve against the seat. The adjustment allows the temperature of the trap discharge to be altered between 60°C and 100°C, which makes it ideally suited as a device to get rid of large quantities of air and cold condensate at start-up.

Liquid expansion steam trap

Typical application
Because of its fixed temperature discharge characteristic, the liquid expansion trap may be usefully employed as a 'shutdown drain trap'. Here, its outlet must always point upwards, as illustrated in the image below, to enable continuous immersion of the oil filled element. As the trap can only discharge between 60°C - 100°C it will only normally open during start-up. It can be installed alongside a mains drain trap which would normally be piped to a condensate return line.

Installation of a liquid expansion steam trap

Advantages of the liquid expansion steam trap

Disadvantages of the liquid expansion steam trap

Balanced pressure steam trap

A large improvement on the liquid expansion trap is the balanced pressure trap, shown in the image on the right. Its operating temperature is affected by the surrounding steam pressure. The operating element is a capsule containing a special liquid and water mixture with a boiling point below that of water. In the cold conditions that exist at start-up, the capsule is relaxed. The Valve is off its seat and is wide open, allowing unrestricted removal of air. This is a feature of all balanced pressure traps and explains why they are well suited to air venting.

Balanced pressure steam trap with replaceable capsule
Balanced press steam trap with replaceable capsule

Operation of balanced pressure steam trap capsule

As condensate passes through the balanced pressure steam trap, heat is transferred to the liquid in the capsule. The liquid vaporises before steam reaches the trap. The vapour pressure within the capsule causes it to expand and the Valve shuts. Heat loss from the trap then cools the water surrounding the capsule, the vapour condenses and the capsule contracts, opening the Valve and releasing condensate until steam approaches again and the cycle repeats.

Operation of balanced pressure steam trap capsule in open position
Operation of balanced pressure steam trap capsule in closed position

Early bellows type elements of non-ferrous construction were susceptible to damage by waterhammer. The introduction of stainless steel elements improved reliability considerably. The image below shows an exploded view of a modern balanced pressure steam trap arrangement that has considerable resistance to damage from waterhammer, superheat and corrosion.

Typical balanced pressure capsule arrangement
Typical balanced pressure capsule arrangement

Advantages of the balanced pressure steam trap

Disadvantages of the balanced pressure steam trap

The differential below steam temperature at which the trap operates is governed by the concentration of the liquid mixture in the capsule. The 'thin-walled' element gives a rapid response to changes in pressure and temperature.

Bimetallic steam trap

As the name implies, bimetallic steam traps are constructed using two strips of dissimilar metals welded together into one element. The element deflects when heated. (see image on the right).

There are two important points to consider regarding this simple element:

Bimetallic steam trap

The performance of any steam trap can be measured by its response to the steam saturation curve. The ideal response would closely follow the curve and be just below it. A simple bimetal element tends to react to temperature changes in a linear fashion.

Some use combinations of two different sets of bimetal leaves in a single stack, which operate at different temperatures (see image below).

Operation of a bimetel steam trap with two leaf element

A more innovative design is the disc spring thermostatic element shown in the image right below. The thermostatic element is made up of a set of bimetal discs. These discs, if acting directly between the Valve stem and the seat (as with some thermostatic steam traps), cause the discharge temperature of the condensate to change linearly with changing pressure.
By incorporating a spring washer between the discs and a recess in the seat, this absorbs some of the bimetal expansion at low pressure so that a greater temperature change must occur with changing pressure. The spring washer shape is preferred over a coil spring because it develops force in an exponentially increasing rate, rather than in a linear rate. This effect takes place up to 15 bar g until the spring is deflected to the bottom of the recess, and means that the discharge temperature of the condensate will follow the steam saturation curve more accurately. Discharge rates are also improved by the dynamic clack which tends to produce a blast discharge.

Advantages of the bimetallic steam trap

Multi-cross elements as used in the Spirax Sarco SM range of bimetallic steam traps
Bimetallic steam trap with cooling leg

Disadvantages of the bimetallic steam trap

Spirax Sarco