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.
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.
Advantages of the liquid expansion steam trap
- Liquid expansion traps can be adjusted to discharge at low temperatures, giving an excellent 'cold drain' facility.
- Like the balanced pressure trap, the liquid expansion trap is fully open when cold, giving good air discharge and maximum condensate capacity on 'start-up' loads.
- The liquid expansion trap can be used as a start-up drain trap on low pressure superheated steam mains where a long cooling leg is guaranteed to flood with cooler condensate. It is able to withstand vibration and waterhammer conditions.
Disadvantages of the liquid expansion steam trap
- The flexible tubing of the element can be destroyed by corrosive condensate or superheat.
- Since the liquid expansion trap discharges condensate at a temperature of 100°C or below, it should never be used on applications which demand immediate removal of condensate from the steam space.
- If the trap is to be subjected to freezing conditions the trap and its associated pipework must be well insulated.
- The liquid expansion trap is not normally a trapping solution on its own, as it usually requires another steam trap to operate in parallel. However, it can often be used where start-up rate is not an important consideration, such as when draining small tank heating coils.
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 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.
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
Advantages of the balanced pressure steam trap
- Small, light and has a large capacity for its size.
- The Valve is fully open on start-up, allowing air and other non-condensable gases to be discharged freely and giving maximum condensate removal when the load is greatest.
- This type of trap is unlikely to freeze when working in an exposed position (unless there is a rise in the condensate pipe after the trap, which would allow water to run back and flood the trap when the steam is off).
- The modern balanced pressure trap automatically adjusts itself to variations of steam pressure up to its maximum operating pressure. It will also tolerate up to 70°C of superheat.
- Trap maintenance is simple. The capsule and Valve seat are easily removed, and replacements can be fitted in a few minutes without removing the trap from the line.
Disadvantages of the balanced pressure steam trap
- The older style balanced pressure steam traps had bellows which were susceptible to damage by waterhammer or corrosive condensate. Welded stainless steel capsules introduced more recently, are better able to tolerate such conditions.
- In common with all other thermostatic traps, the balanced pressure type does not open until the condensate temperature has dropped below steam temperature (the exact temperature difference being determined by the fluid used to fill the element). This is clearly a disadvantage if the steam trap is chosen for an application in which waterlogging of the steam space can not be tolerated, for example; mains drainage, Heat Exchangers, critical tracing.
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:
- Operation of the steam trap takes place at a certain fixed temperature, which may not satisfy the requirements of a steam system possibly operating at varying pressures and temperatures.
- Because the power exerted by a single bimetal strip is small, a large mass would have be used which would be slow to react to temperature changes in the steam system.
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).
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
- Bimetallic steam traps are usually compact, yet can have a large condensate capacity.
- The Valve is wide open when the steam trap is cold, giving good air venting capability and maximum condensate discharge capacity under 'start-up' conditions.
- As condensate tends to drain freely from the outlet, this type of steam trap will not freeze up when working in an exposed position. The bodies of some bimetallic steam traps are designed in such a way that they will not receive any damage even if freezing does occur.
- Bimetallic steam traps are usually able to withstand waterhammer, corrosive condensate, and high steam pressures.
- The bimetal elements can work over a wide range of steam pressures without any need for a change in the size of the Valve orifice.
- If the Valve is on the downstream side of the seat, it will tend to resist reverse flow through the steam trap. However, if there is any possibility of reverse flow, a separate Check Valve should be fitted downstream of the trap.
- As condensate is discharged at varying temperatures below saturation temperature and, provided waterlogging of the steam space can be tolerated, some of the enthalpy of saturated water can be transferred to the plant. This extracts the maximum energy from the condensate before it drains to waste, and explains why these traps are used on tracer lines where condensate is often dumped to waste.
- Maintenance of this type of steam trap presents few problems, as the internals can be replaced without removing the trap body from the line.
- The flash steam produced whenever condensate is discharged from a higher to a lower pressure will tend to cause an increase in backpressure in the condensate line. The cooling leg allows the condensate to cool down, producing less flash steam in the condensate line and thus helping to reduce the backpressure.
Disadvantages of the bimetallic steam trap
- As condensate is discharged below steam temperature, waterlogging of the steam space will occur unless the steam trap is fitted at the end of a long cooling leg, typically 1 - 3 m of unlagged pipe. Bimetallic steam traps are not suitable for fitting to process plants where immediate condensate removal is vital for maximum output to be achieved. This is particularly relevant on temperature controlled plants.
- Some bimetallic steam traps are vulnerable to blockage from pipe dirt due to low internal flow velocities. However, some bimetallic traps have specially shaped Valve trims that capture the discharge energy to open the Valve more. These tend to give an intermittent blast discharge characteristic rather than a continual dribble discharge, and as such tend to be self-cleaning. These Valve trims are sometimes referred to as dynamic clacks.
- If the bimetallic steam trap has to discharge against a significant backpressure, the condensate must cool to a lower temperature than is normally required before the Valve will open. A 50% backpressure may cause up to a 50°C drop in discharge temperature. It may be necessary to increase the length of cooling leg to meet this condition.
- Bimetallic steam traps do not respond quickly to changes in load or pressure because the element is slow to react.