Balanced pressure steam trap ( BPST )

Balanced pressure steam trap with replaceable capsule

A large improvement on the liquid expansion trap is the balanced pressure trap, shown in Figure 2 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.

FIG:-2 Balanced pressure steam trap with replaceable 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 (Figure 2.1).

FIG:- 2.1 Operation of balanced pressure steam trap capsule

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. The result is the response line as illustrated in Figure 2.3.

FIG:- 2.3 Typical response of a balanced pressure steam trap Y - Y

Early bellows type elements of non-ferrous construction were susceptible to damage by waterhammer. The introduction of stainless steel elements improved reliability considerably. Figure 2.4 shows an exploded view of a modern balanced pressure steam trap arrangement that has considerable resistance to damage from waterhammer, superheat and corrosion.

FIG:- 2.4 Typical balanced pressure capsule arrangement

• Advantages of the balanced pressure steam trap:-

1.Small, light and has a large capacity for its size.

2.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.

3.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).

4.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.

5.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:-

1.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.

2.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.