Balanced Pressure Trap:
As discussed in my previous post Balance pressure trap is classified under temperature operated traps.
Due to robust design of the internals balance pressure steam trap has a good life expectancy.
Balanced Pressure is an improvement on the Liquid Expansion Thermostatic Trap in the sense that the above Trap is self-adjusting to varying operating steam pressure conditions within its designed pressure range. That means that the above trap is capable of discharging the Condensate with small amount of sub-cooling (irrespective of steam pressure), at the same time it does not allow blowing of live steam.
The accompanying sketch (refer Figure-1, Exhibit 34.8) below shows the schematic arrangement of the constructional details of a Pressure Balanced Bellow. The thermostatic flexible element (as in case of Liquid Expansion Trap) is replaced with a suitably designed Bellow, which is sensitive enough to respond to the internal and external pressures acting onit. The above Bellow instead of being filled with oil is evacuated and then filled with a small quantity of some sort of liquid, which has a boiling point somewhat lower than water. This allows the filled liquid (on seeing the steam) to boil off and generate an internal vapour pressure in excess of the steam pressure surrounding it, by an amount required for expanding the bellow to close the flow passage. That means that the live steam filling the trap must cool down to form a sub-cooled Condensate (a few degrees below steam condensationtemperature) to cause the internal pressure of the bellow to diminish and thereby allowing passage for the Condensate through the discharge port.
Since the above functioning principle is based on the differential pressure (i.e. the difference between the internal and external pressure applied on the bellow) it is independent of the operating pressure of the steam. The above Trap is therefore reasonably responsive to the varying steam pressure conditions. Due to the thin wall of the Bellow element, it is able to respond to temperature changes (due to varying steam pressures) fairly quickly resulting into the response line shown in the curve shown below (refer Figure-1, Exhibit 34.9). The above trap is also fairly quick with regards to free venting of cool air during initial startup, due to the unobstructed passage of flow in the shrunk position of the Bellow.
 Balanced pressure steam trap Figure-1 (Exhibit 34.8) |
 Balanced pressure steam trap Figure-1 (Exhibit 34.9) |
Balance pressure steam trap response curve
Advantages of Balanced Pressure Trap:
- High air venting capacity for startup.
- Self-adjusting, to function satisfactorily under varying steam pressure (within its operatingpressure range) conditions without any external adjustments.
- Does not require freezing protection under subzero ambient conditions when its discharge is piped as a free draining line.
- Simple and compact design for large capacity Trap.
Disadvantages of Balanced Pressure Trap:
- The Trap is generally not suitable for super-heated steam
- Limited resistance to water hammer.
- Some amount of sub-cooling of condensate cannot be avoided.
- Susceptible for corrosion attack, when handling corrosive condensate.