Email this pageTrap Back Pressure
What is Back Pressure?
In regard to a trap or a valve, 'back pressure' is the pressure at just after the trap or valve. In other words, 'back pressure' is the outlet or secondary pressure of the trap or valve. The difference between a trap’s inlet (primary) pressure and 'back pressure' is the 'differential pressure' for operation.
If the condensate is discharged to the atmosphere just after the trap, no back pressure is generated
Even if the condensate is discharged to the atmosphere, if the flow passes through a number of bends, it meets resistance that impairs the flow and back pressure is generated
Counterpressure from the force of a water head forms back pressure
If the pressure increases inside a flash tank into which the discharged condensate flows, the back pressure increases
Back Pressure and Trap Discharge Capacity
If the inlet (primary) pressure remains stable, as the back pressure increases the trap’s differential pressure for trap operation decreases. In the same trap, as the differential pressure for trap operation increases, the trap’s discharge capacity increases (we touched on this in Steam Theory: Traps and Orifices).
Trap Allowable Back Pressure
The subject of back pressure is more complicated than merely its relationship to trap condensate discharge capacity. There is also the issue of trap allowable back pressure.
Allowable back pressure is the maximum back pressure allowed in order for the trap to be able to operate normally, and is expressed as a percentage of the trap inlet (primary) pressure. Generally, as the back pressure increases, the trap loses its ability to operate normally.
So what exactly happens to trap operation when back pressure increases? In many traps, the back pressure exerts force on the valve in the direction that opens the valve, and therefore acts as the valve opening force. If the inlet (primary) pressure remains constant while only the back pressure increases, the valve opening force increases. In some types of traps, this results in the loss of the ability to hold the valve in the closed position and the valve ends up being opened and remaining open.
Differences in Allowable Back Pressures
For example, the allowable back pressure for disc traps is generally 50% to 80% depending on the product design. This means that for an inlet pressure of 1.0 MPaG, the back pressure must be less than 0.5 to 0.8 MPaG. Free float traps, on the other hand, have a relatively high allowable back pressure of over 90%. If the inlet pressure is 1.0 MPaG, free float traps can be used without problem with a back pressure of around 0.9 MPaG.
As we have seen, the allowable back pressure differs depending on the type of trap, so when making trap selection, in addition to considering the condensate discharge capacity it is also necessary to take into account the back pressure at the planned installation location.
If altering a system in order to recover condensate from currently installed traps, the traps will be subject to more back pressure than they were before condensate recovery was implemented, so care must be paid to both discharge capacity and allowable back pressure.
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