Thursday, June 19, 2014

Pressure Reducing Valves for Steam Opimization

pressure reducing valve
Pressure Reducing
Valve (PRV)
A well designed steam system should produce clean, dry steam ready for distribution at high pressure through the steam distribution network. This maximizes the potential to generate and supply quality saturated steam at the lowest total cost.

Most applications require a pressure reduction at the point of use.

Significant benefits include:

1) A reduction in the cost of capital equipment; 2) Plant costs decreases by reducing flash steam; 3) Since saturated steam pressure is directly related to temperature, controlling pressure will automatically control temperature thus avoiding the need for supplemental temperature controls; and 4) The ability to supply optimized steam pressure for any individual application.

Principles of Operation:

Normal positions before start-up are with the main valve closed and the pilot valve held open by spring force or air pressure.

Entering steam passes through the pilot valve into the main diaphragm chamber and also out through the control orifice. 

As flow through the pilot valve exceeds flow through the orifice, control pressure increases in the diaphragm chamber and opens the main valve. 

As steam flows through the main valve, the increase in downstream pressure feeds back through the pressure sensing line to the underside of the pressure diaphragm. 

When the force below that diaphragm balances the compression force of the spring above it, the pilot valve throttles. The control pressure maintained in the main diaphragm chamber positions the main valve to deliver just enough steam for the desired delivery pressure. Adjustment of the spring or air pressure above the pressure diaphragm changes the downstream pressure set point. 

When steam is no longer required, the sensing line pressure increases closing the pressure pilot and the control pressure bleeds back through the control orifice. This allows the main valve to hold the desired reduced pressure, and it may close tight for a dead-end shutoff.