To ensure smooth operation of pressure reducing valves on steam lines, moisture separators should be installed before the pressure reducing valve.
It is well recognized in the plant engineering teams that reducing the steam pressure to a value sufficient to meet the process temperature requirements reduces steam consumption. Therefore, pressure reducing valves (PRV) have become a common requirement across process plants. However, pressure reducing valves or control valves in general are susceptible to damage from the moisture in the steam which can impact the valve’s performance and lifecycle.
Wet steam is a bi-phased steam that contains a mixture of liquid water droplets and gaseous steam. The water droplets travel at high steam velocities. Thus when these heavier water droplets collide with the metal surfaces they will erode valve seats and fittings, a condition known as Wiredrawing. Additionally when these water droplets collide with the sensitive diaphragm of the control it often damages the diaphragm by either deforming it or rupturing it.
Thus to protect the pressure reducing valves from damage from wet steam, a moisture separator must always be installed upstream of the valve. Additionally during regular plant maintenance the steam trap connected to the moisture separator must also be checked to ensure it is functioning well. If this steam trap malfunctions, the moisture collected will flood the separator, making the separator ineffective in removing any more moisture.
Illustration: Wiredrawing effect on a valve seat.