Pressure Reducing Stations
Many industrial plants produce high-pressure steam for the processes to work. In many of these plants, there is excess steam capacity available that can be utilized for other purposes, such as space heating, water heating, etc. but process steam is often generated at a higher pressure than can be used for the other purposes. Whatever the needs for lower pressure steam, a pressure reducing station will be required.
A pressure reducing station is more than just a reducing valve fed off a steam main. A properly designed and installed pressure reducing station takes into consideration velocity, good piping practices, and safety.
A steam pressure regulator reducing station is not just a pressure regulator cut into a steam line. The station needs to be carefully designed and properly installed and the entire station should pitch down slightly in the direction of flow. Done correctly, the station will give many years of quiet, satisfactory, performance.
Steam at high pressure has more heat energy than steam at a lower pressure. If dry saturated steam is reduced in pressure, this extra energy causes the steam to become superheated at the lower pressure. In practice though, steam is never completely dry. When the pressure of wet steam is reduced, the extra energy causes the steam to become drier; however this effect is very small. For a typical pressure drop, the improvement in dryness is perhaps only 2%. Steam at the end of a distribution main may well be wetter than 90% so the 2% improvement will still not result in dry steam. Consequently improvements can usually still be made by fitting a separator as part of the standard pressure reducing station. A separator also reduces the possibility of valve “wiredrawing”, the erosion of the valve face which frequently occurs when the valve spends much of its life working close to its seat.
The layout of a typical pressure reducing station is as listed:
- Isolation Valves – enable equipment to be isolated for maintenance work or to save energy
- Separator and Steam Trap – remove entrained water and condensation
- Strainer – ensures that dirt or scale does not block the pressure reducing valve
- Pressure Reducing Valve – reduces the pressure of the system
- Diffuser – a pipe which smooths the flow of steam as the volume is increased
- Safety Pressure Relief Valve – ensures that a maximum pressure is not exceeded
- Pressure Gauges – used to check pressures
- Bypass Valve (optional) – can be used to supply steam while the reducing station is being maintained. This may also be a source of miss operating resulting in energy losses