Email this pagePressure Reducing Valves for Steam
Reducing Steam Pressure
The pressure of the steam that is generated at high pressures in a plant is reduced to meet the needs of the products and applications for which the pressure is to be used. The pressure of the steam is lowered by throttling down the size of the steam passageway. In simple terms, this is how pressure reduction for steam is achieved.
For merely reducing the steam pressure it is possible to use the valve in a fixed partly-open position, or by inserting an orifice plate for the steam passing through. However, this results in any fluctuations in flow rate being accompanied by a corresponding fluctuation in pressure. A pressure reducing valve is designed to automatically adjust the amount of valve opening in order to allow the pressure to remain unchanged even when the flow rate fluctuates.
Advantages of Pressure Reducing Valves
While it is possible to maintain a constant pressure by using the combination of a control valve, a pressure sensor and a controller, a pressure reducing valve offers the advantage of being able to control pressure through fully-automatic self-operation, requiring no type of motive power. It offers the further advantage of extremely rapid response action, as it operates by automatically sensing the pressure.
Types of Pressure Reducing Valves for Steam
The mechanism that automatically adjusts the pressure in pressure reducing valves uses the balance between the steam pressure and the adjustment spring. At present, this is universal on all pressure reducing valves manufactured. However, when it comes to the mechanism that determines the amount of valve opening, there are two types to be found:
- Amount of valve opening determined directly by the elongation and retraction of the adjustment spring -> Direct Acting
- Amount of valve opening determined by the force of the steam -> Pilot Operated
An overview of the features of each type of valve-opening mechanism follows.
Direct Action
- Pros: Compact, affordable, simple construction
- Cons: Usable range of flow rates is more limited compared to the Pilot Operated Types, tendency for secondary pressure to drift from set pressure (offset) when flow rate or primary pressure fluctuate
Pilot Operated
- Pros: May be used across a broader range of flow rates than the Direct Acting Types, no tendency for secondary pressure to drift from set pressure (offset) when flow rate or primary pressure fluctuate
- Cons: Larger, more expense and of more complicated construction than Direct Action Types
When to Use Each Type of Pressure Reducing Valve
From their characteristics we can see that the purpose for use and the applications are different between Direct Acting pressure reducing valves and Pilot Operated pressure reducing valves.
The steam flow rate in steam transport piping fluctuates greatly depending on the operational status of the equipment. In addition, the amount of steam used by each piece of equipment at start up differs greatly from the amount used by the equipment during its steady operational state. Only pilot operated pressure reducing valves can handle these types of conditions. As a result, when pressure reducing valves for steam are mentioned, one is typically referring to pilot operated types.
Direct acting pressure reducing valves, which have the advantage of being compact and light, can be used on compact equipment that has only minimal fluctuations in load. They are perfect for cases that typically call for manual operation due to the necessity for extremely unpredictable minute adjustments of a globe valve that is fixed in a partly-open position.
We must remember to select the proper pressure reducing valve for the circumstances, based on an understanding of the various characteristics of each valve type.