What is Piston valve?
Piston valves are used for fully open or fully closed for on/off regulation on steam, gas, and other fluid services. These valves are mostly used on fluids that cause excessive seat wear. Piston valves are mostly actuated manually, but sometimes other actuation modes such as hydraulic and electric are used.
Piston valves are usually used where the valve body is permanently installed and to reduce maintenance. Piston valves are not designed for throttling applications and they must be used in fully opened or closed positions.
When the valve is fully opened, only the bottom face of the piston is exposed to fluid and the rest of the body is protected by the upper sealing rings. So, the sealing surfaces are protected from erosion by the fluid flow.
Examples of piston valves are:
- The valves used in many brass instruments
- The valves used in pneumatic cannons
- The valves used in many stationary steam engines and steam locomotives
A piston valve can also refer to a 2-way 2-position, pilot-operated spool valve. The term is extremely popular among spud gun enthusiasts who often build homemade piston valves for use in pneumatic cannons. Valves are typically constructed primarily from pipe fittings and machined plastics or metals.
The inside of a piston valve contains a piston that blocks the output when the valve is pressurized and a volume of air behind the piston. When the pressure behind the piston is released, the piston is pushed back by the force of the pressure from the input.
This allows the valve to be opened by a much smaller pilot valve, with speeds faster than possible with just a manually operated valve. Functionally these types of valves are comparable to quick exhaust valves.
This type of piston valve is also sometimes referred to as a back-pressure valve.
How does a piston valve work?
The construction of a piston valve is similar to a globe valve and they are used to shut off and regulate. These valves provide positive shut-off. The shut-off assembly comprises the metal piston, two resilient valve rings, and a metal lantern bush.
The sealing surface consists of the outer vertical surface of the piston and the corresponding inner surfaces of the sealing rings. This provides a sealing surface compared to the globe valves of conventional design. At the beginning of the opening, a displacement uncovers the small flow section. Which controls the low rate of flow.
Most automatic valves are of double disc type, the double-disc design can be used to equalize the pressure across the disc and it can be varied to change the flow characteristics through the valve or to change the operation of the valve such as by decreasing the response time of the valve’s action.
A piston balance valve allows the force exerted across the seating area to be as equal as possible while being directed in the opposite direction. The double-disc arrangement has the flow coming from the left.
In the closed position, the fluid pressure would press against the bottom of the top disc and against the top of the bottom disc equalizing flow pressure. Even when the valve is open there is an equalizing force that gives the valve a smooth operation.
Application of Piston Valve
Piston valves are used primarily for saturated and superheated steam and hot water service in power plants, refineries, pulp and paper mills, etc. Specific applications include steam headers and manifolds, condensate manifolds, desuperheaters, steam trap isolation, etc.
Advantages of Piston Valve
- They provide flow control.
- A good tightness is ensured by the environmentally tight contact of the metal piston to the non-metal seal packs.
- The circumference of the piston can also be used for liquids containing solid particles, as it travels by sealing packs during opening and closing.
- They are also suitable for gaseous fluids.
- Since the pistons move in non-metal gaskets, they are not easily worn. Worn gaskets can be easily renewed.
Disadvantage of Piston Valve
- Pressure losses are high due to their structure.
- They contain dead volume. Residues may accumulate inside the valve.
- In large nominal dimensions, enormous forces act on the opening and closing spindle. This force makes it difficult to close. It is appropriate to switch to “Balance piston” type valve with nominal sizes larger than DN 65.