Piston Check Valves

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How Piston Check Valves Work

Piston check valves consist of a piston, usually in the form of a cylindrical plug, that sits within a valve body that’s shaped like a globe valve.

When the forward-flow fluid pressure on the inlet side is high enough to overcome the piston's weight and any spring force (i.e., the cracking pressure), the piston moves upward, and the fluid flow path opens. As the fluid slows, the piston begins to move back towards its seat. This pushing effect is due to both the piston's weight and, if present, the spring's weight. If the fluid moves backward, the pressure forces the piston fully into its seat, and flow stops immediately. 

Benefits of Piston Check Valves

One of the key benefits of using piston check valves is their sealing performance, achieved by a linear piston that fits precisely into the valve seat. This makes piston check valves a more effective sealing solution than swing check valves. Because of their high-pressure and high-temperature capabilities, they are ideal for steam, air, and gas systems. The space above the piston, whether or not a spring is used, serves as a dashpot or cushion to prevent the seal from slamming shut violently (which minimizes seal damage and also reduces water hammer). 

Piston check valves are commonly used for applications such as

• High-pressure water or steam lines.
• Air compressors and nitrogen systems.
• Feedwater systems where backflow could damage pumps.

Key Factors in Piston Check Valve Performance

Orientation, pressure drop, cracking pressure, and cleanliness are three key factors impacting the performance of these check valves.

Spring-Loaded Piston Check Valve or Not?

Here’s a quick table to help you determine if you need a standard or spring-loaded piston check valve.

FAQs about Piston Check Valves

What is the difference between a "Lift" check valve and a "Piston" check valve?

Sometimes these terms are used interchangeably because both types of valves use a lifting motion where the disc moves perpendicular to the seat. However, a piston check valve specifically uses a guided cylindrical piston that often includes a dashpot (a chamber that cushions the movement), whereas a standard lift check valve might just use a simple free lifting disc. Piston check valves are suited to high-pressure applications, while lift check valves are suited to low-pressure applications.

Can I install a piston check valve vertically?

Only if it is spring-loaded, a standard gravity-actuated piston check valve depends on the weight of the piston to drop back into the seat. If a piston check valve is installed vertically with flow moving upward, the piston may not be able to reseat properly or could get stuck in the open position when flow stops. Needless to say, this is bad.

Why is the pressure drop higher in a piston check valve than in a swing check valve?

In a swing check valve, the fluid flows relatively straight through the valve body. However, in a piston check valve, the fluid must travel an "S-shaped" path: up over the seat and around the piston. This change in direction creates turbulence and resistance, resulting in a higher pressure drop across the valve.

What causes a piston check valve to "chatter"?

Chatter (a rapid opening and closing sound) usually happens when the valve is oversized. If the flow rate isn't high enough to keep the piston fully lifted, it will cycle and bounce against the seat. To fix this, you may need a valve with a lower cracking pressure or a smaller diameter valve to increase flow velocity. While less common, turbulent flow may also cause a piston check valve to chatter.

How do I prevent "water hammer" with these valves?

Piston valves are naturally good at reducing water hammer because of their linear travel. However, for maximum protection, choose a spring-assisted valve. The spring ensures the valve closes rapidly once flow stops. This, in turn, prevents the reverse surge of liquid from gaining momentum.

Can these valves handle "dirty" fluids?

Generally, no. Piston check valves require a close fit between the piston and the valve body/guide for reliable performance. If there is scale, grit, or debris in the fluid, it can get trapped in the clearance space, causing the piston to "hang up" and stick in the open or closed position.

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