# Stop Check Valve

Written by Matt Milbury. Posted in Valve

A stop-check valve is essentially, two valves built into one. It can act as a globe valve to isolate or control the flow rate. It also acts as a check valve by preventing reverse flow. This is critical to prevent boiler flooding by preventing steam and condensate from back flowing into the generator.

Stop Check Valve Placeholder

These valves are usually used in high pressure & temperature applications or wherever steam is being utilized. This means it is commonly found in power plants or steam generation facilities.  When there are multiple boilers or generators installed in parallel, a stop check valve is usually the first valve on the discharge line which is used to protect the individual boiler. ASME Boiler & Pressure Vessel Code requires the installation of a stop valve when there are multiple boilers connected to a header system. When it is installed in this configuration, it may be known as a non-return valve or a boiler stop valve. Additionally, they can be found in geothermal, upstream oil and gas, refineries and cogeneration plants.

Like globe valves, a stop check valve is available as a straight, pass through valve and an angle body valve. Because these valves look like globe valves, it is extremely important to ensure that the orientation of the valve is in the right direction when installing. The valve design varies from manufacturer so it is important to reference the manufacturer’s installation, operation and maintenance manual (IOM).

The minimum velocity required for a full disc lift in a stop check valve can be estimated the following way, where:

$$V = j r^2 sqrt{v}$$

$$V$$ = liquid flow, ft/s (m/s)

$$v$$ = specific volume of the liquid, ft3/lb (m3/N)

$$j$$ = 55 (210.0) globe, OS&Y blocked bonnet

= 75 (286.4) angle, OS&Y blocked bonnet

= 60 (229.1) Y-pattern, OS&Y bolted bonnet

= 140 (534.7) Y-pattern, threaded bonnet

$$r$$ = ratio of port diameter to inside pipe diameter

Note that this may require a valve that is smaller in diameter than the nominal pipe size of the piping. If this happens, reducers should be used to ensure the diameters match. It is important to use the correct type of reducer to ensure that the process will not get slugs of liquid or vapor.

The importance of having a correctly sized valve is not based on pressure drop since the pressure drop for a large partially open valve will be about the same as a correctly sized valve that is fully open. Sizing the valve correctly will minimize maintenance costs during its life.