What is a non-return valve?
The main function of a non-return valve (NRV), otherwise known as stop or check valve, is to allow steam flow in one direction, and automatically prevent backflow. In other words, they prevent steam from flowing back to the main header of the boiler, or back to a boiler that has failed or is off-line.
An NRV can also be used to isolate a boiler during blowdown and shutdown, and to assist in returning a boiler to operation after any planned or unplanned maintenance.
Why are they needed?
The non-return valve is really just another safety device. Although the NRV should not be relied upon for primary shut-off, it does restrict the flow of steam back from the header into the boiler, which could be disastrous.
Per ASME standards, one NRV is required for one-boiler, high pressure applications (above 15 psig). For high-pressure applications involving multiple steam boilers with manholes that are connected to a common steam main, the steam connection from each boiler must be fitted with two stop valves. The first valve must be automatic and the second should be outside-screw-and-yoke (OS&Y).
How do they prevent backflow?
Non-return valves are flow sensitive, and rely on the steam flow to open and close. The internal disc opens and allows the flow of steam through the valve. The disc will begin to close as forward steam flow decreases or is reversed, depending on the design.
As you can see, non-return valves are critical for any steam boiler system, temporary or permanent. Be sure to check with your temporary equipment supplier to ensure that they include an NRV with their boiler rental package.