Boiler contractors see these valves all the time when working on equipment. Generally the steam relief valve is often little understood, often incorrectly installed, and usually neglected. A little refresher on these valves might be in order.
How Relief Valves Work
As the pressure of the steam within a boiler approaches the set pressure of the valve, the steam pressure on the underside of the actuating disc approaches the pressure of a spring applied to the outer side of the disc. When equilibrium is passed, the disc starts to lift off its seat. The moment this happens, steam is suddenly released all around the disc to what is called the “huddling chamber.” This chamber increases the area of the disc that sees steam pressure, thus increasing force. This increased area under steam pressure makes the pressure much more unbalanced in the direction of the valve discharge opening and therefore pops the valve into a wide open position. When the valve opens with a “pop” the valve seat is preserved from wiredraw caused by slow opening.
Closure of the valve occurs only after the boiler pressure is dropped several pounds below the set point. The reduction of the area of the disc seeing steam causes the disc to firmly close against the valve seat.
Relief Valve Installation
Proper installation of a steam relief valve seems somewhat simple and is, as long as two areas of concern are followed.
The first area of concern is valve distortion. Valve distortion occurs when the valve is improperly wrenched in, using the valve body instead of supplied wrench flats. Distortion also occurs when the discharge side of the safety relief valve is made to bear the weight of the discharge piping. To prevent this distortion use a short nipple from the valve to an independently supported bell reducer or drip pan elbow. These valves are precision devices and any distortion will affect accuracy and calibration.
The second area of concern is discharge piping. For a safety valve to do its job it must be sized properly to adequately relieve all the steam the boiler is capable of producing while operating at its maximum. All piping to or from a safety relief valve must be at least as large as the valve’s connections. Also, the restrictive effect of elbows and the friction losses in pipe must be taken into account. For this reason, piping runs should be as short as possible and pipe sizes should be generous.
If you need help in replacing or sizing a steam relief valve please contact Stromquist and Company at 1-800-241-9471. All others can order this product from one of our affiliates at CGNA.