- Created on Tuesday, 03 June 2003 03:36
- Written by Topog-E Gasket Co. of Tulsa, Oklahoma
Boiler safety valves are little understood, often incorrectly installed, and usually neglected. That is a general statement, and as with most general statements, must not be taken as universally true. However, there is enough truth there to warrant some discussion of the valves. The terms "pop valve," often applied to safety valves, is not a bad term in that it accurately describes the action of the valve.
As the pressure of the steam within a boiler approaches the set pressure of the valve, the steam pressure on one side 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. This escaping steam strikes an additional shoulder around the circumference of the disc thus increasing the area of the disc that sees steam pressure. This sudden increase in area under pressure makes the pressure much more unbalanced in the direction of opening, which pops the valve to the wide open position. This pop action prevents wiredraw of the seat due to slow action
Closure of the valve occurs only after the boiler pressure has dropped several pounds below the set point. This is due to the increased area of the disc that sees steam pressure after the valve unseats. To ensure tight closure after a valve has popped under pressure, the boiler maximum operating pressure should be set several pounds below the set pressure of the safety valve. Some manufacturers recommend setting ten percent or more below.
Proper installation of a safety valve is a result only of careful adherence to the manufacturer's instructions. In general these will concern themselves with two areas: distortion of the valve itself and proper piping.
A safety valve body is part of the precision of the whole device. Distort the body, and the accuracy of the valve's calibration is destroyed. Care must be taken to apply wrenches in only the place required for installation.
Similarly, on the discharge side, the valve body must not be made to bear the weight of the discharge piping. Distortion could occur. A short nipple from the valve body should enter the mouth of an independently supported large bell reducer or a drip pan elbow leading through large piping to a safe discharge area or preferably, the outdoors.
All piping to or from a safety valve must be at least as large as the safety valve's connection and the restrictive effect of elbows and the friction losses in pipe must be taken into account. For this reason, runs should be as short as possible and pipe sizes should be generous.
For a safety valve to do its job, it must be sized so that when open it will adequately relieve all the steam the boiler is capable of producing while operating at its maximum.
The purpose of the valves is safety, therefore, they are worth a little attention to give them their best chance to work.
There is some discussion in the field about whether or not it is wise to periodically pop a steam safety valve. Since there is the possibility of a valve becoming incapable of proper operation due to scale coating inactive parts. The Hydronics Institute in their Operation and Maintenance Manual for Steel Boilers recommends an operations check on safety valves at the beginning of the heating season and every six months. Their recommended method is to raise the pressure until the valve pops. Whether by this, or if the hand lever is used, the pressure should be as high as practical to blow the seat and other parts clean of foreign matter.
This article was taken from Topog-E's Boiler Maintenance booklet, for more information visit their web site @ www.topog-e.com.
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