Seat damage can often occur when a valve is first lifted as part of the general plant commissioning procedure, because very often, dirt and debris are present in the system. It should be noted, however, that a soft seal is often more susceptible to damage than a metal seat. Resilient or elastomer seals incorporated into the valve discs are often used to improve shut-off, where system conditions permit.
Unlike ordinary isolation valves, the net closing force acting on the disc is relatively small, due to there being only a small difference between the system pressure acting on the disc and the spring force opposing it. In addition, for a reasonable service life, the mating and sealing surfaces must have a high wear resistance. Typical figures required for an acceptable shut-off for a metal seated valve are 0.5 μm for surface finish and two optical light bands for flatness.
The disc must articulate on the stem and the stem guide must not cause any undue frictional effects. Characteristics of the safety valveįor a metal-seated valve to provide an acceptable shut-off, the sealing surfaces need to have a high degree of flatness with a very good surface finish. The seat tightness is affected by three main factors firstly by the characteristics of the safety valve, secondly by the installation of the safety valve and thirdly, by the operation of the safety valve. Seat tightness is an important consideration when selecting and installing a safety valve, as not only can it lead to a continuous loss of system fluid, but leakage can also cause deterioration of the sealing faces, which can lead to premature lifting of the valve.
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