Valve Seatings

The efficacy of a valve's seating and subsequent sealing are critical criteria in selecting a valve for a certain process function. Valve seatings are the areas of the seat and closure member that make contact with each other to close. Because the seatings are subject to wear during the sealing process, their sealability tends to deteriorate with use.

Metal Valve Seatings 

Operational wear is not restricted to soft-seated valves; it can also occur in metal-seated valves if the process system is conveying corrosive or particle-containing fluid. Metal seatings can be deformed by trapped fluids and wear particles. 

Corrosion, erosion, and abrasion exacerbate the damage. The surface finish will deteriorate when the seatings wear in if the wear-particle size is excessive in comparison to the size of the surface imperfections. A coarse finish, on the other hand, tends to improve as the seatings wear in if the wearparticle size is modest in comparison to the size of the surface imperfections. 

The wear-particle size is determined not only by the material type and condition, but also by the fluid's lubricity and the contamination of the seatings with corrosion and fluid products, both of which diminish the wear-particle size. As a result, the seating material must be resistant to erosion, corrosion, and abrasion. If the material fails to meet one of these conditions, it may be wholly inappropriate for its intended purpose. For example, the fluid's corrosive activity considerably increases erosion. 

A material that is extremely resistant to erosion and corrosion may also fail totally due to inadequate galling resistance. On the other hand, the best material may be too expensive for the type of valve under consideration, necessitating a compromise. 

Periodic Sealant Application

Certain valves have the capability of introducing sealants into the valve seat and stems on a regular basis in order to maintain an effective seal over an extended period of time. Sealants sprayed into the gap between the seatings after the valve is closed can plug leakage holes between metal seatings. 

The lubricated plug valve is a metal-seated valve that solely relies on this sealing mechanism. In some other types of valves, the injection of a sealant into the seatings is used for creating an emergency seat seal after the initial seat seal has failed.

Soft Seatings 

Soft seats are quite effective, although they are limited in their application at high temperatures and pressures. Manufacturers of proprietary soft seats will specify the maximum and minimum design pressures and temperatures that their products can withstand. Some soft seats are also incompatible with certain fluids at certain pressures and temperatures.

Soft seatings may have one or both sitting faces made of a soft material such as plastic or rubber. Soft seated valves can attain extraordinarily high fluid tightness because these materials conform quickly to the mating face. 

Furthermore, the high level of fluid tightness can be achieved repeatedly. On the negative side, the application of these materials is constrained by their fluid compatibility and temperature. 

Soft seating materials have an unanticipated constraint in circumstances where the valve shuts off a system that is rapidly filled with gas at high pressure. The high-pressure gas that enters the closed system acts like a piston on the gas that originally filled the system. 

Compression heat can be high enough to destroy soft seating material.  In globe valves, a heat sink resembling a metallic button with a large heat-absorbing surface is located ahead of the soft seating element to protect it from heat damage. In the case of oxygen service, this design safeguard may not be sufficient to keep the soft seating part from igniting. 

To avoid such failure, the valve inlet route may need to be extended beyond the seat passage, forming a pocket in which the high temperature gas can gather away from the seatings. The fundamental consideration in constructing soft seatings is to keep the soft seating element from being displaced or extruded by fluid pressure.