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Pumps & Systems, March 2007
Properly choosing an elastomer seat for a butterfly valve requires taking time to carefully determine the application parameters. Here are the primary considerations involved in selecting a seat.
Butterfly valves are a very common type of valve found in many applications. They are relatively compact, simple, and easy to maintain, and they come in a multitude of materials. Selection of the housing, disc and shaft is straightforward in most applications.
However, selection of the seal or seat material in valves with elastomer seals is probably the most difficult and important decision in specifying a valve. There are several things to keep in mind as you select an elastomer for a butterfly valve seat. The major factors to consider are temperature, pressure, chemical compatibility, agency approvals, wear and cost.
First, it is important to remember that the user is the application expert and the manufacturer, or his representative, is the expert on what seal or seat options are available. The manufacturer should have resources and experience to help you make a decision, but he will only know as much about your process as you tell him.
One of the most basic process application parameters is temperature. The temperature information should include the minimum and maximum continuous process temperatures as well as the ambient temperature around the valve as it operates. Short duration high temperature conditions, such as might occur during startup, should be identified for consideration by the valve manufacturer.
Elastomers are not good conductors of heat and can typically resist temperatures well above normal operating limits for short periods of time. The position of the valve disc during these short duration temperature events is important as well. The seat of a valve with the disc in the open position is not highly stressed and is less likely to fail from a temperature spike than the seat of a valve with the disc in the closed position exposed to the same temperature spike. The valve manufacturer's application engineering staff should be able to use the temperature information provided by the user to determine which seat or seal material is suitable.
Another basic process application parameter is the process pressure. The most critical pressure is that which the valve must seal against. This pressure differential from one side of the disc to the other determines which seat or seal elastomers are suitable. The manufacturer must know the minimum and maximum pressure differentials to properly select a seat material.
Perhaps the most difficult and critical factor in selecting the seat or seal elastomer is determining what chemicals the seat must resist. All substances that the seat may be exposed to during installation, in service, and during cleaning and maintenance must be considered. Incidental exposure to cleaning agents is frequently overlooked with disastrous results. The valve manufacturer should have an expert chemist available to discuss chemical compatibility.
The user should also keep in mind that there is wide deviation in the actual rubber recipes used by valve manufacturers. Just because "Brand A" has an EPDM seat suitable for an application does not mean that the EPDM used by "Brand B" will perform the same.
In addition to providing expertise, the manufacturer should be able to provide samples of the seat material for compatibility testing by the user. Table 1 shows general chemical compatibility of materials commonly used to make elastomer seats for butterfly valves.
 Many butterfly valve applications require seats which are made from Food and Drug Administration (FDA) approved ingredients or require a specific color (most commonly white). These seats are typically more expensive and usually do not perform as well as the equivalent "general use" seat. You should carefully consider this requirement before specifying it for a butterfly valve seat.
Wear is a problem in some applications and can be completely disregarded in others. If the material flowing through the valve is abrasive, then wear resistance of the elastomer seat is very important. Wear resistance is difficult to measure and nearly impossible to compare among valve manufacturers. Moving to highly abrasion-resistant compounds frequently means a decrease in the flexibility of the seat, especially at low temperatures. The user should consider wear resistant valves such as those with inflatable seats for applications where wear is a critical factor.
The final factor to consider is cost. After thoroughly describing the application and considering the above factors, elastomers which are not suitable can be eliminated from consideration. It is then simply a matter of selecting the least expensive seat from among those remaining in consideration.
Unfortunately, choosing a seat material in the real world is seldom this simple. Many processes involve multiple materials or chemicals and temperature ranges. The ideal seat for one chemical in a process may be incompatible with another in the same process. The application temperature may affect the chemical resistance of a seat. The ability of a seat to withstand low temperatures may affect its ability to withstand abrasion.
Without question, the more demanding the application, the more the seat will cost! However, by carefully reviewing the application and taking advantage of the valve manufacturer's expertise, the user can select the optimum seat for the application. A reputable valve manufacturer will also be able to discuss molding "special" seats to address special application requirements.
Properly choosing an elastomer seat for a butterfly valve requires the user to take the time to carefully determine the application parameters. Temperature, pressure, chemical compatibility, agency approvals, wear and cost are the primary considerations in selecting a seat. Once the manufacturer or his representative is presented with this application information, they will be able to recommend the optimum seat for the valve application, giving you a long, trouble-free life from your butterfly valve.
Robert Hansen is product manager for Posi-flate.
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