Valves
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.
Below is a table showing general chemical compatibility of materials commonly used to make elastomer seats for butterfly valves.
COMPOUND CHEMICAL GROUP
Buna-Nitrile, NBR, Buna-N
Polyurethane
Butadiene Acrylonitrile Copolymer
Urethane Polymer
EPDM, EP
Ethylene Propylene
Copolymer and Terpolymer
Fluoroelastomer Fluorocarbon Polymer
GENERALLY RESISTANT TO
Many
Hydrocarbons, Fats,Oils,Greases, Hydraulic Fluids, Chemicals Ozone,
Hydrocarbons, Moderate
Chemicals, Fats, Oils, Greases
Animal and Vegetable Oils, Ozone, Strong and Oxidizing Chemicals
All Aliphatic, Aromatic and Halogenated Hydrocarbons, Acids, Animal and Vegetable Oils
Silicone
Organic Silicone Polymer
Moderate or Oxidizing Chemicals, Ozone,
Concentrated Sodium Hydroxide
GENERALLY ATTACKED BY
Ozone Ketones, Esters, Aldehydes, Chlorinated and Nitro Hydrocarbons
Concentrated Acids, Ketones, Esters, Chlorinated and Nitro
Hydrocarbons
Mineral Oils and Solvents, Aromatic Hydrocarbons
Ketones, Low Mole Weight Esters and Nitro Containing Compounds
Many Solvents, Oils, Concentrated Acids, Dilute
Sodium Hydroxide
This seat developed wear from abrasive material flowing through it.
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 amongst 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 temperatures 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. And, 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 moulding “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.
This seat has extensive surface checking and swelling due to chemical incompatibility. It also had a severe loss of strength resulting in failure.
44 Solids & Bulk Handling • November 2010
For more information please contact Posi-flate on tel: 01908 564 455 or visit:
www.posiflate.com
www.solidsandbulk.co.uk
Many butterfly valve applications require seats which are made from Food and Drug Administration (FDA) approved ingredients or require a specific colour (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 amongst 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.
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