Select the Right Instrument-System Valve

Start by matching valve type to desired function.

By Michael Adkins, Swagelok Company

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A valve manufacturer's representative as well as product catalogs and product test reports can be valuable resources in refining your choice.

Keep Five Pointers in Mind

1. Know your application. When choosing a valve, you must have on hand certain pieces of information, including the chemical composition of the system media and the full range of pressures and temperatures the valve may experience over its life. Make sure your valve choice can accommodate these parameters. Don't go with hunches or approximations. Consult the product data.

2. Check for material compatibility. It's possible to have the right valve but the wrong materials of construction. Valves often come in a standard set of materials, but others often are available. Always check the product catalog to identify temperature and pressure ranges, as well as compatibility with different system media (chemicals). When in doubt, consult your manufacturer's representative.

 
Proportional Relief Valve Figure 6. When actuated by
over-pressurization, this valve provides a measured
release of fluid.

3. Factor in the maintenance schedule. Different valves have different maintenance schedules. Your system parameters, including the number of times the valve is cycled, will affect this schedule. Your maintenance team must be able to manage the schedule. This seems like an obvious point but it's often overlooked. Are you willing to service that valve once every 20 days when it's 100 feet in the air?

4. Understand pressure drops. Almost every valve or other component produces a drop in pressure. You must check the cumulative pressure drop, to avoid risks of ending up with too little pressure at a certain point in the line. Every valve is rated with a flow coefficient, Cv, which describes the relationship between the pressure drop across an orifice, valve or other assembly and the corresponding flow rate. The higher the Cv, the lower the pressure drop. Ball and needle valves of the same size will produce very different pressure drops -- very little for the ball valve but significant for the needle valve (or other globe valves).

5. Consider cost of ownership. The true cost is the purchase price plus the expenses of owning and maintaining or replacing a valve over time. To calculate the cost of ownership, you must know how long the valve will operate in your particular system between maintenance checks. Maintenance costs should include replacement parts, labor and downtime. Some valves are much easier to maintain than others; some can be serviced in place, while others must be removed from the process line. Also, given your valve choice, what are the chances of unscheduled maintenance and downtime?


MICHAEL ADKINS is product manager, general industrial valves, for Swagelok Co., Solon, Ohio. E-mail him at michael.adkins@swagelok.com.

 

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