Valve Selection - Tips & Traps
Know your application
When choosing a valve, you must have certain pieces of information in hand, including the chemical composition of the system media and the full range of pressure and temperatures over the course of the valve's life. Make sure your valve choice can accommodate these parameters. Don't go with hunches or approximations. Consult the product data.
Check for material compatibility
It is possible to have the right valve but the wrong materials of construction. Valves will often come with a standard set of materials, but there are alternatives. You should 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.
Know your maintenance schedule
Different valves have different maintenance schedules, and your system parameters, including the number of times the valve is cycled, will affect this schedule. The valve's maintenance schedule needs to be manageable for your maintenance team. This seems like an obvious point but it is often overlooked. Are you willing to service that valve once every 20 days when it is 100 feet in the air?
Understand pressure drops
Most every valve or other component produces a drop in pressure. You need to be aware of the cumulative pressure drop because otherwise you may end 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. A ball and needle valve of the same size will produce very different pressure drops. A ball valve will produce very little pressure drop, whereas a needle valve or other globe valve will produce significant pressure drop.
Consider cost of ownership
The true cost of a valve is not its purchase price. The true cost is the purchase price plus the cost of owning and maintaining or replacing that valve over time. To calculate the cost of ownership, you must know how long a valve will operate in your particular system between maintenance checks. Maintenance costs must be figured not only in replacement parts, but also in labor downtime. Note that some valves are much easier to service than others. Some can be serviced in place; others must be removed from the process line. Also, given your valve choice, what are the chances of unscheduled maintenance and downtime?
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