Charlie Bald
Senior Development Engineer
Fluid Control Div.
Parker Hannifin Corp.
New Britain, Conn.

Selecting solenoid and control valves seems like a straightforward process. Armed with orifice size or pressure differential, engineers usually choose the smallest or least-expensive valve. Little do they realize that competing valves often have different flow rates and pressure ratings, despite sharing similar body and port dimensions.

The fact is, orifice size and pressure both affect flow rate. Therefore, system designers should first establish the required flow rate. From there, investigate prospective valves that satisfy the ratings, then narrow the search to ones that best match application and cost requirements. This should give the most appropriate valve and help avoid costly repairs and replacements down the road.

Valve selection
Historically, designers tend to select valves based on port or orifice size. It’s easy to peruse manufacturers’ catalogs using orifice size as a point of reference. And it typically finds an inexpensive offering: the smaller the valve, the more attractive it is to purchasing managers.

But this approach is problematic for two reasons. First, catalogs have limited space and only show ratings for a few applications. Second, internal construction affects valve performance, so orifice size may not be the best indicator for a suitable valve.

Orifice size alone does not necessarily describe flow capacity because some flow paths are more efficient than others. A better approach sizes valves based on the empirical flow coefficient, Cv, which accounts for flow rate, pressure drop, and fluid properties.

It’s no secret that two different valves can have the same size orifice yet quite different flow rates and Cvs. Internal features such as solenoid stroke, springs, plunger geometry, orifice configuration, and flow passages combine to determine a valve’s flow rate and Cv. And valves with different internal geometries likely have different flow rates.

To demonstrate the point, Parker conducted a performance-based study last year to compare its 204/304 Series valves with versions offered by other manufacturers. In all, we collected more then 70 different valves and conducted 101 tests against 17 features, among them flow coefficient.

One finding was that different-size valves can have the same flow rates. This reinforces the “buyer-beware” aspect of valve selection — don’t overpay for large but poorly performing valves.

We also learned that pressure ratings at 68°F greatly exceeded ratings listed in manufacturers’ catalogs. In turn, different pressure ratings corresponded to different coil wattages. An accompanying chart lists catalog and actual flow rates for valves with 1/8-in. orifices. It shows that catalog ratings may list nominal flow rates but, when tested, values can differ widely, despite similar design features.

Calculating flow coefficients
To simplify sizing, calculate the desired flow coefficient first and then work with valve manufacturers to complete the selection process. Flow coefficient for 60°F water and a 1-psi pressure drop is defined as:

Cv = Q (ΔP/S)0.5

where Q = flow capacity, gpm; S = specific gravity (water = 1); and ΔP = pressure drop, psi.

Regression analysis of manufacturers’ catalogs shows that Cv is roughly proportional to the square of orifice diameter. This is especially true for small two-way, direct-acting valves.

The resulting flow curves shed light on Cv differences for the same orifice diameters. For example, the “Rating valves” graph shows various flow rates and diameters for same-size valves with large ac coils. Notice how Cv varies for a given orifice size.

Based on this understanding, engineers can customize valves for specific applications. For instance, using a higher-powered coil in a large valve can boost the overall pressure rating, despite the listed orifice size or flow coefficient.

To illustrate the point, say an application requires a two-way, normally closed valve with a 0.28 Cv. One can start with a valve with a 1/8-in. diameter orifice, 11-W coil, and 200-psig pressure rating. Substituting a 22-W ac coil, however, creates a harder-driving valve with a 520-psig pressure rating, but still with a 0.28 Cv.

Occasionally, valve manufacturers use short stroking to give smaller valves higher pressure ratings. A good rule of thumb is for valve stroke to be one-quarter of the orifice diameter. This approach delivers a better flow coefficient for a given orifice size.

For example, a valve with a 0.125-in. orifice diameter would typically need a 0.031-in. stroke. Designing the same valve with a 0.016-in. stroke may improve its pressure rating. In side-by-side tests, however, the larger stroke offers fewer flow restrictions inside the valve. Ideally, small valves should meet high-pressure requirements without short stroking.

Design check list
Users often begin the selection process knowing little more than orifice size and pressure differential. But the best approach is to consider six key factors:

• Flow coefficient.
• Pressure rating.
• Port size and type.
• Valve type, such as two, three, or fourway operation.
• Coil wattage (preferred or limits) for the application.
• Ambient and media temperatures.

Refine the selection process by considering the following additional criteria:

Service and cycling data. This includes the valve’s operating speed, life expectancy, time energized and de-energized, as well as normal, fast, or continuous cycling.

Position. Valves can be normally open or closed, or require directional control.

Electrical considerations. Specify the valve’s voltage and frequency, and whether it uses ac or dc power. Also note the electrical connections, such as pipe thread, grommet, AN or DIN connector, strain release or auto terminal.

Fluid medium. This includes the fluid’s temperature, viscosity, aggressiveness, physical state, chemistry, and contamination (particularly if there are foreign bodies in the fluid).

Pressure. Indicate if the valve requires a maximum, differential, or back pressure.

Ambient conditions. Specify humidity, temperature, and elemental exposure for the application.

Vibration and shock. Note if the valve will be subject to external forces, vibrations, or shock loads.

Valve manufacturers can offer an endless array of creative designs — far too many to fit within the pages of a catalog. Orifice size may seem to be a good starting point but many other factors are involved with valve selection. Desired flow rate is actually a better starting point for opening a dialog with manufacturers to determine the best valves. It may take a little extra time, but it helps reduce unnecessary and costly repairs and replacements down the road.

Make Contact:
Parker Hannifin, Fluid Control Div., parker.com/fcd