Solenoid Valve Selection: Don't Be Fooled by Flow Rate

Flow rate values might not mean what you think they do. Applying ISA's two-coefficient formula can help optimize selection.

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Solenoid valves are widely used throughout the chemical processing industries. However, selecting the best valve for a given application can be difficult. Flow rate, a critical parameter for selection, can be calculated using various approaches. However, many of these methods are imprecise or yield non-dimensional results.

Complicating the situation is the fact that there is no single industry-accepted method for calculating gas flow rates for solenoid valves. Some valve manufacturers experiment with multiple formulas to determine which one will result in the most favorable- looking values for their valves. This practice can confuse end users, wasting time and money and compromising safety and efficiency when the wrong valve is specified.

This article summarizes the key issues involved in selecting solenoid valves for chemical processing applications, and discusses their flow properties and the methods most often used to calculate gas flow rate. In addition, it recommends a two-coefficient formula developed by ISA (Instrumentation, Systems and Automation Society), and shows how to use it to determine the best solenoid valve.

Solenoid valves 101

It is important to point out that in chemical processing applications, solenoid valves typically do not handle the medium directly. In many cases, the valve acts as a safety shutoff device for a larger system. Performance in this capacity is critical. A valve might be in the system but unused for long periods of time, even up to a year. When it is called upon to shut down the system, it must act immediately and predictably.

Solenoid valves use electricity to open and close an orifice in the valve body, allowing or preventing the flow of a given medium. In its simplest form, this type of valve is opened and closed by a plunger that is raised and lowered by the energizing and de-energizing of a solenoid. The magnetic field created by the solenoid's coil turns the plunger's stop into an electromagnet, attracting and raising the steel plunger. A corresponding spring compresses as the orifice opens. Upon de-energizing, the magnetic field is discontinued and the spring returns, forcing the plunger back onto the orifice and shutting off flow.

Solenoid valves can also be used to control or actuate other processes. For example, they can control the movement of other control devices such as large process valves that manage the application's media, or energize components such as desiccant dryers or venting systems, which require air pressure.

The following must be considered when selecting a solenoid valve:

  • Valve type (two-, three-or four-way)
  • Voltage
  • Pressure
  • Medium handled
  • Materials (stainless steel, brass, plastic, or other material)
  • Port size fluid connection
  • De-energized position (normally open or closed, etc.)
  • Ambient conditions
  • Approval(s) required (UL, CSA, FM, CENELEC)
  • Coil/connection type
  • Current draw limitations
  • Duty cycle (continuous or intermittent)
  • Electrical enclosure type
  • Life expectancy (minimum expected cycles per year)
  • Maximum and minimum operating pressure differentials
  • Orifice diameter
  • Allowable internal leakage
  • Ratings (NEMA 4, 7, 9, etc.)

    Finally, there is flow rate to consider.

    Ups and downs of gas flows

    Let's review the basics. Flow rate is the quantity of a medium that passes at a given point during a given period of time. It is measured using the term CV, which represents the quantity of water in gallons per minute that will pass through a valve with a 1 psi (pounds per square inch) pressure drop across it. This measure is used to determine a valve's capacity at higher pressure drops, but also applies to both liquid and gas flows at pressure drops small enough that density changes are negligible.

    Another, more accurate term for gas flow is "compressible fluid flow." Compressibility refers to a gas' ability to change density with temperature and pressure changes. Flow rates for compressible fluids are significantly more difficult to calculate than those of incompressible fluids, simply because of their properties.

    The density of a compressible fluid flow fluctuates a great deal at higher temperatures and, especially, pressures. At lower pressures, the behavior of water, steam or inert gases may be very similar, but it changes wildly for gases when extrapolated to high pressure drops.

    In cases in which a flow becomes compressible within the valve, effects might include cavitation, flashing or Mach number effects, making consistency in calculations even more difficult.

    Flow path within the valve also can change the CV value (see Fig. 1). A straighter, more direct path means that the medium will have fewer fluid stresses applied to it as it passes through the valve. A tortuous (or convoluted) path will have the opposite effect. In fact, two solenoid valves that are the same in every way except internal construction and flow path can have completely different CV ratings.

    Figure 1. Flow Path Determines Rate

     

    Flow rate in solenoid valves can be affected by a multitude of factors, including internal construction. If a valve's flow path is uncomplicated (top), the medium will flow through it without stress. A tortuous flow path (bottom), as with this five-port valve, will result in a lower flow rate due to greater stress on the medium as it travels through the valve.

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