Gases & Instrumentation - March/April 2008

fluctuations such as those caused by thermal expansion. A powered solenoid valve with a low differential pressure producing relatively low flow will have a greater internal temperature rise than the same valve with a higher pressure differential and higher flow. The larger resultant flow rates actually cool the internal components of the valve. At a given power setting, seals in a valve without internal thermal compensation will expand over time and as temperature increases. As the seal expands, flow is reduced as the cylindrical area above the orifice decreases (Figures 2 and 3).

that the orifice is subject to thermal expansion that will affect performance.

Flow is proportional to the cylindrical area above the orifice (= Π x d x h). This area is referred to as the effective metering orifice, and flow stays proportional until the cylindrical area becomes greater than the machined orifice or other limiting restriction. Valves that incorporate thermal compensation features can reduce seal expansion and control the area above the orifice, maintaining repeatability and minimizing hysteresis.

Closed Loop Feedback Does Not Solve All Ills

Even in a closed loop system, the thermal expansion of the valve can be greater than the ability of the control circuit to correct for it.

In a miniature valve with a relatively large orifice, such as 0.065 inches, thermal expansion comprises a small percentage of the total flow. Compensation is usually unnecessary in this instance. In small orifice valves, however, such as in the 0.003 to 0.030 inch

Figure 2. Valve in open state

Figure 3. Heat swollen elastomer reducing flow

Internal Thermal Compensation a Must

Internal thermal compensation should be a foremost requirement when selecting proportional solenoid valves. In typical “spider spring” poppet-style proportional valves born of digital valve heritage, published repeatability and hysteresis numbers are often incorrectly related to the accuracy of the machined orifice. Engineers must realize

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