Suppose the pilot pressure is from 3 to 15 psig for the illustrated pneumatically operated, diaphragm actuated control valve. Assuming the control valve is trimmed for a linear response and the travel position indicator is calibrated in percentage, what would be the approximate pilot pressure if the position indicator showed the valve to be 75% open? Illustration GS-0051
• Signal range of 3–15 psig means the valve does not start moving at 0 psig but at 3 psig • A linear trim/response means valve travel is directly proportional to the signal over the usable span • The usable span is the difference between upper and lower range limits (15 − 3)
• What fraction of the full 3–15 psig span corresponds to 75% valve travel? • After you find that fraction of the span in psi, what must you do with the lower range value (3 psig) to get the actual pilot pressure? • Look at the choices: which one matches a value that is offset above 3 psig rather than starting from 0?
• Calculate the span: 15 psig − 3 psig • Multiply the span by 0.75 (75%) • Add the lower range limit (3 psig) to that product before comparing to the answer choices
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