Suppose the pilot pressure range 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 valve position with a 9 psig pilot pressure? Illustration GS-0051
ā¢ Pilot pressure span from 3 to 15 psig (not from 0 psig) ā¢ How a linear valve trim means equal pressure changes give equal travel changes ā¢ Relating percentage of pressure span used to percentage of valve travel when the indicator is calibrated in percent
ā¢ First, calculate the total pilot pressure span between fully closed and fully open, based on the 3ā15 psig range. How many psig is that? ā¢ Next, find how far 9 psig is above the starting point of 3 psig. What fraction of the total span does that represent? ā¢ If valve travel is linear with pilot pressure, what does that same fraction represent in terms of percent valve opening?
ā¢ Be sure you subtract from 3 psig, not from zero, when finding how far along the range 9 psig is ā¢ Confirm that a linear response means a straight-line relationship between pilot pressure and valve travel ā¢ Match the fraction of span you calculated to the closest percentage in the answer choices
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