Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve HEAT EXCHANGER FORMULAS & EXAMPLE
H) What is the maximum close-off pressure of the control valve selected in Part G?
From the HB Control Valve Selection chart, the maximum close-off pressure for the selected valve is 85 PSI ΔP if no positioner is used, and 135 PSI ΔP if a positioner is used.
I) For the selected control valve, is a positioner required to completely shut off the flow of steam to the heat exchanger?
When the control valve is completely closed, the pressure drop across the valve is at its maximum value:
Δ P MAX = Steam Supply Pressure – Heat Exchanger Pressure = 100 PSIG – 0 PSIG = 100 PSI
Thus, the control valve must have a close-off pressure capability of at least 100 PSI. Without a positioner, the maximum close-off pressure of the valve is 85 PSI. Therefore, a valve positioner is necessary to provide the required closing force to the actuator diaphragm. In a normally-closed valve, the valve is held closed by a spring force. The spring pressure is set so that the valve will stay closed against an inlet pressure of 85 PSIG. The opening action is performed by the 3-15 PSIG air signal to the actuator diaphragm. When the air signal is 15 PSIG, the valve will completely open against the spring pressure. When the air signal is 3 PSIG, the valve will stay closed provided that the inlet pressure does not exceed 85 PSIG. If the inlet pressure exceeds 85 PSIG, the valve will open and a positioner will then be required to decrease the air signal pressure below 3 PSIG to allow the valve to fully close.
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