When the transfer pump is operating and all transfer valves are open, if the supply tank and pump inlet pressures remain the same; the pump outlet pressure rises to its normal lever or above; and meter pressure is substantially above supply tank pressure, but there is no flow, the condition is probably due to a defect in the vapor eliminator or the ________.

• A. Differential Valve
• B. Check valve
• C. Pressure switch
• D. Regulator

Answer: (A)

When a transfer pump runs with all valves open but no liquid is moving, look for a restriction downstream of the pump rather than a problem at the source. In this scenario, the pump is able to raise outlet pressure, and the meter reads a pressure well above the supply, yet there is no flow. That combination means the pump is building pressure but cannot push liquid through the downstream path.

The vapor eliminator is part of that downstream path, and its operation depends on a differential valve to maintain the proper pressure difference across it. If the vapor eliminator or the differential valve is defective, the pathway for liquid can be blocked despite the pump’s pressure rise. The result is high outlet and meter pressures with zero flow because something in the downstream control-portion of the system isn’t allowing liquid to pass.

Other components described wouldn’t produce this exact symptom pattern. A check valve failing closed could create backpressure but usually shows different behavior; a pressure switch is an electrical control device and doesn’t by itself create a mechanical no-flow condition; a regulator would alter downstream pressure rather than block flow while leaving upstream pressures elevated.