There is a right way and a wrong way to stop an AODD pump. Most operators discover the wrong way through a diaphragm failure — and because the failure happens some time after the bad shutdown practice, the connection isn’t always made. The Fluimac Phoenix manual specifies a precise stop sequence, and deviating from it has direct consequences for diaphragm life and pump integrity.
The Wrong Way: Closing the Fluid Valve First
The most common incorrect stop procedure is closing the downstream fluid valve while the pump is still running. This creates a deadhead condition — the pump continues to stroke against a closed valve. With every stroke, the pump builds pressure in the discharge manifold until either the air supply pressure is overcome and the pump stalls, or the pump continues to cycle at very high discharge pressure.
When the pump stalls at deadhead with the diaphragm extended on the discharge side, all of that discharge pressure is applied across the diaphragm membrane in a static, sustained load. This is different from the cyclic loading during normal operation. The diaphragm is held at full extension under pressure, which stresses the flex zone in a way that promotes creep and delamination — particularly in PTFE-faced diaphragms where the PTFE layer can separate from the backing elastomer under sustained load.
The worst outcome is a stall at full discharge pressure with the pump left in this condition for an extended period — overnight shutdowns where someone “stopped” the pump by closing the outlet valve and walking away are a common cause of premature diaphragm failure.
The Correct Stop Sequence
The Phoenix manual specifies the correct stop procedure as follows:
Step 1 — Close the air supply valve. Shut off the compressed air supply to the pump first. The pump will complete its current stroke and stop. At this point, the pump is no longer generating discharge pressure.
Step 2 — Close the suction valve. Once the pump has stopped, close the suction-side isolation valve to prevent backflow from the suction source into the pump body.
Step 3 — Close the discharge valve. Close the discharge isolation valve last. At this point, the pump is isolated on both sides with no trapped pressure load.
This sequence ensures the pump stops under its own inertia, with no pressure load applied to the diaphragm by a closed downstream valve.
Stopping for Short Periods
For brief interruptions — shift changes, line adjustments, short maintenance windows — the minimum acceptable stop is to close the air supply. The fluid valves can remain open if the pump will restart shortly and if backflow from the system into the pump is not a concern. If the suction source is under positive pressure (flooded suction, elevated tank), close the suction valve to prevent the pump from filling up and siphoning during the stop.
Emergency Stops
In an emergency requiring immediate pump shutdown — chemical spill, diaphragm failure, process upset — close the air supply first. If the failure has already caused process fluid to enter the air circuit (diaphragm failure symptom: fluid exiting the air exhaust), do not attempt to restart the pump. Isolate both fluid valves and the air supply, then assess whether the diaphragm has failed and whether the pneumatic exchanger has been contaminated.
Long-Term Shutdown and Storage
For shutdowns longer than a few hours, or where the pump will be stored, the Phoenix manual requires the pump to be flushed with a compatible neutral liquid to remove process fluid from the wetted path. Leaving corrosive or crystallising chemicals in the pump during a shutdown causes chemical attack on wetted components, deposits that foul ball seats on restart, and in cold climates, possible freeze damage to absorbed moisture in the diaphragm elastomer.
After flushing, discharge the air supply to atmospheric pressure and store the pump in its normal orientation. For ATEX Zone 1 pumps, confirm the pump is fully de-energised (air supply closed and bled) before any maintenance work begins on the fluid circuit.
If you need guidance on commissioning and shutdown procedures for a Fluimac Phoenix installation, contact us at info@autoflotechnology.com.