APCO Double-Acting Throttling Device (DAT)



APCO Double-Acting Throttling Device (DAT)

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Throttling devices have been used on vertical turbine pumps for decades. Old style, one-way throttling devices are only able to regulate flow to restrict air out. Restricting air out helps minimize shock with each pump start, preventing damage to the pump, valves and piping. This establishes back pressure on the rising column of water, thereby reducing shock, pressure surge and water hammer when the water column hits the closed pump discharge check valve.

However, on pump stop, the pump discharge check valve closes and the vertical turbine air valve opens to allow air to re-enter the pump column to prevent vacuum and/or water column separation. Because an old-style, one-way throttling device restricted air re-entry at the same rate as air out, vacuum and water column separation can occur, damaging pump seals, packing, and gasketed joints. Additionally, the pump may be called to start while the column is still full, putting a severe strain on the pump shaft, motors and electrical controls.

Operation of Double Acting Throttling Device

The PTFE tapered plug of the throttling device is pushed ahead into the throttle position by a very light-weight, stainless steel spring. The plug stays in this position due to the force of air being discharged from the pump suction on pump start. On pump stop, the tapered plug moves from its throttle air-out position to the full-air-in position due to negative pressure inside and atmospheric forces of air rushing into the valve.

The desired tapered plug position for throttling air out is regulated by turning the stainless steel adjusting screw. When satisfactory throttling of air is achieved, the adjusting screw is locked in place with the stainless steel lock nut.


0.5-8” (15-200mm)


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