Reciprocating Compressors – Valve Unloaders

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Inlet valve unloaders are used to deactivate a cylinder end and reduce its capacity to zero. Two of the more common types of unloaders are depressor-type unloaders and plug-type unloaders. Depressor-type unloaders hold the inlet valve open during both the suction and discharge strokes so that all the gas is pushed back through the inlet valves on the discharge stroke. Plug-type unloaders open a port to bypass the inlet valve and connect the cylinder bore directly with the inlet gas passage.

Compressors may be set up with inlet valve unloaders to be used for both capacity control and reducing the compression load during starting and up-set conditions. For example, the capacity of a single stage compressor with two compressor cylinders may be reduced by 25% by unloading the outer end of one cylinder and by another 25% by unloading the outer end of the second cylinder. Unloading the frame ends of the cylinders at the same time that the outer ends are unloaded will reduce the flow to zero and is recommended only for start-up due to excessive heat build-up inside the cylinder after extended operation.

The compressor manufacturer must be consulted if the cylinder is to be run single acting with the frame end unloaded. Many times rod load reversal and proper lubrication may not be achieved while running single acting with the frame end unloaded.

When a cylinder end is deactivated, the pulsation levels in the piping system can increase significantly. If a cylinder may be operated with unloaders, it needs to be analoged in both its operating mode and its
unloaded mode.


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