Liquid slugging can damage the compressor by washing oil out of the crankcase or entering the cylinder, trying to compress liquid can blow the cylinder head.
To protect the compressor from liquid sludges certain system has safety heads fitted.
The safety head has an internal discharge valve plate at the top of the cylinder directly in front of the discharge pipe. This discharge valve plate lifts, making the full opening of the discharge pipe accessible to the cylinder.
The safety beads are spring-loaded to remain in a sealed position for all normal operating pressure but to open in the case liquid entered the cylinder, the valve location is usually in the cylinder head or in the Cylinder walls.
The moving valve plate will not open unless hydrostatic pressure builds the cylinder, it is head down by an up in heavy helical spring.
If the liquid is trapped in the cylinder and the piston starts its compression stroke, the liquid will push solidly against the safety head. Since liquids do not compress, the safety head allows any liquid to be expelled from the discharge, without physical damage to the compressor.
As a piston starts to come back down, the helical spring forces the safety head back into position.
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