Reciprocating Packing Tips
Pressure packing rings are contained in a packing case that is installed in the crank end of a reciprocating cylinder. This case will have a series of cups that provide a sealing surface for the rings during their controlled ride on the piston rod.
The modern packing rings are made of segments that are held together by a coil spring on the outside diameter of the ring. The spring will hold the packing ring against the piston rod in the absence of pressure.
The sealing aspect of a packing ring is activated by the compression process. The pressure generated in the cylinder will hold the rings against the sealing surface of the packing cups, and against the diameter of the piston rod.
1) Always clean and repair the packing case when changing packing rings, and avoid the temptation to replace packing rings without removing the case. Also, inspect the condition of the piston rod, and take corrective action for scratches or defects that can cause leakage.
2) The cups must be parallel flat with lapped sealing surfaces. The rim of each cup and the back of each cup are the surfaces requiring a lapped finish.
3) The internal dimensions of each packing cup was designed with clearances to match the application. Clearances are maintained by restoring cups to their original Cup ID and Cup Depth.
Excessive side clearance can cause the packing ring to wear out faster or break and cause damage. If there is not enough side clearance, the packing ring can stick, fail to seal and cause damage. So, make sure you know the cup specifications before doing packing inspections or repairs.
4) The overall packing case length varies, but most cases use a gasket between the nose cup and the cylinder. You should be careful to keep the length of the case at a dimension that will continue to compress the packing case gasket.
5) The rings will have match marks stenciled into each segment. This keeps the assembly consistent with their placement during the manufacturing process.
Always assemble the segmented rings according to these markings, and install the rings so the markings will face the pressure.
6) Radial cut packing rings are designed to be installed towards the pressure when used in a cup with other types of packing rings. The exception would be when a cup has a tangent to the rod ring with a radial back up ring.
7) Always protect the packing, and keep it clean, during installation. Leaks can develop from minor scratches on the rings.
8) A rough finish on the piston rod can affect the service life of the packing rings. However, non metallic rings can leak when the finish is too smooth to allow the rings to grip the rod.
9) If the cylinder is in a non lubed service, worn rider rings on the piston can cause the rod to drop which will cause the rod to scrape against the packing case. This damage on the piston rod will often create a leak path through the packing rings.
10) Save money on packing rings. Shop around when buying packing and piston rings. The OEM and the OEM Distributors are not likely to make the packing you buy, and you can save money if you buy direct from a manufacturer that understands packing and piston rings.
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