Reciprocating Compressors – Packing
Packing provides the dynamic seal between the cylinder and the piston rod. It consists of a series of Teflon rings mounted in a packing case, which is bolted to the cylinder. The piston rod moves in a reciprocating motion through this case. Figure 11-13 shows a typical packing case. The packing case is constructed of a number of pairs of rings, as shown in Figure 11-14.
The gas pressure is higher on one side of each ring. This compresses the rings against the sealing area. Each pair of rings consists of one radial cut ring and one tangential cut ring. The radial cut ring is installed toward the cylinder (pressure) side. Gas flows around the front face of the radial cut ring and then around the outside of both rings. Since the ring outside diameter is greater than the ring inside diameter, a squeezing force is exerted on the rod. This seals the path between the rings and the rod. The
radial cuts are positioned in the ring assembly so that they do not line up with the tangential cuts. Cylinder pressure will force the ring assembly against the packing case lip, thus preventing flow around the rings.
The amount of pressure differential one set of rings can withstand is limited. Therefore, several pairs must be installed to handle typical field gas compression applications. The basic design of the packing is left up to the manufacturer.
Lubrication is needed to reduce friction and provide cooling. Lubricating oil, which must be finely filtered to prevent grit from entering the case, is generally injected in the second ring assembly. The pressure differential moves the oil along the shaft.
A separate cooling system may be required for high-pressure service (5,000 psi) or where high compression ratios and long packing cases are installed.