Experience With Piston and Piston-Rod Seals for High-Pressure Reciprocating Compressors Injecting Natural Gas on North Sea Production Platforms

Abstract

Summary The need to restrict flaring of associated natural gas has emphasized the importance of reliability of high-pressure reinjection compressors installed on certain North Sea platforms. This paper reviews the contribution made by the development of piston and rod seals used on injection compressors, with reference to three important fields: Beryl, Statfjord, and Thistle. Early North Sea Reinjection Experience During 1977 the first associated natural gas was reinjected in the U.K. oil sector. This operation is performed by two-stage reciprocating compressors. Early experience with these compressors revealed that the piston and rod seals. which are subjected to pressures approaching 400 bar [40 MPa], were a major source of machine failures. These failures consisted of severe scuffing and wear between the bronze sealing elements (piston and packing rings) and the countersurface (liners and piston rods), causing erratic seal life, sometimes of only a few hours' duration, together with badly damaged liners and piston rods. The typical condition of used bronze piston rings is illustrated in Fig. 1, and the operating lives achieved with bronze seals are summarized in Table 1. The most serious consequence of this unreliability was its effect on continuity of oil production, and an immediate development program to eliminate such failures was begun by the platform operator in collaboration with the appointed seal manufacturer. Details of this development are presented, including an analysis of the causes of seal failures and measures taken to remedy them, with particular attention to seal material and design and to lubrication requirements. Field experience gained in the application of the modified seals is summarized in three case histories. details of which arc given in Tables 2 through 4, with comparative results listed in Table 5. Selection of Seal Material At the outset of this development program, a change to nonmetallic seal materials for both the piston rings and the rod packings was considered on the basis of a proprietary grade of filled polytetrafluoroethylene (PTFE), which had already been used successfully for packings and piston rings in various lubricated compressor installations, includingethylene primary compression to 350 bar [35 MPa],ammonia synthesis gas compression to 350 bar [35 MPa], andhydrogen compression in refinery service to 200 bar [20 MPa]. JPT P. 2294^