Modeling methods for perfecting sealing units of piston compressors at the design stage

Abstract

Efficiency, reliability, and operating life of piston compressors depend on the perfection of designs of their constituent units, distinctive amongst which are seals of pistons and glands operating with high and time-variable pressure differentials.Sealing units must meet two basic requirements:• leak-tightness, which is defined by the permissible (safe) degree of gas leakage through the sealing unit under the compressor stage operating conditions set at the design stage; and • operating life, which is characterized by the continuation of operation of the sealing unit with no change in the technical parameters of the compressor stage (efficiency, power consumed, temperature, etc.) at the level as demanded by the operating conditions. The leak-tightness of the seal with conventional split or ganged piston rings at a known maximum pressure differential ∆p on the sealing unit depends on a host of design parameters: number of rings z r and their cross-sectional area b r h r , interring spaces that depend on the dimensions of the pieces connecting the rings or on the ring spacing t r , radial clearance in the piston-cylinder pair δ p-c , running clearance in the ring lock ∆ l , presence or absence of lubricant, etc. To a first approximation, the required number of rings in the sealing unit of the piston can be determined by the proposed empirical relationship √  5∆p ≤ (z r -1) ≤ √  10∆p.(1)The operating life of the sealing unit depends on the materials of the piston-cylinder friction pair, speed of motion of the piston, roughness of the contact surfaces of the friction elements, cooling conditions, overall pressure differential ∆p in the seal as well as on the pressure distribution across the elements of the sealing unit. In medium-and high-pressure stages, the last of the referred factors determine to a large measure the rate of wear of the piston rings and the scheduled time for their replacement.In special-purpose compressors with one-way acting (unidirectional) pistons, the overall pressure differential in the sealing unit of high-pressure stages attains 20-40 MPa, which, in accord with Eq. (1), increases the number of sealing rings to be installed on the piston to 11 ≤ z r ≤ 21 and, consequently, increases the axial dimensions of the cylinders.Experience of designing, computational analysis, experimental studies [1-3], and industrial use of sealing units of domestic piston compressors shows that excessive increase in the number of sealing rings does not reduce external leaking much and does not facilitate uniform distribution of pressure across the rings of the sealing unit.

Experimental study on frictional and sealing performances of packing rings in an oil-free gas compressor

“…There are many studies on self-lubricated sealing rings from theoretical [12][13] or experimental [8][9] viewpoints for the preceding details. However, some aspects of tribological and sealing properties still require further investigation, especially for the running process of packing rings under oil-free lubricated conditions.…”