Deformation of nonmetallic sealing rings in compressors and expansion machines

Abstract

A promising line for upgrading compressors and expansion machines is the use of unlubricated cylinder-piston units, and sometimes other parts of the moving mechanism. This raises a problem over efficient and long-lived seals, which are made of nonmetallic materials having low elastic moduli E.Calculations and test results show that the maximum pressure differences across a single sealing ring are Ap < 3 MPa in the various stages, which means that the low elastic modulus implies that the rings may be deformed.When the rings deform, there are changes in the parameters of the slots between the surfaces through which compressed gas flows: piston ring-cylinder or piston-ring-end surface of piston groove.In general, gas leaks through the seal on three channels (Fig. 1, in which Pl and P2 are the pressures at the inlet and outlet from the slot):-through the ring section fr = ff'Dc~n between the cylinder 3 and the sealing ring 2 with a nominal gap in the slot ~n = ~r; -through the end diffuser channel fe = ~Z(Dc -bg)Sn between the ring and the groove in the piston I with nominal gap in the slot 5 n = ~e; and -through the rectangular section in the sealing ring joint fj [1]. Calculations on the flow rate through the shot usually involve replacing these channels by a single section subject to the condmon f = fr + fe +fj =/r'Dc~n" The nominal equivalent gap 5 n is determined by experiment. To simplify the calculations, one usually assumes a linear variation in the pressure along this channel, which has a constant area of cross section f. The condition f = constant essentially establishes that the gap 5 n is independent of the pressure pattern in the gas flow region. That assumption is correct for metal rings 9 From [2], the effects of the ring height h and gap 5 n on the vertical distribution over the ring are negligible, but when one uses nonmetallic rings, one has to assume that ~n = f(A'), and consequently that the area of the annular slot f* varies over the height of the ring, which may have a substantial effect on the pressure distribution over the height of the ring [2], and also on the actual gas flow rate through the slot and the type of wear on the sealing rings. In the installed state (Fig. 2a), the cross section of the sealing ring is a rectangle of dimensions b x h; with a uniform pressure p on all the faces of the ring, which has previously been pressed on to the cylinder by the expander, the equivalent radial gap ~n may be taken as constant over the height h of the ring. When a compressor or expansion machine operates, there is a gas leak through f*, and the load distribution on the ring corresponds to that shown in Fig. 2b, which we assume for determining the deformation of nonmetallic sealing rings.In the elastic strain range [1, 3], the forces act independently, and the strain in the piston ring from the various factors is equal to the sum of the strains from each of them separately. We estimate the dependence of the sealing ring strain on the gas pressure difference Ap. The ring is firmly pressed on to the...