Back pressure prevention rings

Fig. 7.5 Details of piston seals (a) a piston honed to a mirror finish that is accurately cylindrical and machined to an interference fit to the cylinder, (b) a A ring of bronze or brass, fitting the chamfer of the piston and precisely fitting the diameter of the cylinder (this ring will deform somewhat to create the high-pressure seal above pressures of about 5 kbar), (c) a rubber O-ring which can seal pressures below 5 kbar (above this pressure elastomers tend to become brittle), (d) a backing ring of PTFE, (e) a circlip to prevent rings falling off when the piston is extracted, (f) an accurately machined 45° chamfer.

Shims Shims are used to prevent modules from moving back and forth during pressurization and depressurization. Such movement could wear on the internal O-ring seals. Shims are plastic spacer rings similar to washers. They are typically 0.20-inches thick, and can be purchased from the manufacturer of the pressure vessel or fashioned from polyvinylchloride (PVC) pipe. Shims fashioned from PVC pipe must be cut parallel and free of burrs to work correctly. They are installed between the face of the lead membrane module and the adapter hub (see Figure 6.16) after all the... 

In some oil field applications (deep wells), where temperatures approach 500°F (260°C) and pressures of 20 000 psi are reached, it would be useful to have strain crystallizing rubber seals made out of natural or neoprene rubber. At these pressures, however, the extrusion gap must be closed and the rubber seal must be protected by a series of harder and high modulus rubber back-up rings. Properly designed, this progression of material prevents extrusion, tearing and cutting or other destructive deformation of the rubber seal and distributes loads more uniformly. 

Shims Shims are used to prevent modules from moving back and forth during pressurization and depressurization. Such movement could wear on the internal O-ring seals. Shims are plastic spacer rings similar to washers. They are typically 0.20-inches thick, and can be purchased... 

The flange-spreader tool (Fig. 7-21) is required in remote operations to spread low-pressure valve flanges to prevent damage to the ring gasket during removal or insertion of the valves. The tool is shown in place with the flanges spread and the valve assembly moved back on its cell fixture. 

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