Valve seating and bearing materials can cause many problems in gaseous and
seawater systems. Oxygen systems in particular require a careful selection
of such materials. Some design guidance on material compatibility is given
in the subsection entitled Fluid Compatibility. Guidance can often be
obtained from a valve manufacturer. The piping designer should obtain an
accurate description of every material in any valve, including lubricants in
an oxygen system or in any system which carries gas to the chamber. Suitable
stress values must be selected for the pressure-retaining materials, and
compatibility checks should be made for all materials in the valve.
(3) Pipe Joint Seals. MIL-T-27730 is a specification for
tetrafluoroethylene tape and for its use as an antiseize and sealant of pipe
threads for liquid and gaseous oxygen systems of 2000 psi and less. The use
of this tape is widespread and its use is suitable as an antiseize and
sealant material in any piping system for a hyperbaric chamber. The use of
other pipe thread sealant compounds requires a compatibility analysis and
approval for the proposed system.
(4) Straight Thread Tube Seals. MS-33649 and MS-16142 give boss
dimensions for military straight thread tube fitting O-ring gaskets. Seal
manufacturers can supply O-ring compounds of several types for use with the
dimensions. Elastomeric materials are available that are compatible with all
of the candidate fluids except oxygen. For Oxygen systems, Viton and Kel-F
are preferred.
CAUTION:
Teflon O-rings have shown a significant tendency
to leak because of cold flow of the material.
(5) Flanged Joint Seals. Considerable experience has been gained
concerning the use of flat gaskets, O-rings, and metallic rings as seals for
pipe flanges. Chapter 12 of Survey Report on Structural Design of Piping
Systems and Components, Rodebaugh and Pickett, (Reference 19) presents a good
summary of this experience. The ASME Code described standardized procedures
for designing flanged joints and these procedures are discussed in Reference
19. Considerable help can also be gained from manufacturers of pipe fittings
and seals concerning the proper joint design and installation procedures for
various fluid systems.
c.
Operations and Maintenance. The plumbing of a piping system for
easy operation of hand operated valves is a major piping consideration
leading to the use of a number of fittings and inclusion of gages and flow
meters. Such additional complexities are almost always identified in the
overall systems analysis and it is thus important that a thorough system
specification be available.
Provisions for maintenance stem from an analysis of potential malfunctions in
the piping system. Moving parts in valves must be examined and replaced
periodically, and gages and meters must be calibrated. The piping layout
must include provision for the maintenance of such components.
d.
Draining and Cleaning. Flasks and dead-end sections of piping in
breathing gas systems should be provided with drains.
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