(3) The threaded cap closure seal (Figure 3-29) is inexpensive and
mechanically simple. Use of an O-ring seal provides automatic radial
compression, hence the cap does not have to be tightened to provide the
preload to the seal, and a large spanner wrench is not required for removal
and installation. Close tolerances must be held to secure the proper
compression of the O-ring, and the chances of seal damage during closure are
greater than for designs which do not involve sliding against the sealing
element.
(4) Threaded Plug Closure Seal. A relatively simple closure which
is widely used is the threaded plug. It is readily available, easy to make
and can be seal-welded if necessary. A plug in a high pressure gas cylinder
is shown at Figure 3-30. Note the use of a seal weld to keep leakage to a
minimum.
(5) Flat Face O-ring Seal.
A closure or seal which is used
extensively in piping systems is the
flat face O-ring seal, shown at Figure
3-31. The ring groove is cut in one
of two mating flat faces and the
assembly is held together by a union
nut. A properly made closure of this
type will easily seal 2,000 psi when
made up hand tight due to the sealing
action of the O-ring.
(6) Large Diameter Closure Seal. When large diameter closures are
subjected to high pressures there can be considerable movement of the various
parts. Under these conditions, the mating clearances can sometimes increase
to a gap which defeats the sealing action of the O-ring. To overcome this
problem, a Floating Ring Seal is sometimes used. See Figure 3-32. The
internal pressure lifts and stretches the metal ring carrying the O-rings and
effectively keeps the clearances to zero. A seal of this type is used on the
fifteen foot door of the Navy's Ocean Simulation Facility at Panama City,
Florida.
(7) Ring Sealed Penetrator Plug. A very convenient way to provide
a penetration in a pressure vessel is to use a ring sealed plug, as shown at
Figure 3-33. The advantage of this device is that a smoothly reamed hole is
all that is needed in the pressure vessel. A spotface on the inside or
outside of the vessel shell is not required for sealing and the reamed hole
can be made by hand. It is also very easy to change the plug if it becomes
damaged or to change the use of the penetration.
b. Door Seals for Compression Chambers.
(1) The most common seal for chamber doors is the rectangular
Neoprene
packing. It is usually 3/4 x 1 inch and is retained in a groove
which is
slightly narrower at the top than the bottom. The door has a lip
which is
forced into the packing by chamber pressure. A seal of this type is
shown in
Figure 3-18.
(2) Coming into use for chamber door seals is the O-ring, but it is
used in a non-standard groove. Normally, an O-ring is held in a rectangular
groove with about 15% compression in one direction and with space for lateral
movement of at least 50% of the ring section, at 900 to the direction of
compression. Under pressure the ring moves to one end of the groove and the
sealing action of the groove is activated by the retained pressure.
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