'O' Ring Guide

Materials & properties
Rapid gas decompression
Although rapid gas decompression
(RGD) — previously known as explosive
decompression (ED) — is a phenomenon
generally found in the oil and gas industry,
it can occur in any application where there
is a rapid drop in gas pressure. Such
damage is found in sealing applications
ranging from paint guns and fire
extinguishers to marine stern glands and
systems containing refrigerants.
How damage occurs
RGD damage is structural failure in the
forms of blistering, internal cracking and
splits, caused when the gas pressure, to
which a seal is exposed, rapidly reduces
from high to low. Although no strict rules
apply, damage should be considered
in a gas or dissolved gas system when
pressure is greater than 5MPa (725psi),
and decompression exceeds 1MPa
(145psi) per hour.
The elastomeric parts in a system are, to
a greater or lesser degree, susceptible
to the permeation and diffusion of gases
dissolving in their surface. In time, the
elastomer becomes saturated with gases.
Under these conditions — as long as the
gas pressure in the elastomer remains
at equilibrium with the ambient pressure
— there is minimal, if any, damage. Thus,
no deterioration in performance of the
elastomeric part occurs (unless caused
by other factors, eg chemical or thermal
degradation or extrusion damage).
When external gas pressure is removed
or pressure fluctuations occur, large
pressure gradients are created between
the interior and surface of the component.
This pressure differential may be balanced
by the gas diffusing /permeating out of
the elastomer, especially if any external
constraints are not removed.
But, if the elastomer cannot resist crack
or blister growth during the permeation
process, then structural failure will result.
Rapid gas decompression damage can
manifest itself in various ways — anything
from internal splits that are not visible on
the surface of the seal, to surface blisters,
fractures and complete fragmentation.
Leader in RGD-resistant elastomers
We have conducted intensive materials
development programmes over the past
30 years to help industry overcome RGD
problems. Much of this work is carried out
in collaboration with plant manufacturers,
oil/gas operators and research bodies.
We offer RGD resistant elastomers,
which are validated by James Walker
Technology Centre. Specific grades are
tested and approved by oilfield operators
and equipment manufacturers, with several
qualified to Norsok M-710 Annex B.
The formulation, mixing, quality control
and processing of these compounds
is rigorously controlled. Today they are
rated as benchmarks by which others
are judged. Albeit each compound has
a broad range of applications capability,
their particular features are as follows:
l FR68/90 — this fluorocarbon-based
material is the first in our new
generation of oil and gas elastomers.
With new polymer architecture, it offers
low compression set, excellent RGD
resistance, plus enhanced resistance
to sour gas, amines and steam. It is
Norsok rated up to at least 8.4mm cross
section ‘O’ rings.
l FR58/90 — fluorocarbon terpolymer
(FKM) material with excellent chemical
and thermal properties, plus good RGD
resistance. It is widely approved and
James Walker’s RGD materials test laboratory
specified for oilfield duties, and has also
achieved the highest Norsok rating of
0000 with 5.33mm section ‘O’ rings.
l FR25/90 — a fluorocarbon
tetrapolymer (FKM) that combines
improved low temperature capability
with excellent chemical properties. It
offers excellent RGD resistance, and
has achieved the highest Norsok rating
of 0000 with 6.99mm and 5.33mm
section ‘O’ rings.
l Elast-O-Lion ® 101 — an hydrogenated
nitrile (HNBR) grade with high
mechanical strength and wear
resistance. It has good resistance to
many oilfield chemicals, including
H 2
S and amine corrosion inhibitors. It
is resistant to RGD and approved to
many oilfield specifications. It achieved
the highest Norsok rating of 0000 with
6.99mm section ‘O’ rings.
l Elast-O-Lion ® 985 — our hydrogenated
nitrile (HNBR) grade with a temperature
capability down to -55°C, but offering
reduced mechanical properties and
RGD resistance when compared to
Elast-O-Lion 101.
l AF69/90 — An Aflas ® (FEPM) based
RGD-resistant grade with excellent
resistance to oilfield media and steam.
For detailed information on RGD-resistant
grades see: Elastomeric seals &
components for the Oil & Gas Industry.
To order or get further details, call your local contact shown on rear cover or listed at www.jameswalker.biz 11