7625 West New York Street | |||
Indianapolis, Indiana 46214-4911 USA | |||
Telephone: (317) 273-6960 | |||
Facsimile: (317) 273-6979 | |||
Order Desk Telephone: (800) 401-1723 | |||
Order Desk Facsimile: (800) 515-9254 | |||
E-mail: info@floteco2.com | |||
Wendell Hull & Associates, Inc.
Forensic Consultants /
Accident Reconstruction 1020 S.
Main , Las Cruces, NM
88005 (505) 523-5623
FAX 523-5709
May 24, 2003
Brian Davidson
FloTec Incorporated
7625 West New York Street
Indianapolis, IN 46214-4911
RE: CGA 870 Sealing Washers and Crush Gaskets
Dear Mr. Davidson:
Pursuant to your request I have reviewed the photos
you transmitted and provide the following observations and data for your
consideration. Wendell Hull &
Associates, Inc., (WHA) is well aware of the CGA 870 style seal design and has
experience with both fire investigation involving these seals and testing of
medical equipment utilizing these seals. For
your convenience I have enclosed herein the WHA Oxygen Capabilities Brochure
along with my current resume.
WHA has been testing medical devices for the industry
for many years and due to our background and experience have encountered CGA 870
seal failures, seal ignitions, and seal leakage both in the laboratory and in
the field on a number of occasions. In our opinion, the CGA 870 style seal is a critical element
of the regulator design and should be maintained within the manufacturer’s
specifications as an integral element of the overall design.
In our opinion this issue is important enough that on at least one
occasion we recommended that the manufacturer add a warning to their product
literature against the user changing this seal except for an approved
design/material. The following
discussion provides a brief summary of our general conclusions pertaining to
this issue:
1)
Based on
our experience, most medical regulator manufacturers design their regulator
inlets to incorporate a "sealing washer" comprised of a metallic ring
capturing an elastomeric seal rather than a "crush gasket" comprised
usually of a Nylon disc. These two
styles of inlet seals (i.e., sealing washer versus crush gasket)
provide distinctly different design features/sealing capabilities as well as
oxygen compatibility:
a)
Design:
The sealing washer design provides a configuration suitable for an elastomeric
material, usually Nitrile rubber (i.e., Buna-N) or Viton, to be used as the
primary seal rather than a plastic material such as Nylon, which is often used
for the crush gasket design. Experience
and testing indicate that elastomeric materials are far superior to plastic
materials for sealing configurations like this and provide positive sealing at
much lower loads (applied torques). Further,
the sealing washer design can be reused many times without affecting its sealing
efficiency. Some key considerations
pertaining to these issues are discussed below:
i.
Lower Torque: WHA testing indicates
that the elastomeric/metal-sealing washer will effectively seal at torque values
less than 10 in-lbs, which can easily be applied by most users.
The design of the sealing washer provides a “pressure activated”
design that allows the elastomeric material to move with pressure application
into the sealing gland and thereby create a positive pressure seal at low
torque. By contrast, the plastic
crush gaskets require 10 in-lbs or more initial torque to affect a seal
(depending on surface roughness); and, because they must "crush", or
deform, to seal, they loose some of their sealing efficiency each time they are
used. Each time the crush gasket is
reused, our experience indicates that a higher torque must be applied to produce
a sealing efficiency similar to the initial use.
Further, our testing indicates that after 5 or 6 uses of the crush gasket
the torques required to produce a good seal are excessive for an elderly patient
and approach 50 in-lbs (depends on the previous torque value applied). The need to apply greater torque to the crush gasket results
from the need to deform the seal more each time it is used.
ii.
Reuse Capability: Our testing also indicates that the sealing washer design is more
robust and can be reused many times without requiring increasing torque to
affect the same seal and without significant deterioration of the sealing
efficiency each time it is used. This
is not true of the crush gasket style seal as discussed above.
Our experience indicates that the crush gasket style seal is often reused
even though they should only be considered a limited use item.
It is noteworthy in this regard that many regulators retrofitted with
crush gasket seals are often returned with bent or broken T-Handles, presumably
due to the difficulty in achieving a leak tight seal at the cylinder valve
interface. As will be discussed
below, leakage of these seals causes an ignition mechanism to be active that
would otherwise be eliminated.
b)
Sealing Capability and Ignition: The sealing capability between the two styles of
seals is different; and, if misused such that low-level leakage results, the
ignition probability would be expected to increase. Some key differences between the two styles of seals that
influence leakage are that the sealing washer style is reusable by design but
the crush gasket is more suited to single use applications.
However, the crush gaskets are generally used interchangeably with
sealing washers and as a result are often reused many times by users.
Further, as discussed above, the sealing washers inherently require lower
torque to affect a proper sealing whereas the crush gaskets require higher
initial torque (as well as an increasing torque after each use).
These differences lead to a greater probability of leakage over time when
the crush gasket is used; and, when weeping/seeping flow of oxygen occurs across
a polymer interface, the ignition probability increases.
The ignition mechanism in question is called “flow friction” and
relates to the heat produced by oxygen flow across surfaces of polymeric
materials.
c)
Oxygen Compatibility: The oxygen compatibility of the materials used for
these seals is also very different, based on standard testing.
In the past the sealing washers were primarily comprised of
Buna-N/Aluminum but recently the trend in the industry has been to move to Viton/Brass
for greater compatibility. Crush
gaskets are typically comprised of Nylon. The
following table summarizes the oxygen compatibility of these materials based on
standard tests:
Material |
Heat of Combustion (cal/gram) |
Autoignition
Temperature (oC) |
Combustion
Threshold (%
O2 or psi) |
Nonmetallics |
|
|
|
Buna-N |
9900 |
173 |
22
% |
Viton |
3300 |
268
- 322 |
56
% |
Nylon |
7500 |
178 |
22
– 28 % |
Metallics |
|
|
|
Aluminum |
7500 |
N/A |
~15
psi |
Brass |
850 |
N/A |
>7000
psi |
As can be seen from this table, the combination of
Viton/Brass provides the best compatibility for the common materials being used.
The Heat of Combustion provides a measure of the energy release by a
material when it burns in oxygen and is an indication of its overall damage
potential. The Autoignition
Temperature (or Autogenous Ignition Temperature (AIT)) provides a measure of the
material’s relative ease of ignition and indicates the approximate temperature
at which a material could be expected to spontaneously ignite in high-pressure
oxygen. The Combustion Threshold is
measured by the Oxygen Index (OI) for nonmetallic materials and the Promoted
Combustion (PC) pressure for metallic materials. The OI of a nonmetallic material is the concentration of
oxygen (percent) necessary for the material to self-support combustion once
ignited. The PC of a metallic
material is the minimum pressure at which the material will self-support
combustion. As is indicated in the
table above, by every category of evaluation, Viton/Brass materials are superior
in compatibility to the other common materials; hence, the industry trend is
appropriately toward the more compatible materials.
It is noteworthy, however, that Buna-N/Aluminum combination sealing
washers have been successfully used in the past.
In our opinion, that is due to their superior sealing effectiveness,
which reduces the probability of heat being generated due to flow friction while
in use. While, Buna-N exhibits similar oxygen compatibility to Nylon,
because it is an elastomeric material and because it inherently provides a
better seal than Nylon, ignition of Nylon crush gaskets by flow friction is more
probable and occurs more frequently based on our experience.
d)
Regulator Fit: Experience indicates that the same crush gasket is used on many
different types of regulators but the fit varies widely with the actual
regulator being used. In some cases
the crush gasket fits loosely due to a too-large inside diameter and in other
cases it must be forced onto the regulator inlet due to an inside diameter that
is too small for the regulator inlet. When
the gasket is too loose the position on the regulator can interfere with
positive sealing. When the gasket
is too tight the gasket may bow which also can interfere with sealing.
In either case, the sealing efficiency is affected.
Further, when the sealing gasket is too loose it is subject to falling
off when the regulator is changed to a new cylinder, which increases the risk of
contamination migrating into the regulator.
When the gasket is too tight, it encourages users to leave the gasket
installed beyond its limited useful life. By
contrast, the standard sealing washers supplied by the manufacturer are designed
to fit the regulator inlet properly and are snug enough to “grab” the inlet
insert during change out but be easily removed and changed if dirty or worn. In my experience, either type seal design may be used
successfully, but only seal designs sized for the regulator and recommended by
the manufacturer should be used. Further,
if a crush gasket is used, then a limited useful life should be expected and
users should be trained to apply greater torque (to ensure a leak tight seal)
and to change the gaskets after each use.
2)
WHA has
investigated several fires associated with either misapplication of the sealing
device or flow-friction ignition of a crush-gasket style design.
Only one fire has been investigated by WHA with a sealing washer style
design but chemical analysis indicated that contamination on the inlet was
probably present at the time of ignition. While
ignition could develop with either style, due to the fact that all nonmetallic
materials are flammable in medical oxygen, the crush gasket style seal exhibits
a higher frequency of ignition and probability of ignition, in our experience.
Some of the key factors are as follows:
a)
Training: Adequate training of
users is obviously limited judging from the fires we have investigated, which
indicated similar misunderstandings by users to the circumstances depicted in
the pictures that you provided me. Some of the conditions your photographs show
are the following:
i.
Two crush gasket seals (one on top of the other) – This configuration
not only hinders the sealing efficiency but also works to defeat the pin-index
mechanism on the cylinder valve. The
regulator inlet insert is sized according to Compressed Gas Association (CGA)
requirements and is therefore limited in its length. Therefore, when the regulator is inserted into the cylinder
valve outlet proper alignment and indexing only occurs when a single seal is
installed. If two seals are
installed, which is shown in your photos and which I’ve also experienced in
the field, then the regulator does not insert into the cylinder valve outlet
properly and an indexed fit does not occur.
Based on experience, ignition is one outcome of this improper
installation.
ii.
One crush gasket seal on top of the cylinder valve plastic dust cover –
This arrangement results from the user failing to remove the dust cover on the
outlet of the cylinder valve before installing the regulator.
This condition would result in many of the same problems as discussed
above. In addition, the dust cover
material would potentially be present directly in the flow stream (possibly
hindering the gas flow) and would be subject to both adiabatic compression
ignition and flow friction ignition. This
condition also would be expected to develop debris that would migrate into the
regulator.
b)
Misuse:
Possibly due to the lack of training, or possibly due to the lack of
appreciation for oxygen safety issues, we have experienced users installing
regulators on cylinder valves with nothing more than vinyl tape over the outlet
of the valve instead of a proper sealing washer or crush gasket.
In these cases ignition resulted and the misuse of the equipment, due to
the failure to provide a proper seal between the regulator and the cylinder
valve, was the proximate cause. In
our opinion, use of a seal that is not recommended by the manufacturer and
designed for the regulator interface, is a similar misuse of a critical element
of the overall design.
The above summary of observations and opinions are based on our present experience in the medical oxygen industry and the testing that we have conducted; and, are provided for your consideration. We obviously have not fully investigated the incidents that you referred to so we provide these comments solely based on our general familiarity with the industry. If you wish to provide us with more specific details pertaining to the incidents in question, we would be happy to investigate more fully.
Sincerely,
Barry Newton, BSME, PE
Wendell Hull & Associates, Inc.