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Yoke Washer Seal Alert

 

 

 

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.