Reducing high pressure oxygen system risk

Managing the risks of a high pressure oxygen system is particularly challenging when the system is used in a life support application. Careful selection of non-metallic sealing materials is of paramount importance in controlling the residual risk to patients – for which the oxygen system is a life line.

Today, piped medical oxygen is an essential life saving and life supporting service provided in every acute hospital. Oxygen is stored at a hospital site as cryogenic liquid or in high-pressure gas cylinders. Over the years there have been many fires associated with these systems, particularly those using high pressure oxygen cylinders, which necessitate the use of pressure regulators to reduce the pressure to a level at which it can be distributed throughout the hospital more safely. Thankfully, incidences of fires associated with medical oxygen systems are rare and the risks associated with oxygen systems are far outweighed by the benefits of medical oxygen to patients. What can be done to minimise the risk to patients in our hospitals when a fire does occur? The answer lies in minimising the residual risk. BS EN ISO 14971 – Medical devices – Application of risk management to medical devices takes it’s definition of risk from ISO/IEC Guide 51:1999, which is: ‘[the] combination of the probability of occurrence of harm and the severity of that harm‘. Only by minimising both variables can the residual risk be reduced to an acceptable level.

Reducing occurrence
The development of standards governing regulator design and testing have gone a long way to reduce the occurrence of auto-ignition, but fires still happen, occasionally, and they always will. A recent search on Google for “Oxygen regulator fire” returned 617,000 results!

The following is an inexhaustive list of factors that can increase the probability of auto-ignition of the components within a high pressure oxygen regulator:
 

A combination of two or more undesirable factors increases the likelihood of autoignition further. Adiabatic compression along with the presence of a contaminant is thought to account for the highest proportion of reported incidents. Poor servicing of an oxygen regulator by a third party agent caused at least one regulator fire, which was one of the incidents that led to the MHRA issuing medical device alert MDA/2003/016.

Careful component design and protection with a 100 µm (maximum) filter reduces the likelihood of auto-ignition significantly. In addition, maintenance and replacement of regulators should be carried out inline with the manufacturers instructions for use.

Auto-ignition testing is invaluable in proving the design of high pressure oxygen system components. However, the auto-ignition test in BS EN ISO 10524-2 does not consider contamination, particle impact, corrosion or wear. It is a type test only, used to prove component designs under ‘normal’ operating conditions.

Controlling severity
In addition to reducing the occurrence, the severity of the hazard must also be managed - the combination of both resulting in a solution with an acceptable residual risk when weighed against the benefits of medical oxygen to the patient. Six years ago the European Industrial Gases Association (EIGA) published document IGC Doc 73/00/E concerning High Pressure Breathing Gas Systems – Toxicity Risks of Using Non-Metallic Materials. This document highlights the risks associated with the use of nonmetallic materials in high pressure oxygen systems and offers guidance on the design of equipment and the selection of materials based on the toxicity of combustion products. This followed a number of accidents where highly toxic gases had been released into oxygen pipeline systems used for breathing applications.

One such incident in the UK prompted NHS Estates to issue safety notice NHSE SN (2003) 01 in 2003. The safety notice stated that all manifold systems were to be checked to ensure that they did not contain materials that would release toxic substances in the event of internal ignition of components. It may have been useful if reference had been made to the aforementioned EIGA document, since this contains a great deal more background and detail on the subject. Maybe this would have prompted an industry wide review of non-metallic materials used in high pressure oxygen service, and you may not be reading this article.

The EIGA document IGC Doc 73/00/E specifically refers to halogenated polymers being those which are of greatest concern in high pressure oxygen regulators used for breathing purposes. Regulators typically have at least two polymeric seals. One is known as the seat and the other the diaphragm. Both are normally made from polymers or elastomers as they must make a pressure tight seal against a metal face. Severity should also be reduced by minimising the mass of polymer/s used in high pressure components. By limiting the amount of potential fuel, the quantity of toxic by-products that would be released during an inadvertent combustion is reduced.

Halogenated polymers
Halogenated polymers are those containing elements in group 17 (the halogens) from the periodic table – namely Chlorine, Fluorine and in some polymers, Iodine. During combustion, halogenated polymers can produce potentially lethal chemicals including phosgene, diphosgene and oxygen difluoride. The table below lists some of the possible by products of combustion of such materials. Upon ignition in pure oxygen, combustion of a halogenated polymer is extremely fierce and rapid, and has the potential to release a concentrated bolus of toxic gases into the pipeline.

BOC has been pro-active in demanding that halogenated polymers are no longer used on high pressure oxygen systems. BOC Medical Asia in particular are contacting all hospitals they supply with oxygen, making them aware of the situation, and providing free guidance and support to those affected. It is understood that Air Products also has a corporate policy of following EIGA’s recommendations.

Why then do we continue to install high pressure oxygen regulators with halogenated polymer seals in our hospitals everyday? Cost may be one factor. Most high pressure oxygen regulators used in manifold supply systems are designed for use in welding equipment – not for breathing by patients in our hospitals! The medical gas market as a whole accounts for only a small portion of the total market for oxygen pressure regulators, so economies of scale make some ‘welding’ regulators the cheap (but not cheerful) option for those aiming for cost leadership in what they see as a commodity marketplace.

The choice to use halogenated polymers for use in most regulators for welding applications is not arbitrary – in fact halogenated polymers are the easiest to incorporate into a design in most cases, offering high flexibility, good chemical resistance and high auto-ignition temperatures. The removal of these polymers leaves a very limited selection of materials for the designer, necessitating more complex designs with tighter manufacturing tolerances to ensure an equivalent degree of reliability is achieved.

This can make the components more time consuming to design and manufacture, and therefore more expensive to produce. It is thought likely that ignorance has also played its part. The mechanical requirements of BS EN ISO 2503 for a welding regulator may be the similar to medical gas manifold regulators to BS EN ISO 10524-2, but materials selection should follow the procedures laid down in BS EN ISO 15001 with risk management practices applied to ISO 14971.

New standards
BS EN 738-2, the old harmonised standard for manifold and line pressure regulators for medical gases has recently been superseded by BS EN ISO 10524-2. The new standard enhances the safety of medical oxygen regulators with new requirements. First of all, it refers back to BS EN ISO 15001, highlighting the need to assess and manage materials selection based on toxicity in addition to the conventional challenges of cost and mechanical performance. BS EN 738-2 made a similar reference to prEN 13159, which was never published, leaving manufacturers with no published standard to which they could refer.

Furthermore, high pressure regulators with aluminium components in the gas stream will not be allowed in ISO 10524-2. Aluminium regulators have been in use in the US for many years, and reports of fires in aluminium bodied high pressure oxygen regulators are not uncommon. This is due to the relatively low auto-ignition of aluminium alloys compared with brass in a high pressure oxygen enriched atmosphere.

The main reason that BS EN ISO 2503 cannot not be applied to medical gas regulators is clear. It does not attempt to manage the risk of toxic gas release upon auto-ignition of a pressure regulator in oxygen service. In addition to lowering the probability of occurrence, this must be done by lowering the severity of harm that may be caused by the hazard of autoignition.

Obviously no appreciable harm is caused if a welding regulator releases a concentrated bolus of toxic gases through the end of a welding blowpipe – but to a critically ill patient the consequences are likely to be extremely serious.

With the arrival of HTM 02-01, managing the toxicity risk with high pressure oxygen systems will be ever more important. Local manifold backup supplies, which may be sited close to critical care areas, can dramatically increase the severity of the toxicity gas hazard. This is because the affect of the any toxic gas release would be augmented by a dramatically smaller pipeline volume between the regulator and patient, reducing the possibility of any significant dilution within the pipeline as the gas flows to the ward terminal units.

Oxygen supplied by a locally sited manifold system is also likely to be shared between fewer patients, increasing the overall mass of toxins delivered to each patient, should a fire occur.

It is encouraging that changes in relevant standards should lead to a wider understanding of the toxicity hazards posed - and maybe one day to an industry wide adoption of safer non-metallic materials for use in high pressure oxygen systems. It is hoped that the information herein gives the reader the ability to make an informed decision about one particular issue affecting patient safety in our hospitals.

References

1 IGC Doc 73/00/E – EIGA 2000.
2 OXHAZ/FAIL.0188 – Oxygen Hazards Analysis Report – NASA 2001.
3 NHSE SN (2003) – Safety Notice -Oxygen cylinder manifolds used to supply oxygen for patient use – NHS Estates ‘03.
4 BS EN 14971:2001 – Medical devices – Application of risk management to medical devices.
5 MDA/2003/016 – Medical device alert – Medical gas regulators and flowmeters MHRA 2003.
6 BS EN 738-2.
7 BS EN 10524-2.
8 BS EN ISO 2503.
9 ISO/FDIS 10524-2.
10 BS EN ISO 15001.