Dr David Pritchard – gas explosion expert (#112)


David Pritchard worked for the UK’s Health and Safety Laboratories where he undertook investigations into gas explosion incidents and how to prevent them.

Can you describe something that has recently amazed you? How did it make you feel?         

I never cease to be amazed when I go walking with a local U3A rambling group around Buxton where I live.  Buxton is nearly completely encircled by the Peak District National Park, so you do not have to go far in most directions before you are in stunning unspoilt open countryside. The northern part of the Peak District is called the Dark Peak as the rock there is a dark gritstone, while the southern part with a light limestone rock is called the White Peak. The Dark Peak has a harsher landscape with lots of open moorland while the White Peak, my favourite of the two areas, has a much gentler landscape with wide open dales. Walking through a dale with a river running through it or walking along a ridge looking down on a dale you cannot but be awed by God’s creation.  On returning from a walk I think how fortunate I am to have this wonderful countryside on my doorstep, I feel refreshed and often very muddy and wet. You just have to remember that God created mud and rain as well as dry ground and sunshine.


What does Health and Safety Laboratories do? What’s the range of investigations?

The Health and Safety Laboratories (HSL) are part of the Health and Safety Executive (HSE), which is the government body for developing policy, providing guidance and enforcing the workplace health and safety regulations.  Its remit is to carry out research on all aspects of health and safety and to give technical support to HSE’s inspectors which includes assisting with the investigation of workplace accidents.

HSL is probably unique among laboratories working in the health and safety area in that it has the staff expertise and facilities to undertake work in most aspects of health and safety.  It can undertake work on engineering problems, fires and explosions and occupational hygiene and medicine. The experimental facilities range from chemical laboratories, large scale rigs for engineering work and outdoor test sites for fire and explosion work.


Can you tell me what your role at HSL involved?

My area of expertise was gas explosions.  I have undertaken experimental work to generate data for validating explosion models and the testing of devices for mitigating the effects of gas explosions.  By the nature of this type of work most of it has had to be undertaken at the outdoor test sites.  I provided advice to HSE’s inspectors on gas explosions.  The major part of this work involved assistance with the investigation of accidental explosions. Investigation work could involve going to the site of the explosion to assess the damage, undertaking testing and examination back at the laboratory of samples and equipment removed from the site and attending court as an expert witness. My other main job was standards work.  I was a member of a number of British and European standards committees that were preparing standards for measuring explosion characteristics and for testing devices for mitigating the effects of an explosion.


I don’t think most people realise the forces and energy involved with an explosion. 

Yes, a gas explosion does generate huge amounts of energy as evidenced by the damage they can cause.  An explosion in a terraced property could well destroy the property, do extensive damage to the adjoining properties and cause damage to nearby properties from the blast wave and material ejected by the explosion. In the case of explosions at chemical plants and refineries they can cause minor damage, for example window breakage, at distances of the order of a kilometre from the plant.

I remember one incident next to a tower block. The boiler room was located underground by the side of the tower block.  The ceiling of the boiler room was the pavement by the side of the block.  Part of the ceiling comprised concrete slabs, about 1m by 2m, in which were inset small squares of thick glass.  The explosion blew the roof off the boiler room.  It was only by chance we met someone who asked if we had seen the concrete slab that was on the roof of the tower block.  Going onto the roof, if my memory serves me right, the tower block was 18 stories high, there was one of the concrete slabs with the glass insets.  Amazingly there was hardly any damage to the slab.


What for you were the hardest cases to work on? How did you cope with investigating tragedies because I’m guessing, unlike the emergency services, that wasn’t part of your training?

I found that the incidents that most brought home the tragedy of the event were the domestic gas explosions.  Going through the wreckage I was coming across all sorts of personal belongings, part of the life of the occupants that had been lost forever.  Though sadly people were killed in some of the incidents in others although their house had suffered extensive damage they had amazingly escaped with only minor injuries. There were also some strange effects of an explosion.  The particular one that sticks in my memory was seeing a house that had been extensively damaged yet there were still pictures hanging intact on the walls that had remained standing.

In most cases by the time I arrived at an incident site the leak had been stopped, the fires put out, the people rescued and the bodies removed, so I rarely experienced the “gruesome” side of an incident.  In all the years I was involved in incident work I can only recall seeing one dead body.  The result was that I was somewhat detached from the human tragedy side and the job was much like any other scientific investigation.

Incident investigation is challenging work though and the hardest part I found was the site work.  Once the call came for assistance I had to drop anything else that I was doing and get ready to travel to the site as soon as possible. The site could be anywhere in the country. They were long working days in dirty conditions, usually in the open and the weather could be cold and wet.  As time goes by all the incidents I have been involved in are tending to merge into one.  What brings back details of a particular incident is when on holiday traveling around in our motorhome I recognise a place where there was an incident or even pass the site.

The incidents that do stick in my mind are those with some memorable event.  One that comes to mind is an explosion in a block of apartments.  A colleague and I were examining the damage inside an apartment when we kept hearing a rustling sound.  We eventually traced it to a cat hiding behind some furniture. After some effort we managed to coax it into a cat box, which we had conveniently found in the apartment.  Another one was an explosion in a house.  The leak was traced to a hole in a section of lead pipe, just where it passed through the floor.  Initially it was thought this had been caused by the pipe flexing and rubbing against the edge of opening through the floor.  Examination of the pipe by a HSL metallurgist revealed what appeared to be teeth marks around the pipe hole.  A further investigation concluded the hole had been caused by a rat gnawing through the pipe.  What was even more bizarre was that this was not the original property on the site. The original one had been destroyed by bombing during the Second World War.  I do hope that the adage of everything occurs in threes does not come true.


After a massive gas explosion, there must be debris everywhere, how do you go about figuring out the cause?

How one would go about investigating an explosion would depend on the type of explosion.  Was it an explosion of an external cloud, did it occur inside a piece of equipment or was it a process that had gone wrong are some of the types. As an example the following is the process one would go through to investigate the explosion of an external cloud of flammable gas or vapour.  The first step would be to identify what had leaked and where it had leaked from.  Note for an explosion to occur there must be a delay between the start of the leak and the ignition to allow time for formation of a flammable cloud. If the ignition occurred when the leak started the result would be a jet flame burning at the point of leakage followed by a more serious fire if the jet ignited surrounding items.  Once the source of the leak was identified then the next stage would be to establish why the leak occurred.  Had corrosion of a pipe caused a hole to develop, had poor maintenance resulted in a leak developing in a joint or valve or had some recent work left a valve open or a flange not properly secured are some possible reasons.  The other question to answer is what caused the ignition.  Sometimes it is fairly obvious what caused the ignition, but in a few cases it is not possible to definitely establish the source.  The pattern of explosion damage can be used to identify where the explosion was initiated, which in turn may establish the ignition source or at least suggest some possible sources.


I remember you once saying that if today someone proposed laying gas pipes going straight into residential homes, they’d never get approval.

I think I was a bit melodramatic in suggesting that if today someone proposed piping a flammable gas to most homes in the country it would not be allowed.  I suspect that when they first started piping gas to buildings, for gas lamps, at the beginning of the nineteenth century little thought was given to safety. Today under the current health and safety regime a detailed risk assessment would be required before any decision was taken on allowing it.  Then it would be a case of deciding if the benefits outweighed the risks.  An interesting question is that if society is happy to accept the risks of a technology that have been around for many years would it also be happy to accept the risks from the same technology if it was introduced today?


What would you say was your biggest achievements or legacy from your career?

A cannot claim any ground breaking achievement during my 34 years working for the Health & Safety Laboratory and its forerunners.  However, I am content with the thought that in some small way I have contributed, along with many others, to improving the health and safety of both workers and the public.

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