Last reviewed 22 April 2016

Justin Tyas examines a case in which a carpet company and an engineering inspection firm were ordered to pay combined fines and costs of more than £25,000 after a large pressure vessel exploded, propelling the vessel’s quarter-tonne lid six metres into the air and just missing a worker.

The National Physical Laboratory (NPL) states that pressure is “generally the result of molecules, within a gas or liquid, impacting on their surroundings — usually the walls of the containing vessel. Its magnitude depends on the force of the impacts over a defined area”. Thus, pressure is the force per unit area, usually expressed in Newtons per square metre (or pascals).

Pressure systems are defined as:

  • a system comprising one or more pressure vessels of rigid construction, any associated pipework and protective devices

  • the pipework with its protective devices to which a transportable pressure receptacle is, or is intended to be connected

  • a pipeline and its protective devices.

There are numerous types of pressure systems and associated equipment, which have a wide range of industrial applications including:

  • boilers and steam heating equipment

  • compressed air systems

  • pressure cookers and autoclaves

  • pipework and hoses

  • valves and gauges.

According to the Health and Safety Executive (HSE), the main causes of pressure-related incidents are:

  • poor equipment and/or system design

  • poor installation

  • inadequate maintenance of equipment

  • deficient repairs or modifications

  • an unsafe system of work

  • operator error, inadequate training and/or supervision.

The principal hazards from pressure systems are from the uncontrolled release of stored kinetic (moving) and thermal energy including:

  • an explosion or release of compressed liquid or gas

  • flying debris including parts from equipment that have failed

  • fire as the result of escaping chemical release

  • contact with released liquid or gas such as steam.


A carpet company used four stock dye pressure vessels, which were, essentially, industrial-sized pressure cookers for processing and dying carpet fibres. During a production run, one of the vessels exploded. Its quarter-tonne lid was torn off from its locking mechanism and hinges and was propelled six metres into the air with such force that it dented one of the roof girders.

No one was injured in the incident but the lid narrowly missed a worker who was standing nearby. It was a reportable dangerous occurrence (under the previous Reporting of Injuries, Diseases and Dangerous Occurrences Regulations) because of the potential to cause death. The incident was investigated by the HSE.

An external company had been contracted to undertake periodic thorough examinations of the pressure vessels.

Statutory requirements

The principal regulations that concern the manufacture, supply and use of pressure equipment are as follows.

  • Pressure Equipment Regulations 1999: cover (with some exclusions) the design, manufacture and conformity assessment of pressure equipment and assemblies of pressure equipment with a maximum allowable pressure >0.5 bar, ie subject to an internal pressure of 0.5 bar above atmospheric pressure. This includes the supply and putting into service of such equipment.

  • Pressure Systems Safety Regulations 2000 (PSSR): concern (with exceptions) the safe design and use of pressure systems. Their aim is to prevent serious injury from the hazard of stored energy as a result of the failure of a pressure system or one of its component parts.

Before using any qualifying pressure equipment (new or otherwise), a written scheme of examination must be in place, and an examination undertaken.

Critically, the PSSR 2000 are applicable to “relevant fluids” including steam at any pressure, mixtures of fluids at a pressure of >0.5 bar above atmospheric and gases dissolved in solvents under pressure. Relevant fluids do not apply to hydraulic oils. Hydraulic systems, while using high pressures, do not store energy in the system and so are not covered by the PSSR.

In 2014, the HSE published a second edition of Safety of Pressure Systems. Pressure Systems Safety Regulations 2000. Approved Code of Practice and Guidance on Regulations, which is aimed at dutyholders including users, owners, competent persons, etc. A simple flow chart/decision tree is provided to assist users/owners as to whether the regulations apply to their pressure systems.

The general requirements of the PSSR appear under regulation 4 through to regulation 15.

  • Regulation 7 Safe operating limits — prohibits the system for being used or allowed to be used, unless the safe operating limits have been established.

  • Regulation 8 Written scheme of examination — requires that before a pressure system is operated, the user/owner must ensure that a written scheme of examination (WSE) has been prepared. The WSE should be drawn up by a competent person, or if drawn up by someone other than a competent person, certified as suitable by a competent person. Guidance is given about the attributes and role of the competent person.

  • Regulation 9 Examination in accordance with the written scheme — the owner/user must ensure that the equipment is periodically examined by a competent person in accordance with the written scheme. They must also ensure the safety of the examiner by performing the required preparatory work.

  • Regulation 10 Action in case of imminent danger.

  • Regulation 11 Operation — concern the safe operation of the system and the action to take in an emergency. Training is a specific requirement of the Provision and Use of Work Equipment Regulations 1998.

  • Regulation 12 Maintenance — concern the type and frequency of maintenance required.

  • Regulation 13 Modification and repair — to ensure the safety of the pressure system including its protective devices.

  • Regulation 14 Keeping of records, etc — including where a system is sold or changes hands, the previous owner has a duty to pass over all documents held under this regulation to the new owner/user.


The carpet company pleaded guilty to breaching regulation 12 of the PSSR and was fined £10,000 and ordered to pay the costs of £1174.

Regulation 12 of the PSSR states: “The user of an installed system and the owner of a mobile system shall ensure that the system is properly maintained in good repair, so as to prevent danger.”

The engineering inspection firm pleaded guilty to breaching regulation 9(2) of the PSSR and was fined £13,000 and ordered to pay the costs of £1111.

Regulation 9(2) of the PSSR states: “Where a competent person undertakes an examination for the purposes of paragraph (1), he shall carry out that examination properly and in accordance with the scheme of examination.”


The failure of the pressure vessel and the ensuing dangerous explosion was a result of a malfunction of the regulator and pressure relief valve. An investigation by the HSE determined that the owner (carpet company) had not ensured that suitable and sufficient maintenance of the vessel’s safety devices was being completed. In addition, the periodic statutory thorough examinations had not been completed for three years.

A written scheme of examination was in place at the carpet company, which included the vessel that failed. An engineering inspection firm was carrying out periodic thorough examinations of other pressure systems for the carpet company. However, it failed to complete these on four stock dye pressure vessels for a number of years, including the vessel in question. Therefore, it failed to carry out the required statutory examinations.

Lessons learned

This was a serious incident with the potential to cause death or serious injury. If a pressure system fails, it could explode violently, releasing vast amounts of kinetic and thermal energy.

This incident should be a wake-up call for all dutyholders of pressure systems including owners, users and competent persons. There are clear standards within the regulations to ensure that pressure systems and their associated safety devices are maintained, so that they do not have the potential to cause danger when operated within their defined safety limits by suitably trained personnel.

In addition, there is a requirement to ensure that this is augmented by periodic thorough examinations by a competent person. The written scheme of examination must be drawn up by a competent person with the necessary knowledge, experience and independence to undertake the functions required of them.