Written Scheme Of Examination

Under the Pressure Systems Safety Regulations 2000 (PSSR 2000), a Relevant Fluid is defined as a fluid that can present a danger due to the energy stored under pressure or its physical properties.

A fluid is classed as a relevant fluid if it falls into either of the following categories:

1. Steam

   • Steam is always a relevant fluid under PSSR 2000, regardless of pressure or temperature.

2. Any gas, or mixture of gases, which is at a pressure greater than 0.5 bar above atmospheric pressure

   • This includes compressed air, nitrogen, oxygen, carbon dioxide, and many industrial process gases.

This definition also includes:

• Liquids that are kept under pressure and would release gas if pressure were reduced, such as liquefied petroleum gas (LPG), refrigerants, or ammonia.

• Air systems, including compressed air receivers and pipework, when operating above 0.5 bar gauge.

Fluids not normally considered relevant fluids under PSSR include:

• Liquids that are not heated above their boiling point and do not generate gas when pressure is released (for example, water in a cold-water system).

In summary, under PSSR 2000, a relevant fluid is steam, gas, or a pressurised fluid capable of releasing gas, where its pressure and stored energy could pose a risk if the system fails.

The pressure × volume by-product (P × V) is a value used in pressure-system safety to indicate the amount of stored energy within a pressure vessel or system. It is particularly relevant under the Pressure Systems Safety Regulations 2000 (PSSR 2000) when assessing whether a system has the potential to cause danger.

The calculation is straightforward:

Pressure (bar) × Volume (litres)

Pressure is taken as the maximum allowable pressure above atmospheric pressure (bar gauge), and volume is the internal volume of the vessel or system in litres.

The P × V by-product helps determine:

• Whether a pressure system is considered significant enough to present a risk

• Whether the system falls within the scope of PSSR

• If a Written Scheme of Examination (WSE) is required

As a general guide under PSSR:

• For gas systems (excluding steam), a pressure system is normally considered relevant if the P × V exceeds 250 bar-litres

• Steam systems are always within scope, regardless of P × V

For example:

• A compressed air receiver rated at 10 bar with a volume of 50 litres has a P × V of 500 bar-litres, which exceeds the 250 bar-litre threshold and would therefore fall within PSSR.

In summary, the pressure × volume by-product is a simple method of estimating stored energy in a pressure system and is used to decide the level of regulatory control and inspection required to ensure safety.

Minor Pressure System;

Minor systems include those containing steam, pressurised hot water,
compressed air, inert gases or fluorocarbon refrigerants which are small
and present few engineering problems. The pressure (above
atmospheric pressure) should be less than 20 bar (2.0 MPa) (except for
systems with a direct-fired heat source when it should be less than 2 bar
(200 kPa). The pressure-volume product for the largest vessel should be
less than 2 x 105 bar litres (20 MPa m3). The temperatures in the system
should be between -20 °C and 250 °C except in the case of smaller
refrigeration systems operating at lower temperatures which will also fall
into this category. Pipelines are not included.

Intermediate Pressure System;

Intermediate systems include most storage systems and process
systems which do not fall into either of the other two categories.
Pipelines are included unless they fall into the major system category.

Major Pressure System;

Major systems are those which because of their size, complexity or
hazardous contents require the highest level of expertise in determining
their condition. They include steam-generating systems where the
individual capacities of the steam-generators are more than 10 MW, any
pressure storage system where the pressure-volume product for the
largest pressure vessel is more than 106 bar litres (100 MPa m3) and any
manufacturing or chemical reaction system where the pressure-volume
product for the largest pressure vessel is more than 105 bar litres (10
MPa m3). Pipelines are included if the pressure-volume product is
greater than 105 bar litres.

Under the Pressure Systems Safety Regulations 2000 (PSSR 2000), a Competent Person is an individual or organisation with the necessary knowledge, experience, and independence to carry out specific duties related to the safe management of pressure systems.

The role of the Competent Person is critical to ensuring that pressure systems are examined properly and do not present a danger.

Definition of a Competent Person under PSSR 2000

A Competent Person is someone who:

• Has sufficient technical knowledge of pressure systems, including design, operation, and failure modes

• Has practical experience in examining pressure vessels, pipework, and safety devices

• Is independent and impartial, meaning they must not be subject to commercial or operational pressures that could influence their judgement

This may be:

• An individual engineer

• An inspection body

• An insurance inspection company

• A specialist engineering organisation

Duties of a Competent Person

Under PSSR 2000, a Competent Person is responsible for:

• Drawing up or certifying Written Schemes of Examination (WSE)

• Carrying out examinations in accordance with the WSE

• Assessing whether a pressure system is safe to operate

• Specifying examination intervals

• Reporting defects that could give rise to danger

If a dangerous defect is found, the Competent Person must:

• Issue a written report

• Specify timescales for remedial action

• Notify the enforcing authority if there is an immediate risk

Independence Requirement

A key requirement under PSSR is that the Competent Person must be sufficiently independent from the day-to-day operation and maintenance of the pressure system. For example:

• The system owner or operator may not be considered competent if their role compromises impartiality

• Maintenance contractors may act as a Competent Person only if independence can be demonstrated

Summary

A Competent Person under PSSR 2000 is a qualified, experienced, and independent expert who ensures pressure systems are examined safely, legally, and effectively, helping prevent failures, accidents, and legal non-compliance.

Under the Pressure Systems Safety Regulations 2000 (PSSR 2000), inspection periods stated in a Written Scheme of Examination (WSE) are decided by a Competent Person using a risk-based assessment, SAFed Guidance PSG 01 – Guidelines on Periodicity of Examinations, and are not fixed time limits.

How Inspection Periods Are Determined

Inspection intervals are set by considering the likelihood of failure and the potential consequences if failure occurs. The Competent Person assesses several key factors:

1. Type of Pressure System and Fluid

• Steam systems usually require more frequent examinations due to high stored energy and risk

• Gas and compressed air systems are assessed based on stored energy (P × V)

• Liquefied gases often require shorter intervals due to rapid expansion risk

2. Operating Pressure and Temperature

• Higher pressures and temperatures increase stress on equipment

• Systems operating close to design limits generally have shorter inspection intervals

3. Pressure × Volume (P × V) By-Product

• Systems with higher stored energy are examined more frequently

• Lower P × V systems may justify longer periods between examinations

4. Design, Age, and Construction

• Older systems or those with unknown history often require shorter intervals

• Materials of construction and fabrication standards are considered

• Weld quality and corrosion allowance affect inspection frequency

5. Condition and Maintenance History

• Good maintenance records and previous satisfactory examinations can justify longer intervals

• History of defects, corrosion, or repairs usually leads to shorter intervals

6. Environment and Duty Cycle

• Corrosive, wet, or aggressive environments increase deterioration risk

• Systems subject to frequent pressure cycling or intermittent use may need more frequent inspections

7. Protective Devices and Controls

• Reliability of safety valves, pressure switches, and control systems is assessed

• Protective devices often have separate inspection intervals from vessels or pipework

Typical Examination Periods (Guidance Only)

While not fixed in law, common industry practice includes:

• Steam boilers: 12–14 months

• Steam receivers: 24–48 months

• Compressed air receivers: 24–60 months

• Safety valves: 12 months

These are guidelines only and can be increased or reduced based on risk.

Summary

Inspection periods in a Written Scheme of Examination are:

• Set by a Competent Person

• Risk-based, not time-based

• Influenced by pressure, fluid, stored energy, condition, and operating environment

• Reviewed and updated following each examination

This approach ensures inspections are proportionate, effective, and focused on preventing danger, as intended by PSSR 2000.