“An inspection booking does not replace a Written Scheme of Examination. The scheme defines what must be examined before the examination takes place.”
An air receiver can be easy to overlook in a busy garage, workshop or factory. Yet if the pressure system needs a Written Scheme of Examination (WSE), that document must be in place before the system is operated.
The scheme identifies the safety-critical parts of the system, the examinations they need and the maximum time between examinations. A competent person must then examine the system in accordance with it.
This guide explains when a scheme may be required, how the 250 bar litre calculation works for compressed-air receivers and what to do if your site has pressure equipment but no current WSE.
Quick Answer: When is a Written Scheme of Examination required?
A compressed-air receiver and its associated pipework are likely to need a Written Scheme of Examination when the receiver’s pressure in bar multiplied by its internal capacity in litres is 250 bar litres or more. Other systems, including steam equipment, must be assessed on their own characteristics and exemptions. Where a WSE is required, it must be suitable for the system and in place before operation; the system must then be examined by a competent person in accordance with it.
Explore Velocity Safety’s PSSR examinations and Written Scheme support.
Key Takeaways – PSSR Written Schemes
- A WSE identifies which parts of a qualifying pressure system need examination, what the examination involves and its maximum interval.
- For a compressed-air receiver, pressure × internal receiver volume of 250 bar litres or more is a key WSE threshold.
- The 250 bar litre calculation is not a universal test for every kind of pressure system; steam equipment needs separate consideration.
- The user of a fixed system, or owner of a mobile system, is responsible for establishing the scope of the scheme.
- A competent person must draw up the scheme or certify that it is suitable.
- The scheme must be in place before the system operates, and examinations must then follow the scheme.
Jump to:
What Is a PSSR Written Scheme of Examination?
A Written Scheme of Examination is a document setting out how the safety-critical parts of a qualifying pressure system will be examined under the Pressure Systems Safety Regulations 2000 (PSSR).
According to HSE guidance, a suitable scheme typically identifies:
- The equipment and parts of the pressure system covered
- Which parts need examination
- The nature of the examination and any testing of protective devices
- Preparatory work needed to examine the equipment safely
- Any examination needed before first use, where appropriate
- The maximum interval between examinations
- Critical parts that require examination after certain modifications or repairs
- The competent person certifying the scheme and the certification date
The WSE is specific to the pressure system. A generic inspection checklist or an annual compressor service sheet does not establish the scope and examination requirements of that system.
What Does the 250 Bar Litre Rule Mean?
For a compressed-air receiver, multiply the pressure in bar by the receiver’s internal capacity in litres.
For example:
| Receiver example | Calculation | Result |
|---|---|---|
| 100-litre receiver at 8 bar | 100 × 8 | 800 bar litres – above the 250 bar litre threshold. |
| 25-litre receiver at 10 bar | 25 × 10 | 250 bar litres – at the threshold, not below it. |
HSE identifies a compressed-air receiver and its associated pipework at 250 bar litres or more as a system likely to require a WSE.
Check the actual receiver details and the wider system rather than estimating from the size of the compressor motor. If there are several vessels, their configuration and the applicable exceptions need proper assessment.
250 bar litres does not define all of PSSR
It is particularly relevant to the WSE and examination requirements for certain pressure systems containing a relevant fluid other than steam. Smaller systems may still have other safety duties. Steam systems must be considered separately, irrespective of this pressure-volume calculation.
What Equipment May Need a Written Scheme?
Compressed-air receivers are a common reason for garages, engineering workshops and manufacturing sites to need a WSE. HSE also lists examples such as steam boilers, steam sterilising autoclaves, gas-loaded hydraulic accumulators and certain refrigeration and LPG systems.
Whether a particular item needs to be included depends on the pressure system, the fluid it contains, any applicable exceptions and the danger that could result if a part failed.
A scheme may cover:
- The air receiver or other pressure vessel
- Associated pipework where failure could be dangerous
- Protective devices such as safety valves
- Other parts whose failure could release stored energy and cause injury
HSE notes that the compressor associated with an air receiver does not normally need to be included in the WSE itself. This is one reason to define the system and its examination scope accurately rather than calling every part of the installation a “compressor inspection”.
Who Is Responsible for the Written Scheme?
For a fixed pressure system, the user has responsibility for defining which items form the system and which need to be included in the scheme. For a mobile system, that responsibility generally rests with its owner.
They can obtain technical advice when establishing the scope. A competent person must then draw up the WSE or certify a scheme prepared by the user or owner as suitable.
The competent person needs the knowledge, experience and independence appropriate to the system and examination. The user or owner must also ensure that the system is subsequently examined by a competent person in accordance with its scheme.
Read HSE’s guidance on written schemes of examination.
How Often Does a Pressure System Need Examining?
There is no single examination interval that applies to every pressure system. The WSE sets the maximum interval for the relevant parts of that particular system.
The competent person considers factors such as its design, condition, operating environment, previous examination findings and the potential consequences of failure. Different parts may require different examination intervals.
Protective devices should be covered by the scheme. Their correct operation matters because a safety valve that is damaged, obstructed or incorrectly set may fail to protect the system against excess pressure.
The scheme also needs review as the equipment changes or ages. Replacing a vessel, altering the installation or carrying out a significant repair may require the scope or examination arrangements to be reconsidered.
What If Your Site Has No Written Scheme?
First, establish what pressure equipment you have and gather the information needed to assess it:
- Equipment type, manufacturer and model
- Receiver or vessel internal volume
- Operating pressure and safe operating limits
- The fluid or gas in the system
- Details of associated pipework and protective devices
- Previous WSEs, examination reports and maintenance records, if available
- Any modifications, damage or known defects
A competent person can use those details to assess whether a WSE is required and what it must cover.
Where a WSE is required, it must be in place before operation. An examination then needs to be arranged in accordance with it; simply obtaining the document does not complete the examination duty.
Velocity Safety provides pressure-system examinations and support with preparing or reviewing Written Schemes of Examination. See the PSSR service and request an assessment.
Final Takeaway
If you operate a compressed-air receiver, check its pressure and internal volume. A result of 250 bar litres or more is a clear reason to establish its WSE requirements promptly.
For other pressure systems, including steam equipment, do not rely on that calculation alone. Identify the system, check the applicable PSSR requirements and ensure a suitable scheme and the examinations it specifies are in place.
Unsure whether your compressor or pressure system needs a WSE?
Velocity Safety supports businesses with PSSR pressure-system examinations and Written Scheme preparation or review across the UK.
Send us the equipment type, receiver volume, operating pressure, fluid and any existing documentation. We can advise on the next assessment and quote for the work required.
📞 Call: 01254 933 486
📩 Email: info@velocitysafety.co.uk
🌐 Or contact Velocity Safety online.
Frequently Asked Questions
What is a Written Scheme of Examination under PSSR?
It is a document identifying the parts of a qualifying pressure system that need examination, the nature of those examinations and the maximum time between them. It must be drawn up or certified as suitable by a competent person.
Does a 250 bar litre air receiver need a WSE?
A compressed-air receiver and its associated pipework with a pressure × internal volume value of 250 bar litres or more are likely to require one. The threshold includes 250; it does not apply only when the calculation exceeds 250.
Does PSSR only apply above 250 bar litres?
No. The pressure-volume exception concerns particular requirements for certain non-steam systems. It does not describe the whole scope of PSSR, and steam systems require separate assessment.
Does a small air compressor need a Written Scheme?
It depends on the receiver, operating pressure, system arrangement and applicable exceptions. Check the internal receiver volume and pressure rather than judging by the physical size or motor rating alone.
Does the compressor itself go in the WSE?
Not normally when it is associated with an air receiver, according to HSE guidance. The receiver, relevant pipework and protective devices may need to be covered. The competent person should assess the particular system.
Who can prepare a Written Scheme of Examination?
A competent person can draw up the scheme, or the user or owner can prepare one for a competent person to certify as suitable. The person selected must have appropriate knowledge, experience and independence.
How often must a PSSR examination take place?
The WSE specifies the maximum interval for the relevant parts of the system. There is no universal annual examination date for every pressure system.
Is a compressor service the same as a PSSR examination?
No. Routine servicing helps maintain the equipment. A PSSR examination is performed by a competent person in accordance with the Written Scheme and addresses the parts identified by that scheme.
Can Velocity Safety help if we do not have a WSE?
Yes. Velocity Safety offers PSSR examination services and support with preparing or reviewing Written Schemes. Provide the equipment details and any existing documents when requesting an assessment.
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