Valve Criticality Analysis (VCA)

Human Factors Engineering (HFE) involves applying what we know about human capabilities, limitations and characteristics to design. It can be defined as “Setting people up to succeed through design“. Design that takes account of human factors will reduce human error.

Failing to apply Human Factors Engineering (HFE) principles during the design of plant or equipment can result in manual valves (or other items that a person has to operate, view or maintain) being located in a position that is difficult or dangerous for the person to access.

Reach access - VCA -
Image courtesy of IOGP Report 454 (August 2011)

This has several consequences:

  1. The person is unable to access the valve when required during operations. This is an issue if the valve has to be operated quickly, for example, in order to control an incident that is escalating, such as a leak or fire.
  2. If inaccessible, then the valve may not be maintained in accordance with requirements.
  3. The valve may require scaffolding or other means to access it for inspection or maintenance, increasing the cost and duration of maintenance activities.
  4. A person may injure themselves whilst trying to access the valve (e.g. stretching, crawling underneath equipment, or failing from a ladder).

Due to space constraints and the complexity of modern chemical, process or nuclear power plants, not every valve or instrument that people will interact with can be positioned at the ideal location for human interaction. So, if compromises have to be made, how do we ensure that they are the best compromises for safety and operability? And how do we justify those compromises to our stakeholders?

An extremely useful tool is available to inform the placement of valves. This is a structured approach that considers how important each valve is. Commonly known as Valve Criticality Analysis, or VCA, it is often used in the oil, gas, nuclear, marine and defence industries.

Purpose of Valve Criticality Analysis

Valve Criticality Analysis provides structure to the decisions that determine the location and accessibility of valves.

The aims of VCA are to:

  1. Ensure that safety-critical or operationally critical valves are easily accessible.
  2. Ensure that the design of the facility reflects the frequency of human interaction with equipment, and that frequently-maintained equipment is easy to access (or maintain/replace).
  3. Ensure that people can adopt a good working posture when operating valves.
  4. Ensure that the operation of valves does not require excessive muscular force.

This will improve safety, operability and maintainability when operating these valves. Although the focus tends to be on “valves”, the approach is equally suited to reviewing the location and access to instrumentation or other equipment that people interact with.

Valve Criticality Analysis (VCA) -
Valve location, accessibility, orientation and ease of use can be informed by Valve Criticality Analysis

Essentially, in this analysis, each valve is assigned a criticality rating which then informs its access requirements (i.e. those valves that are critical to operations or safety, and those that are accessed most frequently, should be easy to access and operate).

VCA is typically applied in the Front-End Engineering and Design (FEED) and Detailed Design phases of a major project, or as soon as initial layouts are available.

Key inputs to Valve Criticality Analysis are a good understanding of the tasks that people are required to perform, (in normal operations and in emergency situations), as well as an understanding of the capabilities and limitations of the people who will perform these tasks.

Benefits of Valve Criticality Analysis

VCA formalizes the decision-making process, using an agreed set of criteria. It removes the need for lengthy debate and discussion on where to locate valves. VCA provides clear and consistent guidance to designers and can reduce the time required to review proposed designs. Operational and maintenance activities are easier and quicker to perform; and human failures are less likely.

Conducting a VCA at the appropriate time in a project can remove the need for costly and time-consuming re-work (for example, preventing the need for changes when the plant has been constructed).

VCA can reduce project (capital) costs by eliminating unnecessary pipework, stairs or work platforms that would provide rapid access to non-critical valves that are rarely or never used. Valve criticality analyses support effective human performance by designing plant and equipment around humans and their tasks.

Valve criticality ratings

Valves are typically placed into one of three categories:

  • Category 1: valves that are critical for safety or operations; or used frequently for routine operations or maintenance (e.g. more than once every 6 months). This category also includes valves that have a high likelihood of failure. The consequences of a lack of quick access would be serious. These valves should be positioned so that they are visible and accessible from ground level or a permanent platform accessed by stairs, to enable rapid and unencumbered access.
  • Category 2: valves not critical for routine operations or emergencies, but need to be accessed (less than once in six months). It is preferable that these valves are positioned at ground level, or on a platform. However, where compromises have to be made, alternative access may be considered for Category 2 valves, such as the use of a fixed ladder to access a working platform (rather than stairs), or the use of scaffolding. The design should allow for the planned access arrangements (e.g. space for a temporary platform or scaffolding).
  • Category 3: valves that are not critical for operations and are infrequently used. For example, they may only be used for commissioning, start-up activities or rarely-performed maintenance tasks. As these valves are rarely operated, their position and orientation is less critical. Permanent access is desirable, but not essential. Access to these valves may be compromised due to project costs or timescales.

The lower the criticality rating, the more freedom that a designer has in determining the layout and access requirements. If a company has a standard for HFE, the access and positioning requirements for each Category of valve should be clearly specified, along with the clearances around each Category of valve, labelling requirements, maximum forces required to operate etc. For example, Category 1 valves should be placed at waist/chest height, rather than below the knees or above the shoulders.

Approach to VCA

The following approach is usually undertaken as a small workshop with a team of operations, maintenance and safety personnel; chaired by a human factors specialist if available. On a large project, the design will be divided into sections and a series of VCA workshops may be required. This approach is used in the chemical and process industries, but could easily be modified for other industries:

  1. Prior to the workshop, agree which valves and instruments would typically sit in each Criticality Category. This will provide default ratings for common valve types and reduce debate in the workshop. (See Table 1 below).
  2. Provide an overview presentation of the area (using computer 3D models if available) to familiarize workshop participants.
  3. Review drawings that contain proposed piping and instrumentation (in the process industries these are often called Piping and Instrumentation Diagrams, or P&IDs).
  4. Assign each valve a Criticality rating (Category 1, 2 or 3).
  5. Produce a summary table, for example showing for each valve: the criticality, frequency of use, Category assigned and notes (such as exceptions to the default ratings). If changes to existing equipment are required, the table should include an Action and the Actionee.
  6. Annotate drawings with the Criticality ratings for Category 1 and 2 valves (for example, draw a red square around Cat 1 valves and a red circle around Cat 2 valves).
  7. These workshop outputs should then inform the design and layout of piping and instrumentation.
  8. Where the design cannot comply with these requirements, a deviation should be raised and discussed with the client or company representative. For example, in a brownfields project, it may be more difficult to meet the access requirements due to congestion or existing piping and process constraints.
  9. Check compliance with the workshop outcomes as the project progresses. As more detailed 3D models become available, the location, visibility and access of valves should be reviewed – particularly those in Category 1.

On large projects, where it may not be possible for detailed review of all valves and equipment by a human factors professional, training should be provided to design teams so that they understand the principles of VCA and know when to raise concerns with a human factors professional.

Applying VCA to equipment other than valves

The VCA approach is traditionally used to inform the access requirements for manually operated valves in the chemical and process industries. However, the approach can be applied more widely to controls and displays (or anything else that humans interact with), to ensure that they are accessible and visible, according to their function and frequency of use.

The approach can also be modified to inform the design and layout of equipment in other industries.

Valve criticality categoryExample valves
Category 1Control valves
Isolation valves
Relief valves and depressuring valves
Trip and anti-surge control valves
Emergency shutdown valves
Liquid cargo transfer valves
Category 2Condensate drain valves
Service oil valves
Hydraulic service valves
Potable water valves
Manual valves for normal startup/shutdown
Category 3Valves used for commissioning
Valves used for decommissioning
Valves used to isolate tanks for inspections
Valves for pressure tests
Valves used in dry dock only
Table 1: Typical example valves in each of the three criticality categories. Creating a similar table for your company or industry prior to a Valve Criticality Analysis (VCA) workshop will greatly reduce discussion time in the workshop itself and increase consistency between workshops.

Further reading

Human factors engineering in projects. The Second Edition (June 2020) is free to download from here. This revised publication was jointly developed by the Energy Institute (EI) and International Association of Oil and Gas Producers (IOGP), and supersedes the first edition of IOGP 454 Human Factors Engineering in Projects (August 2011).

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