Degrees of Separation

Andy Brazier explains why process isolation is more complicated than you might think

PROCESS plant and equipment will often contain hazardous substances and conditions. If intrusive work has to be carried out, typically maintenance, there is a chance that hazards may be released that cause harm to people in the vicinity. In order to reduce the risks, properly planned and implemented isolations are required that provide a barrier between the source of the hazard and the plant or equipment being worked on. Since I first published this guidance in IChemE’s Loss Prevention Bulletin, I’ve had the opportunity to review and discuss isolations on a number of occasions. I’ve been asked if the paper needs to be updated, and it doesn’t. But given the continued interest, it is worth sharing with the wider chemical and processing engineering audience.

These isolations are usually achieved in the first instance by closing valves, but more robust methods may also be required such as inserting spades or removing pipework depending on the nature of the hazard and the work to be carried out. Also, part of the isolation procedure includes removing any hazard contained within the equipment, which may require draining, venting and/or purging with an inert substance.

In 1997 a document was published titled The Safe Isolation of Plant and Equipment. Whilst endorsed by the Health and Safety Executive (HSE), it had actually been developed by the Oil Industry Advisory Committee (OIAC). In other words, the regulator was not telling industry how to isolate plant and equipment, and instead industry was explaining what it did (or at least what it thought it should do).

The original document set out a method for determining what form of isolation should be used – ranging from single valve through to “positive isolation” using spades and blanks – depending on the duty of the plant and the task being performed. It formalised the use of “double block and bleed”, pointing out that valve integrity needs to be proven if an isolation is to be relied upon.

The original OIAC document was updated and published by HSE as Health and Safety Guidance Document (HSG 253)¹ and many companies have developed their own, in-house isolation standards. It appears that in the years since 1997, HSE has accepted industry’s suggestions and converted them into requirements. Also, companies agreed to comply.

Given that the requirements have been clear for some time and accepted by industry it would be reasonable to assume that compliance has been achieved; but experience has shown that companies’ isolation procedures often fall short of the requirements, particularly for proving the integrity of valves. Also, people often do not fully understand why adherence to a strict sequence of steps is required when isolating plant and equipment.

An ‘interesting’ example

Isolating a pig receiver in preparation for opening the door provides an excellent example to demonstrate the potential difficulties in achieving a suitable isolation. It is universally accepted as a critical activity requiring a high degree of integrity. Figure 1 demonstrates that the receiver will need to be isolated from the pipeline at both its inlet and in its kicker line. Given that the pipeline is likely to be live and possibly at high pressure, there can be no doubt that a proven isolation will be required. A double block and bleed arrangement can be used for this.

A level of complexity is introduced when we recognise that there is more to the isolation than simply shutting valves. In this case, the receiver will need to be depressurised, drained, purged and vented. Closed vent and drain systems are often required so that hazardous material can be removed in a safe way. These create additional process connections that require isolation. A further complication is created by overpressure protection provided by a relief valve connected to the vent header. This will need to be isolated, but should remain available for as long as possible to ensure protection is provided.

The sequence of steps required to isolate this pig receiver is summarised below. It should be noted that this is actually a simplified scenario because in reality there may be additional items to consider including a balance line, multiple drain points and purge connections.

Sequence of events