Right along the oil and gas supply chain – from extraction through to delivery to the end user – safety is the number one concern.

When an operator wants to isolate a pipeline, whether this is part of the routine operation of a facility, to carry out maintenance, or in response to a potential crisis, it’s critical that the valve used is 100 per cent effective.

To isolate pipelines in oil, gas and petrochemicals applications, double-block-and-bleed (DBB) valve systems have been mandatory since 1990 following the events of the Piper Alpha disaster. These arrangements feature two independent valves in the pipeline itself with a third valve allowing operators to drain the space in between the two valves.

The benefit of this is that, if the first valve in the system leaks, the second valve will stop it traveling down the pipeline to the area where it might potentially become a hazard, whilst an open ‘bleed’ valve will prevent the leak even reaching the second valve.

Conventionally, in order to achieve a DBB system, two standard isolation ball valves are installed along with a separate facility for bleeding the pipework in between. However, there are down-sides to these traditional installations.

Leak paths

The first is around fugitive emissions – the small amounts of material that leak out through the flange connections between separate components. These are a major cause for concern across the industry, particularly for onshore facilities where the material may be escaping into populated areas.

Potential leak paths are also presented by any join in the pipework, whether this is a connection between a pipe and a component or two separate sections of pipework. Therefore, increasing the number of joints required for a valve installation significantly increases the likelihood of media escaping from the system.

If you multiply the additional potential leak paths created by installing three valves instead of one for isolation duties across an entire oil refinery, it’s easy to see how it could present a substantial issue.

Size and weight

The second challenge is that more components will take up more space and add to the weight of an installation.

Requiring an additional valve unit to be installed as well as a T-section to allow the intervening pipework to be bled, more than doubles the space required compared with a single valve system, which can be an issue where space is of the essence.

This significant size increase can often make the installation unfeasible, especially where multiple valves along the line need to be upgraded.

One area in which this is a particular concern is floating production, storage and offloading (FPSO) facilities, where space and weight are at a premium. In the world of upstream oil and gas production, these facilities have gained a lot of ground against traditional oil and gas platforms as they allow energy companies to access increasingly remote locations more cost-effectively.

These applications require a large amount of equipment to be housed on the facility, which has a finite amount of weight and space. Therefore the physical dimensions of all installations have become a critical concern and engineers are keen to shave off size and weight wherever possible – needless to say, without compromising on safety levels.

Single-unit DBB valves

To tackle the inherent challenges with traditional double block and bleed system – which requires three times more connections than are needed for a single unit – a potential solution is the single cartridge DBB valve manifold.

Incorporating all three valves into a single housing increases the safety of the system by removing connections between separate units, resulting in fewer potential leak paths.

Another key advantage is that the whole system can be built to have the same face-to-face length as a standard single isolate valve, as specified in the relevant international standards – API 6D and ANSI B16.10.

Not only does this mean the system can be easily installed into an existing system without any re-working of the surrounding pipes, but also that the space required for a DBB system is reduced by more than half. This frees up room for other equipment and easier personnel access or, in the case of floating facilities, saves critical weight and space.

We pioneered the single-unit double-block-and-bleed valve, which we named Oliver Twinsafe, in the early 2000s and, since then, the range of sizes and designs of single unit DBB valves available has grown rapidly. We recently manufactured our 200,000th DBB valve, demonstrating the popularity they have gained across the industry.

As global energy businesses face challenges like increasingly inaccessible oil and gas reserves and the need to reduce fugitive emissions, technological innovation at ground level will play a central role in delivering solutions. Single cartridge DBB valves are just one example of how the valve industry has developed in recent years in response to industry needs.

Paul Shillito M.Sc., M.Phil., D.M.S., C.Eng., F.I.Mech.E.

Technical Director

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