Flow-stopping explained: How we work on live gas pipelines

At Squire Energy, we don’t just install new pipes and gas connections, we also work on projects that involve existing ‘live’ gas supplies. In these cases, we must meet the relevant safety and regulation requirements applicable that arise when working with live gas.

Simply turning the gas off can be costly and disruptive, so instead, wherever possible, we use temporary flow-stopping methods, which allow the gas to continue flowing whilst we carry out our work. Crucially, we have to stop the flow in a specific area of the pipe run without disrupting the gas supply in the whole of the pipeline. There are two main methods of flow-stopping that we use, isolation by means of ‘bagging off’ (or ‘bag off’) insertion and isolation by means of squeezing (or ‘squeeze off’) the pipe. The method we choose depends principally on three things; the pressure of the gas flowing through the pipe; the material that the pipe is made from, and the pipe diameter.

In this blog, we explain the difference between the two flow-stopping methods and give an insight into how the flow-stopping methods and process works.

Isolation by Bagging Off (‘bag off’) method

Whether we are dealing with connections to metallic gas mains, welded steel, cast-iron or polyethylene (PE) pipes, it’s advisable (and preferable) not to turn off the gas if possible, as the entire pipe system would then need to be extensively re-tested before being turned back on. Therefore, in the case of steel or cast-iron pipes, using a method of temporary flow-stopping with a bypass installed is essential. For this type of work, we use a bagging off isolation process. Due to the risk of bag failure, this process is only suitable in low-pressure pipes.

The first step is to ensure that all relevant and approved safe operating procedures are in place to carry out the work that we need to do safely. We check that the operation and working environment have been properly and thoroughly risk assessed and that methods of working are in place and understood by everyone involved. Everything that is required for the process must be provided, usually in a kit for the specific pipe size. So, the first job for the team is to check the equipment is in date and that everything is the right size for the project.

Once all the safety checks have been carried out, it’s time to get to work!

For metallic gas mains, the first part of the process (after the required amount of ground has been excavated safely) involves attaching a drill to the pipe to form a small bore opening and then setting up a bypass for the gas to flow through and around the main pipe. Once this has been done, further drilling is carried out and bespoke strengthened bags are inserted. The bags are inflated, forming a seal against the internal wall of the pipe causing the flow of gas to stop, temporarily. We use two bag tubes, one on either side of the section of pipe to be cut out or isolated or where the proposed new connection is to take place. Having two bags on each side creates a double block and bleed isolation. This is also an added safety measure to ensure that if one of the inflatable bags fails, there is a backup in situ to prevent the gas from flowing through to a potential open end. Before any isolation works are carried out, the required pressure and soundness (tightness) tests are carried out, along with a ‘decay’ test, to help confirm the known existing offtakes supplied by the gas main continue to receive a supply.

For certain large diameter plastic or PE gas mains, the above method of bag insertion may also be the preferable method employed for temporary isolation.

Once the work on the pipe has been successfully carried out, the bags are deflated, and the bypass is removed. Before the bypass is removed, it’s essential that the team check the pressure. If a positive pressure is detected and the gas is flowing through the pipe correctly, the bypass and bags are removed.

Isolation by Squeezing Off (‘squeeze off’) method

Since their introduction in the 1960s, the use of polyethylene (PE) pipes have dominated the gas distribution industry and most of our pipes are now made from this versatile plastic polymer material. There are a number of benefits to using PE pipes; they don’t corrode (unlike metal) and they can be relatively easily fused (jointed) together using a process called Polyethylene pipe fusion (read our blog here), which takes away the need for mechanical (screwed or welded) joints, and is considerably easier to handle.

The flexibility associated with pipes manufactured using PE also means that a different method can be used for temporarily stopping the gas flow. This method is called ‘squeezing off’. The initial preparation stages for this process are the same as for a ‘bagging off’ isolation normally used for metallic pipes, in that safety, preparation and a full understanding of the specialist and specialised works involved are the top priority. For squeezing off PE pipes, a bypass is set up to ensure uninterrupted flow around the area in question, by fusing bespoke tees to the pipe and using a drill to form a small diameter opening through the pipe wall. After the bypass is in place and tested, and the numerous safety checks are carried out, the squeezing off operation may commence. This involves clamping – with bars placed into a clamping machine around the PE pipe, at the location before and after the area where the work is being carried out. The PE pipe is then compressed (closed / ‘squeezed off’) by means of operating the hydraulic clamp. The pipe walls are squeezed together until the inside surfaces touch, effectively and temporarily closing off the pipe so that no gas can flow. Once the work has been completed, we may insert two shells (known as a re-rounding tool), that are the same diameter as the pipe, to force the pipe back to its usual size. Releasing the hydraulic squeeze off clamp allows the PE pipe to reform to its original shape and size and for the flow of gas to resume.

Squeezing off PE service pipes and then cutting/capping the ends is also a method often employed to permanently isolate a gas supply where its use may no longer be required.

The above descriptions are a simplified explanation of the processes involved in both methods and several steps and checks have been omitted in order to shorten the full description of the operation, but we hope that this will provide the reader with a general idea.

Safety Measures

Understandably, the flow-stopping process must be carried out very carefully as we are dealing with live gas, whether the works are for a new connection or an isolation. One of the most important tasks the Squire Energy team carry out throughout the process is to continually monitor the pressure of the gas in the pipes to ensure it is maintained. All our operatives are always fully briefed on the requirements of the task and we ensure that every person involved is correctly trained, accredited, and has the necessary experience and knowledge of what is required and how/why the operation is to proceed.

As is applicable to all our works, there is a substantial amount of pre-operation preparation involved in any job involving live gas supplies and the temporary/permanent disruption to the flow of gas. On a project involving flow-stopping, our team will calculate all of the distances between the different components required to ensure these meet the regulatory requirements. The method of working (known as a routine operation or non-routine operation) is submitted to the gas network operator for prior approval, along with all risk assessments and method statements before the work is carried out. When we arrive at site, the team have been fully briefed and then physically mark out the distances so everyone working on the project is aware of where everything is, and where everyone needs to be to keep the operation running smoothly and safely.

If you would like to find out more about some of the projects we have worked on click here.

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Disclaimer – This guide is intended for general information purposes only, it has not been written by one of our operatives, as a technical instruction or for any replacement of any manufacturer’s instructions, training or industry procedures. Some parts of the process may also have been omitted, as they are deemed applicable only in certain bespoke situations and are of considerable length and detail. We hope you will find this guide of interest.

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