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At Stark, we not only install new pipes and gas connections but also work on projects involving existing ‘live’ gas supplies. In such cases, we must meet the relevant safety and regulatory requirements associated with working with live gas.

Utilising temporary flow-stopping methods

Instead of simply turning off the gas, which can be costly and disruptive, we prefer to employ temporary flow-stopping methods whenever possible. These methods allow the gas to continue flowing while we carry out our work. The key objective is to stop the flow in a specific area of the pipe run without disrupting the entire gas supply in the pipeline. We primarily use two flow-stopping methods: “bagging off” insertion and squeezing the pipe. The choice of method depends on the gas pressure, pipe material, and diameter.

In this blog, we explain the difference between the two methods and provide insight into how they work.

Isolation by Bagging Off (‘bag off’) method

When dealing with connections to metallic gas mains, welded steel, cast-iron, or polyethylene (PE) pipes, it is advisable to avoid turning off the gas if possible. Turning off the gas would require extensive re-testing of the entire pipe system before turning it back on. Hence, for steel or cast-iron pipes, we use the bagging off isolation process, which involves temporary flow-stopping with an installed bypass. Please note that this process is suitable only for low-pressure pipes due to the risk of bag failure.

Safety checks and preparation

Before starting the work, we ensure that all relevant and approved safe operating procedures are in place. We conduct thorough risk assessments and ensure that everyone involved understands the methods of working. We check that all necessary equipment, specifically tailored to the pipe size, is provided and in proper condition.

Once the safety checks are completed, we proceed with the following steps:

  1. Excavate the required amount of ground safely.
  2. Attach a drill to the pipe to create a small bore opening.
  3. Set up a bypass for the gas flow around the main pipe.
  4. Carry out additional drilling and insert bespoke strengthened bags.
  5. Inflate the bags to form a seal against the internal wall of the pipe, temporarily stopping the gas flow.
  6. Use two bag tubes on each side of the section of the pipe to be cut out, isolated, or connected to a new connection.

Safety measures and testing

Having two bags on each side creates a double block and bleed isolation, adding an extra layer of safety. We conduct pressure and soundness tests, along with a “decay” test, before any isolation work to ensure the existing off-takes supplied by the gas main continue to receive gas.

For large-diameter plastic or PE gas mains, the bag insertion method may also be used for temporary isolation.

After completing the work on the pipe, we deflate the bags and remove the bypass. Before removing the bypass, we ensure there is no positive pressure (indicating no gas flow) within the pipe. If there is no positive pressure, the bypass and bags are removed.

Isolation by Squeezing Off (‘squeeze off’) method

Polyethylene (PE) pipes are extensively used in the gas distribution industry due to their corrosion resistance and ease of fusion. To temporarily halt the gas flow in PE pipes, we utilize the squeezing off method, which differs in its preparation stages from the bagging off method used for metallic pipes.

Preparation and safety checks

The flexibility of PE pipes allows for a different method to temporarily stop the gas flow. This method is known as “squeezing off.” The initial preparation stages for this process are similar to the “bagging off” isolation used for metallic pipes. Safety, preparation, and a comprehensive understanding of the specialized work involved are top priorities. For squeezing off PE pipes, a bypass is set up to ensure uninterrupted flow around the targeted area. Customized tees are fused to the pipe, and a small diameter opening is created through the pipe wall using a drill.

Squeezing off process

After setting up and testing the bypass and conducting necessary safety checks, we commence the squeezing off operation. It involves clamping the PE pipe using bars placed into a clamping machine, both before and after the work area. The hydraulic clamp is then used to compress the pipe, bringing the inner surfaces together and temporarily blocking the gas flow. Upon completing the work, re-rounding tool shells matching the pipe diameter may be inserted. These shells restore the pipe to its original size. Releasing the hydraulic squeeze off clamp allows the PE pipe to regain its shape and size, enabling the gas flow to resume.

Squeezing off PE service pipes and subsequently cutting or capping the ends is also a commonly used method for permanently isolating a gas supply when it is no longer required.

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

Safety precautions and regulatory compliance

Working with live gas necessitates the careful execution of the flow-stopping process. The Stark Works team diligently monitors the gas pressure in the pipes throughout the operation. We thoroughly brief our operatives on the task requirements and ensure that they possess the necessary training, accreditation, experience, and knowledge to carry out the operation effectively.

Compliance and pre-operation preparation

Compliance with regulations and thorough pre-operation preparation are crucial in projects involving live gas supplies and temporary or permanent disruption to gas flow. For flow-stopping projects, our team calculates the distances between various components to meet regulatory requirements. We submit the proposed working method, whether routine or non-routine, to the gas network operator for approval, along with comprehensive risk assessments and method statements. Upon arrival at the site, our team is fully briefed and physically marks out the distances to ensure a smooth and safe operation.

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 as a replacement for 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 find this guide interesting.

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