Pressure Testing and Backpressure: Keeping the Plug and Pipe Bladder Under Control

Procedure for Pressure Testing with a Plug

A pressure test on a new or repaired pipe section follows the same sequence, regardless of whether a mechanical plug or a pipe plug is used for isolation. First, map out the section: which pipes and branch lines are included, where the plug is to be positioned, and where pressure can originate from—both from the pump and from a high point in the pipeline.

The pipe is filled slowly, and air is vented at the highest point as the water enters. Trapped air in the pipe represents hazardous energy during pressurisation and leads to unreliable readings because the air compresses instead of building pressure in the water.

Pressure is increased in increments, typically half a bar at a time, with checks on the plug position, seal tightness, and pressure gauge at each step. Once the test pressure is reached, it is stabilised for the duration required by the test procedure, and the pressure is read before any other action is taken.

Depressurisation is carried out just as slowly as pressurisation, and pressure is released through a valve—never by loosening the plug. After the test, the plug is removed, the opening is closed, and both the testing and removal are documented.

Backpressure: What the Plug Can Withhold and How to Manage It

Backpressure is the pressure acting on the rear of the plug from water or air within the isolated section. This must be known before installing the plug: a pressure of just a few metres of water head sounds minor, but can push a pipe plug out of the pipe with great force if it is not secured.

As a rule of thumb, select a plug whose documented backpressure limit provides a good margin above the pressure that can actually occur—not just barely above it. Backpressure can increase unexpectedly when a pump starts, when a valve is closed rapidly, or when a high-lying section drains into the isolated section.

Pipe plugs normally only withstand low backpressure (often below 1 bar, depending on type and diameter), whereas mechanical plugs may be approved for higher pressures. These figures are product-dependent and must be verified in the data sheet—never use a generic figure from a different plug or diameter.

Test pressure and pressure testing require a solution that is sized and approved for the test pressure. A standard working plug or pipe plug must not be used as a barrier against test pressure unless explicitly approved for that purpose by the manufacturer.

Water Head Backpressure: How to Calculate It

Backpressure is not only what the pump delivers. A high-lying section draining down towards the plug creates static pressure that can be far greater than expected. One metre of water head corresponds to approximately 0.1 bar, so 10 metres above the plug creates approximately 1 bar of backpressure even before the pump starts.

Use the table as an estimate when evaluating whether the plug is suitable. Then add a safety margin for rapid valve operations and pump starts, which can cause momentary pressure spikes.

Pipe Plug or Mechanical Plug for Pressure Testing

Pipe plugs are quick to install and well suited for shut-off during draining, repair, and brief work in an unpressurised pipeline. They normally withstand only low backpressure, and the limit varies by type, diameter, and inflation pressure, which must be checked in the data sheet for the individual plug.

Mechanical plugs are expanded against the pipe wall and are available in versions approved for higher pressures. They require a clean and dry pipe, a seal matched to the pipe material, and the torque specified by the manufacturer. If the plug is installed crookedly, it can slip or damage the pipe wall.

For actual pressure testing, select equipment with a documented pressure rating, preferably test heads or isolation tools designed for the purpose. A standard working plug or pipe plug must not be used as an isolation barrier against test pressure unless explicitly approved by the manufacturer for that specific pressure and diameter.

Safety: Confined Space, Pipes, and Work Zone

Working in and around pipes is working in a confined space. There may be oxygen deficiency, gas, or fumes in a pipeline, and a plug can seal off a section where fresh air cannot enter. Never enter a pipe without atmospheric monitoring, ventilation, a safety harness, and a designated standby person outside who can call for help.

Never stand on the side of the plug facing the pressure, and do not position yourself in line with the pipe end during inflation or pressurisation. Sharp pipe edges must be removed before inserting the plug to avoid cutting the rubber seal.

Monitor the inflation pressure using a pressure gauge on pipe plugs, and keep an eye on the pressure throughout the work. If the pressure drops, the seal is leaking, and the plug must be removed and inspected—do not simply top it up uncritically.

Finally, document what has been isolated, where the plug is located, and when it was removed. A forgotten plug in a completed pipeline is an error that is first discovered during pressure testing or operation—and it can ruin an entire pipeline section.

Documentation and Handover

Record what has been isolated: section, plug type, location, documented backpressure limit, test pressure, date, and operator. This record belongs with the welding log and test report so that the next person working on the section can see what has been done.

Upon completion, verify that all plugs and pipe bladders have been removed and that openings have been closed again. A plug left behind in a finished pipeline is only discovered during pressure testing or operation, and it can ruin an entire section.

Upon handover to the operations manager, the test report must be included, and any deviations noted so they are not solely retained in the memory of the site personnel.