Hot tapping lets engineers create a new branch on a live pipeline without taking the system offline. It is an elegant bit of engineering, but the work is not finished the moment the cut is made. Before the connection is handed back, it must pass a pressure test. This is the stage that proves the joint is sound, the isolation is tight, and the pipeline is fit to carry on running.
If you manage a site and have not watched a hot tap before, the pressure test can feel a little opaque. This guide explains what is actually happening, why it matters, and what quality looks like from the outside.
Why pressure testing matters
A hot tap introduces a new fitting to a line that is already under load. The fitting is welded or clamped to the pipe, an isolation valve is mounted on top, and a cutter drills through the wall to open the connection. Each of those steps creates a potential leak path.
Pressure testing verifies three things at once: the new fitting and its welds are holding, the isolation valve is seating correctly, and the joint can withstand normal running pressure with a safety margin on top. Without a test, you are trusting the workmanship blind. With one, you have physical evidence that the connection is fit for service, along with documentation that regulated sites need to keep on file.
The standard sequence UK contractors follow
Most reputable hot tapping companies UK wide follow a broadly similar method. Figures vary with pipe material, product, and design pressure, but the shape of the process is consistent.
The contractor isolates the new fitting using the tapping valve, cleans around the joint, and selects a test medium. Water is the default because it stores little energy if something fails. The system is then pressurised in stages rather than straight to the target, so any weakness shows up early. Once the hold pressure is reached, the line is monitored for a set period while the engineer walks the joint, checks for weeping, and applies leak detection fluid to flanged or threaded components. The system is then vented under control and the result is logged.
Typical test pressures and hold times
The table below gives a rough guide to what facility managers will usually see. These figures are indicative, not prescriptive: the actual values are set by the relevant design code and the site’s written scheme of examination.
| System type | Test medium | Typical test pressure | Typical hold time |
|---|---|---|---|
| Low pressure water main | Water | 1.5 × maximum working pressure | 30 to 60 minutes |
| Industrial process water | Water | 1.25 to 1.5 × working pressure | 30 to 60 minutes |
| Compressed air | Air or inert gas | 1.1 × maximum working pressure | 10 to 30 minutes |
| Steam line fittings | Water (hydrostatic) | 1.5 × maximum working pressure | 30 minutes or more |
For the regulatory backdrop, the Health and Safety Executive sets out dutyholder obligations in its Pressure Systems Safety Regulations 2000 guidance, which remains the primary reference point for UK sites.
Reading the result
The outcome is binary: pressure either holds or it does not.
A steady reading across the full hold period means the joint is sound and the valve is tight. A gradual drift downwards typically indicates a minor leak at a gasket or thread, which is located, resealed, and retested. A sharp drop signals something more serious, such as a poorly seated valve or a weld defect, and will trigger further investigation before any retest.
A competent contractor will issue a written record showing start pressure, end pressure, ambient temperature, hold duration, and the pass or fail result. On larger systems, this is backed up by a chart recorder or digital log.
What quality looks like on the day
The signs of a professional job are straightforward. The team explains the test plan before pressurising anything, uses calibrated gauges with visible stickers, ramps pressure gradually, ropes off clear exclusion zones, and investigates every weep however small. The paperwork is completed on site, not promised for later.
Pressure testing sits alongside the broader inspection regime that underpins pipeline repair services and connects directly to valve repair services, since the new isolation valve becomes a permanent part of the system.
To discuss a hot tapping job on your site, call the RDS Pipeline team today on 01277 500510.
