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Method Statement 3.11.1 - Pressure Testing of Mains

Document Information

Field Value
Document Reference MS 3.11.1
Issue Number 2
Issue Date 1 June 2026
Next Review 1 June 2027
Prepared By Sean Ashton
Approved By Aaron Mason, Director
Task No Task PPE to be worn Plant required Reference is made to the Team Pack risk assessment
3.11.1 Pressure Testing of Mains As defined by on-site risk assessment Works vehicle and hand tools. Compressor. N/A

Hazards

  • Manual Handling
  • Stored Energy in the Pipe
  • Defective Plant and Equipment
  • Towing Techniques
  • Incorrect Loading
  • Insufficient Space

Environmental Impacts

  • Noise
  • Dust
  • Light from Night working

Customer Impacts

  • Noise
  • Dust
  • Access

Training Required

As Per specific contract training matrix

Additional Plant and Equipment

  • Winch/Tractor Winch
  • Pipe Bursting/Slip Lining Rig
  • Mechanical Excavator

Permits Required

As Per specific contract specifications and Client requirements.

COSHH Assessment Required

  • 2 Stroke Oil
  • Diesel/Petrol
  • Line Marker Spray
  • Lubricating Spray

Sequence of Operations and Control Methods

Cleansing of Pipes

  • On completion of construction and before any disinfection internal surfaces of pipes shall be cleaned thoroughly by swabbing.

Precautions prior to testing

  • Prior to testing, the main shall be anchored adequately and the thrust from bends, branches, or other pipe ends transmitted to solid ground or suitable temporary anchorage. No thrust shall be transmitted to existing pipes.
  • Pipe ends shall be adequately capped off and restrained.
  • Testing shall not be carried out against closed valves or air valves.
  • Before the main is tested the trench shall be backfilled as much as possible.

Selecting test lengths

  • Generally do not test lengths greater than 1000m. The main should be divided into sections so that the pressure at the lowest point does not exceed the system test pressure and the pressure at the highest point is at least equal to the Maximum Design Pressure.

Preparation for equipment

  • Refer to set up design drawings / Procedures prior to commencement. Example below.

!Pasted image 20250916192424.png - The Pump should be adequately sized and capable of applying the test pressure in under two hours. - The pressure gauges or transducers with a logging facility and display shall be as shown above and be in the range 5-16 bar, temperature range of 0-50 degrees Celsius, and ability to read pressure to 0.02 bars. - All pressure sensing systems shall be calibrated in line with manufacturer's recommendations and the calibration date marked on the instrument. - All instruments shall be within calibration date prior to being used. - Seals and non-return valves should be checked prior to the test. Valves for air bleeding should be sited at high points. - Water for testing should be discharged into another test section to reduce waste. Appropriate checks are required before discharging to a river or lake.

Recording results

  • The test details and results shall be recorded in line with the adopting water company requirements and should include, but not limited to, the following:
  • Vertical elevation of the test section showing heights and location of air valves, gauges and filling points
  • Plan of the site
  • Start and finish of each test (date and time), the test pressures applied and pressures recorded during the test
  • Causes of failure and any remedial action taken
  • Date and outcome of final test

After the test

  • After testing, pipelines should be depressurising slowly. All air venting facilities should be open when venting pipelines. Water used for testing shall be disposed of safely.

Safe working during testing operations

  • In cases of large height differences between top and bottom of the test section the test length may have to be shorter to reduce the head of water to a tolerable risk.
  • A specific Risk Assessment should be carried taking into account the possible hazards associated with a test failure; an example may be not to put a test end just uphill of a school, major road, and electricity sub station.
  • Large volumes of water can cause a lot of destruction and move equipment and people.
  • The safest place to fill a test section is at the lowest point to enable the air to escape ahead of the water.

Testing of Polyethylene mains

  • Unless specified by the Designer use Potable Water to fill the main and remove all the air in the main by charging the main with water at a rate that allows air to discharge freely through the vents.
  • After the main is fully charged with water close all vents.
  • Leave the main to stabilise overnight or for a minimum of 2-3 hours.
  • The recommended System Test Pressures (STP) is as follows:
Rated Pressure Test Pressure
Up to 10bar 1.5 x rated pressure
12 bar - 16 bar pressure 1.5x working pressure (or 5 bar + working pressure, whichever is the least)
  • The maximum rated pressure is the maximum pressure that the system, including fittings, can withstand continuously in service.

Pressurising the main

  • The rate at which the pressure is increased to the recommended STP is important. The size of the pump is important as the time taken to pressurise the main is required to be recorded and determines the test time.
  • Monitor and record the pressure rise and the time taken to reach STP. This shall be known as time tl.
  • Record and plot the pressure against water volume in litres or against time. If the plot is producing a curve, then it is likely that air is present within the test section. If significant volumes of air are encountered terminate the test, remove the air and re-commence the test.

Interpreting the pressure test

  • Once the STP has been reached, isolate the main by closing the air vents/valves and allow the pressure to decay.

The following three-point test analysis shall be used to determine the result of the test. This may be done manually or using a logger and computer based analysis.

  • The time taken to reach the STP is tl.
  • Take the first pressure reading P1 at t1 where t1 is equal to tl.
  • Take the second reading of pressure P2 at t2 where t2 equals 7tl
  • Take a third reading of pressure P3 at t3 where t3 equals 15tl.

Allow for creep within the polyethylene main and make corrections to values of t1, t2, and t3 as follows:

  • t1c=t1+0.4tl
  • t2c=t2+0.4tl
  • t3c=t3+0.4tl
  • Work out the slope of the pressure decay curve between t1 and t2 - slope n1 as follows: n1=(logP1-logP2)/(logt2c-logt1c).
  • Work out the slope between t2 and t3-slope n2 as follows: n2=(logP2-logP3)/(logt3c-logt2c)

For a sound main with no leakage, n1 and n2 should lie between 0.04 and 0.1. Typical values are as follows:

  • 0.08-0.1 for unsupported mains e.g. slip lining or no backfill

  • 0.05-0.08 for mains in intermediate ground condition

  • 0.04-0.05 for mains in compacted backfill

  • The above calculation may be performed by using a pressure time logger and the data downloaded on approved software to calculate slope values.

Early warning of problems

  • This may be achieved as a rule of thumb by comparing the predicted pressure with the actual pressure. The predicted pressure is calculated using
  • P=Pl[2.5x(t/tl+1)]^n
  • Where Pl=test pressure at start of test
  • Tl= The time taken to reach the STP is tl
  • n= slope of pressure decay curve(0.1 for mains not backfilled, 0.4 for backfilled)

Action taken in the event of test failure

  • Where n<0.04 there is probably air in the system:

    • Check air valves working
    • Check shape of pressure decay curve
    • Is main laid in rock and bedded in concrete

    • If n>0.1 there is most likely a leak:

    • Check all bolted joints

    • Check all other exposed joints and mechanical joints
    • Look for evidence of leakage. If n>0.15 there is most certainly a leak.

    • If lines plotted are not straight then determine whether line getting steeper or shallower. If line shallow but now steeper-possibly some air, now escaped, no action.

    • If steep, then normal-no action

    • Was normal then steep-there is a leak
    • Was normal, now very shallow, no action

Testing other materials

  • Once the main has been charged the water loss can be measured using two methods.

Method A (volume pumped in)

  • Maintain the STP pressure for one hour by additional pumping if necessary and record the quantity of water required to maintain the pressure at STP.

Method B (volume drawn off)

  • After the main has been charged to STP, close all valves and disconnect the pump from the pipeline.
  • Monitor the pressure in the pipeline for a further hour without adding any further water into the main.
  • Re-establish the original test pressure (STP) by injecting a measured quantity of water into the test section.

Carefully draw off the water into a calibrated container until the pressure registered at the end of the pressure test (as in 2) is reached again.