Installing "Selfseal" Test Plugs

The test plug is usually screwed into a BSP socket welded to a pipe. Some valves, pumps or other items of plant are provided with suitable tapped bosses which can be used. Screwed pipe tees can also be used.

It is suggested that the socket is welded to a pipe and a hole then drilled in the pipe before fixing the test plug. An 8mm hole will be suitable for all sizes of probes.

It is also suggested that the plug be positioned in the lower half of the pipe where possible. If fitted in the top, a pocket will be formed which can trap air and, when the pressure probe is inserted, the air will enter the probe. This could affect the pressure reading. Where this is unavoidable a spare probe can be used to bleed off the air before taking the readings.

Further reasons for suggesting this location are:-

  1. The formation of the air pocket can encourage corrosion at the boundary between the air and the water.
  2. It is usually easier to reach the underside of a high level pipe and insert the probe during testing.
  3. Where flexible pipes are used to connect to the measuring instrument, they will hang down rather than kink or pull the probe.

For low level pipes, the above considerations may not apply and the performance of the test plug itself will not be be affected by the angle of fixing. If the socket is fitted on the underside of the pipe there may be a collection of sediment or scale.

This will not affect the operation of the plug but may cause blockage of the pressure probe. Ideally therefore the preferred position must be for the plug to be located at the side of the horizontal pipework where possible, and somewhat towards the bottom on high level pipework. Wherever possible a pair of test plugs should be fitted at the same height.

It will be appreciated that any pressure measurement is affected by the standing water height in the system and the static head above the point of measurement. If readings are being taken between flow and return, to determine the pressure drop across an item of plant, the difference in height of the measurement points will be included in the readings and must be allowed for. Where they can be at the same height this factor can be ignored and the calculation simplified.

Always fit test plugs so that they are accessible to probes. Thermometer probes can be up to 229mm (9") long.


Operating Procedure, pressure/temperature readings

  1. Slowly remove the cap from the plug. If, while doing so you feel or hear gas or liquids escaping, quickly retighten the cap. Determine at this time if the plug has been used improperly, and if necessary replace the plug as soon as possible.
  2. Having determined that the plug is operating correctly, remove the cap.
  3. Select the appropriate probe for pressure and/or temperature. Lubricate the probe. Examine the probe for any sharp burrs which would cut the plug core. Remove any burrs before using. Do not use non-standard or damaged probes.

Pressure Probe Insertion:

  • Determine approximate pressure in pipeline and select a gauge of suitable range complete with test plug adaptor/probe.
  • Partially insert the lubricated pressure adaptor/probe into the test plug. If you are not sure of the pressure behind the plug, be prepared to quickly withdraw the probe before rupturing or over-pressurising the gauge. The plug, in its static state and without a cap, can easily withstand over 35 BAR, 500 PSI without leaking. Do not assume that, since the plug is not leaking, that it is safe to quickly and fully insert a low-range gauge. If you guess wrong, then the pressure gauge could explode in your hand.
  • When the pressure gauge needle stops moving up-scale, then fully insert the probe and read the pressure.
  • Do not leave the probe in the plug any longer than necessary. Always screw the adaptor/probe union to the top of the test plug to prevent the internal pressure from ejecting the instrument and probe.
  • When removing the probe, do not place your face near the plug. The plug should not leak when you remove the probe, but always observe safety precautions.
  • As soon as the probe is removed, replace the cap.

Temperature Probe Insertion.

  • Temperature probe insertion is similar to pressure probe insertion with the exception that the operator should always first determine the pressure behind the plug before insertion of the temperature probe. This establishes that the plug is operating within its specification limits. After pressure has been determined follow steps as outlined in the Pressure Probe Insertion paragraphs above.

Using A Test Plug Which Has Been Installed And Unused For Several Years

  • Follow all operating procedures shown above.

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