Transient Gas Meter Gas Pressures at Boiler Start Up

Transient gas pressure issues (non boosted and boosted) can be experienced on low pressure systems operating at 21 mbar notional gas pressure.

The start up of a gas appliance (or significant load change) can cause the inlet gas pressure to dip momentarily BELOW 21 mbar , in extreme cases causing the appliances “under-pressure cut off switch” to shut off before gas pressure recovers via the gas meter/gas network back to 21 mbar. This is your classic nuisance “trip”.

This is a “control issue” and by slowing down/removing the sensitivity of the client booster/appliance take-off should mitigate/solves the issue. The Clients pipe designer (meter to appliance) should therefore keep in mind to cater** for possible the momentary (transient) pressure dip to prevent nuisance trips of his gas appliances.

Note this pressure dip may be exacerbated if a rotary positive displacement gas meter (“rotary” gas meter) is used which can take a second or two to fully respond (the rotors need to spin up) – and the pressure regulator on the meter takes a second or so to open fully (as evidenced in the video).

** If you have only a limited pipe volume between gas meter and your appliance and/boosters/compressors are used, we would recommend considering:

  • The pipe from meter to booster/appliance is sensibly sized – e.g. one at least one or two sizes up from design minimum to effect an accumulator reservoir (increase volumetric capacity) to smooth out the pressure dip transients
  • Any actual booster/compressor supplied has a “soft start” or programmable ramp up ability – (typically 6 second ramp). Whilst the appliance/burner may take 100 or so seconds to ramp up it’s the booster sudden start that can cause issues – before the appliance takes gas.
  • The under-pressure cut off switch for the appliance / booster is fitted with a damper/local reservoir (there are proprietary fittings available) to slow down the sensing of lowering inlet pressure.
  • The under-pressure cut off switch for the booster is located sensibly (e.g. not in the throat of the booster suction pipe).

If you have only a limited pipe volume between gas meter and your appliance and/boosters/compressors are used, we would recommend considering:

  • The pipe from meter to booster/appliance is sensibly sized – e.g. one at least one or two sizes up from design minimum to effect an accumulator reservoir (increase volumetric capacity) to smooth out the pressure dip transients
  • Any actual booster/compressor supplied has a “soft start” or programmable ramp up ability – (typically 6 second ramp). Whilst the appliance/burner may take 100 or so seconds to ramp up it’s the booster sudden start that can cause issues – before the appliance takes gas.
  • The under-pressure cut off switch for the appliance / booster is fitted with a damper/local reservoir (there are proprietary fittings available) to slow down the sensing of lowering inlet pressure.
  • The under-pressure cut off switch for the booster is located sensibly (e.g. not in the throat of the booster suction pipe).

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