A positive impact on everything we touch
Over 2000 dedicated people nationwide
Providing critical infrastructure for the UK
A positive impact on everything we touch
Over 2000 dedicated people nationwide
Providing critical infrastructure for the UK

Yorkshire Water Services Limited

Commissioning, Supply and Installation of a Gas Bag


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Confined space risks eliminated with the innovative installation of a gas bag.

nmcn were commissioned to supply and install a new gas bag at Caldervale STW on behalf of Yorkshire Water.

Key Information


Commissioning, Supply and Installation of a Gas Bag


Yorkshire Water Services Limited


November 2017 - November 2018



Delivery Business Unit


Benefits at a Glance

Gas bag was installed on a concrete slab which removed the requirement for a chamber.
The elimination of a chamber lead to a reduction of health and safety risks.


The gas bag was required, at a minimum, to have a capacity of 440m3 above ground.  To do this we needed to build and install a raised platform. Other requirements were that it needed to;

  • Be a concrete construction
  • Enable access to remove pipework from the Gas Bag
  • Leave an open gap between lower and upper platforms
  • Be accessible for operations and maintenance
  • Include all necessary hand-railing and access metal works
  • Include all associated blowers and instrumentation (necessary to meet the needs of the biogas system pressure balancing)
  • Include all necessary equipotential earthing and bonding
  • Include lightning protection around the Gas Bag
  • Have suitably sized Catch-Pots at all system low points, as required
  • Ensure all Catch-Pots were located above ground (to eliminate any confined spaces)
  • Ensure all Catch-Pots have hose connections for replenishment (correlating to site connection and/or providing further hose points as required)
  • Have associated pipework of a suitable material, suitably sized and supported
  • Have all necessary isolation, purge points and pressure relief valves between Digesters, Gas Bag and Users
  • Ensure pressure relief valves have mechanical traps
  • Include all necessary cabling and cable ducting


  • Ensure all connections have blank flanges fitted
  • Relocate existing flow meters (if suitable) to the new pipework from each digester to the Gas Bag
  • Provide maintenance access to the flow meters
  • Relocate existing emergency slam shut valve (if suitable) to new pipework to users
  • Have all required MCC modifications
  • Have removable Gas Bag Bypass pipework (to divert flow from Digesters to Flare for maintenance of Gas Bag)
  • Have existing PLC coding modified to control the Gas Handling System and decommissioned Gas Bell
  • Modify SCADA screens to add new Gas Bag and system and remove existing Gas Bell system
  • Extend existing Zone fencing to accommodate new Gas Handling system
  • New Gas Bag compound and pipework areas to be finished in concrete
  • Suitable benched to existing drainage to allow the compound to be self-draining
  • Include an extension of the drainage system, as required
  • Provide new Zone signage for the Digester & Gas Bag compound
  • Have existing foam isolation valves removed and subsequent PLC code from the digester outlets
  • Be located at a safe distance from the Flare and other potentially hazardous systems


A detailed Commissioning Plan was established within four weeks of the contract award and included, but not limited to:

  • Decommissioning of existing Gas Bell
  • Purging and making safe
  • Sludge management and feeding plan
  • Operational interface during commissioning of new system and decommissioning of existing system
  • Connection of all new pipework
  • Pressure testing
  • Full commissioning and testing of the new Gas Bag
  • 21 elapsed day reliability period


Following this we worked collaboratively with Yorkshire water to provide a detailed sludge management and feeding plan.

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