Fuel Supply
An essential part of the infrastructure supporting the operation of a CHP package is the system through which the gaseous fuel is provided.
The gas supply to a CHP unit may need to be uprated from the existing site supply to take account of the increased gas demand. Furthermore, if the supply pressure is too low a gas booster/compressor will need to be added.
Natural gas is by far the most common fuel for packaged CHP units. The supply of natural gas to a user is by pipeline from the national distribution network, much of which is owned and operated by Transco. The gas is put into the system at its connections with the onshore and offshore sources by a number of gas providers or ‘shippers’, which provide gas to the user and pay Transco or other pipeline operators for use of the distribution system.
The installation of a gas-fired CHP package will usually increase the site’s consumption of gas, as the new plant will generate both heat and power and will usually be operational for a large proportion of the year. As well as the increase in total annual gas consumption, there will usually be an increase in the maximum rate of consumption. It is, therefore, important to establish whether the local gas supply system and the existing site gas connection can meet the new peak requirement. If not, it may be necessary to increase the capacity of the existing site connection.
Modifications of this type need to be discussed with the system operator (usually Transco), or with an independent company that offers a service managing all aspects of gas connections. A number of key issues need to be defined during these discussions:
• The anticipated annual gas consumption. It is important to remember that many suppliers quote the fuel consumption of their plant as NCV, while the consumption data supplied for gas connection assessment must be given as GCV.
- The anticipated maximum rate of gas consumption. This is usually expressed in MW or in therms per hour. Again, the data must be quoted as GCV.
- Gas supply pressures. Where the intention is to install a gas turbine CHP package, it is usually more cost-effective to have the gas supply at the supply pressure required by the CHP unit. This avoids the costs associated with installing and operating pressure-boosting equipment.
- The likely route for new supply pipework and the location of metering and pressure regulating equipment. The general preference is for the metering and regulating equipment to be close to the site boundary. This means that the pipework beneath the public highway is owned by the pipeline operator, while pipework on the site is the responsibility of the CHP plant installer and owner.
While gas supply system modification may be an option in some instances, it may be simpler and more effective in other cases to install a separate new gas supply to a CHP plant, while leaving the existing system in place, unaltered. This has the advantage of avoiding disruption to other site functions, and it may allow the new gas supply to be provided at the pressure required by the CHP package.
A new gas connection to the site can be provided either by Transco or by other companies offering the necessary specialist services. The metering and regulating equipment will usually incorporate safety shut-off valves, together with vents designed for the safe release of trapped gas in the event of a problem with the system. The design and installation of the pipework, valves etc. are required to meet appropriate standards and specifications.
It is important to ensure that the pipework routing is clearly marked on the site drawings, and it is usual to install markers on the surface to help confirm pipework location. Once installed, the connection and pipework require little or no maintenance or attention, apart from routine checks.
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