Most of the issues encountered by gas pressure boosting are experienced on standard low-pressure systems (LP) that operate at 21mbarg notional pressure, although often less than this in urban locations where the high local demand and limited available capacity in the gas network lowers pressure.
The sudden start-up of non-typical plant like burners, boosters, compressors, or CHPs can reduce the inlet gas pressure. This causes the under-pressure cut-off switches to shut off before the pressure recovers from the meter or inlet gas service pipe. This is very often a control issue and slowing down the response of the booster or taking the appliance off the gas may solve the issue – allowing the gas network, upstream pipework offtake, meter, and downstream pipework to respond to change of loads in time.
A Rotary Positive Displacement (RPD) gas meter type can exacerbate this issue as the governor on the meter takes a second or two to open. These meter types are often required for ‘larger’ peak hour gas demands or for gas demands that can take a few seconds to respond.
Considerations if boosters and compressors are used
▪ Boosters and compressors should not be considered as a solution to overcome incorrectly sized downstream installation pipework.
▪ The proposal to install a booster or compressor must be notified to installer at the pre-quotation design stage to ensure that correct network analysis is undertaken.
▪ The proposal to install a booster or compressor must be notified to the installer at the pre quotation design stage to ensure that a correctly designed and sized new gas connection, offtake inlet and primary meter works are proposed, approved by the Gas Transporter and subsequently carried out by installer
▪ It is not possible to retro-fit a booster or compressor post-connection if the system has not been designed to specifically accommodate this.
▪ Retro-fitting a booster or compressor post-construction could render the entire installation as `unfit for purpose` and potentially unsafe. This must be understood.
▪ The Gas Transporter can opt to disconnect the gas supply if they deem it to have been incorrectly designed or to have a detrimental effect on their network due to the presence of boosters or compressors were not notified to them at the design stage.
▪ Boosters and compressors supplied by the various approved manufacturers have a “soft start” or programmable ramp-up ability. Whilst the appliance or burner may take longer to ramp-up, if the booster suddenly starts then that can cause issues.
▪ The downstream installation pipe from the primary gas meter to the booster or compressor to appliance must be correctly sized.
▪ The under-pressure cut-off switch for the booster is fitted with a damper or local reservoir to slow down the sensing of lowering inlet pressure.
▪ The under-pressure cut-off switch for booster should be located sensibly.
▪ If the downstream installation pipes between meter and booster or compressor and appliances is relatively short then a pressure accumulator may be installed to increase capacity to smooth out pressure transients.
▪ Boosters or compressors may not be installed in the same room or space as the primary gas meter.
Key issues to consider
If you decide that a gas booster is required for you site, then there a number of considerations that must be taken into account to ensure a safe and compliant installation. Not just this, but an installation that is reliable and has efficient performance.
Should you decide that a gas booster is proposed or is going to be required, then you should provide all of the necessary information as early as possible at the pre-quotation or pre-design stage. One of the most important things for any business or organisation is ensuring that their heating plant operates efficiently to minimise their energy costs and environmental impact. In the scenario of a gas-fired heating plant there are of course a number of factors that have to be taken into account during design to achieve maximum efficiency. Not least of these being the potential requirement to boost gas pressure to the burner.
All gas-fired burners have a legal requirement for a minimum gas pressure to ensure their correct operation. In some examples the incoming gas pressure already available from the Gas Transporter’s network and from the primary gas meter may be sufficient, but in others it may fall well short of the minimum working pressure stated by the appliance manufacturer. Here, you may need to consider the installation of a gas booster – although careful selection is advised.
Assessment and Evaluation
Any assessments or evaluations of requirements for new offtakes and gas connections would likely be based on the information provided to the installer. If it is found that incorrect or incomplete information was provided by you, your business, or 3rd party handling the gas installation, then any design, review, and approval by the Gas Transporter and any installation work carried out would be incorrect.
Should a new connection be completed based on the foundation of incorrect information, or if gas pressure boosting plant and equipment is installed that was not advised prior to the design and assessment stages, then the Gas Transporter may take the appropriate action under their license to operate and ensure that the gas network is kept safe and protected. This action may encompass a complete disconnection of any offtake from the gas supply network. Any costs incurred to rectify these matters would be passed onto the customer, so it is therefore essential that any and all information that you supply is complete, correct, and honest.
Gas booster selection
It is recommended that in any scenario where the booster is located close to the primary gas meter that suitable design is considered to determine the oversizing and selection of an appropriate length or section of downstream installation pipework between the outlet connection on the primary gas meter to the inlet of the booster. This oversized section of pipework acts much like a reservoir to dampen the possible adverse effects of the booster lift on the upstream network, primary gas meter, and control components.
Controlling the Gas Booster
Once a gas booster is installed, they can be configured to run continuously or to use automatic/thermostatic control. The benefits of continuous running are avoidance of frequent pressure fluctuations while reducing the wear and tear on the booster motor, bearings, and drive belt resulting from stop/start operation.
Disadvantages of continuous running are increased levels of noise and energy consumption, both of which can be avoided by switching to automatic or thermostatic operation. Boosters must have a low inlet pressure cut-out or cut-off device that shuts down the booster in the event that the gas supply pressure falls below a pre-determined level. This protects the incoming gas supply from being compromised by the operation of the booster. Often the booster start surge can actually lead to activation of this low-pressure device and can result in unwanted tripping out of the booster.
This can usually be overcome by using a booster motor with a variable speed drive, soft start, or by installing an anti-surge reservoir upstream of the booster. The anti-surge reservoir works by acting as a ‘damper’ between the inlet pipework and the inlet gas pressure switch. The damping effect is created by fitting a restrictor in the connection between the inlet pipework and reservoir – preventing detection by the inlet pressure switch of momentary surges and changes in pressure the booster starts and stops. Of course, care must be taken in sizing the restrictor to ensure that any permanent reduction in the inlet pressure can still be detected by the gas pressure switch within the mandatory three second period.
Variable speed drives are most useful when the booster serves multiple appliances. If one boiler switches off then the downstream gas pressure will increase and will be detected by the pressure transducer to be relayed to the inverter. Once this happens, the inverter will act to reduce motor speed on the booster and achieve stable downstream pressure. These drives can also deliver major energy savings as well as reducing booster start surge issues and wear and tear stress. Generally, the booster is interlocked to the appliances it supplies so that failure of the booster will cause an appliance shut down. Dual-fuel burners should be interlocked with the fuel selection so that the booster only operates for gas firing.
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