When gas pressure needs a boost
Published: 04 February, 2015
When gas boosters are required for heating system there are a number of key issues that must be taken into account to ensure reliable and efficient performance. Bernard Dawson, tTechnical director of Riello, explains.
Ensuring that heating plant operates efficiently is clearly very important for any organisation that is seeking to minimise energy costs and environmental impact. In the case of gas-fired heating plant there are a number of factors that need to be taken into account when designing for maximum efficiency – not least of these being the potential requirement to boost gas pressure to the burner.
All gas-fired burners have a requirement for a minimum gas pressure to ensure correct operation. In some instances the mains gas pressure already available may be sufficient but in other cases it may fall short of the minimum pressure and require installation of a gas booster. Where a gas booster is required, careful selection is essential.
Gas booster selection
Gas booster selection involves calculating the gas volume flow rate and the gas pressure lift required for operation of the burner. The gas flow rate is calculated by dividing the burner firing rate by the calorific value of gas. The pressure lift is calculated by subtracting the gas mains supply pressure from the minimum pressure required at the burner.
In situations where a number of burners are being supplied by a single gas booster, selection involves calculating the total gas flow of all the appliances and the maximum gas supply pressure required by any single appliance.
It is also very important to note that all pipework should be sized as normal, irrespective of whether a booster is being used, aiming for just a 1 mbar pressure drop between the gas supply meter and the appliance. Installing a gas booster is not a way to enable the use of smaller gas pipework and the use of a gas booster will not overcome any existing issues with undersized pipework.
When installing the gas booster it may be necessary to provide your gas supplier with a ‘ramp profile’ so they can ensure the pipework upstream of the meter is correct. The ramp profile is not governed by the booster but by the burner/boiler or other appliance that is consuming the gas.
Gas booster installation
The installation requirements for gas boosters are detailed in IGE/UP/2 ‘Installation Pipework on Industrial and Commercial Premises’, so they must be installed in compliance with this.
Early in the design/selection process it is very important to identify the most appropriate location for the gas booster(s). The chosen area should be well-ventilated, dry, clean and easily accessible for both installation and maintenance purposes. Also, to minimise the length of high-pressure pipework required, the booster(s) should be as close to the burner(s) as is practically possible. Usually this will be somewhere in the boiler plant room.
In terms of pipework runs, the maximum recommended volume of boosted pipework is no more than 25% of the total run between the meter and the burner. If it is necessary to have a higher volume of boosted pipework than the recommended maximum, then we would recommend installation of a non-return valve on the booster outlet.
Where parallel or duty/standby booster systems are in use, non-return valves must be fitted to prevent gas re-circulating around the non-operative booster. When non-return valves are used, the pressure drop across the non-return valve must always be taken into account when selecting the booster and sizing the gas pipework.
Ideally, the gas booster should not be located in the governor or meter room. However, if this is unavoidable it must be positioned so that it does not compromise access to the governor or metering plant. Any electrical equipment used in this location must be suitable for a Zone 2 area with group II A gases.
Similarly, locating the gas booster in an air compressor plant room is not recommended, but if this can’t be avoided then, again, access to the air compressor plant should not be compromised. Also, the air inlet to the compressor must be connected to ductwork that terminates outside the plant room. In the event of a leak, this will avoid the compressor sucking in gases.
Wherever the gas booster is located, anti-vibration mountings and flexible gas inlet and outlet connections are required to minimise noise transmission and pipework stresses. This is the sole purpose of the flexible connections - they must not be used to correct any misalignment of the pipework.
The gas booster(s) should be firmly positioned on a horizontal, flat bed or platform. If connecting the booster requires any changes to pipework sizes then reductions or enlargements in pipework diameter should be smooth and as close to the gas booster as is practically possible.
Controlling the gas booster
Gas boosters can be configured to either run continuously or to use automatic/thermostatic operation.
The key benefits of continuous running are avoiding frequent pressure fluctuations and also reducing wear and tear on the motor, bearings and drive belt through stop/start operation. The disadvantages of continuous running are increased noise and energy consumption – both of which are avoided by using automatic/thermostatic operation. BS 8487 (Design and construction of gas boosters used in association with combustion equipment - Specification) ‘favours’ automatic booster operation but this is not an imperative.
Boosters must have a low inlet pressure cut-out device that shuts it down in the event that the gas supply pressure falls below a set level (minimum 10 mbar). This device protects the gas supply main from being compromised by the operation of the booster. Often the booster start surge can lead to activation of this low pressure device, resulting in nuisance tripping of the booster.
This can be overcome by either using a booster motor variable speed drive/soft start or 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. This ‘damping’ effect is created by fitting a restrictor in the connection between the inlet pipework and the reservoir. This prevents detection by the inlet pressure switch of momentary surges and changes in the inlet pressure as the booster starts and stops. Care must still be taken in sizing the restrictor to ensure that any permanent reduction in the inlet pressure is detected by the gas pressure switch within the mandatory 3 second period.
Variable speed drives are particularly useful when the booster supplies multiple appliances (e.g. several boilers). If one boiler switches off, then the downstream gas pressure will increase and this is detected by the pressure transducer which relays this to the inverter. In turn, the inverter reduces the motor speed on the booster to achieve stable downstream pressure. Variable speed drives can also deliver significant energy savings, as well as reducing booster start surge issues and wear & tear stresses.
Generally the booster is interlocked to the appliances it is supplying so that failure of the booster leads to appliance shutdown. Dual fuel burners should be interlocked with the fuel selection so that the booster only operates for gas firing.
Given all of the factors that can influence the correct performance of a gas booster there are clear benefits to seeking specialist input when getting involved with this highly complex area of specification.
For further information please visit: www.rielloburners.co.uk