Food technology
Some form of mechanical ventilation will normally be incorporated into the design of a food technology area, to supply fresh air for the occupants and to ensure complete combustion of gas. Any mechanical ventilation should be interlocked with the gas supply such that, in the event of a fan failure, the gas supply will be isolated, as described in BS6173/20092 (section 11.1).
A food technology area will normally have a number of domestic-style gas cookers around the room, so it can be difficult to incorporate a cost-effective ventilation solution to remove both the products of combustion and the byproducts of cooking efficiently.
IGEM UP11.2 (section 11.2.4) takes this potential issue into account and states that, where the ventilation requirements might not be met, a carbon dioxide (CO2) monitoring system should be installed. CO2 monitors should have a warning alarm at a level of 2,800ppm, with 5,000ppm being the level at which the gas must be isolated.
Solutions for teaching and food technology areas would be to install a control and monitoring panel by the main teaching area, to ensure the integrity of the gas supply and equipment by use of gas pressure proving. Such devices can also include the facility to isolate the gas in the event of a high CO2 level or ventilation-fan failure. A gas pressure proving system for use in a teaching area should be fitted with an emergency gas-isolation button on the panel, as well as a key switch to control gas availability at workbenches. A countdown timer should also be incorporated, to ensure that gas is not available for out-of-hours, unauthorised use.
A well-designed gas pressure proving system ensures that there is no pressure drop downstream of the control valve – that is, no leakage or open appliances – before allowing gas to flow. Two main methods of gas pressure proving are available: 1) Single-mounted downstream sensor: This method is assumptive, because it takes a snapshot of the gas pressure upstream of the safety shut-off valve (SSOV) when it is first opened, and then looks downstream of the (subsequently) closed SSOV for a pressure drop. Consideration should be given as to whether it is acceptable for a safety system to be assumptive in its design, in view of more recent technological advances towards fail-safe operation. An example would be that, if the gas supply pressure was too high, for whatever reason – for example, because of a failed regulator – then the valve should not be opened at all, as to do so could create a dangerous situation. Single-sensor systems are not fail-safe where there is an installation problem with the SSOV and a gas leak exists. 2) Differential pressure sensing. The other, more recently exploited, method of monitoring the gas pressure is by means of differential pressure measuring. This technique measures the pressure differential across the inlet and outlet of the solenoid gas-supply valve. This provides continuous pressure readings – effectively, during and after the pressure test – so ensuring an accurate safety check is carried out with no assumptions during the testing time. Such a system can constantly monitor the supply pressure without opening the SSOV – and is fail-safe where a gas leak is also present. Systems that monitor CO2, CO and combustible gases – as well as controlling the occupancy scheduled ventilation rates, bench water supply and electrical isolation – are available in a single control panel.