Industrial Apparatus Consultants Ltd - Dilo SF6 gas handling, measuring instruments and accessories.

SF6 gas is man made and an excellent insulator and has become practically irreplaceable in major applications such as high voltage switchgear and particle accelerators.

It is however a significant greenhouse gas, is tightly regulated and must therefore not be released into the atmosphere.

Dilo was founded in 1951 with the development of a metal to metal seal principle incorporated into high pressure tube unions, creating a range of high integrity unions with working pressures to 1000 bar and temperature ratings to 500 deg C. The Dilo logo shows this metal to metal seal principle. Please contact us for further details on this range which is widely used in turbines and chemical plants.

This quality was transferred to the range of couplings produced for SF6 gas handling to ensure absolutely minimal leaks on the range of SF6 service carts developed since 1967. Dilo adopted a policy of high quality and high integrity, which lead to several important choices: No oil lubricated machines in the SF6 circuit, no refrigeration to liquefy the gas for storage, to keep moving parts to a minimum. Thus today there is a full range of SF6 service carts based on oil free compressors, all allowing SF6 gas to be recovered from circuit breakers and liquefied into standard external storage tanks without difficulty. Alongside this product range the necessary measuring instruments were developed. Today Dilo can offer a complete range of devices to handle SF6 gas safely, to check its purity to avoid danger to operators and for reuse, and to offer reliable recycling of the gas. All this is based on the use of high integrity components, which include valves, fittings, hoses and accessories using the Dilo metal to metal seal principle.

Environmental concerns over greenhouse gases means it is no longer acceptable for SF6 gas to be vented to atmosphere. Even small quantities of this dense, inert gas must be recovered. Decomposition products which may occur due to power arcing must be removed by filtering (and care taken to clean the gas chamber if work is to be done on it). If the gas is too contaminated for purification simply by filtering, then it must be returned to a gas supplier for disposal. If is it however simply contaminated with air due to a leak, then the air must be separated out to obtain pure SF6 for reuse. Dilo can carry out this separation. Dilo is also involved in new developments such as SF6/N2 mixtures (GIL) and can supply all the equipment needed for mixing, evacuating the gas zone, filling, and recovering the mixture into storage.

Thus the Dilo range includes gas service carts from small portable units for medium voltage switches with just a few hundred litres of gas, to large units for 400 KV GIS with perhaps 200 Kg SF6 per phase. Bigger units are also available for use with even larger quantities of gas associated, for example, with high voltage particle accelerators. Static storage tanks for SF6 can also be supplied, in different sizes and for either gaseous or liquid storage. These can be used on a specific site, but must not be transported on the roads under pressure. Storage in standard gas bottles is used when transport is required.

All Dilo service carts are made up of similar components. Only the size varies to meet reasonable operation times for different volumes of gas. There is a filter, then vacuum compressor and compressor to remove gas from the switch, down to 50 mbar, and the compressor pumps it into storage. After maintenance, the gas compartment must be evacuated before refilling with gas. This is achieved with a vacuum pump. A pressure regulator controls the filling rate and final pressure of SF6 gas. Recent developments include replacing the vacuum compressor on some service carts with a rotary suction pump to achieve full recovery to 1 mbar. Another option depending on the volume of gas is to choose the compressor for either gaseous or liquid storage of SF6 gas. Carts may also be road trailer mounted to allow easy movement from one site to another.

Finally, the rate of gas flow depends also on the smallest diameter along the path: hoses and couplings can be in 8mm, 20mm and 40 mm nominal bore (DN8, DN20, DN40), the biggest size being reserved for the biggest service carts. Hoses can be rubber, or stainless steel with a wire braided exterior. Self seal valves provide excellent leak integrity and keep hoses dry even if left unused for long periods. Many switchgear manufacturers use Dilo couplings. Adapters can be provided to a wide range of other types of connector, and also for different SF6 bottles.

The measuring instruments cover all those required to confirm the gas quality as required by IEC. Firstly, check for decomposition products. If they are present, then care must be taken when working on the gas zone. Dilo offers handling kits with protective clothing, cleaning equipment, and neutralising solutions. Then check for dew point and SF6 volume percent (as against oxygen measurement). Individual instruments can be supplied but more recently the trend is towards combined units which require less gas as all measurements are taken with one sample (also saving operator time), and allow the data to be saved and downloaded later. There is also a new device to collect the gas used in measurement to avoid all gas emission. If any leak detection is required, then use the battery powered simple hand held sniffer. Finally, in case of indoor or underground substations, continuous monitoring may be required to alarm in case of an SF6 leak. Dilo offers an instrument for this.

Thus Dilo offers a complete service for the handling of SF6 gas required for maintenance of SF6 switchgear, including reuse of the gas, purifying the gas, measuring instruments to allow this, and is continually at the forefront of development to ensure safe handling of SF6 gas to contribute to the environmental concerns of our era.