as can be seen from fig1 and 2 , the cleaning unit shown generally at 10 comprises a pipe 2 detachably connected in a fluid tight manner at both a first access point 4 and a second access point 6 to a gas main 8 . at access point 4 the pipe 2 may be insertable into the main 8 by a selectively variable amount . an openable and closable inlet 12 for purging gas is provided in the pipe 2 close to the point 4 . a first manual valve 14 and first automatic valve 16 , with pneumatic control , are provided in the pipe 2 downstream ( with respect to the gas flow ) of point 4 . on the downstream side of the first automatic valve 16 , a first separation stage 18 is located . this first separation stage 18 comprises a cyclone separator 20 connected by means of a manual valve 22 to a disposal chamber 24 , which is vented to the atmosphere via a further manual valve 26 . opening the valve 22 , periodically , allows separated debris to drop from the cyclone separator 20 into the chamber 24 . opening the valve 26 , periodically , allows the debris to discharge from the chamber 24 . downstream of the first separation stage 18 , a second separation stage 28 is located . the second separation stage 28 comprises a filter 30 connected by means of a manual valve 32 to a disposal chamber 34 which receives separated debris when the valve 32 is opened periodically . the chamber 34 is vented to the atmosphere via a further manual valve 36 when opened periodically . the filter 30 is a coarse filter , which may be made of wire mesh . downstream of the second separation stage 28 , a fine filter 38 is located . the filter 38 is a fine filter , which may be made of paper . downstream of the filter 38 , the pipe 2 divides into two branches , a first branch 2 ′ leading to a roots blower 40 . the blower 40 is driven by a diesel powered engine 42 to which it is connected by means of a safety coupling 44 . diesel fuel is supplied to the engine 42 by means of a fuel valve 46 . the second branch 2 ″ leads via a second automatic valve 48 back to the first branch 2 ′, downstream of the blower 40 . the function of the second automatic valve 48 , which as will be explained later , is spring - loaded ‘ fail - open ’ is to protect the blower 40 from damage due to build - up of excess pressure downstream of the blower 40 . the pipe 2 then returns via a third automatic valve 50 , a non - return valve 52 and a manual valve 54 to the second connection point 6 where it is connected to the gas main 8 . the function of the non - return valve 52 is to ensure that the gas flow cannot reverse . a plurality of sensors s 1 to s 5 are positioned along the pipe 2 to sense the pressure at the points where they ire connected . a first sensor s 1 is positioned betwveen the first automatic valve 16 and the first separation stage 18 , a second sensor s 2 is positioned between the first separation stage 18 and the second separation stage 28 , a third sensor s 3 is positioned between the second separation stage 28 and the filter 38 a fourth sensor s 4 is positioned between the filter 38 and the blower 40 and a fifth sensor s 5 is positioned between the blower 40 and the third automatic valve 50 . the sensors s 1 to s 5 are connected to a central control and monitoring unit ( not shown ). as can be seen more clearly in fig2 a suction head shown generally at 56 and mounted on an inner tube ( not shown ) is introduced into the gas main 8 at the first connection point 4 . the inner tube on which the suction head 56 is mounted is controlled by means of a jack 58 . for ease of transport , the cleaning unit 10 is preferably mounted on a trailer 60 within a housing 62 . preferably debris from a length of the main 8 between the access points 4 and 6 is scraped or otherwise suitably propelled by appropriate means towards the access point 4 , whilst the main is live , so this debris is gathered at a single place in the main at or adjacent to the access point 4 for extraction by the suction head 56 . in operation , prior to start - up of the cleaning process , the first and second manual valves 14 and 54 are closed , as are the first and third automatic valves 16 and 50 . the first and third automatic valves 16 and 50 are spring loaded ‘ fail - shut ’, i . e . in the closed position when the system is not in operation . the second automatic valve 48 is open when the system is not in operation and closed when it is in operation : i . e . it is spring loaded ‘ fail - open ’. as the cleaning process is started , the cleaning unit 10 is initially purged with gas which is injected into the pipe 2 via the inlet 12 , so that the whole system is filled with gas and there is no air present . the manual valves 14 and 54 are then opened and the control unit opens the automatic valves 16 and 50 , the fuel valve 56 and shuts the automatic valve 48 and then the diesel engine 42 is started , to operate the blower 40 . the engine 42 is hand - cranked , for safety reasons , as an electric starter might produce sparks which could ignite the gas . the fuel valve 46 is a solenoid operated valve which is linked to the control system , so that the control system can shut - down the diesel engine 42 in an emergency , by closing down the fuel supply to the engine . while the cleaning system is in operation , the sensors s 1 to s 5 continually monitor the gas pressure to ensure that it is above atmospheric pressure and below a maximum set operating pressure , which is substantially equal to the pressure in the gas main . any loss in pressure below atmospheric pressure or increase above the set operating pressure indicates to the control system that there is an emergency , caused for example by a blockage in the system and the control system shuts down the cleaning process . in the event of an emergency , the following close down sequence is followed ; ( i ) the diesel supply to the diesel engine 42 is shut down ; ( ii ) the second automatic valve 48 is opened to bypass and thus protect the blower 40 by allowing gas pumped thereby to circulate through the pipe branch 21 ″ and valve 48 and back to the blower to ensure there is no excessive pressure build - up whilst the system is in the process of shutting down ; ( iii ) after a set time , typically 5 seconds , the control system closes the first and third automatic valves 1 and 3 and the cleaning system stops operation . by monitoring the outputs of the sensors s 1 to s 5 , the control system is able , in an emergency shut - down as described above , to indicate to the operator where the problem has arisen , which is helpful in achieving a more rapid solution to the problem . the control system is further designed not to allow the cleaning system to operate outside of the system set parameters . for example , the control system can be set to allow the cleaning system to operate with a maximum pressure differential across each of the filters 30 , 38 , thus detecting when a filter is blocked or becoming blocked . either access point 4 or 6 may be specially drilled or may be a known access point normally provided to a gas main . the access points 4 and 6 are sealed after the cleaning procedure is completed and the pipe 2 is detached from the access points .