Description

In 2009 National Grid Gas awarded Drallim Industries the contract for the manufacture of Slamshut Cabinets for their countrywide replacement program. The duration of the contract was seven years with a value of three million GBP.

The cabinets, of which there are sixteen variations, in conjunction with Slamshut valves, monitor the gas pressure and rapidly cut the flow of gas when the pressure in the system exceeds the set point and avoids overpressure conditions.

National Grid had a large number of slam-shut control systems operating on twin-stream and triple stream pressure Regulating Installations (PRIs).  Many installations were over 25 years old and required replacement to ensure that safety systems were kept in good order.

When the station outlet pressure rises, the lead stream slam-shut will close when its set point is reached. If the outlet pressure continues to rise, the slam-shut on the standby stream will then close. If the outlet pressure subsequently falls, the standby stream slam-shut will reopen to maintain gas supplies. If the outlet pressure again rises above the set point (e.g. If the standby stream is faulty), the standby stream slam-shut will again close and will not reopen. If the outlet pressure falls again, the lead stream will now reopen to maintain gas supply. Should the outlet pressure rise above the set point yet again, the lead stream slam-shut will again close and will not reopen.

The principle of operation for a two-stream installation is as follows:

The principle for three stream operation is exactly the same. The streams will close in ascending order of set point. When all streams are closed, should the outlet pressure fall, the last slam-shut to close will reopen first. Each stream will only re-open once.

The control cabinets for the other streams differ slightly from the ‘master’ in their internal pipework and are designated as ‘Slaves’’.