Management of a Liquefied Petroleum Gas Storage and Distribution Centre

Butagaz, a French subsidiary of the Shell Group, is one of the market leaders in liquefied petroleum gas (LPG).

Located in the Midi-Pyrenees region, equidistant from the Atlantic and Mediterranean coasts, the new distribution centre at Castelsarrasin is designed to supply the domestic and industrial consumption needs of the whole region.

Unloading, loading and administrative data processing have necessitated the installation of a sophisticated data management system, which has been provided by Telemecanique. TSX 7 PLCs in a redundant configuration are connected via a Mapway network to Monitor 77 supervisors. The control and monitoring of the various stations uses ISP 70 weight indicators, Altlvar 5 variable speed drives and XBT operator terminals.

The Castelsarrasin centre

The centre is supplied with commercial propane via a rail link from Geogaz Lavera on the Fos Gulf, and from Butagaz in Pauillac, in the Gironde region. The trains consist of twelve 80 T tank wagons, of which 45 T is used. After weighing on a weighbridge the wagons are directed onto 4 tracks and to 6 unloading stations. A gas compressor transfers the propane from the tank wagons to two underground storage tanks each with a capacity of 2500 m³. It takes an average of two days to transfer the gas from a 12-wagon train. Three mixed stations are used to load tank wagons for transfer to other sites, if required.

There are three loading stations for the low capacity (19 T) tanker trucks and high capacity (39 T) trucks which enter the site after being weighed on a weighbridge. The propane is pumped into them using vertical centrifugal pumps, each with a continuous nominal flow rate of 100 m'/hr. The flow is regulated by a pressure valve which delivers the gas to the storage area. A turbine used with two dosing pumps adjusts the quantity of methanol added to the propane while the trucks are loaded. It takes 12 minutes to load a 19 T truck.

Site safety is subject to particularly stringent construction and operating regulations. The fire safety system, supplied by two reservoirs of water, each containing 1300 m³, is pressurized using two sets of motor-driven pumps each handling 600 m^3/hr. A third set, the same as the two others, acts as a backup. Three staff operate the site during the day. Outside normal opening hours trucks are loaded using a "self-service" system controlled by the automation system and monitored by a safety officer.

Objectives

To meet customers' needs, especially in terms of extending the opening hours of the loading stations, it is essential to have a tool which meets the criterion of standby redundancy. It must provide:

• safety of personnel,

• availability of plant.

Solution

The hardware architecture which has been implemented is organized into a number of levels.

• Acquisition of data by various sensors (volume, flow rate, pressure and weight measurement devices).

• At a higher level, two TSX 107 PLCs, in redundant configuration, perform the sequential processing of this data, check that the unloading and loading, storage and filling procedures are strictly adhered to,

transmit weight data to the ISP-70 weight indicators, transmit commands to start the electrically-driven pump sets and control the valves. The processing functions of these PLCs are also used to generate the settings for the two Altivar 5 speed drives which control the dosing pumps. Three XBT-C terminals are used for the operator dialogue. This enables containers to be identified and the truck/driver database to be checked (driver number, codes and authorizations, characteristics and regular checks, etc). They display transaction authorizations, and direct drivers to their allocated loading stations.

• Two PCs with Monitor 77 software control the plant, manage the trains and trucks, manage stock levels, and deal with any potential problems. One PC is active, and the other is passive. They are interchangeable. These supervisors communicate with the TSX PLCs via the Mapway network. Their real-time database checks the data entered on the XBT terminals, and the weighing procedures before and after every operation. Thus, loading of trucks is prohibited in the case of an unauthorized driver or if there is some inconsistency with the information on the database. When a transaction is accepted a bulk cargo document is printed out for the driver. If the tanker is overloaded the document is not printed.

• In parallel with this, processing is performed on a server PC linked to the two Monitor 77 PCs via an IBM PC Token Ring data network. Reports are transmitted daily via a Transpac network to the company's head office, where the invoicing department is based.

Safety

Site safety is based on hardware and procedure redundancy. An approved fail-safe PLC and the process PLCs continuously check each other to ensure that sequences are running correctly. In addition, all emergency shutdowns, gas sensors, level and pressure safety devices (which, if exceeded, place the storage areas in safe mode and automatically trigger the fire safety systems) are double checked.

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