Plant consist of three different compressors type driven by electric motors installed on different natural gas Liquefaction processes: number 6 compressors named V6 (Nuovo Pignone 3BCL525 centrifugal compressor), number 2 compressors named V2 (Nuovo Pignone 3BCL606 centrifugal compressors), number 2 compressor named GB2 (Nuovo Pignone BCL503 centrifugal compressors).

Electric motors operate at fixed rotating speed. Vorecon system is installed between electric motor and compressor to allow the control of compressor speed changing electric motor torque.

Liquefaction plant exploits the freezer cycle concepts as show in the following figure:

Propane gas is used as refrigerant gas in the process.

All liquefaction processes are similar and are realized using two compressors of the same type.

Following figure shows a SIMPLIFIED LAY-OUT OF THE STATION:



This process has a particular start-up procedure because electric motors and compressors are designed to operate at process working condition: propane gas in compressors suction should be close the vacuum pressure and temperature should be 10°C as maximum. In really, when plant starts for the first time, propane gas conditions shall be very different from design conditions: propane temperature shall be close to environment temperature and propane pressure depends on the amount of propane which has been used to pressurize the cycle. These means that, when compressor starts, electric motor overload trip could occurs.

To avoid this some dedicated functions are developed in the control system:

  • Dedicated start-up procedure to allow the maximum closing of antisurge valves before the speed increasing;
  • Flow measure compensation using measured propane gas pressure and temperature to allow the maximum closing of antisurge valves before the speed increasing;
  • Compressor loading procedure can be automatically “paused” if overload is approaching to occur.

SSE supplied the Unit Control Panels (one per compressor) and SSE technician operates during commissioning activities providing also technical assistance during all critical start-up phases of the cycle.

In each panel the PLC SIEMENS 400H in “hot-stand-by” redundancy configuration is installed. Every panel is composed of one SIEMENS WinCC SCADA supervision systems  installed in the computer on front panel.

Control system manages all the auxiliaries and the machine, valves through automatic or manual sequences. It also manages the permissive to start and the compressor trips.
Valves management in automatic modality allows the system to carry out the compressor pressurization before the compressor starts, without the operator’s intervention.



SUPPLY 10 Unit Control Panel (6 for compressor type V6, 2 for compressor type V2, 2 for compressor type GB2)
SUPERVISION Siemens Wincc SCADA system


SSE operates in the field of industrial control solutions and undertakes the design, software development, production and commissioning of Control & Supervisory Systems for the Oil & Gas and Power Generation Industries. SSE main task is to provide and service Control and Supervisory Systems for rotating machinery and associated auxiliary systems.

The controller normally used in these applications is the PLC.

Following the list of PLC generally used in our applications:

  • General Electric Intelligent Platform – 90/30, 90/70, Rx3i, Rx7i series
  • Siemens – 300, 400H, 400HF series
  • Rockwell Automation – RsLogix 5000 series, PlantPax
  • Foxboro – Triconex, Trident
  • Telemecanique – Premium
  • HIMA – Himax
  • Woodward – Micronet plus, regolatori 505, 5009

Anyway, on gas turbine control applications, dedicated PLC are normally used:

  • General Electric – MarkVIe
  • General Electric – MarkVI
  • General Electric – MarkV,
  • Woodward – Micronet plus

At least one HMI (Human Machine Interface) is normally developped for control panel.

SSE make use of SCADA (Supervisory Control And Data Acquisition) supervision software to develop HMI. Following the main supervision software used:

  • General Electric – Cimplicity
  • Siemens – Wincc
  • Rockwell Automation – RsView, FactoryTalk
  • Intouch – Wonderware


  • process controls are developed completely internally on PLC.
  • Dedicated freeze cycle process start-up procedure functions are developed in the control system.


  • Control System readily adapted to any similar application (freeze cycle)
  • Cost Reduction