+98 21 22774509





Chista instrument team has good experiences in different projects
and will be so pleased to present two different types in bellow case studies.

High Speed control system

High Speed control system

Tank Gauging System for LPG tanks

Gauging System for LPG tanks

High Speed control system

Project overview

There were 12 LPG tanks and facilities located in a tank farm and their task is collecting liquefied gases from several refineries then send for ship loading. The facilities were known as Propane & Butane Boil off and Refrigeration Plant (BOG).

Scope of work

LPG is remained in the tanks and stored in very low temperature. Gases are boiling and vapors slightly come up. Since increasing tank internal pressure is not allowed so boiled gases should be collected from top of the tanks and send to five compressors skids to liquefied again and send back to storage tanks. These compressors and related facilities are known as BOG.

Bellow machines and equipments were used for this project.

  • • 3 compressors skid (capacitance: 750 KW – Vertical)
  • • 2 compressors skid (capacitance: 350 KW – Horizontal)
  • • 12 tank (LPG) – double wall
  • • 1 tank gauging system (redundant) – MTBF 94 years
  • • TGS DI & AI: 144
  • • 2 metering system on export line
  • • 12 Pump station
  • • Field Instrument

The most important part of the project is control system with fastest possible cycle time.

Control system capability

Used control system contains three main parts ESD, DCS & FGS with following specification

  1. One Process Control system with redundant processor, power supply and communication module with non-redundant I/O card with full diagnostic and line monitoring function and plant balancing.
  2. One SIL 3 PLC with redundant processor, power supply, communication module, non-Redundant IO card for emergency shutdown and FGS system. All IO cards fully support SOE function with SIL3 certification.
  3. Condition Monitoring System (CMS) was done in 6 industrial PC to monitor CMS temperature, vibration sensors, alarm, trip. A graphical display was used in HMI and can be accessible in separate HMI from other engineering stations.
  4. One fiber optic patch panel was considered in each Control room and local equipment rooms to have faster path for data transmission.
  5. Interposing relays were considered for each marshalling panel interconnection and MCC panels.

used hardware can support bellow services well:

  • • 2500 I/O
  • • 80 SIL3 Loops
  • • Fully redundant system
  • • CPU AS 410 H & FH
  • • Modbus TCP redundant
  • • PCS 7 IL_Library
  • • APL Library


Fast loop was the most important requirement and cycle time should be less than < 100 ms as per project specification. Optimum point for cycle time was 75 ms and we could reach to 76 ms in actual condition. It is very important result in this project.

Tank Gauging System for LPG tanks

Project overview

There were 4 LPG tanks. Two tanks contain liquid propane and two tanks contain liquid butane in an area known as offsite area of a gas refinery. All products come out from refinery and stored in the tanks for ship loading.

Scope of Work

Tanks are double wall and full containment also we have to prevent overfilling by using a very high accurate tank gauging system. Since the tanks used for export also all devices should have custody transfer approval and bellow specification should be followed on this project. Tanks dimensions are 70 m diameter with 40 m height. Storage temperature is around -40°C.

  • • Using two different types level transmitters to reduce failure risk
  • • One tank gauging package to show all tank process data well for each tank.
  • • Three measuring transmitter packages for emergency action to show accurate level and voting in ESD and get action based on 2 out of 3 voting method.

The most important part of the project is control system with fastest possible cycle time.

Tank gauging system specification

Selecting right model of master device is the first step of tank gauging system design and the most important parameter to select master device is fluid dielectric value. It is very low for LPG and makes some problem in echo wave reflection from liquid level. Also radar has lower efficiency and it would be better to use another type.
Used tank gauging system (TGS) has been designed based on servo level transmitter and it is main master device in the system.
Measurement should be done with maximum possible accuracy to let us have total accuracy ±0.5 mm for whole package.

Tank gauging system components

Used system designed based on servo level transmitter contains bellow components:

  • • Servo level transmitter to measure LPG level, density and interface level
  • • Multipoint average temperature sensor to measure temperature in each point from bottom to top of tank and average temperature
  • • A local indicator installed in bottom of tank on the ground lets us operator see tank values easily
  • • Fully redundant output to reduce losing signal risk
  • • Transmission distance: 6 km
  • • Redundant data collector signal in control room
  • • Redundant connection to control system

Supporting package specification

A supporting package was used in side of TGS to reduce overfilling risk. This package measures level only and generate three separate signals for voting by emergency shutdown system in control room. The package works by three different level transmitters, one servo type and two radar types.
As explained already about radar wave reflection and fluid dielectric value, only one device can work there well. It is radar from only one specialist manufacturer in the world.

Supporting package components

Used supporting package for TGS contains bellow components:

  • • Servo level transmitter to measure LPG level accurate and safe
  • • Two radar level transmitter to generate analogue signal for LPG level
  • • A local indicator to show LPG level easily
  • • Voting 2 out of 3 in control system between these signals.


As mentioned in project requirements, system should be fully redundant with ±0.5 mm accuracy. We could reach to ±0.2 mm accuracy in fully redundant system and more than 94 years MTBF as per final engineering calculation.