The Tank Gauging System (TGS) is implemented over the FACILITIES tank farms. Two types of applications for the TGS are considered:
• Operational: Custody Measurement comparison
• Inventory Monitoring
For inventory monitoring application, it is used as back-up to liquid metering system, liquid movement and operation, leak control and data reconciliation.
The Tank Gauging System interface to the DCS for monitoring, alarm/ status indication via a proven communication link (Dual Redundant). TGS data can also be used by other Oil Movement & Storage System (OMS) via DCS.
The TGS measure the level and calculate the volume in tanks continuously, with automatic compensation of Pressure, Temperature, and Density, as required.
The TGS is generally used for continuous operation under normal operating conditions. The TGS system has facilities to prevent unauthorized adjustment, and also provide security to allow sealing of the calibration adjustment.
Tank Instrumentation
For Custody Measurement the system accuracy is of custody grade is as per API MPMS 3.3 , even when used for Inventory measurement.
Radar techniques is advised to be employed for Tank level measurement. Frequency Modulated Continuous Wave principle is advised to be used for Inventory measurements and Operational measurements (Custody Measurement comparison).
Servo gauges are avoided unless particular technical requirement.
Stilling wells are generally used where required and recommendation from VENDOR is sought. Stilling well is perforated with holes which will allow free movement of product through the stilling well to avoid product stratification. API MPMS Chapter 3.1B provides guideline on the perforation requirement. Stilling well should be supported from the tank bottom or from the lowest portion of the tank shell to provide that extra stability.
The TGS Instruments are capable of detecting the level surface even when it is unstable i.e. it should be capable of providing a reading corresponding to the liquid.
The TGS are able to measure level as near to the bottom of the tank as possible. Tank gauging transmitters are not be used to initiate trips but may be used to provide pre-alarms for trips
Where water bottom measurement is required, a separate sensor is advised to be used and connected to main Tank Gauging transmitter.
Temperature probes is to be installed on the top of the tanks. The Minimum number of probes is generally in accordance with API MPMS Chapter 7 (minimum one element per 3 m tank height ).
It should be possible to isolate and remove on-line all electronics for maintenance during normal operations.
All TGS should have the following features:
• Temperature compensation by RTD.
• Tank gauging transmitters
• Density correction, where the composition of the tank contents is not known accurately.
• Pressure transmitters, as required.
• Water bottom sensor (along with anchor weight) to measure water level in the tank bottom.
• For pressurized spheres/ bullets, pressure sensors preferably built-in with integrated valve for isolation
Other instruments, if required for applying the correction factor if specified and are generally supplied by TGS VENDOR.
Controller and Data Acquisition/Managment System
Field Control Unit (FCU) or Control Interface Unit (CIU) are units which communicate with the Field Devices/ Tank Instruments over proprietary or Fieldbus communication. Their main function is to control and monitor various Tank Instrument and keep record of the data for inventory managment. They also act as intermediatory or service provider for display or communication to workstation or to Network in control room.
Local Indication and Power Isolation
- Each tank gauge is be provided with a local indicator which display the tank level measurement via a LCD indicator located at grade level to the tank.
- Level measurement units are normally in millimeters with increments of 1mm. Indicators are advised normally to be located adjacent to the tank at grade.
- Each Tank gauging is to be provided with power isolators to isolate the TGS Level Transmitter located at grade next to the local indicator.
System Functionalities
The system is designed to have the capability to produce calculations for the following:
• Level
• Net volume
• Gross Volume
• Density
• Temperature
• Mass of contents
• Alarms
• Average Temperature and Temperature at different level based on temperature probes over the height.
• Tank filling indication and rate
• Tank discharge indication and rate
Level gauge calibration tables (strapping tables) is entered in the TGS system to determine volumes.
TGS System provides Diagnostic and status /alarm of the Radar as well as have calibration facilities.
The system are capable of generating calibration reports and system diagnostic / alarm report on request.