What is FMS or Flow Metering Skid?
A Flow Metering Skid is a framed or moduled device on which various other assemblies are fitted for the measurement (flow rates) of gas or liquid products. The major purpose of using a metering skid is for custody transfer. They are designed and manufactured to fulfill the lowest uncertainty. At the same time, they optimize operation and maintenance costs. The metering skid includes equipment for flow conditioning, filtration, automated or manual operational sequences, draining, venting, safety, maintenance, lifting, proving, sampling, etc. Mass or Volumetric flow rates as per the client specification are measured by such skids or packages. Depending on the requirements for measurement, this package can also be used for other treatments like flow control and cleanliness of fluid.
Flow Metering Skid Components
The main components of a metering skid (Fig. 1) are listed below:
- The structural framework of the skid along with supporting members
- The piping network
- Applicable process equipment
- The electrical power feed including the earthing system, the MCC and all cabling and trays
- The local instrumentation and control system including the flow computer, personal computers, and printers and the PLC.
Purpose of the metering skids
Metering skids can be used for various purposes as mentioned below:
- Pressure regulation and metering stations
- Fuel gas conditioning systems
- Border metering stations
- Offshore gas and liquid metering
- CNG filling stations
- Biomethane grid injection systems
- Underground gas storage metering and control skids
- LNG metering skids
- Calibration facilities
Application of metering skid
Offshore metering systems are used in FPSO, FSO, Platform / MOPU / TLP, FLNG / FSRU. Onshore metering systems are used in Oil Production Plant, Oil Refinery, Gas Processing Plant, Petrochemical Plant, Terminal, Tank Farm, Pipelines, etc.
Few important considerations for FMS Design
- The type and size of the FMS are based on fluid properties, required system uncertainty, flow rate, pressure drop, maintenance, proving requirements and more. Different types of meters such as the Positive Displacement meter (PD meter), Turbine meter, Ultrasonic Flowmeter (UFM), Coriolis meter, Orifice meter, V-Cone meter, Ventury meter, etc. are used for flow metering.
- A flow meter must be calibrated and validated at a certain interval of operating life to ensure the reliability and uncertainty of the system. The validation methodology and calibration intervals must be carefully decided as it plays a crucial role in the design of the metering system design.
- For the optimum performance and operability of an FMS, the most critical items are the Metering Control System whose design depends on various functional applications and requirements with preferences for Flow Computer, PLC and HMI manufacturers in line with the preferred operator infrastructure.
- In addition to its metering capability related design considerations, the system has to be designed and developed in the context of various external influences like availability, maintenance possibilities, safety procedures, ATEX and Ingress Protection, etc.
- The design must strictly comply to project specifications and applicable international codes, standards and regulations, etc.
- FMS Packages, in general, contains two numbers of metering runs (2 x 100 % configuration) one-meter run as a duty or operating and another meter run as standby. Under normal operation mode, only one stream (Stream-A) shall be in operation as Duty Stream. This is calculated to be able to provide 100% flow rates required to the FMS skid. In this mode, the other stream (Stream-B) and Proving run should be fully isolated.
Operating & Control Philosophy
- Prior to the operation of the FMS Skid, a line-walk is required to ensure that all equipment is in the good installed condition.
- Valve opening-close position shall be ensured to be as per PEFS (P&ID) requirements.
- Stream intended to be placed in operation must be pressure-equalized with the inlet pressure.
- All operation control of the metering stream shall be made using the operation of USV Valves in the inlet, outlet and proving run of each metering stream.
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