Pipeline Metering – Delivery and Receipt Stations
With many pipeline projects underway, be it crude, condensates or propane, correct metering station design is critical to the success of a project and a company’s profits. The following will cover equipment design basics typical for many companies who deliver or receive gas and must measure and consider gas quality. There are specific Federal, State and Local codes and regulations to contemplate and for the purposes of looking at equipment design basics, won’t be covered and assumed must be considered. For further information, one might turn to the AGA for specifications and PHMSA for codes.
Most all meter station designs start with flow rate, looking to maintain an uninterrupted flow of gas and consideration of maximum and minimum flows are to be taken into account. Various metering technologies are available including ultrasonic, gas turbine meters, orifice meters, PD meters and coriolis meters such as those manufactured by Micro Motion. Typically, larger meter stations do not use turbine meters, orifice meters or PD meters, but when used, are seen in low flow rate applications. Multiple meter runs are used when the maximum design flow rates are required for turndown between maximum and minimal flows. Automated valves are used to open or close a meter run as required to maintain flow rates through an individual meter within a range recommended by the manufacturer. With Micro Motion meters, no flow conditioning is required or straight run of pipe is necessary as with other meters, which makes for a smaller footprint skid. A small section of pipe is installed to mount transmitters, probes, and Thermowells.
FMD Prover Mounted to Meter Skid in the Field
Since meter stations are for custody transfer, a meter by-pass with a double block and bleed valve with meter proving connections are included. A trailer mounted ball prover or compact prover is commonly used for meter proving in high pressure applications. How often the meter is proved out depends on state regulations and or contract language between parties. For larger meter stations, the system may be connected to a fixed compact prover where meter proving is continuous from such companies as Flow Management Devices.
Pressure and temperature transmitters, such as the Emerson 5700 series transmitter, temperature thermowells, test thermowells and chromatography probe if using ultrasonic meters, are to be included and gas quality is taken into account. As for gas quality, many companies use a filter-separator before the meter station, and some will do continuous gas sampling. One method is to install a gas analyzer, such as those manufactured by JP3, to monitor gas for H2S, O2, N2, CO2 and other contamination that exceeds any specific quality requirements as agreed between parties. If any agreed parameters for gas quality are exceeded, the system is designed to shut down at chosen presets.
Some applications require odorization and a panel used to automatically meter in mercaptan is included on the meter skid. Automatic shutoff valves are included as well as check valves to prevent any backflow along with pressure and flow control valves, pressure relieving valves and pressure limiting devices. Area classification is to be considered and the NEC and AGA will provide guidelines. Grounding of the meter station to a ground grid is required to meet applicable codes and standards.
Propane Meter Skid with Mercaptan Injection Panel
Piping size is based on flow rate, and design pressure and temperature is based on the most severe condition. The meter station is supplied with an inlet and outlet valve to allow the meter station to be isolated from other plant piping and equipment. Drain collection piping, usually 1”, is included to take liquids from various drain points. In some cases a full drain pan is included under the skid with walk on grating and drain valves.
Meter Skid Full Drain Pan
Spirit Flow-X Flow Computer
Flow computers, such as those manufactured by Spirit IT, are used to accept inputs from the various instruments on the meter station as well as chromatographs and gas analyzers. The meter station data collected by the flow computer is designed to transmit information by phone, cellular, satellite or radio. A UPS system is included to provide backup power if AC power is lost. Flow computers are normally mounted off-skid outside the hazardous zone with junction boxes being mounted on-skid for local power connection and wiring to off-skid equipment. In some cases, equipment such as flow computers can be mounted on skid inside of a nitrogen purged panel.
Platform/Walkway/Handrail around Perimeter of Meter Skid
For safe access to valves, access platforms with handrails and stairs are provided. Depending on grade elevations and slope where meter skids are mounted, stairs are included to gain access to a walkway installed around the skid used for safely maneuvering around the equipment.
As this is a basic equipment overview, there are other considerations and equipment to consider. The best place to start is with a well-qualified company who you can consult with that has you, your project and your company’s best interest in mind and is not looking to “sell” you something. Pulling together a team of stakeholders including those from measurement, project management, operations, and maintenance will ensure all wants and needs are covered, and in the end, you get a system that works.