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Digital Natural Gas Flow Meter: Custody Transfer Multi-Path Sizing Systems
Quick Answer
For natural gas custody transfer, choose a multi-path ultrasonic gas flow meter with at least four chordal paths, size the meter to actual pipeline flow rate and pressure, and plan for flow conditioning plus straight run. Silver Automation Instruments recommends the SU-321 multi-path gas flow meter for DN50 to DN600 lines with accuracy of plus or minus 0.5 percent of reading and ATEX Zone 1 certification. Send us your pipe size, operating pressure in bar g, temperature in degC, and flow range in kg/h or Nm3/h for a sizing check.
Multi-Path Ultrasonic Meters for Custody Transfer
A custody transfer meter measures gas volume for payment between buyers and sellers. In many countries this measurement follows AGA Report No. 9, ISO 17089, or OIML R137. These standards accept multi-path ultrasonic meters because their accuracy depends on sampling the actual velocity profile across the pipe.
A 4-path meter uses four transducer pairs placed at different chord locations. A 6-path meter adds more chord samples and helps where upstream piping is less than ideal. Single-path meters are common for process monitoring but they do not meet legal custody transfer accuracy in most gas applications.
A natural gas distribution station in Thailand had unstable readings because a control valve was only 5D upstream of the meter. We moved the meter upstream of the valve, added a flow conditioner, and corrected the flow computer settings. The comparison against a reference meter improved from about plus or minus 1.1 percent to plus or minus 0.35 percent of reading.
Sizing Inputs That Matter
The meter size is not always the same as the line size. In natural gas service, the line is often selected for low pressure loss while the actual flow rate at normal conditions may use only 10 to 20 percent of line capacity. If the meter is the same size as the line, it can operate at low velocity and lose accuracy.
We ask for pipe diameter from DN50 to DN600, operating pressure in bar g, operating temperature in degC, minimum normal and maximum flow rate in kg/h or Nm3/h, and gas composition or heating value. Actual gas density changes with pressure and gas type. For example, a customer in Vietnam requested a DN100 meter for 18 bar g natural gas at 9200 kg/h. We checked actual gas density at 12.7 kg/m3 and found gas velocity was only 8.2 m/s. A DN80 meter gave a better velocity of 13.1 m/s and reduced total project cost.
Velocity Limits and Meter Sizing
We design multi-path ultrasonic natural gas meters for gas velocity between 3 and 30 m/s at actual flowing conditions. The best custody transfer range is usually 5 to 25 m/s. If the maximum flow gives velocity below 3 m/s, the meter is oversized. If the minimum flow gives velocity above 25 m/s, pressure loss and acoustic noise become a problem.
In practice, we calculate actual flow rate in m3/h from mass flow and density. Density comes from AGA 8 or ISO 12213 gas property calculations. Then we check velocity using the meter bore diameter. A 4-path meter can handle a turndown of 30 to 1 in steady natural gas flow. A 6-path meter may reach 50 to 1. Do not use turndown values from clean liquid applications because gas pressure shifts density and velocity quickly.
Recommended Meter Configuration
For most natural gas custody transfer stations, Silver Automation Instruments recommends the SU-321 multi-pa
Installation Notes That Affect Sizing Accuracy
Meter size and path count are not enough. Installation decides whether the meter meets expected uncertainty. We typically require 10D straight run upstream and 5D downstream for a 4-path meter without a flow conditioner. With a flow conditioner plate, upstream straight run can drop to 5D depending on the conditioner type and pipe layout.
Do not install a control valve, tee, elbow, or thermowell immediately upstream of the meter. A thermowell creates shedding. A control valve creates jetting and swirl. We have seen customers place a pressure regulator too close to the meter and then ask why the flow reading jumps.
For high pressure gas above 40 bar g, pipe wall roughness and coaxial alignment become more critical. Use a full bore meter body and match the internal diameter to the adjacent pipe. Gaskets must not protrude into the flow path.
Outputs, Diagnostics, and Flow Computer Matching
A custody transfer gas meter rarely works alone. It sends flow, temperature, pressure, and diagnostic data to a flow computer. The flow computer converts actual volume to standard volume or energy using gas composition. The SU-321 supports 4-20 mA HART for flow rate and pulse output for total volume. Modbus registers include speed of sound per path, signal gain, and signal to noise ratio. These readings help spot liquid carryover, probe contamination, or transducer fouling before billing error becomes large.
We recommend configuring the flow computer with AGA 8 or ISO 12213 gas data. Send us your typical gas composition and we can help with initial density and heating value setup.
FAQ
Q: What is the minimum path count for natural gas custody transfer?
A: A 4-path meter is the minimum under AGA Report No. 9 and ISO 17089. Single-path meters are not accepted for most legal custody transfer applications.
Q: Can the SU-321 handle bidirectional natural gas flow?
A: Yes. The meter supports bidirectional flow with separate forward and reverse totalizers.
Q: What accuracy can I expect after installation?
A: In a proper installation, a 4-path SU-321 reads within plus or minus 0.5 percent of actual standard volume. Repeatability is plus or minus 0.1 percent. Field verification often shows plus or minus 0.3 percent when straight run and flow conditioning meet the sizing sheet.
Q: Is the meter approved for hazardous areas?
A: Yes. Options include ATEX Zone 1, IECEx, and Class I Division 1.
Q: How do I get a custody transfer gas meter sized?
A: Send us your pipe diameter, operating pressure, temperature, flow range, and gas composition. We return a model recommendation with velocity check, straight run requirements, and price quote.
Contact Silver Automation Instruments at Tel +86-135-1213-1526, WhatsApp 86-135-12131526, or WeChat +86 13512131526. Ask for a natural gas multi-path sizing review for your custody transfer station. The website is flow-meter.com.au
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