Gas Meter Proving Can Ensure A Healthy Bottom Line.
One barrel of oil, for example, is equivalent to approximately 5,600 cubic feet of natural gas. At $7.50 per thousand cubic feet, the natural gas equivalent of one barrel of oil costs $42 or more than the cost of a barrel of oil. In the petroleum liquid industry no good crude oil measurement system would be complete without a prover. This is currently not the practice of the natural gas industry.
Under billing causes loss of revenue and over billing can cause a future collection that will cost the company millions of dollars. For these reasons, gas meter proving is quickly becoming important and necessary to ensure precise measurement of natural gas.
Reasons For Proving A Field Meter
Although the installed field meter has been completely checked and calibrated at the factory, and a performance curve developed, there are many things that can affect the meter on site, causing the meter to lose its calibration.
1. Any meter can have changes during operation that will cause errors in measurement. Orifice plates can become damaged. bearings on turbine meters can wear, and dirt and trash can accumulate on flow conditions. The inputs into flow computers and other electronic devices can be changed by mistake, causing errors in measurement.
2. The calibration medium, and the pressure and temperature are usually different from the Factory calibration conditions. Any change in density, pressure or temperature can cause a shift in the number of pulses produced per unit volume.
3. One of the most important reasons for meter proving in the case of custody transfer is to give both the buyer and seller confidence the volumes they transfer are correct, thus eliminating disputes.
There are many different methods and devices available that can be used to prove the natural gas meter. Some of them are calibrated master meters, sonic nozzles, bell provers, and volume provers. Each device has its advantages and disadvantages.
These proving devices can be divided into two categories: primary and secondary. A primary device is a volume device such as the bell or volume prover that has had its volume checked and verified against a standard such as the NIST in the United States. This device can then be used to prove meters. A secondary device is a device that has been checked against a primary device and is then used to prove another meter. An example of a secondary device is a transfer meter prover using a master meter.
Volume provers, which are relatively new, are primary devices and very accurate. They are, however, very expensive and work best at higher pressures and lower volumes.
Bell provers, which are also accurate, work only at low pressures and cannot be used for large flow volumes. They can be used to calibrate a master meter which can then be used to prove other devices at higher pressures and higher flows.
Bell Provers Expensive To Build And Maintain
The sonic nozzle is precise, plus or minus 0.25%, bur can only check the meter at one flow rote and pressure. The sonic nozzle also causes a pressure drop in the system.
A calibrated turbine master meter can be used to check the field meter through its entire operating range. It can be used at high and low pressure, and the systems using master meters are relatively inexpensive. The master turbine meter can be portable or stationary. However, the master meter can become damaged in use and lose its precision. For this reason, meters with dual rotors that have the ability to check and verify their condition are best suited as master meters.
Since master meters are currently the least expensive available way to calibrate an existing field meter, we will discuss the various master meter systems in more detail. AGA 6 on Transfer Meter Proving using Master Meters is also being updated and rewritten.
The transfer prover system consists of a master meter with the field meter placed in series. Air at atmospheric pressure and at various flow rates is then pulled through the two meters and a comparison is made between the master meter and the field meter. A transfer prover system can be used to calibrate field meters brought into a shop or the master meter prover can be portable and taken to the field.
The insitu master meter prover is a master meter usually placed immediately upstream or downstream and in series with the operating field meter. It is connected on site and is used for proving at actual operating conditions of flow and pressure. The master meter is connected to an existing three-valve manifold.
The high pressure proving system consists of a number of different sized parallel turbine meter runs with provisions for installing a field meter for test downstream of the master meter. A sonic nozzle is placed downstream of each of the two meters in series to limit the flow and to verify the accuracy of the master meter.
Equipment Required For Master Meter Proving
* Turbine Meter
AGA 7 states that gas turbine meters must have a predicted accuracy of plus or minus 1%. Therefore, the device used to check the meter must have an accuracy that is the same or better than plus or minus 1%. AGA 7 also states the repeatability of a turbine meter normally exceeds plus or minus 0.1%. Some manufacturers state their turbine meter is repeatable to plus or minus 0.05% or five parts in 10,000.
Using modern flow computers, the typical curve of a turbine meter can be linearized. During the prove, the volume of both the calibrated master meter and the field meter being proved can be measured precisely during the prove cycle, using pulse interpolation as described in API Chapter 4, Section 6 Pulse Interpolation.
* Pressure And Temperature Transmitters
The pressure and temperature at the meters must be precisely measured at the master meter and at the meter being proved. Because of the small pressure drop between the master meter and the meter being proved, the pressure between the meters is normally measured with a differential pressure transmitter. The pressure at the first meter in series is measured with a precise pressure transmitter and the pressure at the second meter is determined by subtracting the differential pressure between the two meters. The lower the pressure, the more difficult determining precise pressure becomes. On atmospheric transfer meter prover systems, a differential pressure transmitter can also be used to determine the pressure at the first meter. The high-pressure port of the differential pressure transmitter is left open to atmosphere and the low-pressure port is connected to the pressure port of the meter. Atmospheric pressure can also be a manual entry, or a precise atmospheric pressure transmitter can be used.
A temperature transmitter must be placed at least three diameters downstream of each meter. There will be a slight temperature loss caused by the pressure drop between the meters. Since this pressure drop is very small, only a small change in temperature will be experienced. Extremely precise and well-calibrated temperature transmitters must be used to measure this precise differential.
* Piping Per AGA 7
Piping for both the turbine master meter and field meter pre-runs and post runs must follow the recommendations of AGA 7. The post run of the first meter and the pre-run of the second meter in series can be combined, making the piping distance between the two meters a total of 10 pipe diameters. If any manifolds, elbows or bends occur before either the master meter or field meter, a flow conditioner should be installed before the meter in accordance with the flow conditioner manufacturer's recommendations.
* Flow Computer
The calculations used in the flow computer must follow the procedures described in AGA 6 & 7. The report from the flow computer must be presented in such a way that the calculations from raw pulses to final compensated volumes can be verified using hand calculations. The flow computer must be able to linearize the master meter test curve and perform pulse interpolation on the frequency outputs from master meter and the field meter being tested.
* In Situ Proving With A Master Meter Placed In Series With The Field Meter
A calibrated master meter run of sufficient size and pressure rating is connected to a three-valve manifold located upstream or downstream of the meter to be tested. Since this master meter run will have at least one elbow upstream of the meter run, a flow conditioner is recommended. The master meter run must be complete with a flow computer, and pressure and temperature transmitter.
The field meter to be tested and the pressure and temperature transmitters associated with it are connected to the proving flow computer in such a way that the existing measurement is not affected. Normally, the pulses from the meter come from a parallel connection and the analog signals from the pressure and temperature transmitters are connected in series.
* High Pressure Proving
High pressure proving at various flow rates can be done with a high pressure proving system. This system is located where there is sufficient pressure and flow to check all the meters in the system. For example, the best location for high pressure proving in a distribution system is where gas enters the distribution pipeline. The system here can serve two functions. It can measure the gas being purchased and it can be used to prove all the high pressure meters used downstream in the distribution system. Flow rates through the meter being tested can be varied by routing the flow through the other meters in the system. The pressure in a high pressure proving system can be varied as long as it does not affect the downstream system.
Procedures For Atmospheric Master Meter Transfer Proving
1. After the meter to be tested has been properly installed, the system must be checked for leaks. One method is to block the inlet to the system. All valves are put in the full open position and the blower started. Pulse inputs will register in the flow computer as air is pulled out of the piping turning the meters. If there are no leaks, the pulses will stop. Another method is to pressurize the system with both the inlet and discharge blocked. Leaks can then be detected using a leak detector fluid. When the system passes the leak test, it should be recorded on the prove report.
2. The precision of the master meter should be verified before each prove. The manufacturer of the master meter will have a procedure on how to verify the meter is still in calibration. When the master meter condition is verified, its condition should be recorded on the prove report.
3. Enter the meter information for the master meter or select a master meter whose information has already been entered.
4. Enter all the information for the field meter to be proved or enter the serial number for the information to be entered automatically.
5. Set the flow rates for each prove run. Prove runs are normally done at 10%, 25%, 50% 75% and 95% of the meter to be proved maximum flow rate. These are recommended only; the flow rates asked for by the end user should be used.
Proves can be done based on time or on volume. Either method will produce a good prove. In this example, we will use prove runs based on time.
6. Set the time and number of each prove run. The time for each flow run will be determined by the number of pulses generated by the meters. The time must be long enough for a minimum number of pulses to be generated by the lowest frequency output. If the flow computer is capable of pulse interpolation, the time of each run can be shortened in accordance with API Chapter 4, Section 6, Pulse Interpolation standards. The number of proves at a specific flow rate depend on the client. However, it is recommended a minimum of three runs be made to determine if the meter being proved is repeatable.
7. Start the prove, The computer will automatically adjust for the correct flow rate. A time for stabilization will occur at the beginning of the prove and alter each flow rate change.
8. When the prove run runs for the specified time and the run is successful, the information is saved, the blower control valve is automatically adjusted to the next flow rate. Normally, the first run is done at maximum flow setting. If the meter repeats and is within specifications at the maximum flow, it is an indication it will prove at the lower flows.
9. At any time during a prove run or cycle, the prove can be stopped (aborted). Aborting the prove stops the proving run. An aborted prove report is stored or printed.
10. When the prove for all flow rates is complete, the control valve closes, the prove report is printed, and all information is saved in a file on that prove.
Procedures For In Situ Master Meter Proving
1. If the in situ master meter run connection has been designed correctly, a one-inch or smaller by-pass line will be installed on the inlet block valve to allow pressure to be built up slowly in the master meter proving run. A blowdown valve one inch or smaller should also be installed on the master meter run to allow the pressure to be taken off the run after the prove is complete and the valves to and from the master meter run are closed.
2. Slowly open the inlet by-pass pressurizing the master meter run. While this is being done, check for leaks in the system. If no leaks are found, slowly pressure up the system. An increase of one pound per second is the normally recommended pressure increase.
3. Slowly open the inlet to the master meter run. After the inlet is opened, open the downstream valve. The flow rate on the master meter run should increase at a rate not to exceed 20% per minute.
4. Slowly close the line block valve, putting all the flow through the master meter run. The flow rate on the master meter run should increase at a rate not to exceed 20% per minute. This valve must be at double-block and bleed-type valve and be able to verify that all flow through the valve has stopped.
5. The precision of the master meter should be verified before each prove. The manufacturer of the master meter will have a procedure on how to verify that the meter is still in calibration. When the master meter condition is verified, its condition should be recorded on the prove report.
6. Enter the meter information for the master meter into the flow computer or select a master meter listed whose information has already been entered.
7. Enter all the information for the field meter to be proved or enter the serial number for the information to be entered automatically.
8. Normally, the flow rate of the field meter cannot be changed insitu proving. Therefore, the prove will be done at one flow rate.
Proves can be done based on time or on volume. Either method will produce a good prove. In this example, we will use prove runs based on time.
9. Set the time and number of the prove run. The time for the run will be determined by the number of pulses generated by the meters. The time must be long enough for a minimum number of pulses to be generated by the lowest frequency output. If the flow computer is capable of pulse interpolation, the time of each run can be shortened in accordance with API Chapter 4, Section 6, Pulse Interpolation standards. The number of proves at the flowing rote depend on the client. However, it is recommended that a minimum of three runs be made to determine if the meter being proved is repeatable.
10. Start the prove.
11. When the prove run time is complete and the required runs have been made and are successful, the information is saved to be printed out at a later time.
12. At any time during a prove run or cycle, the prove can be stopped (aborted). Aborting the prove stops the proving run. An aborted prove report is stored or printed.
To disconnect the master meter prover, reverse the set up procedure.
Open the block valve allowing the flow to go directly down the line.
Close the valves at the inlet and outlet of the master meter run.
Slowly drop the pressure on the master meter run and disconnect it from the meter system.
(The author can be reached at drudroff @aol.com or email@example.com.)
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|Comment:||Gas Meter Proving Can Ensure A Healthy Bottom Line.|
|Author:||Rudroff, Daniel J.|
|Publication:||Pipeline & Gas Journal|
|Date:||Apr 1, 2001|
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