Calibration and Witnessing of EFM Devices

Randy McCallum
Vintage Petrolem

18th Annual Measurement Short Course
Acadiana Flow Measurement Society
April 29-30
Lafayette, LA

INTRODUCTION

The oil and gas industry has evolved immensely in the past ten years. One of these evolutions has been the widespread use of electronics in the custody transfer area. Many companies are installing EFM as a replacement to chart based systems. Several factors are the reasons for this shift. The most obvious is the increased accuracy over chart based systems. EFM has an overall accuracy of 0.25% versus the 1.0% accuracy of a chart recorder. EFM also offers the ability to communicate remotely to the measuring device enabling timely data acquisition, notification of any alarm conditions and the monitoring of much more than gas sales.

CALIBRATION OF EFM

Due to the increased accuracy of EFM, calibration equipment should be more accurate as well. Equipment used to calibrate EFM should be a minimum of 2 times as accurate as the devices being calibrated. Calibration equipment should also be checked and calibrated on regular intervals to ensure the highest accuracy in the EFM devices. Since many manufacturers are producing electronic flow computers and transmitters, this paper recommends the technician be adequately trained on any devices he or she will be calibrating as per the manufacturer.

CALIBRATION PROCEDURES

This paper will not attempt to give specific directions on calibration since manufacturers vary. It will give some basic guidelines for the technician to follow.

1. Ensure calibration equipment has been checked for accuracy.
2. Check unit for leaks and repair any found before going on the calibration routine.
3. Fix all analog values prior to taking the flow computer out of service. Some flow computers will automatically do this.
4. Remove the unit from service.
5. Perform an "as found, as left" check of the unit's analog inputs, logging any markers used. A three point or five point check is recommended.
6. If the unit deviates or "is off" of the calibration input valve, a calibration of the unit is performed.
7. Calibrate using the manufacturer's recommended procedure. Typically a three point (zero, mid-scale, and full-scale is utilized.
8. Perform an "as found, as left" check of the unit's analog inputs, logging any markers used. A five point check is recommended, if available, as the minimum after a calibration.
9. Inspect orifice plate and seal ring for any damage and replace as necessary before returning unit to service.
10. Return unit to service, unfix all inputs and monitor values to ascertain the unit is functioning properly.
11. Update any data needed in the flow computer: gas composition, new plate size, etc..
12. Collect data for audit trail and post calibration printout.

WITNESSING OF EFM

The technician responsible for the witness of a custody transfer low computer should be as knowledgeable about the measuring device as the person calibrating the unit. An effective witness will ascertain that the person calibrating the unit is capable of doing so, and that all procedures are followed according to company and manufacturer's recommendations. He or she will also verify that the calibration equipment being used is a minimum of 2 times as accurate as the device being calibrated and that such equipment has been recently checked for accuracy. An effective witness will have the attitude that "This is my gas and I want everything done correctly."

WITNESSING PROCEDURES

The witness should make certain that all calibration guidelines mentioned above are followed. The witness is there to represent his or her company to assure that the unit calibration is done correctly and should do so in a professional manner. This can be accomplished by having knowledge about the flow computer being calculated.

1. Ensure calibration equipment has been recently been checked for calibration.
2. Make certain no leaks are present.
3. All analog values are to be fixed prior to taking the unit out of service.
4. Calibrate as per manufacturer's recommended procedure.
5. Check orifice plate and seal ring for damage.
6. Update any data needed.
7. Return the unit to service and unfix all analog inputs.
8. Request a printout of the events file and characteristics file and check for accuracy.

CONCLUSION

The calibration and witnessing of EFM are very similar to the calibration and witnessing of chart based systems. Whether calibrating or witnessing, the technician needs to have a working knowledge of the systems being employed. This includes the calibration recommendation as well as the internal components of the flow computer. The technician has to be able to trouble shoot and repair the unit from the orifice fitting to the solar panel. He or she has to know how the unit calculates the volume it is measuring. This will only be accomplished through education and day to day work.

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