Abstract
This paper discusses Small Volume Proving for Coriolis (SVPs), used in the oil and gas industry to validate the performance of custody transfer meters. Recently Coriolis mass flow meters have been introduced for custody transfer. While these offer reduced maintenance requirements over traditional PD and turbine meters, proving Coriolis flow meters using Small Volume Proving for Coriolis is challenging. This paper presents Volume Proving for Coriolis results for a Coriolis meter that matches or exceeds the most stringent requirements for custody transfer. This is achieved in part by using a novel signal processing technique that reduces the dominant component of the measurement noise. It is associated with so-called Coriolis mode vibration, with negligible loss of dynamic response.
Introduction
In the oil and gas industry, custody transfer is a transaction in which ownership of a quantity of material is transferred from one operator to another, for example from the well operator to the first transportation operator, via pipeline, truck, or ship. Subsequent custody transfer transactions may take place as the crude commodity arrives at the refinery and as refined products leave. The high value of the material, and perhaps the payment of government duties, leads to stringent measurement requirements to verify the quantity of material involved in each transaction.
Current standards require flow meters used for custody transfer measurements to undergo regular verification of measurement performance. A process known as meter proving. Commonly this is carried out using a specialized piece of equipment known as a prover, which is based on the principle of filling a known volume. Examples include bi-directional ball provers and compact pistons or Small Volume Proving for Coriolis (SVP). Generally, large bi-directional ball provers are used for high-volume custody transfer operations. Small Volume Proving for Coriolis is typically part of a mobile truck-mounted system that travels from site to site providing meters on a scheduled basis.
Connected in Series
To carry out proving, the Small Volume Proving for the Coriolis system is connected in series with the meter under test. Fig.1 shows a typical Small Volume Proving for Coriolis (SVP) design. Flow passes through a cylinder of known volume. The prover’s piston contains a poppet valve which, when open, allows flow to continue irrespective of the piston location. Specifically, this allows the piston to be pulled to the upstream position without interrupting the flow. With the piston drawn to the upstream position and the poppet valve closed. After that, the piston moves with the fluid as it sweeps through the calibrated volume.
At the start and end of each “pass”, high-precision position detectors generate electrical signals to indicate the passage of the piston. During the pass, the volumetric flow measurement from the meter is integrated. This is so that a comparison can be made between the known prover volume and the reported volumetric flow from the meter under test.
Testing Range
10:1 flow range. At the highest flow rates, a prover pass can be as short as 1.5s, raising such issues as the noise properties and dynamic response of the flow meter. Consequently, the use of electronic or software damping of the measurement output is commonplace. It is also possible to define a “run” at a particular flow rate as consisting of one or more prover passes. This is where the meter output is summed over all the passes, to increase the effective duration of each run. The performance parameters of interest include the repeatability of the meter over a set of runs (each run consisting of one or more passes) at the same flow rate. As a result the linearity of the meter over the flow range of the test procedure, as explained in more detail below.
Types of Meters
Mechanical meters, such as turbine or Positive Displacement (PD) devices. They have traditionally been the most used in liquid custody transfer applications. Consequently, the most widely used meter output is the pulse output. This is generated using the blades of a turbine meter, or a gear within a PD meter, which are positioned near a Hall effect sensor coil. As the blades or gear teeth rotate and pass the pickup coil, a pulse is generated, the output rate of which is proportional to the rotation speed. This will determine the flow rate of the fluid, with each pulse representing a discrete volume of fluid. As electronic meters have been introduced, the pulse output has been retained. The microprocessor generates pulses to represent the observed flow rate, thus maintaining compatibility with established standards, practices, and equipment.
One limitation in the use of pulse output is its discrete nature. It is only possible to detect an integral number of electronic pulses. This may place a significant constraint on the repeatability of achievable overruns of short duration. Providing that it is unusual for pulse output frequencies to exceed, say, 10 kHz. An interpolation technique known as double chronometry or the double-timing method [2] has been developed to provide an estimate of the fractional number of pulses during a run. Hence, eliminating the issue of pulse discretization error.
History
Over the last 10 years, Coriolis mass flow meters have gained acceptance in custody transfer applications. This offers reduced maintenance requirements compared with mechanical meters due to their lack of moving parts. In 1996, Whitman described field proving of a Coriolis meter using a small volume prover [3]. Recently, the Weights and Measures Division (WMD) of the National Institute of Standards and Technology (NIST) published a series of articles about small-volume provers [4]. This included sections on Coriolis meters.
The American Petroleum Institute (API) Manual of Petroleum Measurement Standards, Chapter 5, Section 6 on the measurement of liquid hydrocarbons by Coriolis meters [5]. These are guides that have an appropriate number of proving runs as well as the number of passes per run for different types of provers. This includes Small Volume Proving for Coriolis (SVP), in a table, reproduced as Table 1. Detailed examination of these documents, however, reveals that guidelines and good practice recommendations are still open to numerous interpretations.
For instance, questions often arise as to exactly how many passes per run are permitted, what pulse output damping to use, and what constitutes a minimum acceptable run time.
Early 1998
An early (1998) and detailed examination of the issues associated with online calibration of flow meters using an SVP is provided in the NEL guidance. This describes trials of a range of flow meters, including PD, turbine, ultrasonic, and Coriolis meters. Table 2, reproduced from [6], shows the effect of changing the number of passes per run in the NEL trials. This demonstrates the gulf in performance at the time between mechanical and electronic devices for single-pass runs. Also, turbine and PD devices showed repeatability of less than 0.01%. while the ultrasonic and Coriolis meter could only deliver 0.66% and 0.12%, respectively. Another issue explored in[6] is the delay in pulse output from electronic meters.
Example
For example, it proved necessary to delay the gating of the meter output pulses to accommodate electronic and software time constants. One advantage of a purely mechanical flow system such as a PD meter is that it exhibits negligible delay in the generation of pulses. However, for electronically based devices, measurement noise, and microprocessor delay may be detrimental to meter performance, especially on short passes. Of course, the use of damping on the pulse output signal reduces noise at the expense of increased delay [7].
Accordingly, proving is considered by some in the industry to be something of a black art. Especially when it comes to the setting of the damping factor for a meter that is difficult to prove in the field. Thus, Vandiver [8], in offering guidelines on methodologies for Coriolis meter proving. Vandiver recommends that “The pulse output damping value should never be set higher than 0.8s”. This recommendation is illuminated by the Coriolis meter frequency output signal shown in Fig. 4 of [6], which shows that the pulse output may not have stabilized to a steady flow. This will occur by the start of the proving run if the damping setting is too high.
Issues of Pulse Output Delay
The issues of pulse output delay and damping have become of sufficient concern within the industry that an API Task Group was formed to investigate. They investigated the effect of microprocessor-generated pulse output delay on field proving of meters. This also includes Coriolis and ultrasonic meters, using SVPs. It is hoped that the current paper will offer some insights into these issues.
The Invensys University Technology Centre for Advanced Instrumentation at the University of Oxford has developed Coriolis metering technology. The development had improved performance characteristics, for example concerning dynamic response. The reader is referred to the companion paper in this special issue on Coriolis mass flow metering. [9] In addition, for a background in Oxford’s transmitter technology, including the use of Field Programmable Gate Arrays (FPGAs). The importance of pulse output generation has been addressed through the development of a patented algorithm [10] with high precision. The precision level is (to one part in 10 million averaged over one second [11]) and minimal delay.
This technology provides a variety of means for ameliorating what are perceived to be the principal difficulties when calibrating a Coriolis meter using an SVP:
1st Issue
The first, which is now rare, is the production of irregular pulse streams from the meter. This was found in many early Coriolis meters that were changing frequency. It is also missing and adding pulses was the technique used as part of the pulse generation software. This gave rise to uneven or intermittent pulse mark-space ratios. Uneven pulses give rise to poor repeatability when interacting with pulse interpolation. This is even to the extent that better repeatability can be achieved without using interpolation. In the Oxford transmitter, the 40MHz clock signal to the FPGA ensures a smooth 50% duty cycle. This occurs at a high pulse output frequency of 10kHz, there are 2000 clock ticks between edges. This ensures that pulse interpolation techniques will improve repeatability. Of course, this technology is entirely immune to the pulse irregularities arising from wear in mechanical meters.
2nd Issue
The second issue arises from damping and time constant. The dynamic response of the meter is pivotal. As previously mentioned, the meter needs to generate a stable output frequency after the initial drop-in flow rate caused by the launch of the piston. This will occur before the first detector triggers the start of the pass. Previous trials on the Oxford transmitter [12] have demonstrated that for the bent tube design used in these trials. Also, this is a very rapid (4ms) step change in flow indicated via the pulse output from the meter within 16 ms of the start of the physical change in flow. It was the step change completed on the pulse output signal within a further 20ms or less.
For a high-frequency straight flow tube, the companion paper [11] in this special issue reports a delay in the change to the pulse output of less than 4ms. It also had an insignificant extension of the duration of step change. Of course, in both these experiments no damping is applied to the pulse output signal as it is considered unnecessary.
3rd Issue
A third issue is also response-time-based. Many Small Volume Proving for Coriolis (SVPs) show a drop in flow during a pass due to the gas plenum pressure dropping as the piston moves. This must be successfully tracked by the meter. This issue is perhaps best considered as a low-frequency flow modulation. Besides being opposed to a step change in flow discussed above. However, as
Applied to the proving problem, the Oxford transmitter connected to a commercial bent flow tube has passed the most stringent interpretation of current proving standards. This was done at a commercial testing laboratory. Specifically, over an 11:1 flow range, covered in eleven steps, using five repeated prover runs each of a single pass only. The meter then achieved a repeatability of less than 0.05% over the range, with an average repeatability of 0.017%. This was achieved without applying any damping on the pulse output signal.
Oxford Research
The Oxford research transmitter collected high bandwidth data during the proving runs, and this paper provides a detailed analysis of the data. It also describes the signal processing techniques used to provide the measurement performance. Furthermore, section 2 describes the proving trials and results obtained by the testing laboratory. In addition, section 3 provides the corresponding data collected within the Oxford transmitter. This data collected demonstrates that they are essentially consistent with those of the testing laboratory. Also, section 4 provides a more detailed analysis of one set of prover runs. It also explains how the signal processing techniques employed can simultaneously reduce noise. The reduction has little loss of response time so pulse damping is unnecessary. Overall, the paper aims to demonstrate the latest advances in Coriolis meter signal processing. They can provide reliable technology that can match the stringent proving requirements of the oil and gas industry.
Oxford transmitter data records
Two main types of detailed records can be collected by the transmitter. One records the raw data from the two flow-tube sensors along with flow-tube temperature information in a so-called “sensor” file, updated at 12kHz. This file type allows the most fundamental reconstruction and analysis of the measurement calculations. It also provides no information on other aspects of meter operation such as flow-tube control or external inputs. The second type of file stores up to 60 channels
Signal analysis
Having demonstrated that broadly speaking the internally recorded data are consistent with those observed via the pulse output. In this section, an analysis is provided of the raw sensor data and the signal processing algorithms used to generate these results.
In virtually all Coriolis mass flow meters, the raw sensor data are generated from two sensors (usually velocity-sensing coils). This provides essentially sinusoidal analog signals derived from the motion of the flow tube.
Concluding Remarks
The primary demonstrated that the latest generation of Coriolis mass flow meters can match those requirements. Insight has been offered into the signal processing issues associated with providing high measurement precision and fast dynamic response. Most specifically with the relationship between the drive and Coriolis modes of vibration, and their influence on the raw flow.
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Acknowledgments
The authors are grateful to Steve Whitman of Coastal Flow Measurement, Inc. and Richard Paton of NEL for detailed technical comments on earlier drafts of the text.
References
Response of a Coriolis flow meter to step changes in flow rate.
Flow Measurement and Instrumentation (2003)
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