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 Inside this issue
 Electronic Uncertainties in Measurement Analog/Digital Converters A frequent misunderstanding is found when Analog to Digital Converters (A/Ds) are discussed. Selecting an A/D or Digital to Analog (D/A) converter with a higher bit resolution does not necessarily guarantee a proportionate increase in accuracy. High resolution is not the same as high accuracy. High A/D accuracy is determined by its stability, linearity and repeatability. You would naturally expect an improvement by using a 16-bit A/D instead of a 12 bit A/D. However neither resolution nor accuracy will improve the measurement result if the sampling period is slow or if there is poor or no compensation for temperature drift over the range of the A/D. How Much Resolution Do I Need? For most measurement applications, where temperature spans are within a 100 deg. F range and pressure spans are within a 1000 psi range, a 12-bit A/D will provide 4 times the maximum discrimination levels required or specified by API MPMS 12.2 (1995). API algorithms round the observed values, regardless of increased resolution. Reducing the Uncertainties of Analog Circuitry It has been Omni's experience that analog circuitry can be designed and tested to exhibit excellent stability and linearity over the entire range of the A/D. To reduce the effect of temperature drift, circuitry should be routinely "burned in" and cycled over the operating range of the electronics. Any drift due to temperature can be trimmed out and minimized for each analog circuit. This can produce a highly stable, linear A/D with temperature drift reduced to less than 10 parts per million (ppm) per deg. F. This approach takes time and is not an inexpensive routine for the manufacturer. Photo-Optical Isolation - How it Works
Optical Isolation Eases Installation Concerns The ability to provide independently photo-optical isolated A/D channels can be accomplished by using independent high-precision V/F (Voltage/Frequency) converters. These devices provide a high-speed (500 msec) linear average representative of the input signal over the complete sample period. For long-term reliability of electronic circuitry and continued equipment operation, there can be no substitute for photo-optical isolation. But for the manufacturer, photo-optical isolation uses more circuit board "real estate" and more input power. Other forms of transient protection can be less expensive for the manufacturer but are not nearly as effective in minimizing the cost of maintenance for the user. The cost of catastrophic failure is sometimes not considered when some product selection criteria are being assembled. Photo-optical isolation also minimizes ground loop effects and isolates and protects the computer from pipeline Electro Magnetic Interference (EMI) and damaging transients. Gas Gathering As a result of flow computers being available with multiple serial ports, direct communication links to gas chromatographs or BTU analyzers provide a closer coupling between totalizing and analysis devices. The full gas analysis information available from a GC (gas chromatograph) on multiple meter runs can be downloaded to the flow computer using industry standard communication protocols. Multiple flow computers can also be multi-dropped to a single GC. With new multi-variable transmitters, it is now possible to bring all the flow process variables from a single transmitter and connect to the flow computer. This can be done with point-to-point connections from Honeywell digital SMV3000 transmitters or by an RS485 Modbus link from a Rosemount 3095MV multi-variable transmitter. An extensive archiving function is essential for any flow computer in gas operations. It permits an audit trail for hourly data and for retroactive reconstruction of flow data as a result of such events as transmitter failures or incorrect orifice plate sizing. Omni's Modbus-based archive registers also reduce the requirement for the host supervisory system or DCS to constantly poll the flow computer for all its data. The SCADA can poll at regular intervals and "backfill" its database with the additional data. This permits the SCADA to scan most frequently only for the most critical data and minimize any communication overhead. Ultrasonic Flow Meters Ultrasonic technology has been used for many years in flow measurement applications. However it has only been within the last few years that the transducer and microprocessor technologies have advanced to the point that high levels of accuracy are being achieved (Better than 0.5% is claimed in gas measurement by Instromet, the industry leader.). As the technology has advanced the full potential for high accuracy custody metering is being realized. According to manufacturers' claims, ultrasonic meters have a number of significant advantages over traditional metering methods: turndown ratios as high as (60:1), no pressure drop, no calibration, minimal maintenance and no moving parts. With the increasing need to save both capital cost and maintenance costs the move to ultrasonic meters appears to make sense. A single 12" gas ultrasonic meter can replace up to 3 12" orifice meter runs with all of the associated calibration and maintenance requirements. Ultrasonic flow meters use a digital Transit Time technique to measure the velocity of the fluid independent of composition variations. Therefore with accurate dimensions being available for the pipe or spool piece in use an accurate determination of volumetric flow is possible. Most ultrasonic meters used for high accuracy installations utilize an array of transducers arranged around a spool piece. All transducers are "wetted" i.e. exposed to the fluid and transmit pulses in opposite or send\receive directions up to 20 times per second. The velocity of the fluid is directly proportional to the speeding up or slowing down of the pulses as they travel with or against the flow. As the gas industry moves increasingly toward the use of ultrasonic meters for custody transfer measurement, the inclusion of an accurate and reliable flow computer enhances the operation of the station. With the addition of real time Gas Chromatograph or density meter data as well as data logging, audit trails and additional diagnostic information an Omni Flow Computer integrated with the ultrasonic meters adds considerable strength to a new metering technology.
RedundancyDuring normal operation, one computer is designated the primary and the other computer the secondary or backup. To ensure synchronization between both devices, important variables such as PID controller settings, control valve positions and proving meter factors must be transmitted from the primary flow computer via the Peer to Peer serial communication link to the secondary flow computer. Should a failure of the primary flow computer occur, the secondary flow computer is automatically promoted to primary and assumes all control and measurement functions. In this case the data flow on the Peer to Peer link reverses automatically and the new master begins to transmit critical data to the slave, assuming that it is functioning. Peer to Peer communication errors can occur during the switch over and are normal. If the flow computer is operational, the Peer to Peer communication errors can be cleared by pressing the 'Ack' key on the flow computer keypad or writing to point 1712 (acknowledge station alarms). Sensing Failures and Switching between Redundant Computers When 'Activate Redundancy' is selected in the Peer to Peer menu, database variables are activated to provide a redundancy switching mechanism which is accomplished by cross connecting 4 digital I/O points from each flow computer (primary and secondary). Helical Turbine Meters The use of dual-bladed helical meters has been popularized in the USA in recent years. However, they have been widely used in the North Sea, Europe and Africa for at least two decades in various industrial and custody applications. The interest in the USA has been the possibility of using a meter that can exhibit reliable linear performance over a wide range of fluid properties and operating conditions particularly in batched crude oil pipeline transportation. The established custody transfer market leader in this technology is Faure Herman Meter. The performance of dual bladed helical turbine meters has been proven to meet the repeatability criteria of API MPMS guidelines for custody transfer service. The helical meter conforms to all existing international petroleum measurement standards including API, IP and ISO and covers both PD and conventional turbine meter regions defined in the API MPMS guidelines. A helical meter's performance has been shown to equal or better the PD meter's performance over a range of low to high viscous fluids. With high turndown ratios, depending on viscosity, and low pressure drop and other mechanical advantages, manufacturers claim that the use of the dual-bladed helical meter with a factory calibration for multiple viscosities can be a very cost effective, low maintenance alternative to the use of PD meters. For most line sizes, because of the low resolution pulse output, the device requires the use of double chronometry proving method, either with conventional pipe provers or with reduced volume or small volume provers. When large viscosity variations occur, the use of a viscosity indexing/flow linearization function greatly improves the performance of either a helical or a PD meter. Both the double chronometry proving method and a universal calibration curve polynomial can be found in the Omni flow computer (c.f. Vol 1.1 Flow Linearization). These above features have been used with success both in the U.S.A. and in Latin America on major pipeline projects. For recommended reading on metering statistics, see ISO 4124:1994, IP PMM-Part 14:1993 and API MPMS Ch 13.2:1994 for further information on statistical control of dynamic measurement systems. |
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