LOW POWER FLOW COMPUTERS Greg Phillips Bristol Babcock Inc. 2000 Governors Circle West, Suite F, Houston, TX 77092 INTRODUCTION Flow computers, themselves, are undergoing an evolution. One challenge for most vendors will be to offer a low power flow computer whose pricing approaches that of a three variable chart recorder. Many companies in the gas transmission, gas distribution and production industry; expect such a flow computer to be an evolution from today s smart transmitter technology, because of improved accuracy and innovation of multi-variable transmitters. That is to say, differential pressure, static pressure and temperature all in one transmitter. REQUIREMENTS The key operations, performed by low power flow computers include calculations, historical data storage, alarm/event logging, network communications for realtime or near-time data acquisition, trending of variables and flow rate to include data editing capability. Each of today s flow computers offers some subset of the following calculations: AGA3: Corrected flow for orifice meters. (Orifice) 1992 edition AGA5: Energy content (BTU) AGA7: Corrected flow for linear meters. (Turbine, PD) NX19: Compressibility AGA8: Super compressibility (Detailed and Gross) The low power flow computer (LPFC) of today requires that it be designed to provide high performance while minimizing the overall EGM installation cost. These electronic gas measurement locations vary from site to site; in most cases commercial power is not available, thus the need for LPFC s that can operate on direct current (DC) power sources. These power sources for remote locations can be battery power, for instance a lithium battery or alkaline battery. The lithium battery can provide longer life over a greater temperature range, while the alkaline battery is limited. The other most commonly used are the solar panel with lead acid or gel cell battery, this power source usually offered as an integral package with the LPFC includes panel, charger/regulator, battery, cabling and mounting hardware. The size and cost of the solar array will depend on the geographical location and power requirements of the LPFC. The energy from the sun creates a photovoltaic effect in solar panels that charges a storage battery powering the flow computer. The output voltage is 6vdc and should provide usually 30 days of autonomy. A typical LPFC offered by most vendors will include the following: Microprocessor capable of performing necessary equations and calculations. Capability to accept smart transmitter or analog transducer inputs. The ability to store historical data, hourly, daily, event and alarm logs. Capable of displaying real-time as well as historical data via a display or handheld. Able to be configured via a laptop computer or terminal interface by menu selection. Class 1, Div. 1, explosion proof, or intrinsically safe certified for hazardous areas. DC power source, internal lithium or alkaline battery, and solar array powered. Figure 1 shows an RTU-style single-run flow computer. This configuration includes a single-board RTU that is programmed to perform flow calculations and interfaces to differential pressure, pressure, and temperature transmitters. The transmitters are included in the package, but mount external to the flow computer. Optional items shown include the handheld terminal and internal modem. The hand-held terminal functions as both an operator interface/configurator and a data transfer medium. This same transfer medium might also be performed by a laptop computer. By adding a private line modem for telephone line communications or a switched network modem for dial-up phone communication wide-area networking to these remote sites can be achieved. FIGURE 1 2003 PROCEEDINGS PAGE 291
Since this type of flow computer is based on a singleboard RTU, it will, typically, include extra input/output points. For example, the unit shown in figure 1 includes eight analog inputs (4-20madc or 1-5vdc), three of which are interfaced to transmitters. The five remaining could be used to interface to additional transmitters for a multirun meter station. This RTU can also accommodate Digital I/O capability to perform run-switching valve status, valve control and gas sampling Instrumentation. While the additional I/O points imply that the flow computer can accommodate two or more meter-runs, if sufficient RAM (random access memory) is not available to store additional historical data, then the processing capability is limited. With today s technology and most vendors using 16 bit or 32 bit microprocessors data storage has not been a problem. FIGURE 2 Figure 2 shows an arrangement that is representative of the new trend in flow computers. DP and pressure transducers not transmitters, have been integrated into the LPFC package. That is to say that the secondary measuring device is mounted internally to the LPFC packaging not external. The amplifier electronics is included on the computer CPU (central processing unit) board. While figure 2 represents a single run application, some vendors can introduce dual-run capabilities by simply adding additional transmitters or one single multivariable transmitter. TOTAL MEASUREMENT ACCURACY A discussion of gas measurement accuracy must encompass all the stages of conversions and calculations there are in a LPFC system. Since each stage introduces error, the overall accuracy of the LPFC depends on the accumulated errors of all stages. The API has defined three stages in a flow computer system as primary, secondary, and tertiary. PRIMARY MEASUREMENT The orifice and the positive displacement meter or turbine has been the two primary methods of gas measurement in the United States. Some estimates put the number above half a million or more. These primary measuring elements impact the LPFC s performance greatly and must be inspected or verified as to their compliance to accuracy specifications. Remembering that the flow computer itself is considered a tertiary device that is responsible for correctly calculating flow based on information that it processes from secondary measuring devices such as transmitters or transducers. All together these three devices are critical to the overall accuracy of measurement we require. Positive displacement meters and turbines should normally be sized to operate between 60% to 90% of their maximum linear capacity. With proper maintenance with respect to mechanical drives, pitting, scratches, or deposits these inspections can normally be somewhat easily performed. On a scheduled basis these meters should be proven to insure accuracy and credibility not only to their mechanical but also electrical frequency output integrity. For instance two types of known provers are pipe and tank type. They provide a known volume against which the volume indicated by the flow meter can be compared. Both these provers are volumetric with respect to comparison results established. With proper maintenance and proving these P.D. and turbine meters should maintain accuracy to within plus or minus 1/4 %. Orifice meters are generally inspected to ensure compliance with ANSI/API 2530 established tolerances. These tolerances include orifice diameter, edge width, edge sharpness, surface finish, flatness, and concentricity. There are several defects that can cause adverse effects on the meter s accuracy such as a bent plate, a nicked bore or rough surface. Also, if it has residue build up or is dirty this needs to be cleaned. SECONDARY MEASUREMENT Most LPFC packages include secondary and tertiary devices: DP, pressure and temperature transmitters that which are interfaced to a digital flow computer. Transmitter specification provides the accuracy of conversion to a 1-5vdc or 4-20mA dc analog signal. To determine the accuracy (or really, uncertainty or probable error ) introduced by all three transmitters, the square root of the sum of the squares method is used. For example, if the accuracy of each transmitter is 0.25%, the total probable error of the secondary stage is approximately 0.43% (this method is accepted by the industry even though it weighs each input DP, pressure, and temperature the same). PAGE 292 2003 PROCEEDINGS
Further errors are introduced when the transmitter outputs are interfaced to A/D s (analog to digital converters). Typical accuracy for the input conversion is 0.025%. Again using the square root method, the total probable error for all three inputs is 0.043% The mathematically correct way to account for the combined uncertainty of the transmitters and analog inputs is to add them together: 0.43% to 0.043% = 0.473%. If you doubt this method, an alternative is to treat each of the three inputs separately. The 0.25% for the transmitter should be added to the 0.025% for the input, for a total error of 0.275% per input. The square root method for all three is still 0.473%. Advantages of figure 2 depend upon the strategy the particular vendor uses to process the data from the three variables DP, pressure and temperature. If the LPFC processing this data uses an A/D (Analog/Digital) converter the accuracy of that data may represent what is stated in the above paragraph using analog transmitter technology. If the vendor utilizes smart transmitter technology with digitized communication of the values presented, first, there is no secondary/tertiary conversion. There is no 1-5vdc or 4-20mA dc output and no A/D conversion, as performed by an analog input. Thus, the typical 0.025% uncertainty is eliminated. In addition, the smart transmitter accuracy is increased. For example, if the smart transmitter had an accuracy of 0.1%, then the square root summed for all three variables would equal 0.17%, compared with 0.473% for the aforementioned system. If this transmitter was to be a multi-variable three in one transmitter such as some vendors supply today, the DP sensor is fully static pressure compensated. TERTIARY MEASUREMENT The tertiary stage comprises the calculations within the LPFC. It is relatively easy to accurately perform an instantaneous AGA3 or AGA7 flow equation as most vendors do not consider this a problem. Compared with the input accuracy, the calculation accuracy is insignificant. However, how often are the calculations done? That is the main concern of manufactures of LPFC s. A further issue is averaging and totalizing over times such as an hour and a day. While some people think that calculations performed inside a flow computer are extremely accurate, the truth is precision can fall off in time-based averaging and totalizing. By using double precision (64 bit) floating-point math for all averaging and totalizing, these averages and totals are usually updated once per second. In general the LPFC of today will require it execute input sampling, alarming averaging, totalizing, PID control if required, and all calculations, except AGA8, once per second. Due to the intense calculation required by AGA8 for compressibility using detailed gas composition this could be performed once per minute. AUDIT TRAIL ALARM/EVENT LOG A requirement apparent to vendors that manufacture LPFC s is the audit trail. This is a log that will keep track of alarms and events that occur within the LPFC system. An example of alarms that may appear in the alarm log would be: System power down System power restore Low system power level DP, Pressure, Temp, and Turbine High alarm High high alarm Low alarm Low low alarm Out of range Rate of change Return to normal Power down Power restore Low RAM battery level Examples of events: Operator sign-on (laptop) Operator sign-off (laptop) Low flow cut-in Low flow cut-off Override mode on Override mode off Maintenance on (calibration) Maintenance off (calibration) Orifice plate change Value change of constant (Alarm and events should have the capability of being reported over the wide area network (WAN). SNAPSHOT LOG Upon certain alarms and events, some LPFC s will not only log an alarm message, but will store the entire list of input, flow, and configurable constant values. This allows the user to see the entire station status, rather than a single message, when an exception condition occurs. INSTANTANEOUS/HISTORICAL LOG In addition to storing alarm and event audit trail information a LPFC will have the capability to store current as well as historical information. A LPFC will typically store 35 days of information, the amount of information stored in an hourly or daily log varies and depends on what the vendor may offer. Information that may be available in the hourly, daily as well as quarter hour logs are: 2003 PROCEEDINGS PAGE 293
Date start Time start Flowing time Average Differential pressure Average Static pressure Average Temperature Average Specific gravity Average BTU Flow extension Flow Rate Energy Rate Alarms occurred Events occurred Compressibility (FPV) Average C-prime Average CO 2 (Carbon dioxide) Average N 2 (nitrogen) These items will also include station identification, station tag and meter identification. COMMUNICATIONS Requirements for LPFC s at a minimum, is to provide a RS-232 Port capable of interfacing with a laptop computer or a handheld device. This interface would enable the user to configure, monitor, and change parameters specific to the measurement requirements of the LPFC. It would also provide the capability to up dump or collect historical logs, such as daily, hourly and audit/event logs. RS-232C is a serial asynchronous communications standard used to connect modems, terminals and printers with serial interfaces. The Electronic Industries Association (EIA) developed the recommended Standard-232 to define a serial communication interface. This standard is referred to as RS-232/ RS232-C, and RS-232-D. The C and D refers to particular versions of the standard. Although RS232C is only specified for use in transmission lengths up to 50 feet, it is often used for greater distances at lower baud rates. An additional RS-232 port has to be made available for network communication for local area networks (LAN) and wide area networks (WAN). LPFC s in the digital communications world are defined as DTE (data terminal equipment) devices. Modems, radio modems and other communication media are considered DCE (data communication equipment). The main difference between DTE and a DCE device is definitions of their respective transmit and receive pins. Pins two and three have opposite meanings, the DTE device transmits to the DCE receive pin, and the DTE receives data from the DCE transmit pin. The other pins defined on a DB-9, DB-15 or a DB-25 pin connector refers to control or handshaking signals. These are used to control the timing between device s for transmitting and receiving data. To better define the operation of digital data exchange RS-232 link signaling is accomplished with voltages that range from +or 3vdc to +or- 25vdc. If the voltage on the transmit or receive lines is positive (between +3vdc and +25vdc) this represents a 0 bit; if the voltage is negative (between - 3vdc and -25vdc), this represents a 1 bit, both with reference to the signal ground pin. RS-232 communication is most used in implementing wide area networks where linking several LPFC s is required over many miles. This wide area link is accomplished via the DTE to DCE (modem/radio modem interface. RS-485 on the other hand is a EIA standard for serial communications that uses a balanced system for signaling and basically the same signaling voltages. The RS-485 link can be used over fairly long distances (1000 ft.) and at high baud rates such as 38.4 KB. This form of communication is normally used for local area networks, offshore platforms, gas plants, etc. The link is established by using a single twisted pair (both transmit and receive on the same set of wires) that is connected to each device and each device on the network having its own distinct hardware and/or software address, (this also applies to RS-232 as well). This forms a bus topology that can be made use of by network protocols. Since it is a very simple and inexpensive topology, RS-485 is used frequently in the field connecting LPFC s over a local area network. TELEMETRY Various types of communication media are available to us today. An example of the types range from the following: (PLM) Private line modem, communications a form of DCE that modulates over leased or private telephone lines. (SNM) Switched network modem, communication over the PSTN (public switched telephone network) modulate over PSTN for cellular or dial-up communication links. (FOM) Fiber optic modem, communication over fiber-optic cable usually 64 micron or 200 micron thickness cable. (Radio/Microwave, Spread Spectrum) These types of communication are capable of extending long distances, with wireless capability. INPUT/OUTPUT CAPABILITY Additional demands have been placed on LPFC s to provide I/O capable to perform various functions. This I/ O capability is performed by analog inputs and outputs (1-5 vdc or 4-20 ma dc) or discrete inputs and outputs (open collector or relay). INPUTS An example of signal inputs that may be interfaced with LPFC s are digital or discrete open/close contact closures. PAGE 294 2003 PROCEEDINGS
WELL TEST In the production industry for example a separator is used to separate condensate, water, and oil from the well. Normally measured in barrels or tenths of barrels, accumulators register the amount of liquid passing through a turbine and produce frequency and or contact closure outputs proportional to the amount of product produced. The LPFC can register these outputs from these accumulators and present hourly, daily and monthly totals. Some LPFC s based on a demand requested either locally through a laptop computer or globally over the network communication media may invoke a well test. While normal EGM calculations are occurring, a well test can be initiated where over a predetermined amount of time usually hours, a totalization of average DP, P, T, flow rate, total volume, condensate total, and water total can be determined. Contact closures from various alarms may be monitored by the LPFC as well. Valve status Security hatch Gas level detection Intrusion alarms High levels Low levels These are just a few examples of inputs that may be monitored by the LPFC. OUTPUTS An example of signal outputs that may be produced by LPFC s are analog and digital. VALVE CONTROL Some LPFC s provide analog outputs in the form of voltage or current, and digital output in the form of contact open collector or pulses. The output selected will depend on the actuator controlling the valve, usually a electronic to pneumatic converter is used. SAMPLERS Almost all LPFC manufactures offer the digital output configured as a standard to pulse a gas sampler. Using a predetermined volume of natural gas the user can configure a rate where which a digital output will activate a sampler. The rate or sampler activation interval is based on the cylinder size and the amount of time it will take to fill the sample cylinder. SUMMARY In recent years, it has become apparent that the gas production, distribution and transmission companies require greater accuracy and low power consumption. A rather extensive amount of gas research and standards committee activity is pointing to higher raw input sampling rates, higher calculation frequencies, more intensive calculations, more data storage, and, in general, considerably more work for the processors used in flow computers. With deregulation, gas marketing and contract agreements are placing their own demands on the LPFC s. They are now used for custody transfer. Specific data must be available for billing and auditing. Flow computers must reside on communication networks to provide data now, not two weeks from now. The contracts also dictate accuracy, which, in turn, dictates sampling rates of DP, pressure, temperature to include AGA calculations. To make matters worse for LPFC vendors, there is no uniformity in gas industry requirements. Every company seems to have a unique need, be it the data that is stored, on what interval, how to do averaging and integration, what information is required over a communication network, the communication protocol, and so forth. In addition, the field measurement personnel have their requirements. The flow computer should be easily installed, calibrated, and started up. Ideally, it should install like a smart DP transmitter, not three transmitters plus a computer. The unit must also be low enough in power consumption to make solar power or batteries viable. Analog output PID (Proportional, Integral, Derivative) control is a continuous signal to the valve, either 1-5vdc or 4-20madc. This output is usually determined manually by the user or automatic by the LPFC in that the valve may be controlled based on a pressure variable or flow rate. This term is based on which variable is chosen to be primary. The selection would be either pressure control with flow override or flow control with pressure override. Digital control uses a pulsed or intermittent voltage output to control the valve. Both outputs analog or digital are based on a variable chosen, pressure or flow and a set point at which the valve will be positioned. Greg Phillips 2003 PROCEEDINGS PAGE 295