Advancements in Flow Computing Benefit Natural Gas Industry

By John Stitzer
Fisher Controls International, Inc.

Introduction

Prior to the advent of electronic flow computers, circular chart recorders were the mainstay of flow measurement in the natural gas industry. A paper chart, which recorded line pressure, temperature and differential pressure across an orifice plate, was manually gathered at each well site on a weekly or monthly basis and integrated to determine total gas flow by the company's gas accounting group. The integration process was tedious, slow and gas flow that was off the chart could not be accounted for. Records were kept in the field office showing verification and calibration information for each chart recorder. Gas samples had to be taken quarterly and analyzed for composition to support chart integration. The customer would be billed based on the calculated gas flow volume.

Today, the benefits of modern flow computers to the natural gas industry are undisputed. They provide real economic advantages and are essential to helping companies comply with government mandates such as FERC 636. Here are a few benefits of electronic flow measurement and control documented in an actual installation:

• Information gathered electronically and remotely from wells has increased overall flow and improved the flow balance for the field.

• Automating the state-mandated well-test procedure has increased production limits over manual testing.

• Short-term gains are being achieved by increasing or decreasing production for the spot market using flow control.

• The ability to control production from each well and of the overall field assures consistent gas delivery, and lets gas be sold at a premium.

• Field operators are now responsible for 50% more sites than before automation, saving contracted service expense.

• Automated reporting saves operators from unnecessary travel to sites, reducing their "windshield" time and costs for road clearing and maintenance in winter months.

• Production information used by the accounting department increases billing accuracy and reduces the billing cycle time by up to two weeks.

• The engineering department uses production information to analyze well performance for maintenance scheduling and for gas reserve analysis.

This progression from manual to electronic remote flow computing and reporting has not come about easy, or quickly, for that matter. Rather it has been marked by a slow, yet steady evolution of hardware and software advancements combined with knowledge gained by manufacturers and the end users based on years of installation experience. Let's look at where we are today.

Technology

The greatest single force driving flow computer technology is the ever-evolving microprocessor. Today, computers based on 32-bit processors are becoming increasingly available and the trend towards more processing power will most certainly continue into the future.

Microprocessors are becoming less generic as designers are taking into account the intended application for each new device. For example, newer microprocessors with enhanced communications capability are especially attractive for flow computing applications.

Power consumption has a definite impact on the initial cost of a flow computer installation, and the lower the power consumption the better. Today's flow computers consume a fraction of the power of their predecessors, typically less than one watt for a basic unit. The resulting reduction in solar panel and battery requirements can significantly lower the installed cost of the flow computer.

Flow computers using newer memory technologies, such as "Flash" ROM (read-only memory), allow firmware upgrades to be made on-line in the field or over remote communications links by means of a simple file download. In the past, a flow computer had to be taken off-line while its memory module was removed and replaced. When flow computers are spread over several hundred square miles, the savings in time and travel afforded by the flash ROM is tremendous.

Today's flow computers are highly reliable, a result of the increased use of surface mounted electronic components, which are better able to withstand exposure to wide temperature ranges, dust, and vibration that is present in the field environments. In addition, these components generally cost less than their predecessors.

Auxiliary I/O, that is I/O not used for flow measurement, is becoming increasing popular in flow computers, brought about largely by the expanding role of the flow computer to perform extended monitoring and control functions. Expect to see flow computers make greater use of digital I/O technologies, such as HART and Fieldbus, as the popularity of smart end-devices increases.

Another technological advance in flow computers is the multi-variable sensor. These all-in-one sensors measure both differential and static pressure within the same sensor body. Some even provide a connection for a temperature probe. Sensors may incorporate "smart" electronics or use the flow computer's electronics for signal processing. A single multi-variable sensor can replace three conventional transmitters at less than half the cost.

High Performance

Today's flow computers are the highest performing, most reliable units ever made. They offer improved accuracy and repeatability of measurement, faster calculations, and greater capability to store data. Improved microcomputer technology is the most obvious reason for these improvements, along with better software and sensor technology.

Smart transmitters, whether multi-variable or single-variable design, have contributed greatly to improving flow computer performance. With smart transmitters, the process variable is converted to a digital value within the transmitter and compensations to the process value are made there. The resulting digital value is read directly by the flow computer's microprocessor. Smart transmitters usually have an accuracy approaching three times that of a typical analog transmitter's accuracy.

A smart transmitter retains the factory calibration and characterization of the sensor within non-volatile memory. In a conventional transmitter, if damage occurs to the sensor, the complete unit must be sent back to the factory for re-characterization. Another important advantage of a smart transmitter is that it can have a stability specification of four times that of a conventional analog transmitter. This means less error is introduced into the flow calculation and the frequency of calibration is reduced.

The faster processing speed of modern flow computers lets them do much more than their predecessors. While performing AGA calculations was about all that older flow computers could handle, today's units can be perform more calculations and do them more often. This has allowed flow computers to assume additional tasks such as expanded monitoring, alarming, and complex control.

The ability of 32-bit processors to access increased amounts of memory, as well as higher density memory chips, lets modern flow computers store amounts of data previously unheard of. This lets users accumulate more data points for longer periods of time which is useful for audit trail history and for analyzing the performance of a well.

Ease of Installation and Use

Today's flow computers are easier and less costly to install due to their smaller size and greater use of the multi-variable sensor. By eliminating the need for separate differential pressure and static pressure transmitters, multi-variable sensors reduce tubing, mounting, and wiring requirements. Some multi-variable sensors are designed to be used with a special manifold that attaches to the orifice flange and eliminates the need for external tubing entirely.

Ease of use is an important factor contributing to increased operator acceptance of flow computers. The PC has set the standard for friendlier man-machine interfaces. Gone are the days of the dedicated man-machine interface hardware commonly required by older flow computers. Not only did they provide limited information display, but could be used with only one brand of flow computer. Users can now select the type of PC that best suits their needs, and manufacturers have extensive options on how information is displayed and input.

Efficient retrieval of flow computer data is concern of users and manufacturers are addressing this problem. For on-site retrieval, the cable connection between the flow computer and PC is the traditional method. A newer method uses a low-power RF link between the flow computer and PC to let the user download data while remaining in their vehicle. It is possible to poll several flow computers from a single location using this method. While not yet common, these methods are in use today by some companies.

Customize to the Application

Newer flow computers are better able to adapt to changing application requirements, as manufacturers move away from the inflexible, dedicated devices of a decade ago. Newer flow computers can be configured to perform either orifice flow or turbine flow measurement using AGA-3 or AGA-7 calculations. Even the version of AGA-3 calculation, such as 1985 or 1992, can be selected on some flow computers. This gives users tremendous flexibility in deciding where and how the units are employed.

It wasn't long ago that flow computers were expected to measure flow and little else. Today's flow computers are true measurement AND control devices offering capabilities that were at one time exclusive to high-end RTUs. For example, control valve regulation based on PID algorithms is now commonplace among flow computers. Some devices even employ override control for two or more loops, adjusting each loop to maintain desired pressure and flow rates.

Some manufacturers have given flow computers logic and sequencing control capabilities to extend their range of applications even further. With this capability, the flow computer can make logical decisions based on measured or calculated values and provide an appropriate action or response.

Still another way to adapt the flow computer to the application is through programs that the user creates and runs on the flow computer. These programs are generally written in a higher level language such as C and loaded into the flow computer's memory through a PC. Manufacturers who offer this capability generally provide or recommend software "tool kits" for doing this.

Modular and Expandable

In the past, users had two choices when shopping for a flow computer; purchase a unit which met their current needs and then replace the unit when their needs changed, or purchase a unit which would potentially meet their future needs and incur the additional cost up front. Unfortunately, the second alternative assumed that the user understood what their future needs would be, which wasn't often the case. Today, users can purchase a unit that satisfies their current needs and yet be expanded to meet their future needs. "Scalability" is the buzzword for this increased expandability.

Whereas flow computers used to be almost exclusively single meter run devices, many are now capable of measuring flow on two or more meter runs. The user need only purchase an additional multi-variable sensor and connect it to the flow computer to take advantage of this feature. Likewise, the need to connect to field devices can be as simple as plugging in I/O boards or modules and connecting field wiring.

Modular hardware also facilitates field maintenance and repair. Today's flow computers use small, compact circuit boards that are easy to remove and replace. Even most multi-variable sensors are easier to remove and replace than individual transmitters.

Not only has the performance of flow computer software improved dramatically over the years, but today it is much easier to upgrade. Flash memory has allowed manufacturers to offer users upgrades to enhance the performance of, or add new functionality to, units installed in the field and made it easier to transfer software from one unit to another.

Flexible Integration

Early flow computers were designed to be chart replacement devices where operators had to go on-site to retrieve the data. If a user wanted to retrieve data remotely, they had to develop the necessary interface software and assemble their own communication system, or contract with a third party to do the work. Today, flow computer manufacturers are taking on this task by working with leading host software developers to create drivers that can link their flow computers to the host packages making for cost-effective SCADA systems.

Just as open architecture is becoming the goal of most industrial control systems, open, non-proprietary systems are expected to prevail for remote gas monitoring and control applications as well. A new industry standard has emerged that will have a major impact on vendors and end users. Called "OLE for Process Control (OPC)", it is a standard that benefits both manufacturers and end users by defining a standard interface for data access. OPC provides for a high degree of interoperability between client and applications supplied by different vendors. The OLE technology is currently available on Microsoft's NT and Windows 95 operating systems.

Another area of integration improvement is the use of Cellular Digital Packet Data (CDPD) communication to link flow computers to a host. Digital cellular phone networks free companies from having to install expensive private communications systems to link their remote sites to the host. They simply utilize the capabilities of their existing cellular carrier and pay for service on a monthly basis, thereby avoiding large capital equipment outlays. This technology also lowers the cost of communication because data and voice can be transmitted on the same channel which reduces the number of channels required to serve an area. As digital cellular coverage becomes more widespread, this will be an increasingly popular method of communicating flow data.

For areas without good cellular coverage, an alternative communications system may soon be available employing low orbit earth satellites. This advanced technology should work very well, but initial costs may not be competitive with conventional communication methods.

The Internet, while not yet widely used as a means to obtain flow computer data, nevertheless promises users tremendous benefits in both convenience and cost savings. Users will have access to an entire field of flow computers from a PC located at their home, office, or on the road. Expensive, specialized host software packages will no longer be required to retrieve and display flow computer data. Add to this the reasonable cost of Internet services and you have a very attractive alternative to conventional SCADA systems. Expect to hear much more about this in the future.

Another integration problem for users is how to take raw data from flow computers and fit it into their corporate accounting and reporting system. This problem is compounded if more than one brand of flow computer is used in the company's operations since there is no industry standard for data formatting. This problem is being solved with the advent of flow data editing packages which take raw data from the flow computer and allow the user to format it according to their needs. These packages work with most major brands of flow computers.

Approvals/Compliance/Testing

Rules, regulations, and requirements mandated by local, state, and federal agencies are challenging flow computer manufactures to ensure that their products are in compliance. In addition, users are increasingly asking manufacturers to document the performance of their products under "real-world" conditions.

Approvals are the most rigorous form of qualification that flow computers must meet. Approvals are awarded by agencies such as the Canadian Standards Association (CSA), Underwriters Laboratories (UL), Factory Mutual (FM), and Industry Canada. To obtain an approval, manufacturers must design their products to meet the requirements of the approving agency. Products are then submitted to the agency for testing. Once it is determined that the product meets the agency's requirements, an approval certificate is issued.

Compliance to flow measurement standards is determined by the flow computer manufacturer and is measured against the requirements issued by an industry committee or organization. No third party certification process is required, however the manufacturer may choose to submit their product to an independent testing lab for verification. Examples of compliance standards are AGA Reports 3, 7, and 8 and API Chapter 21.

Independent testing and benchmarking of flow computers is gaining popularity with manufacturers as users are demanding to know how well one flow computer performs against another in a simulated "real-world" environment. Although there is currently no industry standard for testing, there are laboratories that perform manufacturer-specified flow computer testing such as the Colorado Experimental Engineering Station, Inc. (CEESI) and the Southwest Research Institute (SWRI).

Summary

Natural gas flow computing has come a long way from the days when chart recorders were king. Advanced microprocessor-based electronics, smart sensors, sophisticated applications software, friendlier operator interfaces, and innovative communications technology have given companies greater knowledge of, and control over their field operations. This has increased their productivity and profitability, while at the same time ensured compliance with government regulation. As greater numbers of wells are automated, these benefits will continue to be realized throughout the natural gas industry.