CHANGING OPERATIONS PROMPT MAJOR GAS PLANT AUTOMATION

Michael J. Mullally, Kathleen D. Goens
Exxon Co. U.S.A.
Houston

Exxon Co. U.S.A., Houston, has automated operations at its Katy, Tex., gas-processing plant to meet changing business demands.

The project consisted of a central computer communicating with electronic flow measurement (EFM) devices, on-line chromategraphs, and the plant's process control system.

The project sought to improve measurement by providing current "accounting quality" production information and to improve the accuracy and accountability of the gas-dispatching function provided by the Katy plant.

The plant's existing measurement equipment-gas samplers and chart measurement-were replaced with electronic devices and connected to a central computer system that manages the information needed to operate a major gas-processing facility. The completed system has been in operation since late 1991.

As a result, production accounting information is generated on site and is available in real time. In addition to providing current information, the computer also provides operations personnel with an effective tool to analyze historical information.

Operations at the Katy gas plant, Waller County, tex., have changed dramatically over the last 5 years as the Katy gas field production has declined. The lean-oil absorption plant which was designed for 1 bcfd of gas currently processes around 500 MMcfd with third-party processing (the processing of pipeline gas delivered from outside the Katy field) accounting for more than 80% of the plant's throughput (Table 1).

Third-party processing has added more receipt and delivery points (approximately 27 meters). Fig. 1 reflects the plant's configuration following completion of the automation project.

Change has also come in the form of monthly processing elections and monthly swings in gas price. These changes have made accuracy and timeliness in receipt and disposition of gas more important to profitability.

In today's business and regulatory environment, timely and accurate information is important to both the processor and the pipeline customers. Better "tools" such as those installed for this project are needed to provide up-to-date production and gas dispatching information.

TEAM COMPOSITION

Since this project involved such technically diverse areas as gas and liquid measurement, computers, networking, and process control, Exxon in June 1989 pulled together a team with varied backgrounds to design and implement the system.

The team consisted of the plant chemical engineer, a computer-electronics engineer, a plant instrumentation-measurement technician, contract software applications engineers, and several plant operators.

Critical to the project's success, these people pooled their knowledge to design an effective data-gathering system that provides accurate, real time operation and accounting information.

The team recognized that lag time in integrating charts was a substantial contributor to the problems in managing gas dispatching. Volumetric data from the charts are not available for a week after each calendar month. Thus daily volume totals were 1-5 weeks old.

Connecting the EFM equipment to an integrated computer system would eliminate these time delays.

As part of the project, the team also recognized the need to provide an automatic method for updating the EFM devices with new energy content, specific gravity, and contents (mole %) of carbon dioxide and nitrogen because the energy content of third-party inlet gas often varies significantly from day to day.

This recognition prompted the decision to utilize online chromatographs for BTU measurement instead of gas samplers.

To improve the gas-dispatching function, the team decided to expand the plant's distributed control system (DCS) so that the amount of gas delivered to each of the nine residue-gas pipelines could be more accurately controlled.

After the team identified 55 meters for EFM and 24 different composition gas streams for chromatograph analysis, its members recognized that this large amount of data required a computer to manage and display information to the gas-plant operators effectively.

Once installed, this computer could also perform supervisory control algorithms to enhance plant performance further.

EFM; CHROMATOGRAPHS

Almost every U.S. producer and transporter has switched or plans to switch from chart measurement to EFM.

EFM of gas uses electronic differential and static transmitters connected to a microprocessor which performs the AGA gas-flow calculation to determine flow rates and then integrates these flow rates to calculate an accumulated volume.

While these calculations are simple enough, the demands placed upon an EFM device become more complicated when accuracy, reliability, communications requirements, and accountability are also considered.

Even with proposed new API standards for EFM, each vendor's attempt at meeting these requirements has resulted in a significantly different EFM device.

Since many EFM devices are completely configurable and programmable, the burden of accuracy often falls on the user. In order to measure gas accurately, an operator must pay attention to details in both the physical construction of the meter run and in the devices used to perform the measurement.

Even more care must be taken if the measurement is to be custody-transfer quality. When large volumes are involved, a small mistake can result in large adjustments.

Selection of a suitable custody-transfer EFM device is not to be undertaken lightly. The Katy automation project team spent 3 months analyzing the various EFM devices on the market. In particular, the team was looking for a highly accurate device that could provide a significant audit trail.

Since the accuracy of the custody-transfer measurements were important to the Katy automation project team, the decision to connect smart transmitters to the EFM was made (Fig. 2).

The output of these transmitters is electronically compensated to minimize any nonlinearity that may result from temperature and pressure deviations. Several devices on the market today have this accuracy. Most support handheld communicator or laptop PC communications for calibration and configuration.

The following list provides some of the important measurement accuracy and audit-trail requirements used for the project:

Differential-pressure transmitters accurate to 0.1 psi
Static-pressure transmitters accurate to O.5 psi
Use of resistance temperature devices (RTDS) for temperature measurement
Full AGA gas-flow equation performed every second
14 days of hourly and 35 days of daily flow information stored in memory
Simultaneous communications to central computer, printer, handheld communicator, and a customer telephone connection.

On-line chromatographs were used for compositional analysis of each of the inlet and outlet streams to the plant as well as several of the process streams,

For custody transfer, the chromatograph must meet the following specifications:

Be reliable and have low maintenance costs
Have auto calibration capability
Be accurate and repeatable to O.5% of the measured value for mole % for each component
Calculate the BTU/cu ft within an accuracy of O.5 BTU
Calculate 24-hr averages for all values (for example, mole %, BTU, sp gr., etc.)
Have serial-communications ability to a host computer
Log alarms/deviations
If multi-stream, have double block and bleed capability to ensure no cross contamination of streams.

CENTRAL COMPUTER

The computer system installation and the networking of the various end devices were the most technically complex aspects of the project (Fig. 3).

The information generated by the flow computers and chromatographs is most useful when it is transmitted to and received by the central computer. This computer utilizes a "real time" data base to store and display information for operators, engineers, and management,

In addition the computer performs supervisory control functions automatically to control inlet and outlet flow rates to satisfy Customer requirements while optimizing plant performance.

Installed in July 1991, the host computer uses four serial communications ports for data acquisition from the EFM equipment, the distributed control system, and two different brands of gas chromatographs. All four of these devices communicate via different protocols so they each require their own communications software.

Custom communication drivers were written for the DCS and the two types of chromatographs while vendor software was purchased for the EFM communications. Communications between the EFM and computer utilized standard shielded cable and limited-distance modems, while both of the chromatograph systems utilized RS485 communications.

RS485 communications (when possible, depending on the particular vendor's hardware and software options) requires less hardware for greater distances than the more commonly used RS232 communications.

The information in the real time data base is displayed on PCs in both tabular and graphical format. The plant's control room operators are the primary users since the plant is staffed 24 hr/day (20-in. color monitors were installed in the plant control room to reduce eye strain).

The plant superintendent, engineering technician, engineers, and accountants (less frequent users) also have access and utilize standard 14-in. monitors.

Access to the computer is accomplished with direct links, fiber optics, and telephone dial-in capabilities. Because the control room and the plant office, which are located about 400 yards apart, have different grounding systems, a fiber optic link between the two buildings was installed to alleviate all ground loop problems.

The plant engineers' and accountants' offices are located on the north side of Houston about 60 miles from the control room. Hence, a dial-in communication link was installed.

The engineers and accountants run a terminal emulation package layered under a graphics package to display the very same graphs and reports available to the control room operators. The two control room operator terminals are the only terminals that can make operational changes.

A multi-tasking computer was used simultaneously to acquire data, store data, execute applications programs, and support the graphics display consoles. In order to obtain the desired performance, the computer system has 52 mb of memory and two disk drives providing almost 2 gigabytes (gb) of disk space.

The computer applications include the following:

Data acquisition
Archival of accounting information from the flow computers and chromatographs
Allocation of gas and liquids to each of the third parties and the Katy field.
Flow management controls to help operations dispatch the gas from the five inlet master meters to the 22 different residue gas meters at the plant tailgate.

During development of the algorithms for the application programs, plant operators were enlisted to help specify design requirements (Fig. 4). In particular, their expertise was essential to the design of the flow management software.

Incorporating the operators' design recommendations ensures the effectiveness as well as the acceptance of the system.

DATA ACQUISITION

From each EFM meter, the computer system collects three different time classes of data. In addition to retrieving instantaneous flowrate information every minute, the computer system also retrieves daily and monthly totals.

Similarly, the computer system collects the current analysis, daily averages, and monthly averages from the chromatograph when they become available.

It takes approximately 6 min for a chromatograph to complete an analysis on an individual stream (a gas sample from a particular pipeline). Because some of the chromatographs may analyze up to six streams, some streams are analyzed every 6 min while other streams are analyzed every 36 min.

In addition to collecting and archiving the stream analysis, the computer System performs some range validation. If the computer determines the analysis to be unreasonable, it logs an alarm.

If the computer believes the analysis is reasonable, it then down loads the CO, content the nitrogen content the specific gravity, and the energy content (BTU/cu ft) to the appropriate EFM.

In addition to flow rate and chromatograph data, the computer retrieves some 300 different process variables from the distributed control system (DCS) every 30 sec.

By integrating the process information into the computer system, the plant operator is able to view all plant operations from a terminal. Once the supervisory control algorithms are applied, the computer system provides the flexibility to download new setpoints to the DCS.

EFM and on-line composition analyses on all the gas inlets, fuel, product sales, flare, and residue-gas pipelines allow a complete mass balance around the plant.

Once every 5 min the computer calculates the portion of gas liquids, shrinkage, fuel, flare, and residue gas attributable to each plant customer. This information is then made available not only to operations, technical staff, and management, but also to each pipeline customer.

Tracking operations on a real time basis gives the operators and the pipeline customers the opportunity to work together to affect the final outcome of the monthly accounting process.

GAS DISPATCHING

Gas dispatching is the process of moving the right amount of gas from one pipeline to another pipeline at the right time.

Katy has long been a central exchange point for gas in Texas. As a result, flow rates between two pipelines can change 100-200 MMcfd from one day to the next and sometimes from one hour to the next.

Automation has helped streamline the dispatching process. The complexity of managing the large number of changes in gas flow from five inlet pipelines to nine outlet pipelines without the aid of computer made dispatching more time consuming for the control room operators than monitoring of gas-processing operations (Fig. 5).

Gas dispatching is complicated by the number of potential disposition points and the number of changes in flow rate.

Each inlet pipeline can request that gas be delivered to each of the nine outlet pipelines. Each outlet can then request that gas be redelivered or traded to any another of the eight outlets.

Some gas outlets have up to 4 meters in service; in total, there are 22 residue meters connected to the tailgate (Fig. 6). To manage this process, each request (also called a "transaction" or a "nomination") is recorded for audit purposes, then gas flow is changed simultaneously on multiple pipelines but without upsetting the gas-processing operation.

It is not unusual to have as many as 90 transactions (requests for changes in gas flow or disposition) per day.

In the past, each transaction required several steps on the part of the operator logging the transaction, estimating the flow on two pipelines, making the change on two flow controls, then re-estimating the flow on each pipeline.

The new system (Fig. 7) is designed so that the operator handles each transaction once by entering the transaction information into the computer. The computer then automatically adjusts flow targets and aids in tracking the results.

Since the gas actually delivered from one pipeline to another is never exactly equal to the amount requested, a strategy for controlling the flow of residue gas is required. The control strategy must perform the dispatching function without upsetting overall plant pressure.

CONTROLLING SOFTWARE

In order to accomplish this goal, the flow to each of the residue-gas pipelines is managed in a stepwise fashion. The control software performs this by implementing the following control algorithms:

Plant pressure control software manipulates the total residue flow leaving the plant
Total residue control software manipulates the flow to each pipeline proportionate to the pipeline's current flow and the change in total residue flow target
Flow management software swaps gas between pipelines to compensate for any new transactions or changes in flow from other pipelines
Pipeline balancing control analyzes the differential pressure for each of the meter runs a pipeline has in service and then determines the target rate for each meter (a pipeline may have as many as four meters with none to four meters in service at one time
The control software "down loads" the flow target for each meter to the digital flow controllers.

In order to close the loop, the EFM calculates the flow rate every second then updates an analog signal that provides closed-loop feedback to the digital flow controller. The flow controller then manipulates the flow control valve to meet its flow target.

The flow target sent to each flow controller is determined by the computer using the algorithm described previously. (Exxon decided to keep the flow computer's "cash register" function separate from the flow control function, both for reliability and accountability.)

The amount of gas available at the tailgate of the plant is determined by the plant pressure-control software in the main computer.

If the pressure is failing (too much gas is leaving the plant), less gas is available. If the plant pressure is increasing (more gas arriving than leaving), then more gas is available.

The pressure-control software which executes once per minute raises the flow target on the total amount of gas leaving the plant when the plant pressure is too high and lowers the total amount of the gas leaving the plant when the plant pressure is too low,

Every 5 min the residue-control software executes and adjusts the flow targets for each pipeline. Each of the nine pipeline-flow targets is adjusted for changes in nominations and deliveries while the total residue-flow target is kept constant.

The computer estimates each pipeline's share of the total amount of gas leaving the plant at that instant. This is determined from nominations and the amount of gas available for each nomination.

The amount of residue gas attributable to each inlet pipeline (determined with mass-balance calculations) is used to determine how much gas is available to meet each of that inlet's nominations. Each residue pipeline's flow target is then a function of the amount of gas nominated by each inlet and how much gas (or how close to the amount nominated) is actually delivered by that inlet.

The plant pressure-control software has proven successful. Before this algorithm was implemented, the 20-50 MMcfd inlet swings resulted in plant pressure swings of 100 psig. Under this control algorithm, these same inlet swings now result in plant pressure swings of 10 psig.

The gas dispatching and nomination process still requires surveillance by operators and the plant's pipeline customers. This process is beyond the computer's control because a nomination or request for a fixed amount of residue gas cannot be achieved if inlet flows change or other pipelines cannot take the amount nominated.

Automation can control many processes very effectively, but in some cases, it can only perform a stewardship role. Since the computer provides the ability to track the data, it can still help improve processes not directly controlled.

INFORMATION SYSTEM

Despite the complexity of the equipment installed and the large amount of data now available as a result of this project, the control room operator uses a few screens to operate the plant and dispatch the gas.

This project is primarily an information system with exception of one control application-gas dispatching. EFM improves measurement accuracy.

An integrated computer system, however, is the key to providing information to the people who need it the most. It neatly sorts, displays, and prints reports on gas composition, dispatching, and flow information for plant operators, accountants, and customers.

Plant operators have access to custody-transfer quality flow information available at the click of a button, and accounting has monthly dispatching and meter totals from the previous month available on the first day of the month.