PLC/PC MONITORING, CONTROL SYSTEMS A BOON FOR SMALLER NGL PLANTS

Milton R. Gill, Linden B. Sisk, Wayne F. Warren
Texaco Inc.
Houston

Terry W. Simpson
Texaco U.S.A.
Velma, Okla.

Experience at two plants operated by Texaco U.S.A. indicates that installing monitoring and control systems based on programmable-logic controllers and personal computers (PLC/PC) at smaller NGL plants can improve plant economics and processing of customers' gas and increase personnel safety.

Such a system gives management a means to track the performance of a plant while making significant steps to enhance employee motivation, morale, and safety.

ACCESS TO INFORMATION

Many smaller NGL plants (< 100 MMcfd) continue to rely on traditional control panels and various remote instruments to monitor extraction processes as well as engines and compressors.

The design of these systems generally assumes that little operator intervention will be required.

When an alarm occurs, the operator is provided little assistance in determining the underlying problem beyond identification of the particular measurement which exceeded its limit.

Some data might be displayed on strip-chart recorders, but most are available only in real time on a panel or at remote locations.

The situation in smaller plants with PLC/PC monitoring and control systems is much different and more like that in a larger plant for which a more expensive distributed control system (DCS) has been justified.

The operator has access to a large amount of information that he or she can use to analyze alarms as well as optimize plant operation in terms of safety income, and maintenance costs.

In fact, because of his advanced awareness of the state of the plant, he generally functions more in an active than a reactive mode.

With a significant amount of data available on which to base decisions, the operator can quickly see the results of his actions on the entire plant without leaving his station.

PLC/PC SYSTEMS

A system developed for Texas Velma gas plant in Oklahoma is typical of systems for small plants. A diagram of the system is shown in Fig. 1.

The extraction process and remote booster compressors have been monitored and controlled by the system for almost I year. The engine room is currently, being added to the system.

The system uses radio-frequency links to communicate with the remote boosters, but telephone links could also be used.

The main hardware component of the monitoring and control system is the PLC which acquires data from many points and also services as a controller.

In addition, there are loop controllers to control essential processes in the event of a failure of the main system. These controllers were installed before the new system was introduced, which eliminated the need for installing several pressure transducers.

Access to the data in the PLC is through the operator's PC and the parallel office PC. Both PCs have separate printers to produce alarm logs and daily reports.

PLCs are conceptually straightforward to program with a high level language called "Ladder Logic." Challenges arise in interfacing with such components as loop controllers, gas chromatographs, and other items, but suitable drivers can be found or written.

The PLC has a reputation for high reliability, which is particularly important in NGL plant operations.

The software which is run on the PC to provide access to the PLC is one of the many commercially available operator-interface packages. The packages are generally characterized by the ease with which they can be programmed to produce a wide variety of screens.

The printed reports prepared by the PC are generated either by the operator interface or by a commercial spreadsheet program.

SYSTEM DISPLAYS

The "look and feel" of the PLC/PC system can be conveyed by reviewing several of the PC screens from the Velma system.

The schematic diagrams of the process form the basic set of screens for the system, as seen in Fig. ". This screen shows the cryogenic expander and the demethanizer.

The data include temperatures, pressures, and loop controller set points, and actual operating points, all of which are updated approximately each second.

In Fig. 3, a loop controller screen is shown with a square window (left of lower center) selected for changing a set point.

A digital and bar graph representation (right in Fig. 3) and a real-time trend (upper left) for each controller is displayed to keep the operator updated on the controlled loops.

A typical alarm screen modeled after a traditional annunciator panel is shown in Fig. 4. A shutdown alarm blinks red and a non-shutdown alarm blinks yellow until acknowledged, and then the alarm is highlighted continuously until cleared.

Alarms are also shown in a text format at the bottoms of other screens as illustrated in Fig. 2.

A gas-chromatograph analysis screen is shown in Fig. 5. The most recent analysis results and times for three streams are given at the top of the screen, and the corresponding 24-hr averages for the last day are given at the bottom.

An example historical trend is shown in Fig. 6 for several measurements over a 20-min period. There is considerable flexibility for changing the presentation parameters to give trends ranging from minutes to weeks in length.

Finally, a plant-balance screen is shown in Fig. 7. This screen shows the source of all incoming gas and the disposition of all residue gas and NGL as well as the amount of gas flared and used for fuel.

These screens suggest the displays that an operator can have available. Many other screens can be generated, and in fact an interested operator or engineer can learn to develop custom screens using the operator-interface screen generator.

OPERATION

The comprehensive overview of the monitored processes is the most striking feature provided by a PLC/PC system.

The schematic diagrams become a mental model of the plant and provide numerical values for important measurements which are monitored. A quick scan of these diagrams rapidly reveals the status of the plant, which is further aided by such graphical data as the liquid level in the demethanizer (Fig. 2).

Additional status information for an operator coming on duty for his or her shift can be easily obtained from the electronic mail feature of the system. The operator can be updated on the activity of the previous shift, and any problems which need further tracking can be highlighted.

Alarms generate the most tension in the operation of an NGL plant. When more than one alarm occurs, the order in which they occurred is indicated on the alarm printer (Fig. 1).

The measurements in alarm blink on the schematic diagrams to help the operator quickly sense their relative locations. All of the information in the system is then at the operator's disposal to help him track down the trouble.

If a loop controller set point must be changed, the new value can be entered directly from the PC keyboard, even if the controller is at a remote location in the plant.

Even though the old annunciator panel at Velma remains in place and operational, it is essentially ignored by the operators who rely totally on the PLC/PC system.

Real-time trends are useful in determining the root cause of an alarm. In fact, if trends are being closely monitored, an operator might observe changes that can be corrected before an alarm occurs.

Also, real-time trends are useful for monitoring a problem which is being watched for possible further action and for tuning PID loops. (PID = proportional integral derivative control, an algorithm used to control an analog variable; e.g., in a control valve to regulate flow in a pipeline.)

Active, opposed to reactive, operation of a plant is easy with a PLC/PC monitoring and control system. The operator can change any parameter and within minutes see the effect on the plant operation.

For example, this encourages trial-and-error efforts to keep the methane-ethane ratio at the optimum value or to reduce the number of compressors required to maintain line pressure.

SOFTWARE DEVELOPMENT

For more complex control problems, off-line advisers can be used to provide the operator with a suggested parameter value, which can then be fine-tuned manually.

An "off-line adviser" is an optimization program which looks at critical outputs of the system and suggests calculated critical PID setpoints to the operator.

In the future, this approach will be accomplished more and more by on-line advisers.1

An on-line adviser is integrated into the system so that it actually makes the setpoint changes.

Careful monitoring of engines and compressors can payoff quickly if even a singly catastrophic failure can be avoided. Historical trend curves can be most helpful with this task.

In addition, compressor diagnostic software (CDS), currently under development, has significant potential for compressor monitoring.

This software is being developed by Southwest Research Institute, San Antonio, under the sponsorship of the Gas Research Institute, Chicago, and the Pipeline and Compressor Research Council, Dallas.

CDS can detect valve or piston ring leakage that wastes horsepower, monitor important parameters such as rod load, and provide an alarm when appropriate.

The Clean Air Act will result in some engines being equipped with air-fuel ratio controllers and possibly ignition controllers in order to control emissions.

It would be possible to monitor these systems and to provide reports if continuous compliance is required.

PLC/PC monitoring and control systems, can help provide a safer workplace. Activated strobe and horns used with H,S and combustible gas alarms are sometimes difficult to locate in noisy environments or from closed control room-.

A much safer approach when an alarm occurs is to display a map of the plant on a green with the activated location- indicated. The operator then knows immediately where the alarms have occurred and can take the necessary action to be certain that all personnel are out of the affected area.

In addition, the level of safety in a plant generally depends on the overall awareness of the status of the plant. Better monitoring results in a safer plant.

Small plants processing less than 10 MMcfd such as Texaco's Enville gas plant in Oklahoma, are frequently manned only 8 hr/day. A PLC/PC system recently installed at the plant, which formerly had only pneumatic controls, can provide a record of the plant operation during the unmanned period.

This can be particularly helpful when several alarms have occurred, which might have shut down the plant. Knowing the order in which the alarms occurred can be helpful in restoring, normal operation.

Further, processes which are affected by diurnal variations in temperature can be modified automatically during night hours to keep the plant more nearly optimized.

In addition, an operator of a small plant can have a PC monitor in his or her home connected to the plant system by modem. This allows him to respond to alarms from his home during the night hours.

The potential benefits of this approach range from preventing an extended shutdown during the night hours to more efficient operation from rapid response to minor alarms.

CUT TRAINING TIME

Training is an on-going process in most plants. A PLC/PC monitoring and control system can cut training time dramatically, and result in better trained personnel with greater self-confidence.

The schematic diagrams provide a tangible model of the plant rather than abstract descriptions which have traditionally, been used by default.

Further, the trainee can develop a sound understanding of the plant by observing the interactions among the various parameters from changes in the loop controller set points. Limited "What if..." exercises promote this learning.

Motivation of plant personnel is essential for the efficient and safe operation of a plant. Increased levels of motivation can result from competition among shifts to produce the most income.

With electronic meters, on-line gas chromatographs (GCs), and available data, an operator can determine the income he has generated on his shift. This competition could be carried a step further by accounting for the energy used to produce the income, for example, the number of compressors required during the shift.

Motivation also results from the increased self-confidence developed by better training and from having a better understanding of the process being controlled. Essentially, the operator knows that he can make a difference.

Finally supervisory personnel can use the office monitor to assist the team effort in solving problems when alarms occur.

In particular, they can monitor H,S and combustible-gas alarms that could have serious personnel consequences. At other times, they can maintain general surveillance of the plant.

QUANTIFYING BENEFITS

Although the tangible benefits of PLC/PC monitoring and control systems are easy to list, quantifying them is at best approximates Typically good data and statistics on a plant are unavailable for the period prior to the installation of a system. But even if data were available, a long period of time would be required fully to characterize the operation of a plant.

The following tangible benefits are therefore listed without quantitative results:

Process optimization. The extraction process is more nearly optimized resulting in greater NGL production and residue gas closer to specification.
Flared gas. The amount of gas flared at remote boosters because of shutdown compressors is decreased as a result of more rapid response to restore the compressors to service.
Upsets. There are fewer upsets resulting in less lost production.
Unmanned-plant shutdowns. There are fewer extended shutdowns during the night that result in lost production.
Maintenance. The number of high-cost maintenance projects for engines and compressors will be decreased because of improved monitoring and diagnostics.
Training. Training costs are decreased as a result of the system providing a much more rapid means to describe the process and provide hands-on experience.
Operator time. All reports are generated automatically which frees the operator for other tasks.
Emissions. The risks of exceeding emission levels in continuous compliance situations are decreased.

The intangible benefits fall into the categories of improvements in personnel safety, in training, in motivation, and in morale.

All of these items affect plant profitability, but the cost of a system could be justified alone on the basis of improved plant safety.

HOLDING DOWN COSTS

The hardware and software costs for PLC/PC monitoring and control systems will range from $50,000 to perhaps $80,000 depending upon the number of points monitored and the need for special drivers.

System design, programMing, and installation will typically be as much as 150% of these costs and can be greater in some cases. If pressure transducers must be installed to produce electrical signals, additional costs ranging from $200 to $400/measurement will be necessary.

System costs can quickly escalate out of control without a concerted effort by both plant operators and suppliers to contain costs.

A fundamental strategy to minimize costs is to develop basic systems which can be used in a number of plants. After initial development of a pilot plant, the cost will be significantly lower for other plants.

Further, similar systems in different plants have other advantages. Maintenance might be less expensive if there are fewer different system designs to be maintained.

Also, someone transferred to a different plant will have less difficulty adapting to the new situation if the monitoring and control system is basically the same as the one in the previous plant.

Despite the tendency to customize a basic system for small differences among plants, software is frequently more expensive to modify than are plant procedures to change. The best strategy is frequently to adapt an operation to use standard software, unless cost is not a primary consideration.

These principles can be realized by developing an overall strategy as early as possible in the development of these systems.

This strategic, however, is usually not forthcoming until one or two pilot systems have been completed and the significance of the benefits and the magnitude of the costs for a high degree of customization have become apparent.

REFERENCES

Chou, K., Clay, R. M., Gamez, J. P., Berkoiwitz P. N., and Papadopoulos, M. N., "Multivarible control system installed at ARCO West Texas gas plant," OGJ, Nov. 16, 1992, F,. 33.
Smalley, A. J., Harris, R. E., Kothari, k. M., and Berry, A, R., "Optimizing Compressor Operation by Effective Application of Performance - Models," PSIG 23rd Annual Meeting Minneapolis, Oct. 24-25, 1991.