Automation Helps Keep NGPL In Driver's Seat.
To design and operate a reliable control system, it is necessary to understand the basic operation of the equipment to be controlled; what information is required for safe control and operation; and how best to gather and process that information. The limitations of the control system will be the determining factor on how information is gathered and processed. The principles of engine and compressor operation are well known, and NGPL has been able to successfully apply these principles to develop practical automated control systems, using the available technology of the day. This ability has allowed NGPL to maintain remote operation of its facilities for the past 25 years.
For typical natural gas transmission, and storage service, compressors can be divided into two basic types, reciprocating and centrifugal. Drivers for these compressors can be electric motors, gas-fired turbines, or reciprocating engines. For the purpose of this discussion, we will limit ourselves to the topic of controls as they apply to reciprocating type compressors driven by reciprocating engines.
In order to properly control and monitor a driver/compressor unit, mathematical models must be developed to solve for the unknown or difficult-to-measure performance parameters. Unknowns such as theoretical horsepower and theoretical compressor capacity are readily derived from OEM supplied data, actual field test results and easily obtained operating data.
Calculating the actual compressor horsepower and capacities is a somewhat more difficult task, as there are many uncertainties that are difficult to account for. These uncertainties include pulsation, valve leakage, ring leakage, pressure drop through the compressor and valves, and uncertainty in the data. In past years it has been difficult to calculate and account for these uncertainties directly. As more powerful computers have been made available, it has become easier to make these direct calculations, however, the programs are expensive, and require large amounts of data, which may be difficult to obtain or simply be unknown.
A simpler empirical approach has been developed and used by NGPL for many years. Because of the relatively narrow range of pressures encountered in transmission service the differences in the actual and theoretical calculations may be accounted for by using easily calculated correction factors based on compression ratio. By calculating these factors from actual test data and plotting them versus compression ratio, a line or curve may be drawn through the data points. This curve is expressed mathematically as a polynomial equation, and the coefficients derived allow the actual compressor horsepower and capacity to be easily calculated for any given conditions.
Information about the health of the compressor may be tracked in a similar fashion. For example, from test data compressor efficiency may be calculated at known points. When these known points are plotted against the compression ratio, the resulting curve becomes the expected compressor efficiency for any given ratio. When this is compared to the actual compressor efficiency calculated during operation, any disagreement between these numbers will provide an alert to a potentially abnormal condition. This empirical method has proven too be highly practical, providing accurate results and is well-suited to the simplest of control systems.
At the heart of any engine control system is fuel measurement. Because of its importance, NGPL has orifice meters installed at each individual unit. All operating schedules for air manifold pressure and ignition timing, as well as the brake horsepower calculation are based on fuel flow to the engine.
Through test data, it is a simple matter to calculate the engine brake horsepower, and fuel flow. By plotting the fuel flow vs. brake horsepower for a series of known points, a curve is developed with coefficients that allow calculation of brake horsepower for any given fuel flow. In a similar manner, schedules for air manifold pressure and ignition timing are developed.
A comparison is made between the calculated compressor horsepower and the measured fuel horsepower. If these two numbers agree, this indicates a normally operating engine. If there is disagreement between these values, it is an indication that the engine is using either too much or too little fuel for the present load condition, and is a warning of an abnormal condition. Depending on the amount of disagreement between these values, an alarm or shutdown is issued. Some examples of problems that could result in a disagreement would include:
* mechanical problems with the unit such as a misfiring power cylinder
* improper calibration of control system components
* a compressor volume pocket or end unloader may be out of position
* fuel gas composition has changed significantly.
In the mid-1970s, NGPL began to automate the compressor stations along its Gulf Coast System. This early automation system utilized the existing pneumatic control panel, located at each unit in the engine room. This panel functioned as an interface between the supervisory computer and the unit, and also provided monitoring of the safety shutdown system. It had no ability to initiate control functions other than safety shutdowns. The pneumatic panel received its commands in one of three ways. Commands could be entered directly at the panel, remotely from the compressor station control room, or remotely from the gas control office in NGPL's Chicago or Houston office.
Data for the control calculations, and unit status, were passed to a "supervisory" computer located in the control room of each compressor station. Remote commands were passed through a pneumatic/electric interface to individual engine control panels. Many of these control panels are still in service, with updated supervisory computers, and continue to perform satisfactorily.
First Generation PLCs
In the mid-1980s, NGPL began a program of replacing obsolete equipment along its Amarillo System. As part of that program, the pneumatic engine control panels were replaced with electronic control panels. A programmable logic controller (PLC) was installed for each unit in the compressor station control room, while an operator interface panel was installed in the engine room. The PLC performs all the functions of the old pneumatic panel, with the advantage of increased reliability, and reduced maintenance. One of the most significant features of this system is the ability to make real time AGA fuel flow calculations using actual measured data, thereby eliminating many of the assumptions required by the pneumatic control system. As with the pneumatic base control system, a supervisory computer monitors the units and initiates certain control functions, as well as receiving direct commands from the local operator or remote commands from Gas Control.
As part of Kinder Morgan's Midstream Operations, NGPL is continuing to upgrade its control equipment. New PLCs, which can be installed directly in the engine control panels, allow control functions to be moved away from the control room, and out to the engines themselves. By decentralizing the PLCs, the benefits of increased reliability, and reduced installation and maintenance costs, are immediately realized. As with the previous systems, further refining of the control calculations, and increased precision of control provide improved unit monitoring and performance. This translates into lower maintenance costs and improved fuel economy.
Further refinements to the control calculations also continue to be made. An example would be that at certain locations where gas chromatography data is available, the fuel flow calculation is made in units of energy flow (Btu/hr) instead of volume flow (MMscf/hr). This automatically provides correction for changes in composition of fuel gas, and improves performance.
Future enhancements will include "smart" load step calculations, whereby the PLC performs load and capacity calculations ahead of any control changes to anticipate the effect on the unit. Improved emission monitoring capability will allow the PLC to consider the effects of exhaust emissions, and make appropriate changes to the air and timing schedules.
Most new horsepower installations in the natural gas transmission industry consist of gas-fired turbine, or electric motor driven compressor sets. Many companies as well as NGPL have replaced much of their existing reciprocating horsepower with more modern unit types. However, the large bore, slow speed, natural gas-fueled reciprocating engines will remain in service for many years into the future. Regardless of the type of driver, the control and operating philosophy developed by NGPL has proven its reliability, and practicality on a wide variety of installations. NGPL, with its many years of operating experience, and its proven ability to develop and operate efficient and reliable control systems, is well-positioned to remain a low cost and dependable transporter, and storage service provider in the natural gas industry.
Michael Bohannan, Field Engineer, Kinder Morgan Inc., Lakewood, CO
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|Title Annotation:||Natural Gas Pipeline Company of America|
|Comment:||Automation Helps Keep NGPL In Driver's Seat.(Natural Gas Pipeline Company of America)|
|Publication:||Pipeline & Gas Journal|
|Date:||Oct 1, 2000|
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