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Designing Pipeline Safety
For more than 48 years, the Institute has conducted the Gas Machinery Research Council research and development program. Critical to this long-standing program for the gas industry was the development in the early 1950s of unique design tools for solving the problems of vibration and pulsation in natural gas compressor station pipelines. This capability contributed to the safe and reliable mechanical design of pipeline compressor systems and became a significant factor in the rapid expansion of the nation's pipeline system. Since its inception, this program has supported technology development activities that have allowed this capability to evolve over the years in response to changes in the marketplace. This capability continues to be used in the design of as many as 150 systems each year.
Technology Today® talks with Edgar R. Dupré, manager of the Fluid Machinery Design Services section in SwRI's Mechanical and Materials Engineering Division, as he shares his more than 35 years of experience with these important analysis tools.
Tell us about the compressor system design tools and why they are important.
During and immediately following World War II, the oil refining, petrochemical, and natural gas transportation industries in the United States grew extensively. This expansion led to the installation of numerous gas processing plants and, in particular, gas transmission compression facilities and pipelines. Machinery and piping for these facilities were sometimes hastily designed and installed. Because this equipment consisted primarily of reciprocating compressors and pumps, severe vibration problems often occurred. These vibration problems and the resulting potential piping failures presented grave safety concerns because of the flammable fluids flowing through the pipelines under high pressure.
Vibration problems increasingly became safety and economic issues, so 15 companies of the Southern Gas Association joined together in 1952 to form the Pipeline and Compressor Research Council (PCRC), known today as the Gas Machinery Research Council (GMRC). This group of engineering firms, machinery manufacturers, and operating companies required a program that would enable the industry to design, install, and operate compression and pumping equipment safely and efficiently.
The PCRC subsequently commissioned SwRI to develop a tool that could evaluate these compression systems at the design stage so that corrective measures could be developed prior to installation and operation. The first analysis was performed in November 1954 for a gas transmission station in Refugio, Texas. Since then, more than 10,000 analyses have been performed for compressor stations, gas processing plants, refineries, and petrochemical complexes throughout the world.
Although originally seen as a design tool to optimize new installations, this capability is also used to evaluate existing systems to develop corrective measures or to analyze new operating conditions. This usage accounts for about half of the studies performed over the past 10 years.
How does the simulation process work?
The simulation process involves creating a model of the compressor and its piping system, based on drawings and other data provided by the client prior to the analysis. Once the accuracy of the model is verified, a point-by-point review is conducted of the fluid pressure waves throughout the piping. These pressure waves, caused by the reciprocating motion of the compressor piston, can be amplified by the acoustical characteristics of the piping system a condition referred to as resonance. These pressure pulses couple into the physical (mechanical) piping system, resulting in sometimes dangerous vibration leading to stress and failures. Pressure pulses also affect the opening and closing of the compressor valves, thus increasing the power requirements of the machinery.
The analysis process includes modifying the model to shift the acoustical resonant frequencies of the piping system away from the excitation frequencies generated by the compressor. By controlling, adjusting, and damping the acoustical resonance, the pressure pulsation energy can be minimized, resulting in safe and efficient compressor operation.
The Institute also performs other work related to gas compressors, such as prediction and analysis of piping mechanical response, and analysis of thermal-induced piping stresses, compressor valves, foundations, and crankshafts.
Although the design tool is potentially accurate, it relies on the expertise of the designer or engineer to provide sound recommendations to the client. In short, SwRI is in the advice business. We are consultants in all phases of compressor system operation, analysis, and design.
What is unique about the technology development aspect of this program?
Every design project pays a small royalty to the Gas Machinery Research Council for the use of this technology. The royalty is used to fund an annual research and development program, which addresses new technical challenges in the industry and problems discovered in the field. One of the key features is a continual effort to expand our expertise and evolve our design tools.
There have been numerous developments over the course of this program that are considered quite commonplace today and that we, as compressor system designers, often take for granted. As with all technology, however, there is a first time for everything.
Some of the fundamental developments were in the area of gas piping acoustics. One of the most prominent developments of the program was the use of the "junction theory," whereby pressure pulsation filters could be designed without passing higher frequency acoustical energy. Another important discovery was the phenomenon of "unbalanced acoustic forces," which can cause damaging vibrations in the cylinder manifold piping around the compressor units. Equally important, of course, was the development of methods to control these forces.
In addition to technological acoustical developments, significant advances were made concerning the vibration and mechanical characteristics of compressor and pump machinery. Computer programs were developed to predict the mechanical resonance characteristics of the compressor cylinder manifold and related piping systems. These early programs laid the foundation for the sophisticated finite element analyses in use today.
What new tools are you using?
One of the more recent changes in the industry is the installation of a new class of high-speed compressors. In the traditional slow-speed compressor, the gas velocities are low and a simple linear model is adequate. In high-speed compressors, however, the valve area and internal cross section of the gas passages inside the cylinder are much smaller compared to cylinder volume, which means that gas velocities through the valves are much higher.
New tools were required to more accurately represent the effects of compressor cylinder interaction. Precise models of valve losses are required for the accurate prediction of cylinder performance. Unlike prior models, SwRI's new interactive model incorporates a dynamic valve model that allows more accurate prediction of cylinder performance.
Another new tool worth mentioning, although it has been in use at SwRI for about four years, is the digital data acquisition system. This is an automated method for pulsation data documentation that was formerly done by recording PolaroidTM pictures off an analyzer scope. In addition, a new analog-digital hybrid system enables much more accurate calibration of the compressor cylinder model, producing more consistent and repeatable data.
Who are your clients, and why do they continue to come to SwRI?
The success of the overall program is due, in a very great part, to repeat business. The main ingredient in repeat business is a satisfied client one who believes in the tools and in the expertise of the engineering staff that use them.
Our clients range from the largest oil production and refining companies to nationwide gas transportation and power companies to worldwide plant engineering companies to foreign and domestic gas compression and pump manufacturers, chemical plants, natural gas and oil production facilities, processing and gas gathering facilities and gas storage installations. Although the majority of our clients are domestic, we have conducted analyses for installations in more than 60 foreign countries and on every continent except Antarctica.
With all the changes going on in the gas industry, what is your view of this program's future prospects?
The use of natural gas for home fuel and power generation is increasing every year. Further, natural gas is a clean burning and ecologically acceptable fuel. The use of other petroleum products and hydrocarbon gases is also on the increase. As a result, gas compression systems will continue to be installed, repaired, modified, and improved during the next 10 years or more. With such intensive and widespread industry activity, the need for analysts and designers such as those in SwRI's Fluid Machinery Design Services section will continue for years to come. Naturally, a reliable and trusted design tool is also essential.
At SwRI, we continually try to improve our productivity and quality. This constant pursuit of excellence enables us to provide superior client service, and is the key to maintaining our leadership in compressor and pump system dynamics, analysis, and system design.
Comments about this article? Contact Ed Bowles at (210) 522-2086 or firstname.lastname@example.org.
Published in the Fall/Winter 2000 issue of Technology Today, published by Southwest Research Institute. For more information, contact Maria Martinez.