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By Drew Robb
The Turbomachinery Laboratory at Texas A&M University hosted another record- breaking event despite a three-month delay due to Hurricane Harvey. The 45th Turbomachinery and 33rd International Pump User’s Symposia (TPS 2017) took place in December In Houston, Texas. The exhibition hosted 359 companies and spanned 216,000 square feet. Some 4,620 delegates representing 46 countries visited the exhibition or attended technical sessions. The Symposia comprised 15 short courses, 23 lectures, 16 tutorials, 23 discussion groups and 21 case studies. Topics included compressors, steam and gas turbines, expanders, pumps and drivers, and auxiliary equipment, such as couplings, bearings, gearboxes, dry gas seals and annular seals.
The big news from the show, however, was the retirement of Dr. Dara W. Childs, Director of the Turbo Lab since 1980 and chair of the TPS advisory committees. He was honored with a banquet dinner during the Symposia (pp. 13).
The keynote by Jody Elliott, President of Oxy Oil & Gas, covered Enhanced Oil Recovery (EOR). Oxy has over 2 million horsepower of compression assets:30% centrifugal compressors, 60% reciprocating compressors and 10% screw compressors. All are used in upstream operations. Some of these machines are involved in EOR compression. Oxy is the largest operator in the Permian Basin, which spans New Mexico and Texas. CO2 is used to boost oil production in developed fields. The CO2 mixes with and releases the oil, then it is separated and reinjected in a closed-loop process. Most of the CO2 comes from natural under- ground domes, but captured emissions from power plants are also employed. Pipelines transport CO2 from external sources to the oil fields where it is used for EOR. Elliott reported that 5% of total U.S. domestic oil production is already coming from CO2-based EOR. Much of this is from the Permian Basin.
It is all about maximizing value from existing oil fields. Primary oil production only recovers about 15% of avail- able oil reserves in conventional fields. A method known as waterflood EOR is used to extract another 30% of the oil. After that, CO 2 EOR is needed to take out another 15%. “Other technologies will add to that to get above 60% of the actual oil present,” said Elliott. “For unconventional resources, primary oil production only brings out 6.5%, so that is a big opportunity for EOR.”
Garry Studley, Process Simulation Engineering, Dresser-Rand business, part of Siemens Power and Gas, delivered a session on optimizing component selection in synchronous motor compressor trains. Compressor trains driven by constant speed synchronous motors need to be designed to withstand high levels of oscillating torque during startup. The importance of accurately predicting peak torque levels at resonance is critical for proper component selection. Studley offered various tips during the tutorial: The motor and shaft line need to be designed for startup and steady-state operation. The peak torque capacity of the flexible coupling elements is typically the weak- est link. The shaft diameter and the overall stress concentration must be considered to determine weak-link locations. Integral flanges are best suited for the motor and gear shaft ends as they have good torque capacity and fatigue characteristics.
Supercritical SCO2 power cycles
Another tutorial by Timothy Allison, Manager of Rotating Machinery Dynamics at Southwest Research Institute (SwRi) delved into supercritical (SCO2 ) power cycles. A fluid is considered to be supercritical if its pressure and temperature are greater than its critical values. This produces a very dense fluid, which acts both like a liquid and a gas. From a thermodynamic power cycle perspective, fluids such as SCO2 can be superior to water and air, he said. Its higher density, means smaller equipment, lower costs, lighter weight and better enthalpy. SCO2 takes less energy to compress, but when it expands, it produces an abundance of energy. Its low viscosity means there is little pressure drop in the system. CO2 power cycles offer high efficiency and power density relative to incumbent steam Rankine and air Brayton cycles for power generation over a wide range of applications, said Allison. These include waste heat recovery, concentrating solar power (CSP), geothermal, nuclear and fossil energy. Above 500°C, SCO 2 is 2% to 4% more efficient than steam. Below that, a conventional Rankine cycle is probably a better option. As CSP deals with temperatures of up to 1,000°C, it is ideal for SCO2. While many of these applications have not progressed beyond the laboratory stage, waste heat recovery has made the greatest headway with several commercial projects already operating. “Compact machinery reduces material costs and is beneficial in low space and potentially low weight applications,” said Allison. However, the combinations of pressure, temperature and density in SCO2 power cycles lies outside the experiential base of existing turbomachines. Common challenges in the design of SCO2 turbomachinery include rotordynamics, pressure containment, sealing, thermal management, and transient/off-design operation.
Many large steam turbines (STs) are deployed in oil and gas applications. They are used to drive compressors and pumps, and produce electricity. The Prelude floating LNG vessel, for example, only has STs driving its compressors. STs in LNG were the norm until the eighties when gas turbines became more popular. Within the sector, many different steam turbine conditions exist. What worked for one application may not be good for another. An ST in a refinery may operate at 40 bar and 400°C at less than 20 MW, whereas a cracked compressor in ethylene service may need steam operating at 100 bar, 510°C and 85 MW. Condensing STs run below atmospheric pressure at their exhaust, while back pressure STs run above atmospheric pressure. Whichever type is involved, the API 612 standard requires sizing for worst steam conditions in power requirements. “As a contractor, we don’t want a to- tally new product, but at a minimum, a referenced part” said Emmanuel Bustos, Head of Rotating Machinery Department at TechnipFMC. “We check the casing and blades size, tip speed and existing applications, especially for the last blades.”
Controls dedicated to centrifugal and axial compressors require different valves, which are the final element in the control loop. These include anti-surge (recycle), suction throttle, hot gas bypass and quench control valves. Wayne Jacobson, Global Technology Manager Compressor Control Corp. (CCC) explained the objective of each type of valve, its ideal location relative to the compressor and the optimum performance characteristics for the valve. Anti-surge control valves, he said, are heavily influenced by piping layout as this influences controllability. The discharge volume of the compressor should be reduced to minimize the system’s dead time and lag time. “Make the distance as short as possible between the compressor and the recycle take off, and from there to the check valve,” said Jacobson. He offered several tips: In multistage compressors, anti-surge valve calculations are more complicated and can lead to oversizing of valves down- stream in multistage compressors. When the impellers are matched, the surge limit line and the choke line will likely be covered by one anti-surge valve. Fast and precise stroking of the anti-surge valve is needed (two seconds or less stroke to time to open and close).
Tips on the use of suction throttle valves (STV) were also provided. As the STV closes, it creates a higher pressure drop across it. This causes the inlet pressure to the compressor to drop accordingly with the consequence that the pressure ratio across the compressor rises. This causes the flow through the compressor to fall. For performance maps, closing the STV leads to the curve shifting vertically downward. This shift should not go down to the point where a surge can happen. Another compressor session authored by Patrick Smith, Principal Engineering Associate at Air Products & Chemicals, offered strategies to prevent catastrophic compressor failures during transient oper-ating conditions. “Simple changes in machinery condition monitoring, machinery protection and maintenance strategies could have prevented many failures,” said Smith. “Hot gas bypass valves are needed when a machine trips.”
Brüel & Kjær Vibro held a press conference on the evolution and application of API 670. Steve Sabin, Brüel & Kjær Vibro’s SetPoint Product Manager, who served as secretary of 4th and 5th edition task forces, summarized the development of the standard since its beginning.
“If a standard is good, it will have many editions and updates,” said Sabin. “The 6th edition will be coming out in a couple of years.”
API 670 is an instrument standard measuring vibration. It highlights a major upgrade compared to the early days of put- ting a screwdriver on a bearing housing to listen for vibration. In those days, it was all about being outside and inferring what was going on inside. Then along came Don Bently with the idea of putting proximity probes inside a bearing housing that could measure shaft vibration. Initial systems were complex, composed of many different cable lengths, each with specific parts. In addition, the ST, gearbox and compressor of the same train had different probes and systems. API 670 was introduced, in part, to provide standardization and simplification. The first version specified how many sensors were needed and where so OEMs could standardize (Table). “API standards reflect acceptable operating practices rather than catering to the least common denominator,” said Sabin. The 6th edition is likely to include changes related to SIL, wireless guidance, cybersecurity, rolling element bearing monitoring, cylinder pressure sensors, and over- speed enhancements. “Wireless is good for condition monitoring but is not yet good enough for safety systems,” said Sabin. “Wired systems are still the standard for machinery protection.”
Range versus efficiency
Striking the proper balance between range and efficiency was the topic of choice by Jim Sorokes, Principal Engineer at Dresser- Rand, a Siemens business. “A balance between peak attainable efficiency and overall operation range must be addressed when specifying, designing and selecting centrifugal compressors,” said Sorokes. Engineers, overly dependent on computers and CFD data, can occasionally be- come lost in a computational haze, he said. Such data must be interpreted correctly against the fundamentals of aerodynamic design. Compressor user requirements can be widely variable because there are many styles, each with a different performance characteristic. “Range and peak efficiency are typically mutually exclusive,” said Sorokes. “You can attain high peak efficiency at the expense of flow range or vice versa. But in most cases, it is a compromise.” Component designs are based on the choices made: impellers, guide vanes and diffusers. However, the impeller is by far the most critical decision, followed by the guide vane. Users and designers, then, must com- promise between machine requirements and what the OEM can deliver. To achieve that, both should each a strong agreement on nomenclature. For example, one party might be talk- ing about turndown while the other responds about aerodynamic stability. There- fore, it is important to define terms and not make any assumptions. Another example: different flow coefficients used within the same firm — one by the European branch that was different from what was used in the U.S.