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VIBRATION, ONLINE WASHING, COMPRESSOR SIZING, PERFORMANCE TEST CODE SHORTCOMINGS AND A CALL FOR STRONGER ETHICAL STANDARDS
The 42nd Annual Turbomachinery Symposium and 29th International Pump Users Symposium took place in early October in Houston, Texas. Almost six thousand attendees enjoyed a program consisting of short courses, tutorials, technical sessions, discussion groups and 322 exhibits.
The keynote session was opened by Dr. Dara Childs, Director of Texas A&M’s Turbomachinery Laboratory which organizes the event. He introduced Vincent Volpe, Dresser-Rand’s CEO and President, who discussed the challenges to business due to the cost pressures of short-term commodity price volatility acting against a backdrop of strong long-term demand. He showed bullish energy predictions for the next 20 years, with almost all of the gains coming from markets outside of North America and Europe.
“It is difficult to react to the spikes in commodity prices and sustain an economic model,” said Volpe. “This makes it hard for customers to decide whether a project will be viable or not.”
As a result of this pressure he considers a risk of returning to the old days of procurement when bids were submitted according to client specification and contracts that largely went to the lowest bidder. Capex was king. This procurement-driven picture was displaced in the late nineties by vendor alliances and a renewed emphasis on lifecycle costs. In other words, Opex became a far more important a factor.
“Unfortunately, Capex is becoming the preeminent factor in procurement, which could set the industry back 30 years,” said Volpe.
He conceded that price pressure was not going away, so offered a solution of renewed vendor partnerships as well as customer and vendor alliances to break the cycle. But he said this can only be achieved by using technology and stressing total system and package costs (Capex and Opex) rather than individual components. If each vendor engineers its products to integrate well with those it is commonly associated with, overall structural costs can be brought down considerably in a sustainable fashion without the need to squeeze suppliers’ margins.
“We can reduce overall cost of ownership by having suppliers, OEMs and academia work together,” said Volpe. “With effort, we can reduce the product development timeline from seven years to less than four.”
This means getting together at the outset to generate a decent functional spec, with all parties investing time and resources outside of the project work itself. As this requires tighter partnerships, he ended with a call for a greater level of ethics.
“As engineers we all have an obligation to speak when something is not right, which takes courage,” said Volpe. “Also, do not share one client’s or supplier’s ‘good ideas’ with another as this is intellectually dishonest and it may be illegal.”
As usual, the Turbo Symposium featured a variety of technical sessions. First up came a talk on torsional vibration. This can induce power fluctuations in synchronous motordriven reciprocating compressor trains. Such vibration is particularly important in remote island mode-operated gas turbine generators.
Torsional vibration in recip-driven compressors at the Point Thompson project in Alaska, for example, led to gear tooth surface failure due to micro-pitting. Mitigation techniques included the implementation of a chemically accelerated surface treatment to minimize micro-pitting and a new coupling that reduced pulsation by 25%.
Klaus Brun, Program Director at Southwest Research Institute (SwRI) followed with a dissertation on online water washing. He began with the basics: gas turbine axial compressor blades foul due to the deposit of dirt particles during operation.
This fouling significantly affects the gas compressor’s aerodynamic performance and efficiency, thus forcing the operator to regularly shut down the unit for offline water washing of the compressor; alternatively, online washing technologies can be used to clean the compressor during operation to minimize shutdowns and optimize availability.
To gain an improved understanding of the effectiveness of online cleaning technologies, specifically the dirt removal and redeposit processes, SwRI conducted a number of tests of blade fouling. These tests quantified the effectiveness of washing fluids as well as how much redeposition of foulant occurs in downstream stages.
“Water washing is a subject that everyone has an opinion about,” said Brun. “We wanted to find out how effective it really is and which liquid is best.”
The study compared the effect of highpurity water, de-ionized water, regular water and commercial detergents on fouling and found no real difference between them. As for claims that some proprietary cleaning fluids can minimize redeposition, again the experiments found no obvious difference between the various fluids.
Of particular interest was the question of whether online water washing caused noticeable blade erosion. According to SwRI, the project measured only 0.00047g of weight loss on a compressor blade after eight hours of constant online water washing. The conclusion: if washing is performed only for 10 minutes or less per day, it can be regarded as a negligible cause of erosion.
“Online washing is a viable means of cleaning compressor blades, but as some areas are not cleaned well, it doesn’t replace offline washing,” said Brun. “Cleaning appears to primarily be a droplet impact function and does not depend on cleaning fluid. You can probably do it for one minute and get most of the washing done.”
A key point to note is that dirt removed from the inlet guide vanes (IGVs) and early blade stages was found to redeposit downstream during testing. On one Frame 5 machine, for example, heavy salt deposits on the IGVs and first couple of stages got moved down to later stages.
The American Society of Mechanical Engineers (ASME) Performance Test Code on Compressors and Exhausters (PTC 10- 1997) has served worldwide as the standard for determining centrifugal compressor thermodynamic performance during factory acceptance and field performance testing for many years. However, its value has recently come into question during efforts to use it in the selection of compressors for high-pressure carbon dioxide and hydrocarbon mixture reinjection. In that case, the application of PTC 10 has been found to result in significant errors in the estimation of compressor polytropic heat and efficiency.
According to Mark Sandberg, a Consulting Machinery Engineer at Chevron Energy Technology Co., the result of ongoing research has been to identify portions of the test code procedure that are subject to errors caused by unique process conditions. He proposed an alternate evaluation method of test results and predictions to reduce errors in the calculation of derived properties relevant to the determination of compressor performance parameters.
Further on this subject, Rainer Kurz, Manager of Systems Analysis at Solar Turbines, suggested a probabilistic methodology as a framework for better compressor plant designs using Monte Carlo simulations and risk analysis.The goal is to more accurately define process uncertainty and its impact on machine performance requirements.
“The use of statistical and probabilistic tools allows us to better take into account the uncertainty in design conditions, the performance of components, as well as ambient conditions and demand,” said Kurz. “This approach allows the design of plants based on their most likely scenarios, as opposed to traditional designs that tend to work best under what are considered to be unlikely worst case scenarios.”
This methodology can be used on entire pipeline systems, complete plants or offshore platform operations. The turbomachinery can then be integrated into an overall plant system, which Kurz said was how we needed to be looking at things today. This parallels Volpe’s comment above.
“Once integrated, equipment hardly ever operates at its design points,” said Kurz. “The true operating point in compressor stations, for example, is typically far away from its design point. This adds fuel consumption and leads to higher emissions, which in turn makes them more expensive to purchase and operate than necessary.”
How does this occur? Designers are schooled to increase their degree of accuracy by adding safety factors and tolerances at every step of the design process. This leads to major errors in sizing due to design uncertainties such as ambient conditions, equipment performance, degradation over time, process requirements, and the performance of other components such as piping, compressor stations upstream and downstream, coolers and separators.
This can bring about power margins as large as 34.5% using traditional designs. Kurz believes that 5-to-10% is sufficient and that this can be verified using the Monte Carlo method. “But it requires close collaboration between the OEM, the EPC contractor and the end user,” he said. “While far more interaction is necessary, beginning at an early stage, can have a positive impact on Capex, Opex and safety.”