Centrifugal Compressor Rotordynamics in Wet Gas Conditions

LectureA new technology challenge in centrifugal compressor design and operation is the condensate phase management. End users (especially in offshore and subsea operations) are more and more interested to have a Wet Gas Compression system which is able to tolerate liquid in the process gas. Authors’ Company has initiated for several years a research program aiming to investigate the impact of the liquid phase on centrifugal compressor operability (mainly thermodynamics, rotordynamics, erosion, axial thrust). As an introduction, the Authors’ Company past experiences and more recent experimental tests [Ransom D. et al. 2011], [Bertoneri M. et al. 2012], are reviewed in order to show how the rotordynamic behaviour of a centrifugal compressor may be affected by the wet gas. However in the core, this paper is focused on the novel rotordynamic experimental outcomes of a wet gas single stage compressor test campaign. The machine was equipped with the following special instrumentation: Pressure and temperature probes along the flow path and internal seals; Magnetic lamination installed on the shaft end to allow for stability test through a magnetic exciter; Load cells installed in the thrust bearing; Torquemeter installed at the compressor coupling. The explored test conditions were: Wet gas = Air and Water mixture up to 3% of Liquid Volume Fraction (LVF); Suction pressure levels = 10, 15, 20 bar-a; Maximum Continuous Speed = 13500 rpm. The compressor went through an extensive test campaign where the following aspects were thoroughly investigated: Rotordynamic behaviour during steady state wet operation; Rotordynamic stability (through magnetic exciter); Transient phenomena: response to liquid load variations (LVF up to 8%), start-up/shutdown from wet conditions, start-up with stratified flow into suction pipe. The compressor dynamic behaviour was monitored both from lateral viewpoint (using no contact probes located close to bearing locations) and axial/torsional viewpoint (through the special instrumentation described above). Overall the compressor was able to withstand a huge amount of liquid phase, with an increased vibration level with respect to dry conditions but still in the safe area, both in steady and transient tests. Finally, major differences were found only at high flow – high liquid/gas density ratio conditions where an unexpected subsynchronous vibration (SSV) was showing up. The nature of this SSV was deeply investigated and finally it was fixed through a balance piston seal geometry change

Full Load Testing Of A 12.5 MW Vertical High Speed Subsea Motorcompressor

Lecturepg. 79-92In late 2006 Authors’ company was awarded a contract for the motorcompressor for the Ormen Lange Subsea Compression Pilot. This Pilot represents the first subsea motorcompressor unit ever built in a fully marinized version and tested in a water pit, which was developed specifically for the project at the customer’s site (Nhyamna, Norway). The prototype unit is a 12.5MW (16763HP) integrated motorcompressor, which runs up to 10.5krpm in a vertical configuration with the following features: Single casing; High-speed motor rigidly coupled to a multistage centrifugal compressor (3 journal bearings shaft line); Canned Active Magnetic Bearings (AMBs); Process gas used as cooling fluid for the Electric Motor; Internal separation system (to protect the bearings and the electric motor from the intrusion of solid and liquid materials); Fully marinized AMBs control system; The design and validation plan for this complex equipment were developed through a four years program where many challenging milestones were achieved. The Ormen Lange Subsea Compression Pilot motorcompressor has successfully completed an intensive full load testing campaign in the Authors’ Company plant including the following: Static-dynamic tuning of the AMBs; Mechanical Running Test; Compressor Performance Test (ASME PTC-10 Type 2 + Full Load test); Electric Motor Performance Test; Overall cooling system check; Landing test (partial landing and full speed delevitation test). The test campaign has fully demonstrated the functionality of this prototype unit capable of operation in the service conditions. The Next step will be the shipment to the costumer site where the submerged testing will be conducted. This paper will provide a detailed description of the main test results of the subsea motorcompressor covering all aspects of the test program including rotordynamic behaviour, compressor thermodynamic performance, axial load variation versus operating conditions (monitored thanks to the thrust AMB), electric motor cooling system behaviour, electric motor performance, AMBs and control system operability. Of special interest is the rotor delevitation test from full speed consisting in the two ton rotor running at 10.5krpm dropping onto the mechanical emergency bearings until the machine reached a complete stop condition. This test, which was repeated 5 times, provided a full-scale verification of the landing numerical simulation developed by the authors (Ransom et al., 2009)

Tilting-Pad Bearings: Measured Frequency Characteristics Of Their Rotordynamic Coefficients

Lecturepg. 33-45This paper reviews a long standing issue related to the stiffness and damping coefficients of tilting-pad (TP) bearings; namely, What is the nature of their frequency dependency? A research project was implemented at the Turbomachinery Laboratory (TL) at Texas A&M University (TAMU) around 2003 to examine the issue, applying procedures that had been developed and used to investigate the rotordynamic characteristics of annular gas seals. Those seals, using a smooth rotor and a honeycomb or hole-pattern stator were predicted to have strongly frequency-dependent reaction forces that could not be modeled by a combination of stiffness, damping, and inertia coefficients. Measurements confirmed the strongly frequency dependent nature of their stiffness and damping coefficients. Subsequent test have examined the following bearing types: (i) Two-axial-groove bearing, (ii) pressure dam bearings, (iii) Flexure-pivot-pad tilting-pad bearing (FPTP) in load-on-pad (LOP) and load-between-pad (LBP), (iv) Rocker-pivot-pad TP bearing in LOP and LBP configurations at two different preloads and 50 and 60% offsets, and (v) a spherical seat bearing in LOP and LBP configurations. Representative test results are presented for some of these bearings. In addition, this paper includes experimental results for 5-pad and 4-pad tilting pad bearings (with similar features to TAMU configuration iv) tested at the GE Global Research Facility (GRC) as part of an independent research initiative from GE Oil and Gas. Frequency effects on the dynamic-stiffness coefficients were investigated by applying dynamic-force excitation over a range of excitation frequencies. Generally, for all bearings tested at TAMU and GRC, the direct real parts of the dynamic-stiffness coefficients could be modeled as quadratic functions of the excitation frequency and accounted for by adding a mass matrix to the conventional [C][K] model to produce a frequency-independent [M][C][K] model. Additionally, the direct damping could be modeled by a constant, frequency-independent coefficient. Consequently, these experimental findings from two independent sources support the use of synchronously reduced force coefficients for characterizing the dynamic performance of tilting pad bearings in Oil and Gas applications, as prescribed by API 617 7th edition (Process Centrifugal Compressors) and more generally by API684 Rotordynamic Tutorial

Development Of A High Pressure Rotordynamic Test Rig For Centrifugal Compressors Internal Seals Characterization

Lecturepg. 46-59The current centrifugal compressor design for the Oil & Gas market is more and more challenging since the cost requirements and the presence of many competitors is pushing towards casing size reduction and rotational speed increase. The first requirement basically leads to increase the number of wheels per rotor and the second to cross more critical speeds requiring the proper degree of damping. The two consequences together lead also to increase the rotor flexibility ratio (defined as the ratio between the Maximum Continuous Speed and the first critical speed as per the Fulton diagram and API617 7th ed. [1-2]) and finally the rotordynamic stability is very much challenged. The centrifugal compressors rotordynamic stability is then strictly related to the internal seals’ dynamic behaviour and for this reason the authors’ Company decided several years ago to develop internally a High Pressure Seal Test Rig to measure seals’ stiffness and damping. The rig is now in operation. This paper aims to describe the main test rig capabilities, the applied identification procedures and the preliminary test results on a long labyrinth seal (smooth rotor - straight toothed stator). Due to the pressure level (500bar design pressure), the test rig plant appears like a high-pressure industrial plant equipped with the testing cell (a 1:1 scale high pressure compressor) and all the relevant auxiliaries: a 400 kW electric motor (driven by a VFD), a speed increaser gear box, a high pressure reservoir (6 m 3 ) with a volumetric compressor to fill it, high pressure pipes and valves. The testing cell is composed of a high-pressure compressor casing with stator parts capable to regulate the seal inlet swirl and a rotor running on Active Magnetic Bearings (AMBs), which serve as exciters (5kN MAX Force over a 0-330 Hz frequency range per axis) and displacement transducers. Special instrumentation is installed into the testing cell in order to measure the main test parameters: seal upstream/downstream gas pressure, upstream temperature and swirl and mass flow. Industrial high-pressure instrumentation is installed on the plant for regulation and monitoring purpose. Maximum test pressure is 350bar and maximum rotational speed is 15000rpm. Test gas is nitrogen. The AMBs control-system capabilities have been tuned to define several alternative excitation patterns and the relevant state of the art identification techniques have been applied. The first seal tested is a long labyrinth seal to simulate a centrifugal compressor balance piston seal. Test results and comparison with a commercial bulk flow code predictions will be fully described. Finally, the future test program will be showed

Design Validation of High Speed Ratio Epicyclic Gear Technology in Compression Systems

LectureLecture 5: Epicyclic gear technology is a key factor to support thcompression growth strategy in electrified applications, due to the ever increasing transmission ratio required to meet the high compressor speeds. The paper collects the experience of the authors in developing a unique product through its conceptual design, as well as its mechanical and rotordynamic assessment, up to its complete validation with a full speed full load test in a complete unit arrangement. The main target of high pressure ratio compressors is to fulfil the required compression service with one casing less than traditional technology. The new compressor technology results in machines which run much higher in speed. To achieve a compressor ratio of ~30 in a single casing, the rotational speed is increase by roughly 40% respect to a traditional compression train, and therefore it becomes the most critical parameter. The specific requirements of higher speeds at high powers dictate the need for the very high speed ratio gearing, which was critical to the success of the new compressor technology. The power transmission was jointly developed by the gear and compressor manufacturers. The methods used to evaluate the gearbox configuration options are detailed in this paper, along with the gearbox specific technology design challenges faced. A detailed overview of the selected epicyclic transmission technology is provided and how the use of the fundamental configuration was able to provide advantages over more traditional arrangements when combined with some application specific design features. Specific address will be provided to the tooth design principles applied, bearing technology and rotordynamic modelling and performance. The paper will close with details of transmission performance testing from the gearbox manufactures test bed to the full load string test in a complete compressor unit arrangement, at different operating conditions

Biogeography and Character Evolution of the Ciliate Genus Euplotes (Spirotrichea, Euplotia), with Description of Euplotes curdsi sp. nov.

Ciliates comprise a diverse and ecologically important phylum of unicellular protists. One of the most specious and best-defined genera is Euplotes, which constitutes more than 70 morphospecies, many of which have never been molecularly tested. The increasing number of described Euplotes taxa emphasizes the importance for detailed characterizations of new ones, requiring standardized morphological observations, sequencing of molecular markers and careful comparison with previous literature. Here we describe Euplotes curdsi sp. nov., distinguishable by the combination of the following features: 45±65 μm length, oval or elongated shape with both ends rounded, narrow peristome with 25±34 adoral membranelles, conspicuous paroral membrane, double-eurystomus dorsal argyrome type, 6±7 dorsolateral kineties and 10 frontoventral cirri. Three populations of the novel species have been found in brackish and marine samples in the Mediterranean and the White Sea. We provide the SSU rRNA gene sequences of these populations, and an updated phylogeny of the genus Euplotes. Using the molecular phylogenetic tree, we inferred aspects of the biogeographical history of the genus and the evolution of its most important taxonomic characters in order to provide a frame for future descriptions. Ultimately, these data reveal recurrent trends of freshwater invasion and highlight the dynamic, yet convergent, morphological evolution of Euplotes

The ‘placebo effect’ in the conservative treatment of plantar fasciitis: a systematic review and meta-analyses

Purpose: The study of the placebo effect is key to elucidate the ‘real effect’ of conservative interventions for plantar fasciitis. The aim of this meta-analysis was to quantify the impact of placebo in the different conservative treatments of plantar fasciitis. Methods: A systematic literature review was performed on double-blind placebo-controlled trials (RCTs) according to PRISMA guidelines on PubMed, Embase, and Web of Science. The meta-analysis primary outcome was the 0–10 pain variation after placebo treatments analyzed at 1 week, 1, 3, 6, and 12 months. The risk of bias was assessed using the RoB 2.0 tool, while the overall quality of evidence was graded according to the GRADE guidelines. Results: The placebo effect for conservative treatments was studied in 42 double-blind RCTs on 1724 patients. The meta-analysis of VAS pain showed a statistically significant improvement after placebo administration of 2.13/10 points (P < 0.001), being highest at 12 months with 2.79/10 points (P < 0.001). The improvement of the placebo groups was higher in the extracorporeal shock wave therapy studies compared to the injection studies (2.59 vs 1.78; P = 0.05). Eight studies had a low risk of bias, 23 studies had ‘some concerns,’ and 4 studies had a high risk of bias. The GRADE evaluation showed an overall high quality of evidence. Conclusion: This systematic review and meta-analysis demonstrated that the placebo effect represents an important component of all conservative approaches to treat plantar fasciitis. This effect is statistically and clinically significant, increases over time, and depends on the type of conservative treatment applied to address plantar fasciitis

STALL INDUCED AERODYNAMIC FORCING AND ROTOR VIBRATIONS IN A MULTISTAGE CENTRIFUGAL COMPRESSOR

LectureThe Oil & Gas industry is looking with increased interest at solutions for improving operating flexibility of centrifugal compressors. The stable operation of a compressor stage or machine is generally limited at the left of operating range by the occurrence of a local aerodynamic unsteady phenomenon, the rotating stall, which usually precedes the surge. Rotating stall could cause, depending on the actual operating conditions, severe sub-synchronous vibrations to the rotor which may compromise rotordynamic behavior, preventing the machine from operating at very low flow rates. An accurate characterization of rotating stall phenomena, and their impact on rotordynamic stability, may represent an important step forward in centrifugal compressor design and performance predictability, insofar as it allows to correctly predict the real operating range of the machine. In recently published works the authors presented a procedure which allows reconstructing the pressure unbalance due to the diffuser rotating stall, to estimate the rotating force acting on the shaft and, by means of a rotordynamic model, the vibration at the bearings. In addition to this, a criterion to scale the rotating force coming from model test conditions up to fullscale machine conditions has been developed and successfully validated. In this framework a thorough work has been performed to apply the aforementioned procedure to an LNG multistage compressor. Firstly, the stages which equip the machine were tested as single scaled-down stages in a model test rig, in order to fully characterize their dynamic behavior while approaching the left limit and operating in stall condition. Then, the full scale machine has been equipped with dynamic pressure probes in different locations along the gas flow path, and has been tested according to ASME PTC-10 standard; this allowed to capture the stall inception and its evolution and finally to get the rotating pressure pattern acting on the rotor. A noticeable agreement was obtained between the force resulting from the pressure field integration and the one obtained through a proper scaling of the test data. Finally, the calculated stall force has been used as an input in a rotordynamic model of the whole compressor: the predicted Subsynchronous Vibration (SSV) estimated at the displacement probe location has been compared with the measured value showing some differences which can be related to the proximity of the first rotor mode

Nutritionally enriched tomatoes (Solanum lycopersicum L.) grown with wood distillate: chemical and biological characterization for quality assessment

: Bio-based products are nowadays useful tools able to affect the productivity and quality of conventionally cultivated crops. Several bio-based products are currently on the market; one of the newest and most promising is the wood distillate (WD) derived from the pyrolysis process of waste biomass after timber. Its foliar application has been widely investigated and shown to promote the antioxidant profile of cultivated crops. WD was used here as additive for the cultivation of tomato (Solanum lycopersicum L.) plants. The application improved quality (chemical) parameters, minerals, polyphenols, and lycopene contents of tomato fruits. The extracts of WD-treated and untreated tomatoes have been chemically and biologically characterized. The 1 H-NMR and ESI-mass spectrometry analyses of the extracts revealed the presence of different fatty acids, amino acids and sugars. In particular, the WD-treated tomatoes showed the presence of pyroglutamic acid and phloridzin derivatives, but also dihydrokaempferol, naringenin glucoside, cinnamic acid, and kaempferol-3-O-glucoside. When tested in cells, the extracts showed a promising anti-inflammatory profile in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. Furthermore, the extracts displayed a slight vasorelaxant activity on rat aorta rings (either endothelium-denuded or endothelium-intact) pre-contracted with phenylephrine or potassium chloride. PRACTICAL APPLICATION: Wood distillate has been used for tomato plant growth. Tomatoes showed improved nutritional parameters, and their extracts displayed antioxidant and anti-inflammatory activities