Retrofit Of Gas Lubricated Face Seals In A Centrifugal Compressor.

LecturePg. 75-84There are significant advantages in using gas lubricated face seals (dry gas seals) in centrifugal compressor service. Foremost among these are the elimination of the seal oil system resulting in lower maintenance, increased safety, and higher operating availability. For these reasons, one of four identical compressors at an installation having severe problems with seal oil contamination was selected for trial conversion to dry gas seals. The rotordynamic engineering portion of this job was done by the compressor manufacturer in conjunction with the gas seal supplier. The seal assembly was designed so that it would be essentially a drop-in conversion from a mechanical standpoint. Rotordynamic studies indicated that the conversion would result in a "better" machine. Unfortunately, sustained operations were not possible, due to excessive vibration levels at startup with the new seals. The shaft vibration exceeded 0.007 in, peak-to-peak, at a subsynchronous frequency of 4900 rpm (the machine rated speed is in excess of 10,000 rpm). These levels were sufficiently high to cause extensive damage to all internal labyrinths. The midspan labyrinths were wiped open in excess of 0.060 in, radial. Analysis of tape recorded data indicated that the vibration was due to a rotor/bearing system dynamic instability. Additional computer simulations of the compressor rotordynamics revealed that the oil seals had provided sufficient damping to the system to bound the instability. This extra damping was not being provided by the gas seals. Bearing redesign to increase stability and realignment of the rotor within the bundle to remove suspected excitation appear to have eliminated the problem

Impact Of Electrical Noise On The Torsional Response Of VFD Compressor Trains

LectureThe popularity of VFD motors in compressor trains has increased in recent years. Increased process flexibility is a primary consideration for implementation of VFD systems. However, this comes at a cost of increased mechanical and electrical complexity. Problems rarely experienced with single speed motors have become more likely with VFD's. These include impacts on the torsional behavior leading to component failure and extended downtimes. VFD's can excite the torsional system in several ways. This paper examines the impact of white noise on the torsional response of VFD compressor trains and contrasts it against the response to single frequency harmonic excitation. The characteristics of white noise, noise modeling characteristics for torsional response, and noise generation techniques are examined. Generation of a frequency banded noise signal is presented using a commonly used math modeling code. Two case studies are presented representing VFD compressor trains found in the LNG industry. Both experienced torsional problems related to the VFD and, in one case, led to a coupling failure and release of parts from the train. In the examination of these trains, a single frequency harmonic (SFH) sweep and banded Gaussian white noise (GWN) encompassing the dynamic response envelop of the 1st TNF, are used as inputs to the torsional analysis

Subsynchronous Vibration Problem And Solution In Multistage Centrifugal Compressor.

LecturePg. 65-74The investigation of a subsynchronous vibration problem encountered in a six stage centrifugal compressor is discussed. At a running speed of approximately 9000 rpm, a subsynchronous vibration (at 4200 rpm) of nearly two times the synchronous vibration level was encountered. A systematic program was undertaken to identify the problem and correct it. A detailed analysis of the floating ring annular oil seals, balance piston labyrinth seals and impeller aerodynamic cross coupling was conducted. The oil seals were identified as the primary cause of the subsynchronous vibration due to lock up, and a modified seal design incorporating circumferential grooves was developed. This radically reduced the seal cross coupled stiffness. Further, a modified bearing design was investigated to increase the rotor logarithmic decrement. Changes were implemented in the compressor with the result of no subsynchronous vibrations for the operating conditions of the compressor thus far

Paying for parking : improving stated-preference surveys

This article describes an experiment which introduced random ranges into the variables used for the design of a stated preference survey and its effects on willingness to pay for parking. User behaviour at the time of parking was modelled to determine their willingness to pay in order to get to their final destination more quickly. Calculating willingness to pay is fundamental during the social and economic assessment of projects. It is important to correctly model how car parks and their users interact in order to get values which represent reality as closely as possible. Willingness to pay is calculated using a stated preference survey and by calibrating multinomial logit models, taking variable tastes into account. It is shown that a value with a low variability can be obtained for willingness to pay by correctly establishing the context of the choice and randomly changing the variables around an average value

Ovine pedomics : the first study of the ovine foot 16S rRNA-based microbiome

We report the first study of the bacterial microbiome of ovine interdigital skin based on 16S rRNA by pyrosequencing and conventional cloning with Sanger-sequencing. Three flocks were selected, one a flock with no signs of footrot or interdigital dermatitis, a second flock with interdigital dermatitis alone and a third flock with both interdigital dermatitis and footrot. The sheep were classified as having either healthy interdigital skin (H), interdigital dermatitis (ID) or virulent footrot (VFR). The ovine interdigital skin bacterial community varied significantly by flock and clinical condition. The diversity and richness of operational taxonomic units was greater in tissue from sheep with ID than H or VFR affected sheep. Actinobacteria, Bacteriodetes, Firmicutes and Proteobacteria were the most abundant phyla comprising 25 genera. Peptostreptococcus, Corynebacterium and Staphylococcus were associated with H, ID and VFR respectively. Sequences of Dichelobacter nodosus, the causal agent of ovine footrot, were not amplified due to mismatches in the 16S rRNA universal forward primer (27F). A specific real time PCR assay was used to demonstrate the presence of D. nodosus which was detected in all samples including the flock with no signs of ID or VFR. Sheep with ID had significantly higher numbers of D. nodosus (104-109 cells/g tissue) than those with H or VFR feet

Direct ionization and electron capture in M

This article discusses direct ionization and electron capture in M-shell x-ray production by fluorine ions

A Fluorescence Reporter Model Defines “Tip-DCs” as the Cellular Source of Interferon β in Murine Listeriosis

Production of type I interferons, consisting mainly of multiple IFNα subtypes and IFNβ, represents an essential part of the innate immune defense against invading pathogens. While in most situations, namely viral infections, this class of cytokines is indispensable for host survival they mediate a detrimental effect during infection with L. monocytogenes by rendering macrophages insensitive towards IFNγ signalling which leads to a lethal bacterial pathology in mice. Due to a lack of suitable analytic tools the precise identity of the cell population responsible for type I IFN production remains ill-defined and so far these cells have been described to be macrophages. As in general IFNβ is the first type I interferon to be produced, we took advantage of an IFNβ fluorescence reporter-knockin mouse model in which YFP is expressed from a bicistronic mRNA linked by an IRES to the endogenous ifnb mRNA to assess the IFNβ production on a single cell level in situ. Our results showed highest frequencies and absolute numbers of IFNβ+ cells in the spleen 24 h after infection with L. monocytogenes where they were located predominately in the white pulp within the foci of infection. Detailed FACS surface marker analyses, intracellular cytokine stainings and T cell proliferation assays revealed that the IFNβ+ cells were a phenotypically and functionally further specialized subpopulation of TNF and iNOS producing DCs (Tip-DCs) which are known to be essential for the early containment of L. monocytogenes infection. We proved that the IFNβ+ cells exhibited the hallmark characteristics of Tip-DCs as they produced iNOS and TNF and possessed T cell priming abilities. These results point to a yet unappreciated ambiguous role for a multi-effector, IFNβ producing subpopulation of Tip-DCs in controlling the balance between containment of L. monocytogenes infection and effects detrimental to the host driven by IFNβ

Multiple outer-shell ionization effect in inner-shell x-ray production by light ions

This article discusses multiple outer-shell ionization effect in inner-shell x-ray production by light ions

Is there a common water-activity limit for the three domains of life?

Archaea and Bacteria constitute a majority of life systems on Earth but have long been considered inferior to Eukarya in terms of solute tolerance. Whereas the most halophilic prokaryotes are known for an ability to multiply at saturated NaCl (water activity (a w) 0.755) some xerophilic fungi can germinate, usually at high-sugar concentrations, at values as low as 0.650-0.605 a w. Here, we present evidence that halophilic prokayotes can grow down to water activities of <0.755 for Halanaerobium lacusrosei (0.748), Halobacterium strain 004.1 (0.728), Halobacterium sp. NRC-1 and Halococcus morrhuae (0.717), Haloquadratum walsbyi (0.709), Halococcus salifodinae (0.693), Halobacterium noricense (0.687), Natrinema pallidum (0.681) and haloarchaeal strains GN-2 and GN-5 (0.635 a w). Furthermore, extrapolation of growth curves (prone to giving conservative estimates) indicated theoretical minima down to 0.611 a w for extreme, obligately halophilic Archaea and Bacteria. These were compared with minima for the most solute-tolerant Bacteria in high-sugar (or other non-saline) media (Mycobacterium spp., Tetragenococcus halophilus, Saccharibacter floricola, Staphylococcus aureus and so on) and eukaryotic microbes in saline (Wallemia spp., Basipetospora halophila, Dunaliella spp. and so on) and high-sugar substrates (for example, Xeromyces bisporus, Zygosaccharomyces rouxii, Aspergillus and Eurotium spp.). We also manipulated the balance of chaotropic and kosmotropic stressors for the extreme, xerophilic fungi Aspergillus penicilloides and X. bisporus and, via this approach, their established water-activity limits for mycelial growth (∼0.65) were reduced to 0.640. Furthermore, extrapolations indicated theoretical limits of 0.632 and 0.636 a w for A. penicilloides and X. bisporus, respectively. Collectively, these findings suggest that there is a common water-activity limit that is determined by physicochemical constraints for the three domains of life