1,720,961 research outputs found
Geometry Influence on Two-Phase Flow in Safety Valves
No full textA careful design of pressure safety valves (PSV) is an essential requirement for safeguarding of
industrial plants; reliable correlations are available for PSV design in the cases of liquid or gas discharge.
Instead, if two-phase flow is possible, especially for low vapour quality (less than 10%), PSV design
becomes very difficult owing to complex thermal hydraulic phenomena that happen between the two
phases. Currently there are some calculation methods, based on different simplifying hypotheses, that try to
predict the two-phase flow rate through a PSV knowing the inlet fluid conditions (pressure, quality or
temperature) and the outlet pressure; however, none of them is acknowledged as being reliable for every
situation and, therefore, there are not standards for PSV design under two-phase conditions.
The PSV size is one of the most important parameters used for choosing between the two main prediction
models, Homogeneous Equilibrium Model (HEM) and Homogeneous Non Equilibrium model (HNE).
This paper shows the results of an experimental research carried out with steam-water two-phase flow
through two PSVs having the same orifice diameter but with different discharge coefficients and inlet
geometries. The experimental data are compared with the results of a calculation method based on the
homogeneous model with non equilibrium hypotheses and an other one presented in API Recommended
Practice 520, developed with equilibrium hypothese
Two phase pressure drop influence on pressure safety valve performance
No full textA reliable design of pressure safety valves (PSV) is essential for the safety of industrial equipment. However when two-phase flow is possible, especially for low vapour quality (less than 10%), the design becomes very difficult owing to complex thermal hydraulic phenomena that occur between the two phases. Furthermore, when a circuit for the recovery of the discharged flow is also present, the built-up back pressure can be rather high thus decreasing the predicted flow rate. As a consequence, for a correct PSV design, accurate methods for the calculation of the pressure drop in two-phase flow are of the utmost importance.
At present, various methods, based on different hypotheses, are available. In particular, some methods provide more equations, specifically taking into account the local fluid flow pattern; others are based on general hypotheses which are assumed valid for every flow pattern.
This paper shows the results of an experimental research carried out on a two-phase steam-water system.
The experimental data are compared with the results of three different pressure drop calculation methods, and the performance of the models is evaluated as a function of the main process parameter
Water two-phase flow through pressure safety valve with variable backpressure: check of calculation methods to estimate mass flow-rate and critical flow conditions
No full textA careful design of pressure safety valves (PSV) is an essential requirement for safeguarding of industrial plants; reliable correlations are available for PSV design in the cases of liquid or gas discharge. Instead, if two-phase flow is possible, especially for low vapour quality (less than 10%), PSV design becomes very difficult owing to complex thermal hydraulic phenomena that happen between the two phases. Moreover, in this situation, the prediction of the critical flow onset (which occurs when the fluid velocity becomes equal to the sound velocity in the fluid) is very important for a correct design because it involves a choked flow situation. Currently there are some calculation methods, based on different simplifying hypotheses, that try to predict the two-phase flow-rate through a PSV knowing the inlet fluid conditions (pressure, quality or temperature) and the outlet pressure; however, none of them is acknowledged as being reliable for every situation and, therefore, there are not standards for PSV design under two-phase conditions. This paper shows the results of an experimental research carried out through a PSV with steam-water two-phase flow. The experimental data are compared with the results of a calculation method based on the homogeneous model with non equilibrium hypotheses and an other one presented in API Racomanded Practice 520, devoloped with equilibrium hypotheses. The performance of the models is evaluated as a function of inlet and outlet parameters; an analysis of model critical flow prediction capability is presented too, with some uncertainties caused by the test procedures (here the mass flow-rate is imposed
Comparison among Three Prediction Methods for Safety Valves Design in Two-Phase Flow in the case of a Small Valve
No full textIn the case of two- phase flow discharge, pressure safety valves (PSV) design becomes difficult due to complex thermal-fluid dynamic phenomena that occur between the two phases. Currently, there are two main types of prediction models, the Homogeneous Equilibrium Model (HEM) and the Homogeneous Non Equilibrium model (HNE), used for developing methods to calculate the theoretical mass flux G(t); this value has to be corrected by an experimental coefficient k(d), termed as two-phase "discharge coefficient", for obtaining the actual two phase mass flow-rate value W(r). Generally, for each method a different way of calculating k(d) is proposed, but various studies are looking for a general correlation that considers all the two-phase flow aspects. This paper will compare some experimental results obtained with a steam-water system and a small PSV (office diameter phi(or)=6 mm) with the predictions of three methods, an HEM, an HNE and a more recent method, called HNE-DS, proposed by the ISO working group on PSV sizing
Perdite di carico concentrate e ripartite in efflusso bifase: confronto tra le previsioni di correlazioni teoriche e dati sperimentali
No full textLe perdite di carico nel caso di efflusso monofase vengono valutate con correlazioni di calcolo ritenute unanimemente affidabili. Nel caso bifase, invece, la complessa fenomenologia dell’efflusso e la sua conseguente difficile modellazione ha indotto alla formulazione di diverse correlazioni, più o meno affidabili a seconda delle caratteristiche termodinamiche del fluido e geometriche della tubazione. Il maggior numero di correlazioni è stato proposto per il calcolo delle perdite di carico ripartite, mentre per la valutazione delle perdite di carico concentrate sono disponibili solo alcune correlazioni suffragate da pochi dati sperimentali.
In questo articolo sono presentati i risultati di una ricerca sperimentale, condotta in condizioni bifase acqua e vapore, per lo studio delle perdite di carico concentrate, su una curva a 90° (i=10 mm), e ripartite, su un tratto di tubazione di i=10mm.
I dati sperimentali sono confrontati con le previsioni di diversi metodi di letteratura in funzione dei principali parametri caratteristici dell’efflusso per ottenere informazioni utili ad un loro utilizzo ottimale. Infine si è cercato di sviluppare nuove correlazioni semiempiriche per le perdite di carico concentrate, partendo da quelle già disponibili e più affidabili per le perdite di carico ripartit
Studio teorico-sperimentale sulle perdite di carico concentrate e ripartite in efflusso bifase
No full textUn corretto dimensionamento di una valvola di sicurezza non può prescindere dalla conoscenza della pressione di scarico. Nel caso in cui sia presente una uscita convogliata, diventa necessario valutare le relative perdite di carico che vanno a sommarsi alla pressione finale di scarico nel determinare la contropressione. In condizioni di efflusso bifase il calcolo delle perdite di carico è molto complicato, le correlazioni disponibili più complesse e meno affidabili rispetto alle condizioni monofase e la propagazione dell’errore ha maggiori effetti sul calcolo della portata effluente. Inoltre, la portata scaricata in condizioni bifase è molto più sensibile alla variazione della contropressione che può anche incidere sul raggiungimento delle condizioni di criticità.
Come noto, le perdite di carico possono essere classificate in ripartite e concentrate: nelle condizioni di efflusso bifase, mentre per il primo tipo sono disponibili diverse correlazioni, in genere di discreta affidabilità, che fanno riferimento o meno al flow pattern, nel caso delle perdite concentrate la situazione è più complessa e le correlazioni disponibili meno sicure. In questo articolo saranno presentati i risultati ottenuti da una ricerca sperimentale, condotta in condizioni bifase acqua e vapore, per lo studio delle perdite di carico concentrate su due curve a 90° (i=10 mm) e su un tratto di tubazione di i=10mm. I dati sperimentali sono stati confrontati con le previsioni di diversi metodi proposti in letteratura, anche in funzione delle condizioni di prova; infine si è cercato di sviluppare un’altra correlazione partendo da una già disponibile impiegata per il dimensionamento delle valvole di sicurezz
Dimensionamento delle valvole di sicurezza in bifase: confronto tra tre diversi metodi
No full textIl corretto dimensionamento delle valvole di sicurezza è un’esigenza essenziale per la protezione negli impianti industriali: esso però risulta molto difficoltoso in presenza di un fluido in bifase o prossimo alla saturazione, a causa dei complessi fenomeni termofluidodinamici che avvengono tra le due fasi, poiché l’effetto combinato delle condizioni iniziali del fluido e della tipologia della valvola condiziona la fenomenologia dell’efflusso e l’eventuale raggiungimento dell’equilibrio termodinamico.
I metodi di calcolo attualmente disponibili, tutti espressi in funzione delle condizioni del fluido in ingresso (pressione, temperatura e/o titolo) e della pressione di uscita, sono basati su diverse ipotesi di partenza, che ne limitano l’utilizzo nel caso in cui queste non siano verificate: di conseguenza modelli validi in certe situazioni possono fornire risultati non corretti in altre. In questo articolo verranno valutate le previsioni di tre modelli omogenei sulla portata effluente attraverso una valvola commerciale con orifizio di diametro=10mm: il primo è un modello di equilibrio tratto dai lavori di Leung (già allo studio dell’ISO come riferimento normativo), il secondo, sviluppato da Henry-Fauske, assume come riferimento iniziale l’ipotesi di non equilibrio ed infine il terzo, denominato dagli autori HNE-DS, partendo da un’ipotetica situazione di equilibrio effettua delle correzioni per tener conto del ritardo nella nucleazione delle boll
Size and geometry influence in pressure safety valve design in two phase flow. Comparison among three prediction methods
No full textIn the case of two- phase flow, especially for low vapour quality (less than 10%), pressure safety valves (PSV) design becomes very difficult owing to complex thermal hydraulic phenomena that occur between the two phases. Currently, there are some calculation methods, based on different simplifying hypotheses, that try to predict the two-phase flow rate through a PSV knowing the inlet fluid conditions (pressure, quality or temperature) and the outlet pressure; however, none of them is acknowledged as being reliable for every situation and, therefore, there are not standards for PSV design under two-phase flow conditions. The PSV size is one of the most important parameters used for choosing between the two main prediction models, Homogeneous Equilibrium Model (HEM) and Homogeneous Non Equilibrium model (HNE).
This paper shows the results of an experimental research carried out with steam-water two-phase flow through two PSVs having the same orifice diameter (10 mm) but different discharge coefficients and inlet geometries. The experimental data are compared with the results of three calculation methods: the first one is based on equilibrium hypotheses and is the reference of API Recommended Practice 520; the second one is a calculation method based on the HNE hypotheses; the last one is a recent method, called HNE-DS, introducing some correction parameters in the HEM hypotheses to consider the boiling delay and is under the consideration of a working group ISO on sizing of PSV. The results show that the PSV geometry and the discharge conditions are important factors in choosing the more suitable model for the sizing of small PSV
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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