Pubblicazioni

Articoli pubblicati con la partecipazione degli ingegneri CST

Copertina Manutenzione - Feb 2018
Titolo:  Miglioramento dello stato vibrazionale in un impianto LDPE
Rivista: Manutenzione & Progettazione
Autore: Cosimo CarcasciMarco Sacco

Una delle principali cause di rotture e di conseguenti fermate inaspettate negli impianti industriali sono le vibrazioni dei vari macchinari e tubazioni.
I compressori alternativi sono, per loro natura, particolarmente inclini a questo tipo di problematiche, pertanto una particolare attenzione deve essere posta in fase di design della macchina e del layout delle tubazioni per
prevenire il presentarsi di questi inconvenienti.
Il presente articolo mostrerà una tipica situazione in cui la criticità di questi fenomeni si è presentata quando la macchina aveva già molti anni di esercizio alle spalle, ma grazie alla stretta collaborazione tra l'end-user, CST, quale società di ingegneria, e le imprese coinvolte nella realizzazione delle modifiche proposte, le suddette problematiche sono state controllate e ridimensionate fino a raggiungere livelli ritenuti accettabili anche nel lungo termine.


compressor tech-oct2017
Titolo: CST grows from small service provider to full compression system engineering company in 15-year span.
Rivista: Compressor Tech 2
Autore: Cosimo Carcasci

From service engineering provider to full scope design, engineering and execution of complete compressor systems. The article retraces the history of C.S.T., from its origin, when the company was initially composed of just four people and worked as a service engineering provider mainly to Nuovo Pignone (now GE Oil & Gas) for reciprocating compressors, to today, when, in the span of 15 years, C.S.T. has transformed into an international player for critical machine applications in the Oil & Gas and other industrial sectors. The Company staff is nowadays of about 50 people and while at the beginning the sales of the company were almost 90% in Italy, in the near future C.S.T. expects to reach 50% of sales from foreign markets, offering a quick mobilization capability for any project all around the world.


Manutenzione - Copertina 03 2017
Titolo: Prevenzione delle problematiche negli impianti LDPE
Rivista: Manutenzione - Tecnica & Management
Autore: Cosimo Carcasci, Marco Sacco

Le vibrazioni sono una delle principali e più diffuse cause di danno e di guasto sulle macchine rotanti. I compressori alternativi in particolare sono spesso affetti da problemi di vibrazione, dal momento che queste sono un inevitabile effetto del moto alternato e delle pulsazioni di pressione che tali macchine generano.
Durante l’ingegneria di dettaglio vengono solitamente prese misure tecniche preventive per evitare alti livelli vibratori, generati dalle così dette shaking forces: ad esempio design appropriato delle fondazioni, sistemi di bilanciamento delle forze, polmoni smorzatori, orifici e adeguati supporti per le tubazioni. Nonostante questo, ci sono molti casi in cui i problemi di vibrazione si presentano già dal primo avviamento della macchina nell’impianto. Non sempre tali vibrazioni sono da ricondurre a un’ingegneria di dettaglio inadeguata. In molti casi la causa è da ricercare nel non aver prestato sufficiente attenzione all’interazione tra il compressore e il resto dell’impianto.

Quindi il successo del progetto può essere significativamente influenzato da valutazioni e decisioni riguardanti metodi di controllo di pulsazioni e vibrazioni prese fin dalle prime fasi dell’ingegneria di dettaglio, se non addirittura nella stessa fase di specifica. L’utilizzo di un consulente esperto in questo campo e coinvolto direttamente dall’end user assume un ruolo chiave, in quanto dotato di una visione globale ed imparziale ed avente il compito di assicurare sia che vengano seguiti gli obbiettivi di un design a regola d’arte, sia che decisioni critiche riguardo ad efficienza, sicurezza ed affidabilità vengano prese con oculatezza ma allo spesso tempo anche nel minor tempo possibile.


compressor-tech2-may-2017
Titolo: Multy-physics approach in torsional analysis of reciprocating compression train
Rivista: Compressor Tech2
Autore: A. Fusi, F.Grasso, A. Ussi, A. Baylon

Reciprocating compressors generate considerable torque variations that cause torsional vibrations into the drive train that, if excessive, could bring to the failure of the components of the train. If the rotor has a superimposed torsional vibration over the steady rotation, the flux linkages, produced by the induction motor, start to oscillate. This phenomenon modifies the electromagnetic fields across the air gap between stator and rotor, generating an additional torque, which acts at the same time with the rotor’s harmonic excitations. The air gap effect influences the natural frequencies of the electromechanical system. This means that the electric behaviour of the motor has a significant role in torsional vibration analysis, and it is necessary to accurately evaluate its contribution. For this reason, a simplified approach was developed to include the air gap effect inside the study of the torsional analysis, adding one spring and one damper in the mechanical model obtained from the equivalent shaft of the rotor.
Journal of Natural Gas Science and Engineering
Titolo: Analysis of valve-retainer orientation influence on a reciprocating compressor for natural gas vehicle refueling stations
Rivista: Journal of Natural Gas Science and Engineering
Autore: Isacco Stiaccini, Giovanni Galoppi, Francesco Balduzzi, Marco Sacco, Sabrina Santoni, Giovanni Ferrara

In natural gas vehicle refueling stations, the reciprocating compressor is the key element in defining both the refueling process efficiency and the operating costs of the whole station. Great attention is therefore devoted at increasing both the performance and reliability of the machine. The valve-retainer is a component that plays a key role in determining the compressor performance, by introducing significant pressure losses, which affect the suction and discharge phases. However, the valve-retainer is essential for the proper orientation of the valve during the compressor operation and guarantees a simple and reliable design of the machine. Nevertheless, present design guidelines for the valve-retainer are focused mainly on reliability and maintenance criteria, with scarce regard to the fluid-dynamics behavior of this component. In order to assess the influence of the valve-retainer on the performance of a reciprocating compressor for refueling natural gas powered vehicles, attention is given in this study to the retainer orientation that determines the flow path of the process gas inside the compressor plenums. No previous work was found dealing with the assessment of the influence of the valve-retainer geometric features on the fluiddynamic behavior of the plenum. The present analysis was carried out by taking advantage of a hybrid time-frequency domain numerical model, able to provide a significant increase of the accuracy level and reduced computational costs. The model was also integrated with FEM acoustic and CFD steady-state simulations to characterize the acoustic response of the plenum and the pressure losses, respectively. As a case study, a double acting compressor with two different orientations of the valve-retainer was analyzed by simulating the interaction between the compressor and its plenums. The compressor performance was evaluated in terms of indicated power, specific power, suction and discharge mass flow profiles and pressure oscillations inside the compressor plenums. The robustness and accuracy of the approach allowed one to assess that the influence of the valve-retainer orientation on the compressor performance is indeed negligible. It was observed that a 6% variation of the pressure loss across the plenum between the two configurations led to an absorbed power variation lower than 0.2% and a variation of compressed gas mass lower than 1%.
Titolo: Root cause analysis and solution of recurring fatigue failures in the cooler pipe bends of a high pressure LDPE compressor
Evento: EFRC 2016 - Dusseldorf
Autore: Cosimo Carcasci, Marco Sacco, Fabio La Monica, Emanuele Cintolo

Machinery vibration is one of the main causes of damage and failures in industrial plants, and reciprocating compressors are particularly subject to vibration-related phenomena as alternating forces and pressure pulsations are inherent in their design. Vibration problems may also arise after a machine is installed and is set in operation due to inadequate attention to interaction between the compressor and plant as a whole during the design stage. This paper will examine a case that occurred in a polyethylene plant located in Ragusa, Italy, where high vibration issues resulted in broken pipes and welding failures. In particular, the area that was most affected by vibration was the inter-stage cooler and the piping in the immediate vicinity. New, more effective supports were studied and the exact positioning of the supports was based on the results of a dynamic forced-response FEM analysis; the vibration measurements were also used to tune the FEM model, and subsequently to identify, through the model, the key points where a stiffer structure was required. As a result, many of the existing supports were reinforced and additional supports were positioned where they would be more effective according to the FEM analysis. High damping rubber pads were also added between the supports and the piping in order to further lessen the impact of the vibration. Finally, also the cooler structure and foundations were reinforced.
Titolo: Controllo delle emissioni nei compressori di gas.
Rivista: Manutenzione T&M - Febbraio 2015
Autore: Alessio Butini

Gli idrocarburi sono trattati ogni giorno dall’ industria di processo e, nelle fasi di compressione, i trafilamenti di gas sono eventi sempre presenti e purtroppo dannosi sia dal punto di vista della produzione sia per l’impatto ambientale. Il monitoraggio continuo delle fughe e la manutenzione predittiva possono rappresentare una soluzione efficace sotto diversi punti di vista.
Titolo: La Formazione, collante tra progettazione e manutenzione.
Rivista: Manutenzione T&M - Febbraio 2014
Autore: Marco Giaconi, Agnese Giannoni
 
Al fine di ammortizzare al meglio i costi di acquisto e gestione delle macchine e degli impianti il loro ciclo di vita si sta sempre più allungando. Cresce anche la necessità realizzare prodotti sempre più flessibili, al passo con le possibili variazioni dei parametri operativi, con la tecnologia e le richieste sempre più esigenti dei clienti; affinché questo possa essere possibile è necessario quanto meno un legame saldo e continuo tra progettista e operatore. Purtroppo il legame tra le due figure, in particolare tra progettista e manutentore, tradizionalmente non è mai stato molto stretto: il primo spesso legato alla teoria e con pochi riscontri diretti dal campo, il secondo impegnato per lo più direttamente in campo a cercare di tenere efficienti e funzionali le macchine. Un ruolo importante può essere svolto dal docente dei corsi di formazione il cui lavoro non può essere limitato a trasmettere informazioni e conoscenza, ma dovrebbe anche avere la funzione di rendere possibile un agile ed efficace scambio di informazioni tra il campo e l’ufficio tecnico.
Titolo: Managing Transient Operating Conditions of Hyper Compressors
Evento: ASME 2014 Pressure Vessels & piping Conference PVP2014-28046; July 20-24, 2014, Anaheim, California, United States
Autore: Marcello Agostini, Cosimo Carcasci, Alessio Cristofani
 
Special reciprocating compressors are required to reach the very high pressures necessary to feed the reactors used in LDPE plants. The working conditions of the compressors are determined by the reactor operating pressures and are conside-red for evaluation of the loads, torsional, pulsation and vibra-tion conditions. Performance and safety are key considerations and ope-rator experience is essential for optimal and safe plant opera-tion. Plant automation contributes to safety and optimizes pro-cess conditions. The compressor start up and shutdownprocedures, commonly detailed in maintenance manuals, are analyzed studying the operating parameters to draw up guidelines for avoiding damage to plunger sealsandkeeping loads within design limits,without compromising the design performance. The multiple gas compositions of co-polymers and the fact that gas expansion can cause a reduction in temperature, need adequate consideration to avoid damage caused by polymer inside the compressor cylinders.The paper will also give recommendations on the neces-sary procedures to reach the operating pressures with correct distribution ofthetotal pressure ratio between the two stages. These will also include the requirement of minimum pressure on the plunger head to maintain contact with the crosshead and avoid failure of such a brittle component
Titolo: Brown-Miller Algorithm for Nonlinear, Multiaxial Fatigue Analysis.
Rivista: CompressorTech Marzo 2013
Autore: Riccardo Traversari, Alessandro Rossi, Marco Faretra

The crosshead to piston rod threaded connection is one of the most critical parts of the whole reciprocating compressor. Traditionally, this critical connection is dimensioned by the classic method of the “Load/Deformation diagram” or “triangle of elasticity’s” to evaluate mean and fluctuating stresses on the thread and then the relevant safety factor with the specific Smith fatigue diagram.
A better approach is to make a Finite Element Analysis, which can be performed simulating the threaded connection as a cylindrical surface, neglecting the real thread geometry or, even better, simulating the real thread geometry. But, even the more refined elastic FEM fatigue analysis, with Von Mises monoaxial equivalent stress, tipically estimates stresses that often exceed the material yield strength in the tread root radius, thus resulting in unrealistically poor safety factors even in the subsequent fatigue analysis. Also a biaxial fatigue analysis for infinite life in the linear range shows the same problem even for well proven applications.

This paper, after a review of the aforementioned methods, describes a new advanced approach to the design of the threaded connection of the crosshead to piston rod based on an elastic-plastic, multiaxial nonlinear, non proportional FEA, using the Brown-Miller algorithm with the Morrow correction of mean stress. This new calculation method allows taking in the correct consideration also the possible local plastic deformations, evaluating their real effect on the thread strength.

Titolo: A Multi-phase CFD Study of a liquid slug Ingestion in a Reciprocating  Compressor.
Rivista: Proceedings of the ASME 2013 Pressure Vessels & Piping Conference PVP2013;  July 14-18, 2013, Paris, France. Original published and copyright of ASME.
Autore: Riccardo Traversari, Bernado Bettini, Cosimo Carcasci, Andrea Fusi

The fluid handled by reciprocating compressors has to be strictly limited to gas and, under no circumstances, can there be a liquid fraction ingestion, as this event can cause the famous “liquid hammer effect” that can generate catastrophic failures. A study was conducted to evaluate the quantitative effects of the ingestion of a liquid slug into a reciprocatingcompressor cylinder with the goal of calculating the pressure distribution on the piston during such an event. The Multi¬phase CFD ANSYS FSI software, utilized to simulate the behavior of a mixture of gaseous and liquid hydrocarbons during the delivery phase, has shown that the pressure inside the cylinder increases up to five times the normal pressure. This new approach can be helpful in the design stage or for root cause failure analysis of large compressors such as the ones utilized in the natural gas, refining, and LDPE industries

 

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