Pilot-operated Relief Research Articles

Effect of Dynamic Pressure Feedback Orifice on Stability of Cartridge-Type Hydraulic Pilot-Operated Relief Valve

Current research on pilot-operated relief valve stability is primarily conducted from the perspective of system dynamics or stability criteria, and most of the existing conclusions focus on the spool shape, damping hole size, and pulsation frequency of the pump. However, the essential factors pertaining to the unstable vibration of relief valves remain ambiguous. In this study, the dynamic behavior of a pilot-operated relief valve is investigated using the frequency-domain method. The result suggests that the dynamic pressure feedback orifice is vital to the dynamic characteristics of the valve. A large orifice has a low flow resistance. In this case, the fluid in the main spring chamber flows freely, which is not conducive to the stability of the relief valve. However, a small orifice may create significant flow resistance, thus restricting fluid flow. In this case, the oil inside the main valve spring chamber is equivalent to a high-stiffness liquid spring. The main mass–spring vibration system has a natural frequency that differs significantly from the operating frequency of the relief valve, which is conducive to the stability of the relief valve. Good agreement is obtained between the theoretical analysis and experiments. The results indicate that designing a dynamic pressure feedback orifice of an appropriate size is beneficial to improving the stability of hydraulic pilot-operated relief valves. In addition, the dynamic pressure feedback orifice reduces the response speed of the relief valve. This study comprehensively considers the stability, rapidity, and immunity of relief valves and expands current investigations into the dynamic characteristics of relief valves from the perspective of classical control theory, thus revealing the importance of different parameters.

Effect of the resonance suppression damping on the stability of a cartridge pilot-operated relief valve

The pilot-operated relief valve is a complex hydraulic component. The instability phenomenon during its application may cause some unexpected problems, such as poppet vibrations or pressure fluctuations, etc. Despite the abundant use of relief valves in pressure-control applications across various fields, there is a lack of quantitative stability criteria. This paper focuses on deriving the quantitative stability criteria of a cartridge pilot-operated relief valve. After establishing the detailed mathematical model of such a valve, the effects of the main parameters on its stability are analyzed through the Routh–Hurwitz stability criterion. The analysis results indicate that the unreasonable match between the chambers and the poppets may break the stability of the valve. However, limited to the internal space of the cartridge-type valve, some chambers are too small to match the minimum manufacturing mass of the poppets. To ensure the stability of the pilot-operated relief valve, a dynamic pressure feedback orifice is typically designed between the main stage and the pilot stage as a resonance suppression damping element. So that the contradiction between the stability criterion and the sensitive parameters will be resolved if the orifice is small enough. As a result, the stability of the pilot-operated relief valve may be considerably improved. Finally, simulations and measurements were utilized to verify the correctness of the predicted results.

Study on EOPs for PORV-Break LOCA in Generic PWR Simulator

After the Three Mile Island Unit 2 accident, regulatory bodies were concerned about the safety measures in design and operation corresponding to the operator’s decisions and procedures for handling such off-normal transients. Several recommendations were proposed to analyze transients and accidents, improve and revise emergency operating procedures (EOPs), and conduct functional training. In this work, procedural paths were systematically studied to identify the problems in the diagnosis associated with a pilot-operated relief valve (PORV)–break loss-of-coolant accident (LOCA) as well as to suggest new indications for improving the EOPs. Operational parameters during PORV-break LOCA and pipeline-break small-break LOCA were analyzed using a generic pressurized water reactor simulator to compare and justify the symptoms between these two events. It was found that suggesting further indications mainly in the reactor cooling system and containment symptoms may improve the diagnosis of a PORV-break LOCA from the pipeline-break small-break LOCA. This paper presents a practical approach to evaluating diagnostic procedures to better understand operator recovery actions corresponding to reactor system response in dealing with a PORV-break LOCA.

Effect of the balanced piston on the stability of hydraulic pilot-operated relief valve

The stability of a pilot-operated relief valve is investigated through mathematical modeling and experimental measurement. A comprehensive mathematical model of a cartridge-type pilot-operated relief valve is developed, which is designed with a balanced piston at the front of the pilot poppet. The dynamic behavior of such a valve was then investigated using a frequency-domain analysis method. The balanced piston was found to have a marked effect on the stability of the relief valve. The local stability of the valve under various annular clearances was further discussed by the normalization method. The results indicate that an appropriately sized balanced piston can improve the stability of the relief valve’s subsystem. However, when the annular clearance is large, the subsystem’s dynamic characteristics do not change obviously, although it does raise the pilot poppet damping ratio to some extent. Furthermore, numerical simulations and experimental studies were produced to validate the above conclusions, and excellent agreement between the measurement and the model was obtained.

CFD-Based Cavitation Research and Structure Optimization of Relief Valve for Noise Reduction

The cavitation flow of the cartridge pilot-operated relief valve’s main valve port was numerically simulated and studied using Computational Fluid Dynamics (CFD) in our research, and the relief valve structure was modified to decrease cavitation noise. The findings indicate that cavitation in the relief valve occurs mostly downstream of the main valve port and is linked to the wall. Cavitation generation is influenced by the jet at the valve port. The closer the high-speed jet at the valve port is to the valve’s inner wall, the more readily cavitation occurs. Conversely, there is less cavitation. This paper reports the effect of the forms and parameters of the main port structure on the jet angle and the morphology of cavitation. The structure of the spool’s annular groove has a considerable influence on the cavitation attached to the spool wall and the valve sleeve wall, whereas the outlet position of the relief valve mostly impacts the intensity of cavitation near the valve sleeve wall. Based on this, the relief valve’s optimized structure is designed, with which the maximum vapor volume fraction and total vapor volume of the cavitation flow are considerably decreased for various inlet pressures and spool openings. The results of the experiment show that following optimization, the noise of the relief valve drops dramatically, confirming that optimizing the structure has a beneficial impact on decreasing cavitation noise.

Study of PWR hot leg creep rupture and RCS depressurization strategy during an SBO accident

Abstract Preventing the leakage of radioactive materials is important to nuclear safety. During a station blackout accident in pressurized water reactors, the hot leg creep rupture caused by hot leg countercurrent flow occurs before the reactor pressure vessel failure that caused by lower head rupture. The secondary fission products barrier is lost after hot leg creep rupture. An analysis for this phenomenon was done using the Modular Accident Analysis Program version 4.0.4 code. A station blackout accident for CPR1000 is simulated and the occurrence and influence of hot leg creep rupture phenomenon are analyzed in detail. After that, a sensitivity analysis of the opening of different pressurizer pilot-operated relief valves at five minutes after entering severe accident management guideline (before the hot leg creep rupture occurs) is studied. The results show that reactor pressure vessel failure time can be extended by at least 4 h if at least one pilot-operated relief valve is opened and direct containment heating phenomenon can be eliminated if at least two pilot-operated relief valves are opened.

Characteristic Analysis of Digital Emulsion Relief Valve Based on the Hydraulic Loading System

In the hydraulic loading system, the performance of digital relief valve plays an important role in the dynamic response of load. However, the research on large-flow emulsion relief valve is still far from perfect. In this paper, digital relief valve is taken as the research object. Based on pilot-operated relief valve, a digital control scheme using a linear stepping motor is adopted to regulate the working pressure of relief valve. The structure of relief valve is analyzed and optimized from the aspects of dynamic and internal flow field characteristics to obtain a good working performance. To obtain its accurate working characteristic, the structural model and digital control system of relief valve are established by AMESim and Simulink, respectively, for electrohydraulic cosimulation. The results show that digital relief valve has a better characteristic of real-time dynamic pressure regulation. Therefore, the digital control system could improve the dynamic performance of relief valve, and the design of digital relief valve structure is reasonable and feasible. The simulation method employed in this paper provides a better theoretical basis and reference for the comprehensive research of digital large-flow emulsion relief valves based on the hydraulic loading system.

Pulse safety device with adaptive controller for technical systems with high reliability requirements

To ensure the protection of technical systems against excessive pressure, pulse-safety devices (pilot-operated relief values, or PORV) are used, which differ from spring-loaded safety valves in their increased throughput and high accuracy of operation at set pressures. At the same time, PORVs are less reliable due to the complexity of the design. Their reliability depends on the operability of two valves - pulse and main ones. The pulse valve that controls the entire safety system is still the weakest PORV element. This is especially true for systems that have high safety and reliability requirements, for example, for aerospace technology and the nuclear industry, because the resource of sensitive elements decreases sharply when working in extreme operating conditions. The article proposes a PORV design with redundancy of the valve sensing element by a system in the form of a metal bellows and a slide valve. The reliability values of the standard and proposed PORV designs are analyzed.

Effects of Structure Parameters on Abnormal Opening of Pilot-Operated Relief Valve Under Alternating Pressure

Hydraulic vibration excitation has been widely regarded as a promising method of excitation because of its high power density and large output force. However, the alternating pressure in the hydraulic vibration exciter could cause the pilot-operated relief valve (PRV) to open abnormally, which presents a new challenge to the normal operation of the PRV. To determine the abnormal opening characteristics of the PRV under alternating pressure, the effects of structure parameters of the PRV (including diameters of orifices 1 and 2, volume of the pilot valve inlet, volume of the main valve spring chamber, area ratio of the main spool, and main spring pre-compression force and stiffness) on its abnormal opening displacement under alternating pressure were numerically investigated. The calculation results indicate that the abnormal opening of the PRV will be effectively decreased by appropriately increasing the diameter of orifice 1, decreasing the pilot valve inlet and main valve spring chamber volumes, and increasing the area ratio of the main spool and the main spring pre-compression force. The influence of the orifice 2 diameter on the abnormal opening of the PRV is dependent on the diameter range of orifice 1. The reasonable diameters of orifices 1 and 2 are in the range of 0.8-1.2 mm. The influence of the main valve spring chamber volume is more significant than the pilot valve inlet volume. The influence of the main spring stiffness is not significant.

Station black out concurrent with PORV failure using a Generic Pressurized Water Reactor simulator

Station Black Out (SBO) is an accident situation that refers to the total loss of offsite power, along with the unavailability of onsite power, which results from the failure of all Diesel Generators (DG). Probabilistic Safety Assessment (PSA) spans a number of methods that include modeling of event-trees and simulation of accidents scenarios, aimed to quantify risk and ensure safety in nuclear power plants. PSA also deals with prediction of future accidents and calculation of failure probabilities that has been done in this study. A SBO accident was simulated using a Generic Pressurized Water Reactor (GPWR) simulator from KEYMASTER™. The accident scenario consists of two stages; the first stage belongs to normal SBO, in second stage SBO accident with additional failure of Pilot Operated Relief Valve (PORV) opens and it stuck open has been considered for the pressurizer. A comparison of the two stages was made by plotting variables on the same graph. The research has been carried out to analyze the hot and cold leg temperatures, Steam Generator (SG) pressure, SG Narrow Range (NR) level, SG water-level-percentage (PCT), Pressurizer pressure, Fuel Temperature, and containment pressure. Simulation results suggest that failure in closing PORV has negligible impact on hot and cold leg temperatures, results in an overall less pressure in SGs, but higher pressure in the pressurizer. Additionally, containment pressure did not exceed the maximum approved pressure of 8.7kg/cm2, but was approaching the Advanced Pressurized Water Reactor’s (APR-1400) design pressure of 4.218kg/cm2. Finally, nuclear fuel temperature exceeded Probabilistic Risk Assessment (PRA) limit of 726.7°C for both scenarios. The research results will provide help in future for better understanding of accidents in APR-1400 reactors.

Testing and Modeling of an Acoustic Instability in Pilot-Operated Pressure Relief Valves

Pressure relief valves (PRVs) are included as an essential element of many compressor piping systems in order to prevent overpressurization and also to minimize the loss of process gas during relief events. Failure of the valve to operate properly can result in excessive quantities of vented gas and/or catastrophic failure of the piping system. Several mechanisms for chatter and instability have been previously identified for spring-loaded relief valves, but pilot-operated relief valves are widely considered to be stable. In this paper, pilot-operated PRVs are shown to be susceptible to a dynamic instability under certain conditions where valve dynamics couple with upstream piping acoustics. This self-exciting instability can cause severe oscillations of the valve piston, damaging the valve seat, preventing resealing, and possibly causing damage to attached piping. Two case studies are presented, which show damaging unstable oscillations in a field installation and a blowdown rig, and a methodology is presented for modeling the instability by coupling a valve dynamic model with a 1D transient fluid dynamics simulation code. Modeling results are compared with measured stable and unstable operation in a blowdown rig to show that the modeling approach accurately predicts the observed behaviors.

Comprehensive Nonlinear Modeling of a Pilot Operated Relief Valve

This paper is directed toward a comprehensive nonlinear modeling and simulation of the performance of a class of a pilot operated relief valves. A mathematical model is deduced to predict the performance of the valve in the steady state and transient modes of operations. The developed model takes into consideration most nonlinearities of the valve and is studied within the MATLAB-SIMULINK environment. The validity of the proposed model is assessed experimentally in the steady state and transient modes of operations. The detailed modeling has resulted in a good agreement between simulation and experimental results. During the simulation studied, it was found that, nonlinearity occurs due to three factors: the pressure changes cause nonlinear velocity changes of the flow rate, the throttling area of the valve restriction usually changes nonlinearly, and the discharge coefficient of the throttling area of the valve restriction does not remain constant. In the transient mode of operation, the simulation studied identified some critical parameters which have a significant effect on the transient response of the valve. Most of the model’s parameters can be evaluated readily by direct measurement of the valve components dimensions thought the Coulomb friction factor and bulk modulus are tuned to match the model to the measurements.

Research of the Pilot Operated Relief Valve for MRF System

Combining with the rheological characteristics of MRF and the traditional structure of pilot operated relief valve, the key technical problems existing in this type of valve when using MRF instead of mineral oil were analysed. To solve its key technical problems, A novel pilot operated relief valve for MRF system has been designed. Adopting MRF valve for using the pilot valve, designing reasonably the relative position and the size of the working gap in the MRF valve, and optimizing design the structure of the main valve of the pilot operated relief valve for MRF system, can effectively improve the damping force of the MRF valve and reduce the erosion and noise of the pilot operated relief valve for MRF system.

Pressure compensation method of under water hydraulic system with hydraulic power unit being under atmospheric circumstance and pressure compensated valve

Based on the analysis of the-state-of-the-art of pressure compensation of underwater hydraulic systems (UHSs), a new method of pressure compensation of UHSs, whose hydraulic power unit is in the atmospheric circumstance, is proposed. And a pilot-operated relief valve with pressure compensation is realized. The pressure compensation precision is guaranteed by direct detection. Its dynamic performance and stability are improved by a dynamic feedback. Theoretical study, simulation and experiment show that the pilot-operated relief valve with pressure compensation has a fine property of tracking underwater ambient pressure and meet the requirement of underwater ambient pressure compensation.

Riser-Relief Valve Dynamic Interactions (Extension to a Previous Model)

Further investigation of the dynamic stability behavior of a typical pilot-operated relief valve is reported. The present study is an extension to Botros et al. (1997) model, which includes mapping of the oscillating frequencies and amplitudes with riser dimensionless length L/D; inclusion of the effects of a wedge-O-ring seal in the model; detailed analyses of the field tests revealing unknown values for model parameters. These model refinements resulted in a better agreement between simulation results and field measurements. Analysis of piston oscillation frequencies and amplitudes indicates that the piston oscillation frequency mirrors the riser’s one-quarter-wave resonance frequency for lower values of L/D. At L/D = 20 and higher, two modes of oscillations started to emerge with two distinct frequencies. Maximum oscillation amplitudes occurred at L/D corresponding to one-quarter wave. Wedge-O-ring seal mechanism helps in suppressing piston oscillations; but it must be used in conjunction with a proper lubricant, otherwise the piston may jam partway during the upward stroke.

Riser-Relief Valve Dynamic Interactions

The dynamic stability behavior of a typical pilot-operated relief valve has been investigated both numerically and experimentally. The valve piston equation of motion has been solved in connection with the conservation equations of the oscillating flow in the attached riser. The boundary condition at the riser tip is of a choked flow type and had to be given special attention in oscillatory flows. Pertinent quasi-steady flow equations have been formulated to describe the different phases of relief along the piston lift. A model describing the process of pressure relief is described briefly, followed by a parametric study of the effects of various factors on the oscillation levels and characters. Field measurements were taken on an NFS 8 pilot-operated dual-outlet relief valve mounted on a riser assembly of L/D = 15 in a natural gas compressor station.

Post-facta analysis of the TMI accident (I): analysis of thermal hydraulic behavior by use of RELAP4/MOD6/U4/J2

A thorough analysis of the thermal-hydraulic behavior of the TMI-2 reactor during the first three hours of the accident on March 28, 1979 was carried out by use of RELAP4/MOD6/U4/J2 computer program. The analysis includes the loss of feedwater transient, the small break loss-of-coolant accident (LOCA) through the pressurizer pilot-operated relief valve (PORV) and the distribution of hydrogen generated by the zirconium-water reaction. It took account of the effects of the pressurizer heaters and of the presence of the hydrogen. The calculated result shows good agreement with the accident record, and the effects of some key parameters have become better understood. This analysis shows that the tops of active fuel rods would have begun to be exposed at 6490 s into the accident and the core mixture level remained at a height of 3.4 to 4 ft from the bottom of the core for about 40 min.