Valve Dynamics Research Articles

Dynamic behavior of valves with pneumatic chambers for reciprocating compressors. 1st report. Calculation.

This paper describes the dynamic behavior of valves with pneumatic chambers for reciprocating compressors. These are known as 'damped valves' and are capable of reducing the impact on the valve seat and valve stopper. The characteristics of the dynamic behavior of the damped valves were clarified by calculating newly derived governing equations of valve dynamics. From the calculated results, it becomes apparent that the volume of the pneumatic chambers and the clearance between the pneumatic chamber and the valve have a large influence on the impact speed of the valve. Furthermore, the valves tend to close later for a higher compressor speed and to oscillate at a larger amplitude for a lower density of gas such as hydrogen. These tendencies show that the selection of the specification of damped valves is very important. The stiffness of the valves spring and the lift of the valve also affect valve behavior as with valves without pneumatic chambers.

Performance of four-nozzle flapper hydraulic servovalves

The static characteristics of four-nozzle flapper valves are investigated with both constant supply flow and constant supply pressure. The ranges, in which linearization of these characteristics would be valid, are determined. Linearized and nonlinear mathematical models are derived and reduced to a dimensionless form to simulate the valve dynamics in both cases of no-load and blocked-load operation. Numerical solutions of the nonlinear models are carried out to make an assessment of the effects of input step size and fluid line parameters on valve response. Solutions for harmonic inputs are also made to determine the valve frequency response. The bandwidth attained is shown to suit practical applications, with the cutoff frequency governed by the allowable stall-pressure attenuation, especially in constant supply flow operation. Comparing the results with the solutions of the linearized models, decision could be made regarding the cases in which the latter could be relied upon in the analysis and design of four-nozzle flapper valves.

A digital simulation method of the pressure pulsations in reciprocating compressor-piping systems. 1st report Calculation method.

One method of analyzing pressure pulsations is to use an analog simulation which can consider the interaction between a compressor and the attached piping. Conventional digital pulsation analyses, in which suction/discharge gas flow is one-sidedly applied as a forcing function, do not consider the interaction effects. This paper describes new digital simulation methods which can account for a compressor-piping interaction. In these methods, non-return valves and the stroke of the valves of compressors are expressed by nonlinear damping and nonlinear stiffness respectively. The dynamic equation of the total system is constructed by assembling the characteristic equations of each element using the finite element method. This governing equation is integrated time historically using the Newmark-β method ; and then the flow rate and pressure variation are obtained. By using the above methods, the following three types of digital simulations were carried out for a compressor-piping model : (1) conventional digital simulation (2) new digital simulation equivalent to the analog simulation (3) new digital simulation including the valve dynamics. From these calculations, it is demonstrated that this present method has the same capability as the analog simulation. Furthermore, it is shown that this new method can deal with more detailed interactions by considering the valve dynamics, which is difficult to account for in the analog simulation.

The dynamics and sensitivity analysis of a plunger-type pressure control valve.

This paper is concerned with an analytical study of the dynamics of a plunger-type pneumatic pressure control valve, already proposed by the authors, which maintains a constant secondary pressure lower than the primary pres­ sure, in spite of the change of the primary pressure or the fluctuation of the load connected with this valve. At first, the dynamics of the secondary pressure and the plunger movement, caused by changes of the primary pres­ sure and the load, is analyzed by means of the state vari­ able method. Furthermore, for improvement of responsibi­ lity, the conditions for adjusting system parameters are found through the root locus method. Next, the sensitivi­ ty analysis is applied to the investigation of the influ­ ences of parameter variations on the dynamics of the con­ trol valve. By means of simulation studies of sensitivity functions, we have shown which parameters are really sig­ nificant and also which are not. Throughout these analy­ tical and simulation studies, guidelines for improving the performance of a plunger-type pressure control valve are obtained.

On the fluid dynamics of the aortic valve

An aortic-valve model is developed, having a quadratic cross-section, two rigid cusps and two wedge-shaped aortic sinuses. The flow through this valve is assumed to be one-dimensional, just as the flow behind the cusps should be one-dimensional. The resulting model equations are two nonlinear ordinary differential equations of second order for the valve opening area as a function of time in two different ranges.This model allows the size of the aortic sinus to be varied; it also permits a computation of the pressure at both sides of the cusps (unlike previous models of this kind, which consider the flow behind the cusps as stagnant). The computed valve motion due to this pressure difference is in good agreement with experimental results, although no vortex with circular streamlines is postulated in the aortic sinuses. Obviously such vortices trapped in the sinuses are not important for the valve closure, which is controlled solely by the flow deceleration.

A modified catheter system for retrograde left ventricular catheterization in aortic valve stenosis.

Conventional retrograde catheterization of the left ventricle using a single large catheter through severely stenotic aortic valves or prostheses has recognized difficulties. An alternative method is described. This method incorporates a coronary angioplasty guide catheter and coronary probing catheter attached to separate pressure transducers allowing simultaneous recording or aortic and LV pressures. This technique was used in seven patients with aortic stenosis in whom a conventional approach had failed. The movable probing catheter permits alterations of the catheter orientation and angulation while the guiding catheter stabilizes the system within the high velocity jet formed in the ascending aorta. This stabilization and the small size of the coronary probing catheter permit exploration of the valve surface and allow for insertion through a tilting disc valve or a severely stenotic valve without significantly altering hemodynamic function. In our laboratory this technique allowed us safely and reliably to assess aortic valve dynamics in each of the patients in whom the technique was attempted without resorting to either transseptal or direct left ventricular puncture.

Computational Method for the Analysis of Valve Transients

A computational method for valve dynamics is discussed. The method consists of a hydraulic part and the equations describing valve dynamics. These are coupled together using a boundary condition in the hydraulic model. Some important parameters for the boundary condition are determined experimentally. The hydraulic equations are solved using the method of characteristics. Some example calculations for a boiling water reactor are presented.

Analysis of the design and dynamics of aortic bioprostheses in vivo

An understanding of the in vivo design and dynamics of the present bioprosthetic valves should provide the information necessary for an improvement in their efficiency and durability. Three types of commercially available bioprostheses were prepared with radiopaque markers and implanted in the aortic position in calves. One week later, under light general anesthesia, the animals were studied to determine the in vivo design, shape, configuration, and motion of the bioprostheses. This information was then compared to that previously obtained from the natural aortic valve in vivo. The following observations were made: (1) In all three types of bioprostheses, the three leaflets opened and closed simultaneously in less than one thirtieth of a second; (2) there was no detectable flexion of the stent posts in any of the three types of valves; (3) in all of the bioprostheses studied, the greatest flexion occurred along the attachment of the leaflets; (4) the systolic and the diastolic geometry of the three types of valves was completely different for each type of valve, and none duplicated the geometry of the natural aortic valve; (5) the open configuration of the leaflets was different for each type of prosthetic valve and different from the natural aortic valve; (6) the zone of leaflet bending varied in size and extent for each of the valves. It is concluded that the differences between the design of bioprosthetic valves and that of natural valves are probably a major factor in increasing the stresses in bioprostheses. Hence stress failures could be reduced and durability increased by redesigning bioprostheses to duplicate more closely the design of the natural aortic valve.

Fluid dynamics of the mitral valve: physiological aspects of a mathematical model.

This paper presents a computational method that can be used to study the fluid computational method that can be used to study the fluid dynamics of the natural or prosthetic mitral valve in a computer test chamber with contractile muscular walls that model the left side of the heart. Having solved the equations of motion and predicted the performance of the valve, the computer presents the results in terms of flow patterns (streamlines), pressure-contour plots, and graphs of velocity, pressure, flow, tension, and distance as functions of time. Predicted echocardiograms and phonocardiograms are also generated by the computer. These predictions are compared with the results of animal experiments, and the method is used to study the diastolic role of the chordae tendineae, the pathophysiology of mitral valve prolapse syndrome, and the fluid dynamics of pivoting disc valves. Our results support the hypothesis that the chordae have a functional role during diastole. The results also establish a computer model of mitral prolapse, which should be useful in functional studies of that condition, and show that the position of the pivot point and the curvature of the occluder can have a profound effect on the angle of opening of pivoting disc valves. These studies illustrate the applications of the method in physiology, pathophysiology, and prosthetic valve design.

Analysis of the dynamics of artificial heart valves

Analysis of the dynamics of artificial heart valves

Influence of valve dynamics on compressor performance

The valves of piston compressors can cause volumetric losses if they do not function properly and these losses originate from the condition where the force of valve springs differs from the optimal force which can be calculated from equations (1), (2) and (3). This problem is particularly critical with suction valves and the purpose of this report is to determine the factors which influence these losses. These determinations were realised via a series of computer simulations giving various operating conditions. The results show that the gas force A∗ (equation (1)) and the Mach number Ma∗ (equation (5)) determine the level of losses for a given force difference. Fig. 4 shows the kind of volumetric losses that can be expected. The most critical conditions occurred with a low A∗ value and high Mach number, hence careful attention should be given to dynamic design for such conditions. This study has revealed that unnecessary losses can be avoided if spring forces are maintained at about 50 to 150% of the optimal force of springs.

Computer Aided Design Studies of a Multiple Cylinder Refrigeration Compressor

A multi-cylinder refrigerant compressor was analytically modeled and simulated on a digital computer. The modeling includes the kinematics of the mechanical compressor, the thermodynamic processes in the cylinders and the flow process of the refrigerant gas in and out of the cylinders. In addition the modeling includes the valve dynamics in controlling the gas flow, the thermodynamic processes in the common suction and discharge plenums and the interaction of the cylinder processes through the common plenums. The remainder of the refrigeration system was treated as pressure-temperature drop. The analytical model was compared to experimental data in terms of cylinder pressures, suction-discharge plenum pressures and temperatures, valve motions and the average mass flow rate of the refrigerant gas. The correlation between the comparison variable was quite good with some error in the prediction of suction valve closing time. The analytical model was used to predict the effect of increased compressor speed on the performance of the compressor. The speed was increased from 500 rpm to 2700 rpm with a decrease in volumetric efficiency from 87 percent to 60 percent, an increase in mass flow rate from 0.0378 kg/s to 0.151 kg/s. Valve impact speeds were also calculated to predict degradation in valve life.

Normal aortic valve function in dogs

Previous trileaflet aortic prostheses failed from fatigue and flexion stresses because their design was not based on the physiology of the normal valve. The dynamics of normal aortic valves were studied in terms of movement of aortic valve commissures and leaflets in vivo. Eight dogs were placed on total cardiopulmonary bypass; through an aortotomy, radiopaque platinum markers were placed on the commissures at the level of leaflet coaptation and at the center of the leaflets' free edge. Three dogs were studied immediately and five dogs after 10 or more days. Marker movement in the beating heart was recorded under fluoroscopy on videotape. The aortic root expands 12 ± 0.4 percent (mean ± standard error of the mean) during systole at a blood pressure of 120 80 mm Hg; it expands 8.5 to 24 percent at blood pressures ranging from 50 30 to 240 180 mm Hg. The relation between aortic root diameter and blood pressure is similar to a classic tension-radius relation of the aorta. The aortic root diameter is controlled by blood pressure and by the pressure gradient across the valve. The functional elastic modulus of the aortic root is 4.4 × 10 5dynes/cm 2. Expansion of the aortic root begins 20 to 40 msec before the valve opens. The velocity of leaflet opening is 97 cm/sec. The orifice of the opened valve is circular and maximal at the beginning, gradually decreasing to the point when the valve snaps shut. Commissural expansion gives a smooth opening, less shear stress on the leaflet surface, less flexion stress at the center of the leaflet and less fatigue strain on the leaflet, thereby maximizing the life and efficiency of the aortic valve. If these characteristics were incorporated into aortic prostheses, prosthetic failure due to fatigue and flexion stresses should be reduced.

電気油圧サーボ系のシミュレーションと最適化

In this paper we discuss the time optimal control problem of an electrohydraulic servo system. This system is described by the 16th order nonlinear ordinary differential equations with multi-discontinuities. The control variables are inputs to the two servovalves with different nonlinear flow gains and dynamics. The performance index to be minimized is made up of integrated square errors of allowable system pressures, load displacement and velocity around the origin, and response time.This problem is solved by applying a hillclimbing algorithm to the 4th order Runge-Kutta method simulation of the state equations. As the pattern of this control is bang-bang, it is important to find the switching criteria as well as stability conditions. In this numericall experiment, a square root function is taken as closing control to a valve, and the amplitude of a feedback variable is limited to ensure reasonable stability.In spite of the several assumptions made throughout with the system, such that the valve dynamics is simple and the liquid flow is steady, the computational results have shown fairly good coincidence with the experimental results.

Measuring the Oscillating Flow from an Electro-Hydraulic Servo-Valve Using an Indirect Method

Recently published work concerning the dynamic flow-rate through servo-valves is discussed. It is shown that there is some disagreement amongst workers concerning the effects that fluid inertia and valve discharge coefficients have on the dynamic performance of these devices. The problem of measuring pulsating flow is discussed and the various commercially available transducers are shown to have shortcomings when used in conjunction with servo-valves. An alternative indirect dynamic flow measurement technique is described. The technique is evaluated using an analogue model. It is concluded that the method is capable of determining the dynamic volumetric flow-rate. The difficulties involved in identifying the valve dynamics from these measurements are discussed. The method is applied to determine the volumetric flow-rate in an experimental rig consisting of a servo-valve controlling a rectilinear motor used to position an inertial load.

Design and development of a steam turbine driven circulator for high temperature gas-cooled reactors

Design optimization of high temperature gas-cooled power reactors with integration of the entire primary coolant system in a prestressed concrete reactor pressure vessel (PCRV) leads to stringent power, size, and environmental requirements for the primary coolant circulation equipment. To meet these requirements, the series steam turbine helium circulator (SSTHC) was conceived and developed by Gulf General Atomic. The SSTHC consists of a single stage axial-flow helium compressor driven by a single stage steam turbine. The compressor and the drive turbine are integrally attached to a single vertical shaft and overhung from a central bearing and seal housing. The drive turbines are in series with the power-producing main steam turbine. This paper discusses the design points of the compressor, drive turbine and auxiliary Pelton wheel drive, as well as the design requirements for the bearings and seal system. A general outline of the SSTHC development program carried out at Gulf General Atomic is given. The following areas are included in the development program: aerodynamics, compressor noise, primary coolant shutoff valve, water bearings and rotor dynamics, seals, blade vibration, and disk catcher. Further, a comprehensive series of transient tests on a circulator have been carried out.

Pattern of blood flow within the heart: a stable system.

Flow patterns within the cardiac ventricles during diastolic filling have been studied in dogs and sheep by cineradiography with fine stream dye injection, and by endoscopic cinephotography. The results show that stable flow patterns occur with little evidence of gross mixing or lateral dispersion. The major flow patterns are expanding vortex systems behind the cusps of the mitral and tricuspid valves. These vortex systems appear important in normal valve dynamics.

Angle of traction of the papillary muscle in normal and dilated hearts: A theoretic analysis of its importance in mitral valve dynamics

Angle of traction of the papillary muscle in normal and dilated hearts: A theoretic analysis of its importance in mitral valve dynamics

Transient Response of a Time-Optimized Hydraulic Servomechanism Operating under Cavitation Conditions

The transient response of a time-optimized hydraulic servomechanism operating under cavitation conditions is analysed in this paper. Factors included in the analysis, in addition to actuator cavitation, are: valve dynamics, valve hysteresis, oil compressibility, actuator leakage and non-linear load friction. After the basic equations have been derived, the transient response is determined graphically using a modification of the technique introduced by McCloy in which the load-friction curve is superimposed on the valve pressure-flow curves. On this plot the load trajectory is determined, step-by-step. The transient response of the system is then determined. Experimental verification of the analysis is included.

The Dynamic Characteristics of Oil Hydraulic Control Valve

This paper presents a new method for the calculation of the fluid flow forces acting on a valve. Its theory is based on the momentum theory and can explain the inertial force of fluid which was not considered in the former momentum theory. As an example, the dynamics of a single stage relief valve was studied analytically and experimentally. The transient response of the valve was calculated with the original nonlinear form and with a linearized form. Numerical results show the linearization does not carry much error and the linearized equations show that which are useful to explain experimental results.