GB1583245A - Reducing corrosion erosion in a hydraulic valve - Google Patents

Description

(54) REDUCING CORROSION EROSION IN A HYDRAULIC VALVE (71) I, DAVID WILLIAM KIRKBRIDE, a citizen of the United States of America, of 706 North 44th Avenue, Yakima, Washington 98902, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of reducing corrosion erosion in a hydraulic valve, and a hydraulic valve so constructed.

It has recently been determined that, in high-pressure hydraulic systems, using phosphate-base hydraulic fluids, whether installed in airplanes, land vehicles, steam turbine controls, or elsewhere, the metering edges of metering passageways of their valves are'subjected to metal removal, because of severe electrokineticinduced corrosion erosion. When this electrokinetic-induced corrosion erosion occurs at the control or metering -edges of rrietal components of a hydraulic valve or other metering device in a hydraulic system, high leakage flows soon result, which often lead to a loss of control of the valve flow gain and pressure gain, instability of the mechanism under control, and destructive overheating of the entire hydraulic system.

Efforts to reduce the hazards of fire led to the use of these phosphate-base fluids in commercial airplanes, starting in 1958, and steam turbine control systems, in the early 1960's. As a consequence, in reference to airplanes, the personnel of British European Airways of England in 1965 began. experiencing increased internal leakage, when operating the new British-manufactured Trident airplanes, which employed'powered controls. It was found that extreme erosion was taking place at the metering edges of the servo valves and other hydraulic valves.

Chemists and physicists, who were experts in fluid flow, and also hydraulic engineers, from both Great Britain and the United States, all concluded, at that time in 1965 or thereafter, that this erosion was due to cavitation. For example, in a technical paper by W. Hamilton of the A.F.R.Ae.S. and the Technical Service Dept. of Hawker Sidley Aviation Ltd., published in the British periodical, Aircraft Engineering, December 1966, the problems, efforts and findings were described.

Water was added to the phosphate-based hydraulic fluids to cope with the extreme erosion problem. However this addition of water only reduced erosion in the presence of non-contaminated hydraulic fluids. When the hydraulic fluids became contaminated with chemical halides, chlorine and fluorine, the addition of water no longer reduced the erosion.

In 1967 the same problems were experienced in American-manufactured steam turbine control systems using phosphate-base hydraulic fluids. It was found that periodic filtering of these fluids through fullers Earth filters reduced the erosion to a rate that has been accepted, though it is not considered desirable.

Also in 1967, American-manufactured airplanes, using phosphate-base hydraulic fluids, were found to have developed the same erosion patterns. These erosion patterns and resulting prdblems are still continuing on today.

In 1969, scientists of The Boeing Company in Seattle, Washington, determined that the erosion patterns or problems were not caused by cavitation, but by electrochemical corrosion, now referred to as electrokinetic-induced corrosion erosion. Their findings are set forth in the Boeing Scientific Research Laboratory Report D1-82-0839, entitled Corrosion of Servo Valves by Electrokinetic Streaming Current, written by J. Olsen, T. R. Beck, and D. W. Mahaffey and issued in September 1969.

Moreover, it has been found that severe electrokinetic-induced corrosion erosion takes place when the hydraulic valves are nearly shut off in the null or neutral position and the fluid flow is small and at a high velocity. In addition, it was noted that the phenomenon of electrokinetic-induced corrosion erosion does not take place at all flow positions of the valve. At flows of hydraulic fluid beyond the null position or close to null position, and up to full flow, substantially no electrokinetic corrosion erosion takes place.

Since 1965, the efforts of engineers and scientists of more than 25 companies in the United States, Europe, and Japan have been involved in attempting to find a solution or solutions to reduce or eliminate the electrokinetic corrosion erosion taking place in the metering passageways of hydraulic valves and other metering devices controlling the flow of phosphate-base hydraulic fluids. During this time, and right up to the present moment, the specification and designs for hydraulic valves and other metering devices show square or rectangular, sharp-cornered, sharp-edge, orifices in the sleeves of the hydraulic valves, the slides moving to increase or decrease the controlled opening of the orifice or metering passageway.

The directions, set forth on the detailed drawings of at least two companies, for the fabrication of metering passageway structures, state that the metering edges must be sharp and free of burrs.

The designer, while thinking about this electrokinetic corrosion erosion problem, must still provide a servo valve that has good response. The valve cannot, under most conditions, have a flat spot or dead zone at its neutral position, because the valve, when operated, must react quickly to changes. The square or rectangular orifice of servo valves provides a lineal change of flow as the valve opens. The sharp edge of the orifice in the fixed sleeve and the sharp edge of the mating slide have generally provided the most satisfactory combination of shapes for the machinist to produce, machine, and trim, in order to provide the flow and the change of flow desired. To obtain the desired results, the metering edges of the slide. must mate with metering edges of the ports in the sleeve within 1 to 3 ten thousandths of an inch (0.00254 mm to 0.00762 mm).

Most design work on metering passages of hydraulic valves has been performed in the past to improve a valve's performance under high flow conditions and not under low quantity flow conditions at high velocities in and near the null position of hydraulic valves. The valve shapes have been adopted to reduce pressure drops under high flow conditions and not to improve low quantity flow conditions at high velocities in and near the null position of hydraulic valves.

Today, the severe results continue to occur because of electrokinetic-induced corrosion erosion of the valve components. In extreme breakdowns, the entire hydraulic system fails, when the maximum hydraulic pump capacity is needed to maintain the leakage flow, thus leaving no capacity for hydraulic control. Yet phosphate ester hydraulic fluids will continue to be used to reduce the hazards of potential fire and to reduce the dangers of those fires that do get started, especially in reference to airplanes. Consequently, this invention has been directed to the successful provision of metallic hydraulic valve components and methods of their manufacture, to create overall hydraulic systems in which electrokinetic-induced corrosion erosion failures are avoided. The success being realized centers on providing improved metallic hydraulic valve components to especially control the hydraulic flow, thereby avoiding severe electrokinetic streaming current and its generation of wall current which induces corrosion erosion.

The problem of the invention was to improve the prior valves, by reducing the electrochemical corrosion erosion phenomena, thus to increase the useful working life of the valve.

To improve the prior hydraulic valves, a quantitative analysis was made and a mathematical equation was developed and validated by comparing analytical results with test data presented by persons at The Boeing Company (refer to the experiments noted in The Boeing Company reference Dl-82-0839). The equation which was developed was::

ip=Wall current (ip)l=Wall current at a selected point away from the metering edge E=The relative dielectric constant EO=The absolute dielectric constant in a vacuum t=Electrokinetic potential Q=Flow through metering passage A=Minimum flow area at metering edge RJ=Potential Reynolds number upstream of the metering edge at a selected point away from the metering edge RO=Potential Reynolds number at metering edge 2.85=Exponent representing the power law relationship inherent in statistical analyses To perform calculations with this equation, some of the data for the terms is derived as set forth in the following description of the preferred embodiments.

Through studies of the parameters of this equation, the selective shapes for the metering passages and metering edges were developed and their dimensions were determined from multiple solutions of this equation. As a number of these selective shapes and a range of dimensions have been derived, it has been and will be possible to develop metering passageways, which will suppress the wall current in varying effective degrees, as the overall electrokinetic streaming current is controlled.

According to one aspect of the invention, there is provided a method of controlling the flow of phosphate ester based hydraulic fluid flowing at low flow -and high velocity through the critical region of a flow-channel immediately upstream of a metering point between a pair of relatively movable valve members when the members are in a null position to thereby suppress production of severe electrokinetic streaming current that induces electrochemical corrosion erosion that otherwise wears away sharp edges at said metering point, wherein one of said valve members has a parallel slide fit with a wall of the other member, said metering point comprising a point of minimum separation between said members and constituting a portion of said flow channel, and said members having opposed surfaces that define said flow channel, which method comprises moving one of said member relative to the other, said opposed surfaces being shaped to eliminate from each of such surfaces at said metering point sharp metering edges bounded by surfaces having an included angle of no more than 900, such opposed surfaces being shaped so that immediately adjacent to and upstream of said metering point the opposed surfaces converge with the included angle between said opposed shaped surface being not more than 45" thereby limiting production of streaming current and resulting erosion of said each of said surfaces at said metering point.

According to another aspect of the invention, there is provided a valve capable of controlling low flow at high velocity of phosphate ester fluid without inducement of significant corrosion erosion of metering edges of said valve, said valve comprising a first member having a chamber with a wall and a port through said wall forming a portion of a flow path through said valve, a second member having a land parallel to and movable relative to said wall with a slight clearance therebetween, said second member being movable to a null position for blocking flow through said port except for low flow at high velocity through said clearance, said land and wall having metering surfaces thereon on opposite sides of said clearance that establish a point of metering when the members are in the null position, said metering surfaces being of a minimum length of 5 thousandths of an inch and so shaped that when combined with an opposed surface of the other member they form portions of said flow path therebetween which flow path portions at a location immediately adjacent to the point of metering having included angles of not more than 45 , each portion of each metering surface having an included angle with an adjacent portion of the respective metering surface that is greater than 90".

In one embodiment of the valve of the invention, for metering the flow of phosphate ester-based fluid, in a sleeve, i.e. a non-moving member of the valve, the orifice of the intake passageway, which serves as the metering passageway, when viewed in a geometric plane parallel to the axis of the intake passageway, is formed with a rounded metering edge, and, when viewed in a geometric plane perpendicular to the axis of the intake passageway, is formed both with spaced parallel metering sides and arcuately curved concave ends, with transitional rounded tangential corners between them. Also in this embodiment, the slide or the moving member of the valve has a leading cylindrical circumferential edge, which serves as part of the hydraulic fluid metering passageway, such edge being formed with a rounded metering edge configuration.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a partial perspective view, with portions removed for illustrative purposes, illustrating a known hydraulic valve subassembly used in an airplane hydraulic control system, wherein a variable metering passageway defined by the relative motion of a slide within a sleeve has sharp metering edges, Figure 2 is a partial perspective view, with portions removed for illustrative purposes, illustrating a known hydraulic valve subassembly in another embodiment, in the form of a disc valve comprising two valve discs that are rotatable relative to one another, wherein a variable metering passageway defined by the relative motion of the discs has sharp edges, Figure 3 is a schematic cross-sectional view of part of a slide and sleeve servo valve of an airplane hydraulic control system, indicating the severe electrokinetic streaming current that creates the wall current and causes unwanted.electrokinetic corrosion erosion of the metering passageways of a valve when the valve is in its neutral position or nearly shut off, or during a very low flow rate, the positive and negative signs indicating the positions of ppsitive and negative ions, at a particular moment in the electrokinetic streaming current, Figure 4 is a view similar to Figure 1 but showing a first embodiment of the invention in which some of the metering edges of the metering passageways are contoured by rounding on both the sleeve and slide, the dotted lines indicating a projection of the metering passageway through the sleeve Figure 4A is a sectional view of part of the valve shown in Figure 4, to show more clearly the rounding of the metering edges, Figure 5 is a view similar to Figure 2 but showing a second embodiment of the invention in which some of the metering edges of the metering passageways are contoured by rounding on both the discs, Figure 6 is a view similar to Figure 4 showing a third embodiment of this invention, indicating how more of the metering edges of the metering passageways are rounded, beyond those illustrated in Figure 4, to also include the passageway through the sleeve, as illustrated with the dotted lines indicating a projection of the passageway through the sleeve, Figure 6A is a partial perspective view of a portion of the passageway through the sleeve shown in Figure 6, illustrating how the corners of the passageway are rounded, Figure 7 is a view similar to Figure 5 showing a fourth embodiment of this invention, indicating how more of the metering edges of the metering passageways are rounded, beyond those illustrated in Figure 5, to also include the passageway down through the top disc of the valve, Figure 8 is a partial perspective view and Figure 9 is a partial cross-sectional view, which together illustrate how the metering edges of the sleeve passageway are contoured by rounding;Figure 9 including reference lines to illustrate stream lines of the hydraulic fluid flow, the equal potential lines of such flow, the equal velocity lines of such flow, and a central reference line common to both views, Figure 10 is a view similar to Figure 6, illustrating a fifth embodiment of this invention, indicating how more of the metering passageway is contoured, beyond that shown in Figure 6, to also include an arcuate wall of the passageway through the sleeve, with the dotted lines indicating a projection of the passageway through the sleeve, Figure 11 is a view similar to Figure 7, illustrating a sixth embodiment of this invention, indicating how more of the metering passageway is contoured, beyond that shown in Figure 7, to also include an arcuate wall of the passagway through the valve disc, Figure 12 is a view in section, looking down into part of the passageway through the sleeve of Figure 10, to illustrate the arcuate side of the passageway, Figure 13 is a view in longitudinal section of a portion of the passageway through the sleeve of Figure 12 as viewed in a plane perpendicular to the plane of Figure 12, and also a portion of the slide, with the slide being shown in some of its positions of movement relative to the sleeve, i.e. the position of null opening, and the positions of further opening, Figure 14 is a graph showing the flow gain curve based on the flow rate through the valve, in relation to the opening of the metering passageway as determined by the slide positions of the structure of Figures 12 and 13, Figures 15 and 16 are views similar to Figures 13 and 14, showing a seventh embodiment in the form of a multiple-port, multiple-direction hydraulic valve, Figure 15 illustrating the contoured or rounded metering passageways between the sleeve and slide, and Figure 16 illustrating the multi-direction flow gain curve based on fluid flow rate through the valve, in relation to the valve slide positions during changes in the metering passageways, Figure 17 is a graph indicating how the wall current density fluctuates, when created in the presence of a severe electrokinetic streaming current, Figure 18 is a combined sectional view of part of sharp-edged sleeve and the adjacent land of a slide of a hydraulic valve which is now used in airplanes, with a graph illustrating both the equal potential or equal velocity curves, and the stream lines, Figure 19 is a view which is similar to Figure 9 and which is to be used in comparison with Figure 18, and illustrates the rounded edges of the sleeve and the adjacent land of a slide of a hydraulic valve according to the invention, with a graph illustrating both the equal potential or equal velocity curves, and the stream lines, Figure 20 is a graph indicating, by comparison, the improvement in the fluid flow, with respect to eliminating the basis of the severe electrokinetic streaming current and its accompanying wall current, in relation to the relative variation in the Reynolds number at various distances away from an orifice configuration of a metering passageway, the top curve being representative of a metering passageway with sharp edges and the lower curve being representative of a passageway with contoured or rounded edges, Figure 21 is a graph which was derived by the present applicant, indicating how the wall current density varies in relation to the variation in distance away from the respective orifice openings of a metering passageway having sharp edges and corners, Figure 22 is a graph, similar in purpose to the graph of Figure 21, also in relation to a valve with sharp edges and corners showing how the applicant's Figure 21 graph compares with the Figure 22 graph which was derived from the data presented by the above-discussed research by The Boeing Company;Figure 22 also shows at the bottom of the graph, the line showing the minimum wall current density generated by the contoured or rounded metering passageway of the invention; wherein the flow conditions for the graphs of Figures 21 and 22 were the same in respect of the differential pressures across the orifice openings of the metering passageways and also in respect of the flow quantities of hydraulic fluid approaching these orifice openings, Figure 23 is a graph illustrating hydraulic fluid leakage rates at various times during a test period, with a first curve indicating the leakage rates occurring in a known hydraulic valve having sharp edge and corners in its metering passageway, and a second curve indicating the leakage rates in hydraulic valve of this invention having contoured or rounded edges and corners in its metering passageway, Figures 24 to 28 are partial schematic views showing how the edges and corners in hydraulic valves of this invention may be shaped, using respectively, a single bevel, a multiple bevel, a bevel and radius, a combination of radii, and a compound curve, and Figure 29 is a view of part of a full flow hydraulic valve, where hydraulic fluid after leaving. an inflow passageway enters a surrounding distribution chamber having its discharge edges contoured in a like manner with the edges of the land on the slide.

The applicant, in working toward a solution of eliminating the electrokineticinduced corrosion erosion of hydraulic valves handling phosphate-base hydraulic fluids, came to an early conclusion in his studies that in order, first, to understand and to determine quantitatively the operating conditions when this corrosion erosion of the metal valve components at their metering passageways occurred, and, secondly, thereafter, to determine how to favourably change these operating conditions to reduce or to prevent corrosion erosion, he must first derive and complete a quantitative analysis of the hydraulic fluid flow, its shear, and its electrokinetic streaming current with its accompanying wall current. Thereafter he derived an equation.Using the analysis and the equation, he designed the various embodiments of his hydraulic valve to obtain much better operating periods of his hydraulic valves, with these periods increasing as more of the former sharp edges and corners of the then currently used hydraulic valves were reformed or created originally, to be contoured differently using radii, bevels, and curves.

Quantitative Analyses The electrokinetic-induced corrosion erosion in the presence of the phosphate-base hydraulic fluid takes place when a hydraulic valve is at its null or neutral position or close thereto at a low quantity flow rate and very high velocity, when the opening through the metering passageway is quite small. The Boeing Company Scientists Messrs. Olson, Beck, and Mahaffey, as part of their studies, developed a two-dimensional metering passage that simulates a valve at its null position. This is shown in Figure 5-1, page 5-Il, of The Boeing Company reference 2(D1-82-0838) entitled "Corrosion of Servo Valves by an Electrokinetic Streaming Current". Tests producing erosion were made and the amount of metal removed was measured. The electrical current required to remove the metal was determined.The results of the study are shown in Figure 11-4, page 11-105, of this same reference. The data provided an excellent yardstick that was used to measure or determine the accuracy and worth of the results of the quantitative analyses made by the applicant. Because the data was available and would provide a good practical check, the applicant used the dimensions of the metering passageway developed by these Boeing scientists, and also the same hydraulic operating conditions of pressure and flow used in their experiments, as the bases for his quantitative analyses.

Quantitative Analysis Theories of Electrokinetic Streaming Current and its Associated Electrical Wall Current In respect to electrokinetic streaming current, the production of its associated electrical wall current is inversely proportional to the electrical conductivity of the phosphate-base hydraulic fluid. The following electrical parameters are used in accounting for this relationship.

g Electrokinetic potential E The relative dielectric constant tO Absolute dielectric constant in a vacuum ip Wall current.

The wall current ip is found by combining the electrical parameters noted above with the following set of fluid flow parameters.

ip=f(EEoNlX) The role that the fluid parameters play is best shown by taking the total differential of the ip (wall current).

df df dip=d(EEof)+ d(X) d(EEof) d(X) It is to be noted, d(EEog)=0 when only one fluid is used.

This relationship points out, that in the quantitative analyses to determine wall current and the corrosion erosion, the electrical parameters remain constant, and only the flow parameters are the variables.

This relationship also shows that in order to solve the equation ip=f(EEo{X), the set of-flow parameters, "X", must be defined and the relating function determined.

From a study of textbooks and technical papers discussing electrokinetic streaming current, this key reference text was selected: "Electrostatics in the Petroleum Industry" Elsevier New York 1958 Edited by A. Klinkenberg and J. van der Minne.

This text by Messrs. Klinkenberg and van der Minne confirms the conclusions stated above.

In studying flow in the metering passage, the applicant noted that the hydraulic fluid flow, as it approaches the metering edges, is accelerating at a high rate and the state of flow changes from laminar to turbulent. In this key reference, Messrs. Klinkenberg and van der Minne discuss the effects of turbulent flow on their page 47, where they say: therefore as long as the electric double layer is confined with a region of laminar fluid flow, the streaming electrical current is proportional to the pressure gradient, and no detailed knowledge of the potential distribution within the double layer is . needed. The problem becomes more complicated for hydrocarbon liquids in turbulent flow, where the laminar sublayer fluid flow may become thinner than the electric double layer.

Messrs. Klinkenberg and van der Minne, further note, on their page 55, that their calculations and equation show that, with turbulent hydraulic fluid flow, the boundary layer of the hydraulic fluid flow-is, under certain flow conditions, of a thickness much less than the thickness of the electrical double layer. Beyond this text of Messrs. Klinkenberg and van der Minne, the applicant has not found a treatise or scientific paper dealing with turbulent hydraulic fluid flow and the resulting electrical effects with respect to reference to hydraulic valves. Therefore from the scientific data available, the applicant determined the following hypotheses: In making a quantitative analysis, the hypotheses regarding the electrokinetic streaming current and its related wall current are as follows: 1.The distribution quantity of the mobile charges entrained in a turbulent hydraulic flow regime is considered a random variable, which is dependent upon the turbulence; 2. The electrical potential developed between a bound charge and a given region within the hydraulic fluid flow path is directly proportional to the mobile charge density of the region and inversely proportional to- the distance between the region and the bound charge; 3. The magnitude of the electrical potential between the bound charge and opposite charges within the wall is inversely proportional to the magnitude of the electrical potential between the bound charges and regions of mobile charges within the hydraulic fluid flow; and 4. The electric wall current density is a function of the electrical potential between the bound charges and the opposite charges within the wall.

In making quantitative analyses, it is possible to arrive at a statistical theory regarding the wall current density: The wall current density is inversely proportional to the bound charge density for a given region within the hydraulic fluid flow and directly proportional to the distance between the wall and the region.

Therefore, the magnitude of the electrical wall current density varies directly with the random distribution of mobile charges.

This preceding equation is the source of the following equation used in presenting a statistical distinct average, exhibiting a random variation with time and space, which is directly dependent on flow conditions.

Tp=Time average wall current density Tl=Time large enough so that tp is the same for any larger time for steady mean flow ip=Instantaneous wall current density i'p=Fluctuating wall current density Therefore ip=Tp+i'p Figure 17 shows the wall current density ip as a function of time at a fixed location at a wall. Figure 17 further indicates that the instantaneous wall current density is a random quantity, which is superimposed on the mean wall current density. The variation in the instantaneous wall current density is directly proportional to the turbulent hydraulic fluid flow.

Previously, a statistical relationship was developed, from which a family of partial differential equations may be derived. However, the usefulness of these equations from an engineering point of view is somewhat restricted. There would be no way to solve these equations even for simple flows. Thus, a more direct approach, utilising a flow model, which may be somewhat physically inexact, but which allows for approximating solutions, was undertaken. The selected hydraulic fluid flow model provided a method for calculating the effect of turbulent hydraulic fluid flow on the deviation of the instantaneous wall current density from the mean or average wall current density, and therefore it consequently accounted for this average wall current density. The hydraulic fluid flow model also reflected the hydraulic fluid flow channel shape and its effect on the wall current densities.

A hydraulic fluid flow model was developed using the analytical tools developed in accordance with the potential flow theory, which in turn employed the conformal mapping shown in Figures 9, 18, and 19, to correlate changes in the hydraulic fluid flow and changes in the shape of the passageways or channels and their related metering passageways. In reference to these figures, the applicant, in developing or deriving the potential Reynolds number, employed the lengths of arcs of equal potential or equal velocity which interconnected to two boundaries of the fluid flow metering passageway. A nominal velocity factor was used, based on the absolute fluid flow through a channel.

(EEOt)=Electrical parameters (previously described), Q Nominal velocity factor, A R,=Potential Reynolds Number upstream of the metering edge, R0=Potential Reynolds Number at the metering edge, ,B=Exponent representing the power law relationship inherent in statistical analyses.

The exponent is described, along with the method of determining its value, in the text Electronic Components and Measurement, printed by Prentice Hall, and written by Bruce D. Wedlock and James K. Roberge. They stated in their third chapter the following: "On log. log. coordinates, the functional form Y=AXA will plot as a straight line if A and p are constant". Therefore the value of fi to be used in the flow model analysis was determined from the experimental data in the above-quoted Boeing Company reference. The value for p was determined as 2.85.

The mathematical equation, therefore, used in the quantitative analyses

Employing the shape and dimensions of the two dimensional metering passage developed by the Boeing Scientists, with a diameter of 0.09 inches (2.286 mm), a gap of 0.001 inches (0.00254 mm) and a flow of 900 cubic centimeters per minute, a conformal mapping plot or graph was developed by the applicant as illustrated in Figure 9. Using data from this plot, the Reynolds numbers were determined. For presentation and use, the Reynolds number of the hydraulic fluid flow approaching the particular metering edge is normalised to the flow just passing the metering edge or in the gap. The Reynolds number in the gap, R0 is set at unity, i.e. one. The Reynolds number of the flow approaching this metering edge, Rj, is noted as the number of times it is greater or less than unity. Figure 20 shows the ratio of the Reynolds numbers, RI R0 at points in the hydraulic fluid flow paths approaching this metering edge.

The wall current associated with the severe electrokinetic streaming current may now be calculated, using: the mathematical model developed; the Reynolds numbers ratio from Figure 9; the electrical constants employed by the Boeing scientists as referred to above; and the nominal velocity factor determined from the area and flow test data also recorded by the Boeing scientists. The wall current determined by the- equation previously set forth is shown in Figure 21.

To compare the theoretical data of the quantitative analyses of the applicant, with the experimental test data of the Boeing scientists, the data presented in Figure 21 is superimposed on the Boeing data and presented in Figure 22.

Attention is drawn to point out the closeness of the relationship of the theoretical data to the experimental data.

The applicant considers his theoretical analyses as an excellent engineering tool to use in developing a shape for a metering passage that will control the hydraulic fluid flow through a valve, thereby reducing or eliminating the erosion. It is then readily observed that the Reynolds number and rate of change of the potential Reynolds number of the hydraulic fluid flow approaching this metering edge of a valve must be reduced. This also therefore means the rate of change of the velocity of the hydraulic fluid approaching the point of metering must be reduced. Such a reduction is accomplished by increasing the length of the fluid flow channel through which the final reduction in the area of the cross-section of the channel takes place as it approaches the point of metering.This quantitative analysis shows very clearly that any change that increases the length of the flow channel adjacent to the metering edge will reduce the rate of change of the velocity of the hydraulic fluid flow and this reduction in turn will change and reduce the electrokinetic streaming current and thereby will reduce the wall current which causes the corrosive erosion of the metering passageways.

The above-mentioned increase in channel length may be obtained by beveling the metering edge: with a single bevel; or a series of bevels; or a combination of bevels and radii; or a single radius; or a compound curve. The most efficient way to accomplish thisis to radius the metering edge or edges. The maximum gain in reduction of wall current is obtained using a radius greater than 0.007 inches (0.1778 mm) or greater, although gains are realised from .003 (0.0762 mm) to .007 inch (0.1778 mm). The Boeing Company Document DI-82-0839 indicated that erosion was initiated seven thousandths upstream of the point of metering.

Figures 9 and 19 are conformal mapping plots of metering passages with a radiused metering edge. Figure 20 presents the data of the potential Reynolds numbers ratio that may be attained. The wall current resulting from this shape is presented in Figure 22. Also included is the data from Figure 21 for easy comparison. As will be noted, there is a very marked reduction in wall current, which means that there is a very significant reduction in or elimination of electrokinetic corrosion erosion. Once a radiused surface structure of ten thousandths of an inch is reached, for all practical purposes, as indicated in these Figures 20 and 22 and elsewhere, no additional useful erosion resistance or avoidance benefits would be realized if a larger radius were to be used.In reference to the respective sized resulting shaped surface structures that are formed leading to the metering cross-section at the null position, if the shaped contour is on a radius of .003 inches, the resulting shaped surface is the arc of a quarter of a circle having a length of .005 inches. If the shaped contour is on a radius of .01 inches, the resulting shaped surface is the arc of a quarter of a circle having a length of .016 inches. The practical length range of these improved shaped surface structures is considered to be within the range of .005 to .020 inches, in respect to improving the flow channel and avoiding the corrosion erosion. The equations developed by the applicant, as noted, are for rounded metering passages, with a smooth surface finish.

It will be commented at this stage that the turbulence in the hydraulic fluid flow in rectangular sharp-cornered orifices which are currently used in airplane hydraulic valves and which are machined by electrical discharge, is magnified by the shape of the orifice and the rough surface resulting from the electrical discharge machining. The flow in the regions of the sharp corners is disturbed by the interaction of the vertical forces in the flow from the intersecting surfaces at the corners. Valves in service at the present time experience higher rates of erosion in the regions of the sharp corners, than at the center of a metering edge.

The increased turbulence of concern at the sharp corners is produced by the hydraulic fluid flow occurring in the region up to 0.005 inches (0.127 mm) from the metering edge. The interaction of the flow forces at the intersection of the surfaces at these sharp corners, produces greater turbulence than at the center of the metering edge. These flow forces are semi-isotropic in direction. They are indeterminate in detail finite numbers. The major troublesome force is that which is perpendicular to the wall surface. If this force is made small enough, it may be assumed that the total force will not significantly affect the turbulence. When a sharp corner is replaced by a rounded corner of constant radius, the vertical forces will intersect at the center line passing through the point of equal distance from the surface, or the center line of the radius.

The cross-section for each vertical plane may be shown by conformal mapping. Figure 9 shows a conformal map for a metering passage with a 0.005 inch (0.127 mm) radius inside corner and a 0.007 inch (0.1778 mm) radius metering edge.

With this combination at the point of intersection, the velocity is reduced to one fortieth (1/40) of the velocity at the metering edge. The flow forces at this relatively low velocity will be so small, that the effect on the turbulence will be negligible, and consequently any additional corrosive erosive wall current will not occur or will be negligible.

Reference is now made to Figures 1 and 2 which show known hydraulic valves.

In Figure 1, a hydraulic valve 30 comprises a stationary sleeve 36 and a movable slide 42 with metering land 40 and actuating rod 44. Numeral 34 denotes an intake fluid flow passageway with sharp exit edges 32 at opening 35 into the bore 43 of the sleeve. Leading edge 38 of land 40 is also a sharp edge. In Figure 2, a flat plate or platen valve 50 has a top platen 56 with an inflow passageway 54 with sharp edges 52 and a lower platen 62 with an outflow passageway with sharp edges 58. The projection of the outline 58 of the passageway 60 is shown at 58'.

In contrast to the sharp edges and corners of the hydraulic valves in Figures 1 and 2, the various embodiments of this invention that are illustrated in the Figures 4, 4A, 5, 6, 6A, -7, 10, 11, 15, 24, 25, 26, 27, 28 and 29, do not use sharp corners or sharp edges. Reference numerals in the figures of the invention denote components similar to those of Figures 1 and 2. As indicated previously, the maximum gain in the reduction of wall current is obtained using a radius of 0.007 inch (0.1778 mm) or greater. Figures 4 and 4A show a first embodiment of a hydraulic fluid flow control.

In a hydraulic valve 30, exit rounded contour 32 has a 0.007 inch (0.1778 mm) radius in the inflow hydraulic fluid passageway or port 34, which is in turn formed in a sleeve or body 36. Also, the leading contour 38 has the 0.007 inch (0.1778 mm) radius, on a land 40 of the slide 42 which is moved by control rod 44. Although the maximum gain in the reduction of wall current is obtained using a radius of 0.007 inch (0.1778 mm) or greater, it is to be understood that worthwhile gains in reducing wall current are obtained when radii in the range of 0.003 inch (0.0762 mm) to 0.007 inch (0.1778 mm) are utilised in forming the edges of the metering passageways. Reference 32' denotes the projection of the edges 32 of the passageway 34.

Also as illustrated in Figure 5, the same changes may be made in a platen or flat plate hydraulic valve 50. Exit edges 52 of the inflow hydraulic fluid passageway 54 in a top platen 56, and the entry edges 58 of an outflow hydraulic fluid passageway 60 in a bottom platen 62, are all formed with a radius, preferably a radius of 0.007 inch (0.1778 mm), or in the range of 0.003 to 0.007 inch (0.0762 to 0.1778 mm).

Figures 6 and 7 show further embodiments of the hydraulic fluid flow control, which has even better control over the possibility of electrokinetic corrosion erosion. In addition to the portions formed with a radius as shown in Figures 4 and 5, more portions are rounded, to provide better control of the flow. All the passageways 34, 54 and 60 have no sharp corners, as a radius, preferably 0.005 inch, is followed when they are formed. This creates the rounded corner structures 46 shown in Figures 6 and 6A, and the rounded corner structures 64 shown in Figure 7. References 32' and 46' denoted the projection of the edges 32 and 46.

Figures 8 and 9 illustrate in more detail how the Figures 6 and 7 embodiment is made to include edges having a preferable radius of 0.007 inch (0.1778 mm), and corners having a preferable radius of 0.005 inch (0.127 mm), or in the range of 0.003 to 0.005 inch (0.0762 mm to 0.127 mm).

Figures' 10 and 11 are somewhat similar to Figures 6 and 7 and show yet further embodiments which provide the control necessary to eliminate electrokinetic corrosion erosion which would otherwise be caused during a flow of phosphate ester-base hydraulic fluids. The leading edge of each metering passageway is in the form of a convex arcuate portion 48 in Figure 10, and a like portion 66 in Figure I l; this convex arc extends to become tangent at each of its ends to the respective corner radius structures 46 and 64.

In using the Figures 10 and 11 embodiment, three improvements are obtained: The first improvement is due to the geometry of the arc portions 48 and 66 which serve as metering edges and their cooperation with the non-arcuate planar vertical surface 43 of the slide 42 adjacent the land 40 in Figure 10, or the nonarcuate planar surface of the opening in the movable top platen 56 in Figure Il; The second improvement will be described with reference to Figures 12 and 13 showing enlarged portions of valves 30 or 50; side wall flow turbulence is reduced by employing the geometry viewed in Figure 13 in shaping the valve components.

Figure 13 shows that the angle or radius of the leading edge (reference 38 in Figure 10) of the slide land (40 in Figure 10) is increased with respect to the vertical convex arc surface (48 in Figure 10) of the inflow passageway, until it is at a tangent to the radii of the round corner structure 46; The third improvement lies in the three-dimensional geometry of valve 30 in Figure 10; this geometry comprises the combination of the arched leading surface 48 of the passageway 34, the rounded metering edges 32 at the opening 35 of the passageway 34, and the rounded metering edges 38 between the land 40 and planar surface 43 of the slide 42.

The cooperation of these geometry features results in the optimum flow gain performance illustrated in Figure 14. Moreover, as such cooperation occurs during operation of the servo hydraulic valve 70 as illustrated in Figure 15, the overall dual direction flow gain plot of Figure 16 becomes a straight line through the neutral valve position. This is indicative of the most desirable performance sought in the operation of a hydraulic servo valve, especially in an aircraft hydraulic control system.

In contrast, as noted previously, in order for the aircraft hydraulic servo valves which are used at the present time to match this performance while utilising their sharp edge and sharp corners, the distance between the metering edges of the slide must be trimmed, by machining, within a tolerance 1/10,000 inch (0.00254 mm). In contast, in utilising valve components incorporating the above-mentioned threedimensional geometry of the Figures 10 and 11 embodiment, the machining tolerances can be ten times larger, i.e. 1/100 inch (0.0254 mm), and consequently easier to achieve.

~ ~~ Employing the same flow conditions in regard to quantity, pressure and temperature, of a phosphate ester-base hydraulic fluid, a passageway incorporating the known sharp edges and sharp corners was comparatively tested with a passageway- according to the present invention having edges and corners with a radius, with the leading side of the inflow passageway having a convex- arc portion tangent to the radius of the corners (as in Figures 10 and 11). The results of this comparison are illustrated in Figure 23 in which the dotted lines show a valve with sharp edges and corners and the solid line represents a valve with rounded edge and corners.

Electrokinetic corrosion erosion of the sharp-edged and sharp-cornered passageway continued, causing excessive leakage in less than three hundred hours of operation. In contrast, electrokinetic corrosion erosion of the metering passageway of the invention, with radiused edges and corners and the abovementioned convex arc portion, occurred for a short time but then stopped, so that leakage remained constant and at a low value. The constant leakage occurring in this way remained fully within the desirable limits for excellent operation and continued to remain at the set level of minor leakage, as shown in Figure 23.

Although the edges and corners of the passageway have been described above as being formed with a constant radius, Figures 24 to 28 illustrate that these edges and corners may be formed slightly differently, while still gaining the operational advantages of the metering passageways of the invention, which are not subject to excessive electrokinetic corrosion erosion.

Figure 24 shows a single bevel, and Figure 25 shows a multiple bevel. Figure 26 shows a combination bevel and radius, and Figure 27 shows the combination of radii. Figure 28 shows a compound curve. Each of these embodiments improves the fluid flow so the electrokinetic streaming current and its associated wall current are sufficiently controlled, so that electrokinetic corrosion erosion is controlled, limited, or eliminated. When the valve of the invention is used in hydraulic control systems of airplanes, the longer successful operating hours achieved by using the valve of the invention will materially add to the safety of the flight operations and reduce the costs of maintenance.

Figure 29 shows part of a full flow hydraulic valve 72 where the hydraulic fluid on leaving the inflow passageway 34 enters a surrounding distribution chamber 74 which has discharged edges 76 contoured in like manner with the edges 78 on the slide 42. These edges 76 and 78, during the low fluid flow, increase the length of the hydraulic fluid flow channel as referred to above, to reduce corrosion erosion

Claims (14)

WHAT I CLAIM IS:

1. A method of controlling the flow of phosphate ester based hydraulic fluid flowing at low flow and high velocity through the critical region of a flow channel immediately upstream of a metering point between a pair of relatively movable valve members when the members are in a null position to thereby suppress production of severe electrokinetic streaming current that induces electrochemical corrosion erosion that otherwise wears away sharp edges at said metering point, wherein one of said valve members has a parallel slide fit with a wall of the other member, said metering point comprising a point of minimum separation between said members and constituting a portion of said flow channel, and said members having opposed surfaces that define said flow channel, which method comprises moving one of said members relative to the other, said opposed surfaces being shaped to eliminate from each of such surfaces at said metering point sharp metering edges bounded by surfaces having an included angle of no more than 900, such opposed surfaces being shaped so that immediately adjacent to and upstream of said metering point the opposed surfaces converge with the included angle between said opposed shaped surfaces being not more than 45" thereby limiting production of streaming current and resulting erosion of said each of said surfaces at said metering point.

2. A method as claimed in Claim 1, in which one of said opposed surfaces opposes the other of said opposed surfaces over a length of from .005 inch to .020 inch.

3. A valve capable of controlling low flow at high velocity of phosphate ester fluid without inducement of significant corrosion erosion of metering edges of said valve, said valve comprising a first member having a chamber with a wall and a port through said wall forming a portion of a flow path through said valve, a second member having a land parallel to and movable relative to said wall with a slight clearance therebetween, said second member being movable to a null position for blocking flow through said port except for low flow at high velocity through said clearance, said land and wall having metering surfaces thereon on opposite sides of said clearance that establish a point of metering when the members are in the null position, said metering surfaces being of a minimum length of 5 thousandths of an inch and so shaped that when combined with an opposed surface of the other member they form portions of said flow path therebetween which flow path portions at a location immediately adjacent to the point of metering have included angles of not more than 450, each portion of each metering surface having an included angle with an adjacent portion of the respective metering surface that is greater than 90".

4. The valve as claimed in Claim 3 in which one of said metering surfaces has a radius of between .003 inch and .012 inch.

5. The valve as claimed in Claim 3 or 4 in which one of said metering surfaces opposes the other of said meteiing surfaces over a length of from .005 inch to .020 inch.

6. The valve as claimed in Claim 3, 4 or 5 in which at least one of said shaped surfaces is in the form of a bevel.

7. The valve as claimed in Claim 3, 4 or 5 in which the shape of at least one of said shaped surfaces comprise a series of bevels at different angles.

8. The valve as claimed in Claim 3, 4 or 5 in which the shape of at least one of said shaped surfaces comprises a compound curve.

9. The valve as claimed in any -one of Claims 3 to 8 in which each of said metering surfaces is so shaped that no portion thereof has an included angle with any other adjacent surface portion of the same metering surface that is less than 1150.

10. The valve as claimed in any one of-Claims 3 to 9, in which the internal corners of the port through the first member are formed on a radius of a minimum of .003 inch.

11. The valve as claimed in any one of Claims 3 to 10 in which said land and said wall are circular and one of said shaped metering surfaces extends circumferentially about said land.

12. A method of controlling the flow of phosphate ester based hydraulic fluid flowing at low flow and high velocity through the critical region of a flow channel immediately upstream of a metering point between a pair of relatively movable valve members when the members are in a null position to thereby suppress production of severe - electrokinetic streaming current that induces electrochemical corrosion erosion that otherwise wears away sharp edges at said metering point, wherein one of said valve members has a parallel slide fit with a wall of the other member, said metering point comprising a point of minimum separation between said members and constituting a portion of said flow channel, and said members having opposed surfaces that define said flow channel, which method comprises moving one of said members relative to the other, said opposed surfaces being shaped to eliminate from each of such surfaces at said metering point sharp metering edges bounded by surfaces having an included angle of no more than 90 , such opposed surfaces being shaped so that immediately adjacent to and upstream of said metering point the opposed surfaces converge with the included angle between said opposed shaped surfaces being not more than 45" and the flow channel formed by said shaped surfaces is such that the rate of change of the velocity of the fluid approaching the said point of metering will be lower than would occur in a flow channel leading to sharp metering edges bounded by surfaces having an included angle of 90" thereby limiting production of streaming current and resulting erosion of said each of said surfaces at said metering point.

13. A method as claimed in Claim I substantially as hereinbefore described with reference to any of Figures 3 to 29 of the accompanying drawings.

14. A hydraulic valve substantially as hereinbefore described with reference to any of Figures 3 to 29 of the accompanying drawings.