GB2278185A - Fluid pressure regulators - Google Patents

Abstract

A fluid pressure regulator provides features which enable an extended regulator life in high pressure service. Dual flexible diaphragms (35, 40) provide chemical resistance in contact with the working fluid and abrasion and fatigue resistance in contact with a backup plate (4) and piston assembly (5). Provision is made for preventing fretting corrosion between the piston (5) and the backup plate (4) by coating at least one of the surfaces, if necessary, to assure that dissimilar metals only are in contact in that couple. <IMAGE>

Description

FLUID PRESSURE REGULATORS This invention relates to fluid pressure regulators and more particularly to high pressure fluid pressure regulation.

Dispensing fluids for industrial applications requires accurate control of pressure in order to provide accurate distribution of the fluids in the process. For paints, adhesives and other high viscosity fluids, distribution pressures of approximately 3,000 psi (2,109,300 kg/m2) are frequently required. This has become increasingly true as suppliers of these fluids have minimised the solvent content of these mixtures in response to demands for reduction in health and fire hazards in the workplace. As solvent contents have decreased, the abrasive nature of the suspended solids has become more significant and has begun to adversely affect the service life of the fluid pressure regulators employed in the system.

Typically, a fluid pressure regulator consists of an inlet, an outlet and a valve placed in the connecting path between the inlet and outlet. A valve closure element is usually biassed against the valve seat and is controlled by a stem or other mechanism which is, in turn, adjustably biassed counter to the closure element by means of a spring acting on a diaphragm and/or piston which enables the regulator to maintain a constant outlet pressure despite fluctuations in inlet pressure. The piston is adjustably spring biassed and is reciprocable within a cylindrical bore in the regulator cover plate. Without a diaphragm, the bore requires a circumferential lip seal in order to prevent leakage of the fluid between the piston and the bore.To function properly against such a seal, the piston requires a very fine finish of the order of 10 microinches (10 ym) or less.

Such a finish is expensive to produce and is very easily damages by corrosion or mechanical injury. Moreover, in the presence of highly abrasive low solvent suspensions, both the seal and the piston finish deteriorate due to sliding contact.

For high pressures, a combination of diaphragm with piston provides more positive sealing. Durability of the diaphragm compared to the lip seal is generally superior since the diaphragm is exposed to flexure rather than sliding wear.

Reduction of the solvent content has increased the viscosity of the working materials so that they require higher pumping pressures and, consequently, regulators designed for those pressures. Regulators which were designed to perform in the range of 1000 psi to 1500 psi (703,100 - 1,054,650 kg/m2) experienced short service life using the high solids/low solvent materials presently available. Increasing wall thicknesses and spring stiffness alone is not sufficient to upgrade a medium pressure regulator for use in the 3000 psi (2,109,300 kg/m2) range of service pressures commonly encountered.

Typically, high pressure regulators employ a diaphragm as well as a piston in a bore of the diaphragm backup plate.

To protect the diaphragm from cutting, the edges of the piston and the bore of the backup plate are commonly given a radius. The diaphragm commonly consists of fabric mesh reinforced rubber for flexibility and a layer, bonded onto the pumped fluid side, of a chemically resistant material.

"O" ring seals are commonly used between the diaphragm, the stem, and the regulator housing.

These features are illustrated in Figure 1 which presents a cross-sectional view of the diaphragm/piston interfacial area of a typical prior art regulator. The stem 6 and the piston 5 are bolted together to capture the one piece bonded diaphragm 1 and "O" ring seal 7 between them.

Diaphragm 1 is composed of a fabric reinforced rubber layer 3 and a chemical resistant layer 2. Piston 5 reciprocates within a bore 8 of backup plate 4. For ease of assembly, a chamfer 11 is provided as well as the relatively loose fit between bore 8 and piston 5 which are also radiused as previously described. "Oe ring 9 provides a seal between diaphragm 1 and housing 14.

In high pressure service, this regulator will potentially experience the following problems: 1. The short guide length of the bore between the chamfer and radius on the backup plate can allow the piston to cock slightly within the bore, and can thus permit localised high pressure contact between piston and backup plate and non-uniform flexure of the diaphragm; 2. The large gap, being the diaphragm, provided by the radii of the backup plate and the piston, allows wedging of the diaphragm into the large gap and consequent excessive abrasion and flexural wear on the diaphragm; 3. The relatively thin backup plate is subject to slight deflection due to high pressure and thus leakage of the working fluid around the seal between the housing and the diaphragm, and 4.The one piece bonded diaphragm, because of the different flexural moduli of the two layers, experiences intensified local stresses which cause early failure; 5. In cases where a resonance causes vibration of the piston within the backup plate bore, fretting corrosion may become a significant problem and thus exacerbate the other shortcomings of this design in a high pressure application.

According to the present invention, there is provided a fluid pressure regulator comprising: a housing body having a fluid inlet, a fluid outlet, and a fluid passage communicating therebetween; a variable valve positioned within said fluid passage and having a valve stem, an orificed seat and a closure element biassed toward said orificed seat; means for adjusting a clearance between said seat and said closure element in order to control fluid outlet pressure;; means, including first and second separate, imperforate, radially coextensive diaphragms in mutual service contact, for maintaining a substantially constant outlet pressure which is independent of fluctuations in fluid inlet pressure, and a piston reciprocably movable within a bore of a housing body cover, wherein either the bore of the housing body cover or the outer cylindrical surface of the piston is provided with a coating for prevention of fretting corrosion of surfaces, which coating is a plated metal coating so that contact is only between dissimilar metals.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to Figures 2 and 3 of the accompanying drawings, in which: Figure 2 is a cross-sectional side view of one form of a high pressure fluid regulator according to the present invention; Figure 3 is a fragmentary cross-sectional view to illustrate detail of important features of the regulator.

Figure 2 shows a high pressure regulator 20 of the present invention. It comprises a housing body 14, a housing cover which also serves as diaphragm backup plate 4, a bonnet 60, a diaphragm tensioning spring 64 bounded at the top and bottom by spring keepers 66 and 62, bonnet bolts 68 and an adjusting screw 70. The bonnet bolts 68 secure the bonnet 60 through the diaphragm backup plate 4 and are threaded into the housing body 14. The plate 4 is also secured to the housing body 14 by backup plate bolts 22. The piston 5 is secured to the spring keeper 62 and stem 6 such that a diaphragm assembly 30 is trapped between the piston 5 and stem 6. non rings 7 and 9 provide fluid seals between the diaphragm assembly 30 and stem 6 and diaphragm assembly 30 and housing body 14, respectively. "O" rings are used in this example but it should be understood that any appropriate seal ring arrangement is satisfactory.

High pressure fluid enters the regulator through an inlet 15, passes through a variable valve 17 which comprises a closure element 26 and valve seat 27, passes around the stem 6 and exits through an outlet 16. Pressure regulation is accomplished by turning the adjusting screw 70 to compress the diaphragm tensioning spring 64. This drives the spring keeper 66 downward so as to increase the spring pressure on the spring keeper 62 which forces the piston 5 and rigidly connected stem 6 downward to displace the closure element 26 from the seat 27, thus opening the variable valve 17. The system pressure downstream of the valve 17 is inversely proportional to the amount of pressure drop through the valve 17. This downstream pressure acts upon the diaphragm assembly 30 and piston 5 to force them upward counter to the direction of force exerted by the diaphragm tensioning spring 64.Thus, after a very brief "hunting" interval, the two opposing forces are balanced and steady state operation is achieved. So long as the inlet pressure remains constant, and so long as the outlet demand remains constant, the positions of all these movable elements remain stationary. Should inlet pressure decrease, the resulting decrease in downstream pressure will cause piston and stem to move downward, thereby further opening valve 17 and restoring the outlet pressure to the set point. The converse is true for increases of inlet pressure as well. Desired pressures are set by turning the adjusting screw 70 to force the upper spring keeper 66 downward to increase pressure and to pull the keeper 66 upward to decrease pressure.

Referring to Figure 3, the diaphragm assembly 30 comprises a backup member which is a flexible elastomeric diaphragm 35 lying against the piston 5 and backup plate 4, and a chemically resistant diaphragm 40 lying against the flexible diaphragm 35 and shielding it from any corrosive properties of the working fluid. It should be noted that the diaphragms 35 and 40 are not bonded together but are radially coextensive and in mutual flat surface contact with each other. This non-bonded contact improves the flexibility of the diaphragm assembly by permitting limited slippage between the diaphragms during flexure. This reduces the tensile and compressive stresses experienced by the diaphragm surfaces and thus improves the fatigue lives of the diaphragms.A gently tapered relief 16 on the backup plate 4 around the circumference of the bore 8 together with a small radius 18 on the piston 5 combine to produce a very small gap behind the diaphragm 35 and they thus permit the diaphragm to flex over a longer distance and thereby to flex less sharply. Note that the chamfer 11 is still provided for ease of assembly. However, the fit of the piston 5 within the bore in the backup plate 4 is sufficiently close so that, when combined with the larger guide length resulting from increasing the thickness of the diaphragm backup plate 4, there is virtually no tendency for the piston to cock in the bore. This eliminates damage caused by non-uniform flexing and by misalignment of regulator components.

A coating "A" on the bore of the backup plate 4 and a coating "s" on the lateral circumferential surface of the piston 5 are provided to ensure that surfaces which may be subject to vibratory oscillation against each other are not of the same metal. This reduces the tendency for fretting corrosion to occur, thereby extending the service life of the piston 5 and of the backup plate 4. It should be noted that it is not necessary to coat both surfaces in order to satisfy the different metal requirement. For example, if the piston 5 were made of steel and backup plate 4 were made of bronze, the different metal requirement would be met. If, however, both were made of bronze or both were made of steel, it would be desirable to coat one or the other to eliminate the fretting corrosion problem.

Depending on the circumstances, coatings such as hard chromium electroplate, nickel electroplate, electroless nickel or other relatively hard plated surfaces may be used. By avoiding the use of similar metals or soft metals on the contacting surfaces the tendency toward fretting corrosion due to reciprocating or oscillating vibratory motion under high pressures at the contact surfaces will be reduced and service life of the regulator improved.

Finally, the increased thickness of the backup plate 4 reduces the tendency of the backup plate to deflect under pressure and thereby prevents initiation of leakage past "0" ring seal 9 between the diaphragm 40 and the housing body. This freedom from deflection also contributes to the alignment stability of the piston which was earlier discussed from the viewpoint of improved guide length.

Provision of a coating on the piston and/or the bore of the backup plate provides resistance to fretting corrosion which can shorten regulator life.

A similar fluid pressure regulator is also shown and described in co-pending Application No. 9125737.8 (Serial No. 2 251 055 A), to which reference is accordingly directed.

Claims (6)

CLAIMS:

1. A fluid pressure regulator comprising: a housing body having a fluid inlet, a fluid outlet, and a fluid passage communicating therebetween; a variable valve positioned within said fluid passage and having a valve stem, an orificed seat and a closure element biassed toward said orificed seat; means for adjusting a clearance between said seat and said closure element in order to control fluid outlet pressure;; means, including first and second separate, imperforate, radially coextensive diaphragms in mutual service contact, for maintaining a substantially constant outlet pressure which is independent of fluctuations in fluid inlet pressure, and a piston reciprocably movable within a bore of a housing body cover, wherein either the bore of the housing body cover or the outer cylindrical surface of the piston is provided with a coating for prevention of fretting corrosion of surfaces, which coating is a plated metal coating so that contact is only between dissimilar metals.

2. A pressure regulator according to claim 1, wherein the means for adjusting the clearance between the seat and the closure element comprises a valve stem protruding through said seat and contacting the closure element; and means for variably biassing said stem counter to the bias of the closure element.

3. A fluid pressure regulator according to claim 2, wherein the means for variably biassing said stem comprises a diaphragm back-up plate which also serves as a regulator housing body cover and which captures the outer periphery of the first and second diaphragms between itself and said housing body; a cylindrical bore in said back-up plate coaxial with said stem; a piston reciprocably fitted in said bore and fixed to said stem such that the inner portions of the first and second diaphragms are captured between the piston and the stem and such that any bias applied to said piston is transmitted directly to said stem and a threaded adjusting device for applying bias to said piston through a diaphragm tensioning spring.

4. A pressure regulator according to claim 3, wherein said bore is of sufficient length within the housing body cover and of sufficiently close fit to the piston to prevent cocking of the piston within the bore.

5. A pressure regulator according to any one of the preceding claims, wherein said closure element is biassed by means of threaded means for pre-loading a spring to establish a force on the piston to adjust the degree of opening of the variable valve and for varying that opening in response to changes in fluid inlet pressure.

6. A fluid pressure regulator, substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.