US3789668A

US3789668A - Overload protected bourdon tube

Info

Publication number: US3789668A
Application number: US00283003A
Authority: US
Inventors: R Bissell
Original assignee: Dresser Industries Inc
Current assignee: Dresser Industries Inc
Priority date: 1972-08-23
Filing date: 1972-08-23
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05
Also published as: CH577168A5; FR2197168A1; JPS4965878A; GB1394980A; IT990176B; FR2197168B1; BE803725A; DE2341809A1; CA988736A

Abstract

A sealed Bourdon tube responsive to externally applied pressures for effecting displacement within operating ranges correlated to the magnitude of pressure being applied. Preconditioning of the Bourdon tube includes a permanent fluid fill with an incompressible liquid of controlled volume corresponding to the decreased internal tube volume when the tube is subject to application of a predetermined overpressure. The confined fluid thereafter presets an overload limit beyond which the tube cannot subsequently be further displaced irrespective of the increasing magnitude of pressure being applied.

Description

United States Patent [191 Bissell Feb.5,1974

OVERLOAD PROTECTED BOURDON TUBE Robert D. Bissell, Orange, Conn.

Dresser Industries, Inc., Dallas, Tex.

Aug. 23, 1972 Inventor:

Assignee:

Filed:

Appl. N0.:

US. Cl. 73/418, 73/395 Int. Cl. G011 7/04 Field of Search..... 73/418, 411, 420, 431, 395,

[56] References Cited UNITED STATES PATENTS 2/1922 Mirk ..73/3686 8/1969 Arbon ..73/368.6

Primary Examiner-Donald O. Woodie! Attorney, Agent, or Firm-Daniel Rubin ABSTRACT 10 Claims, 6 Drawing Figures PATENTH] B 74 O /n P R ESS I00 PRESS FIG. 4

. 1 OVERLOAD PROTECTED BO CROSS-REFERENCE TO RELATED APPLICATIONS 7 BACKGROUND OF THE INVENTION The field of art to which the invention pertains includes the art of measurement and testing as more specifically directed to pressure sensitive units and their construction.

Bourdon tubes are commonly used as the principal operating element in by far the majority of pressure responsive devices available in todays commercial markets. These tubes incur highly predictable and wellknown deflection behavior in response to pressure differentials to which they are subjected, i.e., pressure differentials between inside and outside of the tube. Within a variety of available design criteria, the tubes can be readily adapted for particular ranges and levels of operating pressures. In designing a tube for any par ticular operating range, it is customary to allow a safety factor of on the order of fifty percent above maximum operating pressure below which the tube is intended to be safety operated. Depending on application and operating pressures, wall thickness of the tube can vary from about 0.006 inches to about 0.100 inches. In actual use, it is known to operate these tubes from a variable pressure source communicating either internally or externally of the tube, the latter being exemplified by US. Pat No. 3,338,101.

However constructed, continued effectiveness of the tube is dependent on restricting operation within the intended limits of pressure levels imposed by its design. Should the tube in its lifetime be subject even once to excessive levels of pressure beyond that allowed by the design safety factor, an overstress can occur resulting in a kink or other set to the tube wall. Thereafter the imposed set will adversely affect operating capability of the tube causing unnoticed distortion of instrument accuracy. Should the overstress unknowingly approach rupture level of the tube, subsequent rupture can occur even within the intended operating levels. While causes of applied overpressure are usually both inadvertent and unavoidable as a result of some malfunction at the pressure source, the consequences is for all practical purposes a destruction of the tube and the instrument of which it is a part.

To avoid the foregoing effects, it is customary in accordance with the prior art to position one or more mechanical stops in the deflection path of the tube. By this means, a limit to displaceable movement is imposed on the deflecting tube via a mechanical stop corresponding in position to a predetermined level of safe overpressure. While generally effective, the stops, as might be expected, are difficult to position accurately. At the same time, they are generally ineffective against tube rupturing at the unsupported sections thereof but will generally protect the tube against rupture up to about 300 percent of nominal maximum operating pressure. A mechanical form of internal overload protection is disclosed in German Pat. No. 748,236.

SUMMARY or THE INVENTION The invention relates to pressure gauges and more particularly to improved Bourdon tube constructions therefor adapted for external pressurization. In accordance herewith, the tube is pre-conditioned to include a built-in overload stop that limits deflection movement beyond which it cannot be displaced irrespective of the magnitude of increasing pressure subsequently applied. This is achieved by supplying a permanent fluid fill to the tube with an incompressible fluid of controlled volume corresponding to the decreased internal tube volume at a predetermined level of safe overpressure. Thereafter, the tube provides characteristic deflection movement in response to externally applied pressures of value below the limit imposed by the confined fluid, but becomes totally unresponsive at all pressure levels exceeding the imposed limit. By virtue of this phenomenon, the tube cannot deflect in keeping with pressure levels otherwise capable of producing overstress as to eliminate the adverse effects previously associated therewith.

It is therefore an object of the invention to provide a novel construction for a Bourdon tube.

It is another object of the invention to provide a Bourdon tube having a novel internal overload stop for withstanding excessive pressures without incurring the adverse effects of overstress.

It is another object of the invention to provide a Bourdon tube having an overload stop as in the previous object that is responsive to pressure changes applied externally of the tube.

It is a further object of the invention to provide a pressure responsive device utilizing a novel Bourdon tube construction as in the aforementioned objects.

It is still another object of the invention to provide a novel method for pre-conditioning an externally pressurized Bourdon tube to provide a built-in overload stop against the adverse effects of overpressure to which it might be subjected.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional elevation of a first embodiment transducer unit utilizing a Bourdon tube in accordance herewith;

FIG. 2 is a sectional elevation of a second embodiment transducer unit utilizing a Bourdon tube in accordance herewith;

FIG. 3 schematically illustrates the method for limit load pre-conditioning of the Bourdon tube; and

FIG. 4(a), (b), and (c) are simulated sectional views taken substantially along the lines 4-4 of FIG. 3 at various indicated levels of pressure applied externally against the Bourdon tube.

Referring now to FIG. I, there is shown an encapsulated transducer 10 exemplifying use of a Bourdon tube in accordance herewith and adapted for operation with an optionally detachable or permanently mounted indicating gauge apparatus 11. For this embodiment transducer 10 is comprised of a one piece, generally cylindrical tubular housing 15 having an internally elongated well bore 18. The housing is of jointless one piece solid stock dimensioned to withstand pressure levels without rupturing comparable to the piping system or pressure vessel with which it is being used. At its upper end 16 the housing is integrally sealed while per se open at its lower end 17. Formed about the latter is a thread 20 for mounting the transducer into a tapped aperture of the receiving device or system. External flat sides 21 at and near the top are preferably squared to accommodate indicating apparatus 11 whereas external sides 22 immediately below sides 21 include suitable flats for wrench engaging purposes. A drilled internal aperture 25, extends coaxially upward from bore 18 and is adapted to provide a bearing support for a rotatable pin 26 to be described. Internally at the lower end bore 18 merges with a counterbore 27 for accommodating an annular perforated disc 28 and an annular enclosure ring 29 securing a flexible imperforate diaphragm 30 therebetween. Secured on the inside face of disc 28 is a cylindrical collar 34 to which the lower end of helical Bourdon tube 35 in accordance herewith is securely affixed. The upper or free end of the Bourdon tube is secured to an axially floating cylindrical collar 36 which, with an annular magnet 37, are rotatably supported via pin 26 in aperture 25.

Bourdon tube 35, for reasons as will be understood, is sealed at both ends and is immersed in an incompressible fluid 19, such as silicone or the like, completely filling bore 18. By this means, system pressure as shown by arrow 38 is exposed to the underside of diaphragm 30 through ring bore 39. Via a plurality of angularly displaced small apertures 40 in disc 28, pressure changes applied against the diaphragm are transmitted to fluid fill 19 in bore 18. In turn, pressure changes against the fluid are exerted externally against the sealed Bourdon tube causing it to deflect with a winding and unwinding motion that effects a corresponding angular displacement of magnet 37. For these purposes diaphragm 30 should be capable of displacing sufficient fluid volume for deflecting the Bourdon tube through at least its normal operating range. Preferably the ID of Bourdon tube 35 is slightly greater than the OD of

collars

34 and 36. This enables accommodating reduction in tube ID in response to wind-up motion thereof such that at maximum operating pressure, the tube ID and collar OD substantially coincide. It should be appreciated that as thus far described, the arrangement affords absolute rather than gage pressure sensitivity by virtue of its isolation from the effects of atmospheric pressure changes. Use of a diaphragm 41 in the housing wall communicating atmospheric pressure to the fluid fill, or appropriately venting the tube internally to atmosphere can convert the unit to gage pressure sensitivity when required.

As an optional feature, the axial length of collar 34 can be extended as shown in phanton outline to a termination juxtaposed to the face of collar 36 defining a gap 44 of on the order of about 0.005 inches. The longer collar affords both enhanced viscous dampening as well as an effective form of temperature compensation. Should temperature changes to which the transducer is exposed have the effect of altering fluid volume at a different or greater rate than that the volume of bore 18, the spring rate of diaphragm 30 will effectively introduce a temperature imposed pressure error onto tube 35. By replacing fluid volume with increased volume of collar 34 and constructing the collar of a material such as quartz, having a cubical coefficient of thermal expansion substantially lower than that of the housing, the effect of temperature related error is substantially eliminated. This can likewise be controlled to a limited extent by maintaining the volume of bore 18 at a minimum required to accommodate the liquid fill about the Bourdon tube. Hence, increasing collar length enables a balancing of volume changes between the liquid and surrounding parts such that net volume acting against the diaphragm as a factor of temperature becomes essentially zero.

Indicating apparatus 11, as illustrated in FIG. 1, is essentially of conventional design but including a rectangular collar 45 adapted to slip onto the head of transducer l0 whereat it can be secured, if desired, by means of a set screw 46. Rigidly attached and hermetically sealed to the collar by staked-over flanges 47 is a cup shaped annular casing 48. Frontward of the casing (topside as illustrated) is a transparent crystal 50 overlying an annular gasket 51 and secured to the casing by means of an annular bezel ring 52. An axially located pin 55 rotatably supported in bearing aperture 56 carries a stud 57 on which is secured a bushing 58. Attached to the busing is a magnet 59 and a spring washer 60 which with a nut 61 secures a rotatable pointer 62 for displacement relative to dial plate 63.

The second transducer embodiment for accommodating a C-shaped Bourdon tube in accordance here with will now be described with specific reference to FIG. 2. In this construction, transducer housing is comprised of two

semi-ovular sections

71 and 72 joined together by an encircling weld 73. Extending radially outward of the housing on opposite sides thereof are ears 76 for accommodating receipt of bifurcated indicator collar 77. Bourdon tube 80 in accordance herewith is of a conventional C-type and, like Bourdon tube 35, is sealed at both ends in order that fluid fill 81 internally of the housing can act externally against bushing the tube. For these purposes, the fixed end of the Bourdon tube is secured to the unseen side of a block 82 whereas its free end (not shown) connects to a gauge movement 83 in a conventional manner. Movement 83 customarily includes a pinion 84 driving a pinion shaft 85 to which a magnet 86 is secured.

Extending through the underside of housing 70 is a pressure tight plug 90 having a bore 91 that merges with a counterbore 92. Sandwiched between the plug and a socket 94 having pipe threads 95 is a diaphragm capsule 96 which may, for example, be of a type disclosed in US. Pat. No. 3,202,063 secured in pressure tight relation therebetween by means of a peripheral clamp 97. System pressure in this embodiment is again indicated by arrow 38 and is transmitted through socket bore 100 communicating with offset bore 101 which in turn is exposed to the underside of diaphragm capsule 96.

Each of the

Bourdon tubes

35 and 80 in accordance herewith are pre-conditioned for presetting an overload limit beyond which the tube cannot be operated as will now be described with specific reference to FIGS. 3 and 4. For the respective constructions shown pre-conditioning of tube 35 is preferably conducted prior to its insertion into transducer well 19 whereas for tube 80 it is preferably effected post insertion within housing 70, the latter being represented schematically in FIG. 3. Considering the second embodiment transducer first, tube 80 while at zero pressure and having a cross section as substantially shown in FIG. 4(a) is filled with incompressible fluid, such as liquid silicone or glycerin, via a valve 102 controlling filler tube 103. The tube connects to the otherwise sealed Bourdon tube through block 82. With valve 102 in its open relation, incompressible fluid is supplied through a valve 104 to the housing cavity surrounding the Bourdon tube. Cavity pressure acting externally against tube 30 is then gradually increased causing internal tube volume to gradually decrease in a changing cross section fromthat illustrated in FIGS. 4(a) to 4(b) to 4(0). The latter simulates a cross section at some arbitrary predetermined pressure level above operating range but below the overstress level as for example, 150% of rated operating pressure. While the internal volume is thus being decreased, valve 102 is continuously maintained open for escape or release of internal fluid previously supplied.

On reaching the tube relation of FIG. 4(0), valve 102 is closed trapping an internal fluid volume precisely corresponding to a minimum volume to be subsequently permitted in response to whatever levels of external overpressure are thereafter applied. During normal operation, the tube is not expected to exceed 100% of rated capacity. Should an overpressure be applied inadvertently or otherwise, the tube is permitted by virtue of the internal fluid to deflect normally only insofar as the pressure does not exceed the pressure level correlated to the trapped fluid volume. At such time, attempts to effect additional deflection from further pressure increases tending to further reduce the tube cross section is positively precluded by the confined fluid. Consequently further deflection cannot occur beyond the imposed fluid limit such that application of pressure overstress otherwise likely to effect partial or total destruction of the tube and/or instrument is positively prevented. Not only is the tube protected by this arrangement but at the same time overload protection is simultaneously afforded

diaphragms

30 and 96. That is with the tube prevented from further deflection at its overload limit, the externally surrounding fluid is likewise prevented from transmitting further pressure increases thereby supporting the diaphragm against any additional displacement that higher pressure would otherwise impose.

For the embodiment of FIG. 1, the transducer could be modified to accommodate tube pre-conditioning in the foregoing manner. However, as shown, the tube is preferably placed in a separate chamber for similar preconditioning after which it is removed for placement into the transducer capsule. Respecting the preconditioning procedure, it should be appreciated that the sequence in which fluid is supplied internally of the tube is interchangeable. That is, it can be performed either prior or subsequent to reaching the 150% pressure level by either bleeding or initially supplying fluid at that point respectively, it being important only that fluid volume and tube volume correspond at the desired pressure level.

In operation, system pressure exposed to the underside of either diaphragm 30 or diaphragm capsule 96 is transmitted through the fluid in the housing cavity for application externally against the sealed Bourdon tube. In response to an increasing pressure within operating range, the free end of the Bourdon tube undergoes normal deflection for rotatably displacing the magnet secured thereto. With indicating device 11 supported on the transducer, its coacting magnet 59 effects a magnetic coupling with the transducer magnet in the process of which pointer 62 is displaced relative to dial plate 63. Should an overpressure be encountered, tube deflection continues normally until reaching the pressure level at which pre-conditioning of the tube was imposed. Thereafter, further increases in pressure levels applied against the Bourdon tube do not induce further tube deflection which instead is precluded by the fluid internally of the tube restricting it against any further decrease in internal volume.

By the above description there is disclosed a novel Bourdon tube construction and method for effecting such construction whereby the tube is pre-conditioned to provide an overload stop against the effects of overstress. Requiring only a trapping or predetermined controlled volume of incompressible fluid internally of the tube, a built-in overload limit is preset beyond which the tube cannot subsequently be operatively displaced irrespective of the magnitude of pressure being applied. At the same time, the tube retains its conventional deflection characteristics at all pressure levels below that set by the pre-conditioning in accordance herewith. With a simple yet effective technique, the previous adverse effects induced by overstress to the Bourdon tube is thereby readily and inexpensively eliminated. Complete overload protection is thereby provided not only to the tube but simultaneously to the pressure transmitting diaphragm as well.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall-be interpreted as illustrative and not in a limiting sense.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

ll. In a Bourdon tube sealed at both ends for response to external pressure variations, an overload limit comprising a controlled volume of an incompressible fluid means internally of said tube for limiting response thereof to a predetermined value of increasing external pressure, said controlled volume being less than the internal volume of said tube when tube is exposed to values of external pressure below said predetermined value.

2. In a Bourdon tube according to claim 1 in which the controlled volume of said fluid means within said tube corresponds substantially to a decreased internal volume of said tube when exposed to said predetermined pressure value.

3. In a Bourdon tube according to claim 2 in which said predetermined pressure value is intermediate the tube values of operating limit and overstress.

4. In a Bourdon tube according to claim 3 in which said fluid means is of a silicone composition.

5. In a Bourdon tube according to claim 3 in which said fluid means is of a glycerin composition.

6. Pressure responsive apparatus comprising in combination:

a. means defining an enclosed chamber adapted for communication with a variable pressure source;

b. a Bourdon tube mounted within said chamber for responding to the pressure variations from said source acting therein externally on said tube;

c. means operably connected to said Bourdon tube providing an output indicia correlated to deflection changes incurred by said tube; and

d. incompressible fluid means internally of said tube for limiting deflection response thereof to a predetermined value of increasing pressure.

7 8 7. Apparatus according to claim 6 in which the voltube values of operating limit and overstress. ume of said fluid means within said Bourdon tube cor- 9. Apparatus according to claim 8 i which i fl id responds substantially to a decreased internal volume means is of a silicone corn sition. of sa1d Bourdon tube when exposed to sa1d predeter- 10 A Po 1 8 h mined pressure value pparatus accor mg to c aim in w sa:

8. Apparatus according to claim 7 in which said prefluid means is of a glycerin compositiondetermined pressure value is intermediate the Bourdon

Claims

1. In a Bourdon tube sealed at both ends for response to external pressure variations, an overload limit comprising a controlled volume of an incompressible fluid means internally of said tube For limiting response thereof to a predetermined value of increasing external pressure, said controlled volume being less than the internal volume of said tube when tube is exposed to values of external pressure below said predetermined value.

6. Pressure responsive apparatus comprising in combination: a. means defining an enclosed chamber adapted for communication with a variable pressure source; b. a Bourdon tube mounted within said chamber for responding to the pressure variations from said source acting therein externally on said tube; c. means operably connected to said Bourdon tube providing an output indicia correlated to deflection changes incurred by said tube; and d. incompressible fluid means internally of said tube for limiting deflection response thereof to a predetermined value of increasing pressure.

7. Apparatus according to claim 6 in which the volume of said fluid means within said Bourdon tube corresponds substantially to a decreased internal volume of said Bourdon tube when exposed to said predetermined pressure value.

8. Apparatus according to claim 7 in which said predetermined pressure value is intermediate the Bourdon tube values of operating limit and overstress.

9. Apparatus according to claim 8 in which said fluid means is of a silicone composition.

10. Apparatus according to claim 8 in which said fluid means is of a glycerin composition.