US2742179A - Thermal safety plug - Google Patents

Description

pri 17, 1956 c. B. LIVERS 2,742,179

THERMAL SAFETY PLUG Filed Jan. 21, 1955 /5- 501. DE 1? log new Ill/ll: SOLDER INVENTOR. C. B. L/VERS ATTORNEY r 2,142,112 r SAFETY PLUG:

Carlos B. Livers, North Hollywood, Calif., assignor to Bendix Aviation Corporation, North. Hollywood, Calif., a corporation of Delaware 1 v Application January 21,193,. Serial No. 483,301 1 Claim. (craze-:39

This invention relates" to safety plugs that blowout in response to a rise in tempe'ra-ture above a predetermined value to relievepressure in asystem and prevent damage thereto. The invention is particularly useful in, although not limited to, hydraulic power systems to unl'oad'the pump ifthe temperature of the fiuidlrisesto a value such that continued operation under pressure might damage the pump.

An object of theinvent-ion is to provide a simple and reliable thermal safety plug that is relatively insensitive to pressure while at the same time providing a large passage for escape of fluid when it opens.

Other more specific objects and features of the invention will appear from the description to follow.

Patented Apr. 17, 1956 screwconnection tothedevicetobe protected- The main portionof thev bodymemberlll. may be. of .hexagonal external shape, as shown in Fig. l, for application of a wrench. The. construction disclosed provides both fluid connection between. the chamber. 11 and the pressurefluid circuitof the. device to, bepmtected, and thermal connection to. insure. that the body member 10 will follow the temperature of the device- The-body 10. isprovidedwith two coaxialvent holes or. passages, 12 and 13, respectively, which. are of different diameters. and. extend: in opposite directions from the chamber. 11 to the. exterior of the body. These vent holes 12 and 13 are normally closed by a single plug 14 having a: largev cylindrical end 14a. loosely fitting the passage. 1'2 anda. small cylindricalend 14b looselyv fitting the passage 13. .Eachof theendportions 14a and 14b is secured in sealing relation with,.its associated passage 12 andi13 by a film of solder 15.fillingfthe. clearance space.

Pressure fluid inthe chamber, 11 exerts a pressure force on theplug14 actingin upwardv direction (with reference to. Fig. 2. 01 the drawing) and. with force that is. proportionalto-the. pressure .andthe difierentialarea of the two It is old to provide in pressure systems a vent hole nor- 7 mally closed by a loosely fitting plug soldered in place with a low-melting solder that melts in response to a dangerous rise in temperature to permit release of the plug in response to pressure thereon and vent fluid to reduce the pressure. In many instances, the vent hole must be quite large to handle the full fluid flow of the system and reduce the pressure to a safe value. The pressure force acting on the plug is proportionate to its area, and in a high pressure system this pressure force on a large plug may be so great that the solder fails below the desired temperature because of deterioration of the strength in response to aging and/ or softening at a temperature below thefusing temperature.

This problem is solved in accordance with the present invention by providing two diametrically opposite vent holes of diiferent diameters closed by a single plug having end portions of diiferent diameters slidablyfitting in the respective vent holes and soldered in place with a solder having the desired fusing temperature. With this construction, the total resultant pressure force tending to eject the plug is proportional to the differential area of the two end portions. Therefore, the vent holes can be made as large as necessary to handle the flow, while at the same time the blowout force can be made as small as desired by choosing the relative diameters of the two holes such as to yield a desired differential area.

A full understanding of the invention may be had from the following detailed description with reference to the drawing, in which:

Fig. 1 is an end view of one embodiment of the invention.

Fig. 2 is a longitudinal section taken in the plane II-II of Fig. 1.

, circuit of a pressure fluid device to be protected. Thus,

the chamber 11 extends through one end 10a of the body member, and the adjacent end portion 10b is threaded for end'portions, 14a. and lfib, of the. plug. This. difierential area can be made as srriall as desired by varying the relative diameters of the end portions 14a and 14b of the plug and their associated passages 12 and 13. In other words, the pressure force tending to eject the plug is not determined by the absolute area of the vent passages 12 and 13, and they can be made as large as desired to provide the desired free flow of fluid therethrough.

The diiferential area between the plug ends 14a and 14b is so chosen, relative to the pressure that is normally existent in the chamber 11, as to produce a resultant pressure force on the plug 14 that is ample to eject the plug when the solder 15 has melted in response to a dangerous rise in temperature, but insufficient to eject the plug prior to melting of the solder.

In practice, in a fitting as shown in Figs 1 and 2 in which the threaded portion 10b is of /s" pipe size, the large end portion 14a may have a diameter lying within the limits .131 and .132", and the diameter of the passage 12 may lie within the limits .133" and .134", thereby providing a minimum clearance of .001" and a maximum clearance of .003 to be filled with the solder 15. If this fitting is designed for a working pressure of 2000 p. s. i.,

the diameter of the small end portion 14b of the plug may able to insure ejection of the plug in response to the.

pressure force resulting from the diiferential area between the two end portions. On the other hand, it is undesirable to have an excessive clearance to be filled by a large quantity of solder.

In manufacture, the surfaces of the passages 12 and 13 in the body member and the surfaces of the plug 14 are tinned with the solder prior to assembly. Then the plug is inserted in the body member and the assembly heated above the melting point of the solder to cause it to melt and flow between the plug and the passage surfaces by capillary attraction. While the assembly is heated, any necessary additional solder can be added at the outer ends of the plug to completely fill the clearance.

Solders of diflerent compositions, having ditferent fusing points, are well known, and a suitable composition is chosen according to the maximum temperature at which it is desired to have the plug blow out. In some hydraulic applications, it is desirable to have the plug respond to any temperature in excess of about 280 F., and a suitable solder composition for this temperature consists of:

7 Percent, Tin 45 Lead 32 Cadmium '18 Bismuth In the form of construction shown in Figs. 1' and 2, in which the chamber 11 is closed at one end, there would be little circulation of fluid between the chamber and the device to which it is connected, and thermal conduction is chiefly through the threaded connection between the safety'plug and the device on which it is mounted. In

some instances, the body may be located within a.

' ends for connection into a pipe line, so that 'the fluid flows through the passage 21, and the temperature of the body 20 is determined by the transfer of heat thereto from the fluid. The plug 14 can be extended through the passage 21 if the fluid velocity is low, but it is preferable to extend it through a lateral recess '22 which is in communication with the passage 21, thereby leaving the latter unobstructed. A high velocity flow past the plug 14 could apply a side thrust to it that would prevent its ejection when the solder melted.

Although for the purpose of explaining the invention a particular embodiment thereof has been shown and described, obvious modifications will occur'to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim: g

A thermal safety plug comprising:' a body member defining a chamber adapted to be connected to a pressure fluid device to be protected and having a pair of coaxial cylindrical vent passages of difierent diameters extending in opposite directions from" said chamber through said body member; a plug member extending through said chamber and having opposite cylindrical end portions extending into said respective vent passages, each said end portion being of slightly smaller diameter than its associated vent passage, whereby it has a free sliding fit therein; and an annular body ofsolder of predetermined melting point lower than those of said body and plug members filling the clearance space between each end portion of said plug member and its associated body passage.

No references citedk

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