US3739796A

US3739796A - In-line explosion arrester

Info

Publication number: US3739796A
Application number: US00139337A
Authority: US
Inventors: L Jablansky
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1971-05-03
Filing date: 1971-05-03
Publication date: 1973-06-19
Anticipated expiration: 1990-06-19

Abstract

This invention relates to a system for detecting an explosion in a pipeline carrying molten explosive material and halting the spread thereof by blocking the propagation of the detonating wave by a detonation arresting means that is electrically activated by an electronic switch which is sensitive to the initial detonation.

Description

ilnite States Patent 1 Jalblansky 1 lN-LINE EXPLOSION ARRESTER [75] Inventor: Louis Jablansky, Fair Lawn, NY.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC

[22] Filed: May 3, 1971 [21] App]. No.: 139,337

[52] US. Cl 137/68, 251/63, 317/18 R,

317/33 SC, 317/151 [51] Int. Cl. Fl6k 3/00, Fl6k 17/36 [58} Field of Search ..l37/67-71; 3l7/DIG. 8

[56] References Cited UNITED STATES PATENTS 1,284,197 11/1918 Larner et a1. 137/68 ELECTRONI C SWITCH June 19, 1973 2,399,843 5/1946 Adams 137/67 X 3,022,793 2/1962 3,332,432 7/1967 3,509,942 5/1970 Lindberg 137/67 X Primary ExaminerMartin P. Schwadron Assistant Examiner-Richard Gerard Attorney-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl and Ernest F. Weinberger [57] ABSTRACT This invention relates to a system for detecting an explosion in a pipeline carrying molten explosive material and halting the spread thereof by blocking the propagation of the detonating wave by a detonation arresting means that is electrically activated by an electronic switch which is sensitive to the initial detonation.

4 Claims, 9 Drawing Figures V lN-LINE EX PLOSION ARRESTE R Patented June .19 1973 3,739,796

7 Sheets-Shee t 1 SWITCH ELECTRONIC INVENTOR, LOUIS JABLANSKY 2 "5 Zak Zia, \AWM- Alfrs.

Patented June 19, 1973 7 Sheets-Sheet 2 mmkmumm Patented June 19, 1973 7 Sheets-Sheet 5 KOP ZOFMQ QEPOMJM lllllllllllllllllllllllllllll INVENTOR, LOUIS JABLANSKY Patented June 19, 1973 3,739,796

7 Sheets-Sheet 4 FIG. 4

INVENTOR, LOUIS JABLANSKY W 8%; 20? t. Y AZ ,4/7/x" v Patented June 19, 1973 7 Sheets-Sheet 5 m QI INVENTOR,

LOUIS JABLANSKY Patented June 19, 1973 3,739,796

'7 Sheets-Sheet 6 I N VENTOR LOUIS JABLANSKY :1 14 9 49. fiz d M V! 4 7W3. f

Patented June 19, 1973 'r Shets-Sheet v INVENTOR) LOUIS JABLANSKY @www m IN-LINE EXPLOSION ARRESTER The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION This invention relates to a system for arresting detonations in a continuous process explosive pipe-line such as those used in the manufacture of molten TNT and similar explosive materials.

Prior art devices using explosive arresters were not capable of stopping a detonation from propagating in a conduit containing molten explosive such as tri-nitrotoluene. Devices used in the past, have attempted to arrest the propagation of an initial explosion in a line carrying the explosive by controlling and limiting the diameter of the pipe and/or by the insertion of loops in the production line. Both of these methods have been found to be of limited merit. In manufacturing operations, where molten explosive is being transported from one processing operation to another, limiting the conduit to a critical diameter which will inhibit the propagation will not permit adequate volume of material to flow; likewise where loops of sufficient number are in troduced into the line to substantially inhibit propagation of a detonation their resultant effect in restricting the volume of flow through the conduit has proved objectionable.

SUMMARY OF THE INVENTION The present invention is a system for interrupting the propagation of a detonation wave in a conduit carrying explosive material such-as molten tri-nitro-toluene by attenuating the aforesaid wave to a minimum energy level, either upstream and/or downstream from the explosion point, thereby rendering the initial explosion ineffective in initiating and propagating further explosions along the line.

One of the objects of this invention is to provide quenching action in an explosive train in approximately one millisecond.

Another object of this invention is to provide an arresting device for quenching pipeline explosions which is suitable for more than one type of explosive.

Another object of this invention is to provide a system for interrupting the propagation of a detonation wave in a conduit containing amolten explosive which system does not restrict the normal material flow through the conduit.

Another object of this invention is to provide a means for interrupting the detonation wave in an explosive carrying pipeline without substantially limiting the physical size or shape of the line.

Another object of this invention is to provide a simple, cheap and compact means for effectively quenching an accidental explosion in an explosive carrying pipeline.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings.

. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic and block diagram of an in-line explosion arrester, which closes a passageway when activated by an electronic sensing circuit and switch.

FIG. 6 is a sectional view of aforementioned in-line explosion arrester illustrated in FIG. 4, and taken along line 6-6.

FIG. 7 is a cross-sectional expanded, elevational, view of one of the two detonation arresting means schematically illustrated in FIG. 2.

FIG. 8 is a partial sectional and plan view of the detonation arresting means of FIG. 7 taken along line 88.

FIG. 9 is a cross section of the detonation arresting means taken along line 9-9 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2, a detonation sensor breakwire 10 is coiled about the longitudinal circumference of the liquid explosive pipeline 12 having a product flow 14. The detonation breakwire sensor 10 is electrically connected to a pair of input terminals 18 and 20 of an electronic switch circuit 16. Circuit 16 maintains an in-line explosion arresting means 26 in FIG. 1 and an alternative arresting means 28, 28 in FIG. 2 non-operative so long as the breakwire sensor is intact. When an explosion causes the breakwire 10 to rupture anywhere along pipe-

line

12 or 12, the electronic switch circuit 16 will energize the detonation arresting means 26 and 28 through detonator leads 23 and 25 and to arresting means 28' through a parallel connection at

junctions

27 and 29. In the arrangement shown in FIG. 1 the in-line explosion arresting means 26 prevents the propagation of the explosive train by blocking the line. A barrier attenuates the detonation wave to a minimum energy level rendering the wave incapable of further explosive propagation. In FIG. 2 the explosion arresting means 28 and 28' cause the explosive sectional pipe-line 12' to separate from line 12 thereby opening up the explosive pipe-line 12 causing a critical discontinuity in the explosive fluid and thereby preventing propagation of the explosion. In both arrangements described above the arresting means will interrupt the propagation of the detonating wave in approximately one millisecond after rupture of the breakwire sensor.

FIG. 3 is an electrical schematic of the electronic switch 16 illustrated in FIGS. 1 and 2. A power source 17 is connected through a switch 19 and charging resistor 30 across the series combination silicon controlled rectifier 34 and electric detonator 52, also shown on FIGS. 4 and 7. Electric detonator leads 23 and 25 are connected to the switch 16

output terminals

22 and 24. A capacitor 35 is connected in parallel with the series combination of the silicon controlled rectifier and electric detonator 52 at

junctions

42 and 21. A voltage dividing resistor 32 is connected at one end to the common junction 42 between the limiting resistor 30, an anode lead 36 and capacitor 35, and the other end to the common junction 46 between the trigger electrode 40 and the electrical connection 47. The other end of the electrical connection 47 is connected to terminal 18. Junction 21 is also connected to terminal 20. The low resistance path of the breakwire sensor when connected to terminals 18 and 20 keeps the trigger electrode of silicon controlled rectifier 34 at a low potential and therefore in a cut-off non-conducting condition. When the breakwire sensor 10 is broken the potential at junction 46 is raised to a voltage approaching anode potential causing the silicon controlled rectifier 34 to conduct thereby discharging the energy stored in capacitor 35 to the electric detonator 52, and initiating the detonation arresting means 26, 28 and 28.

FIG. 4 shows the details of an in-line explosion arrester 26 illustrated in FIG. 1. A threaded detonator holder and closure 48 has an axial bore 51 which is filled with an insulating cement which holds detonator leads 23 and 25. A smaller axial bore 53 permits the passage of

leads

23 and 25 to a larger axial bore 55 which contains an electrical detonator 52. The detonator holder 48 is coaxially threaded into a bushing 54 which is in turn threaded into one end of the detonation trap housing 60. A space 56 is provided between the threaded portion 49 of the detonator holder 48 and an inner bore 57 of the threaded bushing 54. This inner space 56 permits gases generated by the detonator 52 to expand, thereby minimizing any shock wave generated by detonator 52. A hollow steel cylindrical plug 68 is operatively positioned below the threaded bushing 54 and slidably engages the inner wall of the longitudinal bore 61 of housing at one end of its outer peripheral surface. Steel plug 68 at said first end has a first annular O-ring groove which contains a first O-ring 64 and a transverse bore on one side of said first end for holding a shear pin 62. The shear pin 62 fits in a shear pin housing groove 63 permitting the bottom surface 67 of the threaded bushing 54 to mate with the bottom surface of the threaded housing bore 69. Steel plug 68 has a first threaded bore 59 on an open end and a second smaller axially aligned bore 81 on a closed end. A steel threaded insert 70 is threaded onto the first threaded steel plug bore 59 closing the second plug bore 81 and retaining therein a low density polyurethane foam 72. The steel threaded insert 70 has a central axially aligned bore 79 and inserted therein is a compressible wafer 58 oppositely disposed and in line with the electric detonator 52. The purpose of the compressible wafer 58 is to absorb some of the shock caused by detonator 52. The purpose of the polyurethane foam 72 is to further attenuate the detonation wave and to prevent spalling of wall of plug bore 81. The TEFLON sleeve transverse port member 66 retains the other smaller closed end of steel plug 68 in a cup shaped recess 66' therein in housing 60 and lines up the transverse bore 71 with transverse bores of the 60' housing 60. A second annular O-ring groove 65' is located in the bottom peripheral surface of the TEF- LON sleeve 66 transverse to the longitudinal axis 86 on one wall of the transverse bore 71; a second O-ring 64' is located in the aforesaid second O-ring groove 65'. O ring 64' acts as a seal to prevent the molten explosive product from entering into the space between housing 60 and TEFLON sleeve 66. A third annular O-ring groove 65" is located transverse to the longitudinal axis 86 on the other wall of transverse bore 71 and has therewithin a third O-ring 64" which acts as a seal to retain the molten explosive material within the transverse bore 71 and the housing 60. An aluminum decelerating stop 76 which is circular in cross section when viewed along line 6-6 is attached to the underneath side of the transverse bore wall 71' by screws 84 and is slightly smaller in diameter than the inside diameter of the longitudinal central bore wall 61 of housing 60.

Pipe flanges

74 and 74 are circumferentially welded at 73, 73' to housing 60 so that the flange bores 75, 75 are axially aligned with the transverse bore 60'. Fourth annular O-ring grooves 65 and fourth 0" rings 64" are located in the flange mating faces 77, 77 to form a seal with mating flanges (not shown) which are hermetically attached to pipe line 12 so that the explosive molten material may be maintained therein. Housing 60 has a threaded boss 83 on its other end and to which a threaded tapered stop 82 is screwed thereon. The wall of tapered bore 80, located adjacent to clearance space 78, gradually slows decelerating stop 76 which has been given motion by electrical detonator 52, effecting a closure of transverse bore 60' with a minimum of shock impact thereby safely attenuating the approaching detonating wave along pipe-line 12.

An initiating pulse from the electronic switch 16 through

leads

23 and 25 will activate electric detonator 52 causing gases to be generated above wafer 58; the pressure of these gases creates a force against steel plug 68 causing shear pin 62 to shear and the TEFLON sleeve 66 to move in a direction in line with longitudinal axis 86 closing the transverse bore 60 and thereby attenuating any subsequent detonation wave from being propagated along explosive line 12.

In FIG. 5, a cross-sectional view taken along line 55 of FIG. 4, we see the concentric arrangement of polyurethane foam 72, the retaining walls of steel plug 68 surrounding the foam 72, and the TEFLON sleeve wall 66 intermediate to the steel plug 68 and the surrounding housing 60.

FIG. 6 shows a cross-sectional view of the housing 60 and the decelerating stop 76 when the detonation arresting means 26 is in its open state. Housing 60 is circumambient the decelerating stop 76 and separated therefrom by clearance space 85. A plurality of screws 84, 84' holds the decelerating stop 76 affixed to the transverse bore wall 71' (not shown).

FIG. 7 shows, in partial cross-section, one of two identical detonation arresting means 28 and 28' which upon receipt of an initiating signal from electronic switch 16 will cause the sectional pipeline 12 in FIG. 2 to separate from pipe line 12. Upper flange 90 has an axial first central bore 104 on its outer surface 91 and concentric therewith a second central bore 106; pipeline 12 is positioned in the upper flange first bore 104 and welded to the outer surface 91 at weld 99. In a similar manner lower flange 92 has an outer surface 93 welded to sectional pipeline 12' at weld 99. Sectional pipeline 12' is axially aligned with pipeline 12 by the lower flange first central bore 112 and a concentric second central bore of smaller diameter 108. The upper flange 90 is connected to the lower flange 92 by a plurality of

bolts

102, 102', 102" and 102" (see FIGS. 8 and 9) which pass through upper

flange clearance holes

105, 105, 105" and 105" into aligned

holes

103, 103', 103" and 103". A circular upper fuze ribbon channel is positioned immediately adjacent to electric detonator hole 109 which holds the electric detonator 52. A matching circular lower fuze ribbon channel 97 is positioned opposite to the upper fuze ribbon channel 95 so that when both flanges are assembled together a closed annular channel is formed for containing the detonating fuze ribbon 94 therein. A circularly shaped upper flange tongue 98, of smaller diameter than the

fuze ribbon channels

95 and 97, mates with an annular channel 96 located in the lower flange 92. An annular O-ring channel 101 of smaller diameter than the upper flange tongue 98 is intermediate to a central bore 104 and the upper flange tongue 98 and located in the same flange face as the upper fuze ribbon channel 95. An O-ring 100 is positioned within the O- ring channel 101 and space formed between channel 101 and lowerflange inner surface 111. The O-ring 100 in contact with the inner surface 111 of the lower flange 92 acts as an effective seal preventing the molten explosive in pipe line 12 and sectional pipe line 12 from escaping or from contacting detonating fuze ribbon 94 or the electrical detonator 52. A threaded hole 110 is opposite to and connected with electric detonator hole 109 thereby permitting detonator holder 48 and electric detonator 52 to be operatively positioned immediately adjacent to the detonating fuze ribbon 94.

FIG. 8 shows a plan and partial cross-section taken along line 88 of FIG. 7 of the upper flange outer surface 91 with a plurality of

screws

102, 102, 102", 102" positioned between pipeline l2 and the outside diameter of the upper flange 90.

FIG. 9 shows a view of the upper flange inner surface taken along line 9-9, a cross-section of detonating fuze ribbon 94, electric detonator 52, detonator holder 48, and a plurality of

screws

102, 102, 102 and 102" in

clearance holes

105, 105, 105" and 105" respectively. This view shows the concentric positions of the O-ring 100, O-ring groove 101 and the detonating fuze ribbon 94 about the second central bore 106. Detonator holder closure 48 and electric detonator hole 109 are axially in line and radially positioned with respect to detonating ribbon 94 so that the electric detonator 52, located in electric detonator hole 109, is adjacent to detonating ribbon 94.

An initiating pulse from the electronic switch 16 through

leads

23 and 25 and 23' and 25' in parallel connection at

junctions

27 and 29, as shown in FIG. 2, will cause the electric detonator 52, as shown in FIG. 9, in detonation arresting means 28 and 28' respectively to tire the detonating fuze ribbon 94. The mild explosive force of detonating ribbons 94 and 94' (not shown) in arresting means 28' will cause the lower flange halves 92 and 92' (not shown) of detonating arresting means 28 and 28' to simultaneously separate from the upper flange 90 and 90 (not shown causing line 12' to be opened, thereby preventing the propagation of the detonation wave along pipeline 12.

From the above description it will be evident that the invention provides a system for rapidly detecting an explosion in an explosive material carrying pipe-line and arresting the detonation wave thereby preventing further propagation of the explosion.

I wish it to be understood that I do not desire to be limited to the exact detail of construction shown and described for obvious modification will occur to a person skilled in the art.

What is claimed is:

l. A system for detecting an initial explosion in a pipeline carrying explosive materials and arresting said explosion prior to the propagation of a detonation wave along said pipeline which comprises:

a breakwire sensor operatively positioned around said pipeline;

an electronic switch having a pair of input terminals electrically connected to said breakwire for maintaining said electronic switch non-operative when said breakwire is intact, and a pair of output terminals for supplying a voltage pulse when said breakwire is broken, and which includes;

a power source;

a silicon controlled rectifier having an anode terminal connected to a positive terminal of said power source through a series connected charging resistor and switch, a trigger electrode connected to the common junction of a first terminal of said pair of input terminals and to one end of a voltage dividing resistor, said voltage dividing resistor having its other end connected to the common junction of said charging resistor and said anode terminal, a cathode terminal connected to a first terminal of said pair of output terminals;

a load capacitor having one terminal connected to the common junction of a second terminal of said pair of input terminals and a second terminal of said output pair of terminals and the negative side of said power source, and a second terminal connected to the common junction of said anode terminal, said voltage dividing resistor, and said charging resistor, whereby said capacitor discharges its stored energy to said pair of output terminals when said breakwire sensor is broken, causing said silicon rectifier trigger electrode to be more positively biased, triggering said silicon rectifier; and

means for arresting a detonation disposed proximate said pipeline electrically coupled to saidpair of output terminals and responsive to said voltage pulse.

2. A system for detecting an initial explosion in a pipeline carrying explosive materials and arresting said explosion prior to the propagation of a detonation wav'e along said pipeline which comprises:

a breakwire sensor operatively positioned around said pipeline;

an electronic switch having a pair of input terminals electrically connected to said breakwire for maintaining said electronic switch non-operative when said breakwire is intact, and a pair of output terminals for supplying a voltage pulse when said breakwire is broken; and

means for arresting a detonation disposed proximate said pipeline electrically coupled to said pair of output terminals and responsive to said voltage pulse, and which includes;

a housing having a longitudinal bore internally threaded on one end, a threaded boss on a secend end, and an interconnecting transverse bore perpendicular to said longitudinal bore;

means for connecting said explosive pipeline to said transverse bore;

an electrical detonator;

means for holding said electrical detonator oppositely positioned and axially aligned to said longitudinal bore cited in claim 2 wherein said means for blocking comprises:

a low friction transverse port sleeve member, slidably positioned within said longitudinal bore, having a cup shaped recess on a first end and a transverse bore wall on a second end concentric with said housing transverse bore thereby permitting said explosive materials to pass therethrough when said port member is in an open position, and blocking said pipeline when said port member is in a closed position;

a steel hollow plug located in said cup recess having a first threaded bore on a first open end and a second smaller axially aligned bore on a closed end;

a steel threaded insert having a recess therein and threaded into said first threaded bore of said steel hollow plug seals said second axially aligned bore of said steel plug;

a polyurethane foam within said second axially aligned bore of said steel plug further attenuates the detonation wave;

a compressible wafer located in said steel threaded insert recess and oppositely disposed from said electrical detonator absorbs shock from said electrical detonator;

means for sealing said sleeve-transverse member from said transverse bore so that said explosive material remains in said pipeline;

means for sealing said steel plug to said longitudinal housing bore so that gases from said electrical detonator will not escape in the intervening space therethrough; and

means for retaining said steel plug and said transverse port sleeve member in an open position so that said transverse housing bore is open to pass explosive material when said electrical detonator has not been'activated by an output pulse from said electronic switch output terminals.

4. A detonator detecting and arresting system as recited in claim 2 wherein said means for decelerating comprises:

a tapered stop having a partially threaded first section internal bore screwed upon said housing threaded boss with its axis concentric with said longitudinal bore, a second tapered section having a first internal diameter equal in size to the diameter of said longitudinal bore and a second section smaller diameter; and

a decelerating cylindrical stop having an outside diameter smaller than the inside diameter of said longitudinal bore and larger in diameter than said second tapered section diameter of said tapered stop, said stop is attached to the underneath side of said blocking mean,, thereby permitting closure of said transverse bore with a minimum of shock impact.

Claims

1. A system for detecting an initial explosion in a pipeline carrying explosive materials and arresting said explosion prior to the propagation of a detonation wave along said pipeline which comprises: a breakwire sensor operatively positioned around said pipeline; an electronic switch having a pair of input terminals electrically connected to said breakwire for maintaining said electronic switch non-operative when said breakwire is intact, and a pair of output terminals for supplying a voltage pulse when said breakwire is broken, and which includes; a power source; a silicon controlled rectifier having an anode terminal connected to a positive terminal of said power source through a series connected charging resistor and switch, a trigger electrode connected to the common junction of a first terminal of said pair of input terminals and to one end of a voltage dividing resistor, said voltage dividing resistor having its other end connected to the common junction of said charging resistor and said anode terminal, a cathode terminal connected to a first terminal of said pair of output terminals; a load capacitor having one terminal connected to the common junction of a second terminal of said pair of input terminals and a second terminal of said output pair of terminals and the negative side of said power source, and a second terminal connected to the common junction of said anode terminal, said voltage dividing resistor, and said charging resistor, whereby said capacitor discharges its stored energy to said pair of output terminals when said breakwire sensor is broken, causing said silicon rectifier trigger electrode to be more positively biased, triggering said silicon rectifier; and means for arresting a detonation disposed proximate said pipeline electrically coupled to said pair of output terminals and responsive to said voltage pulse.

2. A system for detecting an initial explosion in a pipeline carrying explosive materials and arresting said explosion prior to the propagation of a detonation wave along said pipeline which comprises: a breakwire sensor operatively positioned around said pipeline; an electronic switch having a pair of input terminals electrically connected to said breakwire for maintaining said electronic switch non-operative when said breakwire is intact, and a pair of output terminals for supplying a voltage pulse when said breakwire is broken; and means for arresting a detonation disposed proximate said pipeline electrically coupled to said pair of output terminals and responsive to said voltage pulse, and which includes; a housing having a longitudinal bore internally threaded on one end, a threaded boss on a second end, and an interconnecting transverse bore perpendicular to said longitudinal bore; means for connecting said explosive pipeline to said transverse bore; an electrical detonator; means for holding said electrical detonator oppositely positioned and axially aligned to said longitudinal bore; means for blocking said transverse bore slidably positioned in said longitudinal bore and operatively responsive to said electrical detonator; and means for decelerating said means for blocking, thereby permitting the closure of said transverse bore and said attached pipeline with a minimum of shock impact while attenuating the detonating wave in said pipeline.

3. A detonator detecting and arresting system as recited in claim 2 wherein said means for blocking comprises: a low friction transverse port sleeve member, slidably positioned within said longitudinal bore, having a cup shaped recess on a first end and a transverse bore wall on a second end concentric with said housing transverse bore thereby permitting said explosive materials to pass therethrough when said port member is in an open position, and blocking said pipeline when said port member is in a closed position; a steel hollow plug located in said cup recess having a first threaded bore on a first open end and a second smaller axially aligned bore on a closed end; a steel threaded insert having a recess therein and threaded into said first threaded bore of said steel hollow plug seals said second axially aligned bore of said steel plug; a polyurethane foam within said second axially aligned bore of said steel plug further attenuates the detonation wave; a compressible wafer located in said steel threaded insert recess and oppositely disposed from said electrical detonator absorbs shock from said electrical detonator; means for sealing said sleeve-transverse member from said transverse bore so that said explosive material remains in said pipeline; means for sealing said steel plug to said longitudinal housing bore so that gases from said electrical detonator will not escape in the intervening space therethrough; and means for retaining said steel plug and said transverse port sleeve member in an open position so that said transverse housing bore is open to pass explosive material when said electrical detonator has not been activated by an output pulse from said electronic switch output terminals.

4. A detonator detecting and arresting system as recited in claim 2 wherein said means for decelerating comprises: a tapered stop having a partially threaded first section internal bore screwed upon said housing threaded boss with its axis concentric with said longitudinal bore, a second tapered section having a first internal diameter equal in size to the diameter of said longitudinal bore and a second section smaller diameter; and a decelerating cylindrical stop having an outside diameter smaller than the inside diameter of said longitudinal bore and larger in diamEter than said second tapered section diameter of said tapered stop, said stop is attached to the underneath side of said blocking mean,, thereby permitting closure of said transverse bore with a minimum of shock impact.