An arming unit

Abstract

An arming device 10, for example for use with a shock wave interrupt device or shock tube comprises a waterproof housing which includes a safety catch. When the safety catch is released, it opens a pa

Description

Field This invention relates to an arming unit and more particularly, but not exclusively, the invention relates to an arming and safety unit, for example for use with a trigger device, such as a shock wave interrupt device, which is also known as a shock tube or an electrical switch trigger device. Background Shockwave interrupt devices, or shock tubes, have been used in trigger devices to divert and arrest a shockwave created in a low energy explosion in a signal tube. They typical consist of a metal housing with a central bore leading to a chamber which dissipates a shockwave to prevent an explosion. Energy from the dissipated shockwave is relayed from the shock tube to one or more low energy explosive signal tubes. The trigger device therefore acts as a safety break to provide personnel time to evacuate to remote safe position. The trigger device needs to be activated or triggered before an explosive device is able to be deployed. Shock tubes are usually deployed in water and are activated at a specific depth. Actuation of a reciprocating member causes a diverting means to be opened when a pre-set hydrostatic pressure head is exceeded. These shock tubes are typically deployed overboard a vessel when the shock tube is set up in a ‘safe mode’ and a safety catch is set to place the shock tube is in a safe state to handle so that whilst the device is not submerged, the shock tube is arranged such that a diverting means diverts a shockwave to a closed chamber. When deployed the safety catch is actuated, at a pre-set depth of water, this causes displacement of a reciprocating member which opens a pathway to one or more signal tubes. This allows the shockwave to pass from a primary signal tube to one or more secondary signal tubes which in turn activate one or more pre-positioned remote explosive devices. Thus the shock tube safety and arming unit ensures that an explosive train can only commence when it is armed by exposure to water and so ensures safe operation which allows an explosive train to be built whilst onboard a vessel prior to deployment for subsea salvage or destruction of unexploded ordinance by always ensuring a safety ‘break’ between an imitating charge which provides a shockwave in a lead line and a series of explosive charges for ultimate safe destruction or removal of a subsea obstacle or unexploded items such as mines or torpedoes. Likewise electrical switching devices are used to connect electrically activated charges to a remote current source, such as a hand crank generator or magneto. Prior Art Our granted UK patent GB 2 574 485 describes a trigger device which consists of a safety and arming unit with a hydrostatic switch in which a pin is held in place in a piston to interrupt a signal transmission from a lead line shock tube to a disrupter side. This device was successful but had to be submerged to at least 10m in order for it to activate. It was therefore limited and was not able to be used in shallow water or ponds where often water depth is less than 3m. To create a variant of hydrostatic device that activates at these depths either needed a very large diaphragm to exert more hydrostatic force and therefore overcome the spring force which would have been expensive and bulky. Alternatively, a device with a very light spring was proposed but this risked being accidentally depressed due to shock if the device was dropped. The present invention therefore arose in order to provide a cheaper and reliable alternative and which is capable of being used with electrical switch trigger devices. Summary of the Invention According to a first aspect of the invention there is provided an arming unit comprising a waterproof housing which includes an arming device connected to a safety catch which when released opens a pathway to deploy at least one actuator thereby placing at least one arming device in an armed condition. In some embodiments the safety and arming unit may be used with shock tube. It could be used on a non-latching electrical switch or simply used as an actuator for a trigger device. An advantage of the arming unit is that it comprises very few components and is simple to fabricate and repair. Another advantage of the arming unit is that it is robust and may be quickly reset and reused for subsequent firings. Optionally the arming unit may be used to isolate an electrical signal from an initiator (current source) or one or more disrupter tubes from a lead line. In some embodiments the safety catch is retained in a safe condition by a latch which is activated by water, for example by a chemical reaction. In one embodiment the latch is retained in a safe condition by a water-soluble means, whereby when the water-soluble means dissolves, the safety catch is released to open a pathway to deploy the at least one shock tube or trigger device. In some embodiments the water-soluble means includes a water-soluble pill which prior to dissolving resists movement of the latch. In some embodiments the latch includes a spring-loaded plunger. Optionally the spring-loaded plunger has a waist which receives the water-soluble pill. For re-use the spring loaded plunger is ideally retractable by hand or a tool, such as a key or handle for retracting the plunger, may be provided. Alternatively, an automated retraction means, such as a motorised winder, may be provided for resetting the plunger for subsequent use. In some embodiments the housing is in the form of a right circular cylinder. Preferably at least one connector tube is connected to the housing for receiving a shock tube. Preferably multiple connector tubes are connected to the housing each for receiving a shock tube. In some embodiments three connector tubes are connected at an angle of 120° one to another. In some embodiments the housing comprises at least two parts which interconnect by way of an external thread on a portion of a first part and an internal thread on a portion of a second part. An optional lock means is provided to prevent relative rotation of the at least two threaded parts. It is appreciated that the arming unit and the housing are used repeatedly in wet and saline conditions and so are prone to corrosion and rust, so ideally components are formed from marine grade stainless steel, such as SAE 316 stainless steel or a variant thereof. In an alternative embodiment, when released by the water-soluble means, the plunger is deployed and closes a swich or makes a circuit thereby enabling the arming and safety unit to be used in an electrical system to trigger an electrically activated explosive charge. Preferred embodiments of the invention will now be described, by way of examples only, and with reference to the Figures in which: Brief Description of the Drawings Figure 1 is an overall tube of a waterproof housing of a shock tube with an arming unit attached thereto; Figure 2 is a sectional view through the waterproof housing and indicates a lead line leading to the shock tube and a disrupter line leading from the housing; Figure 3A is a sectional view through the waterproof housing showing the shock tube in a safe mode with its plunger in a retracted position; Figure 3B is a diagrammatical plan view through the shock tube showing the position of the lead line and first and second disrupter lines; Figure 3C is a photograph showing an overall view of the shock tube with an arming unit; Figure 4A is a diagrammatical overall view through an example of an electric safety and arming unit; and Figure 4B is a diagrammatical sectional view through the example shown in Figure 4A. Detailed Description of Preferred Embodiments of the invention The embodiment shown in Figure 1 has 3 shock tube connections: a lead line 1 and two split disrupter lines 2 and 3. Another variant is a so called inline configuration (not shown) which comprises a single (inlet) lead line and a single (outlet) disrupter line. Figure 1 also shows an arming unit 10 for a shock tube. The arming unit 10 comprises a waterproof housing 20 which includes an arming device 22 connected to a safety catch 24 which when released opens a pathway 26 to deploy at least one actuator 28 to place at least one shock tube 20 in an armed condition. In another embodiment the arming unit is adapted for use with an electrical system in which a plunger (not shown) presses a GO switch to complete the circuit. The arming unit 10 is supplied typically assembled to a shock tube safety device with flexible tubes 2, 3, which are typically polyvinylchloride (PVC) tubes cemented into an outside case 2A and 3A respectively. Each shock tube 2, 3 is then inserted into the PVC sleeve and crimped in place by the user (as part of the creation of the explosive train) - this creates a waterproof seal. Plunger 30 has a held proud of the ‘armed’ position by a water-soluble pill 32 which is similar to the type used in life jackets. The plunger 30 has a critical waisted section 34 which engages the water-soluble pill 32. When the unit 10 is placed in water, the outer casing 23 of housing 20 is vented by vent 24 located in an end cap 25 which is screwed into a top portion of housing 20 to allowing water ingress. When deployed the water-soluble pill dissolves 32, allowing the waisted section 34 of the plunger 30 to pass therethrough due to an axial force in direction of arrow A applied by spring 28. The waisted section 34 which engages and is entrapped by the water-soluble pill 32 ensures that the plunger 30 is retained in a safe position held and is only subsequently released when the water-soluble pill 32 has dissolved. The plunger 30 has two positions: safe (when retracted) as shown in Figure 2 and armed (when extended). When retained in the ‘safe’ position, that is with the watersoluble pill 32 intact, the plunger 30 blocks an orifice 42 to present a blind passage to shock tube lead line 1. This has proven successfully to interrupt or block transmission of a signal from the shock tube lead line 1 to the, or each, disrupter tube 2, 3. The plunger 30 is able to move only in the direction of arrow A and in the opposition direction when retracted. A suitable rebate or guide means (not shown) ensures the plunger 30 is not able to rotate. One reason for this is to ensure that eh waisted section is always presented in the correct orientation to receive the water-soluble pill 32. This is achieved by way of cap head screws 64 and 65 which are located in linear slots 66 and 67 respectively. This configuration ensures only axial movement of the plunger 30 is possible and so prevents the rotation of the plunger 30. When the water-soluble pill 32 has dissolved, the plunger 30 is spring released, under the action of spring 28, to the ‘armed’ position. In this configuration the orifice 42 aligns with the shock tube lead line 1 and the disrupter tube(s) 2, 3 thereby allowing for signal transmission. In a preferred embodiment the axes of shock tube lead line 1 and the disrupter tube(s) 2, 3 are arranged at 120° pitch to one another in the plunger in the ‘armed’ position. This is the maximum angle to ensure consistent initiation of each separate shock tube 2 and 3. Referring to Figure 3B there is a 2mm bore provided in a central cavity 50. It has been found that this spacing is critical to ensure that the shock is successfully transmitted to the next shock tube. The diameter of the central cavity 50 is larger than the internal diameter of the shock tube lead line 1 and the disrupter tubes 2, 3. It is also noteworthy that shock tubes are supplied in different sizes therefore it is important that the central cavity 50 is correctly dimensioned for use with the respective shock tube. Colour coding of an arming unit 10 and shock tube lead line 1 and the disrupter tubes 2, 3 ensure that only the correct shock tube lead line 1 and disrupter tubes 2, 3 may be connected to a specific type of arming unit 10. A pocket within the lower portion of the plunger is critical to interrupt the signal transmission because it relieves the pressure of the shock tube and therefore prevents the housing casing from cracking. The pocket also creates a barrier to arrest the signal and prevent it from transmitting along the shock tube line to the disrupter. Referring briefly to the embodiment shown in Figures 4A and 4B, there is shown an example of an electrical arming and safety unit 10 which when deployed closes a switch thereby connecting a remote current source (not shown) to explosive charges (not shown). This triggers an electrically activated explosive charge in an electrical system. Figures 4A and 4B are views of an example of an electric safety and arming unit 10 in which like parts bear the same reference numerals as the other Figures. The safety and arming unit 10 shown in Figures 4A and 4B operates in a similar manner to the shock tube in Figures 1 to 3. The invention has been described by way of examples only and it will be appreciated that variation may be made to the aforementioned embodiments without departing from the scope of protection as defined by the claims.

Claims

1. An arming unit comprising a waterproof housing which includes at least one arming device connected to a safety catch which when released deploys at least one actuator thereby placing the at least one arming device in an armed condition and wherein the housing comprises at least two parts which interconnect by way of an external thread on a portion of a first part and an internal thread on a portion of a second part.

2. An arming unit according to claim 1 wherein the safety catch is retained in a safe condition by a latch which is activated by a chemical reaction with water.

3. An arming unit according to claim 2 comprising a water-soluble means, wherein the water-soluble means includes a water-soluble pill which prior to dissolving resists movement of the latch.

4. An arming unit according to claim 3 comprising a spring-loaded plunger, wherein the spring-loaded plunger has a waist which receives the water-soluble pill.

5. An arming unit according to claim 4 wherein the latch is retained in a safe condition by the water-soluble pill which when dissolved causes the safety catch to releases the spring-loaded plunger.

6. An arming unit according to claim 5 wherein when the spring-loaded plunger is deployed it opens a pathway in at least one shock tube.

7. An arming unit according to any preceding claim wherein the housing is in the form of a circular cylinder.

8. An arming unit according to claim 7, when dependent on claim 6, wherein at least one connector tube is connected to the housing for receiving a shock tube.

9. An arming unit according to claim 8 wherein two connector tubes are connected to the housing each for receiving a shock tube.

10. An arming unit according to claim 9 wherein three connector tubes are connected at an angle of 120° one to another.

11. An arming unit according to claim 5 wherein when the spring-loaded plunger is deployed it opens causes a switch to close in an electrical circuit.

12. An arming unit according to any preceding claim wherein a lock means is provided to prevent relative rotation of the at least two parts.

13. An arming unit according to any preceding claim wherein the housing is formed from marine grade stainless steel.

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