Abstract:
A valve mechanism which is resiliently biassed towards a first position but is normally retained in a second position by means of a linkage including a chemically sensitive device which is arranged to release in the presence of a contaminant.

Description:
FIELD OF THE INVENTION 
   This invention relates to a valve intended for installation at the inlet of a fluid tank, and in particular, to a valve which is intended to prevent the contents of the tank from becoming contaminated, as a result of being filled with the wrong fluid. 
   BACKGROUND OF THE INVENTION 
   In large multiple fuel tank installations, for example on petrol station forecourts, there may be a number of storage tanks intended for different grades or types of fuel. Unintentional contamination of one fuel by another, can easily take place if care is not taken when the fuels are delivered and, can cause considerable damage, for example if diesel fuel finds its way into a petrol tank, or vice versa. Since all of these fuels are commonly delivered by tankers with multiple fuel compartments, and all of the storage tanks have similar filler inlets, such an error can easily occur if the tanker operative is in a hurry. 
   International patent application no. WO 00/55598 (Masstech International Limited) describes a device for detecting the presence of a chemical contaminant, the device comprising an indicator element which is held in a first position by means of a failure element which is held in tension, the failure element being made of a material which fails in the presence of the chemical to be detected, thereby releasing the indicator element from its first position and allowing it to move into a second position in order to provide an indication of the presence of the contaminant. 
   The failure of the failure element may occur by shearing or stretching of the element, or, more typically, may occur when the surface of the element is degraded by the contaminant, in such a way as to release it from engagement with another part of the mechanism. 
   SUMMARY OF THE INVENTION 
   The present invention utilises a similar arrangement, in order to trigger the movement of a valve member, in the presence of a contaminant. Accordingly, the present invention provides a valve mechanism which is resiliently biassed towards a first position but is normally retained in a second position by means of a linkage including a chemically sensitive device which is arranged to release in the presence of a contaminant. 
   Preferable the valve is biassed towards a closed position but is normally held open by the retaining linkage. 
   Preferably, the chemically sensitive device comprises an elongate tie member, at least one end of which is frictionally engaged by a cooperating member which connects it to the remainder of the valve mechanism, and whose surface is chemically degraded by the contaminant so that the frictional engagement is lost. For example, the failure element may be made from a material which is dissolved by the contaminant, or may have a surface whose co-efficient of friction changes in the presence of the contaminant. This may result from a change in viscosity of the surface. 
   In a preferred embodiment, the valve mechanism includes a movable closure element mounted in a conduit, which is resiliently biassed towards a position in which it closes the conduit, but is held in the open position, against the resilient bias, by means of a chemically sensitive device of the type described above. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is a perspective view of a device according to the invention, showing some hidden detail; 
       FIG. 2  is an enlarged side elevation of part of the device of  FIG. 1  and 
       FIG. 3  is a plan view of the device of  FIG. 1 , again showing some hidden detail. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , the device as illustrated includes a circular butterfly valve member  2  pivotally mounted on trunnions  4  and  6  in a conduit  8 . As shown, the valve is in the open position in which the butterfly is parallel to the axis of the conduit, and is shielded from flow turbulence forces at the inlet  10 , by a transversely extending fixed vane plate member  12  which also provides an Intake port for housing  60 . 
   The trunnion member  6  is connected to an actuating mechanism  14  mounted on the outside of the conduit  8 , in a housing  16 . As shown in more detail in the enlarged view of  FIG. 2 , the mechanism comprises a plate member  18  forming a crank arm, and having axially extending pins  20  and  22  mounted at opposite ends of a diameter of the plate which, in the open position shown, is at 45° to the axis of the conduit. 
   The upper pin  20  is connected by means of a chemically sensitive device comprising a “chemical fuse”  24 , described in more detail below, to a fixed pin  26  of the mechanism, whilst the lower pin member  22  is connected by a tension spring  28  to a further fixed pin member  30 . 
   In the example illustrated, the “chemical fuse” member  24  comprises a tubular member of material which is chemically sensitised to a contaminant, in such a way that its surface characteristics change rapidly in the presence of the contaminant. The tubular member is provided with end caps  30  and  32  having axial bores which are a close interference fit on the ends of the tube, and transverse bores to receive the locating pins  20  and  26 . 
   In operation, if a contaminant fluid enters the conduit, a sample portion of fluid will enter the housing  16  via intake aperture  35 , and will exit housing  16  via apertures  34 , into the low pressure zone downstream of vane member  12 . This through flow fluid will contact and react with the surface of the “chemical fuse” member, so that the outer surface will rapidly degrade. Under these conditions end caps  31 ,  32  become a slightly looser fit, until the tension spring  28  overcomes the retaining force of the fuse member. At this point the spring will deflect the butterfly member  2  into the main flow stream. Flow forces will then assist the spring to rapidly close the butterfly valve over the full 90° movement. 
   As illustrated, the housing  16  of the valve member is provided with drain plug  36  so that in the event of a shut-off condition contamination fluid upstream of the closed butterfly member  2  can be drained away externally. This plug can also be used to take fluid samples. 
   In addition, the entire housing can be dismantled, to replace the “chemical fuse” when it has been triggered. 
   It will be appreciated that as illustrated, the device is arranged to close a filler Inlet in the event of detection of a predetermined substance. However, it could equally be employed in a context where it was required to open in the presence of a predetermined substance, for example to allow it to escape through an overflow.