to the auxiliary vent connection 16 the burst disc mechanism 20 and the cam shaft 24 may of course be arranged such that the cam shaft passes through the wall of the elbow piece in any convenient direction suitable for access by a service technician preferably the cam shaft 24 terminates outside of the wall of the elbow piece with a splined end 32 an operating handle or wheel 33 is slid over the splines to enable manual opening and closing of the combined quench valve and bypass valve of the present invention typically by a service technician it is preferred that the operating handle or wheel 33 is detachable from the cam shaft so that it cannot be tampered with by un trained personnel it may be preferred that the operating handle or wheel is retained by the service technician rather than being left at the site of the cryogen vessel a locking arrangement or a special spline arrangement may be used to reduce the temptation to tamper with the valve using makeshift operating tools such as spanners or pliers fig4 schematically illustrates a possible arrangement for leading the cam shaft 24 through the wall of the elbow piece 10 while ensuring that the cryogen gas recuperation path remains sealed from atmosphere as illustrated a bush 34 is let into a hole in the wall of the elbow piece and is secured to the wall for example by adhesive the cam shaft 24 passes through the bush and so through the wall of the elbow piece 10 one or more o rings 36 are provided sealing between the bush.