The two subsea control pods are functionally identical. When a pilot control valve (rams close for example) is operated on the hydraulic control manifold a pilot signal is sent down both umbilicals so that the associated SPM valve in each pod 'fires'. If the pod selector valve is set on yellow then power fluid is sent only to this pod and it is only through the SPM valve in this pod that the fluid will reach the ram operating piston. The pod selection has no effect on the pilot system.
Once the yellow pod SPM valve 'fires', the power fluid passes through it to a shuttle valve, the shuttle piston of which moves across and seals against the blue pod inlet. The fluid then passes through the shuttle valve to move the ram to the close position. Fluid from the opposite side of the operating piston is forced out through the 'ram open' shuttle valve and vented through the 'ram open' SPM valve and into the sea.
Note that if the blue pod was now selected to open the rams; then the power fluid would flow to the ram through the 'open' SPM on the blue pod but the fluid from the 'close' side of the piston would be vented through the yellow pod SPM since the 'close' shuttle piston would still be sealing the blue pod inlet port.
The shuttle valves should be located as near as possible to their relevant ports on the BOP stack since the system is redundant only down as far as the shuttle valves. Fig 9.16 shows a NL Shaffer shuttle valve.
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