Posted 01 July 2010 - 09:41 AM
Hard to fathom what the designer was considering, but I can offer my opinion. I'd say that control of the downstream pressure is VERY important since four PCV's are devoted to this purpose. Furthermore, with one in each half of the circuit failing closed, it is more important that the downstream equipment not be over pressurized rather than under pressurized. Continuing, I'd say that PCV2 is normally "in control". It is maintaining downstream at 800kpa. PCV3 and PCV4 are closed since they can easily satisfy their requirements and then close. PCV1 is also doing its job by maintaining the inter-valve pressure at 850kpa.
Now let's look at what happens if a valve fails. If PCV1 fails, PCV2 is ineffective. PCV3 and PCV4 would maintain downstream pressure at 750kpa, a drop of 50kpa from normal. Probably not a big problem. If PCV2 fails, then PCV1 will maintain downstream at 850kpa, an increase of 50kpa from normal. Again, probably not a big problem. If PCV3 fails, you wouldn't even know. If PCV4 fails, then PCV3 will be actively operating to maintain the 800kpa pressure in parallel with PCV2. So downstream pressure is maintained between 750 and 850kpa even if a control valve fails. A pretty robust control system.