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Pipe pressure loss calculation

I am calculating various pressure drops in a pipe and am and am having trouble calculating the pressure at a certain point, for a set case. This is the pipe...

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A and B are vents, through which air passes and travels on up the pipe. I am working out the pressure at points a and b for three cases.

1) both vents open and pumping air
2) vent A open, vent B closed
3) vent A closed, vent B open

I have worked everything out no bother for cases 1 and 2 but 3 is stumping me. My engineering brain knows that the pressure at a should be negative, because of the vacuum effect of the flow through B onwards, but I can't work out how to do the calculation. So far, setting the flow rate at A to zero just makes the frictional losses zero at this point, and leaving it at the same pressure as at B.

I have accounted for losses due to the fittings (inlets) but I want know know how to add on a pressure loss for vacuum effects. I hope this makes sense.

Thanks in advance.

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