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... http://i50.tinypic.com/141mikg.jpg A and B are vents, through which air passes and travels on up the pipe, where is it discharged to atmosphere. 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 there should be a vacuum effect at a, because 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 ... which isn't right! I have accounted for losses due to the fittings (inlets) in my calculation. I would like to know a method to calculate pressure at a when vent A is closed. Thanks in advance.