Well, here's the way it works in multiple product pipelines.
LIGHT BEING REPLACED BY HEAVY
Upstream head losses increase significantly, as the heavy oil is introduced into the pipeline at the initially high light oil flowrate.
As more heavy oil is introduced, the pipeline's flowrate drops rapidly. As pipeline flowrate drops, head losses all along the pipeline, but especially in the downstream segments still containing light oil, become less and suction and discharge pressures no longer fall.
With the addition of more heavy oil upstream, the pipeline flowrate continues to fall, but more slowly. The pipeline's flowrate is now so low that head losses, even in the heavy oil segments become less and suction and discharge pressures there begin to rise, but do not change much in the light oil segments, where the light is soon to be replaced with the heavy oil. The decreasing head losses, from the decreasing flow, balance the increasing head losses from the heavy oil now arriving at those segments.
As the last of the light oil is forced out, head loss tends to increase in that segment with the replacement of the light by the heavy oil, but pipeline flowrate reduces even further, almost cancelling that tendency, but not quite, and the as the last of the light oil leaves the pipeline and the heavy finally exits, the net delivery head settles at the steady state delivery head of the heavy oil.
HEAVY BEING REPLACED BY LIGHT
As you replace a heavy oil with a lighter oil, the head losses of the pipeline segments containing the light oil become less and suction pressures correspondingly increase with the lighter fluid upstream's lesser head loss.
As the lighter fluid enters the pumps, the total discharge head increases rapidly with the pump's suction pressure rise, but the pump differential head will remain the same, as long as the viscosities of the light oil and heavy oil are similar.
The continually decreasing head losses in the upstream segments eventually start to bring up the flow along the entire pipeline, but as the flow is brought up, the head losses all along the pipeline increase until a new equilibruim point is reached.
As the heavy oil continues to moves out, flow of both the light and heavy oil in the entire pipeline begin to increase rapidly, as do the head losses due to increasing velocity begin to overpower the decreasing head loss due to the replacement of the heavy denser oil in the downstream segments, as the last of the heavy oil is forced out.
When the light oil finally exits the pipeline, the last inceases in head due to the heavy oil finally leaving the pipeline are slightly larger than the decrease in head due to the increasing velocity and the ultimate equilibrium point is established at the steady state delivery head of the light oil.
BigInch
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