I've been doing some pipeline modelling in PipeSim recently, and I've found a really puzzling holdup-vs-flow curve, which runs counter to every other holdup-vs-flow curve I've seen before. I'm wondering if anyone else here has
(1) seen something similar,
(2) an idea of if it is realistic
(3) why such an odd curve occurs
In general, I'd expect my curves to look like the first one in this uploaded file - large holdup at low flowrates, and low holdup at low flowrates, following an asymptotic-like decay.
In the problematic case (which is a gas-condensate case with little liquid and low gas flowrates), I'm seeing the second sort of curve, which is low at the start, high in the middle, and low at the end.
The odd thing is that using three standard mechanistic correlations (Olga, Shell and Ainsley), I'm getting the second curve, but if I use the default Pipesim correlation (Beggs and Brill revised), I get the first curve. Three out of four correlations suggest the second curve is right, but I'm a little cautious about going with the "unusual" behaviour.
Can anyone here shed any light on this odd behaviour? Nearest I can figure out, I've somehow exceeded the validity range of the mechanistic correlations, or else there's a mechanism other than deposition and entrainment going on which kills off the holdup at low flowrates.
Any thoughts would be appreciated.
Thanks,
- Steve.