Multiphase Desander – Multiphase Operation (0%, 50%, and 90% GVF) (B-FSM092)
We’ve covered all the theory and background and models – so let’s look at the operation of a multiphase desander at varying gas void fractions (GVF) and turndown scenarios. Posts B-FSM-089 and B-FSM-090 covered capacity, turndown, and performance for hydraulic and pneumatic flows, respectively.
Turndown Performance at 0% GVF
The baseline scenario is shown in the graphic below. A 10” multiphase desander is operating on hydraulic (liquid only) flow. The design point is 13,000 BPD which gives a 25 psi pressure drop. At the process conditions listed the separation size is 24 microns. At 20 minutes the flow drops to 5,500 BPD which reduces the pressure drop to 5 psi (minimum operating point) and the separation size coarsens to 38 microns. The flow then returns to design point, then at 30 minutes the flow increases to 18,500 BPD (maximum operating point). The pressure drop and separation size at the high end are 50 psi and 19 microns, respectively.
Turndown Performance at 50% GVF
Adding gas to the inlet stream makes the graph a bit more complicated. The graphic below shows operation at 50% gas. This is the same desander and same fluid/solid properties. The new design point at 25 psi drop has a capacity of 11,800 BPD liquid and 18.7 MMSCFD gas. The separation size has decreased to 16 microns due to lowering of mixed fluid viscosity and density. At 20 minutes a drop occurs again, with the low point at 5 psi drop – which yields 5,000 BPD liquid and 8 MMSCFD gas with 26 micron separation. The correlating high point at 30 minutes is 16,800 BPD liquid with 27 MMSCFD gas and 13 microns separation.
Turndown Performance at 90% GVF
At 90% gas, shown on graphic below, understanding the operation becomes more challenging. The baseline design point has decreased from 25 psi drop to 5 psi drop, due to critical velocities within the cyclone vortex. The baseline is now 3200 BPD liquid with 48 MMSCFD gas, which gives 13 micron separation. The low operating point is 2 psi drop (2000 BPD liquid plus 30 MMSCFD gas) and high point is 12 psi drop (5100 BPD liquid plus 72 MMSCFD gas). The separation sizes at all point is made finer due to the presence of gas.
There is no simple number for turndown with a multiphase desander due to the compressibility of gas which changes with pressure and temperature. We state turndown in terms of pressure drops, not flow rates. For an estimate the multiphase desander has a turndown of 5, however changing inserts increases the effective turndown to >20.
Valve and accumulator sizing and selection for multiphase desanders will be topic of the next article.
- Rawlins, C.H. 2017. “Separating Solids First – Design and Operation of the Multiphase Desander”, paper 185658-MS presented at the SPE Western Regional Meeting, Bakersfield, CA, 23-27 April.