FSM: Liquid Desander – Particle Travel (B-FSM052)

The purpose of a desander is to capture sand particles from the inlet flow stream. The details below, which accompany the graphic, provides a description of particle travel from inlet stream to capture – in this case all the way to the secondary accumulator. If only the integral accumulator is used, then the final particle capture site is in the chamber just below the apex.

Inlet Flow to Cyclone

Particles are traveling with the inlet flow through the inlet piping (distributed according to turbulence in the flow regime)
Inlet flow enters the desander vessel, and are directed into the cyclonic insert (either single or multi-cone)
The inlet section of the cyclone converts pipe flow to tangential or rotating flow
Solids particles impact the side wall at the inlet then slid along this wall around the cyclone to begin their rotating flow pattern for separation
The vortex finder serves to prevent short-circuit of particles from the inlet to the overflow stream

Cylinder-Cone-Apex

The heavy solid particles spin along the inner wall of the cyclone cylinder, while the light particles are swept to the overflow
When the flow interacts with the cone section, rotational forces increase keeping the heavy particles sliding along the cone inner wall – further intermediate size particles are capture from the high “g-forces”
The apex is the concentration point for all captured particles – particles are spinning the fastest at the apex
Upon exiting the apex, rotational flow is fully arrested, and the particles fall by gravity into the integral accumulation chamber

External or Secondary Accumulator

All capture particles fall by gravity after the apex – that is through the integral accumulator, through the isolation valve assembly, and into the secondary accumulator
Falling solid particles displace water in secondary accumulator – the displaced water goes back up through the apex to report to the overflow stream
Since solids collection in the accumulator is by gravity settling – no oil ingress takes place with the solids
Primary sand cleaning takes place, if the accumulator is pre-filled with clean water
Sand level in the accumulator is (nearly) flat – which is handy for using a level switch to determine when to discharge the solids

The next article will discuss operation of a secondary accumulator.

References

Rawlins, C.H., “Particle Transfer Between the Cyclone and Accumulator Sections of a Desander”, paper SPE-191147-PA (accepted). SPE Production & Facilities, 2018.