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FSM: Liquid Desander – Hydrocyclone versus Desander Hydraulics (B-FSM049)

Air-core in lower cone section of 2” hydrocyclone

One last comparison between hydrocyclones and desanders is pressure profile and hydraulics. The hydrocyclone (and desander) is a simple device with a complex flow profile.

The graphic below shows the flow pattern for the hydrocyclone (left) and desander (right).

Similarities

  • Both devices have a coupled free-forced vortex flow: an outer irrotational (free) vortex surrounds an inner rotational (forced) vortex
  • The free vortex spirals down (towards the apex) while the forced vortex spirals up (towards the vortex finder)
  • Large/heavy particles are swept to the underflow while small/light particles report to the overflow (remember cyclones are a classification device that separate based on particle weight – not size)
  • Both devices have combined axial, radial, and tangential velocity profiles. The magnitude of each velocity helps determine the separation size of the unit.
  • The tangential velocity (vt) is ~10X the axial velocity (va) and ~100X the radial velocity (vr).
  • The forces (gmax) can range from 50-5000 times that of gravity (g) depending on cyclone diameter and pressure drop
  • The g-force imposed on a particle traveling in the irrotational vortex can be approximated from vt and cyclone diameter (D): gmax=2vt²/Dg
  • Pressure drop (ΔP) is defined as pressure at inlet (Pi) minus pressure at overflow (Po): ΔP=Pi-Po

Differences

  • The hydrocyclone has a free-surface air-core enclosed by the rotational vortex. The desander has a flooded-core instead.
  • For the hydrocyclone pressure at overflow (Po) and underflow (Pu) are atmospheric (zero). For the desander Pi>Pu>Po – the underflow pressure (in accumulator chamber) is between the inlet and overflow pressures.
  • Pressure differential ratio (PDR) is defined as (Pi-Po)/(Pi-Pu). The hydrocyclone has a PDR=1.0 (Po=Pu=0), while the desander has a PDR>1.0.
  • The flow split (S) is the ratio of underflow flow rate (Qu) divided by inlet flow rate (Qi). For the hydrocyclone S~0.25 (function of vortex finder and apex diameters), but the desander always has S=0 (all flow goes to overflow).

The next article will discuss classification performance and the graded efficiency curve.

References

  1. Rawlins, C.H., “Particle Transfer Between the Cyclone and Accumulator Sections of a Desander”, paper SPE-191147-PA (accepted). SPE Production & Facilities, 2018.
  2. Svarovsky, L., “Hydrocyclones”, Technomics Publishing Co. Inc., Lancaster, PA, 1984.

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