Multiphase Desander – Secondary (External) Accumulator Operation (B-FSM086)

A secondary accumulator for a multiphase desander is used for the following reasons;

To minimize liquid loss during solids discharge – very important during subsequent slurry dewatering
To prevent gas loss during solids discharge
For multiphase desander operation >100 psig to prevent high differential slurry discharge across the valve (e.g. minimize erosion)
As a primary cleaning vessel for removing oil from sand

If a secondary, or external accumulator is used, below are the following steps for operation.

Fill with sand (see previous post on Particle Travel) the isolate accumulator. Close valve(s) between desander and accumulator – the desander has sufficient hold-up time in the integral accumulator while accumulator is being discharged.
Level measurement: for applications <900# ASME rating use a vibrating paddle/rod insertion instrument. For applications >900# ASME rating use a radiometric (gamma ray) level measurement.
Vent accumulator: this step removes any flash gas
Purge accumulator: open bottom discharge valve for slurry to exit and introduce utility water to push out the water – water at 15-45 psig and 25-50 gpm is sufficient for most applications
Fill accumulator: close bottom discharge valve and fill accumulator with clean utility water
Re-pressurize and back on-line: open the isolation valve to allow the accumulator to receive solids

The entire isolate-vent-purge-fill-pressurize cycle should take 10-15 minutes at most. The cycle can be fully automated by putting actuators on all the valves – very simple but expensive (I don’t make valves or actuators – so complain to those suppliers about the costs!).

The next article starts the discussion pressure drop and fluid throughput for a multiphase desander.

References

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.