FSM: Liquid Desander – Secondary (External) Accumulator Operation (B-FSM053)

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

To minimize liquid loss during solids discharge – very important during subsequent slurry dewatering
For high-pressure (>100 psig) desander operation 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
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.
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.
Vent accumulator: this is an optional step and only used in very high-pressure applications where flash gas must be removed
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 desander sizing & selection.

References

Rawlins, C.H., and Wang, I. I., “Design and Installation of a Sand Separation and Handling System for a Gulf of Mexico Oil Production Facility,” SPE Production and Facilities, paper 72999, Vol. 16, No. 3, 2001, pp. 134-140.