Particulate solids settle in process piping when fluid velocity drops below critical velocity (v_c), also called the limiting deposit velocity. This may occur anywhere in the production system.

• Piping: low flow rate, after expansions, dead legs, stagnate zones, etc.
• Process Equipment: separators, vessels, tanks, etc.
• Vessel internals can create local stagnation zones to collect solids

The transport of solids in piping at liquid only flow has been covered in previous posts;

• See post B-FSM-028 (link here)
• Vertical Upflow: Use 2 times Stokes settling limit
• Horizontal Flow: Use methods from Durand, Turian, Wilson, etc.
• A rule of thumb – the teneral minimum horizontal transport velocity is ~3 ft/s (1 m/s) for sand in water in most oilfield conditions

The transport of solids in piping under multiphase flow conditions is too complicated to detail here. See your local flow assurance specialist who may use a computational model.

Production separators and vessels are the main low velocity zone in the production system. This is by design as the slug catcher, free water knock out, and 2/3 phase separator all use gravity to provide oil-water-gas separation. Sand takes advantage of this design to settling in the first spot it can.

References:

1. Rawlins, C.H., “Design of a Cyclonic Solids Jetting Device and Slurry Transport System for Production Systems”, paper 166118, presented at the SPE Annual Technical Conference and Exhibition, New Orleans, LA, 30 September – 2 October, 2013. https://doi.org/10.2118/166118-MS

Next week I will cover movement of solids in separators.