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Liquid Desander – Vessel and Liner Design (B-FSM075)

PACS deoiler vessel with three-chambers

The previous article listed the four main components of a liquid desander system. I’d like to address the core two components – vessel and liners – in a bit more detail in this article. The design features listed are what you should evaluate when designing or purchasing a desander unit.

Vessel Design 

  • The graphic below shows two different type liner style vessels. Each vessel meets ASME Sec VIII Div. 1 rating.
  • The left vessel has welded mounting plates, while the right vessel has sandwiched mounting plates. Both are fully functional and valid designs. The sandwich design is more expensive but allows access to all internal surfaces of the vessel.
  • Inlet nozzle on each is through the side (to middle chamber). The overflow nozzle on the welded plates is through the top chamber side, while the overflow is out the top dished head on the sandwiched plates. Both are functional and valid designs. The overflow side nozzle allows removal of the top flange plate to reach the liners without disconnecting overflow piping.
  • The inlet chamber (between the plates) may have a small size nozzle to allow draining of fluids during maintenance.
  • The sand outlet on both is directly out the bottom. That is the preferred orientation, especially if a secondary accumulator is used (i.e. straight path fall for the sand).
  • The bottom chamber on each is the integral accumulation chamber. This chamber usually has a side nozzle for mounting a level switch.

PACS for Turndown

  • Desander liners have ~3:1 turndown inherent in their operation (see post B-FSM-61 of 13-Nov-18).
  • Higher vessel turndown is achieved by replacing liners with blanks and vice versa. However, that requires shutdown and access to the internals.
  • Online high turndown (>20:1 has been achieved) by using a Package Active Cyclone System (PACS). Photos of deoiler hydrocyclones are shown in the graphic below. The same design is fully valid for desanders.
  • The idea is to segment the vessel plats into 2 or 3 sections. These can be linear sections (shown in header photo) or concentric circular sections. These sections (or banks) of liners are turned on or off as appropriate).
  • You MUST completely isolate each chamber during operation for this to work. That means that while the vessel has one inlet nozzle it will have a quantity of both overflow and underflow nozzles equal to plate segments (i.e. 2 or 3). BOTH overflow and underflow nozzle of the isolated chamber must be open or closed together – to prevent operating short-circuit.
  • Use actuated valves to provide automated operation
  • A PACS vessel is more expensive than a standard vessel.

Liner Design

  • Liners are a replaceable and consumable component of the system. They will wear out eventually and require replacement.
  • Two liner examples are shown in the graphic below. These are metal (Stellite®) and ceramic (alumina). The material choice is generally a cost option.
  • The liners may be installed with o-rings or gaskets – something to seal them in place and protect from breakage.
  • Both require access to top of the vessel for install, inspection, or replacement
  • This is the core IP item from each vendor, so limited information may be available. Each vendor should at least be able to provide M.O.C., weight, and hydraulic capacity curve for each liner they offer.

The next article will discuss flow sheet design for liquid desanders.

 

References:

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

 

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