Solids Control Equipment - Desanders and Desilters

Print

The hydrocyclone is an inverted conical shaped shell into which pressurised slurry is injected tangentially at the top/wide end. This induces the fluid to flow around the circumference of the shell and downwards to a small opening at its apex.

The circular motion develops high centrifugal forces which increase towards the narrow apex. As a consequence of the centrifugal action, solids are concentrated at the wall and move downwards in an accelerating spiral to be discharged through the lower opening. At the same time as the ring of solids and their attached liquid are being discharged the liquid forms a high‑velocity stream spinning upward near the centre of the cone. This creates an area of low pressure which sucks air in through the open centre of the apex. The liquid discharges through the top of the conical shell by way of the vortex finder, a tube which extends down into the hydrocyclone body.

337 - hydrocyclone

Liquid Discharge

A hydrocyclone should be operated with a spray discharge, so that the solids separate to the outside in the downward spiral, and pass over the lip of the apex as an annular ring. In theory, the only liquid leaving the bottom of a perfectly balanced cyclone is that attached to the surface area of the solids particles it separates and discharges. However, in practice, it is difficult to completely prevent discharge of some liquid.

When the feed to a cyclone contains too much solids, or when the apex is too small, it becomes impossible for the solids reporting to the apex to discharge. When this happens the underflow at the apex changes to a rope discharge. Under these conditions the cyclone is extremely inefficient in separation and a lot of liquid may be discharged along with the solids.

Balancing Procedure

1. LINE UP the hydrocyclone to operate on water.

2. FULLY OPEN all discharge apexes. At this point , the discharge should be in the form a thin curtain-like spray of water from each cone.

3. CLOSE each apex slightly until only a slow drop of water occurs.

Notes:  Avoid over adjusting as this will cause plugging when the cones operate on mud.

If balancing is difficult, or impossible, use the troubleshooting guide below.

Troubleshooting Guide

Underflow

Cause

Wear Pattern

Remedy

Spray Discharge

 

Grooving over lower 5 cm

Correctly set up

Rope Discharge

Overload of solids in feed and/or undersized cone apex (possibly caused by partial plugging)

Sharp grooving 1/3 way up cone and erosion of cone top due to solids in overflow.

Increase apex size until spray discharge is achieved. If unsuccessful remove cone and clean-out underflow blockage.

Continuous Flow Discharge

Low inlet velocity which could be due to low feed head. Could also be caused by partial or total plugging of feeder head.

Excessive wear at cone apex.

Strip and clean feed header and regulate pump to provide + 23 m of head.

Plugged Discharge

Underflow plugging at cone apex caused by solids overload and/or restriction in underflow opening.

Sharp grooving 1/8 way up cone and erosion of cone top due to solids in overflow.

Disassemble cone. Clean-out blockage and reset apex size.

Desander Application

Normally 8in, 10in or 12in diameter. Flow rate 1 200 gpm.

The application of desanders will depend upon the type of mud in use. A desander:

shall be run continuously with unweighted water based muds to maintain low mud gradient, in top hole sections only

may be run sparingly with weighted water based mud

shall never be used with oil based mud.

The purpose of Desanders is:

to maintain sand content of mud below 0.5%.

to prevent overloading of desilters.

A feed rate of 300 - 400 gpm shall be used.

Desanders shall be balanced to produce a spray discharge using the procedure outlined above before use.

Cones shall be stripped and cleaned after every period of continuous use.

For optimum performance, the desander overflow should be routed to the desilter suction tank.

Note:   Since, in practice, the cut point of most 12” cones may be of the order of 200 - 220 microns for an unweighted water based mud, utilisation of screens of finer than 80 mesh may negate the need for desanders.

Desilter

The following guidelines for application of desilters shall be applied:

shall be run continuously with unweighted water based muds to maintain low mud gradient.

shall not be run with weighted water based mud systems.

shall not be used with oil based mud.

The purpose of desilters is to maintain low levels of solids in unweighted mud systems.

Each desilter bank shall be assigned a dedicated pump. The feed requirements shall be maintained as follows:

Operating Pressure

depends on mudweight
45psi water
70 psi 11.5 ppg mud

Minimum Feed Capacity

50 gpm for 4in
110 gpm for 6in
400 gpm for 12in

Desilters shall be balanced to produce a spray discharge. Use the procedure outlined above.

Cones shall be stripped and cleaned after every period of continuous use.

 

Add comment


Security code
Refresh