Hydrocyclone are a broad classification of settling devices that include the desilter, desander, and centrifuge. These are all basically the same device in different size which separates solids at different cut points.
Principles Of Mud Movement In Hydrocyclone
A cross-sectional diagram of a hydrocyclone is shown in the movie. The mud is pumped into the cyclone through a tangential opening at the large end of the cyclone. This results in a rotating motion of the fluid, much like a waterspout.
The resulting centrifugal force causes the particles to concentrate on the wall. The dense layer next to the wall travels downward and is ejected at the bottom of the cone. The remainder of the mud travels upward and exits through the overflow end.
At the overflow end, a short pipe extends down into the hydrocyclone past the mud intake. It stops mud from short-circuiting directly to the overflow. In effect, it taps the light portion of mud in the cyclone, allowing only the dense sludge to spirally slide down to the underflow.
If mud is fed to cyclone at too high a pressure, some of the mud will exit the overflow without having time for the solids to segregate. If the feed pressure is too low, poor separation of solids occurs. This makes it imperative that the hydrocyclone be operated at manufacturer‘s recommended pressure. Four-inch desilter are generally designed to operate at 40 – 45 psi, and six-inch desanders at 35-40 psi.
Tips for purchase and using desilter
The cut point of a solids-separation device is usually defined as the particle size at which half of the particles go to the underflow and half to the overflow. In general, the cut point is reduced as the hydrocyclone diameter is reduced. six-inch cyclones are rated at 30-35 microns cut point, 4-inch at 15-20 microns, and taken on minimum-viscosity suspensions at optimum operating conditions. As a result, they are lower than will be attained on field muds.
For a maximum removal efficiency, the underflow discharge should be in the form of a spray. A rope-shaped discharge means that some of the solids which should come out in the underflow are forced to go to the overflow.
The removal efficiency is also a function of the viscosity and density of the mud. Since the settling rate of a particle is decreased by increased viscosity and density of the mud, the percent of particles of any given size which reaches the underflow is decreased. In other words, an increasing amount of the larger particles goes to the overflow and are returned to the mud system. This means the de-silting should be started before the drilled-solids build up in a mud and increase the viscosity and density.
Solids that are not removed from the system at the first opportunity tend to be reduced in size as they are re-circulated. As they reduce in size, they not only become too small to be removed by a De-silter, but they also quickly build viscosity which reduces the efficiency of the De-silter in removing the larger particles.
When water is added to the mud, it should be added upstream from the desitler to reduce the mud viscosity and improve the efficiency of the desilter. The
Desander or fine screen shale shakers should be used ahead of the desilters to remove the larger particles that can clog or overload the desilter. Clogged discharge on the underflow of a cyclone probably accounts for more inefficient desilter performance than all other problem.
The desilter unit should be sized to handle 125 to 150 percent of the mud circulating rate. Each hydrocyclone will process from 50 to 70 gal/min. The suction, feed, and overflow lines should be sized with generous diameters and minimum lengths in accordance with good centrifugal pump installation practices.
When operated in a conventional manner, a large fraction of the particles in the Barite range is discarded by desanders and desilters. It follows that large quantities of Barite will be discarded when these devices are used on weighted muds. Normally, this will cause prohibitive costs of mud maintenance.