A drilling fluid vibrating screen is a vibrator used for solid/liquid separation. It is an important device in the solids-control system because efficient operation of other surface solids separation equipment is critically dependent on proper functioning of the vibrating screen. The purpose of using a vibrating screen is to recover drilling fluid and remove large solids as most as possible. With rapid development of drilling new technology, the requirement for drilling fluid solids control is increasingly higher. Research for the vibrating screen is paid close attention. A well-designed vibrating screen should have high flow capacity for drilling fluid and high solids-conveying speed. If the
large separated solids can be conveyed off the screen with a high velocity, the fluid and smaller particles will have more chance to pass through the screen. Whether the solid is in submersed condition or not, it should have an appropriate conveying velocity. In submersed condition, the solids with low conveying velocity tend to cover the screen pores, and the flow capacity of the vibrating screen will reduce, even the drilling fluid will run off the screen. When separated from the drilling fluid, the solids with low conveying velocity will increase the chance for smaller solids passing through the screen, to even accumulate, so the load of the screen will increase, and its lifetime will reduce. Thus, the kinematic law of solids on the vibrating screen plays an important role in its fluid handling capacity.
Much research has been done on vibration principle, vibration pattern, kinematics, dynamics and vibration measurement of a vibrating screen, but little work has been done on conveyance law of solids. The results of vibratory screening technology developed for such industries as mining and parts manufacturing are not applicable to screening of drilling fluid for three reasons:
- Unlike other industries, the main material processed by vibrating screen is drilling fluid rather than solids;
- The drilling fluid vibrating screen has high normal acceleration;
- The solids is submersed by drilling fluid, and when separated from drilling fluid, it is also coated by drilling fluid.
The purpose of this topic is to make a thorough study on kinematic behavior of solids on the vibration screen, and make a trial to get new technique about sieving process, so it can help in structure design and choice of basic parameters, and improve screening efficiency.
KINEMATIC LAW OF SOLIDS IN SUBMERSED CONDITION
When the drilling fluid containing drilled solids flows onto the vibrating screen, the liquid/solid separation process begins. The drilling fluid gradually passes through the screen, the solids convey toward the discharge end. The liquid along the screen disappears through the screen cloth as it reaches a point, called the liquid endpoint. At first, the solid particles are submersed by drilling fluid, then gradually become uncovered; at last, drier solids will be conveyed off the screen.
A solid has a complicated motion on an inclined vibrating screen. From the research report, in submersed condition, some solids have suspension motion, while others have driving motion. The driving motion is a motion in which the solids slide or roll on the screen surface or fly into drilling fluid. The two kinematic modes exchange continuously. Because of the action of liquid flow and the screen, the solids that keep contact with the screen surface can enter the mode of suspension motion, whereas the solids suspended in drilling fluid can subside on the screen surface and then enter the mode of driving motion. Although exchange occurs, in a large area, the two kinematic modes have stability, not disorder.