Mechanical solids control equipment selectively separate solids either through size differences, mass differences or both. Shale shakers use screens; hydrocyclones and centrifuges use centrifugal force. Understanding what are the key design factors and optimum operating limits for each of these devices will enable one to determine whether a particular device is needed, how to properly size the unit and how to properly use it.
Basic Information Of Shale Shaker
The characteristics of shale shakers are: (1) vibratory motion, (2) shale shaker capacity, (3) deck slope and number of decks of screens, and (4) screening area.
Vibratory Motion Of Shaker
Each is differentiated by the positioning of the vibrator(s) with respect to the center of gravity of the deck. Circular motion is produced when the vibrator is placed at the center of gravity of the deck; whereas, elliptical motion is produced when the vibrator is placed above the center of gravity. For elliptical motion, the screen deck must be sloped to convey solids; however, sloping reduces fluid capacity. Linear motion is produced when two vibrators are synchronized to rotate in opposite directions. This linear motion produces a uniform conveyance of cuttings. Screen decks can either be flat or sloped slightly uphill.
Capacity Of Shale Shaker
The throughput capacity of shakers depends on three parameters: amplitude and stroke, vibratory motion and vibrator speed. Amplitude is defined as the maximum screen displacement perpendicular to the position of the screen. Stroke is defined as being twice the amplitude. Vibratory motion, defined previously, is either circular, unbalanced elliptical or linear. The vibratory speed is the speed at which the vibrator moves. For a shale shaker to perform effectively, it must separate as well as convey solids. Both are functions of acceleration (g factor where g = acceleration equal to the force of gravity).
This g factor is defined by the equation below:
G factor = (stroke, inches)(RPM)2 / 70, 400
A higher G factor gives better solids separation; however, it will also reduce screen life. Proper screen tensioning is critical with high g shale shakers. Most circular-motion shakers have an acceleration of 4-6 g’s. Most linear motion shakers have an acceleration of 3-4 g’s.
Screen Decks Of Shaker
A shale shaker screen deck (or basket) is vibrated to assist the throughput of mud and movement of separated solids (see Figure 2). Shale shakers that use an elliptical motion usually have divided decks with screens placed at different slopes in order to provide proper discharge of cuttings. Sloped-deck units can have one screen covering the entire deck length, or have a divided deck which has more than one screen used to cover the screening surface, or with individual screens mounted at different slopes. Multiple-deck units have more than one screen layer. In a 2 or 3-deck unit, mud must pass through one screen before flowing through the second, etc.
Shale shaker screening is the primary means of solids separation. If the shale shaker is not working correctly or if the screens are incorrectly sized or torn, efficiency is drastically reduced. Screening action depends on the vibrating action to make mud flow through it. Vibration under mud load creates stresses on the screen and if the screens are not properly installed and supported, they will quickly wear or tear.
- Shale shaker screens are available in square, rectangular and layered design. API has set standards on screen identifications. Screens are labelled with the following: separation potential (216, 250, 284), conductance and area. This notation is necessary because so many variables are possible in screen manufacture – wire size being the most significant.
- Weave of wire cloth and mesh count are two interesting design variables. (Figure 3 shows four weaves: plain square weave, rectangular opening, plain dutch weave and twilled square weave.) The square and rectangular weaves are the types most often used.
Since the thickness (diameter) of the wire used to weave a shale shaker screen can be varied for the same mesh, actual aperture or opening dimension in either direction can be used to describe a screen. Where the opening is small, a micron scale eliminates the use of decimals or fractions.
There are 25,400 microns to the inch. Thus, an opening of 0.0213 in., which is roughly the opening in a 30-mesh shale shaker screen, has a dimension of 541 microns. Both open area and conductance are terms used to describe and compare screens. Although percent open area is related to the ability of a screen to handle flow rate, conductance is a much better measure of the amount of fluid that will pass through a screen. The flow rate of a shale shaker is directly related to the area the liquid can fall through.
Mesh Count is the term most often used to describe a square of rectangular screen cloth. Mesh is only an indicator of the size opening as it is the number of openings per linear inch counting from the center of a wire. A mesh count of 30 x 30 indicates a square mesh having 30 openings/in. in both axis directions; a 70 x 30 mesh indicates a rectangular opening having 70 openings/in. in one direction and 30 openings per inch in the other.
A square mesh screen will generally remove more solids and make a finer cut than a rectangular mesh having one dimension the same as the square mesh, and the other one larger. The main advantage of the rectangular mesh screen is that it does not blind as easily. Another advantage of a rectangular mesh is that it can be woven with heavier wires, which offer longer screen life. Also, it has a higher percentage of open area and higher conductance which increases the fluid volume capacity of the shale shaker.
Basic Information Of Hydrocyclone
(Figure 4 shows a cut-away drawing of a hydrocyclone.) It has no moving parts, only the mud moves. Mud enters the upper, large cylindrical section tangentially to cause spiralling fluid flow. As fluid spirals toward the smaller end, this creates centrifugal force to make particles move toward the outer wall and then downward toward the opening at the bottom of the cone. Mud returns to the system out the top center through the vortex finder opening.
The so-called cut-point of hydrocyclones is the size of particle (sand in water) that has 50:50 chance of either exiting at the bottom of the cone for discard or returning to the mud through the vortex finder. A published cut-point is not directly applicable to muds because of differences in fluid viscosities and particle size, shape, and composition.
(Figure 5 shows cut point for a mud based on the amount of feed head. Cut point is a function of cone size, mud viscosity, feed pressure, or centrifugal forces developed due to velocity.) Without sufficient feed pressure from the pump, the hydrocyclones will not work as well as they should.
To operate a hydrocyclone in balanced condition, the flow out the bottom discharge opening should be a slow spray of liquid and solids. An opening which is too large will discharge too much liquid; an opening which is too small will retain too many solids. When a mud is loaded with drill solids, the cone may eject the solids in a rope. This means the cut point is not as small as it would be if it were operating in spray discharge. Roping is not the normal operating mode, because solids are not being removed at top efficiency. If roping is a continual problem, a larger number of desilter cones or finer shaker screens should be used to lighten the load.
Hydrocyclones should not be operated for very long on a weighted mud because barite will be discarded due to its higher density. For example, a barite particle of 50 microns diameter will be processed the same as a drill solid particle of 74-microns diameter.
Basics Information of Decanting Centrifuges
Decating centrifuges are devices that rotate the mud in a bowl to generate high centrifugal forces causing solids to move toward the outside for removal.
Decanting centrifuges have a cylindrical-to-conical shaped bowl that rotates at approximately 1000-4000 rpm. A conveyor screw is geared with the bowl and turns at a slightly slower speed to scrape the centrifuged solids out of the pool onto the beach for discharge (see Figure 6).
Design and control parameters for a decanting centrifuge are: bowl diameter; shape and speed; water and mud feed rates; and pool depth. Mud is usually fed into the machine by a positive displacement pump. If the feed mud is weighted, it is usually commingled with feed water to cut the mud viscosity within the machine.
In a typical barite recovery mode, the underflow is put back into the mud and the overflow is discarded as a way of getting rid of very fine solids in a weighted mud. In another mode, the underflow is discarded and the overflow is returned to the system.
This process would return expensive fluid to the system while discarding solids. This overflow can also be further processed by another higher g-force machine in which the underflow is discarded to get rid of the fines before returning the overflow to the mud. This operation depends on density of the mud and the relative value of the liquid versus solid phase of the mud.
Basics Information of Centrifugal Pumps
Centrifugal pumps contain an impeller inside a housing. Mud is pumped at constant head by the spinning action of the impeller. Head is controlled by impeller diameter and speed. Flow rate is controlled by head loss in the suction and discharge piping. Impeller velocity is kept constant by its drive motor, regardless of fluid density.
Choosing the correct pump and motor for a specific application requires consulting pump curves. A pump’s size is normally defined by the size of the discharge and suction flanges. A 4 x 5 pump has a 4-in. discharge and 5-in. suction flange. To evaluate a pump, one must know the pump size, its rpm, and impeller diameter (see Figure 7).
The following rules apply to centrifugal pumps:
- Centrifugal pumps do not have a fixed flow rate.
- Discharge head is independent of mud density.
- Discharge head is changed by changing the impeller speed and impeller size.
- Discharge pressure is linearly proportional to mud density.
- Power consumed increases as mud density increases.
- Power consumed increases as flow rate increases.