The characteristics of shale shakers are: (1) vibratory motion, (2) amplitude and frequency, (3) deck slope and number of decks of screens, and (4) screening area.

1. Vibratory Motion - There are three types of shale shaker screening motion: circular, unbalanced elliptical and linear (see Figure 11) 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.

2. Amplitude and Frequency - 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- i"ch/es>(RPM)2

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.

Figure 11 Shale Shaker Motion

Figure 11 Shale Shaker Motion

3. Slope and Number of Screen Decks - A screen deck (or basket) is vibrated to assist the throughput of mud and movement of separated solids (see Figure 12). 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.

Figure 12 Deck Slope & Number of Decks

Figure 12 Deck Slope & Number of Decks

Screening Area - 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.

1. 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.

2. Weave of wire cloth and mesh count are two interesting design variables. (Figure 13 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.

Figure 13 Screen Cloth Weaves

PLAIN SQUARE WEAVE

PLAIN SQUARE WEAVE

RECTANGULAR OPENING

RECTANGULAR OPENING

PLAIN DUTCH WEAVE

TWILLED SQUARE WEAVE

1. PLAIN SQUARE WEAVE

Providing a straight through flow path with the same diameter warp and snute wires in and over and under pattern. This is the most common weave producing the same mesh count vertically and horizontally.

2. RECTANGULAR OPENING

Provides maximum open area and tends to prevent binding or clogging of material. Does not build up on the longer openings and smaller dimension controls the sizing of material.

3. PLAIN DUTCH WEAVE

Provides a tapered opening reducing flow rate. Warp wires are heavier in a plain weave and shute wires are driven close and crimped at each pass.

4. TWILLED SQUARE WEAVE

With the pattern of over two wires and under two wires, this weave produces a diagonal effect. To provide greater strength and corrosion resistance, a larger diameter wire can be woven.

Since the thickness (diameter) of the wire used to weave a 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 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.