As with all equipment, agitation and mixing equipment (Mud agitator) must be sized and installed properly. Poor performance will result from improper sizing of equipment and improper installation.
Mud Agitator Design Parameters
The following information must be known to properly size an mud agitator system: . Mud Tank and compartment dimensions . Compartment shape . Compartmentduty (solids removal, testing, suction, storage, or pill/slug) . Maximum mud density expected Once this information is collected, the design process focuses on the size and type of impeller and the amount of energy required. As mentioned earlier, there are two basic types of flow patterns for mechanical mud agitators: radial and axial. Choosing the Right Impeller:
Radial flow impellers, as used in the drilling industry, are typically fabricated from mild carbon steel (stainless steel is less often used and generally not required, but is available for certain situations) and have rectangular blades (typically three or four per impeller) mounted in a vertical position on some type of hub. In square and rectangular compartments, properly sized radial flow will produce some axial flow when the fluid impacts the compartment side walls. As with all square or rectangular compartments, dead spaces or spots will occur. Complete elimination of dead spots is not practical; however, when properly sized, they are negligible. The impeller should be mounted about 3 to 6 inches (7.5 to 15 cm) from the tank bottom. If the impeller is positioned too far off bottom, the flow will not sweep the tank bottom properly and dead spots can occur. Dead spots not only reduce usable tank volume (thereby decreasing the effective circulating volume) of drilling fluid, but can also increase drilling-fluid costs by allowing barite or other commercial solids to settle. Additionally, the settled solids will increase the time and expense of cleaning tank compartments before rig demobilization or when displacing or converting a drilling fluid from one type to another. It cannot be emphasized enough that good agitation results from proper agitator sizing and impeller placement.
In deep tanks, or where two or more impellers are on the same shaft, the impeller shaft should be stabilized at the tank bottom. Stabilizers are typically short lengths of pipe with an internal diameter large enough to accept an agitator shaft without hindering rotation. The stabilizer pipe will usually have drainage holes cut in it and be welded perpendicular to a small portion of flat steel plate that is affixed to the tank bottom, or the stabilizer may be directly welded to the tank bottom. Stabilizers will limit excess side loads on the bearings, can extend gearbox output bearing and oil seal life, and help prevent the shaft from bending. This is especially useful in situations where rig components are stored in the tanks during rig moves.
With axial flow impellers, there is more flexibility in the choice of shaft length than with radial flow impellers. A well-designed installation will usually have the axial flow impeller mounted two thirds to three quarters of the impeller diameter from the tank bottom. Proper placement of the impeller will make best use of the tank geometry to deflect the fluid flow from the impeller, along the bottom and against the compartment walls. The impeller shaft should be stabilized in deep tanks.
Since axial flow impellers are positioned higher in the tank, higher liquid levels must be maintained to prevent vortex formation (Figure 10.10) and entrapment of air in the mud. If the axial flow impeller is mounted too low, bottom scouring may occur, which could lead to excess erosion of the tank bottom. Another consequence of an improperly mounted impeller is ineffective agitation of the fluid at the surface, which can lead to poor homogenization of the fluid.
Variable pitch impellers (contour, or swept-face, impellers) depart from flat and axial flow impellers by incorporating more than one contact angle on the impeller face. They require less horsepower to move the same amount of fluid as axial or radial impellers; therefore, they can have larger blades or smaller gearbox/motor combinations, or higher torque-capacity gearboxes with smaller motors than radial or axial impeller agitators. Often, they are used in extremely large-volume compartments. Because they impart a more efficient movement of fluid as it contacts the blade surface, less shear force is imparted. Shear is desirable in some compartments of a mud system and not needed in others. When dealing with ‘‘freshly made’’ fluid or in a slug or pill compartment, shear aids in speeding the process of blending and homogenization. Compartments predominantly designated for long-term storage of drilling fluid, such as large-volume holding tanks, will not need strong shear forces. Contour-type impellers have the added benefit of requiring less horsepower per unit of fluid displaced, so they are ideally suited for this purpose.
The placement and sizing of impellers, whether radial, axial, or contour, are extremely important. If all other phases of design and installation are correct and impellers are improperly placed, the result can negate the efforts of an otherwise efficient design. Be sure to consult the manufacturer for proper sizing and placement of all components.
Compartment(Mud Tank) Shape
Agitators work best when they are placed in symmetrically sized round or square compartments(as viewed from the top). Round compartments (Mud Tank) are ideal for many reasons, including their having less dead space in which solids can settle.
When fitted with a center drain or clean-out, ‘‘round compartments’’ are easier to clean and require less wash fluid than rectangular and square tanks. They are symmetrical; therefore, mixing is usually thorough and
optimal; however, there are some drawbacks to round compartments. They use space less effectively and will therefore occupy more room for the same capacity compared with rectangular or square tanks. Round tanks require baffles to prevent solid-body swirl and promote good suspension patterns, which raises fabrication cost.
Mud Tank and Compartment Dimensions
Proper agitator sizing is based upon the amount of fluid to be stirred. Therefore, knowledge of tank dimensions is required. Under most circumstances, all compartments other than the sand trap require agitation. Some systems convert the sand trap to an active compartment; in this case, agitation is required. This can be problematic, considering that many systems have shakers mounted above that compartment, with little or no space allotted for mechanical agitators. If such a contingency is anticipated prior to tank fabrication, arrangements can be made to place the agitators where they will not interfere with shaker placement. Alternatively, mud guns may be used. They will provide fluid movement to stir the compartment.
Mud Tank Internals
An important consideration when constructing mud tanks is the placement of internal piping. If these are positioned wrong, effective agitation may be impossible. The best advice when installing pipes in any type of system is to use common sense. Think about what effect the piping will have on the flow patterns within the compartment. The flow
path of the agitated fluid should not be obstructed by pipe or structural support members.
Round Tank Baffling For a round or cylindrical tank, baffles are essential. Baffles convert swirling motion into a flow pattern to help suspend solids and maintain homogeneity. Baffling can also help prevent vortex formation. In both cases, baffles promote efficient application of power. Baffle width should range from one tenth to one twelfth of the tank diameter and be positioned at 90 increments around the tank. Baffles are generally more effective when placed a short distance from the vessel wall. A gap of 1⁄60 to 1⁄70 of the vessel diameter is recommended between the baffle and the vessel wall.
Square Tank Baffling
Good fluid suspension in a properly sized square tank is similar to a fully baffled circular tank. The sharp corners of square and rectangular tanks
induce nearly the same motion as baffles in round tanks. However, as the length to width ratio of a rectangular compartment increases, the chance for dead zones increases in the far ends of the tank. Strategically placed
baffles at the midpoint of the long section of the compartment will counteract this negative effect. When the ratio exceeds 1.5 : 1, more than one agitator is recommended.
Some manufacturers recommend that baffles be installed around each impeller to enhance agitation and prevent air vortices. A typical steel plate baffle is ½ to ¾ inch thick by 12 inches wide and extends from the tank bottom to at least 6 inches above the top mud agitator blade (about 1 to 2 cm thick by 30 cm wide and extends 15 cm above). Four baffles are installed around each agitator at 90-spacing along lines connecting the agitator shaft center with the four corners of a pit (Figure 10.11). For a long rectangular pit with two or more agitators, the tank is divided into imaginary square compartments and a baffle is pointed at each corner (either actual or imaginary).
Regardless of what style of agitator or impeller is used, proper sizing of components is critical. Once compartment size has been determined, the impeller diameter and corresponding horsepower requirements must be calculated. If the maximum mud weight to be used with the rig is not known, it is best to base all calculations on 20 lb/gal fluid (2.4 specific gravity [SG]). This will give a sufficient safety factor to allow agitation of most any fluid without fear of overloading the motor. Most oilfield agitators range in shaft speed from 50 to 90 rpm.
Turnover Rate (TOR)
Impeller sizes are determined by calculating the TOR (sometimes called time of rollover) for each compartment. This is the time, in seconds, required to completely move the fluid in a compartment (Table 10.1) and can be calculated by knowing the tank volume and impeller displacement:
TOR = (Vt⁄D)×60
. Vt=tank volume, in gallons or liters
. D=impeller displacement, in gpm or lpm (as displayed in Table 10.2).
For flat and canted impeller applications, TOR should range between 40 and 85 seconds. As the TOR approaches 40 seconds, the chance for vortex formation and possible air entrainment increases. At values greater than 85 seconds, proper suspension may be jeopardized and solids will begin to settle.
Table 10.1 Typical Turnover Rate Values, in seconds
60-Hz Impeller Displacement D Values
|Diameter Flat Canted Contour|
For contour impeller applications, values must be significantly faster (i.e., smaller numbers) to achieve the same results, but because of the impeller design, air entrainment is less probable. In symmetrical compartments, the fluid has a nearly equal distance to travel from the center of the impeller shaft or from the impeller blade tip before it contacts the vessel wall. Agitators should be placed where the shaft is centered in the tank or compartment.
When defining the area in which to mix, it is best to work with symmetrical shapes like squares or circles (as viewed in a plan drawing or overhead view of the tank layout). Rectangular tanks should be converted to nearly square compartments if possible. Maximum fluid working volumes in compartments should not be higher than 1 foot (about 3⁄10 m) from the top of the tank. This will allow for a little extra capacity in emergencies, slightly out of level installations, and/or fluid movement on floating rigs.
Working volume for square or rectangular tanks is calculated by knowing dimensional values for length (L), width (W), and height (H; in feet for gallons, in meters for liters):
Vt = L × W(H− 1)× 7.481
The working volume for round tanks with flat bottoms is:
Vt = Π r²(H−1)× 7.481
Vt = Π r²(H−0.3)× 1000
For round tanks with dish or cone bottoms, calculations for working fluid volume are based on straight wall height (i.e., this height is measured from the tank top to where the tank joins the cone or dish at the bottom). This leaves adequate free space above the maximum fluid operating level. In all cases, if H<5 feet (1.5 m), a radial flow impeller should be specified.