Page 232 - Shale Shakers Drilling Fluid Systems

214 SHALE SHAKERS AND DRILLING FLUID SYSTEMS

equipment, however, it is suggested that centrifu- center to the center of gravity of weight). The
gal pumps be started with the discharge valve unbalance provides the amplitude at which the
partially closed to restrict initial pump output and vibrating screen will move (see Chapter 3).
load demand on the motor. Two counter-rotating shale shaker motors will
produce a linear force that should be located
through the center of gravity of the shaker bas-
Electric Motors for Shale Shakers ket. The resultant motion is perpendicular to a
plane drawn between the rotating shafts directed
Shale shaker motors are generally three-phase through the center of gravity of the machine. The
induction motors that are explosion-proof hav- shale shaker motor should be selected to meet or
ing NEMA Design B or similar characteristics exceed the desired stroke of the machine, centrifu-
(Table 12-10). gal force, and acceleration (G's). Adequate horse-
A shale shaker motor usually has four magnetic power is required to perform the work and to
poles (1800 rpm synchronous shaft speed at 60 ensure synchronization. Synchronization results in
Hz), but may sometimes have six poles (1200 rpm) opposing forces from two counter-rotating vibrators
or two poles (3600 rpm). that cancel each other and double directional forces.
The motor should have independent, third-party Stroke, which is independent of motor speed, is
markings indicating its suitability in explosive or the peak-to-peak displacement imparted to the
potentially explosive environments. It is recom- machine. Dampening may occur in the system
mended that these motors be suitable for Class I, affecting the total stroke. Stroke is a function of
Division 1, Groups C and D, and Group IIB atmo- the unbalance (or torque) of the motor and of the
spheres. The motor also should have the proper total weight of the shaker basket, including the
operating temperature or code designation for the weight of the motors and the live load. The stroke
anticipated ambient temperature. equals two times the motor unbalance, multiplied
A 50-Hz motor driving a shale shaker vibrator by the number of motors, divided by the total
should not be operated at 60 Hz since the centrifu- weight. Note that the motor unbalance is a func-
gal force output will increase by 44%. This will tion of the eccentric weight setting. For example,
likely damage the bearings and the vibrating screen. the motor unbalance is 50% of the maximum
A 60-Hz motor driving a shale shaker vibrator can unbalance if the eccentric weights are set at 50%.
be operated at 50 Hz with the understanding that The centrifugal force output of the vibrating
the centrifugal force output will decrease by 31%. motor (Ibs.) is equal to the shaft speed squared
If, at 60 Hz, the centrifugal force is 1000 Ibs., the times the unbalance (in.-lb.), divided by 35,211.
centrifugal force will only be 690 Ib at 50 Hz. Once again, the vibrating motor's centrifugal force
For a given frame size, higher speed motors will output is a function of the eccentric weight set-
have high horsepower ratings, low slip, high start- ting. For example, the centrifugal force is 50% of
ing torque, and low bearing life. Conversely, lower the maximum centrifugal force if the eccentric
speed motors will have lower horsepower ratings, weights are set at 50%. Typical acceleration rates
high slip, low starting torque, and long bearing life. for vibrating screens are 4 to 8 G's.
Electric industrial vibrators are rated in centrifu-
gal force output, frequency, unbalance (working
moment), and horsepower. Centrifugal force is Electric Motors for Centrifuges
created by a rotating offset mass operating at a
given speed. The unbalance is caused by torque Most centrifuges use the same NEMA Design B
resulting from the offset eccentric weight times the explosion-proof motors used for centrifugal pumps:
moment arm [product of] (distance from the shaft either 1450 rpm at 50 Hz or 1750 rpm at 60 Hz.

TABLE 12-10. Electric Motor Specifications for Shale Shakers
US DESIGNATION IEC DESIGNATION
Terminology Explosion-proof Flame-proof
Hazardous Location Class I, Division 1, Eexd Gas Group IIA
Rating Group D
Hazardous Location Class I, Division 1, Eexd Gas Group IIB
Rating if Hydrogen Groups C and D
Sulfide is Encountered