Slurry pump impellers are balanced to meet the acceptance criteria determined by the dynamic effects from variables such as speed, peripheral velocity and shaft stiffness. The slurry impeller unbalance mass (mo) is the result of there being an eccentricity (e) between the impeller center of gravity (cg) and the center of rotation (cr) (see Figure 1). This eccentricity is unavoidable with the variations in alignment of a casting pattern or the positioning of a reinforcing into an elastomer mould. This eccentricity may be very small but could still amount to an unbalance that exceeds the acceptance criteria for the slurry pump impeller.
There are two options for balancing impellers; single-plane and dual-plane.
Slurry pumps generally operate at slow to moderate speeds and are typically designed to have sufficiently large bearing assemblies with relatively rigid shaft extensions to the impeller. This is the reasoning behind why the majority of slurry pump impellers only require single-plane balance.
Single-plane balance is when the amount of unbalance is measured in a single plane and the balance correction is allocated to that plane. In this case, the impeller would be treated as a single disc and the balance correction may consist of a crescent-shaped portion of material removed from the outside diameter of both the front and back shrouds at the same location angle (see Figure 1). Single-plane balancing may be conducted by use of either static or dynamic balancing equipment.
Table 1 – Balance quality grades for various rotors
|G250||Large crankshaft engines|
|G100||Car reciprocating engines|
|G6.3||Centrifuge drums, Fans, Flywheels, Machine Tools, Pump Impellers|
|G2.5||Gas & Steam turbines|
|G1||Tape Recorder drives|
1. Static balancing equipment is commonly used to position the impeller on a freely rotating horizontal arbor and measure gravitational force needed to maintain the slurry pump impeller in a stationary position to determine the unbalance. There are also calibrated static balancing machines that position the impeller vertically and use pressure sensors to determine the unbalance more accurately.
2. Dynamic balancing equipment is used to position the impeller on a machine driven arbor and measure vibrations caused by centrifugal forces to determine the unbalance. This method has the most sensitivity and accuracy for measuring unbalance in either single-plane or dual-plane.
Slurry pumps are also designed for lighter duty services that may allow for higher operating speeds and/or less rigid shaft extensions to the impeller. Slurry pumps designed for these services may be more sensitive to the dynamic effects of impeller unbalance and in some instances may require a dual-plane balance.
Dual-plane balance is when the amount of unbalance is measured in two planes and the balance correction is allocated to each of those planes. In this case, a closed slurry pump impeller would be treated as two separate discs, typically the front and back shrouds. The balance correction may consist of a crescent-shaped portion of material removed from the outside diameter of both the front and back shrouds but at different location angles. Dual-plane balancing can only be conducted by use of dynamic balancing equipment, as described above.
Slurry pump impellers can be balanced to meet an acceptance criteria defined as the permissible residual unbalance (Uper) which is the product of the unbalance mass (mo) and impeller radius (Ro). ISO 21940-11 (previously ISO 1940-1) is the global industry standard for calculating this Uper value using parameters such as impeller weight, rotational speed and balance grade level (G).
Table 1, above lists recommended balance quality grades for various rotors from ISO 21940-11. Pumps are grouped with flywheels and machine tools under G6.3. This is acceptable in water pumps or where impellers have an infinite life, ie. they do not become unbalanced over time due to abrasion, corrosion or cavitation. The use of a G6.3 grade exclusively would not usually be justified in slurry pumps since when handling abrasive solids the impellers are expected to have a limited life due to erosion and corrosion. They become unbalanced, cause vibrations, degrade in performance and must eventually be replaced in order to reset the pump to the original operating conditions.
As an example, if we consider G6.3 value specified for slurry pumps, we can calculate that a small 4/3 slurry pump impeller, running at 2300 r/min must be balanced so that it will generate a centrifugal force equal to not more than 17% of its weight. On the other hand, a large and heavy duty 20/18 slurry pump impeller, running at 400 r/min, must be balanced to produce a centrifugal force equal to not more than 3% of its own weight. Applying G6.3 to a large impeller would therefore add significantly to its cost but the effects of these efforts would only last until the impeller starts to wear.
Experience has proven that G values higher than G6.3 can be used with safety on slurry pump impellers larger than the 4/3 slurry pump. As a rule of thumb, slurry pump impellers balance requirements will fall between G40 on the high (large amount of residual unbalance) side and G6.3 on the low (small amount of residual unbalance) side.
Hebei Tobee Pump Co.,Limited