Static Balancing Criteria and Acceptance Standards for Slurry Pump Impellers

Static balancing criteria

Slurry pump impellers are virtually all statically balanced to an acceptance criteria and this is generally carried out by machining off a crescent shaped portion on the outside diameter of the impeller shrouds (see FIGURE 1). A residual unbalance generally still remains which may result in unwanted dynamic effects and which is the key to establishing appropriate acceptance criteria. Since dynamic effects are described by values such as speed, peripheral velocity and shaft stiffness, a determination of acceptance criteria must include not only the impeller’s dimensions but also its allowable operating conditions.

There are three useful ways to specify the maximum residual unbalance of an impeller.

1. Static unbalance is defined, as the mass m0 required at the outside diameter to stop any

rotation when held in a frictionless bearing. The value of m0 fully describes the static unbalance but not surprisingly, a fixed value of m0 has different effects on different impellers (large effect on a small or fast rotating impeller and a small effect on a large or slow rotating impeller) and thus m0 by itself is not a suitable acceptance criterion. A slightly more refined way is to look at the percentage of m0 relative to the impeller mass M. 

2. Another method is to recognise the notion that there is a distance e between the impeller’s centre of gravity (cg) and the axis of rotation. With the impeller rotating at an angular speed w, the cg’s tangential velocity (Vt) is quoted as a limit rather than an actual value and forms the basis of ISO 1940’s G-grade as shown in Table 1. The convenience of this approach is that for a range of impellers with the same peripheral velocity the resulting m0/M is constant. Note that, since we have included the dynamic parameter via angular velocity w, a particular impeller’s actual G value is zero at rest and increases directly with speed.

2. A third method is to look at the centrifugal force caused by unbalance and particularly at its percentage of impeller weight. This ratio, called unbalanced centrifugal force factor kc, should be <1 as it assures that the impeller of a horizontal pump has an uninterrupted downward force and does not cause load reversals on bearings. Again, for a particular impeller, the kc value is not a constant but depends on the speed of operation.

Unbalance acceptance criteria

In TABLE 1, 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 pumps, we can calculate that a small 4/3 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 20/18 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 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. Impellers having a diameter to width ratio of >6 may be statically balanced in a single plane, using balance rails, on a motor shaft arrangement (or by using a commercial balancing machine and making a single plane

correction).