Q. What is an acceptable amount of residual unbalance in an impeller after it has been balanced?
A. High levels of residual unbalance in rotating parts can generate high unbalance forces, resulting in excessive bearing and shaft loading and inducing high levels of vibration. Balancing methods and residual unbalance limits for impellers are described here.
Note: Other rotating parts may be subject to similar limits. However, drive system components such as motors and couplings are addressed by other standards and may need other considerations.
Pump impellers are typically balanced in accordance with ISO 1940 balance quality grade G6.3 or better (see Figure B.1). This figure indicates the center of gravity displacement or residual unbalance acceptable for balance grade 6.3. (See ISO 1940 for values relating to other balance grades.)
For balancing slurry pump type impellers, refer to the latest edition of ANSI/HI 9.6.4, Rotodynamic Pumps for Vibration Measurements and Allowable Values.
Depending on component geometry, it may be satisfactory to perform a single-plane spin balance. Components are typically single-plane balanced if the ratio of diameter to width D/b is 6.0 or greater. Two-plane (or dynamic) balancing is typically performed otherwise.
Figure B.1 is used by entering the graph at the maximum expected service speed, such as 3000 rpm, and reading the acceptable residual unbalance, 0.021 kg-mm/kg (0.85 oz-in/oz). Multiply this number by the impeller weight in kg (oz) and the result is the allowable unbalance of the impeller in kg-mm (oz-in).
Balancing-machine sensitivity shall be adequate for the part to be balanced. This means that the machine is capable of measuring unbalanced levels to one tenth of the maximum residual unbalance allowed by the balance quality grade selected for the component being balanced.
Balancing machines are capable of measuring unbalance independent of its speed. When the value for allowable unbalance is determined from Figure B.1, it is not necessary to operate the balancing machine at the same speed as the pump speed.
Figure B.1
Q. The corrosion resistance of polymer materials makes them good choices for many pump parts. However, lower temperature and pressure limits may be a concern. What are the normal temperature and pressure limits for polymers?
A. Two types of polymer materials-thermoplastics (TP) and thermosets (TS)-are used in polymer pumps.
Thermoplastics are usually thin-wall members with lower allowable temperatures and pressures. They are lower-weight pieces and melt when excessive heat is applied to them. Thermosets can be heavy-wall, high-pressure, large components and char when excessive heat is applied.
Typical characteristics of pump construction of two polymer types are as follows. These characteristics are a preliminary guide only. Depending on design, these values may be higher.
Both thermoplastics and thermosets are usually internally reinforced by the addition of glass or carbon fibers. The addition of the reinforcing fibers can double the tensile strength in many polymers. When selecting materials for a liquid, the limiting criteria may be the attack of the liquid on the reinforcement-not the polymer.
More information on selecting polymers suitable for a wide range of liquids can be found in ANSI/HI 9.1-9.5, Pump - General Guidelines for Types, Definitions, Application, Sound Measurement and Decontamination. Since this guide is for a pump (pressure boundary parts), the limiting temperature is shown only for continuous applications. This guide does not apply to lined pumps. If a polymer is not recommended, it is identified as NR. When no data was available a dash (-) is indicated.
Polymers are also used for non-pressure parts like wear rings and bushings, and may have higher temperature ratings than shown.
Q. Measurement of flow rate through reciprocating pumps is often difficult due to the pulsating flow. What method can provide accurate flow measurement for these pumps?
A. Methods that measure over a longer period of time will do the trick. These include the following:
Flow Rate Measurement Methods
Weight
Measurement of flow rate by weight depends on the accuracy of the scales used and the accuracy of the measurement of time. A certification of scales shall become part of the test record, or, in the absence of certification, the scales shall be calibrated with standard weights before or after test. Time intervals for the collection period must be measured to an accuracy of one-quarter of 1 percent.
Volume
This is done by measuring the change in volume of a tank or reservoir during a measured period of time.
The tank or reservoir can be located on the inlet or discharge side of the pump, and all flow in or out of the tank or reservoir must pass through the pump. In establishing reservoir volume by linear measurements, consider the geometric regularity (flatness, parallelism, roundness, etc.) of the reservoir surfaces as well as dimensional changes due to thermal expansion or contraction or distortion resulting from hydrostatic pressure of the liquid.
Liquid levels shall be measured by means such as hook gauges, floats and vertical or inclined gauge glasses. In some locations and under some circumstances, evaporation and loss of liquid by spray may be significant and greater than the effects of thermal expansion or contraction. Allowance for such loss shall be made, or the loss shall be prevented.
Displacement Type Meters
Meters responsive to displacement include piston meters, wobble plate meters, rotary vane meters and the like.
For more information, please see ANSI/HI 6.6, Reciprocating Pump Tests.
Pumps & Systems, November 2009