Corrosion Prevention, Rotodynamic Pump Speed & How Harmonics Affect VFDs

Q. What are the best materials to use in pump construction to minimize part corrosion? A. Factors to consider in the selection of materials for wetted pump components include user's experience, life cycle costs, regulatory agency requirements (i.e. limits on lead content in bronzes that contact drinking water), required pump life, duty cycle (operating hours per period), corrosive and/or erosive properties of the fluid, hazardous nature of the fluid, the potential for cavitation, and contamination of the fluid. Corrosive and/or erosive properties of fluids may vary with temperature, concentration of chemicals or solids, the properties of the solids, velocity, and the extent of entrained gasses. Some frequently used materials are listed below.

• Bronze-fitted pump:The casing is cast iron, and the impeller and impeller rings are made of bronze. This combination is commonly used for fresh water at ambient temperatures.
• All bronze pump: All pump parts in direct contact with the pumped liquid are made of manufacturer's standard bronze. This type is often used for pumping seawater.
• All iron pump:All pump parts in direct contact with the pumped liquid are made of ferrous metal (cast iron/ductile iron, carbon steel or low-alloy steel). This pump is commonly used in hydrocarbon services and some chemical applications.
• Stainless steel fitted pump: The casing is made of materials suitable for the service. The impellers, impeller rings and shaft sleeves (if used) are made of corrosion-resistant steel with suitable properties for the application. This type of pump is also used in hydrocarbon and chemical services.
• All stainless steel pump:All pump parts in direct contact with the pumped liquid are made of corrosion-resistant steel with suitable properties for the specific application. This pump is commonly used in chemical applications.
• Rigid polymers/composites:All pump parts in direct contact with the liquid are made of rigid polymers or composites (plastics), either as coatings or as structural material. This pump type is commonly used in chemical services.

For more information on materials of construction for pumps, see ANSI/HI 9.1-9.5 Pumps—General Guidelines. Q. When and how should speed for a rotodynamic pump be monitored? A. Monitoring pump speed is a vital part of overall condition monitoring for many reasons. Knowing the pump speed is important for avoiding a known critical speed and for monitoring head and flow rate, net positive suction head (NPSH) and power. Monitoring pump speed is also important for vibration analysis, avoiding speeds that are harmful to the drive system design and maintaining system control. There are two types of systems drive pumps: constant speed and variable speed. For constant-speed systems, the motor speed is unlikely to change significantly unless a major electrical problem has occurred. While the load of a rotodynamic pump varies with rate of flow, the changes in speed associated with the load changes are relatively slight (less than 2 percent of full load speed for NEMA Design B motors). However, changes due to high or low voltage or loss of power to one phase on a three-phase motor may be significant. Variable-speed systems rely on speed change to control head and rate of flow. These systems can have the same problems as constant-speed systems but may also have unintended speed changes resulting from a faulty drive or speed-control problem. When possible, blocking certain speed ranges may be necessary to prevent operation at or near rotor or structural critical speeds. Common methods of measuring speed are strobe light, revolution counter, tachometer or electronic counter. Any of these devices should be able to measure within 0.1 percent. For more information on condition monitoring, see ANSI/HI 9.6.5 Rotodynamic (Centrifugal and Vertical) Pumps — Guideline for Condition Monitoring. Q. What are harmonics, and how do they affect variable frequency drives? A. Harmonics are undesirable voltages and currents that may appear on electrical power systems as a result of certain types of nonlinear loads, such as computers, electronic lighting, welding supplies, uninterruptible power supplies (UPS) and variable frequency drives (VFDs). When installing VFDs, operators should consider the existing harmonic content and account for any harmonic impact the newly installed VFD may have on the electrical system or the electrical utility. Harmonics can be a concern for any size VFD, whether it is a 500-horsepower (HP) unit in a building or a 75-HP unit in an irrigation application. Operators should review the existing installation—not just the current project—with consultants and drive representatives in order to find the most positive installation solution. A full-wave bridge rectifier or converter converts alternating current (AC) power to direct current (DC) power, no longer a sinusoid waveform, and the capacitor bank acts as a filter smoothing this converted voltage. All AC/DC converters used in different types of electronic systems can increase harmonic disturbances by emitting harmonic currents directly into the electrical grid. Harmonic distortion can cause overheating of supply cables and distribution transformers (reduced insulation life), LED and lighting disturbance, nuisance tripping of circuit breakers, ancillary electronic equipment damage and potential faulty readings, and damage of backup generators. An active filter cancels out most harmonics on the current line. The active filter injects equal and opposite harmonic current onto the line similar to the way that noise-canceling headphones inject noise-canceling signals into the air around the wearer's ears. As the noise-canceling signal from the headphone results in the cancellation of noise, the active filter cancels out the harmonics. For more information on variable frequency drives, see the Hydraulic Institute Guidebook Variable Frequency Drives: Guidelines for Application, Installation and Troubleshooting.