This month’s Q&A is provided by the Hydraulic Institute (HI). HI is a leading developer of pump standards, technical reports and guidelines, data books, and educational material. As an ANSI-accredited standards developer, the Hydraulic Institute continuously reviews and updates its pump standards and adds new subjects as requested by the pump community. The organization encourages users to submit questions, and HI will respond based on existing HI standards, technical documents, and educational products. Please direct your inquires to [email protected].Q: What are some of the key things a user must consider when specifying a pump application?A: Pump users can look at any of the frequently used data sheets provided by HI, which do a fine job of identifying the customary list of requirements, such as operating conditions and performance requirements, site location, liquid characteristics, bearings, seals, materials of construction, and a substantial listing of other database entries, all of which are important to properly specify a pump. Yet they also need to take the next step and understand that while a pump may be purchased as an individual component, its successful operation depends on many interdependent factors, including how well the system was designed, the nature of the initial installation, and the manner in which the system is operated and maintained.The two general categories of pumps used today are kinetic pumps (centrifugal, regenerative turbine, and special effect) and positive displacement pumps (rotary and reciprocating). The flow produced by a centrifugal pump is a function of the resistance offered by the system in which it operates. Positive displacement pumps move a defined volume of liquid and their flow is directly related to the operating speed. The pressure at which a positive displacement pump operates is a function of the system resistance, and precautions must be taken to ensure that the pump discharge pressure is not excessive.Selecting a pump that is undersized or fails to meet the process guarantees is a bad decision, which obviously has a high degree of visibility. Selecting a pump that is oversized is also a bad decision, but may go unnoticed until the maintenance records or an energy audit is performed. Adding excess margin to the design will result in an oversized pump that is typically controlled with a bypass line, which recirculates unused, pressurized flow, or with a highly throttled control valve, which artificially shifts the system curve by inducing a high backpressure. When either of these actions forces the pump to operate far from its best efficiency point (BEP), the penalty will be higher energy consumption and an adverse impact on the service life of the pump.Q: How do different pump operating conditions affect the seal selection process?A: The ultimate responsibility for successful selection, installation, and operation of seals rests with the end-user. The correct application of mechanical seals is a complex process, and consideration should be given to the operation of the equipment, seal chamber design, flush arrangements, liquid properties (chemical compatibility), product (suction and discharge) pressure and temperature, shaft speed, seal arrangement, misalignment requirements, and available utilities. The manner in which the pump is operated — continuous duty, intermittent duty, hot standby, cold standby, on-off operation with spare pump, pump idle, or turning gear operation — is an important criterion that must be communicated to the vendor during the selection process.The effects of operating a centrifugal pump at rates of flow that are above or below the pump’s BEP will influence the service life of a seal. Design characteristics for both performance and service life are optimized around the BEP — the point at which the hydraulic efficiency is highest — and the liquid enters the impeller vanes, casing diffuser (discharge passage) or vaned diffuser in a shockless manner. Flow through the impeller and diffuser vanes is uniform, free of separation, and well controlled. When a pump is operated away from the BEP, a number of different phenomena can occur, such as increased vibration, increased temperature rise, increased shaft deflection, reduced NPSHA margin, and an increase of internal recirculation within the pump. Excessive shaft deflection at the faces of a mechanical seal will reduce the seal life. Most process pump manufacturers limit the allowable operating region (AOR) to operating conditions where the shaft deflection at the seal faces is 0.002 in or less for pumps with rolling element bearings. Since most seal designs and all compression packed pumps permit greater deflections, the continuous rate of flow limits (both maximum and minimum) are application specific.Q: What are the common types of corrosion found in pumping applications? How can users approach the corrosion issue such that they are employing effective material of construction at a reasonable price point?A: The most common application for pumps is moving cold water, and corrosion is not a serious problem here. The standard materials for water applications are cast iron casing and bronze impeller, wearing rings, and mechanical seals. Some people use lower cost cast iron impellers, but they are more likely to corrode due to the high flow velocity of water through the impeller.The more difficult corrosion problems occur when pumping corrosive chemicals. The solution is usually the use of more costly metal alloys such as stainless steel — Monel, nickel, titanium, etc. This problem usually requires the expert guidance of experienced metallurgists or corrosion engineers. The Corrosion Handbook and Hydraulic Institute Standard ANSI/HI 9.1-9.5 "Pumps-General Guidelines" are good references. Non-metallic materials have also been used successfully when pumping corrosive chemicals. NACE International (www.nace.org) is another excellent source for information regarding corrosion. Q: How concerned should users be about vibration in a pumping system? What can users do to limit the impact of vibration on a pumping system?A: Users should be seriously concerned about vibration in pumps since excessive vibration will cause premature failure of pump bearings, seals, and shafts, leading to unscheduled shutdown of pumps and costly repairs.To avoid this problem, the vibration level of pumps should be measured periodically and increasing levels of vibration should be given special attention. ANSI/HI 9.6.4 "Vibration Measurements and Allowable Values" provides a guide for allowable field vibration levels for different pump types and ANSI/HI 9.6.5 "Condition Monitoring" provides recommendations for frequency of taking measurements.Increasing vibration with time is usually caused by scale buildup or erosion of impeller surfaces and can be corrected by cleaning the impeller and rebalancing. However, some problems are caused by resonance when operating close to the natural frequency of vibration of the pump shaft or structure. This is more common with multistage pumps and requires a detailed analysis of the pump design to correct the problem. Q: What are the main concerns users should have with piping systems as they pertain to a pumping application? Are there any common mistakes you see users making in regard to piping for pumping applications?A: A well designed piping system must deliver the liquid to a pump in a smooth, evenly distributed flow pattern at a reasonable flow velocity and it must not impose excessive pipe strain or hydraulic loads on the pump’s nozzles. The inlet piping is most critical for successful pump operation and must be designed to avoid uneven distribution or flow characterized by strong local currents, swirls, or entrained air. The ideal approach is a straight pipe, coming directly to the pump, with no turns or flow disturbing fittings close to the pump. Failure of the inlet piping to deliver the liquid to the pump in this condition can lead to noisy operations, random axial load oscillations, premature bearing or seal failure, cavitation damage to the impeller and inlet portions of the casing.Outlet piping flow characteristics normally will not affect the performance and reliability of a pump; however sudden valve closures may cause excessively high water-hammer generated pressure spikes, which are reflected back to the pump, possibly causing damage. Where sudden valve closures are a possibility, a transient analysis may be needed.Excessive forces placed on pump nozzles due to improperly supported equipment, such as valves or other components, improperly restrained expansion joints or uncompensated thermal expansion can result in misalignment of the pump and driver, binding or rubbing of the pump rotor, and in extreme cases, breakage of pump nozzles, pump feet, or damage to the baseplate structure.A life cycle cost analysis of the piping system will identify areas where pipe size can impact energy costs. The pipe diameters are based on factors such as: (a.) the required lowest flow velocity for the application to avoid sedimentation; (b.) the required minimum internal diameter or the application when solid handling is required; (c.) the maximum flow velocity to minimize erosion in piping and fittings; and (d.) standard diameters used in the facility. Decreasing the pipeline diameter has the following effects: initial costs of piping and components will be lower; initial costs of the pump will be higher; and related electrical supply items therefore will increase. Energy costs, the greatest portion of the total cost of ownership, will increase as a result of higher power usage caused by increased friction losses. Q: What are some best practices users can employ to ensure the longevity of a pumping system?A: Users should educate themselves on pump basics and have a clear understanding of how a pump interacts with the system in which it is installed. A centrifugal pump will operate at the point where its performance curve and the system curve intersect. Positive displacement pumps deliver a (nearly) fixed volume of liquid at a pressure that is determined by the system. In addition, it is essential that the users identify all known operating conditions and communicate them to the vendor. Don’t select a pump that is larger than is required for the application, and avoid using extreme throttling or bypass lines to control the pump, as this wastes energy and typically shortens the service life of the pump. Discuss with the pump vendor the preferred operating region (POR) and the allowable operating region (AOR) for the pump selected, and provide sufficient NPSH Margin to cover the entire anticipated operating region.Also, assess which applications can benefit from the use of variable speed drivers, and check with the vendor to be sure the pump and motor are able to operate at all anticipated (reduced speed) flow points. "Variable Speed Pumping: A Guide to Successful Applications" describes the basic principles of pump, motor, and drive technology and will help users analyze where and how variable speed pumping can lead to cost savings from both reduced energy consumption and increased pump system reliability. For More Information: www.pumps.org
