Agitators play key roles in chemical processing. Their proper operation and maintenance can improve process reliability, leading to reduced downtime and costs
Agitators, which are relied upon to dependably and cost-effectively meet production goals, are commonly used throughout the chemical process industries (CPI). Despite their prevalence and importance, agitators are rarely the focal point of process hardware. In fact, a properly functioning agitator is usually a case of “out of sight, out of mind.” It is when an agitator is not working properly that it garners the most (unwanted) attention. The purpose of this article is to outline best practices for agitator operation and maintenance to ensure reliability and reduce the costs and headaches associated with process downtime.
Agitators share significant characteristics with other rotating equipment, particularly pumps. The most apparent similarity between pumps and agitators is the production of fluid motion via impeller rotation. However, the scope of process objectives to be accomplished by agitators is much broader than that of pumps. While the primary purpose of pumping is to transport material from point A to point B, agitators are used to achieve numerous objectives that include blending miscible liquids, increasing mass transfer rates between immiscible phases, contacting reactants, suspending and dispersing solids in liquids and aiding process control by promoting heat transfer. To meet these objectives, an agitator must be designed, installed, operated and maintained properly.
A key point that should be made at the outset is that equipment vendors provide detailed instructions via installation, operation and maintenance (IOM) manuals, and these documents should serve as the primary information source to ensure safe, reliable and effective agitator operation. Furthermore, adherence to vendor guidelines is required to avoid invalidating the manufacturer’s warranty. Such specific information as provided in IOM manuals will not be presented here; rather, the focus will be on general practices that promote dependable agitator performance.
Figure 1 illustrates the components of a typical agitator. The agitator induces liquid motion through the rotation of an impeller system within the liquid. The impeller is mounted on a shaft connected to the agitator drive that is powered by the prime mover, which is most often an electric motor. The purpose of the agitator drive is to reduce the high motor-output speed to the desired shaft-rotational speed. This speed reduction is usually accomplished by gears, but belts and sheaves are also used.
Figure 2 displays cutaway views of the gear drives shown in Figure 1. The agitator will be mounted on the vessel, beams or an independent structure, and in many cases, a seal is included to contain process material within the vessel. In low-viscosity applications, baffles are employed to eliminate swirl and convert the rotational motion of the impeller into the desired flow pattern throughout the vessel. All of these components must work in concert for an agitator to provide worry-free performance.
Getting off to a good start
Successful agitator operation is rooted in the design process that takes place before an agitator is operational. While smaller agitators may be portable and multipurpose, larger agitators are typically designed to accomplish specific process objectives — for example, blending ingredients to a required uniformity in a specified time, dissolving solids in a stated time or providing a desired rate of interphase mass transfer. The first step of agitator design — specifically, designing to meet process objectives — leads to the specification of a rotational speed and an impeller system that indicates the type, size, number and location of the impellers. Following process design, a mechanical design procedure translates the impeller system and rotational speed selections into hardware that is capable of providing the fluid motion required to meet process objectives while withstanding the mechanical forces that the agitator components will encounter. These forces include the weight of the shaft and impeller system, pressure in the vessel, torque, thrust due to impeller pumping and hydrodynamic forces due to interaction between the blades of the rotating impeller and the material it is moving. These forces, starting at the impeller, are transmitted to the shaft, the agitator bearings and eventually, to the agitator mounting structure. A challenging aspect of agitator mechanical design is that the forces experienced by an agitator are not static; rather, the forces are dynamic and fluctuate with time, particularly the hydrodynamic forces the moving fluid imparts to the impeller system when in turbulent operation.
While the process and mechanical design procedures are not the focus of this article and have been described elsewhere (see Additional Resources box), effective communication is important at this stage to avoid operational problems that can plague an installed agitator. This communication can be complicated by the number of players involved, including: the user; the agitator vendor (who likely acquires the motor and seal from secondary vendors); the vessel vendor; and perhaps an engineering and construction firm. For the agitator vendor to design and deliver an agitator that will provide optimal, trouble-free operation, it is imperative that they are provided with key information that will impact the design procedure, particularly process details that may influence the mechanical design. Providing accurate information can be challenging for the user due to the competing forces of the desire to be quick to market with a product and the long lead times required to design and construct a plant, which can lead to process changes occurring after equipment procurement has been initiated.
Although it is impossible to list all situations that complicate agitator design, Table 1 identifies some of the more common scenarios that can lead to agitator operation problems if not properly taken into account. For example, heating or cooling coils are commonly used in agitated vessels, and their presence can strongly impact agitator performance. When significant heat-transfer area is required and numerous closely spaced tubes are used, the flow pattern produced by the agitator can be altered. The fluid velocities between the tubes are reduced, diminishing the effectiveness of both the agitator and the heat-transfer equipment. Additionally, when internals are in close proximity to an impeller or adversely affect the flow pattern generated by the impeller, the hydrodynamic forces experienced by the agitator can be increased dramatically. During the design stage, it is crucial that the agitator vendor prompts the user to provide the right information and that the user understands that seemingly minor process changes can have a significant impact on agitator design and performance (Figure 3).
Getting up and running
Once an agitator has been designed, built and delivered, it must be installed and commissioned. Efforts made to ensure proper installation can pay dividends, since this is one of the most common failure points (Figure 4). As in the design process, communication between the user, agitator vendor, tank fabricator and, for larger projects, engineering firm, is crucial to proper installation. Subsequently, proper installation is crucial to an agitator’s ability to provide extended, reliable service. Communication may be complicated at this point, because the individuals responsible for agitator installation are likely to differ from those who are responsible for design. So not only is communication among the various parties critical, but good communication within each individual organization must be emphasized as well.
As with all rotating equipment, proper alignment and balancing during installation sets the stage for operating success (Figure 5). For top-entering agitators with long, cantilevered shafts supporting rotating impellers that are subjected to significant hydrodynamic forces, alignment and labanacing are particularly important. Further, since the power draw of an impeller is strongly dependent on its diameter — being proportional to diameter raised to the fifth power in turbulent operation — it is imperative that the vendor provides uniform impeller blade lengths and that the blades are correctly attached to the impeller hub. Small differences in blade length, whether due to improper manufacturing or installation, will translate into significant differences in power draw and the resulting forces on the impeller blades. Imbalanced blade forces lead to shaft deflection and forces that are transmitted throughout the agitator and its mounting structure. Table 2 provides guidelines to facilitate the agitator installation process.
The importance of reliable equipment operation is a given in the CPI as evidenced by the many publications focused on this topic. Compilations available through Chemical Engineering  include a number of articles pertinent to agitator operation and maintenance. With a focus on centrifugal pumps but applicable to all rotating equipment, Chatterjee  identified the importance of communication between end users, project teams and equipment vendors to ensure low cost, reliable and effective operation of installed equipment. To enhance communication and teamwork, Chatterjee outlined a checklist approach that addresses routine maintenance, safety and environment, troubleshooting and repair and replacement considerations. Almasi  discussed proactive monitoring of key parameters of rotating equipment, such as vibration and noise levels, to detect root causes and correct problems early to avoid major upsets and downtime. Although the emphasis is on compressors and gas and steam turbines, the approach is applicable to agitators as well.
While agitation has been applied industrially for a very long time, open documentation of this practice has been common only during the past 50 to 60 years. A landmark publication was the 12-part Liquid Agitation series  published in the mid-1970s. This compilation covered all aspects of industrial agitation, starting with fundamentals, such as dimensional analysis of the Navier-Stokes equations; moving through process design to achieve desired performance in blending, solid-liquid and gas-liquid operations; on to mechanical design to ensure agitator integrity; economic analysis; scaleup procedures; and application guidelines for specific common agitator applications. The seventh and eighth installments [5, 6] of this series focused on hardware and mechanical considerations pertinent to agitator operation and maintenance. A second article series, Advanced Liquid Agitation , concentrated on improvements in impeller technology, computerized selection procedures and advanced experimental and computational methods that can be used to optimize agitator design and performance. A companion article  to this updated series discussed advanced methods of mechanical design including consideration of mechanical seals, fastener locking techniques, material of construction, and structural stiffness that are critical to trouble-free agitator operation.
Professional societies, most notably the American Institute of Chemical Engineers (AIChE; New York, N.Y.; www.aiche.org), have played a key role in disseminating best agitation practices through conferences and publications. AIChE’s Equipment Testing Procedures Committee published a compendium of agitator testing information  that encompasses both process and mechanical considerations, such as shaft runout, gear tooth patterns, seals, noise and vibration, torque and power draw. Undoubtedly, the most comprehensive source of agitation information is the Handbook of Industrial Mixing  that was brought to fruition through the efforts of AIChE’s North American Mixing Forum (NAMF). With authors representing academia, industry, and equipment vendors, this work includes fundamentals (including turbulence and residence time distribution), experimental and measurement techniques, computational fluid mixing, specialized mixing devices (such as rotor-stator devices), typical agitator applications (miscible liquids, immiscible liquids, gas-liquid, solid-liquid or chemical reactions) and agitation in specific industries (including polymers, pharmaceuticals, petroleum and pulp and paper). The two chapters dealing with mechanical design  and the equipment vendor’s role  are particularly pertinent to the present focus on agitator operation. The recently published Advances in Industrial Mixing  provides updates on some topic areas in the original Handbook of Industrial Mixing and completely new subject areas (for instance, micromixers, mixing in the food industry and mixing safety), as well as a chapter on commissioning mixing equipment  that outlines procedures to be used during installation to promote a smooth-running agitator.
Keeping a good thing going
The lifetimes of most agitators are measured in years or decades, requiring maintenance and repair to perform up to expectations and meet process objectives over this timespan (Figure 6). Most routine maintenance activities, such as those listed in Table 3, are relatively obvious and are described in detail in the equipment manufacturer’s IOM manuals. The frequency of these actions depends on the use of the agitator, as well as its physical environment. For example, does the agitator operate continuously or intermittently; is it located outdoors or indoors; is it subject to heat, cold or frequent rain?
Agitators should also be routinely inspected for operating temperature, noise and vibration. If any of these parameters are outside the normal recommended range, catching the anomaly early will prevent additional damage to the agitator and often will result in a less costly repair. Finding the problem source may be tricky, but can often be accomplished by the maintenance crew. In particularly challenging situations, the manufacturer will provide a trained field service technician to offer assistance or ask the user to return the agitator to the manufacturer or a nearby service center for evaluation. Table 4 lists some common causes of agitator operational problems. Note that a number of these causes, such as shaft misalignment and insufficient bolt torque, may be rooted in the installation procedure, reinforcing the benefits of making the effort to get this important preceding step right. Solids buildup is another major cause for concern, as seen in Figure 7.
For agitators equipped with a seal or steady bearing, monitoring these can be an effective indicator of the overall status of the agitator. Like tires on a car, agitator seals and steady bearings will show wear. Uniform wear is expected, but non-uniform wear is likely a sign of misalignment or some other problem. Changes or fluctuations in the temperature, pressure, or level of seal barrier fluid are usually early indicators of problems, and when detected, should be followed up with a thorough inspection of the agitator to identify and correct the root cause of the behavior.
When the going gets tough
When an agitator or other process hardware does not perform up to design expectations, prompt identification and correction of the cause is imperative, particularly when process downtime occurs. This is the role of the vendor’s field-service group (Figure 8). Rapid resolution can often be aided by providing detailed operating information to the vendor. Of particular importance are records of temperature, pressure, power draw and vibration. Additionally, changes in the process or procedures should be communicated, as they may be related to the problem. Have flowrates, process conditions or fluid properties such as viscosity changed? Is the agitator being turned off and on rather than being operated continuously? What has changed that might be responsible for the reduction in agitator performance?
Having spares on the shelf
Agitators typically are a significant piece of equipment in a chemical plant. When purchasing an agitator, it is important to evaluate the impact the agitator has on maintaining process operability. If the process cannot function, or functions at significantly lower efficacy without the agitator, the manufacturer should be contacted to determine how long would be required to obtain a replacement. If the delivery time is too long or the risk too high, it is suggested that a spare agitator or key spare parts be kept on the shelf. Having a spare could save your process from significant downtime, but this peace of mind comes at a price, so a cost-benefit analysis should be used to determine whether a replacement agitator should be kept on hand. Having a spare agitator can pay dividends, but proper storage is required to ensure the spare will be ready when needed. Otherwise, the additional investment may be wasted on a non-functional replacement. Points to consider when storing an agitator are discussed in the following section.
In an effort to help operators keep their agitators and processes up and running, equipment vendors offer reliability planning services to identify key spare agitator components that the user should keep on hand. Agitators are a combination of off-the-shelf and custom components. For instance, the shafts of larger agitators are customized to provide the appropriate overall length and impeller mounting locations for a particular installation. Similarly, impellers are sized to satisfy specific process objectives. Conversely, for many agitators, a standard motor will be suitable. Reliability planning distinguishes between standard components for which the vendor can quickly provide replacements and unique components for which spares should be stocked. Various factors, such as materials of construction, contribute to making an agitator component unique, and the equipment vendor will work with the user to develop a sound spare-parts stocking strategy.
Finding renewed purpose
Given the potentially long lifetime of a well-maintained agitator, it may outlast its original function and be repurposed in a new application. The preceding discussion tacitly focused on new agitators, but the considerations are much the same when an existing agitator is used for a new purpose. However, there are a few additional issues that should be considered in this instance. Good record keeping is of particular importance in this regard so the agitator can be properly evaluated for both its process and mechanical suitability for the new application. Once the pertinent parameters of the new process are known, contact the agitator manufacturer or their local representative and ask if the agitator should be used in the new application. Rarely will the agitator be optimized for the new circumstances, but with an easy change of rotational speed or impeller system (type, diameter, location or number of impellers), the agitator may be successfully used in the new process. If modifications are made, detailed record keeping again becomes important for continued trouble-free operation. Knowledge of changes to the agitator should be incorporated into the maintenance strategy and need to be known if troubleshooting poor performance becomes necessary.
If an agitator is decommissioned from one application and then repurposed in a different one, storage is most likely required (Figure 9). As mentioned in the preceding section, spare agitators and parts must also be stored, and even new agitators require short-term storage if not installed upon receipt. Proper storage is required for an agitator to perform up to expectations when recommissioned. Table 5 lists some key considerations for agitator storage.
From design to installation to provision and storage of spares, many considerations are critical for ensuring agitators reach their full performance potential. Reliable and effective agitator operation starts during the design process, before the agitator even exists, and is followed by proper installation, commissioning and maintenance throughout the agitator’s lifetime that may include reincarnations in a number of applications. By working cooperatively with an experienced equipment vendor and applying the best practices described here, your agitator can provide extended and dependable service. ■
Edited by Mary Page Bailey
1. Chem. Eng., Improving Operability and Reliability, Volume 1: Equipment Systems and Volume 2: Management Systems.
2. Chatterjee, S.K., Improve Rotary Equipment Reliability with Checklists, Chem. Eng., Sept. 2013., pp. 52–57.
3. Almasi, A., Rotating Machinery: What You Should Know about Operational Problems, Chem. Eng., March 2015, pp. 74–78.
4. Gates, L.E., Henley, T.E. and Fenic, J.G., How to Select the Optimum Turbine Agitator, Chem. Eng., Dec. 8, 1975, pp. 110–114. (First of a 12-part series).
5. Hill, R.S., and Kime, D.L., How to Specify Drive Trains for Turbine Agitators, Chem. Eng., Aug. 2, 1976, pp. 89–94.
6. Ramsey, W.D., and Zoller, G.C., How the Design of Shafts, Seals and Impellers Affects Agitator Performance, Chem. Eng., Aug. 30, 1976, pp. 101–108.
7. Fasano, J.B., Bakker, A. and Penney, W.R., Advanced Impeller Geometry Boosts Liquid Agitation, Chem. Eng., Aug. 1994, pp. 110–116. (First of a five-part series).
8. Fasano, J.B., Miller, J.L. and Pasley, S.A., Consider Mechanical Design of Agitators, Chem. Eng. Prog., Aug. 1995, pp. 60–71.
9. AIChE Equipment Testing Procedure: Mixing Equipment (Impeller Type), 3rd ed., prepared by the Equipment Testing Procedures Committee of the American Institute of Chemical Engineers, 2001.
10. Handbook of Industrial Mixing: Science and Practice, Wiley-Interscience, Hoboken, N.J., 2004.
11. Dickey, D.S., and Fasano, J.B., “Mechanical Design of Mixing Equipment,” Chapter 21 of Handbook of Industrial Mixing: Science and Practice, Wiley-Interscience, Hoboken, N.J., 2004.
12. Weetman, R.J., “Role of the Mixing Equipment Supplier,” Chapter 22 of Handbook of Industrial Mixing: Science and Practice, Wiley-Interscience, Hoboken, N.J., 2004.
13. Advances in Industrial Mixing: A Companion to the Handbook of Industrial Mixing, Wiley, Hoboken, N.J., 2016.
14. Dickey, D.S., Janz, E., Hutchinson, T., Dziekonski, T., O. Kehn, R., Preston, K., and Dinnison, J., “Commissioning Mixing Equipment,” Chapter 23 in Advances in Industrial Mixing: A Companion to the Handbook of Industrial Mixing, Wiley, Hoboken, N.J., 2016.
Eric A. Wiget is a regional service manager in the Process and Flow Technologies business unit for National Oilwell Varco (5870 Poe Ave., Dayton, OH 45414; Email: email@example.com). He earned his B.S. in operations management from Wright State University in Dayton, Ohio, and has been in the chemical processing and industrial equipment industry for the majority of his career. Wiget is currently focusing on the management of the industrial service and aftermarket parts business across the Process and Flow Technologies business unit in NOV.
Jason R. Hora is director of services in the Process and Flow Technologies business unit for National Oilwell Varco (same address as above; Email: firstname.lastname@example.org). He earned a B.S.Ch.E. from the University of Kentucky. He has served as an application engineer, regional sales manager and global aftermarket manager for Chemineer agitators over the past 13 years, as well as various other roles for rotating equipment used in the chemical, municipal and oilfield sectors.
Kevin J. Myers is a professor of chemical and materials engineering at the University of Dayton (300 College Park, Dayton, OH, 45469; Email: email@example.com). He earned his B.S.Ch.E. from the University of Dayton and a Ph.D. in chemical engineering from Washington University in St. Louis, Mo. He has been teaching chemical engineering for over thirty years, with an emphasis on transport phenomena and reaction engineering. His research has focused on numerous topics related to mechanical agitation and static mixers.
Eric Janz is the director of research and new product development for the Mixing Technologies business group at National Oilwell Varco (same address as above; Email: firstname.lastname@example.org). With over 20 years of experience in applying mixing theory and practice to difficult processes in the chemical industry, Janz has held various positions in sales, engineering and research and development groups throughout that time. He has multiple publications in the field of mixing and currently holds two patents for mixing equipment. He is also an adjunct faculty member at the University of Dayton, where he received B.S.Ch.E. and M.S.Ch.E. degrees.
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