Editor's Note: This is part two of a two part series about the 'commandments' of increasing pump reliability and life.
Last month, we discussed Commandment 11, Thou shalt always operate centrifugal pumps at their best efficiency point.{C}This article will explain the reasoning behind the second unknown commandment, Commandment 12: Thou shalt avoid prematurely opening pump wear component clearances.
Commandment 12
The premature opening of wear components clearances is especially damaging to a pump's life and reliability. The wear components act as water lubricated bearings and provide stiffness and damping to the pump rotor. The wear components also limit internal recirculation and directly impact overall pump efficiency.
Opening wear component clearances geometrically reduces rotor stiffness and damping, intensifying the relative motion between the rotating and stationary elements caused by normal pump external forces. Additional contact of the wear components caused by this motion increases the clearance, further reducing rotor damping and increasing relative motion. This vicious cycle continues until high vibration, loss of performance or other issues lead to the pump being removed from service.
[[{"type":"media","view_mode":"media_large","fid":"339","attributes":{"alt":"This vicious cycle continues until high vibration, loss of performance or other issues lead to the pump being removed from service.","class":"media-image","id":"1","typeof":"foaf:Image"}}]]
Causes
Wear ring clearances open at an increased rate when the stationary and rotating wear surfaces come into contact. The most common causes of contact are high vibration amplitudes, increased shaft orbit and non-centerline compatibility of stationary close-clearance bores (wear rings, sleeve bearings, balance sleeves) to the true shaft centerline.
In addition to the causes discussed in Part One, some of the major factors that influence the premature opening of the wear ring clearances are described below.
Impeller and Coupling Imbalance
Balance tolerances are typically expressed as a function of the component weight and the pump operating speed (see Equation 1).
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Best-in-class processes achieve a rotor balance < 1W/N for high energy services and < 4W/N for other services. To achieve 1W/N, certain conditions must preexist:
- All keys must be fitted to keyways with no excessive stock or unfilled areas (as would occur when using square instead of full-radius keys)
- The impeller must have an interference fit-up to the shaft
- Shaft concentricity (total indicated runout, T.I.R.) cannot exceed 0.001-in
- Impeller hub turn T.I.R. cannot exceed 0.002-in
- Split rings, when used, must be manufactured to maintain circularity of the bore with exact fit-up of the split ring halves
Angular or Parallel Pump-to-Driver Misalignment-Horizontal Pumps[[{"type":"media","view_mode":"media_large","fid":"342","attributes":{"alt":"Angular misalignment","class":"media-image","height":"170","id":"1","style":"float: right;","typeof":"foaf:Image","width":"307"}}]]
Angular and parallel pump-to-driver misalignment in horizontal pumps is usually the result of unevenness of the pump mounting pads and baseplate, distortion due to thermal growth or poor maintenance processes. Routine maintenance and inspections are often completed on the pump internal elements while the baseplate, barrel or discharge head remain unchecked. As installations age, it is important to periodically check that the baseplate and pump mounting surfaces have not been distorted through external forces, deteriorating grout or other factors. Misalignment due to thermal growth relates predominantly to foot-mounted pumps in high temperature services. In these instances, centerline mounting of the pump should be considered.
Many techniques are used to achieve proper pump-to-motor alignment. State-of-the-art laser systems can provide information for both cold and process temperature conditions.
Angular or Parallel Pump-to-Driver Misalignment-Vertical Pumps
Angular misalignment in vertical pumps occurs as a result of mating faces that are not parallel to each other and perpendicular to the true shaft centerline (see Figure 2). Primary among these conditions are the misalignment of the motor-to-pump shaft, distortion of the discharge head over time and build-up of FME (foreign material exclusion) that can change the relationship between mating faces.
Parallel misalignment in vertical pumps occurs as a result of eccentricities and/or excessive clearance in register fits, non-adjustability of the motor to the discharge head and poor assembly practices (see Figure 3).
Assembling vertical pumps horizontally will also lead to parallel misalignment, even when critical fits are within tolerance, and especially when many column/casing sections are [[{"type":"media","view_mode":"media_large","fid":"343","attributes":{"alt":"Parallel misalignment","class":"media-image","height":"170","id":"1","style":"float: right;","typeof":"foaf:Image","width":"307"}}]]involved. Each component will sit on the bottom of its register fit, resulting in a stack-up of the individual clearances and eccentricities. Better practices include assembling the pump in the vertical orientation where the centerline relationship of each added component can be checked during assembly, or rotating the assembly 180-deg after the installation of each stage piece.
Static Shaft Bow
Shaft straightness is a function of material selection, the process to produce the raw bar and the manufacturing process to achieve the final product. Preferred shaft run-out is maintained at or below 0.001-in and should not exceed 0.002-in. Many specifications allow 0.001-in total indicated runout (T.I.R.) for each foot of shaft length. This is unacceptable, and manufacturers should be encouraged to meet the more stringent criteria.
In addition, neither mechanical nor thermal straightening should be used to restore shaft straightness since internal stresses will relieve themselves under load and the shaft will assume its former shape. Shaft straightening should be prohibited from internal and sub-vendor processes.
Dynamic Shaft Bow
Dynamic shaft deflection relates to operation at or near any of the rotor critical speeds. Most pumps are designed such that the operating speed is a minimum 25 percent removed from the critical speed. As bearing clearances increase, the stiffness and rotor critical speeds decrease. If sufficient margin is not provided in the original design, the critical speed can fall into the operating speed. It is recommended that specifications for new equipment, or when possible for upgrading existing units, require a minimum 35 percent margin between the operating and first critical speeds. This can be accomplished by:
- Increasing the shaft diameter
- Reducing the bearing span
- Upgrading to a material with a higher modulus of elasticity
[[{"type":"media","view_mode":"media_large","fid":"344","attributes":{"alt":"Angular offset","class":"media-image","height":"170","id":"1","typeof":"foaf:Image","width":"307"}}]]
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Component Looseness and Improper Component Tolerances
Strict manufacturing tolerances are required to obtain centerline compatibility of all close clearance components and minimize relative motion between rotating and stationary assemblies. Theoretically, the summation of all tolerances must be less than one-half the bearing clearance to avoid contact (see Equation 2).
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Existing pump components are usually not designed or manufactured to attain these exacting standards. Centerline compatibility is most easily achieved by requiring interference fit-up of mating components in both rotating and stationary elements. Additionally, concentricity of all critical locating bores and turns as well as perpendicularity of mating faces to the true shaft centerline need to be maintained at strict limits.
Material Galling
Certain materials in the martensitic chrome steel group (such as type 410) have high galling tendencies. This can be compensated for by treating the surfaces to a hardness exceeding 50 Rc, so the materials bounce in lieu of galling when contacted. Achieving this value can be accomplished through hardening, direct laser deposition (DLD) or laser hardening of the material itself. When hardening wear components above 50 Rc, note that there should be no differential hardness between the two components.
Another option for martensitic chrome steels is to increase the sulfur content to types 416 and 420F, which increases the material lubricity. Type 416 is sensitive to stress corrosion cracking when subjected to tensile loads and certain boundary conditions, so it should be avoided as an impeller wear ring material. Synthetic materials are also gaining popularity due to their non-galling tendencies and capability of operating at reduced clearances.
Detecting Wear Clearance Opening
Increased wear ring clearances increase internal recirculation, reducing both the total flow delivered to the system and the pump efficiency. Increased clearances will result in the following observable changes:
- Loss of flow and head in an unthrottled system
- Increased motor amps in a throttled system
- Increased vibration amplitudes at the 1X frequency
Thermodynamic testing methods can also be used to detect and trend wear component clearance opening. This testing, exemplified by the British Yatesmeter system, measures the efficiency of existing unit performance with extremely accurate pressure and temperature measurements. By trending internal clearance (inversely proportional to efficiency), pump operating life can be predicted by monitoring internal clearances and evaluating to a pre-established limit. The Yatesmeter can be used as a performance-based monitoring tool to maximize operating life and help avoid mid-cycle failures.
[[{"type":"media","view_mode":"media_large","fid":"347","attributes":{"alt":"The Yatesmeter can be used as a performance-based monitoring tool to maximize operating life and help avoid mid-cycle failures.","class":"media-image","height":"170","id":"1","typeof":"foaf:Image","width":"307"}}]]
[[{"type":"media","view_mode":"media_large","fid":"348","attributes":{"alt":"By trending internal clearance (inversely proportional to efficiency), pump operating life can be predicted by monitoring internal clearances and evaluating to a pre-established limit.","class":"media-image","height":"170","id":"1","typeof":"foaf:Image","width":"307"}}]]
Mitigating Processes
Lomakin Grooving
To increase rotor damping and make the pump more permissive to increasing rotor vibration with time, Lomakin grooving of the wear components is highly recommended. Lomakin grooves are a square grooving pattern of shallow depth. Note that these grooves should be completed on only one of the mating surfaces of close-clearance component pairs.
Lomakin grooving has proven to significantly increase the rotor stiffness over smooth-on-smooth services.
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Inspection and Manufacturing Procedures
Except for material considerations, mitigating the premature opening of clearances involves good operating practices and stringent inspection and manufacturing standards to ensure proper pump assembly and construction. To slow the rate of wear clearance opening, it is important that the original condition of the pump provides the optimum environment for the wear component. This means that all critical bores must be concentric to the true shaft centerline, the shaft must not have excessive run-out and all critical mating faces must be both parallel to each other and perpendicular to the true shaft centerline.
To ensure that these conditions exist, it is often necessary to provide detailed specifications, both for inspection processes and for OEM tolerances. Many pumps are designed for multiple markets, so the default tolerances may not be specifically designed for the intended service. Additionally, specifications for procuring new equipment or inspecting existing equipment are often vague, offering no acceptance criteria for work completed. Investing in more detailed specifications, thorough inspections and improved materials can drastically improve component life and performance.
Pumps & Systems, January 2009