Maintaining Wear Ring Clearances
The premature opening of wear component clearances is especially damaging to the life and reliability of pumps. 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 have a direct impact on overall pump efficiency.
Opening of the 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 taken out of service.
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 and non-centerline compatibility of the shaft and critical locating bores and turns. Anything that could cause a parallel or angular offset of either the wear ring holder or the rotating element could result in accelerated wear.
In addition to the causes discussed in the previous article, some of the major factors that influence the premature opening of the wear ring clearances include:
- Impeller and coupling imbalance
- Parallel and angular misalignment
- Static and dynamic shaft bow
- Component looseness and improper component tolerances
- Materials with high galling tendencies
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 that the materials bounce in lieu of galling when contacted. Achievement of this value can be accomplished via different methods: through hardening, direct laser deposition (DLD), or laser hardening of the material itself. When hardening wear components above 50 Rc, it is important to note that there should be no differential hardness between the two components.
Another option for martensitic chrome steels is to increase the sulfur content which increases the material lubricity, such as types 416 and 420F. Type 416 is sensitive to stress corrosion cracking when subjected to tensile loads and certain boundary conditions, and, as such, should be avoided as an impeller wear ring material. Synthetic materials, such as Graphalloy, Greene Tweed, and Peek, are also gaining popularity due to their non-galling tendencies and capability of operating at reduced clearances.
Detecting Wear Ring Clearance Opening
Increased wear ring clearances increase internal recirculation, reducing both the total ncreased 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
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 been proven to significantly increase the rotor stiffness over smooth-on-smooth services.
Except for material considerations and Lomakin grooving, mitigating the premature opening of clearances involves good operating practices and stringent inspection and manufacturing standards to ensure proper assembly and construction of the pump. 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. As discussed above, 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 original equipment manufacturing tolerances. Many pumps are designed for multiple markets and, as such, the default tolerances may not be specifically designed for the intended service. Additionally, specifications used to procure new equipment or to inspect existing equipment is 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.