Single-stage overhung pumps are an extremely important global resource for the transfer of fluids. This specific style of pump provides a relatively large selection of flow and pressure range offerings and is generally simple to maintain. Processes served can range from heavy to light specific-gravity liquids to temperatures over 427°C in some cases.
The basic design of horizontal OH style pumps according to API 610 has remained relatively constant. They consist of a bearing housing with a radial and thrust bearing, and the impeller overhung on a shaft outside of the bearing housing.
The common API 610 current edition bearing housing retrofit provides several upgrades to legacy-style pumps that may be subject to short mean time between repair schedules or fugitive emission reduction requirements. These retrofits will, in most cases, provide a single-row, deep-groove radial bearing and paired back-to-back angular-contact thrust bearings. Lubrication is improved with oil slinger rings and lubrication channels. Larger oil sumps, fans and cast fins have been added to reduce or remove the need for external cooling water requirements. Finally, the seal chamber and shaft are designed to meet current API 610 deflection limits and Table 7 dimensions. This allows the use of an API 682 seal for upgraded seal technology, improved life and sealing performance.
What is commonly not considered are upgrades to the existing liquid-end components of the pump. Many legacy pumps were designed prior to many industry standards and do not contain several features that ease maintenance and improve reliability of the pressure boundary. The following upgrade opportunities are items that Sulzer has implemented with several end users, resulting in more robust pumping equipment and extended operation intervals between repairs.
IMPROVEMENTS IN NOZZLE LOADING
Legacy equipment may be lacking in the ability to handle external nozzle forces and moments imposed by the connected piping. These forces, when applied in excess, can cause the pump case and/or base to deform, contributing to misalignment, excessive vibration, and potentially wear part contact. Often, these pumps were designed with thin mounting feet, minimal webbing bracing, and flanges that were flat-faced and rated for 150 or 300 psi (10 or 20 bar). These configurations can be upgraded with proper modifications to the existing patterns while maintaining the ability to fit in the existing piping and baseplate installations.
Legacy mounting feet and webbing may be one-inch (25mm) thick or even less, depending on the original design. Newer style API 610-compliant pumps are supplied with much thicker feet and webbing to support API 610 nozzle loading requirements. Pattern alterations have been made to a legacy design to improve the overall stiffness of the pump. In addition to these changes, the flanges for this case were changed from the original American National Standards Institute (ANSI) 300# raised face design to an ANSI 600# thickness with the same drilling and machining dimensions of the original 300# flanges. All of these modifications have helped to improve the overall resistance of the case to deflect in the presence of pipe strain or increase the overall allowance of pipe strain as it relates to the pump alignment.
The final upgrade implemented to improve overall stiffness of the pump and case connections was the installation of gusseting on the vent and drain flanges. Adding the gussets reduces the chance of small-bore pipe weld failures due to vibration fatigue.
CASE SEALING ARRANGEMENT
ANSI/American Society of Mechanical Engineers (ASME) B73.1 pumps, and pre-API style pumps up through API 610 4th Edition, used a non-metallic sheet that would be sandwiched between the cover flange and case. These gaskets could be somewhat unreliable, as the compression across the sealing surface could vary depending on the torque applied to the casing bolting. Additionally, the materials could impose limits for the temperature and chemical composition of the pumped fluid. API 610 5th Edition introduced new design requirements for a confined, controlled-compression, spiral-wound gasket to improve the sealing capabilities of the pump and expand the operational limits for temperature and product.
Many manufacturers of bearing housing retrofits incorporate the use of a spiral-wound gasket in the design of a new case cover. However, the case must also be prepared for this conversion, in most cases. Many legacy-style pumps may have a recess for a non-metallic-style gasket, but that machine dimension was rarely a precisely-controlled depth. Other case styles may not have a gasket recess at all.
If the existing casing dimensions at the casing cover interface will not allow for the incorporation of a spiral-wound gasket groove, additional lagging can be applied to the outer diameter. The casing closure bolt circle is then increased to create the room necessary for the incorporation of a modern API 610 Table 7 seal chamber.
HYDRAULIC RE-RATES AND METALLURGY UPGRADES
In modern refining applications, it is highly likely that the original pump design duty point would change over the course of the expected pump life, due to changing production demands. A hydraulic re-rate could be applied during the redesign phase of a newly-supplied case to optimise the performance of the pump to meet current operational demands.
Volute lip extensions or cutbacks from the original design may suffice. If needed, an alternate volute core box could be created to provide more closely-matched hydraulic profiles. A revised high- or low-capacity impeller may be installed to further optimise the best efficiency point to the desired operating flow rate and differential pressure.
To help combat the effects of corrosion or erosion in pump cases, it may be prudent to upgrade the case metallurgy to better suit the process fluid. These changes may include upgrades from carbon steel to a martensitic, austenitic or duplex stainless steel. Alternatively, as many of these legacy pumps may be cast iron, replacing it with carbon steel, or another weldable alloy, may add decades to equipment lifespan. For highly-erosive conditions, additional steps involving the application of coatings in the flow passageways may also be considered. These may include technologies such as high velocity oxygen fuel (HVOF) sprays, weld overlays and diffusion coatings.
These are some of the modifications in addition to overhung bearing housing retrofits that can be made to API and pre-API style pumps without the need for additional field modification of the existing installation. While these modifications can seem to be a long lead-time and costly up front, they are likely to be dwarfed by the cost of field modifications to base plates and piping installations. In addition to those expenditures, efficiency improvements can often be incorporated to further extend the life of the unit and reduce operating costs all while meeting revised production demands of the facility.