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FIGURE 2 Single stage overhung refinery process pump—API 610 Type OH2. (1) Pump casing, (2) Discharge flange; flanges are standard ANSI B16.5 Class 300, (3) Metal-to-metal casing joint with fully confined, controlled compression gasket, (4) Casing drain at bottom of volute (not shown), (5) Suction nozzle guide vane (featured on some models, (6) Renewable casing and impeller wear rings, (7) Seal chamber with minimum standard dimensions, (8) Cartridge mechanical seal, (9) Impeller—dynamically balanced, (10) Shaft—heavy duty to minimize deflection, (11) Bearing housing, (12) Finned bearing housing cover for air circulation and cooling, (13) Single-row radial and back-to-back mounted angular contact thrust bearings are standard, (15) Labyrinth-type bearing housing closures on both ends, (Flowserve Corporation)

Where casting limitations permit, double volute pumps are used to limit the radial load imposed on the impeller by uneven pressure distributions in the casing. To comply with API 610, the shaft deflection at the wear rings for one and two stage pumps, under most severe dynamic conditions, must be limited to one half the minimum diametrical (internal) clearance specified (the design clearance). For operating temperatures above 500°F (260°C), or where the pump wear rings are made of materials with higher galling tendencies, API 610 requires that 0.005 in. (0.13 mm) be added to the minimum specified diametral clearances. Whenever hardenable wear ring materials are used, the mating wear rings are required to have a differential hardness of at least 50 BHN (Brinell Hardness Number) unless both rings are harder than 400 BHN. This requirement is to minimize the chances of the mating wear rings galling and seizing together if contact occurs.

To aid in maintaining alignment at various operating temperatures, pump mounting feet are located on the case at the same centerline as the shaft for all horizontal pumps. Earlier editions of API 610 also suggested using bearing or seal chamber cooling at elevated pumping temperatures. However, the industry has moved toward less and less cooling water usage because of cost and availability of quality water sources in many areas. Currently, API 610 leaves cooling as an item to be jointly agreed upon by the purchaser and the pump vendor.

In general, pumps meeting API 610 are designed for operation at elevated temperatures. In all services, but particularly in high temperature services, pumps are subject to loads at the nozzles due to thermal movements of piping, fabrication errors in the piping, and movements of the piping supports. These forces and moments tend to "move" the pump relative to its driver and result in misalignment of the pump and driver shafts. This misalignment can increase vibration levels and impose forces on the shafting that contribute to poor reliability. API 610 prescribes minimum forces and moments, by nozzle size, that complying

FIGURE 3 Between bearings, single stage, double section radially split pump—API 610 Type BB2. (1) Pump casing and cover with metal-to-metal confined gasketed joint, (2) Thrust bearing housing with back-to-back angular contact ball bearings, (3) Oil ring lubrication, (4) Cooling insert, (5) Seal chamber and cartridge mechanical seal, (6) Double suction impeller, dynamically balanced, (7) Radial bearing housing with deep-groove radial ball bearing and oil ring lubrication, (8) Labyrinth-type flingers at all bearing housing—shaft openings, (9) Heavy duty shaft to limit deflection at the seals and impeller, (10) Double cover design on some models. (Flowserve Corporation)

FIGURE 3 Between bearings, single stage, double section radially split pump—API 610 Type BB2. (1) Pump casing and cover with metal-to-metal confined gasketed joint, (2) Thrust bearing housing with back-to-back angular contact ball bearings, (3) Oil ring lubrication, (4) Cooling insert, (5) Seal chamber and cartridge mechanical seal, (6) Double suction impeller, dynamically balanced, (7) Radial bearing housing with deep-groove radial ball bearing and oil ring lubrication, (8) Labyrinth-type flingers at all bearing housing—shaft openings, (9) Heavy duty shaft to limit deflection at the seals and impeller, (10) Double cover design on some models. (Flowserve Corporation)

pumps must be able to withstand. Further, it outlines a test procedure to allow verification that a pump meets these minimum nozzle load requirements. The coordinate system for orientation of nozzle loads is in accordance with ISO 1503 standard convention.

Inline pumps have been developed, in part, to avoid the problem of misalignment due to nozzle loads. Figure 6 shows a single stage, overhung, inline pump with a separate bearing frame, Type OH3. Figure 7 shows a high-speed integrally geared inline pump, Type OH6. For a given flange size, inline pumps are required to withstand twice the magnitude of nozzle loads allowed for horizontal pumps. In addition, installation is simplified and less expensive because a block foundation is not required, and the pump mounts in a pipeline similar to a valve, although most users supply a support of some kind for the pump. Furthermore, the vertical arrangement causes the pump to take up much less space. The Type OH6 pump (Figure 7) has a gear box that increases the speed at which the impeller spins and typically produces high heads at relatively low flows. This type of design has many advantages in certain low flow, high head services. Some alternative inline pump designs utilize high-speed motors that eliminate the need for the gearbox. High-speed pumps may also be supplied with inducers in front of the typically radially bladed impellers to improve suction (NPSH) performance. When this is done, the range of operation of the pump may be restricted to avoid off-design flow instability. This should be recognized and the required range of pump operation should be addressed during the applications stage of pump selection.

Pumps of overhung shaft construction are nominally limited by most manufacturers to drivers rated below 500 hp (375 kW). Units with greater power requirements are usually designed with bearings on both ends of the shaft and the impeller—or impellers—in between the bearings (designated by API 610 as between bearings, or Type BB, pumps). Ball bearing construction, in compliance with requirements of API 610, is used to a limit of a bearing Ndm factor of 500,000. The Ndm factor is the product of the pump operating speed (N), in revolutions per minute and the mean bearing diameter (dm), equal to the bearing bore plus the bearing outside diameter, divided by 2 (all dimensions in millimeters). For values ofNdm above 500,000, or where the bearing basic life rating (L10h) does not meet the

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