Pros & Cons of Horizontal Split-Case Pumps

It is the best of pumps; it is the worst of pumps. Horizontal split-case (HSC) pumps are used in most industries around the world to move large quantities of fairly clean fluids—usually water—at low to medium pressures. HSC design geometry poses numerous advantages and disadvantages for users. Advantages of HSC pumps:

• HSC pumps have a smaller footprint compared with a frame-mounted pump of the same rating (in most cases).
• The top half of the casing is easy to remove for inspection of the rotor, and you do not need to disturb the driver or the piping to accomplish this inspection.
• They typically have higher efficiencies than frame-mounted pumps of the same size.
• Because of the dual-eye (dual-suction) impeller design, there are lower net positive suction head required (NPSHR) factors and lower axial thrust.
• This design lends itself to a short shaft with the result of low deflection. This article limits the discussion to one- or two-stage HSC pumps with the impeller situated between the bearings; this design is also known as “between bearings” (API 610- BB-1).
• Typically these pumps offer dual volute construction, which significantly reduces the radial thrust component.

Disadvantages of HSC pumps:

• The pump is sensitive to horizontal elbows on the pump’s suction side. Fluid flow through the horizontal elbows induces asymmetrical pressures on the impeller that lead to shortened bearing and seal life.
• The casing does not lend itself to a confined gasket design, so there are consequential horsepower and pressure limits.
• Because the casing halves are truly not halves from a mass standpoint, the pump’s upper and lower portions will expand and contract at different rates with temperature changes, leading to alignment, bearing life and sealing issues. Compounding the problem is the fact that the pump is foot-mounted. The foot design and differing masses restrict the pump from a temperature aspect, so most manufacturers limit these models to less than 400 F.
• The pump can carry increased costs because it has two stuffing boxes, so two seals or sets of packing are required per pump. Many models are restricted in stuffing box sizes that preclude the use of standard or less expensive seals.
• Many claim that these pumps are more sensitive to pipe strain, but this factor may vary based on model and manufacturer.
• While the dual-suction impeller is a benefit from an NPSHR aspect, the shaft running through the impeller reduces the effective eye area, creating problems with suction specific speed factors and multiple issues with any departure from the best efficiency point (BEP). Recirculation and separation issues when operating away from the BEP cause impeller damage and thrust issues.
• This pump design is sensitive to problems with ring clearances. Issues will arise from either the amount of clearance as it opens up from wear or the differences in clearances from one side of the pump to the other. There are also concerns with what is often referred to as “A” and “B” gaps (clearances involving the impeller to casing).

Common Errors

This pump design is the one I see most often rotating in the wrong direction. It is possible for this pump to rotate in the correct direction while the impeller is installed incorrectly (backward) on the shaft. It is not just sufficient for the pump shaft to turn in the correct direction; the impeller also must be on the shaft correctly. See the original equipment manufacturer’s (OEM) instructions for further guidance. Another common problem is that the wear ring clearances are ignored and/or mismanaged. While other pump designs (such as back pullout, horizontal or vertical) will become grossly inefficient, the HSC pump will experience hydraulic preload or “shuttling” issues. Expect to pay more in energy costs to drive the pump, and expect the bearings and seals to wear out much faster. Replace or renew the ring clearances when the clearances approach two times the original clearance established by the OEM. I recommend to never let the clearance open more than three times the original clearance. As wear clearances open up, the rotor’s critical speed will decrease. A major issue would be if the critical speed dropped to the operating speed range. Most design engineers will have a margin of 25 to 30 percent in their designs. More is better, but ignoring the increasing clearances will eventually destroy the pump. The clearances on either side of the impeller ring to the corresponding casing ring must be within 0.003 inches of each other or the impeller will start to shuttle (axial movement due to differential pressures). I have seen repaired pumps that have two different size wear rings with clearances that are the same, but the outside diameter (OD) is different. The different OD also will create shuttling. Because these designs are more susceptible to unbalanced loading when using a horizontal elbow on the suction, there are many alleged “fixes” that include flow straighteners and suction diffusers. Most of these solutions are “smoke and mirrors” fixes that may actually worsen the problem. If you must have horizontal elbows on the pump suction, have a professional and experienced engineer/firm specifically tailor the flow modifiers for you.

Proper Maintenance

With the pump apart and the rotor removed, make sure the casing mating surfaces are properly prepared. I often see this step shortened or omitted, which will lead to premature casing leaks and more expensive repairs. The majority of casings are manufactured flat on both halves. Some bigger and higher-pressure pumps may have a tapered surface on the upper half. All of the old gasket and whatever adhesive was used must be removed. Remove the casing studs (if applicable) at this time to inspect them and prepare the surface. The surface must be smooth and flat. A good technique in metal surface finishing is to hand-stone the surface. Stones of different finishes can be used in succession, similar to sanding a fine wood surface. Start with a 240 grit stone and work up from there to obtain a smooth and flat surface. Avoid grinders because they tend to follow preexisting contours. A grinder can be used to remove the first layer of gasket and adhesive, but try to avoid any metal removal. I suggest the file as the second step before using the stone. If you have never hand-stoned the pump gasket surfaces, this is the likely cause of the failed gasket. If you are not familiar with the process, consult a skilled machinist or millwright or check the internet. Best practice advice is to make sure the stone is initially dressed and to use medium pressure. Work the surface at different angles, especially when going to the next grit. The stones should be soaked in clean oil and frequently rinsed in oil to avoid loading the stone with the removed metal. To avoid paying for a new set of gaskets from the OEM, some end users purchase a sheet of gasket material, lay it on the casing and “cut” the gasket by using a ball-peen hammer against the edge of the casing as an outline. Do not use this method. If you do, it could be the top reason the gasket leaks in critical areas later. Be sure to use a sharp knife in lieu of the hammer. Additionally, leave a 0.010-inch overhang (extra gasket) at all critical areas such as ring fits and stuffing box clearances. If you are going to make your own gasket, make sure the specification on the material requires a uniform thickness. Using a thicker gasket than specified will not prevent leaks for long. Chances are the surface was not properly prepared.

Other Issues

During maintenance, when lowering the top half of the casing on the lower half, use guide pins of appropriate length set in the lower-half casing to prevent side-to-side movement during the critical last few fractions of an inch. If no guide pins are available, make your own from “all-thread” or ask the local machine shop to make a set. I like to use at least three pins, but four is better. The pins can be different lengths so engagement occurs in a sequence. The use of guide pins is one of the signatures of a true pump professional. Lastly, make sure the impeller’s centerline matches up to the casing’s centerline. The mechanical center should match the hydraulic center. Use shims or shaft sleeve and shaft nut adjustments to accomplish this task. The process will vary from one design to another, but ultimately the pump should operate on center for the most reliable operation.

Final Thoughts

• Balance the impeller and rotor to a specification that makes sense for your operating speed. ISO Grade 2.5 is more than adequate in most cases.
• Do not use heat to remove impellers because they are typically hollow and the cavity likely has fluid inside that could rapidly expand and explode.
• Proper torque sequence on the casing bolts is critical; I recommend doing it in incremental steps. Just because the bolts are torqued does not mean the gasket is properly compressed.
• Axial shaft endplay should be 0.001 to 0.008 inches for most bearing arrangements (see the manual for the exact number).
• Try to have a different hardness on the wear ring sets. One ring should be harder than the mate. I prefer a 50 Brinell difference.
• Check for proper rotor lift from both sides during reassembly and set in the middle of the measured lift dimension unless other factors drive the decision.
• Do not use metal spray techniques to repair ring fits in the casing. Metal spray is OK in other areas, but there is a high differential pressure and stress on these areas. I recommend welding with subsequent machining instead.
• If using dowel pins on bearing housings or casing, never use the same hole twice. Drill/ream for the next size pin or locate the hole in another area.