Written by: Ron White & Ben Roberson, HydroAire Inc.

A “temporary” repair results in five years of service.

Five years ago, an engineered pump service center in the Midwest received a call from a large municipal sanitary district. One of the company’s four submersible pumps had tripped the overloads. This 890-rpm unit operated on variable frequency drives that adjusted the motor speed to match the fluid inflow. Once it had been reset, the 290-horsepower, 42-inch diameter axial flow pump exhibited significant vibration before failing. The pump was removed and sent to the service center where it was cleaned, disassembled and inspected by a team of skilled professionals dedicated to repairing submersible pumps.

Figure 1. 290-horsepower submersible pump upon arrival at the service center

The Assessment

The initial inspection revealed that a large chunk of concrete debris from nearby sewer construction caused the submersible pump to fail. The inspection also showed that the pump had several problems:

• A section of the trailing edge of one of the impeller vanes was broken.
• The rotating ring was missing.
• The impeller hub and lower mechanical seal had cracked and caused damage to the shaft keyway.

Figure 2. The cracked impeller hub

When the municipality inquired about replacement parts from the OEM, delivery was estimated at a 12- to 14-week turnaround time on a new impeller from Europe. The municipality could not afford to be without the pump for an extended period of time. Having a great deal of experience with submersible pumps, the dedicated submersible repair division proposed a temporary solution to get the pump repaired and back into service until the municipality could obtain a new impeller from the OEM.

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With reverse engineering, a pump service facility increased the performance of a service water pump in a nuclear power plant.

When an East Coast nuclear plant wanted to increase the performance of its Layne & Bowler 25 RKCH two-stage vertical lineshaft, wet pit, service water pump, reverse engineering services to upgrade the repair of this unit was chosen. Driven by a 350-horsepower, 1,200 rpm vertical motor, this unit is approximately 45 feet in suspended length. This pump is considered a low suction energy unit with about 3,500 specific speed.

Reverse Engineering: a Proven Alternative

Pump components requiring reverse engineering are rarely new items. They may have deteriorated through in-service use or have been damaged by cavitation or pump failure. Reverse engineering has become a proven alternative for obtaining replacement parts for existing equipment. Replacing these parts requires experienced design and processes to create a component that will meet the form, fit and function of the original part.

Using a portable, coordinate measuring machine (CMM), in conjunction with specialized computer-aided design (CAD) and 3D software, an initial vane layout was prepared using the existing blade.

When reverse engineering hydraulic components, ensuring that the new component will be equivalent to and meet or exceed all the specifications of the original design is critical. In this example, reverse engineering was used, along with the portable CMM, to develop the new castings. In a vertical pump bowl assembly (as in this unit), this is challenging due to the need to interface hydraulics between the vane lay outs of the impeller and the bowl diffuser.

This process requires in-house vane layout hydraulic technology, which may be necessary to re-engineer the deteriorated impeller and diffuser vanes. CAD software was used to overlay the new vane geometry on top of the original blade to ensure that an equivalent component was produced. The hydraulic passageways were polished to increase pump performance.

New, reverse engineered hydraulic component

Reverse engineering with assistance of the portable CMM

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In this case study, routine maintenance of a condensate pump at a nuclear power plant becomes an emergency situation.

When a nuclear power plant pulled its vertical condensate pump for routine maintenance, an emergency situation was not expected. The plant pulled the pump and installed a replacement from storage, but it failed catastrophically after only two days in service.

Requiring a solution for the emergency need, the plant accepted a workscope from a service center that promised a refurbished pump within nine days. The plant shipped both pumps to the service center and sent a condensate system engineer to oversee the work and maintain an open line of communication between the organizations.

This case study highlights the root cause of pump failure for a nuclear power plant and the emergency response required to repair the pump.

Teamwork Critical to Quick Turnaround

One key factor to successfully handling this emergency pump failure was the close teamwork between the plant’s management, an onsite plant engineer located at the repair facility and the personnel at the repair facility. A lesson learned for pump users in emergency situations is that close teamwork and having a customer engineer onsite is critical to facilitating a rapid response.

Root Cause of Failure

The pump failed as a result of having been previously incorrectly repaired, coupled with contributing installation issues, ultimately causing the upper shaft to break. Evidence of the root cause became apparent during the disassembly process. These photos illustrate what the pump service center found.

Ductile Fatigue Failure: The head shaft cracked at the snap ring groove just before the last stage impeller front hub ring turn.

Best practice is to maintain stringent alignment and concentricity between interfacing parts. This ensures correct concentricity and perpendicularity between shaft and bearings and rotor to casing. The service center discovered that the top bowl male fit had been previously repaired by pad welding (see Figure 1), which is an improper practice due to the presence of a sealing O-ring. When a pad weld is performed on a pump that uses the O-ring design, fits and tolerances no longer meet acceptance criteria. It appears that the previous repair provider coated the faces with silicone or another sealant in an attempt to re-establish the proper fits or control leakage (see Figure 2).

Figure 1. Pad welded male fit for the top bowl.

Figure 2. Silicone coating appears to have been used in a previous repair after the male fits were pad welded in an attempt to seal the proper fit between the top bowl and the discharge head.

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Analyzing the poor repairs on a boiler feed pump and how they affect the pump’s performance and reliability.

It is often said that a picture is worth a thousand words. This old adage could not have been truer when a coal-fired power station received pictures from the inspection team at a service center. The plant had pulled a previously rebuilt Worthington boiler feed pump from storage and sent it to the service center for disassembly, inspection and reverse engineering. What the inspection team found was a number of repair defects uncovered from the previous repair process.

Because many repaired pumps initially go into storage as this one did, the consequences of a poorly rebuilt pump may not be revealed until several years later. The unfortunate results can range from reduced pump efficiency and shorter mean time between repair to catastrophic failure and unplanned outage. In this particular case, the repaired pump had been in storage and not been run since the repair was completed. It was therefore in its “as built” condition when it arrived at the service center. The photographs that follow illustrate a number of the defects that were uncovered and how they affect the performance and reliability of the pump.

Shaft

The bearing journal surfaces were not chrome-plated. Lack of chrome plating decreases the shaft and sleeve bearing life. Chrome plating reduces surface friction, reduces wear and helps to reduce contaminants in the oil from the shaft base material.

There was also lack of smooth radii at diameter transitions. Radii help reduce stress concentrations and help prevent shaft failures.

 A newly manufactured shaft with the proper radius

Bearing journals had not been chrome-plated in previous low quality repair.

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Typical vertical pump types include turbine, mixed and axial flow. The Hydraulic Institute classifies these pumps as type VS (Vertically Suspended). Unlike horizontal pumps, these pumps typically have the pumping element (impeller, diffuser, column pipe and pump shaft) submerged in the pumpage. Therefore, the metallurgy of all pump components must be compatible with the pumpage to achieve adequate life (MTBF).

The vertical pump is versatile, both in construction styles and hydraulic capabilities from 1,500 through 10,000 specific speed. It is used effectively in many industries such as nuclear power, fossil power, oil and gas, mining, municipal, general industrial and agricultural markets.

To properly consider repair upgrades, one should be familiar with the operational service of the pump in its specific application. Some vertical pump sensitivities include:

• High Speed-Stator and rotor alignment, cavitation, rotor balance
• High Specific Speed-Intake design, cavitation
• Corrosive Service-Materials compatibility, protective coatings
• Well Pumps-Lineshaft lubrication, start-up and shut-down coordination with valving, variable frequency drive operation, installation

It may be possible to more than double a standard manufactured vertical pump’s life by upgrading the pump repair to a “precision remanufacture.” The pump vibration will be reduced and can be verified upon start-up as proof of the upgrade. The repair and upgrade cost is usually a small item in the pump’s life cycle cost.

Figure 1. (l.) The resultant eccentricity between rotor and stator caused by loose fits between the bowls and columns. (center) The resultant eccentricity caused by a lack of parallelism between the mating faces. (r.) The resultant rotor eccentricities caused by loose fit by threaded couplings.

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