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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More than 15 years ago, a 160 in plate mill was experiencing significant maintenance problems with its descaling pumps; the typical mean time between repairs was only 6 to 8 months. Some rebuilt pumps even failed on start-up.

Descaling is one of the more severe, but critical, services in a steel mill. The pressures are high and the rapid changes in flows and pressures severely impact the pumps. At the same time, the pumps’ performance can significantly impact the quality of the steel produced.

Improvements to these pumps were implemented in various phases over several years. The path was not always straightforward and required close cooperation and teamwork between the aftermarket service provider and mill personnel to implement various upgrades.

Root Cause Analysis-Rotor Condition Analysis

At the start of the project, all of the pumps, which had been in service since the early 1970s, were exhibiting high noise levels along with abnormally high vibration, erosive wear and consistent, frequent maintenance problems.

The first step was to comprehensively analyze the pump rotor in a process called Rotor Condition Analysis. The Rotor Condition Analysis report, coupled with analysis of field operating conditions, provided the forensic evidence to identify the root causes of pump problems. This data, when analyzed in conjunction with the operational data, vibration readings and other field information, enables the aftermarket provider’s engineers to troubleshoot the pump and develop recommendations to solve the identified problems.

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An 1,800 MW power station in the Midwest experienced constant trouble with four of its four-stage boiler feed pumps, which were driven by steam turbines and operated at variable speed to meet the required plant load.

According to the plant, these pumps had always exhibited high vibration. In 2003, the plant approached an aftermarket service provider to resolve this problem. Unfortunately, the pumps were not fitted with vibration monitoring equipment, so vibration trends were not readily available. Since the problem existed on all four pumps, a systematic analysis was recommended to determine the root cause. The plant and repair shop developed a plan to a) undertake a field study and conduct a vibration trend analysis, b) identify possible causes by performing hydraulic and structural analysis, c) analyze suitable modifications and d) rebuild the pump accordingly.

Field Observations

1. The vibration level varied by machine, from 0.3 ips to a maximum of 0.7 ips.
2. Maximum vibration levels were at one times running speed on the inboard bearing housing and two times running speed on the outboard bearing housing.
3. The vibration level was ~0.15 ips at impeller vane pass frequency at five times running speed (the impeller had five vanes).
4. In the past, the suction impellers were repaired or replaced every two years due to impeller vane erosion damage.

Analyses and Results

Engineers believed that the root cause for vibration problems could be attributed to the combined effect of structural stability and hydraulic phenomena. For the purpose of analysis both of these effects were studied separately.

Results of Structural Analysis

The recommendation was that the plant employ a third party consulting firm specializing in vibration analysis to conduct a field and analytical study to determine the full operating deflection shape (ODS) of the pumps. Impact testing was also performed to determine the natural frequencies and mode shapes of each unit. The ODS study revealed the following:

1. Modal impact testing showed a lightly damped natural frequency of approximately 88 Hz on three pumps and 98 Hz on the fourth pump. As the typical running speed was between 4,400 rpm (73.3 Hz) and 5,400 rpm (90 Hz), the pumps were operating at or near critical speeds.
2. Analysis showed that the one times running speed vibration was a result of the entire pump twisting on its pedestals, and not a result of localized motion of the bearing housings (see Figure 1). The pedestals were deforming their cross-sectional shape to produce this twisting motion. The inboard bearing housing moved in-phase with the barrel at this frequency. Applying a fix to the coupling guard or inboard bearing housing could slightly reduce vibration levels, but did not address the vibration’s root cause.
3. The outboard bearing housing vibrated in the vertical direction at two times running speed due to a nearby vertical natural frequency of the outboard bearing housing. In some cases, the amplitude of this vibration exceeded that of the one times vibration on the inboard bearing housing.
4. The outboard bearing housing vibrated in the horizontal direction at vane pass frequency (five times running speed = 375 Hz to 450 Hz). There was a natural frequency at approximately 380 Hz that produced at least some significant vibration over this entire speed range.

Figure 1. Data acquired during testing shows the pump twisting on its pedestals.

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