Minding The Gap

Boiler feed pumps are a critical component in the steam generation process. An online failure of a boiler feed pump can result in inoperability of the plant and a significant loss of revenue. Usually high-energy, multistage configurations, these pumps have stringent design and manufacturing standards that must be followed to achieve reliable operation.

A new unit at a Midwest fertilizer plant in the US experienced a catastrophic boiler feed pump failure shortly after installation. The site was unable to reach a resolution through the original equipment manufacturer (OEM) and began looking for other options. Hydro was recommended as a credible service provider with the technical experience and engineering capability necessary to diagnose and resolve their problem.

The failure was caused by foreign material in the system, but an in-depth analysis showed that a flawed design and non-centralised axial positioning of the impellers contributed to the severity of the event. Using a combination of engineering upgrades and improved standards, Hydro resolved the design flaw and helped the plant achieve successful operation of the equipment.

Understanding the pump

The failed pump is an 8th stage opposed impeller design, meaning that the 1^st through 4^th stage impeller eyes face inboard and the 5^th through 8^th stage impeller eyes face outboard with a crossover chamber connecting the two sides. An inducer prior to the first stage increases the net positive suction head available to the first stage impeller.

The stationary cartridge assembly uses multi-vaned diffusers to recover pressure. During the testing phase, the pump experienced vibration problems that required a modification of the original diffuser assembly design. Inserts were provided on the wall adjacent to the impeller front shroud to close off a large cavity. This insert also provided a closer clearance between the impeller and diffuser shrouds at the impeller exit (Figure 1). A smaller insert was provided on the other side of the impeller to provide the same impeller-to-diffuser clearance at the back shroud.

Figure 1. The modification provided closer clearance between impeller and diffuser shrounds (A) while also closing off the large diffuser sidewall cavity (B).

The failure

The failed boiler feed pump was a new piece of equipment installed in a newly constructed unit. Failure to use high pressure steam blast to clean the piping before bringing the unit online resulted in debris in the system. This debris migrated to the automatic recirculation control (ARC) valve, affecting the valve’s ability to function properly.

The purpose of the ARC valve is to ensure that the pump does not operate below the minimum recommended flow by opening a bypass line when system demand is low. As the ARC valve began to fail, the operation fell below minimum flow. Extreme low flow operation resulted in high radial and thrust loads, substantial vibration and heat generation, hydraulic instability, and ultimately seizure.

The failed pump was sent to Hydro to perform a thorough disassembly, cleaning, and inspection. Great care was taken during the disassembly and inspection to record any condition or information that might suggest the cause of pump failure. A complete dimensional analysis was also completed to aid in the diagnosis.

Inspection results

Disassembly of the failed pump revealed extensive damage to both the thrust bearing and the internal element. The thrust bearing showed significant heat damage and an almost complete loss of the bearing babbitt.

Contact damage was found on the impeller front shroud and diffuser inserts for the 6^th through 8^th stages. At these locations, the set screws that secured the diffuser inserts to the diffusers were sheared, resulting in displacement of the insert (Figure 2). Thermal cracking on the impeller spacer sleeves and damage to the inducer were also observed.

Figure 2. Displaced and fractured diffuser insert.

Taking into consideration the event history, inspection results, and a review of the design, a clear timeline of the pump failure was established. Excessive heat generation and high radial and thrust loads caused the thrust bearing to fail. This failure resulted in an extreme loss of babbitt, increasing the range of rotor axial travel. The greater axial travel resulted in contact between the impeller shroud and diffuser insert in the 6^th through 8^th stages.

The friction between the rotating impeller shroud and stationary diffuser inserts led to shearing of the insert set screws and eventual dislocation of the inserts (Figure 3). Throughout the failure timeline, component contact, damage-related foreign materials, and flow recirculation caused increasing heat generation, rotor instability, and vibration.

Figure 3. The set screws (A) had been displaced due to contact with the impeller. There were also signs of impeller contact damage (B).

This is an abridged version of an article originally published in the March 2021 issue of World Fertilizer magazine. To read the full article, follow the link to the March issue and turn to page 24: http://bit.ly/3vf9ZFG. And to sign up to receive a free regular copy of the magazine, follow the link: https://www.worldfertilizer.com/magazine/world-fertilizer/register/

Read the article online at: https://www.worldfertilizer.com/special-reports/17032021/minding-the-gap/