Skip to main content

Failure investigation, remedies, and mitigation of a centrifugal pump.

 BY LUIS INFANTE & RODOLFO ALVARADO.

A high energy pump at a water injection station in El Furrial, Venezuela exhibited extremely high vibration levels prior to an overhaul. It then suffered a catastrophic failure during startup following overhaul. The hydrodynamic bundle, rotor, and drive end (DE) bearing suffered damage.

 


High energy pump for boiler feed water. Courtesy of Flowserve.

This centrifugal pump is a 3,000 HP, double-case volute, boiler feed water pump type. It has nine stages, outputs 750 gpm of water with suction pressure 1800 psi and discharge pressure 5250 psi. Rated speed was increased from 6000 to 6600 RPM to enhance the hydraulic performance. However, the pump’s actual discharge pressure was about 4,500 psi, well below the target value of 5,000 psi. The coupling was reportedly poorly fitted.

The increased RPM created rotordynamic concerns of getting closer to a critical speed, thus the operator wanted to know about the synchronous regime. The operator also wanted remedial measures and temporary mitigation steps to keep the pump running for 4-6 months until remedies were finally enforced.

The course of action for this investigation was clear since the beginning: conduct an internal clearances analysis and a forced response rotordynamic study.

A thorough study of internal clearances was conducted. Table 1 shows the results from this study featuring a comparison between internal (hydrodynamic bundle) clearances from different data sources, namely: design data, shop measurements, API-610 minimum clearances, and typical clearances for a similar pump with vibratory problems. As a result, the recommended clearance for the center seal, balance piston, eye, and impeller hub seals are shown in the bottom part of Table 1.

Due to the high vibration levels (20 mil DE, 5 mil NDE) reported before the overhaul, a simulation of the pump’s forced response rotordynamic behavior was attempted using a trial unbalance weight located in the coupling. A mass-elastic mathematical model incorporating the effect of the internal seals at 1X and 2X design clearance (as suggested by API 610) was used for this purpose.

One can expect a flexible rotor behavior (first critical speed below operating speed) for this high-speed, long slender shaft, but the stiffening effect provided by the internal seals (Lomakin’s effect) locates the first critical speed in the vicinity of 9,000 RPM, well above 6,600 RPM operating speed. Retrofitting the bearing and coupling with the technology presented below raises this first critical speed to approximately 10,000 RPM, thus discarding the likelihood for resonance despite the steep vibration plot obtained during a ramp up vibration survey.

 


Table 1: Analysis of internal clearances.

REMEDIES AND MITIGATION

Our attention was focused on balancing and the effect on the dynamic stability of the rotor-bearing-support substructure. A bearing and coupling retrofit was recommended with a balancing plane on the coupling and an added mass on the bearing housing. It was discovered that the shaft was excessively long on the DE. Shortening the shaft together with a reduced moment coupling proved to be beneficial in keeping rotor synchronous response away from resonances. Such coupling design has a center of gravity and flex discs moved closer to the bearing.

Tilting pad bearing is a good option for stability and support of this slender high-speed shaft. A pad load distribution analysis showed increased clearances unloading the top pads and the six-pad arrangement offered the best support. Furthermore, a lateral stability analysis revealed six-pad bearing to have the best stability parameters (whirl mode, log dec, amplification factor, undesirable speeds).

Effective mitigation measures that ran for a few months with acceptable vibration levels turned out to be:

1.      Add 150lb to the bearing housing.

2.      Increase internal clearances and bearing clearances (max 6 mil).

3.      Finning the bearing housing ribs for enhanced heat dissipation.

4.      Using the coupling as a balancing plane; adjusting and balancing the coupling.

5.      Correct bearing housing distortion.

CONCLUSIONS

The most likely cause of failure was attributable to tight clearances found in the hydrodynamic bundle’s internal seals. OEM design and even API clearances were considered to be too tight. Reducing internal clearances below API 610 recommendations exposes these pumps to catastrophic failures during start up. For the impeller eye, we recommend a 50% increase (to 27 mil) over API values for stages 1–4 and a 25% increase (to 23 mil) for stages 5–9. Clearances for other rotor locations are shown in Table 1.

Trimming the shaft in the DE side and installing a reduced moment coupling helps in separating the operating speed from the first critical envelope, thus reducing the amplitude of vibration.

 

Source: turbomachinerymag

 

Comments

Popular posts from this blog

Maintenance 4.0 Implementation Handbook (pdf)

WHAT IS MAINTENANCE 4.0? Industry 4.0 is a name given to the current trend of automation and data exchange in industrial technologies. It includes the Industrial Internet of things (IIoT), wireless sensors, cloud computing, artificial intelligence (AI) and machine learning. Industry 4.0 is commonly referred to as the fourth industrial revolution. Maintenance 4.0 is a machine-assisted digital version of all the things we have been doing for the past forty years as humans to ensure our assets deliver value for our organization. Maintenance 4.0 includes a holistic view of sources of data, ways to connect, ways to collect, ways to analyze and recommended actions to take in order to ensure asset function (reliability) and value (asset management) are digitally assisted. For example, traditional Maintenance 1.0 includes sending highly-trained specialists to collect machinery vibration analysis readings on pumps, motors and gearboxes. Maintenance 4.0 includes a wireless vibration sensor conne

27 steps of the Gearbox Repair and rebuilding

 27 steps of the Gearbox Repair and rebuilding: Step 1 Cleaning exterior of Gearbox and identification. Step 2 Remove all bolts from the gearbox. Step 3 Disassembly for Gearbox preliminary evaluation of the condition and repair required Step 4 Mag inspect Gearbox. Step 5 check all Gears. Step 6 Customer communication of health of the Gearbox. Step 7 Parts to be repaired or, reverse engineered parts where needed required for Gearbox rebuild. Step 8 Failure analysis during complete disassembly and evaluation of the component wear and damage. Step 9 Cleaning all internal components and housing. Step 10 Check all bearings diameters in house. Step 11 Check all shafts Step 12 inspect all Gears. Step 13 Set up check line bore of the gearbox. Step 14 Repair and rebuild Gears back to O.E.M Step 15 Replacing all bearings seals and gaskets Step 16 Repair and rebuild all shafts again to O.E.M Step 17 Realigning all gears shafts and bearings back to O.E.M Step

Thermal growth: how to identify, quantify and deal with its effects on turbomachinery

Thermal growth, as used in the field of machinery alignment, is machine frame expansion resulting from heat generation. The generation of heat, of course, is caused by operational processes and forces. Materials subjected to temperature changes from heat generation will expand by precise amounts defined by their material properties. In turbomachinery, thermal growth results from the temperature differences occurring between the at-rest and running conditions. Generally speaking, the greater the temperature difference, the greater the thermal growth. The magnitude of the growth can be calculated from three variables: ∆ T (temperature difference) C   (coefficient of thermal expansion) L    (distance between shaft centerline and machine supports) When machinery begins to generate heat, the temperature difference between at-rest and running conditions will cause thermal expansion of the machine frame, thereby bringing about the movement of the shaft centerlines. This can produce changes in

John Crane's Type 28 Dry Gas Seals: How Does It Work?

How Does It Work? Highest Pressure Non-Contacting, Dry-Running Gas Seal Type 28 compressor dry-running gas seals have been the industry standard since the early 1980s for gas-handling turbomachinery. Supported by John Crane's patented design features, these seals are non-contacting in operation. During dynamic operation, the mating ring/seat and primary ring/face maintain a sealing gap of approximately 0.0002 in./5 microns, thereby eliminating wear. These seals eliminate seal oil contamination and reduce maintenance costs and downtime. John Crane's highly engineered Type 28 series gas seals incorporate patented spiral-groove technology, which provides the most efficient method for lifting and maintaining separation of seal faces during dynamic operation. Grooves on one side of the seal face direct gas inward toward a non-grooved portion of the face. The gas flowing across the face generates a pressure that maintains a minute gap between the faces, optimizing flui

Technical questions with answers on gas turbines

By NTS. What is a gas turbine? A gas turbine is an engine that converts the energy from a flow of gas into mechanical energy. How does a gas turbine work? Gas turbines work on the Brayton cycle, which involves compressing air, mixing it with fuel, and igniting the mixture to create a high-temperature, high-pressure gas. This gas expands through a turbine, which generates mechanical energy that can be used to power a variety of machines and equipment. What are the different types of gas turbines? There are three main types of gas turbines: aeroderivative , industrial, and heavy-duty. Aeroderivative gas turbines are used in aviation and small-scale power generation. Industrial gas turbines are used in power generation and other industrial applications. Heavy-duty gas turbines are typically used in large power plants. What are the main components of a gas turbine? The main components of a gas turbine include the compressor, combustion chamb