Correctly executed impeller vane tipping is critical to boiler feedwater pump performance and reliability, and to avoid problems, pump buyers and users should be aware of this subtle pump quality detail.
Boiler feedwater pumps are critical elements in thousands of electric power generating stations. Historically, engineering procurement construction (EPC) contractors have experienced problems with boiler feedwater pumps during the commissioning and testing of new power plants. On some projects, boiler feedwater pump failures have become a “critical path” constraint to work completion.
At Bechtel Power Corporation, six-sigma tools are used to analyze boiler feedwater pump failures and root cause analyses are used to determine their root causes. With this information, proactive steps are taken to prevent problems before they get to the jobsite. One of these steps is called the “blackbox” review process.
An impeller with defective tipping is shown at the left.
Blackbox refers to the details of a pump design and the manufacturing processes used to produce the pumps for each project. By working closely with the pump suppliers to ensure that designs are properly verified and validated, that engineering requirements are clearly communicated to manufacturing personnel, and that pumps are produced with proven and controlled manufacturing processes, high-quality boiler feedwater pumps can be supplied with no startup problems.
On the right, an impeller that has been reworked. Photos courtesy of Bechtel.
A small, but important, detail of boiler feedwater pump design and manufacturing is impeller tipping – the hand working of impeller vane trailing edges and vane-to-shroud junctions at the impeller outside diameter in order to clean burrs from machining and to achieve appropriate radius geometry for hydraulic performance and mechanical integrity. If the pump supplier does not properly execute impeller tipping, catastrophic pump failures and significant project delays can occur.
Pump Impellers – Any Change Can Create Deviation
Impellers are the heart of all centrifugal pumps. The vanes of a rotating impeller deliver useful energy to the pumped fluid by increasing velocity in a dramatic but controlled manner. This velocity energy is then converted to static pressure energy by diffusion in the pump casing elements. In multi-stage pumps, a series of impellers and casing elements repeats this process on the same flow stream until the desired discharge pressure is achieved.
Most boiler feedwater pumps are multi-stage pumps and have several impellers, which are enclosed-type designs as classified by the Hydraulic Institute and in the range of 1000 to 2000 suction specific speed, Ns:
Ns = (rotating speed, rpm)(flowrate, gpm)**0.5/(head, feet)**0.75
Typically, impellers of this type are produced from single-piece static castings of chrome steel material. Impeller castings include the shape of a hub, two shrouds, and a set of complex vanes or blades, which make up the hydraulic passages. The castings are set up, turned, and bored with precision machine tools. After machining, unwanted material and rough edges remaining at the transitions between the machined and cast surfaces must be removed and smoothed with hand grinders and files. (In some cases, additional material is removed to sculpt the vane shape near the trailing edge to achieve conditions called “underfiled” vanes.) As such, the final impeller geometry is a combination of cast, machined, and hand worked surfaces. The hand working process is often called impeller tipping.
Pump suppliers place great importance on the quality of impeller castings. Some suppliers (and some EPC contractors for that matter) specify precision-type molds, non-destructive examination, documented dimensional inspections, and other quality control measures to ensure correct material integrity and casting geometry. Likewise, impeller machining is closely specified, executed, and monitored. Impeller machining geometry is always carefully communicated to manufacturing personnel using very detailed component drawings, and is routinely dimensionally inspected with micrometers.
Impeller tipping, on the other hand, is often less controlled. Historically, engineers at the pump supplier would specify only general instructions for cleaning the edges and burrs left after machining and provide only general dimensions for filing the vane tips. Most of the actual impeller tipping geometry, including corner radii at the corners and trailing edges, was left to the discretion of the manufacturing craftsman performing the handwork.
Many pump suppliers have recently been party to a consolidation or merger, introducing additional market pressures to lower manufacturing costs. These conditions have moved manufacturing and component production to new and different factories. Labor-intensive practices such as impeller tipping do not transfer easily between factories or to new low-cost supply sources.
Pump Failures – Small Deviations, Big Consequences
Recent experience with moderately high-energy boiler feedwater pumps demonstrates the importance of impeller tipping. The case involves two 50-percent startup pumps, which serve two 425 MW coal-fired plants. The pumps are multi-stage centrifugal pumps in ring-section configuration. Each pump stage produces 1014 feet of head with an impeller peripheral speed of 265 ft/s and absorbs 855 horsepower per stage.
During the commissioning of this power station, two of the startup pumps had to be shut down because of high shaft vibrations. A dismantle inspection of the first pump revealed that the shroud of the first-stage impeller had cracked and broken away while the pump was operating, causing additional component damage downstream of the failure. The second event followed immediately after the first. Cracks were found in the impeller shrouds of the first, second, and third stages. In both cases, impeller cracking had resulted in rotor unbalance and vibration, causing the shutdowns. During the problem-solving and repair processes, unit startup activities were curtailed and the project was delayed.
Root Cause Analysis – Supplier and EPC Contractor Teamwork
The pump supplier and Bechtel Power Corporation worked closely together to analyze the root cause of the failures and develop corrective action plans. Closed forensic examination of the impellers proved that the cracks started at the trailing edges of the impeller vanes near the vane-to-shroud junction and propagated through the vane to the shrouds by a fatigue mechanism. While the fatigue mechanism was apparent, the cause of the crack inception at the trailing edge of the vanes was not so obvious.
The project team investigated several potential causes of the crack inception:
· Cavitation damage (possibly associated with low-flow operation)
· Hydraulic loading (possibly associated with water hammer or undersized cutwater clearance)
· Impeller design (possibly due to insufficient shroud thickness or resonant natural frequency)
· Material properties (possibly associated with foundry carburizing)
· Impeller manufacturing (possibly traced to machining defects or incorrect tipping)
Cavitation damage was quickly dismissed, because trending information showed that the pumps were operated above the minimum flow and with adequate Net Positive Suction Head. Also, the failures had occurred after only weeks of operation, which was too little time for cavitation damage due to impeller recirculation to cause mechanical damage.
Hydraulic loading was a potential cause because one unit had undergone an audible water hammer event during startup activities. However, no subsequent events were heard, which left the second pump failure unexplained. The cutwater clearance was investigated and found to be greater than four percent of the impeller diameter, which was consistent with the supplier’s engineering practice and industry standards such as API Standard 610. Therefore, hydraulic loading was assumed to be a contributing factor, but not the root cause of the cracking.
The impeller design was analyzed in detail. The pump supplier performed a structural resonance frequency analysis using a computerized finite element model. The analysis results showed that stresses were less than one-half the endurance limit based on the nominal shroud and vane thickness, and that the impeller structure had no natural frequencies sympathetic to multiples of running speeds including vane-passing frequencies. It was agreed that the impeller design was fundamentally good, but the inception of the cracks remained unexplained.
The impeller material conformed to the strength specified in ASTM A487 Grade CA6NM. Microstructure and hardness tests found that the material was properly normalized and tempered. Additional material testing at the origin of the cracks found normal micro-hardness as well. The specified material was appropriate and the metallurgy complied with all requirements.
Closer physical review of the impellers revealed two manufacturing quality defects. First, the machined vane-to-shroud radii were too small, because sharp-pointed tool inserts had been used to trim the outside diameter of the impeller. Second, the vane trailing edges were too sharp, with ragged and very thin material sections at the crack inception locations. Further investigation revealed that traditional engineering information and standards had not been fully communicated to an overseas manufacturing facility.
The supplier and Bechtel concluded that machining operations produced thin material sections or burrs at the vane trailing edges, as they always will. These thin sections were not removed and blended by proper impeller tipping, and thereby produced highly stressed material that fractured when the pumps were placed into operation. Small stress fractures on the trailing edges provided the very high stress concentrations, which produced the fatigue mechanism in basically ductile material. In some cases, the cracks propagated into heavier sections of the vanes and down the shrouds. In one case, a piece of shroud broke away.
Correct Actions – Making it Right
After reviewing the system requirements and the latest pump test data, Bechtel agreed that the pumps had head margin and that the impellers could be trimmed by 0.125 inches in order to provide the material to perform proper impeller tipping. The pump supplier reworked all impellers, including new replacement impellers, with correctly machined radii at the shrouds and appropriate radii and blending at the vane trailing edges. Magnetic particle inspection was performed on all the reworked areas to ensure that no small fracture cracks remained. Also, the impellers were dynamically balanced individually after all rework was completed.
The pump supplier also took steps to ensure that their internal processes were corrected. They proactively upgraded their impeller tipping standards and distributed them across the manufacturing facilities by revising their impeller drawing standards. Subsequently, Bechtel had an opportunity to survey one of those manufacturing shops. A new impeller-tipping standard had been communicated and documented for new pumps of this model. Most importantly, manufacturing personnel had been trained and were properly performing impeller tipping.
The startup boiler feedwater pumps were rebuilt with the reworked impellers and placed back into service. They have been operating for more than two years without any further problems.
Preventive Actions – Getting Iit Right Again and Again
The lesson learned from this case is that properly executed impeller vane tipping is critical to boiler feedwater pump performance and reliability. Pump engineers must clearly specify and document tipping geometry requirements, and manufacturing personnel must have the skill to execute the treatment to the defined standards. Pump buyers and users should be aware of this subtle detail of pump quality so that problems can be avoided.
Bechtel Power applied this lesson learned to its existing blackbox review process for all boiler feedwater pumps. Recently, impeller tipping has been reviewed at several design centers and manufacturing facilities of three major pump suppliers. Findings from reviews have shown that pump engineers generally agree on the kind of impeller tipping needed for their designs, but that they rarely agree on how to communicate it to the shops, even within the same company. Bechtel has made helpful suggestions in this regard to ensure that impeller tipping is properly implemented.