Whether a reliability engineer or a purchasing manager in a power plant, no one ever wants to learn that their pump in a mission critical application is beyond repair. Yet, this can happen when health checks of a pump are never scheduled, particularly if, due to a fortunate choice in equipment, the pump has operated reliably for several decades.
This occurred with a primary supplier of backup electrical power, which provides supplemental power to residential and commercial customers in its territory when the power grid becomes overloaded.
Process conditions changed several times over the years as federal emission standards required power suppliers to curtail pollution. But what had not changed was the reliability of the plant’s two twin screw fuel pumps. Neither pump had failed during that time, despite their age of more than 40 years. However, fuel viscosities were reduced several times over the decades as process conditions were altered, and the pumps were not designed to operate in this situation.
The two twin screw pumps provided fuel to two separated turbines, which generated the power. Each ran separately, with only one needed at a time to provide 100 percent of the plant’s output obligations, and the second was backup if needed. However, the reliable, consistent performance of the pumps had given the plant a false sense of security, and the plant had not stocked major or minor repair kits to respond to unexpected downtime if it ever occurred.
The twin screw pump manufacturer conducted a plantwide audit. Elements evaluated included aging equipment, mission critical applications and processes that had undergone dramatic system changes. With all three of these conditions present, a run-to-failure approach on pumps more than 40 years old carried significant risks. Lacking in-stock parts kits, repair options would be more limited and the power plant would have to wait longer for a new replacement pump.
Recognizing the precarious situation, plant personnel partnered with their supplier’s authorized service center to conduct a two-week, comprehensive teardown, inspection and analysis on one of the two twin screw pumps that had been sold new in 1979. The analysis and findings, with recommendations, included:
Pump Case
All four bore locations had areas of galling from screw contact in the suction and discharge areas. The bore inside diameters measured up to 0.0085 inches over the designed high tolerance. This body was cast iron and could not be restored.
Recommendation: Replace
Rotating Assembly
The screw flanks showed excessive wear, which was indicative of an improper timing of the screw shafts to the gears. Both sides of the flanks showed contact. Both long and short shafts had detached coating on the suction and discharge ends of the screw. This matched the heavy contact areas noted above in the casing bores. Outside diameters measured both higher and lower than design depending on the point of measurement. This was a result of wear and raised material on the surface of the screws from flank contact. The flank clearances measured as much as 0.012 inches, which was 0.005 inches over the high tolerance. This added clearance resulted in a loss in delivered flow during operation. The timing gears showed evidence of heavy fretting, and the inside diameter of the gear bores exceeded designed tolerance.
Recommendation: Replace; the repair price was more than 60 percent of the price of a new pump
Bearing Brackets
The front and rear bearing brackets required some minor cleaning to thoroughly flush the seal ports, but they could be used as is once cleaning and deburring occurred.
Recommendation: Reuse
Timing Gear Housing & Front Head
Both of these items were deemed to be usable in their current state, following cleaning and deburring.
Recommendation: Reuse
Bottom line: The pump auditors found that improper timing of the screws had caused excessive wear to the screw flanks, preventing economical repair of the unit. The turbines operated on light fuel oil viscosity. Assuming a fuel oil viscosity of 44 Saybolt Seconds Universal (SSU), the smallest pitch for a 1,250-frame-size pump (0.875 inches) at the highest rotations per minute (rpm) (1,550), and a differential pressure of 1,000 pounds per square inch gauge (psig) (as listed on the nameplate), a new replacement rotating element and reused body would still operate with a volumetric efficiency of only 30 percent, resulting in excessive loss in volumetric efficiency and heat generation.
The audit made it possible for plant personnel to make an educated decision on next steps, which included:
- options for replacement of normal wearing parts where simply reassembling and reinstalling the pump could make sense
- a detailed understanding of the state of all major internal elements, tolerances and wear
- options for executing any repair
- options to replace the pump
In the end, a minor or major repair was not viewed as an effective way to mitigate the power plant’s risk due to the substantial performance losses that would still remain. Thus, the decision was made to temporarily use the reassembled pump as an emergency backup and order a new twin screw pump. The pump supplier’s authorized service center reassembled the pump within a week for that purpose.
Upon successful installation of the new pump, the power plant took its second unit offline and executed the same process, with the same results and success, providing the plant with the peace of mind that comes with knowing a risk of operational failure had been eliminated for many years to come.
Lessons for Operators
It is common knowledge that mechanical seals, bearings and gaskets have a finite life and require periodic and ongoing replacement. A small amount of seal leakage (about 10 drops per hour per seal) is normal and necessary; anything in excess of this requires repair or replacement with a seal compatible with the pump’s operating conditions. Bearings and gaskets should be cleaned, deburred, lubricated and checked regularly for noise and/or abnormal vibrations and rough operation to ensure that all connections are tight.
But it should also be common knowledge that a laissez-faire approach with the pump case and main workings is never recommended for mission critical operations.
Here is why having a user's pump disassembled, cleaned and inspected is a sensible approach compared to running into costly failure:
Timing gears: In the rotating assembly, the timing gears of a screw pump are central to its flow performance. If design tolerances have been exceeded and fretting damage is excessive, the assembly usually must be replaced.
Pump-driver alignment: The system can experience unusual levels of vibration or large variances in operating temperatures if out of alignment.
Pump body: Galling damage from screw contact in the suction and discharge ports can cause bore sites to exceed design tolerances. Restoration may not be possible.
When wear is so excessive that a unit cannot be repaired economically, a complete pump replacement is the best remedy in a worst-case scenario. In better-case scenarios, stocking both minor and major kits can save the day—even when the plant has enjoyed decades of reliable operation with no downtime.
In the end, however, no one knows the screw pump like the team that built it, and health checks conducted by certified technicians set the stage for restoring the factory clearances and design tolerances that equipment was built for—so that end users can focus on what they do best.
