After a roofing manufacturer used large internal gear pumps to supply bituminous flux for years, a number of pumps experienced several shaft failures. These shaft failures were rare in the past and have been brought to the attention of the pump supplier. These pumps typically operate at 100 gallons per minute (gpm) with discharge pressures below 100 pounds per square inch (psi). The system uses a thermal oil heating circuit to maintain the bituminous flux at a proprietary high temperature to reduce the viscosity of the product to a level sufficient to ensure smooth flow and reduce build-up of solid bituminous flux on the walls.
The pumps are powered by a 30 horsepower (hp), 1750 revolutions per minute (rpm) motor and a gearbox that reduces pump speed to 100 rpm at 100 gpm product flow.
These pumps are used to transfer the bituminous flux from the storage tanks in the distillation plant to the final application of the bituminous flux on the production line.
The same pumps have been used at this site for over 20 years. As a rule, these pumps have a much longer service life in such conditions, and such a failure has never been seen in the past. Notably, the company used two versions of the same size pump for two different applications. One is considered a soft pump for asphalt flux application, and the other is considered a hard fill pump for asphalt filling. Rigid pumps have hardened steel rotors and shafts to reduce wear from abrasives in filled asphalt.
When the pump supplier noticed the problem with the broken shaft, the number of failed pumps was 6 in about 12 months. There are 18 pumps in total in this application. Over the past few years, the average number of pump failures over the same period has been one. Some pumps had a broken shaft and the shaft/rotor connection was sheared off, while others had the shaft spinning inside the rotor and the set screw that was supposed to hold the rotor and shaft intact was cut off.
The shear axis is on a rigid pump, and the shaft rotating inside the rotor is on a soft pump. The failure was analyzed and the following results were identified and resolved as directed.
Pump suppliers and manufacturers carefully analyze pumps with broken or rotating shafts. The shaft and set screw were subjected to dimensional checks and metallurgical analysis. Both comply with the manufacturer’s requirements. After the pump was burned to remove residual hardened pitch, abnormal wear in the pump indicated that excessive discharge pressure deflected the shaft and caused the rotor to crash into the pump casing on the suction side (see figures 2 and 3).
There are signs of wear on the suction side of the casing where the rotor is pressed against the casing wall. The suction side of the head plate also shows signs of wear and the rotor is flexing into the head. Other signs of wear on bushings, idlers, and crescents also point to overpressure and subsequent bending and deflection of the shaft. When the rotor presses against the body and head, excessive torque can cause the shaft inside the rotor to break or spin, shearing off the set screw.
According to the manufacturer, “The subsequent overpressure caused the shaft to bend, which in turn caused body wear, head wear, and ultimately failure of the rotor/shaft assembly.” As a result of the analysis, changes were made to the design of the pumps by the manufacturer and an analysis of the procedures for the operation of processes at the facility was carried out.
The pump manufacturer claims a maximum delivery pressure of 150 psi for this pump, due in part to the size of the shaft and the subsequent length to diameter (L/D) ratio of the shaft. Any pressure above 150 psi can cause the shaft to bend and deflect, as seen in the object breaking.
The unit’s standard operating procedures compensate for a maximum allowable discharge pressure of 150 psi, and care has been taken to maintain the temperature and viscosity of the bituminous flux so that the discharge pressure does not exceed this limit. In fact, as the facility’s maintenance team later gathered information about the process, the pressure rarely exceeded 100 psi.
However, it must also be considered whether energy-saving measures may cause the discharge pressure to exceed the maximum allowable discharge pressure of the pump. During plant shutdowns, such as weekends, the temperature of the bituminous flux maintained by the thermal oil heating circuit is reduced to save energy. After investigation, it was determined that the start-up time needed to bring the bitumen to an acceptable temperature and viscosity could not be increased to compensate for the colder weekend shutdown. So, after a weekend shutdown, the asphalt can be too cold and too viscous at start-up, with the pump discharge pressure over the 150 psi limit. There were no temperature and pressure records during these start-ups or pump failures, so this assumption was made.
With these factors in mind, the manufacturer took steps to address the issue of pump shaft deflection while the facility maintenance team focused on addressing the standard operating procedures issue. Each group takes action to eliminate further shaft failures.
The pump manufacturer has modified the pump design to include a larger shaft. Shaft diameter increased by 40%.
This thicker shaft helps reduce shaft deflection by 66%, reducing premature wear and the chance of failure of the rotor/shaft assembly. Larger shafts increase the L/D shaft deflection ratio and increase the maximum allowable discharge pressure to 200 psi.
This design change later became the standard for pumps of this size in all future products from the manufacturer. The shaft diameter on the drive side of the pump has not changed, so existing power transmission equipment such as pulleys and couplings are still applicable to the pump.
Pump suppliers place process monitoring equipment on bitumen flux pumps to measure process circuit temperature, discharge pressure, and motor current. The site maintenance team recorded electronically every two seconds and carried out visual monitoring three times a day for three months.
The facility maintenance team reviewed standard operating procedures specifically related to shutdown and start-up procedures for the bitumen fluxing temperature loop. Initially, the changes were made to prevent a cold start situation.
By analyzing pump wear patterns to determine if the pump was overpressurized, the team was able to focus on why the high pressure was occurring and what steps can be taken to prevent future shaft failure and unnecessary downtime.
The plant management team carefully studied the temperature and viscosity of the product under operating conditions, during downtime and during start-up. They determined that changes in operating procedures could be adjusted to prevent cold start conditions that could increase the viscosity of the bituminous flux and cause high pressure surges. Pump manufacturers agree that larger shafts can be used in pumps of this size and that less shaft deflection and higher pressures can be achieved. The design is now a standard offering for the manufacturer.
By analyzing the root cause of the failure, a team of facility maintainers, pump manufacturers and pump suppliers worked to reduce pump failures due to shaft fractures.
Post time: Jul-21-2023