The alkali regeneration circuit that ensures the removal of total sulfur from the natural gas liquids (NGL) of the natural gas plant has two small volume alkali recirculating seal-less pumps. These pumps are responsible for transporting the lern regeneration alkali containing natural gas (naphtha) from the disulfide separator to the natural gas scrubber settler.
Pump Design Review
These pumps are 178 horsepower (hp) seal-less magnetic drive pumps rated at 294 gallons per minute (gpm) with a total differential pressure of 638 feet (ft).
The process fluid is contained entirely within the pump seal housing and is dependent on the removal of heat generated through the internal lubrication path. Therefore, any slight restriction of the lubrication flow path can jeopardize the condition of the pump’s internal components and lead to catastrophic failure.
In addition, the fluid flows between the inner magnet, the seal housing and reaches the rear of the pump shaft through the shaft bore and then returns to the pump suction port through the thrust balance bore in the impeller (Image 3).
IMAGE 3: Magnetic drive pump lubrication flow regime
The pump shaft connected to the inner magnet is rotated by the magnetic flux circuit between the inner and outer magnets. The outer magnet is connected to the drive motor by another shaft to transmit the rotation to the inner magnet.
Background information
Under normal operating conditions, the pump tripped due to a motor thermal overload fault signal. All process conditions were checked and found to be within normal parameters. Later, the pump started again and immediately tripped due to a motor stall fault signal.
It was decided to remove the pump for disassembly and internal inspection. Key findings included:
1. Excessive friction between the front cover of the impeller and the pump casing. In addition, the impeller shroud has become thin. This is an indication of high thrust motion as shown in Image 1 and Image 4.
IMAGE 1: Casing severe rubbing and caustic crystallization (Images courtesy of Saudi Aramco)
IMAGE 4: Impeller rubbing impact (thinning)
2. The rear thrust pad, thrust washer and sleeve bushing were not in place. The silicon carbide bearing failed and traces were found inside the pump housing ( Images 2, 5 and 6)
IMAGE 2: Rear bush housing assembly (bush carrier) with damaged thrust pad and corrosion erosion impact
IMAGE 5: Signs of chemical attack (corrosion) in the front nondestructive evaluation (NDE) titanium bush housing assembly
IMAGE 6: Process of identifying the casual factors and sequence of pump trip
Research and Findings
Conclusions were drawn based on the results of these examinations, operational trends, alkali laboratory sample analysis and metallurgical analysis. The material analysis showed that the titanium bushing bushings had blue colored particles consistent with the newly introduced process catalyst in 2019. This is considered to be an indication of a corrosion reaction.
It is suspected that the catalyst has filled the standby pump and when it is not running, the pump will not disperse the fluid. The product will then attack the front sleeve bushing as it is mounted in the casing worm and the rear sleeve bushing is located in an inaccessible area of the pump.
Sometimes, the process is affected by high caustic soda concentrations. Material analysis shows that the corrosion rate of the titanium jacket should accelerate when the pump is exposed to higher concentrations of 40% to 50% caustic soda.
As shown in Image 6, it is suspected that the corrosion and loss of the titanium sleeve bushing circumference is sufficient to cause the bolt to lose grip between the bushing carrier and the bracket. As a result, all bushing carriers are free to move radially and axially, causing the thrust pad and thrust washer carbs to shatter. The failure of the rear thrust pad and thrust washer (active bearing) increases the negative thrust and axial movement of the entire rotor towards the pump casing. This leads to severe casing wear, impeller and radial bushing failure.
Recommendation
The intermittent 50% caustic concentration and the impact of the new catalyst corroded the titanium section, causing the thrust bearing to fail.
The user confirmed that the use of titanium was not a common choice, but it was requested by the user during the project design process. In addition, the other pump components were made of duplex stainless steel and were not subject to any corrosion attack. Based on the survey results, user experience and corrosion charts, it was recommended that the bushing housing material be upgraded to a duplex stainless steel material.
Article from “Pumps and Systems Magazine“