Pump operating issue case study: Identifying the poor performance root cause (TiA)
Problem: An alkylation unit’s main fractionator reboiler pumps were experiencing operational problems. Two 100% pumps were available: one online and one 100% spare P-900/901.
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A rotating equipment engineer described the problem: When placed online, the P-900 control valve is 70% open at 130,000 BPD flow. Shortly after start-up, the flow starts decreasing, and the control valve opens. It gets to a point where the control valve is wide open, and the flow is still dropping off. When the pumps are switched, the same behaviour is observed. The pump starts up per design, but the flow quickly deteriorates.
It was concluded that the pump behaviour was indicative of a net positive suction head (NPSH) issue. It was thought that the net positive suction head available (NPSHa) was lower than the net positive suction head required (NPSHr). A new pump with a lower NPSHr was specified.
A replacement pump was selected with input from the original equipment manufacturer (OEM) and was about to be purchased. Management asked the process engineering group to approve the new pump before the order was placed.
In Stratus’ three decades of providing service to the hydrocarbon processing industry, the most common misdiagnosed and undiagnosed engineering issue seen has been hydraulics. Often, this is due to a lack of effective engineering tools, but it can also be due to improper application of available engineering tools.
For this problem, the process engineer used Process Engineering ToolS (PETS) software to analyse the proposed replacement pump. Physical properties, hydraulic pressure drop, and NPSH evaluation were performed. The analyses showed that no pump suction flashing should occur at the operating conditions. No NPSH issue was evident (see Figure 1).
The analyses showed that the NPSHa was well above the installed pump’s NPSHr. Additionally, the proposed replacement pump had very similar NPSH requirements to the installed pumps.
The Fittings Equivalent Lengths Tool was used to obtain the equivalent lengths for the pump suction and discharge piping. The PETS Pump Curve Tool was used to enter the pump curve data. The control valve datasheet information was entered into the Control Valve Tool.
The Pipe System Hydraulics Tool was used to put it all together and model the system (see Figure 2). The model was used to check the system at various conditions. The model showed that the system was process capable, that the existing pump had no NPSH issues, and that there was no flashing in the system. PETS showed the pump operational issues were not due to suction flashing, assuming typical pipe condition.
However, the pump’s operational issues were real. What was causing it? The pump was taken apart to investigate. A pump suction witches hat strainer (unknown and with no visible handle) was installed backwards in the system. Debris was found inside the witches hat (see Figure 3).
Based on the findings, it was concluded that the pumps were starting up fine and after operating a short time the accumulated debris shifted in the inside cone area of the backwards witches hat, impeding flow into the pump. When the pump was stopped, the debris fell to the bottom of the strainer, resulting in a lower pressure drop at start-up but with the debris ready to shift and more fully plug the pump’s inlet the next time the pumps were switched.
Equipment performance problems can be difficult to diagnose. A thorough evaluation considering all possible causes is beneficial. Using the right engineering calculation tool, such as Process Engineering ToolS software, to examine the process is imperative to assist in analysis and making correct decisions. In this case, the investigation prevented the replacement of a pump that was designed properly and process capable.
This short case study originally appeared in PTQ's Technology In Action Feature - Q1 2024 Issue
For more information: Lidia.Suarez@stratusengr.com
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