Due to the myriad engineering, logistical and safety challenges involved in a refining operation, American Petroleum Institute (API) over hung (OH) style pumps are often overlooked as a potential source of improved productivityor as a cause of catastrophic failure, if not operated properly.
Refinery managers, maintenance engineers and production supervisors should adhere to best practices and understand the operational dos and donts for employing pumps properly in refining applications. Paying heed to pump performance can boost operational efficiency, and avoid failures that reduce production or cause catastrophic shutdowns.
Pump fundamentals and monitoring.
One example of a catastrophe occurred recently at a North American refinery that produces about 70,000 barrels per day (bpd). A fire broke out at the bottom of a vacuum tower, forcing a three-day shut down that cost $1.5 million in damages and lost production time. An investigation quickly identified a failed pump as the cause. This is a common occurrence, if pumps are not operated and maintained properly. On average, one out of every 1,000 pumps with a failedmechanical seal leads to a fire. Due to the seriousness and cost of such failures, maintenance engineers and production supervisors need to understand the operation of a centrifugal pump and how to operate it efficiently.
A centrifugal pump is rotating machine comprised of six main parts that work together to keep the pump operating properly (Fig. 1). They include an impeller, pump casing, bearings, bearing frame, shaft and a mechanical seal (Fig. 2). The operation principle of the pump is to convert mechanical energy to pressure. In operation, a rotating impeller accelerates a liquid and as the area of the pump casing expands, the velocity of the fluid is converted to pressure. As a result, pressurized fluid exits the pump discharge. Though the performance of API pumps has improved with enhancements to design and materials, the basic structure has changed little in decades.
| Fig. 1. Centrifugal pump. |
| Fig. 2. The six main parts of a centrifugal |
BEP and pump performance.
Best efficiency point (BEP), the flowrate where a pump has its highest efficiency, is a key factor in pump performance. Few pumps operate at their exact BEP all of the time, because process variables in a production environment are not 100% constant. However, a pump that is properly sized for its application will maintain a flow near peak efficiency. Maintaining a flow between 80%110% of BEP (Figs. 3 and 4) is a good range to maximize efficiency and minimize the risk of excessive wear or pump failure.
| Fig. 3. Centrifugal pump characteristics |
running below recommended minimum flow.
| Fig. 4. Centrifugal pump |
When a pump operates too far off its BEP, forces inside the pump become imbalanced, which can cause parts to deflect and wear excessively.
Operating to the right of BEP, a condition known as runout, means that the flowrate is higher than the pump was designed to maintain. The high flow increases the exit velocity of fluid leaving the pump, thus creating a low-pressure area inside the pump. Operating to the left of BEP occurs when the discharge flow is restricted, causing fluid to recirculate within the pump, also creating a low-pressure area, which can lead to increased radical loading and low-flow cavitation.
In either case, the creation of imbalanced pressure increases the radial loads on the impeller, which can cause shaft deflectionthe bending of the impeller shaft, which increases vibration of the pump. The vibration and imbalance forces can create stress on the pumps internal components, most likely to be seen first in the bearings and/or mechanical seals, the two parts of a centrifugal pump that fail most often.
| Fig. 5. Centrifugal pump characteristics |
running at preferred operating range.
Cavitation and net positive suction head.
Cavitation is another serious problem caused by the lack of adequate net positive suction head (NPSH), the pressure provided at the suction of the pump, less the fluids vapor pressure. When fluid pressure on the trailing side of the impeller blade (opposite the pump intake) falls below the vaporization point of the fluid, vapor bubbles begin to form. When these vapor bubbles reach an area of high pressure inside the pump, they can collapse violentlycausing sudden, uneven axial and radial loading on the impeller. This, in turn, can cause shaft deflection that is random in direction and often severe in magnitude.
While it is easy to reccomend run your pumps at the BEP with adequate NPSH, operation within the oil and gas markets, be it upstream to downstream, is a dynamic process. Process change production rate change, but typically the hundreds, if not thousands, of pumps to support these process do not change and this can lead to improper operations. It is only when a robust condition monitoring program in combination with operations support, can reliable and efficient pump operation be achieved.
Managing pump performance in a refinery.
In a complex refining operation, where the focus is on production, manually monitoring the performance and operation of API pumps is seldom the top priority. A disconnect also often exists between the maintenance and operations functions, which prevents managers from seeing the complete picture of equipment performance. A continous condition monitoring program that involves both maintenance and operations can help refining operations to address both challenges. The North American refinery faced with the $1.5 million pump failure is a case in point. A root-cause analysis revealed that the pumps mechanical seal caused the fireand a review of maintenance records showed numerous repairs and parts replacements consistant with off-BEP pump operations in the weeks and months leading up to the fire. Subsequent to the disaster, the refinery installed a continuous monitoring system in 2009.
The system plays a key role in keeping both the operational and maintenance sides of the refinery up to speed on pump performance and machine health. It monitors temperatures, vibrations, pressure and power to ensure that pumps are operating properly, and also warns of trouble before it occurs.
The monitoring data is distributed to both the control room and a to web-based condition monitoring platform. Simple, easy to understand key performance indicators (KPIs) such as suction pressure, vibration and power data allow any operator to monitor pump operations without special training. When more advanced diagnostics are neccessary, such as vibration spectrums and time-wave forms, the reliability and maintence teams can access the web-based condition monitoring system from any web-enabled device. The system integrates maintenance and operational data in a single dashboard, improving operational efficiency and reducing the need for walk-around manual monitoring. In nearly two years since the system was installed, the refinery has required no unplanned maintenance on its pumps.
While refinery managers, maintenance engineers and production supervisors face continual challenges to maintaining production, they cant lose sight of proper pump operation and maintenance. Both play a key role in preventing pump failures. Condition monitoring systems need to support both machine status and process data to answer why pumps are about to fail, not just when and where. By implementing these best practices, operators can run their pumps like the pros, while boosting production and reducing risk at the same time. HP
|The author |
||Dan Kernan is the manager at ITT Monitoring and Control. He earned a BS degree in mechanical engineering from the University of Rochester and has spent the last decade integrating technologies such as power electronics, embedded sensors and wireless systems with rotating equipment to improve equipment reliability and reduce energy. Mr. Kernan and his team have helped numerous customers in the oil and gas industry implement pumping systems and pump control solutions. He holds three patents or patents pending in technology related to improving pump reliability. |