Centrifugal pumps need to be kept in tip-top condition to handle harsh liquid-terminal operations.
Without question, liquid storage terminals play an integral role in the ongoing success and relevance of a wide array of global industries. These facilities serve as a vital hub in the storage and dispersal of numerous liquids—including vegetable oils and fats, oleochemicals, petroleum products and petrochemicals—all substances that are crucial to the world's transportation and manufacturing sectors.
In short, any liquid that can be transported in bulk—be it by ocean-going tank ship, barge, railcar, tank truck or pipeline—is at some point in its supply chain stored and transferred at a liquid terminal.
While the range of products that may pass through a liquid storage terminal is wide and varying, the operating conditions at most terminals are similar—harsh, often corrosive atmospheres that require around-the-clock equipment reliability. Any downtime has a potentially huge adverse effect on the terminal's operation and profitability.
Transfer pumps used in these conditions are under constant assault and must perform reliably while handling a number of diverse operating characteristics, such as:
- Changes in ambient temperatures and other weather conditions, such as humidity
- Line shock from piping that is not anchored properly
- Piping systems that have sharp bends instead of gentle curves
- Changes in the product being pumped
- Changes in product viscosity
- High volume (unloading a 50,000-gallon tanker) at high flow rates (4,000 gallons per minute)
- Changes in product velocity and force
- Changes in head pressure
Taking all this into consideration, this article focuses on the role of centrifugal-pump technology in liquid-terminal applications. It will also show how proactive steps in both preventive and protective maintenance can keep the pumps running reliably in harsh environments that put the pump's effective operation under constant threat.
The Maintenance Solution
Centrifugal pumps move liquids through the use of centrifugal force. The three common styles of centrifugal pumps are:
- ANSI—These pumps meet centrifugal-pump manufacturing criteria established by the American National Standards Institute (ANSI) in 1977. With that standard in mind, ANSI centrifugal pumps are engineered for operational flexibility and durability and can meet the needs of virtually any fluid-transfer application.
- Standard end-suction—They are ideal for thin liquids and the top choice for most water-pumping applications.
- Self-priming—In these pumps, balanced pumping pressures prevent product recirculation, making them ideal for high-capacity loading and unloading operations.
No matter the operational atmosphere in which these or any type pump is used, a routine maintenance program will extend the pump's life since well-maintained equipment lasts longer and requires fewer and less-expensive repairs. A detailed record of any preventive maintenance that was performed and repairs that were needed should be kept to help diagnose problems and eliminate or minimize future equipment downtime.
Routine preventive and protective maintenance practices should, at a minimum, include the monitoring of:
- Bearing and lubricant condition—Bearing temperatures, lubricant level and vibration should be monitored. The lubricant should be clear with no signs of frothing, while changes in bearing temperature may indicate imminent failure.
- Shaft seal condition—The mechanical seals should show no signs of visible leakage. Any packing should leak at a rate of about 40 to 60 drops per minute.
- Overall pump vibration—Imminent bearing failure can be preceded by a change in bearing vibration. Unwanted vibration can also occur due to a change in pump alignment, the presence of cavitation or resonances between the pump and its foundation or the valving located in the suction and/or discharge lines.
- Pump discharge pressure—The difference between the readings on the suction and discharge gauges will provide the total developed head pressure of the pump. A gradual decrease in the developed head pressure of the pump can indicate that the impeller clearance has widened, which requires an impeller clearance adjustment to restore the pump's intended design performance.
To stay on top of these potential maintenance concerns, a leading centrifugal-pump manufacturer suggests the following basic maintenance regimen. Also worth noting is that maintenance and monitoring intervals should be shortened if the pump is used in severe-service conditions, such as with highly corrosive liquids.
- Check the pump's foundation and hold-down bolts for tightness.
- For oil-lubricated pumps, the oil should be changed after the first 200 hours of operation for a new pump then after every three months or 2,000 operating hours, whichever comes first. For grease-lubricated pumps, the bearings should be re-greased every three months or 2,000 operating hours, whichever comes first.
- Check the shaft alignment.
- Re-grease the motor bearings according to the manufacturer's instructions.
The pump's performance should be checked and recorded in detail at least once per year. Performance benchmarks should be established during the early stages of a pump's operation when the parts are new and the installation adjustments are correct. This benchmarking data should include: