Centrifugal pumps have been commonly used in many services and applications. A common centrifugal pump configuration is the horizontal single-stage pump. The most common pump type is the horizontal, end-suction, top-discharge, centrifugal pump. Other types of pumps, such as multistage pumps, vertical pumps and between-bearing (BB) pumps, have also been used in different applications and services based on specific requirements. This article discusses packaging, installation and commissioning of centrifugal pumps for smooth and trouble-free operation.
Pump Packaging
The baseplate selection for a pump is one of the first steps in pump packaging. This is a key step for reliability and proper pump operation. There have been different configurations, types and materials for the baseplate. The best recommendation is a heavy-duty, strong baseplate fabricated from suitable steel profiles and sections. Torsional stiffness, rigidity and flatness are important for a pump’s baseplate. A fabricated steel baseplate should be sufficiently heavy, properly welded, stress relieved and machined.
All pumps require some small utility piping in the packaging. For example, some piping and connections for lubrication systems, seal systems, drains or vents are necessary. Tubing (even stainless-steel), flexible hoses or similar have been used, but they have presented failures and reliability issues in different pump packages. Small, rigid piping has always been the best option. Small piping with properly selected material and details (the right schedule of pipe/fitting) is better than the above-mentioned tubing, hoses, etc. Small piping should be properly supported as well as sufficiently braced and gusseted to prevent any vibration, high stress points or failures. Corrosion and accident potential are also of concern when regarding small piping; so, heavy walls with conservative (rigid) supports are recommended.
All centrifugal pumps should have a pressure gauge on the discharge side close to the outlet of the pump. This is helpful for troubleshooting and monitoring, as the accurate discharge pressure is a key piece of information. Pressure gauges at the suction are another matter for discussion. On one hand, a pressure gauge at the suction is helpful for monitoring but on the other, the produced disturbances on the suction line are not good outcomes, as the suction piping should be as simple as possible. As a result, many pumps do not have a pressure gauge on the suction side.
Installation
Proper installation usually refers to a correctly implemented foundation, proper anchor bolts, minimum piping loads, proper leveling, appropriate alignment and perfect grouting. Installation of pumps requires great attention. Leveling, proper positioning and alignment are important. As a rule of thumb, the baseplate of a pump train should be leveled (side-to-side, end-to-end, etc.) to within 0.0002 meter per meter (m per m)/0.2 mm per m (millimeter per meter)—for instance, less than 0.4 mm for a 2 m pump baseplate. Regarding the piping of the pump, it is recommended to pipe from the pump suction flange to the suction source, not the other way around.
As a rough indication, the foundation mass should be three to four times the mass of the centrifugal pump’s package. Epoxy grout should always be used for machineries including centrifugal pumps. Anchor bolts should be sufficiently strong and long. More specifically, they should have a length/diameter ratio of 11 to 16. The anchor bolts should be provided with sleeves, or a similar tool, to prevent entry of grout and accommodate relative thermal growth because of the thermal growth difference between the foundation and baseplate. Regarding low piping forces (low nozzle loads), these should be so low that when making piping flanges to pump’s nozzle flanges, no dial indicator should move more than 0.04 mm while tightening or loosening the flange’s bolts.
Commissioning & Operation
The early detection, identification and correction of pump problems are important to ensure continued, safe and productive operation. Immediately after the startup, the produced head and flow rate should be checked and evaluated. The pump should also be checked for excessive vibration, noise, bearing operation, operating temperature and other operational details. These steps should also be checked first; however, if pump hydraulic operation seems normal, the source of a problem could be in the alignment, bearing or seal.
When gas (air) enters a centrifugal pump, it sometimes gets trapped inside the pump (for example in the volute), and this reduces capacity and creates vibration and noise. If a pump is excessively noisy and there is no mechanical problem, then it is likely that gas (air) in the pump is causing the issue. A less common cause could be cavitation. Cavitation usually produces a distinct noise, often different than noise produced by other malfunctions. Another way to determine if the issue is cavitation is to check the net positive suction head (NPSH) margin and monitor fluctuations in suction and discharge pressure.
Rotordynamics & Vibration
Rotordynamics play a major role for any centrifugal pump, particularly for high-speed, large or high-power pumps. Critical speeds should not encroach upon the operating speed range. Most pumps are stiff shaft machineries with the first critical speed above 120% of the maximum operating speed. For variable speed drive (VSD) pumps, the first critical speed should be above 130% of the maximum operating speed. Some large pumps might use flexible shaft concepts. For these, the first critical speed is located below the minimum operating speed, which is typically in the 15% to 25% margin. The second critical speed is usually above 130% of the maximum operating speed.
A good measure to evaluate the pump’s operation and hydraulic is vibration. It is normal for all pumps to vibrate. Even pumps in the best of hydraulic, mechanical and dynamic operating conditions will have some vibration because of minor defects and imperfections. Therefore, each pump will have a level of vibration that may be regarded as normal or inherent. In operation or condition monitoring terms, this is known as “baseline.” However, when pump vibration increases or becomes excessive, some operational issue or mechanical trouble is usually the reason. Vibration does not usually increase or become excessive for no reason. Hydraulic issues, unbalance, misalignment, worn bearings, looseness, etc., can result in high vibration. Some of these root causes of higher vibration than baseline could be tolerated until the next major overhaul or repair. However, some of them can be harmful and dangerous.
High vibration itself could damage pump components, such as seals or bearings. As a rough indication, unfiltered vibration velocity, when measured radially to the shaft (with a bearing-cap mounted vibration sensor for instance), should often not exceed three to four mm/second peak to peak.