Centrifugal pumpsFor decades, the number one pump-style choice by operators of CPI facilities has been centrifugal-pump technology. Centrifugal pumps meet the needs of the many and varied transfer operations that are found within the CPI because their design enables them to handle many fluid-transfer applications. Historians tell us that the first machine that can accurately be classified as a centrifugal pump was a mud-lifting apparatus that appeared in Europe as early as 1475 A.D. [ 1] A straight-vaned centrifugal pump was developed in the late 1600s by Denis Papin, a French inventor and physicist. The curved-vane centrifugal pump, which most closely resembles current-day centrifugal-pump technology, was brought to market in 1851 by British engineer John Appold. Appold’s design, which was three times more efficient than other pump technologies at the time, won him a “Council Medal” at the Great Exhibition at Crystal Palace in London, England, that year. Since their earliest invention, centrifugal pumps have moved liquids through the use of centrifugal force. This makes them kinetic machines in which pumping energy is continuously imparted to the pumped fluid by means of a rotating impeller, propeller or rotor. More specifically, centrifugal pumps use bladed impellers to transfer rotational mechanical energy to the fluid, primarily by increasing the fluid’s kinetic energy, or angular momentum, while also increasing the potential energy (static pressure). The gathered kinetic energy is then converted into usable pressure energy in the discharge collector [ 2]. In other words, a centrifugal pump transforms the energy of “velocity” transferred to the fluid by the impeller into energy of “pressure” in the casing or diffuser(s). Currently, the two most common styles of centrifugal pumps are: • End-suction — These pumps are ideal for thin liquids and the top choice for most water-pumping applications • Self-priming — This type of centrifugal pump has the ability to lift fluid, which gives it an advantage when the source is below the centerline of the pump Both end-suction and self-priming pumps, as well as other pump styles, may also meet the 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. No matter the operational atmosphere where these types of pump are being used, a routine maintenance program will extend the life of the pump since well-maintained equipment lasts longer and requires fewer and less-expensive repairs. In fact, because many CPI pumping systems can often have life spans of 15 years or longer, it is now a valid consideration for the plant operator to perform a life-cycle cost (LCC) analysis that factors in the lifetime costs of maintenance, along with purchase, installation, energy usage, operation, downtime, environmental and other costs when choosing the proper pump technology for the operation. According to the Hydraulic Institute, while energy, at 40%, might represent the highest expected pumping-system-related expense in an LCC analysis, the second-most costly is often maintenance, at 25%, which is well ahead of initial pump cost and operating costs that are both estimated at 10% of life-cycle costs [ 3].
Maintaining an edgeIn order to obtain optimum working life from a centrifugal pump, regular and efficient maintenance is required [ 4]. When the pump is purchased, the pump manufacturer will typically advise the plant operator about the frequency and extent of routine maintenance, but it is the operator who has the ultimate final say about how his facility’s maintenance routine will function, or in other words, whether it will consist of less frequent but more major attention, or more frequent but simpler servicing. The potential cost of unexpected downtime and lost production is also a significant item when determining the total LCC of a pumping system. Again, the facility’s maintenance routine should determine what steps should be followed when an unexpected breakdown occurs, while a post-repair assessment should identify areas where a more-proactive maintenance regime might have prevented the breakdown and resulting downtime. The facility operator must also be certain to keep a detailed record of any preventive maintenance that was performed and repairs that were needed for each pump. This information should be kept in order to create an easily accessible record that can help diagnose problems and eliminate, or minimize, any future equipment downtime. Moving into the nuts and bolts of centrifugal-pump maintenance, routine preventive and protective maintenance practices should include, at a minimum, the monitoring of the following [ 5]: Bearing and lubricant condition. Monitor bearing temperatures (Figure 1), lubricant level and vibration. The lubricant should be clear with no signs of frothing, while changes in bearing temperature may indicate imminent failure.
Quarterly maintenanceThe following should be done every quarter: • Check the pump’s foundation and hold-down bolts for tightness • 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 • Regrease bearings every three months or 2,000 operating hours, whichever comes first • Check the shaft alignment
Annual maintenanceA pump’s performance should be checked and recorded in detail at least once a 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 the following: • The pump’s developed head pressure, as measured at the suction and discharge pressures, for three to five conditions should be obtained. Where possible and practical, a no-flow reading is a good reference and should be included
Diaphragm PumpsThe invention of air-operated double-diaphragm (AODD) pump technology in 1955 was a textbook example of necessity being the mother of invention. The invention of AODD pump technology was the climax in an ongoing search for a way to effectively and efficiently pump substances with a wide range of viscosities — from water to slurries to cement — at a wide range of flowrates. The technology, which was said to have been “conceived out of necessity, born in the arms of innovation, and inspired by sheer will and determination,” operates by displacing fluid from one of two liquid chambers upon each stroke completion through the action of an air valve and a pair of diaphragms that are connected by a common shaft [ 7]. This simple design, where the diaphragms act as a separation membrane between the compressed air supply and the liquid, has stood the test of time. Evidence of this is borne out everyday at CPI facilities around the globe where AODD pumps — whether metal or plastic, ductile iron or stainless steel, clamped or bolted — play a critical role in ensuring that product transfer of thin liquids, corrosives, abrasives, particle-laden slurries and more continues apace in a cost-effective, energy-sensitive, environmentally friendly manner. Like all other types of pumping systems, no matter their method of operation, AODD pumps operate at their best efficiency when maintained properly, either through preventive or protective maintenance. Thankfully for the operators of CPI facilities, the AODD pump’s design means that maintenance is easily and efficiently performed. A preventive-maintenance schedule should be set up for the following AODD pump parts to ensure that the pump is serviced prior to pump wear [ 8]: Diaphragms Valve seats Valve balls O-rings When performing routine AODD-pump maintenance checks, there are three main areas that demand attention: Air valve piston/spool and casing — Ensure that the piston/spool can move freely and remove any debris. Diaphragms — Make sure there is no swelling, cracking or other damage to the diaphragm surface. Balls/seats/O-rings — Make sure that no swelling, cracking or other damage is apparent, and lubricate the shaft, if needed. Another top-of-mind maintenance concern is seal replacement since proper seal installation is critical to pump performance. Care must be taken to ensure that seals are placed in the proper grooves and not damaged during installation. Incorrect seal location will render the pump inoperable, while damaged seals may cause decreased performance and shorter seal life.
TroubleshootingDue to the design of AODD pumps, there are only a few rare complications that may surface during their operation. Fixing these problems can be a simple process in many cases. Below you will find a list of potential problems and solutions that pump users might find in CPI operations [ 7]: Pump will not run or runs slowly— Solutions: Check for obstructions in the air passageways or objects that can obstruct the movement of internal parts. Pump runs but little or no product flows — Solutions: Check for pump cavitation, slow down pump speed to allow material to enter pumping chambers, then increase speed accordingly. Check for sticking ball check valves and, if necessary, replace check valves with proper elastomers. Check to make sure all suction connections are air tight. Air bubbles in pump discharge — Solutions: Check for ruptured diaphragm. Check tightness of clamp bands, especially at the intake manifold. Product comes out air exhaust — Solutions: Check for ruptured diaphragm. Check tightness of large clamp bands. Check tightness of piston plates to shaft, if applicable. Pump rattles — Solutions: Create false head or suction lift. Over the years, the AODD pump technology has been modified to fit specific pumping applications. One of the modifications that has benefited chemical processors has been the introduction of plastic, solid-body AODD pumps. The construction of these pumps allows them to deliver increased product capacity and optimized flow patterns, all while using less air. Their heavier construction also eliminates the “wander” that can plague lighter oscillating pumps and can often result in product leakage. Like their traditional AODD brethren, these unique pumps must be maintained properly in order to reach their full product-transfer potential. A key area that plant operators should focus on for preventive maintenance are the housing bolts. Instead of single bolts pressing punctually against the housing, the bolts are tightened against a diaphragm-sized ring. These rings must be inspected and, if damaged, replaced in order to ensure that the housing bolt force is evenly spread across the pump’s surface, as intended. AODD pumps are well-suited for the CPI thanks to their ability to perform in even the harshest operating conditions, but like all pumps, their operational performance is only enhanced when proper maintenance precautions are taken. So, while AODD pumps can be the answer for many fluid-handling requirements in chemical processing, it is only those that are maintained regularly and properly that have the longest life-cycle and lowest total cost of ownership.
1. Ladislao Reti, Francesco di Giorgio (Armani) Martini’s Treatise on Engineering and Its Plagiarists, Technology and Culture, Vol. 4, No. 3, 1963.
2. “Rotodynamic (Centrifugal) Pumps for Nomenclature and Definitions”, (ANSI/HI 1.1-1.2), Hydraulic Institute, Parsippany, N.J.
3. “Optimizing Pumping Systems: A Guide for Improved Energy Efficiency, Reliability & Profitability”, Hydraulic Institute, Parsippany, N.J.
4. “Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping Systems”, Hydraulic Institute, Parsippany, N.J.
5. “Installation, Operation and Maintenance Manual, Griswold Model 811 ANSI Process Pump”, Griswold Pump Co., Grand Terrace, Calif.
6. Article: Innovation Through Necessity, Wilden Pump & Engineering Co., Grand Terrace, Calif.
7. “Pump User’s Guide,” Wilden Pump & Engineering Co., Grand Terrace, Calif.
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