Chemical Pump Maintenance
Pumps must be kept in optimum shape to handle harsh chemical-processing operations
04/04/2012| Author / Editor: Xavier Rasotto / Wolfgang Ernhofer
Transfer pumps form a key part of any chemical plant. These pumps must operate reliably, often around the clock and handling a variety of corrosive or otherwise difficult materials. This article explains how to plan and carry out routine maintenance of centrifugal chemical pumps so as to avoid pump downtime and consequent lost production.
For decades, the commonest form of pump used in chemical-processing facilities has been the centrifugal pump. These versatile machines use a rotating impeller, propeller or rotor to impart kinetic energy to the fluid being pumped. As the fluid leaving the impeller is slowed by the stationary parts of the pump, this kinetic energy is then converted into pressure energy .
Centrifugal pumps are versatile, with the ability to handle a variety of challenges and potential problems, including:
·changes in ambient temperature and humidity
·line shock from piping that is not properly anchored
·piping with sharp bends instead of the preferred gentle curves
·changes in the viscosity and other characteristics of the liquid being pumped
·large and periodic changes in hydraulic operating point
The two most common styles of centrifugal pumps are:
·End-Suction: Ideal for thin liquids and the top choice for most water-pumping applications.
·Self-Priming: This type has the ability to lift fluid, which gives it an advantage when the source is below the centerline of the pump.
Both of these pump types, and others, are available in ANSI format, which describes centrifugal pumps meeting the manufacturing criteria established by the American National Standards Institute in 1977.
Robust as they are centrifugal pumps will only perform at their best if operators provide the right kind of preventive and protective maintenance. This article will demonstrate how a strict maintenance routine can keep centrifugal pumps running reliably and economically in the harsh environment of a chemical plant.
Making Maintenance Mandatory
Because many pumps have life spans of 15 years or longer, it is becoming common for plant operators to perform life-cycle cost (LCC) analyses of pumping systems. These aid the choice of the most suitable pump for the job by factoring in the lifetime costs of maintenance, along with purchase, installation, energy usage, operation, downtime, and environmental protection.
According to the US Hydraulic Institute, LCC studies show that energy typically represents 40 percent of the lifetime cost of a pump. The second most costly item is often maintenance, accounting for around 25 percent of lifetime cost. Initial purchase cost and other operating costs are both estimated at just 10 percent of the pump’s lifetime cost.
For all types of centrifugal pumps and operating conditions, a program of routine maintenance will extend pump life, since well-maintained equipment lasts longer and requires fewer and less-expensive repairs.
When the pump is purchased, the pump manufacturer will typically advise the plant operator about the necessary maintenance, but it is the operator who has the final say about his facility’s maintenance routine – whether, for instance, it consists of simple servicing carried out often, or major attention at less-frequent intervals.
The cost of unexpected downtime is also significant when determining the total LCC of a pumping system. Again, the facility’s maintenance routine should determine what steps to follow when a breakdown occurs. Once the pump is back in action, a post-repair assessment should identify areas where more proactive maintenance might have prevented the breakdown.
Keep a detailed and easily accessible record of any preventive maintenance performed and any repairs needed. This information can help to diagnose future problems and minimize unplanned downtime.
What to Do and When
Routine preventive and protective maintenance should include the monitoring of :
Bearings and lubricant: Check bearing temperatures, lubricant level and vibration. The lubricant should be clear with no signs of frothing. An increase in bearing temperature may indicate imminent failure. Shaft seal: Mechanical seals should show no visible signs of leakage. Packings should leak at a rate of no more than 40–60 drops per minute.
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, its foundation or valves in the suction or discharge lines.
Differential pressure: The difference between the pressure readings at the discharge and the suction of the pump will provide the total pressure developed by the pump. A gradual decrease in this total pressure can indicate that the impeller clearance has widened. In this case, adjusting the impeller clearance (for pumps with semi-open impellers) or replacing the wear rings (for pumps with closed impellers) will restore the pump’s design performance.
Every three months:
·Check the pump’s foundation and hold-down bolts for tightness.
·Change the oil after the first 200 hours of operation for a new pump and then every three months or 2,000 operating hours, whichever comes first.
·Re-grease the bearings every three months or 2,000 operating hours, whichever comes first.
·Check the shaft alignment.
·Annually: The 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:
°At three to five different flow conditions, including zero flow where this is practical:
°-the total pressure developed by the pump (the difference between the suction and discharge pressures);
°‒flowrate; and
°‒motor amps and voltage
°Vibration signature
°Bearing housing temperature.
°Any change in these figures from those of the year before should be used to re-assess the level of maintenance required to get the pump back to its best performance and keep it there.
All maintenance and monitoring intervals should be shortened if the pump is used in severe-service conditions, such as with highly corrosive liquids or abrasive slurries.
The Importance of Lubrication
All pump bearings will fail eventually. More often than not, however, this will be due to a failure of the lubricating medium rather than to any intrinsic problem with the bearing. It is therefore important to pay particular attention to bearing lubrication so as to maximize bearing life and, by extension, pump life.
For oil-lubricated bearings use only non-foaming and non-detergent oils. The proper oil level is at the mid-point of the bull’s-eye sight glass on the side of the bearing frame. Avoid over-lubrication, which can be just as damaging as under-lubrication: excess oil will cause a slightly higher power draw and generate additional heat, which can cause frothing of the oil. Any cloudiness in the oil generally indicates that condensation has caused the water content of the oil to rise above 2,000 ppm. If this is the case, change the oil immediately.
For re-greaseable bearings never mix greases of differing consistencies or types. Also note that the shields must be located toward the interior of the bearing frame. When re-greasing, ensure that the bearing fittings are absolutely clean as any contamination will decrease bearing life. Avoid over-greasing as this can cause localized high temperatures in the bearing races and create caked solids.
After re-greasing, the bearings may run slightly hotter for an hour or two.
What to Look Out For
Whenever pump parts need to be replaced, treat the repair as an opportunity to examine the rest of the pump for signs of fatigue, excessive wear and cracks. Replace any worn parts that do not meet the following standards:
Bearing frame and foot: Check for cracks, roughness, rust or scale. Check machined surfaces for pitting or erosion.
Bearing frame: Inspect tapped connections for dirt. Clean and chase threads as necessary. Remove all loose or foreign material. Check that oilways are open.
Shaft and sleeve: Check for grooves or pitting. Check bearing fits and shaft runout, and replace the shaft and sleeve if worn or if the tolerances are greater than 0.05 mm (0.002 in.).
Casing: Look for signs of wear, corrosion or pitting. The casing should be replaced if wear is deeper than 4 mm (1/8 in.). Check gasket surfaces for irregularities.
Impeller: Check for wear, erosion or corrosion damage. If the vanes are worn more than 4 mm (1/8 in.) deep, or bent, replace the impeller.
Frame adapter: Look for cracks, warping or corrosion damage and replace if any of these conditions are present.
Bearing housing: Check for wear, corrosion, cracks or pits. Replace housings if worn or out of tolerance.
Seal chamber/stuffing box cover: Check for cracks, pitting, erosion or corrosion, paying special attention to any wear, scoring or grooves on the chamber face. Replace if worn more than 4 mm (1/8 in.) deep.
Shaft: Inspect for corrosion and wear. Check straightness: the maximum total indicator reading (TIR) at the sleeve journal and coupling journal should not exceed 0.05 mm (0.002 in.).
Implementing all of these recommendations may seem daunting, but a proactive maintenance routine will help to avoid unplanned downtime due to pump failures, improve safety and protect the environment.