How To Take Care Of Your Centrifugal Pumps – A Guide To Preventive Maintenance Of Centrifugal Pumps

December 02, 2019

Preventive maintenance of machines, especially centrifugal pumps does not begin with replacement or repair of damaged, torn or defective parts. It is associated with early detection and corrective measures to ensure that all the machines and their components are working at their full potential to deliver high performance and the desired output.


The easiest method for preventive maintenance can be broadly categorized into 4 sections:

  • Selection
  • Installation
  • Operation
  • Maintenance


Each of these segments will help you carry out preventive maintenance tips and ensure that your team regularly follows it to avoid any unforeseen damage to the components or machine.
Selection

Advice the exact nature of the liquid
This includes several factors such as the description of the liquid, its specific viscosity and gravity, operation temperature, reactivity with other liquids, and presence of any suspended foreign particles, if known.
Analyse the discharge conditions
Whenever necessary, maintain the suction lift to a minimum or, provide adequate submergence when the liquid is at or close to its boiling point.
Verify the required capacities
This includes checking the predicted deviations from minimum to maximum.
Check whether the service is intermittent or continuous
By doing so, you can avoid the installation of a light-duty pump on heavy duty service.
Check the discharge conditions
Make sure to supply the manufacturer with an exact range of predicted variations, given if there is a variation in the discharge head.
Make sure enough spare equipment is available
Centrifugal pumps should be regularly inspected for any faulty or damaged components, and worn parts. Make sure that you have spare equipment at any point of time in case of an emergency failure or requirement.
Keep spare parts handy
Some pumps are designed specifically to meet the buyer’s needs. In such cases, the manufacturer recommends keeping a few spare parts handy that are associated with that particular model. Not all pumps are of standard type, and their spare components might not always be available.
Analyse the best suited power for your pumps
It is highly important to maintain a constant power supply when any electrical/mechanical device is in use. Sudden power outage, discontinuing or fluctuating power supply can lead to false circuits and harm components in centrifugal pumps.
Check space limitation
A major reason for rapid deterioration of centrifugal pumps is often installation of pumps in cramped places. This leads to poor maintenance and inspection as it becomes difficult to evaluate every part and corner of the machine due to its restricted confinement area. You can use a vertical pump to gain space or enhance suction conditions which will ensure proper checking and maintenance of the pumps.

Installation

Pump bed plate should be solid
In order to reduce damage caused by vibrations and unexpected lateral shift of the bedplate, make sure that it is grounded firmly. This will not only prevent later shifting but also stiffen the assembly.
Pump foundations should be rigid
The best option for building the foundational layer of centrifugal pumps is concrete foundations that are built up from solid ground. The pump should be positioned straight over or as close as necessary to the main beams or walls, and members when installed on steel work.
Correct alignment of the pump and driver
Alignment plays a crucial role in centrifugal pumps, especially if the pumps have to deal with hot liquids. A proper check should be carried out at operating temperature of the liquid to ensure that the pump is aligned properly with its driver. Usually, the unit is aligned at the manufacturing factory before it is shipped. However, it is possible for the alignment to change or get disoriented during its transit, so a careful check to ensure the alignment is proper should be done.
Make use of liberal piping
The recommended size for pipes in centrifugal pumps is at least one or two sizes bigger than the suction nozzle in use. Avoid using short radius elbows that are near to the pump suction. Also, make sure that you do not use an elbow in a plane which can disturb the flow distribution of the pump.
Provide vent valves at high points of the pump
If the emitted vapours are inflammable or toxic, pipe these vents to a safe exist area.
No strains should be imposed by piping on pump
Piping operation can distort the pump on its bed plate which can easily destroy the alignment of the pump and driver. To prevent this, make sure that no strain is imposed on the pump by the piping operation.
Provide a suitable source for cooling water
It is important to provide a suitable source for cooling water for machinery systems such as pumps. You can use stuffing boxes and water-cool bearings.
Provide a by-pass connection
If the pump can be operated against a shutdown, provide a by-pass. Check that this by-pass should not direct towards the suction, rather to a point where the heat acquired in the pump can be easily dissipated.
Install flow-meters and suitable gauges
Without installing flow-meters and suitable gauges, you will not be able to determine the activity status of the pump.

Operation

Maintain a pump at its recommended flow capacity One of the best practices to follow while operating centrifugal pumps is using them at their recommended flow capacity. Unless it is normal for a pump to be operated at a reduced capacity, avoid using it in such conditions. Otherwise, it may result into shaft failure or high radial reaction. A part of the power consumed by the pump is used to heat the liquid, if there is insufficient flow available in the pump, the excess heat might cause the pump to overheat and cease to run efficiently. Therefore, as a standard rule, the minimum flow for protecting the pump against overheating is about 30 gallons of liquid per minute for every 100 horsepower at shut-off.
Don’t operate a dry pump
When dealing with centrifugal pumps, it is mandatory to have liquids that are necessary for lubrication and cooling the internal clearances. A pump operating dry will usually overheat and seize. Ensure that you never start a pump with its suction valve closed.
Make hourly observations
Preventive maintenance entails activities that are related to analysing the pump and its components at a regular span. Look out for any irregularities in performance, decreased output, lowered quality in performance, bearing temperatures, stuffing box, pressure gauges, and the flow indicator at an hourly basis to check if the pump is operating at its full efficiency and producing consistent output. Frequent observations helps in early detection of faulty components and irregularities in the pump. You can take corrective measures and ensure that the pumps are operating properly without any defects.
Avoid throttling the pump suction to reduce its delivery
This may lead to severe damage to the pump as it leads to pump cavitation. You can throttle the discharge instead, which is much easier and safer as compared to throttling the pump suction.
Don’t stop leakage completely from the stuffing box
Note that some leakage is required for lubrication and cooling of the packing. Don’t force stop the leakage from the stuffing box completely.
Avoid using excessive cooling water in water-cooled bearings
Using excessive cooling water in water-cooled bearings leads to excess cooling of the bearing housing and allows condensation of moisture which tends to contaminate the lubricant.
Use new packing if stuffing boxes wear out
When the packing of the stuffing boxes wears out, many workers add on multiple new packing layers to the old one. Adding one or two rings to the old packing may displace the seal-cage.
Avoid using excessive lubricant with anti-friction bearings
The optimum level for lubricating the bearing with grease is that it should be one third packed with the liquid. In case of oil lubrication, the level should not be more than the recommended level on the oil gauge.
Semi-annual inspection
Regular inspections should be done to ensure the pump is running in its ideal condition and does not have any faulty components. Check for any free movement of stuffing box glands, clean and lubricate the gland bolts and nuts, and inspect if the packing is in proper condition. Check the alignment of the pump and driver, and correct it if necessary. Drain and refill bearings lubricated with oil. Ensure that you lubricate bearings with appropriate amount of grease, having a suitable consistency.
Annual inspection
Perform a complete check up and thoroughly inspect your pumps annually. Remove bearings to check them for any faults and clean them. Remove the packing and inspect the sleeves for any wear. Disconnect the coupling and monitor its alignment. Check and flush the drain, cooling water pumping and sealing water. Once you have done all of the aforementioned, recalibrate all the instruments and operate full tests of pump performance.

Repairs and Maintenance

Don’t casually open a pump for general inspection
Make sure that no workers open the pump for general inspection, unless there is definite evidence that the performance of the pump has declined or there is a noise or driver overload indicated internal trouble.
Pay special attention to examination and reconditioning of metal-to-metal fits
Pay special attention while dealing with stationary parts like casing rings seat in the casing so that you don’t damage it by any chance.
Be cautious while dismantling the pump
Whenever you are dismantling the machine for an inspection or a repair, make sure that you take preventive measures and carefully carry out the entire procedure. Lift the upper half straight up of horizontally split casing pumps to avoid damaging internal parts. Lift the rotor out vertically to avoid any injuries to impellers, wearing rings, etc. Mark all the parts as they are removed. Verify and carefully examine all the parts and joints. Make records of all faults, clearances, and compare with the original statistics of the machinery.
Install new gaskets for complete overhaul
Except for some customized or special designs, most of the centrifugal pump casings have gaskets which might get damaged on opening the pump. Make sure to install new gaskets of the same dimensions as the original, if damaged due to any reason. Accurately trim the inner gasket edges along the internal portion of the pump waterways. Before applying the gasket, ensure that the gasket mounting surfaces are clean.
Examine impellers for cavitation, corrosion or erosion
Again, if there is severe wear of the impellers, replace it with a better material that is available. If any serious signs of cavitation are found, consider relocating the pump and improve its suction conditions or get a customized impeller which will operate properly without any damage under existing suction conditions.
Clean and repaint casing waterways thoroughly
Clean and repaint the casing waterways properly during a complete overhaul. Check the waterways for erosion or corrosion. If there are any severe signs of wear apparent, consult the manufacturer and discuss the possibility of using more resistant materials.
Check all mounted parts on the rotor
Evaluate the concentricity and check the trueness of faces against bores. This will help in preventing the bowing of the shaft when tightening up the shaft nuts.
After mounting on impeller, check concentricity on new wearing rings
Make a slight cut to obtain a true circle after properly evaluating the concentricity of the new wearing rings. Ensure that it is of proper shape.
Careful installation of anti-friction bearings on shafts
The ideal way to mount anti-friction bearings on shafts involves slight heating of the bearings and expand their inner race instead of forcing them on the shaft. If you try forcing the bearing on the shaft, apply force by using an arbor press or by gentle blows of a hammer alternately on opposite sides of a tubular sleeve held against the inner race.
Restore shaft sleeves or shaft to proper service condition at stuffing box
It is possible to rebuild these surfaces by metalizing or wielding, however in most of the cases it is preferred to install new parts as rebuilding isn’t as efficient and effective as installing new parts.
Track records of all inspections and repairs
A complete track record for all inspections and repairs helps form a foundation basis for preventive maintenance which can help cut down on repair and maintenance costs in the long run.

Takeaway
The best solution for avoiding unexpected repair costs or unforeseen damage to production quality is by ensuring that your team is taking proper preventive maintenance measures while working around centrifugal pumps.
Once the aforementioned practices are properly embedded in your organization’s working culture, you would see a drastic reduction in component failures, poor quality production and see your employees working in a much safer environment.
Maintaining pumps isn’t just meant for cost-cutting purposes, but also to enhance safety measures and follow proper procedures that ensure high delivery performance throughout.
Constantly monitoring your machinery can be a little intimidating, however, once you have realized the power of high efficiency delivery and a significant reduction in maintenance expenses, you will most likely incorporate these practices in your business model.