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Why use Computer Maintenance Management Software when Working with Pumps?
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Why use Computer Maintenance Management Software when Working with Pumps?

  • Katarina Knafelj Jakovac

    March 13, 2024

Over 75% of machines in the process industry are centrifugal pumps. The complexity levels of their design cover a wide range of power and production needs.

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Image: Single-stage centrifugal pumps in the process industry

In the US, the centrifugal pump market was valued at over $63.3 billion in 2022 according to research by Grandviewresearch.

The graph shows the market share movement for centrifugal and reciprocating pumps from 2020 to 2022, along with projections until the end of the decade.

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(Source)

It is estimated that the market will grow at an annual rate of 4.9% from 2023 to 2030.

Centrifugal pumps are estimated to have generated a revenue share of over 67.3% in 2022 due to their versatile applications in water supply, fire protection systems, wastewater drainage, food and beverage industry, as well as in the oil and chemical industry.

To optimally utilize the lifespan of centrifugal pumps, it is necessary to adhere to guidelines of good engineering practice for operation and maintenance.

Recommendations relate to the pump itself, auxiliary systems, equipment associated with the pump, fault diagnosis, and the application of computerized maintenance management systems.

Rotating machinery in industrial production represents significant potential for digitalization in the areas of management, maintenance planning, condition monitoring, reducing the number of failures, and managing spare parts inventory.

NOTE: All recommendations are solely for informational and educational purposes. Before any practical application of the recommendations mentioned here, if you lack prior experience or have not worked with a particular machine before, work with individuals for whom it is a daily job, consult experienced professionals, or the pump manufacturer. The blog author bears no responsibility for any unskilled applications of the recommendations mentioned here or for any resulting damages.

1) Best Practice Recommendations for Centrifugal Pump Operation

For each centrifugal pump, it is essential to check the Q-h curve in the manufacturer's pump manual and its actual state during operation in the facility.

The fundamental parameters of the pump include operating parameters such as flow rate, pressure, head, efficiency, and power.

When selecting and operating a centrifugal pump, these data are verified by reading the Q-h curve, where Q denotes the flow rate or capacity [m3/h] shown on the x-axis, and h represents the head [m] shown on the y-axis.

Now let's interpret the Q-h curve of a centrifugal pump. During operation, a centrifugal pump transfers energy to the working fluid, thus achieving flow rate and head.

The required pressure that the pump must overcome determines the operating point on the Q-h curve and the delivery quantity.

As pressure increases, flow rate decreases, shifting the operating point to the left side of the curve. By reducing pressure, the operating point will shift to the right, and flow rate will increase. Figures 1 and 2 highlight the main operating parameters.

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Image: Q-h curve of a centrifugal pump

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Image: Performance curves of a centrifugal pump

  1. Flow Rate/Capacity Q. When selecting a pump, the first step is to determine the quantity of working fluid the pump must deliver. In our example, I have chosen a flow rate of 150 m3/h or 41.76 l/s. The flow rate is marked on the x-axis and a vertical line is drawn to this value at the bottom of the curve.

  2. Head h. It is necessary to know the required head when pumping the working fluid. In this example, let's assume a required head of 50 m, marked on the y-axis. A horizontal line is drawn to the point of intersection with the vertical flow rate line. The intersection of these two values represents the pump's operating point, marked in red.

  3. To achieve different operating points, the impeller diameter of centrifugal pumps can be adjusted. By reducing the impeller's outer diameter (trim), the pump can be adapted to specific delivery volume requirements. Impeller diameters are marked on the right side (219 mm, 208 mm, etc.), and a curve is shown for each diameter on the diagram. Our operating point lies between impeller diameters of 199 mm and 208 mm, so the selected impeller will have the appropriate diameter of 200 mm. Centrifugal pumps may be limited by variable shaft speed, which is an ideal way to control pump flow when multiple operating points need to be satisfied using a single-stage centrifugal pump, thereby avoiding reducing the impeller diameter and adapting the entire system of pipes, valves, etc.

  4. Efficiency η, %. When selecting a centrifugal pump for a specific purpose, efficiency indicates how much energy will be required to operate at a given point. Efficiency curves are positioned to intersect the impeller diameter curves. The higher the efficiency, the less energy is required to pump at the operating point. In this example, the observed efficiency is 81%.

  5. Minimum Flow Rate. Every centrifugal pump requires a minimum flow rate of the working fluid to dissipate generated heat. On the left side of the diagram, the minimum flow rate is marked with a thicker line; operating the pump to the left of this line towards the y-axis is strongly discouraged and may lead to a shorter pump lifespan.

  6. Pump Power. After determining the operating point, the required power can be determined. A vertical line is drawn for the selected flow rate, and where it intersects the impeller diameter curve is the power point. Then a horizontal line is drawn to the y-axis where power is marked. The observed power for this example will be 25 kW.

  7. NPSH. Net Positive Suction Head (NPSH) must be sufficient for the pump to operate correctly. It is the pressure value on the suction side of the pump needed to overcome losses. If the pump does not have adequate NPSH, cavitation will occur during the pumping process, negatively impacting the pump's lifespan. For this example, the observed NPSH will be 4.3m.

In conclusion, operating the pump far outside the Q-h curve or shifting the operating point too far to the right or left may cause serious damage to the pump, excessive energy consumption, and insufficient delivery of the working fluid.

When the pump is started and the pressure relief valve is not fully open, the delivery head is calculated as follows:

  • At a shaft speed of 1750 rpm, the delivery head = impeller diameter squared h=d2h= d^{2}

  • At a shaft speed of 3500 rpm, the delivery head = impeller diameter squared multiplied by 4 h=d24h= d^{2}*4

  • For other shaft speeds, the delivery head is calculated by the formula:

h=d2(shaftspeed/1750)2h = d^{2}*(shaft speed/1750)^{2}

  • The estimated delivery head before startup is calculated by measuring the shaft diameter in mm and dividing it by 100. Then, the obtained value is squared. If the pump speed is 1450 rpm, the squared value is multiplied by 3 and then increased by 10% to convert it to meters.

For example, if the shaft diameter is 80mm:

80 / 100 =0.8 * 0.8 = 0.64 * 3 = 1.92 * 10% + 1.92 = 2.1 m is the estimated delivery head.

Note that for a pump with a shaft speed of 3000 rpm, multiply by 12 instead of 3.

  • The point of maximum pump efficiency is between 80% and 85% of the delivery head. At this point, the rotor experiences minimal radial thrust force.
  • The ratio of shaft length to shaft diameter L3/d4L^{3}/d^{4} must be < 2000 mm to prevent excessive shaft bending.
    L is measured in mm from the center of the rotor keyway to the center of the rear bearing before the coupling.
    d is the shaft diameter in mm below the shaft sleeve in the mechanical seal area.
    The following image shows a cross-section of a single-stage centrifugal pump with dimensions d and L.

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Since most shaft materials have similar moduli of elasticity, changing the shaft material will not solve the problem of shaft bending when the pump operates outside the maximum efficiency range.

  • A pump with a double suction design can operate with 27% lower NPSH or 40% higher shaft speed without cavitation.

  • Doubling the shaft speed will result in a pump capacity doubled, delivery head quadrupled, and an eight-fold increase in power requirement.

  • Stainless steel shafts have much lower conductivity compared to carbon steel shafts. When pumping a high-temperature working fluid, heat transfer to the lubricating oil will accelerate oil aging.

  • If the pump shaft speed is doubled for an extended period, parts wear will increase eightfold.

  • Multistage centrifugal pumps have 2% to 4% lower efficiency.

  • A pre-rotor screw/inducer, to stabilize the turbulent flow of the working fluid before entering the rotor, can reduce the required NPSH by up to 50%.

  • The clearances of open-type impellers are defined by the pump manufacturer. Clearances range from 0.2 mm to 0.5 mm depending on the impeller diameter. For every additional 0.05 mm clearance, you will lose 1% of delivery capacity.

  • The clearances of wearing rings are in the same range, with a 1% loss of delivery capacity for every 0.025 mm increase in clearance.

  • Bearings and rubber seals in grease-lubricated bearing housings have a design lifespan of up to 2000 hours of operation. For pumps operating continuously, this would be 83.3 days. Consider replacing rubber seals with labyrinth or composite type seals to prevent additional stress on the shaft at seal contact points.

  • The axial clearance in ball bearings is 10 times greater than the radial clearance. Therefore, it is essential to mount bearings correctly. If the bearing is too tightly pressed after assembly, the balls will skid rather than roll during operation, causing increased temperature and premature bearing failure. The temperature of the inner race of a properly installed ball bearing should be at least 5°C higher than the lubricating oil temperature in the bearing housing.

  • The lifespan of bearing lubricating oil is proportional to its temperature during pump operation. If the temperature is 100°C, the oil should be completely replaced after 3 months. If the temperature is 90°C, the oil should be completely replaced after 6 months. If the temperature is 80°C, the oil should be completely replaced after 1 year.

  • When selecting a pump, do not choose the one with the largest impeller; leave 5% to 10% for the possibility of replacement.

  • The maximum viscosity of the working fluid for which the centrifugal pump is intended corresponds to the total oil mass increased by a factor of 4 at room temperature.

  • Choose a pump driven by a variable frequency electric motor if possible, for better adaptation to variable working load conditions.

  • Pumps in series must have the same capacity (same impeller diameter and shaft speed).

  • Pumps in parallel must have the same delivery head.

  • Use a blade pump where the required delivery capacity is less than 4.5 m3/h.

  • A centrifugal pump can pump a working fluid containing up to 0.5% by mass of air. If a greater amount of air is present, the pump will cease to pump. Cavitation can occur at any air volume.

  • Use pumps with double volutes if the impeller diameter is 355 mm or larger. This especially applies to vertical pumps with long shafts to prevent excessive shaft movement and damage to bearings and seals.

Recommendations for working with pipelines and fittings in centrifugal pump systems

  • The length of the pipe between the first elbow in the pipeline and the suction pipe must be equal to 10 times the diameter of the pipe. Therefore, if the pipe diameter is 100 mm, the length of the pipe between the suction and the first elbow must be 100 mm X 10 = 1000 mm.

  • Replacing valves with butterfly valves in the pipeline system is equivalent to adding an additional 30m of pipe. On the pressure side of the pump, this will cause operation outside the area of maximum efficiency on the Q-h curve and result in shaft bending. Cavitation may occur on the suction side.

  • After the centrifugal pump and electric motor are aligned, check the tightness of the anchor bolts on the base plate.

  • Check the direction of rotation of the rotor after the pump is assembled. If the rotor is not mounted in the correct direction, the pump will not pressurize the working fluid during rotation.

  • The suction pipe must be at least one size larger than the discharge pipe. Common dimensions are 4'' for the suction and 6'' for the discharge pipe, or 8'' for the suction and 10'' for the discharge, etc.

  • Swirling of the working fluid in the suction pipe occurs if the level of the working fluid is too low or if the level drops slower than 1 m/s, if there is a large amount of dissolved gases in the working fluid, if the outflow velocity of the working fluid from the tank exceeds 3 m/s, or if the working fluid is close to its boiling temperature.

3) Recommendations for diagnosing centrifugal pump failures

  • Damage to the edges of the rotor caused by cavitation indicates low available NPSH, excessive air ingress at the pump inlet, turbulent flow, or internal recirculation of the working fluid.

  • Cavitation-induced damage on the internal surface of the casing and on the tips of the rotor blades indicates improper clearance between the rotor and the casing. Care must be taken during assembly to ensure proper clearance according to the manufacturer's specifications.

  • Braided packings should not be used for pumps with a vacuum at the inlet, as air will enter the pump through the packings and bearing assembly.

  • The sealing system must be degassed and fully filled with sealing medium before starting the pump; otherwise, air will remain trapped in the sealing system.

  • If the specific gravity of the working fluid increases due to temperature change, it may overload the electric motor, so it is necessary to check if the installed electric motor has the appropriate power rating.

Presence of water in the lubricating oil reduces the lifespan of bearings by 48%. Water often forms due to condensation of moisture in the housing. If the oil contains 6% water, the oil lifespan is reduced by 83%.

  • The oil level in the bearing housing must not be lower than the midpoint of the sight glass.

  • The mass of the concrete base plate on which the pump is installed must be 4 times greater than the combined mass of the pump, steel supports, electric motor, and associated fittings connected to the pump; otherwise, vibrations may occur.

  • For pumps driven by electric motors up to 375 kW, concrete foundations must be at least 76 mm wider than the width of the steel supports on which the pump is mounted.

  • For pumps driven by electric motors greater than 375 kW, concrete foundations must be at least 150 mm wider than the width of the steel supports on which the pump is mounted.

  • Pipes must be aligned with the suction and discharge flanges of the pump. The pump should never be aligned with respect to the position of the suction and discharge pipes!

  • If there are pipe reductions on the suction side, the reduction must be connected to the suction flange on the smaller diameter side.

  • Valve spindles, T-joints, and elbows must be installed and positioned perpendicular to the horizontal axis of the shaft, not at different angles.

4) Recommendations for using a computerized maintenance management system (CMMS) to manage centrifugal pump maintenance

CMMS provides numerous benefits for managing maintenance of various types of equipment in different industrial sectors.

Regardless of the type of industrial production in which centrifugal pumps operate, there is a basic set of guidelines that everyone can apply when it comes to improving their maintenance using asset management software.

  • Enter detailed asset information and upload all available documentation (at minimum, spare parts lists, service reports, and diagnostic reports) into the computerized maintenance management system.

  • Prepare and create maintenance plans in advance. The execution schedule for maintenance plans will depend on the type of activity, manufacturer recommendations for centrifugal pumps, and maintenance staff experience.

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Image: Example of preventive work orders for a centrifugal pump in Metrikon software

  • Every malfunction of the centrifugal pump must be recorded in the computerized maintenance management system. Specify repair execution deadlines, circumstances of the malfunction occurrence, and necessary pump information.

  • For each malfunction, open a work order and define: description of necessary work, resources, required tools, spare parts and quantities, roles and responsibilities of personnel.

  • Keep records of when malfunctions are resolved, the cost of resources and parts. If malfunctions are not resolved on time or at all, investigate the causes.

  • All standard operating procedures or SOPs are documents that specify specific steps in the production process and must be continuously implemented. SOPs for centrifugal pumps guide employees on the steps they must take to ensure safe, uninterrupted, and efficient pump operation.

CMMS allows entry of standard operating procedures in the form of task lists or checklists that are available everywhere and to everyone and real-time tracking of whether the SOP has been executed in a manner that the employee digitally confirms the completion of all steps. This achieves standardization of company best practices, reduces the possibility of errors, and raises the level of work quality.

  • By integrating the computerized maintenance management system into daily operations and maintenance of centrifugal pumps, modern companies allocate and track maintenance tasks efficiently.

This will help reduce downtime, as maintenance can be planned and executed much earlier, minimizing disruptions in the production process. CMMS also helps companies track their maintenance costs and plan maintenance budgets, enabling them to make more informed decisions on where, when, and why to allocate resources.

  • Additionally, the computerized maintenance management system helps companies improve the condition of centrifugal pumps and manage overall production assets.

Conclusion

The application of good engineering practice in working with centrifugal pumps, pipelines, and fittings will prolong their lifespan, reduce the number of malfunctions, and enable long-lasting and stable operation. CMMS helps companies in the manufacturing industry improve their maintenance processes and optimize their equipment, leading to increased productivity and profitability. Computer software for managing the maintenance of centrifugal pumps, combined with recommendations for working with and maintaining fittings and machinery systems, will contribute to reducing the number of malfunctions and raising the quality level of the production process.

By tracking the maintenance and performance history of each part of the centrifugal pump, compressor, electric motor, or any other piece of equipment, companies can recognize potential issues before they arise and take steps to prevent them. This reduces the need for expensive repairs and ensures that all equipment operates efficiently.

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
March 12, 2024

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.