Pump FAQs
This collection of frequently asked questions is categorized into four categories for your convenience.
Rotodynamic Pumps / Positive Displacement Pumps / Pump Systems / General Pump Standards
Rotodynamic Pumps
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What items need to be included in regular pump maintenance, and what schedule should be followed?
Pumping systems can be complex, with many moving parts and subsystems that need to be regularly inspected and constantly maintained. Failure to frequently inspect pumping systems can lead to premature failure, losses in efficiency and increased operating costs. Therefore, it is recommended that a monitoring, maintenance and schedule be adopted, and it should include, at a minimum, the following:
- When applicable, gland packings must be adjusted to maintain concentric alignment of the gland follower and maintain specified leakage so that the packing and follower do not overheat.
- Check for any leaks from gaskets and seals. The correct functioning of the shaft seal must be checked regularly.
- Check bearing lubricant level and verify if the hours run show a lubricant change is required.
- Check and verify that the duty condition is in the allowable operating region for the pump.
- Check vibration, noise level and surface temperature at the bearings to confirm satisfactory operation.
- Check that dirt and dust are removed from areas around close clearances, bearing housings and motors.
- Check coupling alignment and realign if necessary.
Note: Additionally, installed auxiliary systems should be included in the maintenance plan so they are monitored and maintained to ensure they function properly.
An inspection and maintenance log should be kept and problems that are identified should be reported immediately. A suggested guide for preventative maintenance for normal applications is given below. Unusual applications with abnormal heat, moisture, dust, etc., may require more frequent inspection and service.
A maintenance plan should include required spare parts to keep on hand. A list of recommended spare parts will depend on normal supplier lead time when ordering parts; whether pumping equipment is used for “normal duty” or “severe duty;” and whether or not there is backup pumping while a unit is down for maintenance. Below is a suggested list of spare parts for pumping units. Note that the items listed for severe duty are in addition to the items listed for normal duty.
For more information about how to maintain pumping systems, refer to ANSI/HI 14.4 Rotodynamic Pumps for Installation, Operation, and Maintenance (2018) by clicking here.
May 2018
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How does a centrifugal pump work?
A centrifugal pump is a type of rotodynamic pump that uses bladed impellers with essentially radial outlet to transfer rotational mechanical energy to the fluid primarily by increasing the fluid kinetic energy (angular momentum) and increasing potential energy (static pressure). Kinetic energy is then converted into usable pressure energy in the discharge collector.
Figure 1 provides a cross section view of a centrifugal pump, which shows the use of a rotating impeller to add energy to the pumped liquid. The liquid enters the impeller axially at a smaller diameter, called the impeller eye, and progresses radially between the vanes until it exits at the outside diameter. As the liquid leaves the impeller, it is collected in a pressure container casing. One design referred to as a volute collects the flow and efficiently directs it to a discharge nozzle.
Figure 2 highlights the discharge nozzle, which is shaped like a cone so that the high-velocity flow from the impeller is gradually reduced. This cone-shaped discharge nozzle is also called a diffuser. During the reduction in velocity in the diffuser, energy in the flow is converted to pressure energy. An optimum angle of seven to 10 degrees is used to most efficiently convert velocity energy to pressure energy.
Centrifugal pumps can have many drivers, but the most common is the electric motor. The motor provides the mechanical energy to pump shaft through a coupling. The radial and axial loads are carried by pump and/or motor bearings. Sealing of the pumped fluid can be done with compression packing or mechanical seals. Additionally, sealless designs are available with canned motors or magnetic drive couplings.
For more information on centrifugal pumps, their construction and typical industries served, refer to HI’s web based Rotodynamic Pumps eLearning Course at www.training.pumps.org.
May 2018
- What are the applications of vertical turbine pumps?
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What is a chopper pump?
Chopper pumps are centrifugal pumps with the capability to handle fluids with a high concentration of solids. Chopper pumps have a cutting attachment added made of hard materials of fixed and rotating elements that macerate solids before entry to the impeller that allows it to handle difficult materials. They cut solids so they pass through the pump more easily and flow out with the rest of the pumped fluid.
A chopper pump’s ability to handle solids gives them more flexibility in what they can pump; this characteristic makes them particularly useful in wastewater treatment plants. Wastewater treatment is split into primary treatment and secondary treatment. Primary treatment is the physical separation of float-able materials and insoluble solids from the wastewater. Secondary treatment is biological treatment of water using microorganisms to remove the remaining solids in the fluid. Both treatments contain solids in the pumped fluid and may require chopper pumps.
Specific steps in the process include pumping scum, mixing the contents of the aeration basin and the anoxic zone, and pumping sludge. Image 1 shows an example of an aeration basin at a treatment facility where chopper pumps mix oxygen with wastewater to encourage the growth of microorganisms to break down solids.
A typical centrifugal pump impeller is more easily clogged by solids, which can halt pumping and cause damage to the system. In particular, stringy materials found in wastewater during the treatment process is especially troublesome to normal centrifugal pumps as the material can tangle the impeller. However, the chopper pump is more effective in dealing with this issue.
For more information on the application of pumps in power plants, refer to HI Guideline Wastewater Treatment Plant Pumps at pumps.org.
September 2018
- I am used to hearing the term “centrifugal” pump, but sometimes hear them referred to as “rotodynamic” pumps. Are centrifugal and rotodynamic terms that can be used synonymously?