Self Priming Centrifugal Pump
Self Priming Centrifugal Pump
What is a Self Priming Centrifugal Pump?
If a pump is placed below the level of liquid to be pumped, the combined effect of gravity and atmospheric pressure ensures that the pump is continuously filled up with the liquid and air does not infiltrate into the pump. In many applications, however, the pumps operate above the level of the liquid and this allows air into the suction line. The entry of air into the pump casing causes various challenges including overheating and pump failure. A self-priming centrifugal pump is a type of pump designed to automatically evacuate the air at the start-up before pumping the liquid. The pump is designed to have the requisite amount of liquid inside the pump’s casing at the start-up before bigging its normal pumping process. To facilitate this, the working parts of the pumps are fitted with close tolerances to hold the liquid within the pump casing. This also helps to prevent the reversal of flow from the discharge side to the suction of the pump when the pump stop running. The constant presence of the fluid in the pump chamber helps in handling the accumulation of air bubbles during the pump working mechanism, simply referred to as “air-pocket”. The self-priming capability of this pump leads to increased operating efficiency in industrial applications where the pumps are used for the repeated yet intermittent pumping process.
Figure 1: Self-priming centrifugal pump (image credits: indiamart.com)
A standard centrifugal pump can be converted into a self-priming centrifugal pump by filling the casing with water and installing a check valve at the bottom of the suction pipework at the initial startup. Self-priming pumps operate by creating a partial vacuum through recirculation of water within the pump head. The partial vacuum allows the entry of water into the suction line.
Other methods can be used to handle the priming problem. A non-return foot valve or an evacuation tank can be used to prevent drainage of fluid in the suction line when the pump starts. A secondary pump can be used to evacuate the air in the suction line. However, these methods have certain shortcomings including the use of an extra piece of extra equipment, piping, and process, and require extra cost. Fundamentally, the self-priming pump evacuates the air from the suction line without the need for any extra devices and, therefore, it is considered the best device to get around this problem.
Working principle of Self Priming Centrifugal Pump
Self-priming centrifugal pump first overcomes the air binding by mixing any residual air present in the pump system with the fluid during the priming process. It does this through recirculation of water in the pressure head to create a partial vacuum to discharge water and to drive out any air. The recirculation causes the air and water in the pump casing to mix at the impeller during the priming cycle. The resulting mixture moves radially through the pump’s body and the air naturally tends to rise while the air-free water flows by gravity back down into the pump casing. In the casing, the air-free water mixes with the remaining air coming from the suction line as the impeller rotates. This process repeats until all the air is evacuated, leading to a partial vacuum in the suction line. Now, the atmospheric pressure outside the suction line is higher than the pressure inside and this forces the water to flow into the suction line and towards the impeller, for the normal pumping process to start.
Figure: Self-priming centrifugal pump working process (image credits: northridgepumps.com)
The rotation of the impeller admits the liquid into the eye of the impeller. The centrifugal force acting on the blades displaces the water radially and axially outwards until it passes through all the components of the impeller. The impeller accelerates the fluid leading to an increase in its kinetic energy before being thrown into the pump casing. In the pump casing, the speed of the fluid slowly decreases due to the gradually expanding flow path, and some of the fluid’s kinetic energy converts into static pressure energy as a result. After leaving the volute casing, the fluid enters the discharge pipe at a very high pressure which allows it to be pumped to the desired location.
Limiting factors for a self-priming pump centrifugal pump
In operation, the self-priming centrifugal pump evacuates air from the suction side to create a vacuum, and the atmospheric pressure forces the liquid into the suction line. This means that the process can only continue for as long as the pressure of the liquid column is less than the atmospheric pressure. if the pressure head of the liquid is equal to the pressure of the surrounding air the priming process will stop and the pump cannot lift water any further. For instance, when pumping water, it is theoretically possible for a highly efficient pump to only self-prime to a height of about 10m from the water source. The exact height limit for different liquids depends on altitude and temperature.
Applications of self-priming centrifugal pump
Self-priming centrifugal pumps apply in various commercial and industrial facilities e.g., power plants, steel mills, wineries, and breweries. They have the following common applications.
- Pumping of water, fuels, raw sewage, industrial wastewater
- Basement floodwater pumps
- Boosting water pressure
- Sewage treatment stations to pump raw sewage into the treatment facility
- Liquid transfer systems: to move product through production lines, empty water tanks, and drums
- Dewatering of construction and mining operations (For such applications, the pumps are referred to as trash pumps)
Advantages of a self-priming centrifugal pump
- Capable of handling a variety of liquids including slurries, corrosive fluids as well as solids
- Suitable for frequent and intermittent processes
- They are easier to maintain
- Have high reliability and do not depend on additional valves to operate
- Have the ability to continue pumping even when located out of the pit (i.e., no longer submerged in a tank)
- Capable of handling solid particles
Disadvantages of a self-priming centrifugal pump
- Inefficient in solid handling because a large clearance needed
- Requires a large volume to assist with the priming process
- Initial priming liquid must be added manually into the pump’s reservoir
- The pump may be large than a standard pump due to the need for a larger reservoir
- The pump must be installed as close as possible to the production line to prevent the liquid inside the pump reservoir from depleting during the priming process. A long suction line means more volume of air that must be evacuated when the pump starts.
- Usually more expressive than a standard centrifugal pump
- Have limited suction lift (max 7.6m)
Troubleshooting self-priming centrifugal pumps
Troubles-shooting a self-priming centrifugal pump is a very simple process if the necessary tools and manufacturer’s guide are available. Some common troubleshooting problems for a self-priming pump include: pump slow to prime or not priming, pump clogging frequently, noisy operation, the pump shuts down, or failing to provide rated flow rate.
Pump slowly to prime or not priming
- There could be a leakage in the suction line (check for leakage by wrapping cling-wrap around each joint and repairing the leakage)
- The suction line is too long or the diameter too large (reduce pipe diameter or construction)
- Pump speed is low (speed up the pump)
Self-priming pump clogging frequently
- the pump is operating too far left on the manufacturer’s curve (speed up the pump)
- Impeller unsuitable for the application (upgrade the pump with sloid handling impeller)
- Fit a strainer to the suction line
Self-priming centrifugal pump is making nosie
- The pump is vertexing i.e., pulling air from the surface of the liquid. sometimes this manifest in the form of a whirlpool on the surface of the water (lower the suction line, increase the diameter of the suction line)
- The impeller is damaged or worn out (replace the impeller)
- The motor bearings are worn out (replace the bearings
- The mounting plate and the foundation lacks sufficient rigidity (reinforce the foundation, mount the pump tightly)
- The suction lift is too high (reduce the suction lift) or presence of foreign material in the pump (disassemble the pump and clean it)
- Pump working in cavitation (Too low Net Positive suction head, inspect suction losses, fully open the inlet valve, correct suction design)
The self-priming centrifugal pump shuts down or fails to provide a rated flow rate.
- Blocked or partially blocked suction line (Can be indicated by a high suction reading)
- The pump needs clearance adjustment
- mismatch of the motor voltage and the line supply voltage (check the motor supply voltage against the voltage specification on the nameplate)
- Collison between the impeller and the pump casing or the impeller lacks a free rotation (dismantle the pump, align the impeller or replace it)
The self-priming centrifugal pump does not start or run
- Shortcomings in pump wiring i.e., loose connections or broken wiring (Fix any loose connection, replace the broken wire, and ensure the pump wiring satisfies the wiring diagram on the motor)
- The fuse is blown or the circuit breaker is open or (replace the fuse, reset the circuit breaker)
- The motor is sorted out
- An open circuit in the thermal overload (Allow the pump to cool down, check the reason for overload, and restart the pump)
Maintenance of a self-priming centrifugal pump
Maintenance of mechanical components
- Ensure the impeller and [pipe work must be regularly unclogged
- Keep the bearing sufferingly lubricated: ensure the required level of lubricating oil is maintained and the bearing casing is kept clean
- Tighten all the mechanical mounting to keep the pump firm and secure. Also, always ensure that the pump flanges and coupling are tight.
- Replace all worn out parts on the pump: impeller mechanical seal, and packing should be checked regularly and replaced if worn out to prevent leakage
Maintenance of electrical components
- The wiring system should be kept free of loose connections
- The stator should be inspected for arching and overheating
- The motor windings should be free of dirt and dust
This article covers the self-priming centrifugal pump, operating principles, and the benefits and drawbacks of the pump. A self-priming pump has been defined as a pump with the capability to evacuate air in the suction line at the startup before commencing its normal pumping operation. After the initial manual priming, the pump always retains a requisite amount of liquid in its reservoir to help with the next priming cycle. Due to the self-priming ability of self-priming pumps, they often find applications in process plants where the pumps are used for repeated yet intermittent operations. The pump has a variety of applications such as irrigation, pumping water, sewage treatment plants, dewatering in construction and mining sites, and liquid transfer systems. The operation of this pump is limited by the pressure of the surrounding air, altitude, and temperature. As the working process is highly dependent on atmospheric pressure, the maxim height to which an efficient pump can self-prime is limited to 10 m above the source of the water. Major benefits of the pump include the ability to handle varieties of liquids as well as solid particles, high reliability, ease of maintenance, and the ability to continue pumping even when not submerged. Some of the disadvantages of the pump include large volume hence large space requirements, inefficient handling of solids, limited suction lift, and high cost. The pump is associated with some common troubleshooting problems including pump slow to prime or not priming, pump clogging frequently, noisy operation, the pump shuts down, or failing to provide rated flow rate. Best self-priming pump’s operation and long service life can be managed though regular maintenance of mechanical and electrical components.