Classifications of Pumps:
Selecting between Centrifugal Pumps and Positive Displacement Pumps
Pumps are in general classified as Centrifugal Pumps (or Roto-dynamic pumps) and Positive Displacement Pumps.
Centrifugal Pumps (Roto-dynamic pumps)
The centrifugal or roto-dynamic pump produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller. Centrifugal pumps include
- Radial, axial and mixed flow units.
- Centrifugal pumps can further be classified as
- End suction pumps
- In-line pumps
- Double suction pumps
- Vertical multistage pumps
- Horizontal multistage pumps
- Submersible pumps
- Self-priming pumps
- Axial-flow pumps
- Regenerative pumps
- Positive Displacement Pumps:
The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head. The positive displacement pump can be classified as:
- Reciprocating pumps - piston, plunger and diaphragm
- Power pumps
- Steam pumps
- Rotary pumps - gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity
Advantages & Disadvantages of Centrifugal Pumps:
The advantages of centrifugal pumps include:
- Weight saving
- Adaptability to high-speed prime movers.
One disadvantage of centrifugal pumps is their relatively poor suction power. When the pump end is dry, the rotation of the impeller, even at high speeds, is simply not sufficient to lift liquid into the pump; therefore, the pump must be primed before pumping can begin.
For this reason, the suction lines and inlets of most centrifugal pumps are placed below the source level of the liquid pumped. The pump can then be primed by merely opening the suction stop valve and allowing the force of gravity to fill the pump with liquid. The static pressure of the liquid above the pump also adds to the suction pressure developed by the pump while it is in operation.
Cavitation in Centrifugal Pumps
Another disadvantage of centrifugal pumps is that they develop Cavitation. Cavitation occurs when the velocity of a liquid increases to the point where the consequent pressure drop reaches the pressure of vaporization of the liquid. When this happens, vapor pockets, or bubbles, form in the liquid and then later collapse when subjected to higher pressure at some other point in the flow. The collapse of the vapor bubbles can take place with considerable force.
This effect, coupled with the rather corrosive action of the vapor bubbles moving at high speed, can severely pit and corrode impeller surfaces and sometimes even the pump casing. In extreme instances, cavitations have caused structural failure of the impeller blades. Whenever cavitations occur, it is frequently signaled by a clearly audible noise and vibration (caused by the violent collapse of vapor bubbles in the pump). Several conditions can cause cavitations, not the least of which is improper design of the pump or pumping system.
For example, if the suction pressure is abnormally low (caused perhaps by high suction lift or friction losses in the suction piping), the subsequent pressure drop across the impellers may be sufficient to 6-19 reach the pressure of vaporization. A remedy might be to alter the pump design by installing larger piping to reduce friction loss or by installing a foot valve to reduce suction lift.
Cavitations can also be caused by improper operation of the pump.For instance, cavitations can occur when sudden and large demands for liquid are made upon the pump. As the liquid discharged from the pump is rapidly distributed and used downstream, a suction effect is created on the discharge side of the pump. Think of it as a pulling action on the discharge side that serves to increase the velocity of the liquid flowing through the pump. Thus, as the pressure head on the discharge decreases, the velocity of the liquid flowing across the impellers increases to the point where cavitations take place.
Perhaps the easiest way to avoid this condition is to regulate the liquid demand. If this is not possible, then increase the suction pressure by some means to maintain pressure in the pump under these conditions.
Factors affecting the efficiency of centrifugal pump:
Affecting the efficiency of Centrifugal Pump group of several factors:
Centrifugal Pump efficiency is mechanical, hydraulic three kinds of volume and efficiency of the product. The efficiency of pumping systems for the pump efficiency and electrical efficiency of the product. Centrifugal Pump Unit caused by the low efficiency of the following main factors.
- Pump it, the efficiency is the most fundamental impact. Pump under the same working conditions, efficiency may be a difference of more than 15%.
- Centrifugal Pump operating conditions below the pump’s rated operating conditions, pump efficiency is low, high energy consumption.
- Motor efficiency in the use of essentially unchanged. So choose a high-efficiency motor of vital importance.
- The major impact of mechanical efficiency and the quality of the design and manufacture. Pump is chosen, the latter part of the management was less affected.
Centrifugal Pump to reduce energy consumption, improve the efficiency of measures to be taken pump group
We have done a Centrifugal Pump efficiency compared before and after the replacement experiments, experiments show that the replacement of inefficient, high-consumption Centrifugal Pump, may improve the pump efficiency about 10%.
- Inverter energy-saving technologies. The design parameter is greater than the actual operating conditions of the Centrifugal Pump, after the installation of frequency control devices, always running on high-performance area.
- Centrifugal Pump, on the main popularization and application of energy-efficient permanent magnet motor speed control and dual-power electric motor and other new energy-saving products.
- Centrifugal Pump choice. The use of new pumps should be selected large manufacturers of pumps, in order to ensure high efficiency Centrifugal Pump.
|← Introduction to Pumps - Mechanism of Work and Equations||Penstock Protection and Supports | Steel, PVC, HDPE Pen stocks →|