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Characteristics of Centrifugal Pumps
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Characteristics of Centrifugal Pumps
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Characteristics of Centrifugal Pumps


    Pumps are generally grouped into two broad categories—positive displacement pumps and dynamic (centrifugal) pumps. Positive displacement pumps use a mechanical means to vary the size of (or move) the fluid chamber to cause the fluid to flow. On the other hand, centrifugal pumps impart momentum to the fluid by rotating impellers that are immersed in the fluid. The momentum produces an increase in pressure or flow at the pump outlet.


    Positive displacement pumps have a constant torque characteristic, whereas centrifugal pumps demonstrate variable torque characteristics. This article will discuss only centrifugal pumps.


    A centrifugal pump converts driver energy to kinetic energy in a liquid by accelerating the fluid to the outer rim of an impeller. The amount of energy given to the liquid corresponds to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller, then the higher the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.


   
        Characteristics
   


    Creating a resistance to the flow controls the kinetic energy of a liquid coming out of an impeller. The first resistance is created by the pump volute (casing), which catches the liquid and slows it down. When the liquid slows down in the pump casing, some of the kinetic energy is converted to pressure energy. It is the resistance to the pump’s flow that is read on a pressure gauge attached to the discharge line. A pump does not create pressure, it only creates flow. Pressure is a measurement of the resistance to flow.


    Head—Resistance to Flow


    In Newtonian (true) fluids (non-viscous liquids, such as water or gasoline), the term head is the measurement of the kinetic energy that a centrifugal pump creates. Imagine a pipe shooting a jet of water straight into the air. The height that the water reaches is the head. Head measures the height of a liquid column, which the pump could create resulting from the kinetic energy the centrifugal pump gives to the liquid. The main reason for using head instead of pressure to measure a centrifugal pump’s energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. End users can always describe a pump’s performance on any Newtonian fluid, whether it is heavy (sulfuric acid) or light (gasoline), by using head. Head is related to the velocity that the liquid gains when going through the pump.


    All the forms of energy involved in a liquid flow system can be expressed in terms of feet of liquid. The total of these heads determines the total system head or the work that a pump must perform in the system. The different types of head—friction, velocity and pressure—are defined in this section.


   
        Gear Pump Operation and Maintenance
   


   
        1. Introduction
   
   
        A gear pump uses two meshing, toothed cogs to force water from the inlet of the pump through to the outlet. Figure No. 1 shows a simplified drawing of an external teeth gear pump on the left along with the alternate arrangement of internally pointing teeth.
   


    2. Gear pump Design


    Gear pumps use toothed gears turning inside a close tolerance housing to draw-in liquid and then squeezing it out ahead of them. Paddle steamers used the same principle of operation. These pumps are positive displacement pumps and anything drawn into them will be forced out. As a consequence they can generate very high discharge pressures. Materials of construction vary from metals of various types and hardness through to plastics of various types and hardness.


    Maintaining the close tolerances between the housing and the cogs is critical to efficient operation. The clearance between the edges of the teeth and the housing and the ends of the cogs and the back and front walls of the housing are very small. Between the teeth and housing it is in the order of 0.1 mm (0.004”) while the clearances between the front and back faces of the gears and the ends of the housing are only 0.025 mm (0.001”). The fine clearances reduce liquid re-circulation back from the high-pressure discharge to the low-pressure suction side and make these pumps one of the most efficient available.


    Gear pumps usually have one shaft penetration through the housing for connection to the drive. The gear shafts on the smaller pumps can be supported in journal bearings within the ends of the housing and are lubricated by the product. On larger pumps rolling element bearings mounted in bearing housings are used. To prevent surface to surface contact wear of teeth the product does the lubrication.


    3. Gear pump Uses


    The design of a gear pump lends itself to use with clean liquids. Insure they draw liquid from well above the bottom of the supply tank in clear liquid space. Both low and high viscosity liquids can be pumped. If food grade products sensitive to shear (i.e. where the churning action of the pump breaks cells and fibres) are to be pumped the size of the pump will need to be increased and the speed reduced.


    The design also produces good suction characteristics and they can be used to draw clean, low viscosity liquids from a good depth or distance. Where high viscosity liquids are pumped, or if drawing from a depth or distance, make it easy for the liquid to flow into the pump. Install large diameter suction lines, keep them short and where possible always put the pump lower than the supply tank so the suction is under positive head pressure from the stored liquid.


    The very fine tolerances prevent pumping anything with a solid or particulate, as it would be squashed between the teeth and destroy the pump. If there is risk of solids being drawn into the pump it is necessary to install a suction line strainer that can be easily cleaned. Use as fine mesh screen as is possible without greatly increasing the suction pressure loses else the pump will cavitate. If the particulate is so fine that it passes through the screen it is better to choose a different design of pump.


    Being a positive displacement pump there deliver very precise quantities for each revolution and this means they have good dosing characteristics regardless of their speed. Gear pumps make good chemical additive dosing pumps provided material compatibility issues are addressed.


    4. Gear pump Installations


    When using a gear pump a pressure relief valve must be fitted to protect the pump if deadheaded against a closed valve or blockage. The PRV can be piped back to the suction side of the pump or into the supply tank.


    Pumps driven by belt drives have the added protection that the belts will slip in the pulleys if the pump is deadheaded. Insure bearings with a heavy-duty radial load carrying ability are installed if the pump is to be belt driven. If a drive coupling is used between the motor and the pump it is critical to align the shafts precisely to within 0.05 mm (0.002”) from motor shaft end to pump shaft end using laser or reverse dial indicator methods. Shaft misalignment produces orbital motion that loads the bearings and distorts the shaft as it turns. Flexible shaft couplings will transmit these loads.


    These pumps require solid, firm mounts on solid metal bases and plinths. If direct in-line drive through a shaft coupling is used the entire pump set must be mounted on a solid steel frame with pump feet positions machined flat to within 0.025 (0.001”) tolerance


    5. Maintenance Issues


    Gear pumps require good, robust installation, a PRV to protect the pump from overpressure and an assured supply of clean liquid. Those with outboard bearings require the bearings to be lubricated. Mechanical seals introduce there own set of problems and if possible select pumps that do not use them. If mechanical seals are fitted it becomes critical that shafts run true and the process pressures and flows are steady and do not fluctuate wildly to load up the bearings and gear teeth unevenly.


    The gear teeth must not be run dry. Unlubricated teeth will rub together and wear away. If these pumps are run dry and temperatures rise the cogs will expand and start rubbing on the housing. This will tear-up the housing and teeth. Either the pump is destroyed or the fine housing clearances are lost which then allows recirculation within the pump. The best protection against dry running is to install a flow switch in the suction line that turns power off to the pump if there is no flow.


    Ball Valve - How They Work


    A ball valve is a shut off valve that controls the flow of a liquid or gas by means of a rotary ball having a bore. By rotating the ball a quarter turn (90 degrees) around its axis, the medium can flow through or is blocked. They are characterized by a long service life and provide a reliable sealing over the life span, even when the valve is not in use for a long time. As a result, they are more popular as a shut off valve then for example the gate valve. For a complete comparison, read our gate valve vs ball valve article. Moreover, they are more resistant against contaminated media than most other types of valves. In special versions, ball valves are also used as a control valve. This application is less common due to the relatively limited accuracy of controlling the flow rate in comparison with other types of control valves. However, the valve also offers some advantages here. For example, it still ensures a reliable sealing, even in the case of dirty media.


    What are the different types of Butterfly valves?


    Each of the three different types of Butterfly valves are adapted to work with different pressures and usage. They include:


    Zero offset Butterfly valves which have the lowest pressure rating and relies on the flexibility of rubber


    Double offset Butterfly valves which are used in slightly higher-pressure systems which, by creating a cam action during operation to lift the seat out of the seal, produces less friction, reducing the tendency for wear and results in high performance


    Triple offset Butterfly valves which are best suited for high-pressure systems.


    Why are butterfly valves good for use as a fluid flow control valve?


    One of the key features that promote Butterfly valves as a fluid flow control valve is that they are smaller and weigh less than the alternatives. Providing they are sized correctly, Butterfly valves can offer a wider control range than similar valves, such as globe or ball valves and take up less space.


    As well as the size and weight advantages, Butterfly valves also have a higher capacity and lower flow restrictions. They are also the valve of choice for economical reasons as they offer lower installation costs. This is down to Butterfly valves being line size and not requiring pipework reducers.


    Applications where Butterfly valves can be used as a fluid flow control valve


    There is a wide range of applications for when Butterfly valves are ideal for use as a fluid flow control valve. They can be used for capacity lines on shops and perform well within slurry and water applications with high volume.


    Some of the applications, where the different types of Butterfly valves that BM Engineering stock, can be used include:
Cooling water, air, gases and fire protection


    Slurry


    Vacuum services


    High pressure, temperature water, and steam services


    Compressed air applications


    Compressed gas applications.


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