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PUMPS- FAGs

What is a pump?
Pump is one of the first mechanical inventions of the mankind mainly conceived for transporting water and water like substances from a low lying area to a place of elevation. Therefore, simply stated, pump employs a suitable principle of working to convey fluids against force of gravity. As industrialization advanced, the technology of pump evolved to suit the needs of the industry in such a way that today there are several types of pumps available to deal with host of fluids from free flowing to highly viscous and from clear liquids to solid laden slurries and even liquids that one may consider un pumpable.
How are pumps classified?
Pumps are classified on the basis of their principle of operation. Broadly stated, there are two distinct types of pumps known as “Dynamic” and “Positive Displacement” Pumps. Under this broader classification, is the detailed classification of pumps.
How does a centrifugal pump work?
As the name itself suggests, a centrifugal pump relies on the centrifugal force (kinetic energy) for transporting the fluid from source of liquid to the point of requirement. Liquid enters the pump at the center of a rotating impeller and gains energy as it moves to the outer diameter of the impeller. Liquid is forced out of the pump by the energy it obtains from the rotating impeller. Centrifugal pumps can transfer large volumes of liquid but efficiency and flow decrease rapidly as pressure and/or viscosity increases. Generally, the centrifugal pumps are designed to handle liquids having viscosity up to 230 cP.

How does a positive displacement pump work?
By definition, positive displacement pump displaces known quantity of liquid with each revolution of the pumping elements against infinite system resistance. This is done by trapping liquid in the void being created mechanically between the pumping elements by the motion of the pumping elements. Generally, the positive displacement pumps become suitable for shear sensitive and viscous liquids as they can handle viscosity up to 1,000,000 cP.

How does one select a suitable type of pump for a given application?
Selection of a pump is dependent on many factors that exist in a system wherein the pump is to be employed. Some of the key consideration factors are listed below:
1. Nature of liquid – its physical & chemical property – For example, abrasion characteristics, % & size of solids, if any, viscosity, shear sensitivity, pH value.
2. Flow rate or capacity desired form the pump
3. Total Dynamic Head against which the desired capacity is to be delivered.
4. Pumping Temperature.
5. Suction Conditions prevailing in the system – whether suction lift, flooded, or vacuum.
6. The service condition for which the pump is to be employed – whether industrial, hygienic, domestic, conformity to CIP requirements etc.

What is cavitation and what are its symptoms?
Cavitation is a phenomenon that occurs due to the vaporization of liquid caused by drop in the liquid pressure below the media vapour pressure. Such a phenomenon can be associated whenever the NPSHA (Net Positive Suction Head Available) in the system is lower than the NPSHR (Net Positive Suction Head Required) of the pump and is known as “True Cavitation”. False cavitation can also occur even when the NPSHA is higher than the NPSHR, due to intake of air in the suction line due to improper installation of suction piping. A pump that cavitates would generally have the following symptoms: 1. Reduced capacity. 2. Higher vibration. 3. Increased noise level. 4. Fatigue failure It is evident from the above that it is undesirable to operate a cavitating pump as the damages to the pumping equipment can become extensive under such circumstances.

What is priming?
Priming is the air handling capability of a pump. In a conventional centrifugal pump, it therefore becomes imperative to fill the casing with liquid prior to starting since in the absence of any liquid, sufficient centrifugal force cannot be generated for creating the requisite pressure differential for the liquid flow to be sustained through the pump casing. This is mainly because of the fact that air, which is present in the suction piping work as well as the involute casing of the pump, is 800 times lighter than water. On the other hand, positive displacement pumps are inherently self-priming. Top

How does one vary the flow rate of a centrifugal pump?
Generally, centrifugal pumps are not employed for applications where flow rates are required to be varied as speed governed flow rates are not possible due to limitation of centrifugal force at reduced speeds. However, it is a standard practice in the industry to throttle the delivery line by using a valve so as to vary the flow rate from a centrifugal pump. Top

How do pumps develop pressure?
In a centrifugal pump, the kinetic energy is converted to the pressure energy and hence speed of operation of a centrifugal pump contributes to higher head or pressure. However, positive displacement pumps do not develop pressure on their own, as they tend to displace liquid against resistance being offered to flow. As a matter of fact, resistance to flow is pressure.Top

What is a suction lift?
Maximum suction lift available (MSLA) from the pump can be expressed as following:MSLA = Atmospheric Pressure – NPSHRThe low pressure created at the pump suction enables the liquid to be lifted up to pump. However as can be seen from the above given equation, the lift is dependent and is limited to the atmospheric pressure (33.9 feet or 1.033 bar at mean sea level). Further, it is also governed by the minimum energy required by the pump to perform its job expressed in terms of NPSHR. NPSHR is dependent on the pump design and its operating characteristics. Suction lift, as a term, is normally referred whenever the pump is drawing liquid from a sump or underground vessel or a tank with the liquid surface exposed to atmosphere. Another important parameter that affects the suction lift capability of a pump is the pumping temperature. For example water at 100 degree centigrade would be impossible to be lifted since it would be in a vapour state.Top

What is total dynamic head?
Total dynamic head (differential or generated head) is a measure of the energy imparted to the liquid by the pump, and is equal to the algebraic difference between the total discharge head and total suction head.The total dynamic head against which a pump operates comprises:1. Total static head.2. Friction losses.3. Velocity head.4. Entrance and exit losses.Where,(a) Total static head is the difference in elevation between the liquid levels of the suction and discharge. If the pump discharges in to a pressure tank, then the total static head is the difference in elevation between liquid levels plus the pressure in the tank expressed in metres of liquid.(b) Friction head is the equivalent head, expressed in metres of liquid, necessary to overcome friction on the interior surfaces of the pipework system including valves, bends and fittings. (c) Velocity head of a liquid moving with a certain velocity is the equivalent static head through which it would have to fall in order to attain that velocity. (d) Entrance and exit losses are usually comparatively small and can be neglected in the majority of the industrial applications.Total dynamic head, where suction lift exists, is the sum of the total discharge head and total suction lift. Where positive suction head exists, total dynamic head is the total discharge head minus the total suction head.

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Page Updated Fri Dec 12, 2008 8:48am EST