MECHANICAL TIPS AND STUDY MATERIAL
The answer is very simple. If we use air as working fluid the head generated is in terms of meters of air. When we use water as working fluid the head generated is in terms of meters of water. As we know that density of air is very less as compared to water, the head produced with air as working fluid will be far negligible than water as working fluid. Hence we remove air from the pump part as it may not be sufficient to suck the water from the sump and will reduce the efficiency of the pump. To remove such difficulty priming is done
CASING OF CENTRIFUGAL PUMP
As we know casing surrounds the impeller and guides the water and also raises the static head. These are designed in such a way that the kinetic energy of water at the outlet of impeller is converted into pressure energy. Mainly three types of casing are used for centrifugal pumps.
Ø Volute casing
Ø Vortex casing
Ø Diffuser casing
We will discuss all the three casing in detail.
1) VOLUTE CASING – Spiral type casing
In such casing the area of flow increases gradually from inlet (throat) to output (delivery pipe). As the area of flow increases gradually the velocity decreases and pressure energy increases.
The only demerit of such casing is formation of eddies where lot of energy is wasted. But on the whole the efficiency of pump can be increased slightly.
2) VORTEX CASING – Circular casing
This is simple arrangement with circular chamber between the impeller and casing. The prime advantage of such construction is that there are negligible eddy losses. Hence energy loss is negligible and efficiency increases. So we can say that vortex casing is better than volute casing.
3) DIFFUSER CASING
These types of casing consist of additional guide blades in-between casing and impeller. These guide blades are mounted on a ring type structure called diffuser. When the water from impeller enters to the guide blade and its velocity is further reduced as the water flows through increased area of flow. Due to reduced velocity of water the pressure energy is increased. It is to be noted here that the efficiency of the diffuser cased pump are greater than the volute and vortex casing pump.
PRIMING – ITS IMPORTANCE
A pump is primed properly before one starts it for operation. It is process of removing air from the suction pipe, impeller and the casing.
Now let’s see how priming is done. To remove the air present, the suction pipe, casing and the delivery pipe is completely filled with water. This removes the air out of the pump.
At this point you must ask a question that – Since head generated by the pump is independent of fluid, WHY CAN’T WE USE AIR AS WORKING FLUID AND WHY PRIMING IS NECESSARY?
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