1.DEF;The margin of pressure over vapor pressure, at the pump suction nozzle, is Net Positive Suction Head (NPSH). NPSH is the difference between suction pressure (stagnation) and vapor pressure.
2.SOME POINTS; If a liquid to be pump is in a tank which is open to atmosphere and it is also at a height above the pump the liquid flow into the pump because of its head in due to the effect of atmospheric pressure on its surface. The pump in this case only adds to the energy of the system .
When a open tank containing the liquid to be transferred is at a lower level than the pump, the energy required to bring the liquid to the pump is provided by atmospheric pressure on its surface but the pump must create the drop in pressure which makes the atmospheric pressure effective.
A discussion of the relevant features of the suction side of a pumping system must include not only the height of the liquid surface above or below the pump and the effect of atmospheric pressure but also the effect of a vacuum or zero pressure on the liquid surface, Vapour pressure and the characteristic of the Suction pipe. Liquid flow through a pipe is impeded by friction over its length by valves or other restriction and by changes of direction.
On the suction side the drop in pressure that can be produced by a pump is limited to that of and almost perfect vaccum . Where comes on the discharge side there is theoretically , no limit to the height through which a liquid can be raised .
If the pump is below the liquid level then HS will be positive if it is above the liquid level HS will be negative. Every liquid has a pressure at which it will vaporize and this pressure varies with temperature . If the combination of pressure and temperature in the suction pipe is such that the vaporization occurs, the efficiency of the pump deteriorate, and a condition can be reached where the pump will will cease to function. The vapour pressure is thus usually shown as a suction head loss in the equation.
3.NOW NPSH-As we know that 1 bar is equal to 10.3 metre theoretically it means any pump can lift water upto 10.3 metre but practically it won't because of the losses.
-(P_a+h)-(losses)= pressure at the eye of the impeller
Where p_a is atmospheric pressure and h is the depth of liquid in a tank
Now,
-(P_a+h)-( h_s+ vapour loss+ friction loss)= pressure at the eye of the impeller
Where h_s is height from tank surface to pump
So the final equation that you already see above
-(P_a+h)- (h_s + frictional losses +vapour losses)=NPSH
Under any given condition losses should not be more than the available head ,till the time above equation is positive pump can easily take suction and when equation becomes zero that was the condition of cavitation and when equation becomes negative pump lose suction. If you use a centrifugal pump for pumping volatile liquid so at the time when pump takes suction there will be pressure drop in the suction line. So due to drop in pressure, vapour start forming. So now you have to reduce the vapour loss for that you need to submerge just the pump in a tank itself so now your equation becomes.
(P_a +h)- (-h_s+ vapour loss + friction loss)
Because h_s is height from tank surface to pump and pump is below the tank surface so negative sign in h_s .
So it means suction lift is added to the equation and your vapour loss becomes minimum.
Note: 1. Friction loss also increases too much if there was too much bend in the pipeline so due to increase in friction losses npsh will be reduced.
2. Just in case you have the pump in which vapour loss and friction loss is constant then in that case how to increase npsh for that you have to deal around h_s .
3. If we are pumping a liquid from a cargo tank then the value of the head(h) keeps on decreasing which means your npsh keeps on decreasing, so this was the only reason that at the time of stripping ,we throttle the discharge valve to reduce the frictional losses .With throttled discharge valve you reduce the flow rate and hence the velocity decreases and we know that frictional losses is directly proportional to velocity.
4. Basically there will be the two important terms NPSH Required and NPSH available. Npsh required means that what you required and it is design concept and npsh available means that what is available to you, and That depends on the place where you are going to install the pump. If the npsh available is less than npsh required then the pump will loose suction. Now you have to find out why your npsh available is less .
There can be many reason for that like;
2.SOME POINTS; If a liquid to be pump is in a tank which is open to atmosphere and it is also at a height above the pump the liquid flow into the pump because of its head in due to the effect of atmospheric pressure on its surface. The pump in this case only adds to the energy of the system .
When a open tank containing the liquid to be transferred is at a lower level than the pump, the energy required to bring the liquid to the pump is provided by atmospheric pressure on its surface but the pump must create the drop in pressure which makes the atmospheric pressure effective.
A discussion of the relevant features of the suction side of a pumping system must include not only the height of the liquid surface above or below the pump and the effect of atmospheric pressure but also the effect of a vacuum or zero pressure on the liquid surface, Vapour pressure and the characteristic of the Suction pipe. Liquid flow through a pipe is impeded by friction over its length by valves or other restriction and by changes of direction.
On the suction side the drop in pressure that can be produced by a pump is limited to that of and almost perfect vaccum . Where comes on the discharge side there is theoretically , no limit to the height through which a liquid can be raised .
If the pump is below the liquid level then HS will be positive if it is above the liquid level HS will be negative. Every liquid has a pressure at which it will vaporize and this pressure varies with temperature . If the combination of pressure and temperature in the suction pipe is such that the vaporization occurs, the efficiency of the pump deteriorate, and a condition can be reached where the pump will will cease to function. The vapour pressure is thus usually shown as a suction head loss in the equation.
3.NOW NPSH-As we know that 1 bar is equal to 10.3 metre theoretically it means any pump can lift water upto 10.3 metre but practically it won't because of the losses.
-(P_a+h)-(losses)= pressure at the eye of the impeller
Where p_a is atmospheric pressure and h is the depth of liquid in a tank
Now,
-(P_a+h)-( h_s+ vapour loss+ friction loss)= pressure at the eye of the impeller
Where h_s is height from tank surface to pump
So the final equation that you already see above
-(P_a+h)- (h_s + frictional losses +vapour losses)=NPSH
Under any given condition losses should not be more than the available head ,till the time above equation is positive pump can easily take suction and when equation becomes zero that was the condition of cavitation and when equation becomes negative pump lose suction. If you use a centrifugal pump for pumping volatile liquid so at the time when pump takes suction there will be pressure drop in the suction line. So due to drop in pressure, vapour start forming. So now you have to reduce the vapour loss for that you need to submerge just the pump in a tank itself so now your equation becomes.
(P_a +h)- (-h_s+ vapour loss + friction loss)
Because h_s is height from tank surface to pump and pump is below the tank surface so negative sign in h_s .
So it means suction lift is added to the equation and your vapour loss becomes minimum.
Note: 1. Friction loss also increases too much if there was too much bend in the pipeline so due to increase in friction losses npsh will be reduced.
2. Just in case you have the pump in which vapour loss and friction loss is constant then in that case how to increase npsh for that you have to deal around h_s .
3. If we are pumping a liquid from a cargo tank then the value of the head(h) keeps on decreasing which means your npsh keeps on decreasing, so this was the only reason that at the time of stripping ,we throttle the discharge valve to reduce the frictional losses .With throttled discharge valve you reduce the flow rate and hence the velocity decreases and we know that frictional losses is directly proportional to velocity.
4. Basically there will be the two important terms NPSH Required and NPSH available. Npsh required means that what you required and it is design concept and npsh available means that what is available to you, and That depends on the place where you are going to install the pump. If the npsh available is less than npsh required then the pump will loose suction. Now you have to find out why your npsh available is less .
There can be many reason for that like;
1. Frictional losses too much
2. Vapour lock in the pump
3. Suction lift too high i.e h_s value is more
4. The water that you are going to pump is too hot i.e vapour loss.
5. If you throttle the suction side then frictional losses becomes too high that is hundred percent chance of Cavitation.
5. Every big pump in engine room is kept at bottom platform to get a high h_s so that the NPSH is positive and pump will not loose suction.
5. Every big pump in engine room is kept at bottom platform to get a high h_s so that the NPSH is positive and pump will not loose suction.
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