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Do Self Priming Pumps have a negative NPSHR
Do Self Priming Pumps have a negative NPSHR
This is rather basic question, but if there is an application where the pump is located above the tank containing the fluid, then the pump must be self priming, right? So the NPSHR of that pump is a negative number, right?
So the NPSHA needs to be a less negative number than the NPSHR of the pump? Example, with friction losses and everything else, the NPSHA is -10 feet. I need to select a self priming pump that has a NPSHR smaller (more negative) than -10 feet, right?
This is for pumping #2 diesel on a supply boat if it matters.
Sorry, but I think you have not understood how NPSHa is calculated.
NPSHa = p-Z+V+f
Where p is the absolute pressure head in the tank feeding the pump. This is 33 ft for a tank open to atmosphere
Z is the height from the tank fluid surface to the impeller center line. A positive Z means the pump has a suction lift i.e. it is above the tank fluid level.
V is the fluid vapor pressure expressed in feet. For water at 68 degF this is about 0.8 ft
f is the frictional loss in the pipes betweent he tank and the pump expressed in feet.
Since the fluid will boil if the NPSHa is lower than the vapor pressure, you cannot have a negative NPSHa. The best you could hope to achieve would be about 1 ft under ideal conditions.
I assume from your example you have a suction lift of about 10 ft. Therefore your best case NPSHa would be about 20 ft, allowing 3 ft for diesel vapor pressure and pipe losses.
A self priming pump simply means the pump can start and pump with no fluid in the suction line. Just like any centrifugal pump you still need to have the pump NPSHr lower than the NPSHa plus a small margin. For diesel fuel a 2 ft margin would be plenty, so you need to select a pump with an NPSHr of 18 ft or less. In-line pumps
NPSHa = p-Z+V+f
Where p is the absolute pressure head in the tank feeding the pump. This is 33 ft for a tank open to atmosphere
Z is the height from the tank fluid surface to the impeller center line. A positive Z means the pump has a suction lift i.e. it is above the tank fluid level.
V is the fluid vapor pressure expressed in feet. For water at 68 degF this is about 0.8 ft
f is the frictional loss in the pipes betweent he tank and the pump expressed in feet.
Since the fluid will boil if the NPSHa is lower than the vapor pressure, you cannot have a negative NPSHa. The best you could hope to achieve would be about 1 ft under ideal conditions.
I assume from your example you have a suction lift of about 10 ft. Therefore your best case NPSHa would be about 20 ft, allowing 3 ft for diesel vapor pressure and pipe losses.
A self priming pump simply means the pump can start and pump with no fluid in the suction line. Just like any centrifugal pump you still need to have the pump NPSHr lower than the NPSHa plus a small margin. For diesel fuel a 2 ft margin would be plenty, so you need to select a pump with an NPSHr of 18 ft or less. In-line pumps
2011-06-01
