Friday, October 5, 2012

Definition of NPSHr (required)

It is the energy in the liquid required to overcome the friction losses from the suction nozzle to the eye of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump's curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH.

An easy way to understand NPSHr is to call it the minimum suction pressure I necessary to keep the pumped fluid in a liquid state.

According to the Standards of the Hydraulic Institute, a suction lift test is performed on the pump and the pressure in the suction vessel is lowered to the point where the pump suffers a 3% loss in total head. This point is called the NPSHr of the pump. Some pump manufacturers perform a similar test by closing a suction valve on a test pump and other manufacturers lower the suction elevation. Know and Understand Centrifugal Pumps


The definition of NPSHr may change in the future. A pump is in a definite state of cavitation with the 3% total head loss definition. Many pump users want a more explicit definition of NPSHr, and higher NPSHa safety margins to avoid inadequate NPSHa and cavitation altogether.

The pump manufacturers publish the NPSHr values on their pump curves. We’re saying that the NPSH reading is one of the components of your pump curves. If you want to know the NPSHr of your pump, the easiest method is to read it on your pump curve. It’s a number that changes normally with a change in flow. When the NPSHr is mentioned in pump literature, it is normally the value at the best efficiency point. Then, you’ll be interested in knowing exactly where your pump is operating on its curve.

If you don’t have your pump curve, you can determine the NPSH of your pump with the following formula:

NPSHr = ATM + Pgs + Hv - Hvp

Where:

ATM = the atmospheric pressure at the elevation of the installation expressed in feet of head.
Pgs = the suction pressure gauge reading taken at the pump centerline and converted into feet of head.
Hv = Velocity Head = V^2/2g where: V = the velocity of the fluid moving through the pipes measured in feet per second, and ‘g’ = the acceleration of gravity (32.16 ft/sec).
Hvp = the vapor pressure of the fluid expressed in feet of head. The vapor pressure is tied to the fluid temperature



The easiest thing to do is to get the pump curve from the manufacturer because it has the NPSHr listed at different flows. Nowadays, you can get the pump curve on the Internet with an e-mail to the manufacturer, you can send a fax, or request the curve in the mail or with a local call to the pump representative or distributor. If you wanted to verify the NPSHr on your pump, you’ll need a complete set of instrumentation: a barometer gauge, compound pressure gauges corrected to the centerline of the pump, a flow meter, a velocity meter, and a thermometer. Definitely, it’s easier to get the curve from your supplier.






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