Introduction

When someone turns on an eleetrie light, the natural tendency is to look toward the light and consider die shine. We tend not to think about the electric wires and the current running through the light bulb. Equally, when someone starts an industrial pump, the tendency is to look toward the discharge piping and consider the pressure and flow. We tend not to think about the suction piping, or the liquid coming into the eye of the impeller. We need to emphasize the necessity to consider what's happening in the suction of the pump. This area is the source of problems, and probably is responsible for about 40% of all pumps going into the shop today.

This chapter is dedicated to NPSH, Net Positive Suction Head. NPSH is what the pump needs, the minimum requirement to perform its duties. Therefore, NPSH is what happens in the suction side of the pump, including what goes on in the eye of the impeller. NPSH takes into consideration the suction piping and connections, the elevation and absolute pressure of the fluid in the suction piping, the velocity of the fluid and the temperature. For the moment we can say that some of these factors add energy to the fluid as it moves into the pump, and others subtract energy from the fluid. There must be sufficient energy in the fluid for the impeller to convert this energy into pressure and flow. If the energy is inadequate we say that the pump suffers inadequate NPSH.

In simple terms we could say that NPSH is the reason that the suction nozzle is generally larger than the discharge nozzle. If there is more liquid leaving the pump taster than the liquid can enter into the pump, then the pump is being starved of liquid.

Think about it this way. When we see a magician pulling a rabbit out of a hat, in all probability there's a rabbit hidden in a secret compartment inside the top hat, or the rabbit is hidden in the magician's coat sleeve. The rabbit does not appear spontaneously. Isn't it interesting that magicians all wear long sleeved topcoats? They always reach into a 'top hat' for the rabbit. When I see a magician pull a rhinoceros from a frisbee, then maybe I'll believe in magic. There is illusion, but there is no magic. Likewise with a pump, the energy must be in the fluid for the impeller to convert it.

Equally, if your body requires more oxygen than the available oxygen in the atmosphere, then you would be asphyxiated. There must be more oxygen available in the air than the oxygen you consume.

To express the quantity of energy available in the liquid entering into the pump, the unit of measure for NPSH is feet of head or elevation in the pump suction. The pump has its NPSHr, or Net Positive Suction Head Required. The system, meaning all pipe, tanks and connections on the suction side of the pump has the NPSHa, or the Net Positive Suction Head Available. There should always be more NPSHa in the system dian the NPSHr of the pump. Let's look at them, beginning with what the pump requires:

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