NPSH CONDITIONS
From the preceding paragraph, we see that a reduction in pressure can cause a liquid to vaporize if it is close to its vapor pressure. The pressure on the liquid entering a centrifugal pump is reduced as it moves from the suction flange to the point at which it receives energy from the impeller. Obviously, we must compare this reduced pressure to the vapor pressure entering the pump to determine whether the liquid will vaporize. This is what we do when we check the NPSH conditions of an application. We call the proximity of the liquid to its vapor pressure its "available NPSH" and the pressure reduction inside the pump, the "required NPSH". We compare the available NPSH to the required NPSH. When the available NPSH is equal to or greater than the required NPSH the pump will not cavitate.

where:

h_sv = available net positive suction head, in feet of liquid

h_sa = total suction head, in feet of liquid, absolute

h_vpa = vapor pressure of liquid at suction nozzle, in feet of liquid, absolute

REQUIRED NPSH
The required NPSH or NPSH_R can be defined as "the reduction in total head as the liquid enters the pump".

The pump manufacturer determines the NPSH_R for each pump by test procedures, and plots the results on the standard performance curves for that pump.

NPSH PROBLEMS
If the available NPSH is not greater than that required by the pump, many serious problems can result; there will be a marked reduction in head and capacity, or even a complete failure to operate. Excessive vibration can occur when sections of the impeller are handling vapor and the other sections handling liquid. Probably the most serious problem is pitting and erosion of the pump parts, resulting in reduced pump life. This is caused by the collapse of vapor bubbles as they pass to the regions of higher pressure. Excessive noise and vibration usually accompany this cavitation phenomenon. As the vapor bubbles collapse, the adjacent walls are subjected to a tremendous shock from the inrush of liquid into the cavity left by the bubble, This shock actually flakes off small bits of metal and the parts take on the appearance of having been badly eroded. This erosion shows up not at the point of lowest pressure where the bubble is formed, but further downstream where the bubble collapses,

The energy expended in accelerating the liquid to high velocity in filling the void left by the bubble is a loss, and causes the drop in head associated with cavitation. The loss in capacity is the result of pumping a mixture of vapor and liquid instead of liquid. Water, for example, at 70⁰F increases in volume about 54.000 times when vaporized, and thus even a slight amount of cavitation will reduce the capacity.

A pump operating with insufficient available NPSH will often pump spurts of liquid. This is caused by the following chain of events: As the pump is started, the liquid accelerates in the suction nozzle until it reaches the capacity at which it is to operate; as it accelerates, the friction losses increase and lower the absolute pressure until the liquid flashes into vapor; as soon as this happens, the pumping action is reduced, and the flow decreases. With the decreased flow, the losses are lower, the absolute pressure is higher, and the liquid does not vaporize. This causes the pump to start pumping again. This increases the flow, reduces the pressure etc. until the whole cycle is repeated. This results in an erratic flow rate with spurts of liquid being thrown from the discharge pipe.

CALCULATING AVAILABLE NPSH OF A PIPING SYSTEM
There are five typical pump installations for which the available NPSH should always be calculated. These are:

1. when the pump is installed an appreciable height above the liquid level;
2. when the pump takes suction from a tank under vacuum;
3. when the liquid has a high vapor pressure;
4. when the suction line is usually long; and
5. when the pumping system is at an altitude considerably above sea level.

The available NPSH can be calculated by use of the following formula:

h_sv = h_psa + h_ss - h_fs - h_vpa Equation 5.2

where:

h_sv         =  available net positive suction head in feet of liquid.

h_psa        = suction surface pressure, in feet of liquid, absolute, on the surface of the liquid from which the pump takes its suction. This will be the atmospheric pressure, in the case of an open tank or the absolute pressure above the liquid in a closed tank.

h_ss       =   static suction head, in feet of Liquid. In other words, the height, in feet of the liquid surface in the suction tank above or below the pump centerline. (Positive if the liquid level is above the pump, negative if the liquid level is below the pump)

h_fs      =   friction head loss, in feet of liquid, between the liquid surface in the suction tank and the suction flange of the pump.

h_vpa      =   vapor pressure of the liquid, at the pumping temperature, in feet of liquid, absolute.

Note that the first three terms in Equation 5.2 equal the total suction head, h_sa, and if we replace the first three terms with h_sa, we get Equation 5.1 which is a mathematical definition of available NPSH.

Each calculation of available NPSH for a piping system requires the five following steps:

Step 1: Determine the suction surface pressure, h_psa. This is the pressure on the surface of the liquid in the suction tank. When the suction tank is open, the suction surface pressure equals atmospheric pressure. When the suction tank is closed the pressure on the surface of the liquid must be measured. In either case, the pressure must be converted to feet of liquid, absolute.

 Step 2: Determine the static suction head, h_ss. This is the height, in feet of the liquid surface in the suction tank above or below the pump centerline. When the liquid level is below the pump centerline, the static suction head is a negative value,

 Step 3: Determine the suction friction head, h_fs. This is the sum of all the friction losses in the suction line from the inlet to the suction flange of the pump, at the specified flow rate. The friction loss factors are from the Pipe Friction Manual of the Hydraulic Institute. Pipe friction per 100 feet of pipe and velocity head are found in Tables 1 through 31 in that Manual and K factors for pipe fittings are found in Tables 32 (a) and 32 (b).

Step 4: Determine the vapor pressure, h_vpa - of the liquid at the pumping temperature, in feet of liquid, absolute. The vapor pressure must be known and converted to feet of liquid.