NPSH of a pump

Normal pump suction head (NPSH) of a pump must be in excess of a certain number, depending on the kind of pumps and the conditions, if damage is to be avoided. NPSH = (pressure at the eye of the impeller - vapor pressure)/(density). Common range is 4-20 ft. 

The conversion of the pump s suction pressure to velocity in the eye of the impeller is called the required net positive suction head (NPSH). As the flow-control valve on the discharge of the pump shown in Fig. 25.1 is opened, the velocity of liquid in the eye of the impeller goes up. More of the pump s suction pressure, or feet of head, is converted to velocity, or kinetic energy. This means that the required NPSH of a pump increases as the volumetric flow through the pump increases. 

The units of NPSH are feet of liquid head. The required NPSH of a pump is due primarily to the conversion of feet of head to velocity in the eye of the impeller. 

When the required NPSH of a pump equals the NPSH available to the pump, the pump will cavitate. 

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

The NPSH v available from or in the liquid system on the suction side of a pump is expressed (corrected to pump centerline) as ... 

The rotative speed of a pump is dependent upon the impeller characteristics, type fluid, NPSH available and other factors for its final determination. The most direct method is by reference to manufacturer s performance curves. VVTien a seemingly reasonable selection has been made, the effect of this selected speed on the factors such as NPSH required, suction head or lift, fluid erosion and corrosion, etc., must be evaluated. For many systems these factors are of no concern or consequence. 

The net head or pressure measured in ft. or m that causes a liquid to flow through the suction side of a pump, enter the pump chamber, and reach the impeller. When the source of liquid is above the pump, NPSH equals the barometric pressure plus the static head, less the entrance head, frictional losses in the suction piping and vapor pressure of the liquid. When the source of liquid is below the pump, NPSH equals the barometric pressure less the static head, entrance head, frictional losses in the suction piping and vapor pressure of the liquid. NPSH is specific for each pump design and application and must be supplied by the manufacturer. 

It is necessary to pump water at 70°F (Pv = 0.35 psia) from a well that is 150ft deep, at a flow rate of 25 gpm. You do not have a submersible pump, but you do have a centrifugal pump with the required capacity that cannot be submerged. If a 1 in. sch 40 pipe is used, and the NPSH of the pump is 15 ft,... 

You must chose a centrifugal pump to pump a coal slurry. You have determined that the pump must deliver 200 gpm at a pressure of at least 35 psi. Given the pump characteristic curves in Appendix H, tell which pump you would specify (give pump size, speed, and impeller diameter) and why What is the efficiency of this pump at its operating point, what horsepower motor would be required to drive the pump, and what is the required NPSH of the pump The specific gravity of the slurry is 1.35. 

The required NPSH of the pump may be read from Fig. 25.2 (regardless of the SG of the liquid being pumped). It shows that at 250 GPM, the required NPSH of 20 ft, will equal the available NPSH of 20 ft. Therefore, at a flow rate of 250 GPM, the pump will cavitate. This calculation has neglected frictional losses in the suction line, which should be subtracted from the available NPSH. 

This results in a temporary loss of pressure at the suction of the pump. This temporary loss of pressure is called the starting NPSH requirement. The more quickly the operator opens the discharge valve of a pump, the more rapidly the liquid accelerates in the suction line. This increases the starting NPSH required. 

The PS discharge head is determined. This is done by adding to the PS pressure rise just computed the net positive suction head (NPSH) of the pump as specified by the vendor. It is always wise to allow an additional head above this value as a safety factor. Here a 50-ft intake head has been assumed for illustrative purposes. 

However, the author recalled a study he had made earlier on the effects of flowrate on NPSH requirements [ ]. According to that study, the most important factor that determines the NPSH requirements of a pump at a given speed is the rate of flow through the impeller. 

Organic HTFs oxidize when heated in the presence of air. This starts at about 150°C and the rate increases with temperature. Heat transfer fluid manufacturers usually recommend a blanket of an inert gas such as nitrogen or argon on the expansion tank to prevent oxidation. This inert gas can also be utilized to pressurize the system in case the fluid will be utilized above its boiling point or the net positive suction head (NPSH) of the pump needs to be elevated. 

We emphasize the need to take into account the head loss A//c, suct(0) in the suction pipe, unless it is negligible, in order to estimate the NPSH. The variations of NPSH(g) are often superimposed onto the head capacity chart of a pump, as depicted in Figure 5.4. The head delivered by the pump decreases with increasing flow rate, while the NPSH value increases. ... 

There is a significant limitation on punp operation called Net Positive Suction Head (NPSH). Its origin is as follows. Although the effect of a pump is to raise the pressure of a liquid, frictional losses at the... 

Equation fl8.19 means that the available NPSH NPSHj is the difference between the inlet pressure, Pinlet, and P which is the vapor pressure (bubble point pressure for a mixture). It is a system curve for the suction side of a pump. It is required that JVPSH > NPSHji to avoid cavitation. All that remains is to calculate or know the pump inlet conditions in order to determine whether there is enough available NPSH (NPSHjiJto equal or exceed the required NPSH (NPSHj. ... 

The need to accelerate the fluid in the suction of a pump is called the starting net positive suction head (NPSH) requirement To calculate this starting NPSH requirement, let s assume ... 

Available NPSH The difference between the pressure at the suction of a pump and the vapor pressure of the liquid. 

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