Pumps NPSH calculation

Example 1 NPSH Calculation Suppose a selected pump requires a minimum NPSH of 16 ft (4.9 m) when pumping cold water What will he the NPSH limitation to pump propane at 55 F (12.8 C) with a vapor pressure of 100 psi Using the chart in Fig. 10-25, NPSH reduction for propane gives 9.5 ft (2.9 m). This is greater than one-half of cold water NPSH of 16 ft (4.9 m). The corrected NPSH is therefore 8 ft (2.2 m) or one-half of cold water NPSH. 

Typical NPSH calculations keep the pump s lowest pressure below the liquid s vapor pressure as illustrated by the following three examples ... 

NPSH calculations might have to be modified if there are significant amounts of dissolved gas in the pump suction liquid. See Suction System NPSH Available" in this handbook for calculations when dissolved gas does not need to be considered. In that case the suction liquid s vapor pressure is a term in the equation. With dissolved gases, the gas saturation pressure is often much higher than the liquid s vapor pressure. 

Methods for the calculation of pressure drop through pipes and fittings are given in Section 5.4.2 and Volume 1, Chapter 3. It is important that a proper analysis is made of the system and the use of a calculation form (work sheet) to standardize pump-head calculations is recommended. A standard calculation form ensures that a systematic method of calculation is used, and provides a check list to ensure that all the usual factors have been considered. It is also a permanent record of the calculation. Example 5.8 has been set out to illustrate the use of a typical calculation form. The calculation should include a check on the net positive suction head (NPSH) available see section 5.4.3. 

Chapter 5 considered pump types and their evaluation and selection. After selecting a pump type, the next step is to size the pump. This requires calculating the flow rate and the pressure rise across the pump or the pump head. The net positive suction head (NPSH), is also important, particularly for centrifugal pumps. NPSH is the difference between the total pressure and the vapor pressure of the fluid at the pump inlet. NPSH will be discussed later. 

Confirm hydraulic calculations on all pump circuits and establish pump NPSH and minimum equipment elevations. 

LAL (low alarm level) = 5 ft from ground and pump center line is 2 ft from ground NOL (normal operating level) = 10 ft above ground level NPSH Calculations (at LAL)... 

When a pump installation is being designed, the avail le net positive suction head (NPSH) must be equal to or greater than the (NPSH)r for the desired capacity. The (NPSH) can Be calculated as follows ... 

For the pumped-discharge case, internal pressure and final fluid height are calculated by Eqs. (26-56) and (26-57). The final fluid level is the point at which the net positive suc tion head (NPSH) equation is satisfied. 

The pump discharge can flow at a higher velocity than the suction line, due in part to NPSH conditions on the suction side of any pump (w hich are not considered directly in these pipe sizing calculations). 

What is the Suction Lift value to be used with the pump curves of Figure 3-36A, if a gasoline system calculates an NPSH of 15 feet available ... 

Figure 32.51 illustrates how system NPSH or NPSH-available is calculated for the usual suction systems outlined. For a centrifugal pump, the basic NPSH is calculated from ... 

Liquid chlorine is unloaded from rail tankers into a storage vessel. To provide the necessary NPSH, the transfer pump is placed in a pit below ground level. Given the following information, calculate the NPSH available at the inlet to the pump, at a maximum flow-rate of 16,000 kg/h. 

Other pieces may have to be elevated to enable the system to operate. A steam jet ejector with an intercondenser that is used to produce a vacuum must be located above a 34 ft (10 m) barometric leg. Condensate receivers and holding tanks frequently must be located high enough to provide an adequate net positive suction head (NPSH) for the pump below. For many pumps an NPSH of at least 14 ft (4.2 m) H2O is desirable. Others can operate when the NPSH is only 6 ft (2 m) H2O. See Chapter 8 for a method of calculating NPSH. 

Obviously the NPSH must be positive, or the liquid would be vaporized and the pump would be filled with gas. Since a pump is designed to transport liquids, if this happened it would just spin in its housing and no transfer would be accomplished. There is an increase in velocity as the liquid enters most pumps. This conversion of pressure energy to kinetic energy may reduce the pressure enough to cause fluids that have a positive NPSH to vaporize. Therefore, each pump has some minimum NPSH below which it will not operate properly. For most pumps an NPSH of 14 ft (4.2 m) of fluid is adequate. Some positive displacement pumps can operate at an NPSH of 6 ft (2 m). Use equation 5 to calculate the NPSH for each pump to be specified. 

Such performance curves are normally determined by the manufacturer from operating data using water at 60°F. Note from Eq. (8-6) that the head is independent of fluid properties, although from Eq. (8-4) the power is proportional to the fluid density (as is the developed pressure). The horsepower curves in Fig. 8-2 indicate the motor horsepower required to pump water at 60° F and must be corrected for density when operating with other fluids and/or at other temperatures. Actually, it is better to use Eq. (8-4) to calculate the required motor horsepower from the values of the head, flow rate, and efficiency at the operating point. The curves on Fig. 8-2 labeled minimum NPSH refer to the cavitation characteristics of the pump, which will be discussed later. 

Figure 4.5 shows a typical relationship between the available NPSH in the system and the NPSH required by the pump as the volumetric flow rate of liquid or capacity Q is varied. The NPSH required by a centrifugal pump increases approximately with the square of the liquid throughput. The available NPSH in a system can be calculated from equation 4.9 having substituted for hfs... 

Calculate the available net positive section head NPSH in a pumping system if the liquid density p = 1200 kg/m3, the liquid dynamic viscosity p = 0.4 Pa s, the mean velocity u = 1 m/s, the static head on the suction side 2, = 3m, the inside pipe diameter di = 0.0526 m, the gravitational acceleration g = 9.81 m/s2, and the equivalent length on the suction side SLes = 5.0 m. 

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. 

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