NPSH Requirements

An example of how to deal with a typical NPSH problem Ls shown in Exhibit 8-2. The required NPSH in this example is 22 ft (6,700 mm). If a horizontal pump is used, the bottom tangent line of vessel A must be a minimum of 22 ft (6,700 mm) above the centerline elevation of the shaft. If a vertical pump is used, the 

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Practically, the NPSH required for operation without cavitation and vibration in the pump is somewhat greater than the theoretical. The actual (NPSH)r depends on the characteristics of the liquid, the total head, the pump speed, the capacity, and impeller design. Any suction condition which reduces (NPSH ) below that required to prevent cavitation at the desired capacity will produce an unsatisfactoiy installation and can lead to mechanical dimculty. 

NPSH Requirements for Other Liquids NPSH values depend on the fluid being pumped. Since water is considered a standard fluid... 

The chart shown in Fig. 10-25 is for pure liqmds. Extrapolation of data beyond the ranges indicated in the graph may not produce accurate results. Figure 10-25 shows the variation of vapor pressure and NPSH reductions for various hydrocarbons and hot water as a function of temperature. Certain rules apply while using this chart. When using the chart for hot water, if the NPSH reduction is greater than one-half of the NPSH reqmred for cold water, deduct one-half of cold water NPSH to obtain the corrected NPSH required. On the other hand, if the value read on the chart is less than one-half of cold water NPSH, deduct this chart value from the cold water NPSH to obtain the corrected NPSH. 

Replacement impeller is not correct casting pattern therefore NPSH required is different. 

The NPSH requirement must be met for all anticipated flows. Maximum flow will usually have a higher NPSH than normal flow. For some pumps, extremely low flows can also require higher NPSH. 

NPSH AVAILABLE MUST BE GREATER THAN NPSH REQUIRED BY THE PUMP... 

Evans plotted the relationship with C = 9,000 resulting in the following graph for initial estimates of minimum NPSH required. 

Figure 1. Use these curves to find minimum NPSH required.

Optionai flow Inducers available for minimum NPSH requirements.)... 

Inducsn An optional helical Inducer Is available to substantially reduce the pump NPSH requirement. 

This available value of NPSHa (of the system) must always be greater b) a minimum of two feet and preferably three or more feet than the required NPSH stated by the pump manufacturer or shown on the pump curves in order to overcome the pump s internal hydraulic loss and the point of lowest pressure in the eye of the impeller. The NPSH required by the pump is a function of the physical dimensions of casing, speed, specific speed, and type of impeller, and must be satisfied for proper pump performance. The pump manufacturer must ahvays be given complete Suction conditions if he is to be expected to recommend a pump to give long and trouble-free service. 

Example 3-8 Closed System Steam Surface Condenser NPSH Requirements, Use Figure 3-44... 

Figure 3-44. Surface condenser condensate removal. Closed system steam surface condenser NPSH requirements. (By permission, Cameron Hydraulic Data, 16th ed. ingersoil-Rand Co., 1979, p. 1-12.)...

Figure 3-45. High altitude process vacuum system, NPSH requirements.

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 NPSH required is approximately 3-5 psi of liquid, above the vapor pressure of the liquid. 

Example 3-6 NPSH Available in Vacuum System, 191 Example 3-7 NPSH. Available in Pressure System, 191 Example 3-8 Closed System Steam Surface Condenser NPSH Requirements, 191 Example 3-9 Process Vacuum System, 192 Reductions in NPSHr, 192 Example 3-10 Corrections to NPSHr for Hot Liquid Hydrocarbons and Water. 192 Example 3-9 Process Vacuum System, 192 Example 3-10 Corrections to NPSHr for Hot Liquid Hydrocarbons and Water, 192 Example 3-11 Alternate to Example 3-10, 194 Specific Speed,... 

When drawing liquid through suction a pump generates a low pressure in the suction area, which, if it reaches vapor pressure, gives rise to gas coming out of solution and causing bubbles to form. An estimate of the margin above vapor pressure, used for many years, is net positive suction head. Two forms are used NPSH-available and NPSH-required. 

NPSH (available from system, A), 160 NPSH (required by pump, R), 160 Operating pressure, 408 Operational check-list, safety relief, 428 Orifice areas, relief valves, 437 Sharp edge, 440 Orifice, flow, 82, 83, 119 Air, table, 107 Overpressure, 403 Causes, 427... 

What is meant by the Net Positive Suction Head (NPSH) required by a pump Explain why it exists and how it can be made as low as possible. What happens if the neces sary NPSH is not provided ... 

A centrifugal pump is to be used to circulate liquid (density 800 kg/m3 and viscosity 0.5 mN s/nr) from the reboiler of a distillation column through a vaporiser at the rate of 400 em /s, and (o introduce the superheated liquid above the vapour space in the reboiler which contains liquid to a depth of 0.7 in. Suggest a suitable layout if a smooth bore 2.5 mm pipe is to be used. The pressure of the vapour in the reboiler is i tcN/nr and the NPSH required by the pump is 2 in of liquid. 

To prevent cavitation, it is necessary that the pressure at the pump suction be sufficiently high that the minimum pressure anywhere in the pump will be above the vapor pressure. This required minimum suction pressure (in excess of the vapor pressure) depends upon the pump design, impeller size and speed, and flow rate and is called the minimum required net positive suction head (NPSH). Values of the minimum required NPSH for the pump in Fig. 8-2 are shown as dashed lines. The NPSH is almost independent of impeller diameter at low flow rates and increases with flow rate as well as with impeller diameter at higher flow rates. A distinction is sometimes made between the minimum NPSH required to prevent cavitation (sometimes termed the NPSHR) and the actual head (e.g., pressure) available at the pump suction (NPSHA). A pump will not cavitate if NPSHA > (NPSHR + vapor pressure head). 

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