Net positive suction head NPSH

The net positive suction head (NPSH) is the absolute pressure in excess of the liquid vapor pressure that is available at the pump suction nozzle to move the liquid into the eye of the impeller. NPSH must always be calculated using units of absolute pressure and then expressed as head. NPSH is a concept entirely different from a pump s suction pressure. The actual NPSH must exceed the required NPSH for a given pump for adequate pump performance. Without adequate NPSH, cavitation and mechanical damage to the pump can occur. NPSH is an important consideration when selecting a pump required to pump liquids from Systems under vacuum or to handle near-boiling liquids or liquids with high vapor pressures. It is usually not practical to specify values of NPSH less than two feet. 

Factors that must be considered in evaluation of NPSH requirements include  

Cavitation is caused by the formation of vapor bubbles in a high-velocity, low-pressure region and by the subsequent collapse when the bubbles move to a higher pressure region. Cavitation can cause excessive erosion and vibration. A liquid moving through a pump will vaporize rapidly whenever the local pressure falls below the liquid vapor pressure. 

Pumps often cavitate in spite of a designer s best efforts to meet NPSH requirements of pumps. This behavior is not well understood, and could occur for a number of reasons, including  

The pressure at the inlet to a pump must be high enough to prevent cavitation occurring in the pump. Cavitation occurs when bubbles of vapour, or gas, form in the pump casing. Vapour bubbles will form if the pressure falls below the vapour pressure of the liquid. 

The net positive suction head available (NPSHavail) is the pressure at the pump suction, above the vapour pressure of the liquid, expressed as head of liquid. 

The net positive head required (NPSHreqd) is a function of the design parameters of the pump, and will be specified by the pump manufacturer. As a general guide, the NPSH should be above 3 m for pump capacities up to 100 m3/h, and 6 m above this capacity. Special impeller designs can be used to overcome problems of low suction head see Doolin (1977). 

The net positive head available is given by the following equation  

Pv = the vapour pressure of the liquid at the pump suction, N/m2, p = the density of the liquid at the pump suction temperature, kg/m3. 

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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. 

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 ... 

The cavitation performance depends, to a large extent, on the conditions at the pump inlet or suction. These are defined by the parameter net positive suction head (NPSH) ... 

The FW storage tank (feed tank) or deaerator storage reservoir must be at the correct height to ensure the proper net positive suction head (NPSH) for the pump. 

The tank should be positioned sufficiently high off the boiler house floor to ensure that the FW pumps are provided with a satisfactory net positive suction head (NPSH). 

For any pump, the manufacturers specify the minimum value of the net positive suction head (NPSH) which must exist at the suction point of the pump. The NPSH is the amount... 

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 ... 

When laying out the diagram, it is only necessary to show the relative elevation of the process connections to the equipment where these affect the process operation for example, the net positive suction head (NPSH) of pumps, barometric legs, syphons and the operation of thermosyphon reboilers. 

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. 

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. 

The next important thing the designer must calculate is the net positive suction head (NPSH). This is the difference between pressure of the fluid at the entrance to the pump (often referred to as the pump suction) and the vapor pressure of the fluid ... 

The pump will be placed on a floating dock that is just a few feet above the river level, so that the net positive suction head (NPSH) will be adequate. The maximum height from the bottom of the barge to the pump will be 20 ft. 

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). 

This standard uses terms that need to be changed or deleted when the standard is applied to HPRTs. In such a context, the word pump should be interpreted as meaning HPRT, the word suction should be interpreted as meaning outlet, and the word discharge should be interpreted as meaning Met. The term net positive suction head (NPSH) is not applicable to HPRTs. 

On large fire water systems, the location of pumps and storage tanks at various plant areas provides greater reliability of protection and results in less pressure drop between the pump and the area of demand. Net positive suction head (NPSH) requirements and friction loss in the piping should be considered in locating fire water pumps. 

Therefore, the liquid level in the overhead condenser would have to be somewhere in the condenser s shell. But then, the liquid in the condenser would be below the reflux drum. How, then, does the liquid get from the lower elevation of the condenser to the higher elevation in the reflux drum We will have to explain this hydraulic problem later. But for now, we can say that most reflux drums are elevated 20 or 30 ft above grade to provide net positive suction head (NPSH) for the reflux pump. Also, most shell-and-tube condensers are located at grade, for easier maintenance during unit turnarounds. 

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. 

Figure 7.16. Recommended values of net positive suction head (NPSH) at various temperatures or vapor pressures (a) NPSH of several types of pumps for handling water at various temperatures, (b) Correction of the cold water NPSH for vapor pressure. The maximum recommended correction is one-half of the cold water value. The line with arrows shows that for a liquid with 30 psia vapor pressure at 100°F, the reduction in NPSH is 2.3 ft (data erf Worthington International Inc.).

The last consideration is the problem of cavitation. The net positive suction head (NPSH) determined in the calculations, must be sufficiently high to prevent spontaneous vapourization of liquid in the event of a drop in suction pressure. 

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. 

Net Positive Suction Head - NPSH. The pressure of liquid inside a pump must always exceed the liquid s vapour pressure, otherwise the liquid will tend to vaporise within the pump, causing cavitation. As the fluid passes on through the pump its pressure increases and any vapour bubbles collapse rapidly causing mechanical damage to the... 

Cavitation corrosion occurs in pumps that have flow conditions that allow bubble formation on the surface of impellers. These bubbles, upon formation, break with enough force to rupture the protective film of the stainless steel. Plants can prevent this by designing a system that avoids bubble formation (i.e., provide sufficient Net Positive Suction Head - NPSH - for the pump), by polishing rotating parts to remove bubble formation sites and by using alloys with greater corrosion resistance and strength88. 

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