Net Positive Suction Head NPSH Available

Net Positive Suction Head (NPSH) Available The available NPSH is the 

Some effort should be made to avoid low available NPSH values (2 to 5 feet) because this may result in the pump manufacturer offering an oversized pump that will operate at part capacity and low efficiency. Whenever possible, excess NPSH over the requirements of the pump should be available. When a pump is handling a boiling liquid, the available NPSH is almost entirely dependent upon the elevation of the liquid level in the suction vessel. 

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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. 

Net positive suction head (NPSH) available or depth from which liquid is to be sucked... 

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 ... 

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). 

NPSH)a available net-positive-suction-head (NPSH)r required net-positive-suction-head... 

The computer program PROG62 sizes the centrifugal pump for the given flow rate and fluid characteristics. The program calculates the hydraulic brake horsepower required by the pump and the actual brake horsepower. In addition, the program computes the available net positive suction head (NPSH) and the pump efficiency. Table 6-7 shows the input data and results of the pump hydraulic design calculation. The available NPSH is 64 ft, and the actual brake horsepower required for the pump operation is 7.0hp, with a pump efficiency of 68%. 

To avoid cavitation, the pressure at the pump inlet must exceed the vapor pressure by a certain value, called the net positive suction head (NPSH). The required value of NPSH is about 2 to 3 m (5 to 10 ft) for small centrifugal pumps, but it increases with pump capacity, impeller speed, and discharge pressure, and values up to 15 m (50 ft) are recommended for very large pumps. For a pump taking suction from a reservoir, like that shown in Fig. 8.5, the available NPSH is customarily calculated as... 

The need for maintaining subcooled liquid in the supply tank is based upon the need for net positive suction head (NPSH), which is the total head of fluid in excess of vapor pressure, at the pump inlet. Years of experience in designing and testing pumps for fluids other than hydrogen have demonstrated the need for NPSH at the pump inlet to ensure proper operation. However, test data for hydrogen suggest that unusual suction performance can be expected. This expectation is especially valid if the available thermodynamic head at the pump inlet is considered. In this case, thermodynamic head is defined as the reduction in enthalpy of the fluid when expanded isentropically. This process involves vaporization, i.e., cavitation of the fluid. 

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. 

Completely submerged pumps can also be used but only for water, since they can easily be damaged or suffer from corrosion in case ofcorrosive fluids like sulphuric acid and oleums. These pumps do not require gland packing and priming tanks. Net Positive Suction Head (NPSH) is always available since... 

Net positive suction head (NPSH) A parameter used in pump characterization to predict or prevent cavitation. NPSH is determined from the physical properties of the liquid and the operating conditions at the pump suction. It represents the excess of the total head of the fluid above its vapor pressure at operating temperature. Each pump has its own demand for a minimum value of this parameter this is the required NPSH and is defined by the pump supplier. The available NPSH is determined by operating temperature and pressure, elevation of the fluid above the pump suction, and the design of the piping system. Available NPSH must always be greater than required NPSH this must be assured by proper design and installation of tire system. 

Available net positive suction head The available NPSH is the net pressure available in a given system, based on vessel pressure and static head, minus the liquid vapor pressure and functional losses in the system. The goal is to maintain equipment heights and minimi /se pump. suaion piping to ensure that the available NPSH is greater than tlie required NPSH. Insufficient NPSH can reduce pump capacity and efficiency and lead to cavitation damage. 

The quantity used to determine if the pressure of the liquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH). The net positive suction head available (NPSHa) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped. The net positive suction head required (NPSHr) is the minimum net positive suction head necessary to avoid cavitation. 

Generally, the glycol/glycol exchanger is placed upstream of the lean-glycol pump. This reduces the glycol temperature at the pump inlet and thus increases the available net positive suction head (NPSH) of fhe pump. 

To express the quantity of energt available in the lit]iiid entering into the pump, the unit of measure for NPSH is feet of head or elevation in the pump suction. The pump has its NPSHr, or Net Positive Suction Head Required. The system, meaning all pipe, tanks and connections on the suction side of the pump has the NPSHa, or the Net Positive Suction Head Available. There should always be more NPSHa in the system dian the NPSHr of the pump. Let s look at them, beginning with what the pump recgiires ... 

For conditions of (1) high suction side (or inlet) friction loss, from suction piping calculations or (2) low available Net Positive Suction Head (10 feet or less), a large open eye on the impeller inlet is necessary to keep the inlet velocity low. NPSH is discussed in a later section. The manufacturer should be given the conditions in order to properly appraise this situation. 

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 avan Net positive suction head available at the pump suction L... 

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. 

Net positive suction head available (NPSH) is the difference between the total absolute suction pressure at the pump suction nozzle when the pump is running and the vapor pressure at the flowing liquid temperature. All pumps require the system to provide adequate (NPSH). In a positive-displacement pump the (NPSH)a should be large enough to open the suction valve, to overcome the friction losses within the pump hquid end, and to overcome the liquid acceleration head. 

A recirculating pump should be chosen so that the developed head is dissipated as pressure drops through the circuit of the system. It is important that the pump and system be properly matched. The fluid being pumped is at or near its boiling point and, therefore, the required NPSH (net positive suction head) is usually critical. The pump should operate at this design level. If it develops excessive head, it will handle more volume at a lower head. At the new operating point, the required NPSH may be more than is available. 

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