Pump cavitation available NPSH

NPSH (net positive suction head) The net positive suction head required to keep a centrifugal pump from cavitating. Cooling a liquid in a pump s suction line increases the pump s available NPSH, as does increasing the liquid level in the suction drum. 

For low available NPSH (less than 10 feet) the pump suction connection and impeller eye may be considerably oversized when compared to a pump not required to handle fluid under these conditions. Poor suction condition due to inadequate available NPSH is one major contribution to cavitation in pump impellers, and this is a condition at w hich the pump cannot operate for very long without physical erosion damage to the impeller. See References [11] and [26]. 

In the above discussion it is assumed that the available NPSH in the system is adequate to support the flow rate of liquid into the suction side of the pump. If the available NPSH is less than that required by the pump, cavitation occurs and the normal curves do not apply. In cavitation, some of the liquid vaporizes as it flows into the pump. As the vapour bubbles are carried into higher pressure regions of the pump they collapse, resulting in noise and vibration. High speed pumps are more prone to cavitation than low speed pumps. 

A centrifugal pump will operate normally at a point on its total head against capacity characteristic curve until the available NPSH falls below the required NPSH curve. Beyond this point, the total head generated by a centrifugal pump falls drastically as shown in Figure 4.6 as the pump begins to operate in cavitation conditions. 

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

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. 

Answer—yes But why Well, the liquid is cooled by 5°F after it leaves the drum. The cooled liquid is not in equilibrium with the vapor in the drum. It has been subcooled by 5°F. This means that the bubble-point liquid has been cooled, without altering its composition. The vapor pressure of the liquid has been reduced. As can be seen in Fig. 25.3, subcooling this particular liquid by 5°F reduces its vapor pressure by about 2 psi. As the specific gravity of the liquid is 0.58, this is equivalent to an increase in the NPSH by 8 ft. Once again, our objective is to increase the flow from 250 to 300 GPM. Figure 25.2 tells us that the required NPSH increases from 20 to 26 ft. However, when we subcool the liquid by 5°F, the available NPSH increases from 20 to 28 ft. As the available NPSH now exceeds the required NPSH by 2 ft, the flow can be increased without risk of pump cavitation. 

The longer the suction line and the larger the diameter of the line, the more mass has to be accelerated. This also increases the starting NPSH required. If the sum of the frictional loss in the suction line, plus the running NPSH, plus the starting NPSH, equals the available NPSH, then the pump will cavitate on start-up. 

The required NPSH, (NPSH)R, is specified by the pump manufacturer, and the available NPSH, (NPSH)A, is determined by the design of the pump suction piping. To prevent cavitation the available NPSH must be equal to or greater than the required NPSH. 

The net head at the suction of the pump impeller must not exceed a certain value to prevent formation of vapor and cause cavitation of the metal. This minimum head is called the available NPSH, and must be greater than the required NPSH,. for a stable pump operation. 

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... 

Pump manufacturers have established guidelines to ensure each pump they supply is not exposed to conditions that result in cavitation. The design standard is called NPSHR or net positive suction head required. The NPSHR takes into account any potential head losses that might occur between the pump s suction nozzle and impeller thereby ensuring the liquid does not drop below its vapour pressure (bubble point). The NPSH is a measure of the proximity of a liquid to its vapour pressure, and must exceed the pump manufacturer s pump NPSHR. There are two process variables that can be adjusted, in case the available NPSH is less than the NPSHR raise the static head and lower friction losses. Conversely, the NPSHR can be reduced by using a larger, slower speed pump, a double suction impeller, a larger impeller inlet area, an oversized pump and a secondary impeller placed ahead of the primary impeller. 

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. 

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. 

Avoidance of cavitation. As with reciprocating pumps, the criterion NPSHa > NPSHr should be obeyed. The available NPSH (NPSHa) for a centrifugal pump may be calculated using equations (9.14) and (9.15) developed in section 9.S.2.4 for reciprocating pumps. In this case however //ja = 0 and NPSHa usually falls with the square of the volume flow rate (Figure 9.28). 

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. ... 

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). 

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. 

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