NPSHa available

This is the energy in the fluid at the suction connection of the pump over and above the liquid s vapor pressure. It is a characteristic of the system and we say that the NPSHa should be greater than the NPSHr (NPSHa NPSHr). 

Hs = Static head in feet (positive or negative) of the fluid level in the suction vessel to the pump centerline. 

Hvp = the Vapor head of the fluid expressed in feet. It is a fiinetion of the temperature of the liquid. See Properties of Water 11 in this chapter. 

Hf = Friction head or friction losses expressed in feet in the suction piping and connections. 

Hi = Inlet head, or the losses expressed in feet that occur in the suction throat of the pump up to and including the eye of the impeller. These losses would not be registered on a suction pressure gauge. They could be insignificant, or as high as 2 feet. Some pump manufacturers factor them into their new pumps, and others don t. Also, changes occur in maintenance that may alter the Hi. If you don t know the Hi, call it a safety factor of 2 feet. 

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If the system pressure were 46 psia, then NPSHa available = -8 + (46 - 44) (2.31/0.58) - 12 = -12 feet, and this is an impossible and unacceptable condition. This means liquid will flash in the line and in the impeller, and cannot be pumped. NPSH must always be positive in sign. 

Have NPSHa available at least 2 feet of liquid greater than the pump manufacturer requires under the... 

The NPSHA available from or in the liquid system on the sucuon side of a pump is expressed (corrected to pump centerline) as ... 

Figure 3-36A represents a typical manufacturer s performance curve. The values of NPSHR given are the minimum values required at the pump suction. As mentioned, good practice requires that the NPSHA available be al least two feet of liquid above these values. It is important to recognize that the NPSHR and Suction Lift Values are for handling water at about 70°F. To use with other liquids it is necessary to convert to the equivalent water suction lift at 70°F and sea level. 

Example 3-7 NPSHA Available in Pressure System, Use Figure 3-41 (b)... 

It is difficult to determine exactly the areas of localized pressure reductions inside the pump, although much research has been focused on this field. It is easy, however, to measure the total fluid pressure (static plus dynamic) at some convenient point, such as pump inlet flange, and adjust it in reference to the pump centerline location. By testing, it is possible to determine the point when the pump loses performance appreciably, such as 3% head drop, and to define the NPSH at that point, which is referred to as a required NPSH (NPSHR). The available NPSH (NPSHA) indicates how much suction head... 

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 ... 

This ty pe of cavitation is also called inadequate NPSHa cavitation. To prevent this ty pe of cavitation, the NPSHa in the system (the available energ) in the system), must be higher than the NPSHr of the pump (the pump s minimum energy 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. 

If the previous system were at sea level, consider the same pump with the same system at an altitude of 6000 feet. Here the barometric pressure is 27.4 feet of water. This is 34 — 27.4 = 6.6 feet less than the sea level installation. The new NPSHa will be 15 ft — 6.6 ft = 8.4 feet available. Referring to the pump curve of Figure 3-36A it is apparent that this pump cannot do greater than 21 feet suction lift as water or 12 feet NPSHr of liquid (fluid). 

Using the example of Reference [6], assume a pump with characteristic curve and added temperature rise data as showm on Figure 3-59 is to handle boiler feed water at 220°F, with a system available NPSH = 18.8 feet. The v apor pressure of w ater at 220°F is 17.19 psia from steam tables and the SpGr = 0.957. Correcting the 18.8 feet NPSHa psia = 18.8 (l/[2.31/0.957)] = 7.79 psia at 220°F. 

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). 

Available lime, 15 26 Available net positive suction head (NPSHA), 21 85... 

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

Calculate the available net positive suction head (NPSHa) for the pump. 

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. 

The primary causes of cavitation due to entrained gas include two-phase suction supply, inadequate available net positive suction head (NPSHa), and leakage in the suction-supply system. In some applications, the incoming liquid may contain moderate to high concentrations of air or gas. This may result from aeration or mixing of the liquid prior to reaching the pump or inadequate liquid levels in the supply reservoir. Regardless of the reason, the pump is forced to handle two-phase flow, which was not intended in its design. 

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). 

For this purpose, commercially available calculation programs are frequently applied. Usually the results are one or more characteristic curves (pump head over volume flow) whereby different load conditions are taken into account. Furthermore the progression of the NPSHa-value can be calculated against the discharged volume flow. Here again the different modes of operation should be considered to determine the worst case . 

NPSHA is an abbreviation of net positive auction head available, it is calculated by Eq. (4). It is a measure of the liquid pressure available at the pump suction to suppress the vaporization or cavitation inside the pump. A hydraulic analysis of pump suction piping will determine the NPSHA. For conservative, NPSHA is usually calculated based on minimum liquid level or maximum suction lift at maximum flow. 

V.3 Net positive suction head available (NPSHA) and required (NPSHR)... 

To prevent pump cavitation, suction pressure and/or static height should be high enough to suppress any vaporization inride the pump. NPSHA is ustmlly calculated to indicate how much suction head is available to prevent vaporization inside the pump. NPSHA is calculated as follows ... 

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