Net positive suction head requirement

Pump Suction. The net positive suction head required (NPSHR) affects the resistance on the suction side of the pump. If it drops to or near the vapor pressure of the fluid being handled, cavitation and loss of performance occurs (13). The NPSHR is affected by temperature and barometric pressure and is of most concern on evaporator CIP units where high cleaning temperatures might be used. A centrifugal booster pump may be installed on a homogenizer or on the intake of a timing pump to prevent low suction pressures. 

D. J. MMethodfor Estimating the Net Positive Suction Head Required by Centrifugal Pumps, ASME 81-WA/EE-32, Washington, D.C., 1981. 

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 ... 

Wliere N = the speed of the pump/motor in revolutions per minute Q = the square root of the flow in gallons per minute at the Best Efficiency Point BEP. For double suction pumps, use A BEP Flow. NPSHr = the net positive suction head required by the pump at the BFiP. 

The Pump s Minimum Rec]uirement Curve. Its ealled Net Positive Suction Head required, NPSHr. 

Figures 3-36A, 3-36B, and 3-36C represent typical and actual performance curves showing discharge total head (head pressure at pump outlet connection for any fluid), required minimum water horsepow er (for pumping water), and capacity or pumping volume of the pump (for any fluid) for several impeller diameters that would fit the same case (housing). In addition the important NPSHr (net positive suction head required by the pump) charac-...

A centrifugal pump is required to circulate a liquid of density 800 kg/m2 and viscosity 0.5 x 10 3 Ns/m" from the reboiler of a distillation column through a vaporisor at the rate of 0.004 m3/s, and to introduce the superheated vapour above the vapour space in the reboiler which contains a 0.07 m depth of liquid. If smooth-bore 25 mm diameter pipe is to be used, the pressure of vapour in the reboiler is 1 kN/m2 and the Net Positive Suction Head required by the pump is 2 m of liquid, what is the minimum height required between the liquid level in the reboiler and the pump ... 

NPSHreqd Net positive suction head required at the pump suction L... 

NPSHR = net positive suction head required (see 1.4.27) in meters... 

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-... 

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. 

To avoid pump cavitation, the net positive suction head available must be greater than the net positive suction head required. 

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 net positive suction head required to prevent cavitation is determined through testing by the pump manufacturer and depends upon factors including type of impeller inlet, impeller design, pump flow rate, impeller rotational speed, and the type of liquid being pumped. The manufacturer typically supplies curves of NPSHr as a function of pump flow rate for a particular liquid (usually water) in the vendor manual for the pump. 

Net positive suction head requirement for centrifugal slurry pumps... 

The net positive suction head requirement has been investigated in an open tank-valve arrangement for a 0.5 by 0.45 m branch pump (impeller diameter 1.4 m) for sands with average particle sizes of 0.3 to 0.4 mm when pumped at slurry densities of 1230 and 1400 kg/m. It was found that the net positive suction head required in meters of slurry was about 50% higher than the corresponding water values. The effect of solids on the pump head and efficiency was less than 3%. 

To ensure operation without cavitation, it is required that the absolute pressure at the pump impeller eye exceeds the vapor pressure by a certain margin. By convention the required absolute pressure at the pump entrance is expressed in terms of head of the mixture being pumped, using the term Net Positive Suction Head Required, NPSHR. The NPSHR is usually specified by the pump manufacturer based on tests with water. Cavitation curves for various rotary speeds are given in Fig. 3 for the pump used here. 

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. 

NPSHR is an abbreviation of net positive suction head required. It is determined by experiments at pump manufacturer s laboratory. Per Hydiaulic Institute Standard, NPSHR is the measui NPSHA w hen pump head is dropped 2% below its value on the performance curve. NPSHR increases with higbet pumping rate, higher pump speed, and for some pumps with smaller impeller [1]. 

From the degassing area discharge, each fuel stream flows to the suction of a variable speed centrifugal sump type pump. Each pump is designed to deliver about 9000 gpm at 20-ft head of pumped fluid. To obtain a reasonable pump speed, the net positive suction head requirement is 11.5 ft. A gas pressure (helium) is maintained o er the pump sump to prevent flooding of upper parts (motor windings, cooling system, etc.) of the pump. 

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