Suction head net positive

The single most common operational problem in a process plant is loss of suction pressure to a centrifugal pump. If the suction pressure is too low, the discharge pressure and the discharge flow become erratically low. The suction pressure, while low, remains comparatively steady. 

The problem described above is called cavitation. A pump that is operating in a cavitation mode may also (but often does not) produce a sound similar to shaking a bucket full of nuts and bolts. A cavitating motor-driven pump always draws an erratically low-amperage flow. This is consistent with the erratically low flow rate. 

Let us assume liquid flows from an 8-in line into the suction of a centrifugal pump. The liquid enters the pump s impeller through a circular opening, called the eye of the impeller, in the center of the impeller. Let us assume that this eye has a diameter of 2 in. 

The velocity of the liquid increases by a factor of 16 (i.e., 8 in 2 in, squared). The kinetic energy of the liquid increases by a factor of 264 (i.e., 16, squared) But where does this large increase in kinetic energy come from Answer from the pressure, or feet of head, of the liquid itself. 

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. 

The absolute pressure (or head) of the liquid at the pump inlet, the suction side, must always be greater than the fluid vapour pressure by the NPSH. Let us use the Mechanical Energy Balance Equation (14), to find the pressure at the pump inlet. 

Point 1 no change, at the liquid surface in the holding vessel u 0, 

Point 2 now chosen to be the inlet to the pump where we need to determine the pressure w3 = it. z3 = 0, p3 is the unknown pressure at pump inlet. 

Note A/ f has to be recalculated based on the shorter length of pipe from the holding vessel to the pump inlet, 8 m, and the fewer fittings in this section of pipework. In this case le = 6.7 m, giving A/+ = 0.24 m H20. 

NPSH available is the difference between absolute pressure (p3) of the liquid at the pump and the fluid vapour pressure (pv), expressed as a head. 

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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. 

D. Florjancic, "Net Positive Suction Head for Eeed Pumps", Sul rReport, 1984. 

E Modulus of elasticity N/ni ibftft NPSH Net positive suction head m ft... 

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 ... 

Severe cavitation damage on the suction side of the pump reveals insufficient water supply to the pump (insufficient net-positive suction head). Such a circumstance could be caused by partially clogged filters or screens upstream of the pump, or simply by insufficient feed of water to the 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 ... 

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. 

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. 

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-...

Net positive suction head (in feet of liquid absolute) above the vapor pressure of the liquid at the pumping temperature is the absolute pressure available at the pump suction flange, and is a very important consideration in selecting a pump which might handle liquids at or near their boiling points, or liquids of high vapor pressures. 

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. 

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) ... 

Fig. 8.79 (fl) Pump performance characteristics. (6) Cavitation erosion zone, (c) Effect of reducing net positive suction head... 

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