Pumps suction head

Normal pump suction head (NPSH) of a pump must be in excess of a certain number, depending on the kind of pumps and the conditions, if damage is to be avoided. NPSH = (pressure at the eye of the impeller - vapor pressure)/(density). Common range is 4-20 ft. 

Skirt (minimum to ensure adequate pump suction head) 12... 

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. 

Head. The tme meaning of the total developed pump head, H, is the amount of energy received by the unit of mass per unit of time (14). This concept is traceable to compressors and fans, where engineers operate with enthalpy, a close relation to the concept of total energy. However, because of the almost incompressible nature of Hquids, a simplification is possible to reduce enthalpy to a simpler form, a Bernoulli equation, as shown in equations 1—3, where g is the gravitational constant, SG is specific gravity, y is the density equivalent, is suction head, is discharge head, and H is the pump head, ie, the difference between H, and H. 

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... 

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. 

Total Dynamic Head The total dynamic head H of a pump is the total discharge head minus the total suction head h,. 

Static Suction Head The static suction head / is the vertical distance measured from the free surface of the hquid source to the pump centerhne plus the absolute pressure at the liquid surface. 

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 ... 

In a submerged-tube FC evaporator, all heat is imparted as sensible heat, resulting in a temperature rise of the circulating hquor that reduces the overall temperature difference available for heat transfer. Temperature rise, tube proportions, tube velocity, and head requirements on the circulating pump all influence the selec tion of circulation rate. Head requirements are frequently difficult to estimate since they consist not only of the usual friction, entrance and contraction, and elevation losses when the return to the flash chamber is above the liquid level but also of increased friction losses due to flashing in the return line and vortex losses in the flash chamber. Circulation is sometimes limited by vapor in the pump suction hne. This may be drawn in as a result of inadequate vapor-liquid separation or may come from vortices near the pump suction connection to the body or may be formed in the line itself by short circuiting from heater outlet to pump inlet of liquor that has not flashed completely to equilibrium at the pressure in the vapor head. 

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. 

The suction head is the available head at the suction nozzle of the pump. 

Suction lift is negative suction head. It exists when the liquid level in the suction vessel is below the centerline of the pump. The pump must aspirate the liquid up from the suction vessel into the pump and then... 

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 ... 

Hi Inlet Head. The los,ses in the pump suction throat to the impeller eye. These losses could be insignificant up to 2 ft, depending on design. 

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. 

NPSH is the pressure available at the pump suction nozzle after vapor pressure is subtracted. It is expressed in terms of liquid head. It thus reflects the amount of head loss that the pump can sustain internally before the vapor pressure is reached. The manufacturer will specify the NPSH that his pump requires for the operating range of flows when handling water. This same NPSH is normally used for other liquids. 

Note when used for pump system balance, this Zhf must be used as a negative number ( — 0.1863) because it is a pressure loss associated with the fluid flowing. For pipe line sizing, the pressure head on the tank of 5 psig and any elevation difference between tank outlet nozzle and pump suction centerline do not enter into the calculations. 

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-...

Discharge static head H (from centerline of pump) Suction static head S, (actually -HS)... 

The total suction head. Figure 3-41, is the difference in elevation between the liquid on the pump suction side and the centerline of the pump (plus the velocity head). Note that the suction head is positive when above the... 

Static head is positive pressure of fluid on pump suction above its centerline (S), (4-). 

This applies because the total head for a pump is total discharge head a( + ), minus ( —) the [suction head, a( + )J, or [suction lift, a( —)]. 

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