Impeller eye of the

The impeller is attached to a shaft. The shaft spins and is powered by the motor or driver. We use the term driver because. some pumps are attached to pulleys or transmissions. The fluid enters into the eye of the impeller and is trapped between the impeller blades. The impeller blades contain the liquid and impart speed to the liquid as it passes from the impeller eye toward the outside diameter of the impeller. As the fluid accelerates in velocity, a zone of low pressure is created in the eye of the impeller (the Bernoulli Principle, as velocity goes up, pressure goes down). This is another reason the liquid must enter into the pump with sufficient cnergt. ... 

It is the energy in the liquid rec]uired to overcome the friction los.ses from the suction nozzle to the eye of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump s curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH. 

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. 

Inadequate NPSHa establishes favorable conditions for cavitation in the pump. If the pressure in the eye of the impeller falls below the vapor pressure of the fluid, then cavitation can begin. 

Inside the pump, the pressure deereases in the eye of the impeller beeause the fluid veloeity inereases. For this reason the liquid ean boil at a lower pressure. For example, if the absolute pressure at the impeller eye should fall to 1.0 psia, then water eould boil or vaporize at about 100°F (see the Tables in Chapter 2 Properties of Water I and II). 

Reply If you could really suck on the milk, then you wouldn t need the straw. -you re actually doing with your mouth on the straw is lowering the atmospi pressure inside the straw, so that the atmospheric pressure outside the straw pu the milk up into your mouth. This is why we say that a pump does not The p-r.ip actually generates a zone of low pressure in. . eye of the impeller, thereby Ic,-" ig the atmospheric pressure inside the suction piping. Atmospheric pressure outside the suction piping pushes the liquid up toward the impeller a maximum of 34 ft under ideal circumstances. 

According to Bernoulli s Law, when velocity goes up, pressure goes down. This was explained in Chapter 1. A centrifugal pump works by acceleration and imparting velocity to the liquid in the eye of the impeller. Under the right conditions, the liquid can boil or vaporize in the eye of the impeller. When this happens we say that the pump is suffering from vaporization cavitation. 

With the pump disassembled in the shop, the damage from vaporization eavitation is seen behind the impeller blades toward the eye of the impeller as illustrated below (Figure 3-1). 

With the pump di.sa.ssemblcd in the shop, with open impellers, the damage is seen on the leading edge of the impeller blades toward the eye of the impeller, and on the blade tips toward the impeller s OD. With enclosed impellers, the damage reveals itself on the wear bands between the impeller and the volute casing. See the illustration (Figure 3-2). 

Stationary guide vanes direct the flow to the eye of the impeller in an orderly fashion. Depending upon the head requirements of an individual stage, these vanes may direct the flow in the same direction as the rotation... 

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. 

A well-established and effective method of ensuring a laminar flow to the eye of the impeller is to provide the suction of the pump with a straight mn of pipe in a length equivalent to 5-10 times the diameter of that pipe. The smaller multiplier would be used on the larger pipe diameters and vice versa. 

In a centrifugal compressor, air or gas at atmospheric pressure enters the eye of the impeller. As the impeller rotates, the gas is accelerated by the rotating element within the confined space that is created by the volute of the compressor s casing. The gas is compressed as more gas is forced into the volute by the impeller blades. The pressure of the gas increases as it is pushed through the reduced free space within the volute. 

Figure 56.1 shows the cross-section of a typical end-suction centrifugal pump where the fluid to be pumped enters the suction inlet at the eye of the impeller. Due to the relatively high speed of rotation, the fluid collected within the impeller vanes is held captive because of the close tolerance between the front face of the impeller and the pump housing. 

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. 

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 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 units of NPSH are feet of liquid head. The required NPSH of a pump is due primarily to the conversion of feet of head to velocity in the eye of the impeller. 

They lack sufficient available NPSH to satisfy the conversion of pressure to velocity, in the eye of the impeller (running NPSH). 

Double suction two incoming streams enter at the eye of the impeller on opposite sides, minimizing axial thrust and worthwhile for large, high head pumps [Fig. 7.2(b)]. 

Single suction the liquid enters on one side at the eye of the impeller most pumps are of this lower cost style [Fig. 7.2(c)],... 

The terms suction and discharge in the context of heads refer to portions of the system before and after the pumping station, respectively. Static suction lift h( is the vertical distance from the elevation of the inflow liquid level below the pump inlet to the elevation of the pump centerline or eye of the impeller. A lift is a negative head. Static suction head h, is the vertical distance from the elevation of the inflow liquid level above the pump inlet to the elevation of the pump centerline. Static discharge head h is the vertical distance from the centerline elevation of the pump... 

The activity inside the pump volute incurs several losses first is the backflow of the flow that had aheady been acted upon but is shpping back into the suction eye of the impeller or, in general, toward the suction side of the pump. Because energy had already been expended on this flow but failed to exit into the discharge, this backflow represents a loss. The other loss is the turbulence induced as the impeller acts on the flow and swirls it around. Turbulence is a loss of energy. As the impeller rotates, its tips and sides shear off the fluid this also causes what is called disk friction and is a loss of energy. All these losses cause the inefficiency of the pump is these losses. 

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