Acceleration, pump suction

Net positive suction head available (NPSH) is the difference between the total absolute suction pressure at the pump suction nozzle when the pump is running and the vapor pressure at the flowing liquid temperature. All pumps require the system to provide adequate (NPSH). In a positive-displacement pump the (NPSH)a should be large enough to open the suction valve, to overcome the friction losses within the pump hquid end, and to overcome the liquid acceleration head. 

If a glycerin-sealed stirrer is used, the rate of solvent evaporation may be accelerated by means of gentle water-pump suction. 

The range of HTS behavior is encompassed by large pipe ruptures at three locations at the core inlet (inlet header), at the core outlet (outlet header), and upstream of a main heat transport pump (pump suction pipe). Breaks at these locations affect the two core passes of a loop in different ways. The core pass upstream of the break (upstream and downstream directions are defined relative to the normal flow direction) always has flow that is accelerated toward the break. The fuel cooling tends to be increased and emergency core coolant refill is rapid in the flow direcdon toward the break. The core pass downstream of the break has its flow reduced by the flow that is diverted out of the break and is therefore more likely to experience degraded fuel cooling. 

L is length of suction line, in ft v is average liquid velocity in pump suction line, in ft/sec C is a constant for different pump configurations and k is a factor related to liquid compressibility. Values of C and k are listed in Table 2. For suction line with different line size, using Eq, (15) to each section separately. Add the acceleration head for each section to obtain the total acceleration head. 

Use an impeller inducer. An impeller inducer looks like a corkscrew device that fits onto the center hub of the primary impeller and extends down the suction throat of the pump. It is actually a small axial flow impeller that accelerates the fluid toward the primarv impeller from further down the suctittn throat of the pump. Some inducers bolt onto the impeller and others are cast into the main impeller. The inducer has a low NPSHr for the system feeding it, and it increases the Ha to the primary impeller. 

It is clear from equation 4.2 that the suction head hs can fall to a very low value, for example when the suction frictional head loss is high and the static head zs is low. If the absolute pressure in the liquid at the suction flange falls to, or below, the absolute vapour pressure Pv of the liquid, bubbles of vapour will be formed at the pump inlet. Worse still, even if the pressure at the suction flange is slightly higher than the vapour pressure, cavitation—the formation and subsequent collapse of vapour bubbles— will occur within the body of the pump because the pressure in the pump falls further as the liquid is accelerated. 

Calculate the available net positive section head NPSH in a pumping system if the liquid density p = 1200 kg/m3, the liquid dynamic viscosity p = 0.4 Pa s, the mean velocity u = 1 m/s, the static head on the suction side 2, = 3m, the inside pipe diameter di = 0.0526 m, the gravitational acceleration g = 9.81 m/s2, and the equivalent length on the suction side SLes = 5.0 m. 

A centrifugal pump is used to pump a liquid in steady turbulent flow through a smooth pipe from one tank to another. Develop an expression for the system total head A/t in terms of the static heads on the discharge and suction sides zd and zs respectively, the gas pressures above the tanks on the discharge and suction sides Pd and Ps respectively, the liquid density p, the liquid dynamic viscosity p, the gravitational acceleration g, the total equivalent lengths on... 

This process is rather slow it may be accelerated by inverting over the dish a large funnel which is connected to a suction pump. In this manner the crystalline cake separates in about two hours. 

The need to accelerate liquid in the suction line of a pump leads to a difficult operating problem, which occurs on start-up. Just before the pump shown in Fig. 23.5 is put on line, the velocity in the suction line is zero. The energy to increase the velocity (i.e., accelerate) the liquid in the suction line must come from the pressure of the liquid at the pump s suction. As the pump s discharge valve is opened, the velocity... 

To avoid pump cavitation on start-up, the experienced operator opens the pump discharge valve slowly. Slowly opening the discharge valve results in reduced acceleration of the liquid in the suction line and a slower rate of the conversion of suction pressure to velocity. 

To review—all the energy needed to accelerate the liquid to the suction of a pump comes from the pump s suction pressure. None of this energy comes from the pump itself. Or, as one clever operator at the Unocal Refinery in San Francisco explained to me, Pumps push, but they do not suck. ... 

This results in a temporary loss of pressure at the suction of the pump. This temporary loss of pressure is called the starting NPSH requirement. The more quickly the operator opens the discharge valve of a pump, the more rapidly the liquid accelerates in the suction line. This increases the starting NPSH required. 

The longer the suction line and the larger the diameter of the line, the more mass has to be accelerated. This also increases the starting NPSH required. If the sum of the frictional loss in the suction line, plus the running NPSH, plus the starting NPSH, equals the available NPSH, then the pump will cavitate on start-up. 

They lack sufficient available NPSH to satisfy the conversion of pressure to acceleration, in the suction line, as the pump is started (starting NPSH). 

Ed Motion losses on the discharge side of a pump Es Motion losses on the suction side of a pump g acceleration of gravity... 

Since the polytropic head has the same units as the pump head times the acceleration due to gravity, it follows that we may apply the dimensional analysis of Section 17.2 to the compressor as well as the pump, with H p replacing gH. This reveals the same independence of the specific volume of the fluid being passed, and the same dependence on volume flow rate at suction, Q, and speed, N. 

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