Nozzles draw-off

The pressure at point A in Fig. 10.4 was 13 psig. This means that the pressure drop in the vapor line from the stripper, back to the fractionator, was 3 psig. In order for the unstripped jet fuel, to flow out of the lower-pressure fractionator, and into the higher-pressure stripper, it had to overcome this 3-psig pressure difference. The 16-ft elevation difference between the draw-off nozzle on the fractionator and the stripper inlet provided the necessary liquid head driving force. 

This equation assumes that before the fluid enters the nozzle, its velocity is small, compared to its velocity in the nozzle. The increase in the velocity, or the kinetic energy, of the fluid in the nozzle comes from the pressure of the fluid. This is Bernoulli s equation in action. The energy to accelerate the fluid in the draw-off nozzle comes from the potential energy of the fluid. This is Newton s second law of motion. 

The coefficient used in the equation above (0.34) neglects friction and assumes the process fluid has a low viscosity. For most process nozzles, these are reasonable assumptions. Detailed information on draw-off nozzle coefficients has been published in Crane.1... 

In summary, the lowest pressure that can be reached at point D in Fig. 11.6 is the pressure at point A. When these two pressures are equal, we say that the draw-off nozzle is limited by cavitation. If we were to lower the pressure downstream of point D, say, by opening a control valve, the increase in flow would be zero. 

Figure 11.7 shows two almost identical draw-off arrangements. The only difference is the elevation of the control valve in the draw-offline. Control valve A is at the same elevation as the draw-off nozzle. The pressure drop across the control valve is 2 psi, or 56 in of water. Let s assume that... 

Increase in velocity through the draw-off nozzle is small, and hence the nozzle exit loss is zero. 

Even after we have made these two unlikely assumptions, the height of hot water in the draw-off sump must still be 56 in above the center-line of the draw-off nozzle. If not, the water would begin flashing to steam, as it experienced a pressure drop of 2 psi, flowing across the control valve. The evolved steam would then choke the water flow, reducing the pressure drop across the control valve until the pressure drop equaled the depth (or head) of water in the draw-off sump. 

Does this mean that control valves should be located well below the elevation of the draw-off nozzle Yes ... 

Does this mean, that the amount of piping, fittings, gate valves, etc. should be minimized on a draw-offline, until the line drops 10 or 20 ft below the draw-off nozzle Yes ... 

Does this mean any frictional losses, due to external piping, at the same elevation as the draw-off nozzle, have to be added to the nozzle exit loss, in determining the liquid level in the sump Yes ... 

The fluid being pumped is hot water. At the desired flow rate of 110 GPM, the manufacturer s pump curve shows that the pump requires 14 ft of NPSH. The elevation difference between the draw-off nozzle, and the suction of the pump is shown on Fig. 25.5, as 46 ft. We really ought to have plenty of running NPSH. But apparently, we do not. 

Well, dear reader, it no longer exists. Figure 25.6 illustrates the true situation. Let s say we are pumping 110 GPM from the pump discharge. But only 109 GPM can drain through the draw-off nozzle. We would then slowly lower the water level in the suction line. The water level would creep down, as would the pump s suction pressure. When the water level in the suction line dropped to 14 ft, the pump would cavitate or slip. The flow rate from the pump would drop, and the water level in the suction line to the pump would partially refill. The pump s... 

Of course, it may simply be that the draw-off nozzle is undersized. To determine whether this is the case, calculate the velocity (in feet per second) V through the nozzle ... 

Lack of available NPSH may also be caused by high frictional loss in the suction piping. If this is the case, a small reduction in flow will not noticeably increase the pressure at the suction of the pump. A properly designed suction line to a centrifugal pump should have a frictional head loss of only a few feet of liquid. However, having a large-diameter suction line, and a relatively small draw-off nozzle, usually will lead to excessive loss of available NPSH. 

Figure 7-76 shows a vertical reboiler with a high draw-off nozzle. Using this figure, the driving force is ... 

Once more, the total system loss (Af), known densities and exchanger length (H4) will give the distance between the draw-off nozzle and the reboiler bottom tubesheet Hi"). If the elevation difference between the draw-off and return nozzle is other than 3 feet, the correct dimension should be inserted in the Hi" equation above. 

For startup, a 2 or 3-inch diameter, gravity-flow by-pass is usually provided from the tower liquid space to a low point of the downcomer, at reboilers with high liquid draw-off nozzles. 

It is desirable to place the draw-off nozzle in a portion of the tray floor that is lowered to form a sump. This lowers the height of liquid on the chimney tray by an amount equal to the nozzle diameter and reduces liquid leakage and the mass of liquid that the tray must support. The outlet nozzle should be flush with the sump floor to ensure adequate drainage. This practice is particularly fruitful in leak-tight applications, because it eliminates th> need for weep holes. 

The above conditions assume two relatively pure liquids. The presence active agent or fine dispersed solids can interfere with the coalescing process and result in a stable emulsion. Many liquid-liquid separators form a stable emulsion at the interface called a rag layer because of these agents and may require draw-off nozzles near the interfoce to prevent accumulation. The rag layer is like foam in liquid-gas systems and is typically stabilized by very fine solids. If the rag is drawn off it may be de-emulsified or broken by filtration, heating, chemical addition, or reveising the phase that is dispersed. 

In most services, the water boot LC will have to be run on manual. If there is more than one water draw-off nozzle, only leave one open to the... 

If specified, reboiler connections are usually located at the bcMtom section of the tower. For the horizontally mounted thermosiphon reboiler, the drawoff nozzle is located just below the bottom tray. For the vertically mounted recirculating thermosiphon reboiler, the draw-off nozzle is located at the bottom head. For both systems, the return nozzles are located just above the liquid level. Exhibit 10-31 shows both of these arrangements. 

Vortex breaker Located in draw-off nozzle sumps, or on the liquid outlet of the bottom of a vessel. Not required for low nozzle outlet velocities. 

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