Draw nozzle

Bottom draw nozzle too small. Pump cavitation problem. Raising tower 10 feet did not help. Flooded the bottom of the tower. Design error in original plant. 

They lack sufficient available NPSH to overcome the frictional losses in the suction piping and the drain or draw nozzle. 

It is positively my experience that the most common reason for pumps cavitation is partial plugging of draw nozzles. This problem is illustrated in Fig. 25.5. This is the side draw-off from a fractionator. Slowly opening the pump s discharge control valve increases flow up to a point. Beyond this point, the pump s discharge pressure and discharge flow become erratically low. It is obvious, then, that the pump is cavitating. 

Many draw nozzles, especially those in the bottom of vessels, plug because of the presence of vortex breakers. Many designers routinely add complex vortex breakers to prevent cavitation in pumps. But vortex breakers are needed only in nozzles operating with high velocities and low liquid levels. Corrosion products, debris, and products of chemical degradation can more easily foul and restrict nozzles equipped with vortex breakers. 

Under normal service conditions, the liquid collector is a separate tray from any liquid distribution device. The basic collector is a chimney tray with a sump (Fig. 9). The chimneys provide passage for vapor rising through the tray. The sump(s) provides for liquid drainage into a draw nozzle (for product draw) or... 

Upon retray, the new trajrs were rotated to their original orientation. Internal piping was required from the new intermediate product draw sump to its draw nozzle and level gage. A flange on the internal draw line leaked, and starved the line of liquid. Poor separation resulted. The internal level gage lines plugged. 

Assembly drawings—nozzle orientations for gas inlet/exit/thermocouples/sam-pling points/bottom drain point Specifications of brick lining and filter mass Sample of filter mass used External insulation and cladding Quantity of dust removed dming shutdown... 

I have a personal dislike of this widely used device. The idea of the vortex breaker is to prevent swirling liquid from conveying vapor into the draw-off nozzle. Sometimes though, trash lost in the tower during a turnaround gets caught on the vortex breaker. Since the draw nozzle typically is located in a sump below the seal pan, it may be hard to inspect. In some cases, it would be best to just cut off the vortex breaker. This is fine if the nozzle exit velocity is less than 3 or 4 ft/s. 

It is positively my experience that the most common reason for pumps cavitation is partial plugging of draw nozzles. This problem is illustrated in Fig. 36.5. This is the side draw-off from a fractionator. 

The vortex breaker is used in almost all liquid-draw nozzles. There are several types of vortex breakers used the most common one is shown as Figure 3.7. 

In this design, vertical baffles are used to break the vortex. The number of baffles used depends on the nozzle diameter. Table 3.11 is often used as a preliminary guideline to design a vortex breaker. The position of fhe draw nozzle is also important a vessel-flushed nozzle may not be always recommended, particularly if the incoming fluid contains inert or gummy material. The internal nozzle projection (50 to 100 mm) is often used to avoid draining of solids along with the liquid stream. 

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