NPSH problems

FIgur 15.8 Pump and forced-circulation reboiler compatibility. (a) Oversized pump, expect NPSH problems (6) well-sized pump. 

Rotary screw (moyno) pumps Stator has one more lobe/screw than the rotor reduce rpm for abrasives starting torque = 4 X initial torque. If particles are present, try to minimize the abrasion by using viscosity > 5000 mPa.s. Backflow or slip is reduced as viscosity increases. NPSH problems are usually not important except for suction lift, pumping from a vacuum and fluid vapor pressures of > 15 kPa. 

A larger suction line sometimes eliminates NPSH problems. Also, many pumps do not have the maximum-size impeller that can fit in the existing pump case. Changing impellers is an inexpensive method to gain capacity if the motor can supply the extra horsepower (see Chapter 10 for debottlenecking motors). 

Trim gas oil (TGO) is a black oil stream withdrawn immediately above the vacuum-tower flash zone. It consists of 20%-50% resid and 50%-80% gas oil. This relatively light material may cause the bottoms pump to cavitate when it overflows its trapout pan at a nonuniform rate. Reduce the TGO pan level to see if this helps the NPSH problem on the bottoms pump. 

Excessive production of trim gas oil High-gravity resid Ejector deficiencies Boltoms-pump NPSH problems Low gas-oil draw temperatures Transfer-line failures... 

Place valves after pumps to avoid net-positive-suction-head (NPSH) problems. 

Cavitation pump running dry. Follow procedures recommended above for insufficient NPSH problem. 

An example of how to deal with a typical NPSH problem Ls shown in Exhibit 8-2. The required NPSH in this example is 22 ft (6,700 mm). If a horizontal pump is used, the bottom tangent line of vessel A must be a minimum of 22 ft (6,700 mm) above the centerline elevation of the shaft. If a vertical pump is used, the... 

FIG. H-21 Suction-type heater, normally installed in tank side nozzle to heat up relatively larger flows of viscous materials using steam, hot water, or other heating fluid in the tubes. These heaters normally have low shell side pressure drops to minimize npsh problems on pumps. For very high viscosity fluids, consult the manufacturer for methods of reducing intake viscosity. An Internal shutoff valve can be used to allow easy bunde removal. Main uses of this type heater Include bunker C, fuel oils, asphalt, molasses, caustic, etc. (Source Armstrong Engineering Associates.)... 

A factor of safety should be applied to ensure that NPSH does not become a problem. 

The number of turbulence or pressure drop producing fittings in the pump suction line should be kept to a minimum. Because of the excessive turbulent and friction loss that they produce, globe valves should not be used in the pump suction line. When NPSH or turbulence is a problem, a turbulence-reducing device should be used. This device should be located as near the pumps suction flange as possible. 

Avoid overheating FW tanks and the generation of steam in the tank. If the FW temperature is too high and there is inadequate net positive static head (NPSH) on the FW pump suction, the pump will not operate and cavitation problems may occur. 

The net positive head required (NPSHreqd) is a function of the design parameters of the pump, and will be specified by the pump manufacturer. As a general guide, the NPSH should be above 3 m for pump capacities up to 100 m3/h, and 6 m above this capacity. Special impeller designs can be used to overcome problems of low suction head see Doolin (1977). 

Therefore, the liquid level in the overhead condenser would have to be somewhere in the condenser s shell. But then, the liquid in the condenser would be below the reflux drum. How, then, does the liquid get from the lower elevation of the condenser to the higher elevation in the reflux drum We will have to explain this hydraulic problem later. But for now, we can say that most reflux drums are elevated 20 or 30 ft above grade to provide net positive suction head (NPSH) for the reflux pump. Also, most shell-and-tube condensers are located at grade, for easier maintenance during unit turnarounds. 

The last consideration is the problem of cavitation. The net positive suction head (NPSH) determined in the calculations, must be sufficiently high to prevent spontaneous vapourization of liquid in the event of a drop in suction pressure. 

The petfotinanoe of a forced circulation rebdler was poor, and was much the same whether power to the pump was on or off. Problem was caused by NPSH required exceeding NPSH available. Ensure pump system compatibility in forced-circulation reboQer systems. 

Since pumps always require extra NPSH on start-up, pumps with a marginal suction head tend to slip on start-up. To overcome this problem, proceed as follows ... 

Low suction pressure. The hydrocarbon in the vessel upstream of the pump may run colder than normal. This lowers the pressure (to bold the liquid at its bubble point) in the vessel. The pump then has to pul up more head and will consequently transfer less volume. This problem has nothing to do with inadequate NPSH. 

Example 18.8 illustrates how to do NPSH calculations and one of the methods listed above for increasing NPSHj . The other methods are illustrated in a problem at the end of the chapter. 

NPSH (Net Positive Suction Head), 627-631 positive displacement, 627 reciprocating, 627 and system curves, 617-618 separation problems Colburn equation, 636-639 Kremser equation, 634-636. 638-639 mass separating agents, 633-639 valves, 623-627 Performance factors, reactors case studies... 

Option Five—Install a condensate pump and condensate drum. Pump the condensate out under level control. The drum must be immediately next to and underneath the reboiler. Also, the drum must be high enough above the new condensate pump to provide sufficient net positive suction head (NPSH) for the pump. This is, of course, the preferred, if not the typical way of handling this very common design problem. 

Let s now assume that we wish to pump 300 GPM, not 250 GPM. If we open the flow[Pg.476]

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