Dimensions inMLT$
sd Design stress ML 1T 2
ss Safe working stress ML 1T 2
Re Reynolds number —
NPSHavail Net positive suction head available at the pump suction
L NPSHreqd Net positive suction head required at the
pump suction
L
5.13. PROBLEMS
5.1. Select suitable valve types for the following applications:
1. Isolating a heat exchanger;
2. Manual control of the water flow into a tank used for making up batches of sodium hydroxide solution;
3. The valves need to isolate a pump and provide emergency manual control on a bypass loop;
4. Isolation valves in the line from a vacuum column to the steam ejectors producing the vacuum;
5. Valves in a line where cleanliness and hygiene are an essential requirement.
State the criterion used in the selection for each application.
5.2. Crude dichlorobenzene is pumped from a storage tank to a distillation column.
The tank is blanketed with nitrogen and the pressure above the liquid surface is held constant at 0.1 bar gauge pressure. The minimum depth of liquid in the tank is 1 m.
The distillation column operates at a pressure of 500 mmHg (500 mm of mercury, absolute). The feed point to the column is 12 m above the base of the tank. The tank and column are connected by a 50 mm internal diameter commercial steel pipe, 200 m long. The pipe run from the tank to the column contains the following valves and fittings: 20 standard radius 908elbows; two gate valves to isolate the pump (operated fully open); an orifice plate; and a flow-control valve.
If the maximum flow rate required is 20,000 kg/h, calculate the pump motor rating (power) needed. Take the pump efficiency as 70% and allow for a pressure drop of 0.5 bar across the control valve and a loss of 10 velocity heads across the orifice.
Density of dichlorobenzene 1300 kg=m3, viscosity 1.4 cp.
5.3. A liquid is contained in a reactor vessel at 115 bar absolute pressure. It is transferred to a storage vessel through a 50 mm internal diameter commercial steel pipe. The storage vessel is nitrogen blanketed, and pressure above the liquid
5.13. PROBLEMS 293
surface is kept constant at 1500 N/m2gauge. The total run of pipe between the two vessels is 200 m. The miscellaneous losses due to entry and exit losses, fittings, valves, etc., amount to 800 equivalent pipe diameters. The liquid level in the storage vessel is at an elevation 20 mbelowthe level in the reactor.
A turbine is fitted in the pipeline to recover the excess energy that is available, over that required to transfer the liquid from one vessel to the other. Estimate the power that can be taken from the turbine, when the liquid transfer rate is 5000 kg/h. Take the efficiency of the turbine as 70%.
The properties of the fluid are density 895 kg=m3, viscosity 0:76 mNm 2s.
5.4. A process fluid is pumped from the bottom of one distillation column to another, using a centrifugal pump. The line is standard commercial steel pipe 75 mm internal diameter. From the column to the pump inlet, the line is 25 m long and contains 6 standard elbows and a fully open gate valve. From the pump outlet to the second column, the line is 250 m long and contains 10 standard elbows, 4 gate valves (operated fully open), and a flow-control valve.
The fluid level in the first column is 4 m above the pump inlet. The feed point of the second column is 6 m above the pump inlet. The operating pressure in the first column is 1.05 bara and that of the second column 0.3 barg.
Determine the operating point on the pump characteristic curve when the flow is such that the pressure drop across the control valve is 35 kN=m2.
The physical properties of the fluid are density 875 kg=m3, viscosity 1:46 mN m 2s.
Also, determine the NPSH, at this flow rate, if the vapor pressure of the fluid at the pump suction is 25 kN=m2.
Pump Characteristic
Flow rate, m3/h 0.0 18.2 27.3 36.3 45.4 54.5 63.6 Head, m of liquid 32.0 31.4 30.8 29.0 26.5 23.2 18.3
5.5. Revisiting the problem of example 5.3, suppose the flow was controlled using a plug-disk globe valve, and the initial design in the example assumed that the valve is fully open. What range of flow rates can be achieved if the valve can be throttled down to one-quarter open? When the valve is one-quarter open, what fraction of the pump work is lost across the valve?
5.6. Estimate the shaft work required to pump 65 gal/min of sugar solution in water (specific gravity¼1.05) if the pump inlet pressure is 25 psig and the outlet pressure required is 155 psig.
5.7. A shell and tube cooler in an aromatics complex cools 26,200 lb/h of naphtha (specific gravity 0.78, viscosity 0.007 cP). The cooler has 347 tubes, 16 ft long, 3/4 inch diameter. If the naphtha is on the tube side, estimate the tube side pressure drop.
294 CHAPTER 5 PIPING AND INSTRUMENTATION
5.8. In a detergent-making process, 1400 gal/h of water flows through a 2-inch pipe system as follows:
Exit from pump, 2 ft vertical, open gate valve, 14 ft vertical, 908 bend, 12 ft horizontal, 1/4 open globe valve, 20 ft horizontal, 908bend, 6 ft hori- zontal, 908 bend, 12 ft vertical, 908 bend, 14 ft horizontal, 908 bend, 4 ft vertical, 908bend, 28 ft horizontal, open gate valve, 3 ft horizontal, entry to tank containing 30 ft of liquid.
a. If the pump and tank are both at grade level, estimate the head that the pump must deliver.
b. If the pump inlet pressure is 25 psig, what is the outlet pressure?
c. Estimate the pump shaft work.
d. If the pump is powered by an electric motor with 85% efficiency, what is the annual electricity consumption?
5.9. A polymer is produced by the emulsion polymerization of acrylonitrile and methyl methacrylate in a stirred vessel. The monomers and an aqueous solution of catalyst are fed to the polymerization reactor continuously. The product is withdrawn from the base of the vessel as a slurry.
Devise a control system for this reactor, and draw up a preliminary piping and instrument diagram. The follow points need to be considered:
1. Close control of the reactor temperature is required.
2. The reactor runs 90% full.
3. The water and monomers are fed to the reactor separately.
4. The emulsion is a 30% mixture of monomers in water.
5. The flow of catalyst will be small compared with the water and monomer flows.
6. Accurate control of the catalyst flow is essential.
Devise a control system for the distillation column described in Chapter 11, Example 11.2. The flow to the column comes from a storage tank. The product, acetone, is sent to storage and the waste to an effluent pond. It is essential that the specifications on product and waste quality are met.
5.13. PROBLEMS 295
