This chapter is from the book
This chapter is from the book
This chapter is from the book
1.7 Summary
In this chapter, you have learned that the three principal types of diagrams used to describe the flow of chemical streams through a process are the block flow diagram (BFD), the process flow diagram (PFD), and the piping and instrumentation diagram (P&ID). These diagrams describe a process in increasing detail.
Each diagram serves a different purpose. The block flow diagram is useful in conceptualizing a process or a number of processes in a large complex. Little stream information is given, but a clear overview of the process is presented. The process flow diagram contains all the necessary information to complete material and energy balances on the process. In addition, important information such as stream pressures, equipment sizes, and major control loops is included. Finally, the piping and instrumentation diagram contains all the process information necessary for the construction of the plant. These data include pipe sizes and the location of all instrumentation for both the process and utility streams.
In addition to the three diagrams, there are a number of other diagrams used in the construction and engineering phase of a project. However, these diagrams contain little additional information about the process.
Finally, the logic for equipment placement and layout within the process is presented. The reasons for elevating equipment and providing access are discussed, and a 3-D representation of a DME plant is presented.
The PFD is the single most important diagram for the chemical or process engineer and will form the basis of much of the discussion covered in this book.
Short Answer Questions
What are the three principal types of diagrams used by process engineers to describe the flow of chemicals in a process? On which of these diagrams would you expect to see the following items:
- the temperature and pressure of a process stream
- an overview of a multiple-unit process
- a major control loop
- a pressure indicator
- a pressure-relief valve
- A problem has occurred in the measuring element of a level-indicating controller in a batch reactor. To what principal diagram should you refer to in order to troubleshoot the problem?
- Why is it important for a process engineer to be able to review a three-dimensional model (actual or virtual/electronic) of the plant prior to the construction phase of a project?
- Name five things that would affect the locations of different pieces of equipment when determining the layout of equipment in a process unit.
- Why are accurate plant models (made of plastic parts) no longer made as part of the design process? What function did these models play and how is this function now achieved?
Problems
- There are two common reasons for elevating the bottom of a tower by means of a “skirt.” One reason is to provide enough NPSHA for bottoms product pumps to avoid cavitation. What is the other reason?
Which of the principal diagrams should be used to do the following:
- Determine the number of trays is a distillation column?
- Determine the top and bottom temperatures in a distillation column?
- Validate the overall material balance for a process?
- Check the instrumentation for a given piece of equipment in a “pre-start-up” review?
- Determine the overall material balance for a whole chemical plant?
- What is the purpose(s) of a pipe rack in a chemical process?
- When would a structure-mounted vertical plant layout arrangement be preferred over a grade-mounted horizontal in-line arrangement?
A process that is being considered for construction has been through several technical reviews; block flow, process flow, and piping and instrumentation diagrams are available for the process. Explain the changes that would have to be made to the three principal diagrams if during a final preconstruction review, the following changes were made:
- The efficiency of a fired heater had been specified incorrectly as 92% instead of 82%.
- A waste process stream flowrate (sent to a sludge pond) was calculated incorrectly and is now 30% greater than before.
- It has been decided to add a second (backup) drive for an existing compressor.
- The locations of several control valves have changed to allow for better operator access.
During a retrofit of an existing process, a vessel used to supply the feed pump to a batch reactor has been replaced because of excessive corrosion. The vessel is essentially identical to the original one, except it is now grounded differently to reduce the corrosion. If the function of the vessel (namely to supply liquid to a pump) has not changed, answer the following questions:
- Should the new vessel have a new equipment number, or should the old vessel number be used again? Explain your answer.
- On which diagram or diagrams (BFD, PFD, or P&D) should the change in the grounding setup be noted?
Draw a section of a P&ID diagram for a vessel receiving a process liquid through an insulated 4″ sch 40 pipe. The purpose of the vessel is to store approximately 5 minutes of liquid volume and to provide “capacity” for a feed pump connected to the bottom of the pump using a 6″ sch 40 pipe. The diagram should include the following features:
- The vessel is numbered V-1402 and the pump(s) are P-1407 A/B.
- The discharge side of the pump is made of 4″ sch 40 carbon steel pipe and all pipe is insulated.
- A control valve is located in the discharge line of the pump, and a double block and bleed arrangement is used (see Problem 1.13 for more information).
- Both pumps and vessel have isolation (gate) valves.
- The pumps should be equipped with drain lines that discharge to a chemical sewer.
- The vessel is equipped with local pressure and temperature indicators.
- The vessel has a pressure relief valve set to 50 psig that discharges to a flare system.
- The tank has a drain valve and a sampling valve, both of which are connected to the tank through separate 2″ sch 40 CS lines.
- The tank level is used to control the flow of liquid out of the tank by adjusting the setting of the control valve on the discharge side of the pump. The instrumentation is similar to that shown for V-104 in Figure 1.7.
A standard method for instrumenting a control valve is termed the “double block and bleed,” which is illustrated in Figure P1.13.
Figure P1.13 Double Block and Bleed Arrangement for Problem 13
Under normal conditions, valves a to c are open and valves d and e are closed. Answer the following:
- Explain, carefully, the sequence of opening and closing valves required in order to change out the valve stem on the control valve (valve b).
- What changes, if any, would you make to Figure P1.13 if the process stream did not contain a process chemical but contained process water?
- It has been suggested that the bypass valve (valve d) be replaced with another gate valve to save money. Gate valves are cheap but essentially function as on-off valves. What do you recommend?
- What would be the consequence of eliminating the bypass valve (valve d)?
- Often, during the distillation of liquid mixtures, some noncondensable gases are dissolved in the feed to the tower. These noncondensables come out of solution when heated in the tower and may accumulate in the overhead reflux drum. In order for the column to operate satisfactorily, these vapors must be periodically vented to a flare or stack. One method to achieve this venting process is to implement a control scheme in which a process control valve is placed on the vent line from the reflux drum. A pressure signal from the drum is used to trigger the opening or closing of the vent line valve. Sketch the basic control loop needed for this venting process on a process flow diagram representing the top portion of the tower.
- Repeat Problem 14, but create the sketch as a PI&D to show all the instrumentation needed for this control loop.
Explain how each of the following statements might affect the layout of process equipment:
- A specific pump requires a large NPSH.
- The flow of liquid from an overhead condenser to the reflux drum is gravity driven.
- Pumps and control valves should be located for easy access and maintenance.
- Shell and tube exchanges may require periodic cleaning and tube bundle replacement.
- Pipes located at ground level present a tripping hazard.
- The prevailing wind is nearly always from the west.
Estimate the footprint for a shell-and-tube heat exchanger from the following design data:
- Area = 145 m2
- Hot side temperatures: in at 300°C out at 195°C
- Cold side temperature: bfw at 105°C mps at 184°C
- Use 12 ft, 1″ OD tubes on a 1-1/4″ square pitch, use a single shell and tube pass because of change of phase on shell side
- Use a vapor space above boiling liquid = 3 times liquid volume
Make a sketch of a layout (plot plan only) of a process unit containing the following process equipment:
- 3 reactors (vertical – diameter 1.3 m each)
- 2 towers (1.3 and 2.1 m in diameter, respectively)
- 4 pumps (each mounting pad is 1 m by 1.8 m)
- 4 exchangers (footprints of 4 m by 1m, 3.5 m by 1.2 m, 3 m by 0.5 m, and 3.5 m by 1.1 m)
The two columns and the 3 reactors should all be aligned with suitable spacing and all the exchangers should have clearance for tube bundle removal.
Using the data from Table 1.7 estimate the footprints of all the equipment in the toluene HDA process.
- For the shell and tube exchangers, assume 12 ft, 1.25″ tubes on a 1.5″ square pitch and assume 2 ft additional length at either end of the exchanger for tube return and feed header.
- For double pipe exchangers, assume an 8″ schedule 20 OD and a 6″ schedule 40 ID pipe with a length of 12 ft including u-bend.
For the footprints of pumps, compressors, and fired heater, assume the following:
- P-101 use 2m by 1m, P-102 use 2m by 1m
- C-101 (+D-101) use 4m by 2m
- H-101 use 5m by 5m
- With the information from Problem 19 and the topology given in Figure 1.5, accurately sketch a plant layout (plot plan) of the toluene HDA process using a grade-mounted horizontal inline arrangement similar to the one shown in Figure 1.9. You should assume that the area of land available for this process unit is surrounded on three sides by an access road and that a pipe rack runs along the fourth side. Use the information in Table 1.11 as a guide to placing equipment.
What do the following symbols (as seen on a P&ID) indicate?
Determine all the errors in the following section of a P&ID.
Figure P1.22 A section of a P&ID to be used in Problem 1.22
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