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Module 5: Flowcharts and Ventilation Systems - Flowcharts -Diagrams
Objective
- Diagnose possible problems of the industrial process system using a flowchart.
Flowcharts can serve many purposes and therefore many levels of sophistication in flowchart preparation exist. Some of the most complex are design-oriented piping and instrumentation drawings (termed P & I drawings), which show every major component, valve, and pipe within the system. Even a drawing for a relatively simple system (or part of a system) can have more than 500 separate items shown on it. Conversely, a simple block diagram used as a field sketch may have only 3 to 5 symbols on the drawing.
Flowcharts for air pollution control studies should be relatively simple. Generally, you need more equipment detail than shown on a simple block diagram, but far less information than provided by the standard P & I drawing. The flowcharts should not be so cluttered with system design details that it is difficult to include present system operating conditions to help identify health and safety risks and performance problems. Since these are primarily "working" drawings, they must be small enough to be carried easily while walking around the facility. The flowcharts should not also require a lot of time to prepare or to revise.
For these reasons, an expanded block diagram flowchart has been adopted for use in these modules. In this type of flowchart, only the system components directly relevant to the study are included. Major components such as baghouses are shown as a simple block rather than a complex sketch resembling the actual baghouse. Most minor components and material flow streams are omitted to avoid cluttering the drawing.
The size of the flowchart is designed so that it fits entirely on a single 8½ in. by 11 in. page and can be carried on a standard clipboard or in a notebook. Furthermore, most of the standard symbols are reproduced on the back of the flowchart sheet.
An example flowchart for a relatively complicated air pollution source, a waste solvent incinerator, is shown in Figure 1. The process equipment in this example consists of a starved air modular incinerator with primary and secondary chambers. The air pollution control system consists of a venturi scrubber followed by a mist eliminator.
The primary and secondary chambers of the waste solvent incinerator have been shown separately because data from each chamber is important to the inspection. However, many components of the incinerator and wet scrubber systems have not been shown because their operating conditions are not central to the potential air pollution emission problems or health and safety problems.
Another flowchart example is shown in Figure 2. This is a simple wet scrubber system serving a recycle operation in a hot mix asphalt plant. Most of the plant is not shown since the scrubber only controls the particulate emissions from the mixing of hot, new aggregate with cold, aged recycled asphaltic concrete. It is apparent in Figure 2 that the duct labeled as section C serves as the discharge point. The liquid recycle pond is shown using an irregular shape and with a slightly different form of cross hatching so that it is easy to differentiate between the pond and the major equipment items.
It should be noted that the symbols for the major pieces of equipment and the symbols for other parts of the system should be located in logical positions. For example, the pond in Figure 2 is placed near the bottom of the sketch, and the stack is in a relatively high location.
The following Example Problems illustrate how flowcharts can be helpful during the inspection of air pollution control systems. They serve as a tool for organizing relevant data and determining what needs further investigation.
Follow these steps when evaluating the overall system:
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Determine whether or not the operating data is consistent and logical.
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Compare current data against site-specific baseline data.
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Determine specific areas that may need emphasis during the inspection.
- Determine potential health and safety problems that may be encountered during the inspection.
Example Problem 1.
Inspection of a Soil Remediation Unit
A regulatory agency is conducting an inspection of a soil remediation unit at a hazardous waste site. This site is an abandoned chemical plant where several nonvolatile carcinogens (chlorinated organic compounds) are present in old lagoons. The plant uses a rotary kiln for destruction of the carcinogens and two side-by-side pulse jet fabric filters for control of particulate matter generated in the kiln. Based on the data shown in Figure 3 (Present Situation) and Table 1 (Baseline Data), determine the following:
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Is the operating data for the system consistent and logical?
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Do any important discrepancies exist between the current and baseline data?
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What areas of the facility should be emphasized during the inspection?
- What health and safety issues should be considered during the inspection?
Solution:
Part i
Determine if the operating data for the system is consistent and logical. There should be logical trends in the gas temperatures, gas static pressures, gas oxygen concentrations (combustion sources) and other parameters along the direction of gas flow.
For this example, the gas temperature and static pressure data are listed in Tables 2 and 3 in the direction of gas flow.
The gas temperature and static pressure trends through the system are both logical. The gas temperatures are at a maximum at the discharge of the combustion source, and they decrease throughout the system. The gas temperature at the fan outlet is not provided for this example. Note that sometimes gas temperature at the fan outlet is higher than that at the fan inlet due to compression that occurs as the gas moves through the fan (the Joule-Thompson effect). The static pressures become progressively more negative as the gas approaches the fan. After the fan, the static pressure of the system significantly increases, as expected. Since the set of plant instruments provides consistent and logical profiles through the system, they are probably relatively accurate.
Solution:
Part ii
Compare the current data against the site-specific baseline data to the extent that it is available.
Step 1. Compare the current temperature data against the site-specific baseline data.
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Evaluate the temperature data for Duct B using Figure 4 and Table 2.
The 160°C temperature drop (from 819°C to 659°C) in Duct B between the kiln and the evaporative cooler is relatively new (see Figure 4). The baseline data indicates that the previous temperature drop was 25°C due to radiative and convective heat losses from the refractory-lined duct. The significantly higher temperature drop presently occurring across this short section of ductwork indicates that air infiltration is probably happening. This air infiltration could reduce the amount of combustion being pulled from the kiln and thereby cause fugitive emissions from the kiln. A check for fugitive emissions should be included in the scope of the inspection.
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Evaluate the destruction efficiency of the rotary kiln using the kiln outlet temperature data. See Figure 4 and Table 2.
The primary function of this portable plant is to incinerate the contaminated soil. It is apparent from the flowchart (Figure 3) that the most useful single parameter for evaluating the destruction efficiency of the rotary kiln system is the kiln outlet temperature monitored by the temperature gauge on the left side of Duct B. The present value of 819°C compares well with the baseline data obtained during the trial burn tests in which the unit demonstrated good performance. Accordingly, it appears that the unit is presently in compliance.
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Evaluate the temperature data for the evaporative cooler. See Figure 4 and Table 2.
The evaporative cooler is important primarily because it protects the temperature-sensitive Nomex® bags used in the downstream pulse jet baghouses. It is clear from the flowchart that presently there is a gas temperature drop of 425°C (765°F) across the evaporative cooler. This fact combined with an observed outlet gas temperature of 234°C demonstrates that this unit is operating as intended. It is not necessary to climb to the top of the unit to check the spray nozzles.
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Evaluate the temperature data for the baghouse. See Figure 4 and Table 2.
The flowchart data indicates there is a severe temperature drop across the baghouse(28°C). This should be included in the field evaluation.
Step 2. Compare the current pressure drop data against the site-specific baseline data.
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Evaluate the static pressure data across the kiln using Figure 5 and Table 3.
The pressure readings are in agreement for the baseline data and the present data.
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Evaluate the static pressure data from the evaporative cooler inlet to the baghouse outlet. See Figure 5 and Table 3.
The baseline static pressure drop is 4.1 in. W.C. compared with a present pressure drop reading of 2.2 in. W.C. Pressure drops across evaporative coolers tend to remain constant. However, the pressure drop across baghouses can vary due to changes in emission loading or a malfunction. An increase in emission loading is directly related to a pressure drop increase. A decrease in pressure drop may result from air leakage at the bag connention points. Air leakage can also occur due to worn or torn bags. The chlorinated organic compounds heated to high temperatures in the kiln could release the acid gas, HCl, into the system, which could damage the bags.
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Evaluate the static pressure data from the baghouse exit to the stack. See Figure 5 and Table 3.
The static pressure increase created by the fan (3.6 in. W.C.) is similar for the baseline and present conditions. The current static pressure drop from the fan exit to the stack (-0.5 in. W.C.) is also in agreement with the baseline value.
Solution:
Part iii
Determine the areas that should be emphasized during inspection. They are as follows:
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Check for air infiltration in Duct B.
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Check for fugitive emissions from rotary kiln.
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Investigate reasons for temperature drop of the pulse jet baghouses.
- Check for air leakage across the pulse jet baghouse and check the bags for damage due to wear and tear and/or acid attack.
Solution:
Part iv
Determine what health and safety issues should be considered during the inspection.
The pulse jet baghouse should be one of the main areas evaluated during the field portion of the inspection. However, this work must be conducted carefully in order to minimize safety hazards. The roof of the unit should be avoided because it is an uninsulated metal surface at 176°C (349°F). The soles of safety shoes could begin to melt and thereby cause a fall. Furthermore, there is a slight possibility of falling through the roof of the baghouse. The gas temperature drop of 28°C across the baghouse indicates severe air infiltration that may be caused by corrosion. If so, the roof may have been weakened. Corrosion is very likely in this process due to the formation of hydrochloric acid and water vapor in the kiln.
The waste being burned in this portable plant includes several suspected carcinogens. This should be noted on the flowchart to serve as a reminder to stay out of areas where inhalation problems or skin absorption hazards could exist.
Summary of Health and Safety Issues
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Avoid roof of pulse jet baghouse.
- Remain aware that chemicals in process are possible carcinogens. Avoid areas where inhalation or absorption may become dangerous.
Example Problem 2.
Evaluating the Performance of a Venturi Scrubber
A company is routinely evaluating the performance of a venturi scrubber serving a hazardous waste incinerator. They are using an Enhanced Monitoring Protocol that is based on the static pressure drop gauge across the venturi. Answer the following questions based on the data shown in Figure 6.
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Is there any reason to believe that the venturi scrubber pressure drop gauge is malfunctioning?
- Is there any reason to be concerned about fugitive emissions from the emergency bypass stack? (The emergency bypass stack has the stack cap covering the outlet.)
The present data and the corresponding baseline data are provided in Tables 4 and 5.
Solution:
Part i
First, evaluate the quality of data before attempting to evaluate the system. There should be logical trends for the static pressures, gas temperatures, and other relevant parameters.
The static pressure and pressure drop data have been combined into a single graph (Figure 7), which can be used to evaluate the static pressures along the entire gas flow path. It is apparent that present static pressure drop data for the venturi scrubber does not make sense. The present mist eliminator inlet static pressure and fan inlet static pressure data suggest that the static pressure drop across the venturi scrubber should be higher than indicated by the gauge. It is quite possible that the venturi scrubber pressure drop gauge is malfunctioning and that the actual static pressure drop is relatively similar to the baseline value of 36 in. W.C.
Solution:
Part ii
There is no reason to suspect fugitive emissions from the emergency bypass stack. The static pressures upstream and downstream of the bypass stack are negative. Accordingly, ambient air could leak into a poorly sealed stack. Untreated combustion gas could not escape through gaps in the stack seal.
A flowchart of the process system can be used to:
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Identify changes in control device performance due to process changes
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Identify instruments that are not consistent with other similar instruments in the system
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Communicate effectively with other personnel
- Avoid potential health and safety hazards
Flowcharts used for agency inspections should be prepared prior to or in the early stages of the inspection. If flowcharts for the system being inspected have been prepared previously, they should be reviewed prior to the on-site work and updated as necessary.
Practice Problems
Flowcharts - Diagrams
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.