Process hazards Reverse Flow

Connections to equipment are typically 50 mm and 80 mm for process vessels and exchangers, according to the size of the equipment. Each connection includes an accessible block valve. Double block valves are provided if required. A check valve should be included if overpressure or other hazard could result from reverse flow during simultaneous drainage from more than one vessel. Individual connections from the equipment are made into the top of the drain header. 

Many accidents occur because process materials flow in the wrong direction. Eor example, ethylene oxide and ammonia were reacted to make ethanolamine. Some ammonia flowed from the reactor in the opposite direction, along the ethylene oxide transfer line into the ethylene oxide tank, past several non-return valves and a positive displacement pump. It got past the pump through the relief valve, which discharged into the pump suction line. The ammonia reacted with 30m of ethylene oxide in the tank, which ruptured violently. The released ethylene oxide vapor exploded causing damage and destruction over a wide area [5]. A hazard and operability study might have disclosed the fact that reverse flow could occur. 

Hazard and Operability Analysis (Hazop) (Kletz, 1992) is one of the most used safety analysis methods in the process industry. It is one of the simplest approaches to hazard identification. Hazop involves a vessel to vessel and a pipe to pipe review of a plant. For each vessel and pipe the possible disturbances and their potential consequences are identified. Hazop is based on guide words such as no, more, less, reverse, other than, which should be asked for every pipe and vessel (Table 1). The intention of the quide words is to stimulate the imagination, and the method relies very much on the expertise of the persons performing the analysis. The idea behind the questions is that any disturbance in a chemical plant can be described in terms of physical state variables. Hazop can be used in different stages of process design but in restricted mode. A complete Hazop study requires final process plannings with flow sheets and PID s. 

The known unknowns are the focus of most hazards analyses. For example, a process may involve the transfer of chemical from a high-pressure vessel to another vessel that operates at lower pressure. The designers of the system may not have considered the possibility that the pressure gradient could reverse, i.e., that the pressure in the first vessel could drop to a value less than that in the second vessel, thus creating the possibility of reverse flow. Such a scenario is a vaUd topic for the hazards analysis team to discuss. This scenario is a known unknown — it may not have been considered, but it is part and parcel of a normal hazards analysis. 

The discussions to do with hazards analyses that have been provided up to this point in this chapter have been predicated on an assumption that the unit being analyzed is a processing operation— typically a section of a refinery, chemical plant, or oil/gas production facility. However, the techniques that have been discussed can be used, when adapted appropriately, to other types of industrial operation. For example, the deviation guidewords of a HAZOP study (the technique is discussed in the next chapter) can be modified to address transportation issues, as illustrated in Table 5.7. Reverse Flow, e.g., becomes Vehicle, Train, or Ship Reverses. ... 

Process fluids (wrong hazards analyses/reverse flow/wrong composition)... 

While working a HAZOP of a water distribution system, we found that reversing flow for emergency reasons created additional hazards to our process. One of the new hazards was critical safety valves and pumping stations could be blocked from contamination from dirt picked up from reverse flow through our filters. 

As with High Flow, the phenomenon of Low Flow is not usually inherently hazardous. However, it can create secondary effects. For example, a low flow of cooling water in a heat exchanger can lead to High Temperature of the process stream. No Flow is usually more serious than Low Flow because its occurrence implies a sudden cessation of a processing activity. Probably, the biggest hazard associated with No How is the possibility of it being followed by Reverse Flow because the upstream and downstream pressures have equalized or even reversed. 

Reverse Flow can create high-consequence hazards because it can lead to the mixing of incompatible chemicals or to the introduction of corrosive chemicals into equipment not designed for them. The cause of Reverse Flow is usually a pressure reversal — a high-pressure section of the process loses pressure process fluids then flow into that section back from low-pressure sections of the process. (The occurrence of reverse flow almost invariably imphes that a check valve and/ or safety instrumented system has failed to prevent the event)... 

The hazards associated with (unexpected) reverse flow can be very serious. One particularly troublesome reverse flow scenario is Reverse flow to a utility header as illustrated in Figure 17.3, which shows two lines. The top line is a utility such as nitrogen, steam, or service air. The lower line shows a process stream containing a hazardous chemical. In normal operation, the utility flows into the process through a check valve (with block valves on either side of it). 

When utility hoses are connected to a process, it is particularly important to make sure that a backflow preventor (check valve) is installed. Otherwise, hazardous chemicals may reverse flow through the hose into another operating area. 

Some plant owners take the view that the complexity of the scheme creates a hazard because of potential misunderstanding of its operation by the process operator. One source of confusion is the use of the reciprocal of the air-to-fiiel ratio. Another is the air flow controller operating in equivalent fuel units. It is possible to reverse the scheme so that the ratio is on the fuel side, which resolves these issues but then means that the fuel flow control operates in equivalent air units. It is possible to reconfigure the scheme so that both ratios are used and both flow controllers work in their own units, as shown in Figure 10.22. 

Nozzle designs with positive shut-off devices have been successfully used. The gas must be free to escape through the nozzle. Material freeze off in the nozzle or malfunction of the positive shut-off device could develop pressure to cause blow-back of the material through the feed zone and hopper or create hazardous conditions. In such cases, conventional free flow and reverse taper type fitted with a heater band for temperature control of the nozzle prevents nozzle drool or freeze off of the material and is used and in nylon processing. Sprue cutter is associated with the nozzle to help the process. A nozzle forms a seal between the injection system and the mold. 

The flare or burn pit should be located remote from the facility and property line due to their inherent hazardous features. They should be well away from high hazard areas or public occupied areas. A location perpendicular to the prevailing wind direction, remote from the major sources of vapor releases and process or storage facilities, is preferred. A crosswind location is preferred since a downwind location may allow vapors to flow back to the plant during times when the wind direction is reversed, while a crosswind location has less possibility of this. See Figure 8.3. 

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