Please enable JavaScript.

Coggle requires JavaScript to display documents.

PROCESS HAZARD - Coggle Diagram

- - - - Pressure reversal – a high pressure section of the process loses pressure, process fluid then flow into that section back from low pressure section of the process
  - - - Blocked-in Pump
      - Polymerization
      - External Fi
  - - - Liquid trap inside the pump while it is still running can increase the liquid temperature because the energy being added by the effect of impeller rotation inside the pump
    - - In polymer chemistry, polymerization, is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks.
      - The inadvertent polymerization of a chemical in a vessel can cause an explosion were it to occur rapidly enough.
    - - A fire on the outside of a vessel can cause the vessel’s internal temperature and pressure to rise rapidly leading to possible vessel rupture due to weakened metal.
      - Relief valves are usually designed to handle the pressure rise caused by external fire.
      - However it is useful to check that there are no unusual external fire situations that might overload the relief valve and that may have not been considered in the desig
  - - - not adequate instrument,
      - lack of automatic alarm or
      - operation running at 100% tank capacity,
      - lack of interlock function
  - - - One means of preventing this problem is to install cathodic protection, i.e., to put an impressed voltage on one part of the system so as to change its potential and reduce the corrosion.
      - The different sections can be electrically isolated from one another by putting insulating gaskets between the flange faces, and by placing insulation under the nuts on the flange bolt
- - - - Do not allow permanent connections between the nitrogen system and either the plant or instrument air systems.
      - Utility nitrogen stations should be clearly marked and have special connectors and hoses which are not common to any other system. Universal air hose connections (crow’s foot) should not be used in nitrogen service.
      - Locations where backup nitrogen is being used should be monitored and alarmed for low oxygen concentration, and signs and barriers installed.
      - Once the problem with the instrument air system has been resolved, the nitrogen to instrument air cross connection must be removed
  - - - The check valve fails to fully close.
      - Process chemicals flow into the utility header from the process line.
      - Process chemicals are then distributed to many other locations in the facility via the utility header
      - The pressure in the utility header falls (say due to an operating upset) so that it is lower than the pressure in the process line.
- - - - A 10-mile section of line was being dewatered following a hydrostatic test. A foam pig driven by compressed air was used to displace the water. The water was being removed from a 12inch bypass line.
      - The pig got stuck; the gas pressure was increased until it reached approximately 400 psig. The end of the temporary trap was also opened up.
      - At this point, the pig was seeing downstream pressure of ambient and an upstream pressure of 400 psig. The differential force was enormous.
      - In order to catch the pig, a large front-end loader was placed in front of the open trap.
      - The pig shot out of the trap, flipped the loader and continued to fly approximately 150 yards in the air, destroying a wooden platform along the way and flipping a front-end loader on to its side
- - - - The dense water could cause a downstream pump to suddenly shut down.
      - BS&W layers can pose a particular problem when switching tanks. If a process unit is taking feed from one tank, then the operator switches to a second tank, thereby the process could suddenly be hit with a slug of water and/or BS&W.
    - - If water is trapped in a confined space and then heated to a temperature well above its boiling point, a violent physical explosion will occur. Such an explosion can occur if water is trapped under a layer of very hot oil.
      - For example, a tank may contain a residual heel of water following a plant turnaround. If, on start-up, very hot oil is pumped into the tank the water will reach a temperature far above its boiling point but will not immediately form a vapor because the liquid covering it suppresses boiling. However, when the water does vaporize it will expand very rapidly and create very high pressures.
      - Storage tanks are not designed for these pressures and so may disintegrate
    - - does have associated hazards, including the following:
        
        Uncontrolled flow of water away from a fire can transport burning liquids to other parts of the facility.
        
        The high conductivity of water can make its use dangerous near high-voltage equipment or power lines.
        
        The sudden application of cold water to hot piping or vessels can severely reduce the strength of the steel—even crack it (a critical factor in the Esso Longford incident
      - Water is a vital firefighting medium. However, the use of water to control fires
- - - - The first is that the firebox is filled with a flammable mixture of hydrocarbon gas and air (oxygen). If a burner is ignited, an explosion will occur.
      - The second concern is that a furnace is operating normally, and then the fuel gas supply fails thus causing the burner flames to go out. If the fuel gas supply is then restored, an unburned mixture of gas and air can enter the hot firebox and ignite
    - - The burner in fired heater system damaging the empty process fluid piping due too the control system indicate low temperature at output fluid stream.
      - One solution to the above problem is to put a flow measuring device on the fluid stream. If the flow of process fluid falls below a threshold value, the heater will be automatically shut down.
    - - When equipment items are used for more than one purpose there is a chance that a high-consequence hazard may be created.
      - For example, on one plant certain storage tanks were used to store a liquid that could freeze at ambient temperatures. Therefore, the tank contents were heated with internal steam coils.
      - However, during turnarounds, the tanks were used to store another chemical. This second chemical was a monomer that could not freeze at normal temperatures and that could, if heated, polymerize rapidly and exothermically. Consequently, if during the plant restart the operators omitted to clear the tanks of this second chemical before turning on the steam coils, the chemicals in the tanks could polymerize, leading to an accident.
    - - Many plants are controlled by distributed control systems (DCSs). A DCS system consists of a set of cards or boards. Typically, about eight signals come from the plant into the board, as shown in figure below. The DCS circuitry processes the incoming signal and calculates one or more output signals which adjust control valve positions
    - - If there is a failure on the input side, two things will happen:
        
        First, the transmitted values to the DCS screen will disappear; the operator will have no knowledge as to what those values are unless someone goes outside to check.
        
        The second consequence of the failure of the input card is that the signals that it controls will fail, usually in their last position. Hence the control valves that are being controlled will “freeze” in place (fail in place). Therefore, if the operation was running smoothly before the input side of the board failed, there should be no sudden change in the operation
      - It is important for the hazard's analysis team to
      - make sure that the output signals do fail in their present condition.
      - If they cause the affected control valves to fail closed or open, the operation will be affected much more dramatically.
      - Fail Open:
        When there is a loss of signal, the valve opens. This is typically chosen to prevent overpressure in the event of a blocked line or in case of catastrophic failure.
      - Fail Close:
        When the signal is lost, the valve will close. This may be chosen in the case of a steam injection well. Uncontrolled steam is extremely dangerous, so the valve closing automatically would be the best option to maintain control.
      - Fail in Place:
        The valve does not change or react when power is lost. This is ideal for applications where the process cannot be shut down or it is undesired to do so.