Hazards in process plant design

3.a) In case of a leak occurring n-heptane, persons who are not wearing protective equipment and clothing should be probhited from polluted areas until cleanup has been completed. The following steps should be undertaken when ever leakage occurring:

  • This should be brought to the notice of the safety personnel.
  • Immediately the sources of heat and ignition should be stopped and removed.
  • Explosive atmospheres must be ventilated.
  • Do not handle the spilled material with bare hands and leaks should be stopped so as to reduce the risk.
  • Use non-sparking tools.
  • Water spray may be used to reduce vapors.
  • For small liquid spills, sand or other non-combustible absorbent material and place into closed containers for disposal.
  • For large liquid spills, build dikes far ahead of the spill to contain the n-heptane for later reclamation or disposal.[1]

In this scenario leakages may arise due to following reasons:

  • Failure of relief valves
  • worn out flange gaskets
  • leakages in delivery piping
  • Leakages in filling line

These kind of leakages may lead to accidental fire and based on the extent of damage, the working area is classified into various hazardous zones.

Hazardous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere, as follows:

Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods;

Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation;

Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a shor

Zone 0: in this case the tanker and the storage tank are considered to be under zone 0.n-heptane is transferred to the storage tank by means of pipe line referred as filling line. Due the presence of large of amount n-heptane in the storage tank, in case of an fire accident the explosive gas atmosphere will be created and prolong for long periods. Due to n-heptane flammability and low flashpoint.

Zone 1: in this case the entire working area is assumed to be under Zone 1. Zone 1 extended 3 meters because of safety purpose due to the spreading of explosive gas atmosphere, the zoning is extended by certain area. The Relief valve present in this region prevents increase in pressure after a certain threshold and at the same time prevents creation of vacuum within the storage tank. In case of an emergency relief valve acts as a fuse. The filling line and delivery piping are used to transport n-heptane from the source to the storage tank and from storage tank to the tanker respectively, the accident that may occur in this region can be handled at normal operating conditions. The leakages may occur in the pipe lines at the joints due to worn out gaskets and in proper maintenance.

Fire Hazards during Transfer to tanker

The transfer of n-heptane from a fixed roof storage tank to a tanker truck is a complex process that involves the active participation of both the representative of a fixed roof storage tank and a tanker truck driver. Here we should take some safety precautions that are necessary.

After the truck is reached the filling area, the tanker truck engine is shut off, a grounding cable is attached to the truck to ground any electrostatic discharge.

A flexible transfer pipe is attached to the tanker and eliminated with n-heptane to remove all air.

Fixed roof storage tank operator opens the storage tank and the driver will open the tank main liquid valve to fill the tank.

The tank driver will control the pressure in the tanker via a pressure building line where n-heptane is vaporized and returned to the storage tank to maintain a pressure differential between the storage tanker and the tanker truck, The pressure differential at least 15 psi.

To compensate for differential expansion, The driver will use a mechanical means to maintain a tight connection at the hose coupler.

The safety features that are typical of storage tank transfer of n-heptane include equipment design such as tanker valves that are interlocked with the truck brake system to prevent fuel transfer from storage tanker, the storage tanker is properly secured before transfer; remote-controlled, redundant liquid valves; storage vessel alarms to prevent overfill; and long drain lines for safety-directing vented n-heptane vapor.

The difficulty of the n-heptane transfer arrangement generates the possible for leakage through equipment failure and human error. One of the particular concerns is that n-heptane transfer equipment goes through a continuous cycle of cool down to cryogenic temperatures and warm up to ambient temperature. This type of thermal cooling can create additional stresses on equipment and sealing devices which could result in decreased reliability over time.[3]

Fires involving n-heptane should be fought upwind from the maximum distance possible. Isolate the area and keep the people as far as possible and deny entry until the fire is extinguished. The area up to half a mile radius is isolated from the source of hazard. The fires must be controlled using hose holders or monitor nozzles especially massive fire in a cargo area. In case this is not possible, the area must be completely isolated and the fire must be allowed to burn. Emergency personnel should avoid low areas and ventilate the place completely before entering the place. Flash back of fires occurs if vapors travel to a source of ignition. N-heptane must be placed away from the fire area so as to prevent it from heat of fire of the explosion. If this is not possible, then water is sprayed from the sides until the fire is put off. Emergency personnel must be aware of the situation and act accordingly. Firefighters must be provided with a full set of protective gears and self-contained breathing apparatus when fighting fires involving heptane.[4]

References:

[1][4] Health guidelines: [Online]. Last accessed 10 December 2009 at: http://www.osha.gov/SLTC/healthguidelines/heptane/recognition.html

[2] R J Brown .area classification. Gases vapours and mists will be covered. [Lecture handout]. From hazards in process plant design and operation, held on 4 November, university of Sheffield.

[3] Fire winds: [online]. Last accessed 11 December 2009 at: http://www.chebeague.org/fairwinds/risks.html

Diagram [1] from hazards in process plant design and operation assignment question paper module (CPE6200) held on 2 to 5 November 2009.

hazardous zones in plant design and operation in transfer n-heptane to tanker

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