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Safety & Environment
HAZOP

Hazard and Operability Study (HAZOP)

The Hazard and Operability Study (HAZOP) is a structured method for identifying hazard scenarios and is well accepted across the chemical industry1. The method begins with identification of “nodes” or a process location where deviations from the design intent will be evaluated. Standard guidewords are used to suggest possible deviations.

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HAZOP Study Terms

Node is a location on a process diagram at which process parameters are investigated for deviations.

      Node examples:

  • Pipeline transferring material from one process unit to another

  • Specific tank or vessel

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 Design Intent defines how the system is expected to operate at the nodes. It provides a point of reference for developing deviations.


Parameter is an aspect of the process that describes it physically, chemically or in terms of what is happening.

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  • Specific parameters: flow, temperature, pressure, etc.

  • General parameters: addition, reaction, maintenance, relief, etc.

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Deviations are discovered by systematically applying the guidewords to each parameter at each node (e.g., more + temperature = higher temperature).

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Selecting the correct node is important for an efficient HAZOP. Too small & you will repeat the same discussion with the same deviations. Too large & the team will be confused about what is the actual deviations being discussed or scenarios may be missed.
 

For HAZOP Studies performed as part of a larger Risk Analysis, it is convenient to define a Node as a specific equipment item such as a column, tank or vessel, pipeline, heat exchanger, pump, etc. All material and energy inputs into the equipment item would be included in the Node. This approach facilitates estimating release rate used for determining incident outcomes.

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Design Intent is typically summarized as a sentence which includes:


• Material or chemical involved include its physical state, quantity, temperature and pressure.
• Activity such as heating, reacting, vaporizing, condensing, freezing, transferring (flow rate), etc.


• Location:
- From tank, vessel, reactor
- At tank, vessel, reactor
- Through pump, compressor, filter
- To reactor, vessel, column

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Guidewords are simple words or phrases used to qualify or quantify the design intent and associated parameters in order to discover deviations.

HAZOP Guidewords

Not all possible deviations are meaningful. It is the HAZOP team’s job to identify meaningful deviations with credible causes.
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Additional HAZOP Study Terms

• Causes are the reasons why deviations may occur. Causes can be equipment failure, human error or external events.

• Consequence may be documented as the Loss Event or Impacts resulting from the Loss Event.

• Recommendations are suggested actions to prevent or mitigate the consequence of deviation, or to obtain further information.
 
Safeguards are the systems in place that reduce the probability of the deviation occurring or mitigate the severity of the consequences. Examples include:
o Dikes
o Drainage
o Instrument systems
o Fire protection systems
o Documented mechanical integrity programs
o Documented sampling or other procedures
 
Preventive Safeguard is one that intervenes between an Initiating Event and the Loss Event – it prevents the Incident from occurring. 
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Mitigating Safeguard is one that is designed to reduce the Loss Event impact.
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HAZOP Documents
The recording process is an important part of the HAZOP Study. The person assigned to scribe the meetings must be able to distill the pertinent results from the myriad of conversations that occur during the meetings. It is impossible to manually record all that is said during the meetings, yet it is very important that all important ideas are preserved.

HAZOP

Each Deviation of a Design Intent is documented with the Possible Causes, Consequences, Safeguards and Recommendations.

Stainless Steel corrosion resistance & industry practices

Stainless steels can be susceptible to certain localised corrosion mechanisms, namely crevice corrosion, pitting, intercrystalline corrosion, stress corrosion cracking and bimetallic (galvanic) corrosion. Localised corrosion is often associated with chloride ions in aqueous environments.

Corrosion resistance relies on a good supply of oxygen. Higher levels of chromium, nickel, molybdenum and nitrogen increase resistance to localised corrosion. The corrosion table is not a exact reproduction of reality, it is a first determination.
The criterion for corrosion in the case of general corrosion is expressed as weight loss by unit of time and surface area, for example grammes by square meter and per hour, or loss of thickness per unit of time, for example mm per year.

The loss of weight by unit of time and surface area is commonly used for laboratory tests, the loss of thickness per unit of time is commonly used in practice.

The general accepted practical limit within a material is considered corrosion resistant, is 0.1mm per year.

Practices used in industry are :
 

  • Corrosion rate less than 0.1 mm/year.  The material is corrosion proof.
     

  • Corrosion rate 0.1 - 1.0 mm/year. The material is not corrosion proof, but useful in certain cases.
     

  • Corrosion rate over 1.0 mm/year.Serious corrosion. The material is not usable.

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