Hazard Identification Techniques

Depending on the extent of the consequences of the potential major hazards, the sources of hazard may be determined by simple means such as checklists, or by more complex methods such as HAZOP, FMEA, F&EI, and so on. The study uses the FMEA method to identify potential hazards related to the hydrogen system. The following section describes these methods.

The word of HAZOP is an abbreviation for "hazard and operability study". HAZOP [ 107] is a simple yet structured methodology for hazard identification and assessment. The basic principle of HAZOP study is the normal and standard condition is safe, and hazard occurs when there is a deviation from normal conditions. The procedure allows the user to make intelligent prediction in the identification of hazard and operability problems.

In a typical HAZOP study, design and operation documents such as piping and instrument diagrams (PID), process flow diagram (PFD), and operating manuals are examined systematically by a group of experts. Abnormal causes and adverse consequences for all possible deviations from normal operation that could arise are identified for each unit of the plant. HAZOP is considered by a multi-disciplinary team of experts who have extensive knowledge of design, operation and maintenance of the process plant. To cover all possible malfunctions in the plant the imagination of the HAZOP team members is guided systematically with a set of guide words for generating the process variation deviations. The list of guide words along with their definition is given in Table 4-1.

Table 4-1 Guide words and their physical significance [107]

Guide words

Meaning

Parameter

Deviation

No

Negation intention

Flow

No flow

Level

Zero level

Less

Quantitative decrease

Flow Level

Temperature

Low flow rate Low level Low temperature

More

Quantitative increase

Flow Level

Temperature

High flow rate High level High temperature

Reverse

Logical opposite

Flow Pressure

Reverse flow Reverse pressure

Part of

Qualitative decrease

Concentration

Flow

Level

Concentration decrease Flow decrease Level decrease

As well as

Qualitative increase

Concentration of impurity Temperature of substance Level of impurity Pressure of substance

Concentration increase Temperature increase Level increase Pressure increase

Other than

Complete substitution

Concentration of desired substance

Concentration zero

Level of desired substance Flow of desired substance

Level zero Flow rate zero

4.3.1.2 Failure Modes Effect Analysis

4.3.1.2 Failure Modes Effect Analysis

A Failure Modes Effect Analysis (FMEA) is a systematic and structured method for identifying product ad process problems, assessing their significance, and identifying potential solutions that reduce their significance. The objective of a FMEA is a look for all the ways a process can fail (failure modes). Each failure mode has a cause and a potential effect. Some failure modes are more likely to occur than others, and each potential effect has a relative risk associated with it. FMEA is an inductive and efficient method for analyzing elements which can cause the failure of the whole, or of a large part of a system. It is good for generating the failure data and information at components level [107]. It has been recommended for use as a hazard identification technique mainly for systems dealing with low/moderately hazardous operations and the ones which cannot support the expensive and time-consuming HAZOP study [3].

The FMEA procedure involves the following steps: identification of each failure mode, of the consequence of the event (s) associated with it, its causes and effects; classification of each failure mode by relevant characteristics, including deductability, diagnosability, testability, item replaceability, and compensating and operating provisions.

4.3.1.3 Fire and Explosion Index (Dow Index)

The fire and explosion index [1] is a step-by-step objective evaluation of the realistic fire, explosion, and reactivity potential of process equipment and its contents. The quantitative measurements used in the analysis are based on historic loss data, the energy potential of the material under study, and the extent to which loss prevention practices are currently applied. It was developed by Dow Chemical Company for fire and explosion hazards. The overall structure of the methodology is sown in Fig. 4.3. The procedure is to calculate the fire and explosion index (F&EI) and to use this to determine fire protection measures and, in combination with a damage factors, to derive the base MPPD (the maximum probable property damage). This is then used, in combination with the loss control credits, to determine the actual MPPD, the maximum probable day outage (MPDO), and the business interruption (BI) loss [1]

Fermenter Control Factor Pressure
Fig. 4.3 Calculation procedures of F&E Index [1]

In the F&E Index calculation the material factor of hydrogen is twenty-one. Appropriate penalty of tank pressure is determined by consulting to Eq. 4-1 and using the operating pressure to determine an initial value.

Penalty = 0.16109 +1.61503(p/6895) -1.42879(p/6895)2 + 0.5171(p/6895)3 (4-1)

For liquid hydrogen (LH2), the low temperature penalty is set to 0.30, hydrogen tanks use carbon steel and are operated at or below the ductile/brittle transition temperature. The penalty for the quantity of flammable/unstable material is calculated as follows: Flammable and combustible liquid or liquefied areas in storage outside the process area receive a lower penalty than those in the process, since there is no process involvement. The penalty is determined by using Fig. 4.4 with total kJ (i.e. quantity of material in storage times combustion heat factor, Hc) in any single storage vessel.

Flammable Liquid Class

Class I Flammable Liquid (F.P. < 37°C) . ■ Class Combustible Liquid (F.P. ^ >=37.8°C)

20 40 60 80 100

Total Quantity of Material (in 10*9 kJ)

Class I Flammable Liquid (F.P. < 37°C) . ■ Class Combustible Liquid (F.P. ^ >=37.8°C)

20 40 60 80 100

Total Quantity of Material (in 10*9 kJ)

Fig. 4.4 Penalty of liquids or gases in storage [1]

The process unit hazards factor (F3) is the product of the general process hazards factor (F1) and the special process hazards factor (F2). The product is used rather than the sum because the contributing hazards included in F1 and F2 are known to have a compounding effect on each other. When penalties are properly applied to various process hazards, F3 is normally not in excess of 8.0. If a higher value is obtained, use a maximum of 8.0 [1]. The F&E index for several hydrogen systems calculated in the study has ranges between 147 to 170, or the degree of hydrogen hazards are classified as "Heavy" to "Severe".

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Responses

  • Birgit
    How to do a fmea study n fermentaton plant?
    5 years ago

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