Nuclear Production of Hydrogen, Fourth Information Exchange ...

USE OF PSA FOR DESIGN OF EMERGENCY MITIGATION SYSTEMS IN A HYDROGEN PRODUCTION PLANT
Initiating event
The initiating event corresponds to a leakage in any mechanical equipment, pipe or flange, located
between the vacuum column C-201 and the decomposition reactor E-207, as shown in Figure 6. Since
the pipeline will be under severe conditions of acidity, pressure and temperature, an occurrence
frequency of 5.23E-5/year is assumed, which is the generic value for catastrophic pipe failure
(including straight pipe and connections) (Bari, 1985).
End states
The “end state” is considered successful (OK) when the primary isolation system works without
complications. If this system fails, but the neutralisation is successful, there will be corrosion of
equipment and mechanical supports, and then, the final state will be “minimal damage”.
In the event of failure of the isolation and neutralisation systems, success with the EFS system,
the violent reaction of water and acid, coupled with the delayed action, would cause severe damage to
plant personnel. This situation is identified as the third end state: “personal injury”.
Finally, if the three mitigation systems failed, the formation of toxic cloud would be unavoidable
and therefore the final state would take this name (toxic cloud formation).
Fault trees
Based on the process architecture of the three mitigation systems: isolation, neutralisation and
flushing; fault trees were developed for each of them.
Failures considered in the system equipment
a) Valves: failure to open and to close, depending on its function and location.
b) Pumps: failure to start and to keep running (when applicable).
c) Catastrophic failure of: low pressure switches, analysers and leak detectors.
d) Failure in the electricity supply.
e) Mixers: failure to start.
f) Tanks: unavailability due to maintenance.
g) All mechanical equipment: unavailability due to maintenance.*
h) Systematic failures in bypass helium lines, which include: failure to close the main helium
line, failure in signal reception and valve actuators.
i) Common cause failures are considered where they apply.
Assigned probability to basic events
Although the process conditions are severe, the materials to be installed during the construction
phase must be the result of engineering studies, reducing the probability of failure at normal levels.
For this reason, generic data were taken for equipment and accessories.
Data for unavailability and lambdas were taken from the Guide to PSA (AICHE, 1989) for the
majority of mechanical equipment, valves and diesel generators; data for pressure switches, analysers
and stirrer, were taken from Table 5.1: “Data on selected process systems and equipment” (Bari, 1985).
The previous tables summarise criteria and parameters for the failure probabilities calculation,
beginning with a list of failure probability model definitions in Table 1, followed by specific data on
mechanical equipment in Table 2, and ending with some unavailability reported values in Table 3.
* Unavailability due to maintenance is not considered for the isolation system, because it is a passive system
sunk in the hydrogen production process, which is only manipulated when the plant is offline. If a human error
(during maintenance) were produced, it would be detected and corrected during the start-up sequence.
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