07 Oct A wide variation in ignition probabilities
A recent project in the GCC has been the catalyst for this article from Peter Stephenson, Business Development Manager, Warringtonfire, which highlights the typical process and hazards/risks that should be considered when assessing the fire & life safety requirements for a desalination plant.
With the global increase in population coupled with the need for industrial and agricultural development to support improvement in living standards, there is a growing need for adequate supplies of potable water that is bacteriologically pure with low salinity. Throughout the world, there is an increasing shortage of good quality fresh water which results in a greater need for desalination of sea and brackish waters. Some governments and water utilities are slow in accepting this solution to their problems which may be due to the unfamiliar technology, lack of expertise and high cost estimates.
The success of the various desalination processes can be attributed to three basic factors which are directly related to and responsible for this success:
1. The correct selection of tried and tested construction materials that are suitable for the conditions found at the plant location. The selection of these materials must consider any inherent corrosive nature of the environment, any adverse conditions that are relevant to the particular site and which might affect the performance of the materials selected and the general feed water characteristics.
2. Selection of the most appropriate desalination process for the relevant location, this may be either a membrane or a distillation system. The process must be suitable for the local conditions and the finished water characteristics required. The choice of energy source must be appropriate for the location and the process. This may include waste heat from a power station or industrial process, or mechanical or electrical energy.
In the case of distillation processes it is vital to make the right selection of design criteria that involves conservative temperatures and heat flux rates, fluid velocities and flow rates. In the case of membrane processes it is important to select adequate pre-treatment regimes that are appropriate for the specific feed water characteristics and to ensure the application of conservative operating pressures and fluid transfer rates to maximise membrane life. The use of energy recovery systems wherever possible is also economically important.
3. Strict adherence to detailed Operation and Maintenance Procedures by well trained and experienced plant operators and maintenance technicians coupled with the implementation of dedicated maintenance regimes involving both scheduled and preventive maintenance programs reduces the possibility of plant breakdown and improves the economy of operation.
Adherence to these three principles should ensure the success of the selected plant by providing the optimum economical life of the equipment together with the lowest possible plant operating costs commensurate with the locality, energy costs and other inherent physical and financial conditions.
What are the risks associated with a desalination plant?
To successfully understand the hazards and risks present at a desalination plant we must first understand the processes and system operations so that a risk analysis can cover all activities of interest. A desalination plant is an industrial facility that uses chemical or physical processes to reduce or recover salt concentrations from water supplies intended for drinking or industrial purposes. Removing salt from the source water renders it fit for human consumption and reduces the occurrence of salt-induced corrosion of any metal in contact with the treated water.
Desalination plants primarily make use of one or more of the following processes for water treatment:
- Reverse osmosis
- Thermal multistage flash distillation
- Mechanical vapor compression
Desalination plants (DPs) have a high level of corrosion risk because they handle large quantities of concentrated saltwater. Plant operating conditions often include distillation, agitation and high salt water flow rates. The prolonged combination of these mechanisms accelerates the corrosion rate and becomes a risk for plant failure etc.
Brackish water and brine cause localised forms of corrosion such as pitting, crevice, galvanic and stress corrosion cracking. Desalination plants may attempt to reduce the corrosion rate by using copper-nickel alloys as a structural material to construct equipment. These alloys demonstrate superior corrosion resistance in aerated and deaerated seawater at various temperatures.
The reverse osmosis method is a popular methodology adopted and will be discussed further. In general, the main treatment unit includes the following stages:
- Seawater intakes and pumps
- Pre-treatment
- Membrane Reverse Osmosis separation unit
- Post-treatment
These stages are generally grouped into two main physical areas on a site and are further split into various subsystems with associated hazards as shown in Table 1 opposite.
Within the industry there is often statistics and data available relating to past accidents and incidents in various plant types. Accidents involving transformers (fire and explosion), accidents causing pipeline leaks and electrical faults are not uncommon on such sites.
Risk assessment
Within the seawater desalination sector a good starting place when looking at site safety is to conduct a Quantitative Risk Assessment (QRA) which will assist in identifying and prioritising the process risks present on the facility and should provide recommendations on cost effective ways of reducing the overall site risk. A QRA will typically include both the risks to life and the capital production losses associated with incidents that could escalate within the site.
The application of a robust risk assessment methodology will define the probability of an event with undesirable consequences and can also give more specific meaning to the frequency with which there may be a major incident resulting in loss of life, or significant financial loss. A site review can be undertaken which would include a brief hazard identification study for those areas significantly different from normal operational sites. The detailed analysis of the frequency of incidents, event hazard ranges, the probability of escalation of an initial event and the calculation of financial risk levels should also be considered.
Across all hazardous sites there is a wide variation in ignition probabilities. These are due to the nature of the equipment present, processes undertaken and the differing sizes of potential incidents. In combination with assessing the likelihood of ignition further analysis should be undertaken including:
Event Consequence Analysis – This defines the risk potential for the site, the consequences of fire related to plant and personnel must be considered. This analysis should also consider the effect of ignited and unignited releases from equipment, pipework and vessels.
Escalation Analysis – It is often important to determine and gain an understanding of which events are likely to lead to further damage to the plant across and potentially beyond the site. Here it is essential to identify the most important contributors to the overall site risk. Escalation analysis, by its nature, tends to produce a large amount of data. A common output of this type of analysis highlights the frequency of an incident in one area escalating to another area.
Individual and Societal Risk Analysis – This assessment identifies the risk of fatality at any position on site and the risk of an accident with a given number of fatalities.
Financial Risk Analysis – The financial risk from an incident is defined by the frequency of an incident and the potential costs of the incident. The costs are divided between the total cost of an incident, including capital losses and production losses, the cost to the organisation and its brand as well as the local community.
A risk assessment of this type produces a large amount of information on the nature of the risks at a facility. In making the assessment, a number of assumptions also have to be made. These include estimates of the levels of equipment, shutdown and intervention probabilities, escalation paths, ignition probabilities and cost implications of events. There are uncertainties associated with all of these, but the approach used provides a best estimate of all of these and is consistently applied across the whole site.
In interpreting the results, it is the order of the risk which should be considered most closely. Similarly, in estimating the financial risk of an event, it is possible to derive an appropriate level of capital investment for reducing the risk by a certain factor. This is not meant to provide a definitive estimate of the capital which should be allocated on a risk reduction option, but rather the order or investment which can be justified; in some cases it is impossible to make much impact on the risk without major (very expensive) modifications to the plant, whilst in others a very simple modification may effectively reduce the risk by an order
of magnitude.
Competent staff
In addition to the general management and administrative skills necessary for the day to day operation of any major facility, the various desalination processes involve a broad spectrum of scientific and engineering technologies complemented by a number of trade crafts required for on-going operation and maintenance. It is vital that the personnel have sufficient theoretical and practical knowledge of chemical and physical sciences, as well as mechanical, electrical and chemical engineering, for the diagnostic control, chemical conditioning and general operation of the various processes.
Successful operation and maintenance of desalination facilities requires skilled mechanics, pump fitters, electricians, instrument technicians, electronic technicians and computer programmers to operate and maintain the equipment and the sophisticated control systems and computerised recorders and controllers. Present day “state of the art” desalination is a highly specialised and constantly expanding field, and the availability of qualified and experienced engineers, technicians and specialists with in-depth knowledge and relevant skills is extremely limited.
It is therefore of particular importance for any major desalination facility owner or operator to include a comprehensive training program to train new employees and to up-grade the skills of existing personnel. Such training may be in-house with company-employed instructors, by contractual arrangement with specialist training organisations, or a combination of both these options.
By engaging a competent and well trained workforce can have a very positive impact on reducing the overall fire
risk rating of a plant. A risk assessment is only one step in the overall approach to risk management and belongs to a dynamic approach. It is recommended that a risk assessment is a living document and should be reviewed on a regular basis to determine whether fire risks have been eliminated or whether other risks have arisen since the last assessment. The significant findings and actions from the assessments must be implemented, formalised and documented.