The following engineering design guidelines are useful for process design engineers or process design consultants who provide engineering design services to clients in the chemical & petrochemical industry, manufacturing, mining & mineral processing, oil & gas, pharmaceutical, renewable energy, semiconductor and water & wastewater industries. These guidelines may provide valuable assistance with respect to the design of process plants.
MANAGING CONFINED SPACES
Confined spaces pose dangers because they are usually not designed to be areas where people work. A ‘confined space’ is defined as an enclosed or partially enclosed space that: is not designed or intended to be occupied by a person, is, or is designed or intended to be, at normal atmospheric pressure while any person is in the space; and is or is likely to be a risk to health and safety from: an atmosphere that does not have a safe oxygen level, or contaminants, including airborne gases, vapours and dusts, that may cause injury from fire or explosion, or harmful concentrations of any airborne contaminants, or engulfment.
Confined spaces often have poor ventilation which allows hazardous atmospheres to quickly develop, especially if the space is small. The hazards are not always obvious and may change from one entry into the confined space to the next. The risks of working in confined spaces include loss of consciousness, impairment, injury or death from: the immediate effects of airborne contaminants, fire or explosion from the ignition of flammable contaminants, difficulty rescuing and treating an injured or unconscious person, oxygen deficiency or immersion in a free-flowing material, such as grain, sand, fertiliser, water or other liquids, falls from a height, environmental factors, for example extremes in temperature, poor lighting, and manual handling.
This code is based on a national model code of practice developed by Safe Work Australia shows how risks associated with working in confined spaces can be managed and mitigated.
SIZING PRESSURE-RELIEF DEVICES
A pressure relief valve protects process equipment from the hazards of high (or low) pressure in a process. It operates by opening at a designated pressure and ejecting mass from the process equipment. The ejected mass contains energy - the removal of the energy reduces the process pressure. Sizing the pressure relief valve incorrectly may lead to undesirable results such as ejecting insufficient amount of mass as a function of time.
This article, published by American Institute of Engineers (AIChE), provides an introduction to sizing a pressure relief valve. The article is provided for educational purpose only and readers are recommended to consult experienced professional engineers to conduct this task.
MAJOR HAZARD FACILITIES (MHFs) ASSESSMENT GUIDELINES
The Work Health and Safety Regulations (the WHS Regulations) require operators of determined major hazard facilities (MHFs) to conduct a safety assessment in order to provide a detailed understanding of all health and safety risks associated with major incidents.
The purpose of this Guide is to assist operators of MHFs to prepare and conduct a safety assessment in accordance with the WHS Regulations.
The guidance has been prepared for operators of MHFs from all sectors: processing, storage and warehousing, notwithstanding the significant differences in complexity. Examples have been given where possible to illustrate possible application to each sector. Applicability will depend on the specific circumstances of the MHF. Operators are advised to refer to reputable texts or engage suitable specialists when choosing to apply a specific technique.
This guidance, prepared by Safework Australia, will provide: assurance to the operator that the potential risk of major incidents will be eliminated or controlled, a detailed understanding of all aspects of risks to health and safety associated with major accidents, the production of a documented safety assessment that meets the requirements of the regulations and which can be used to form part of the safety case submitted for licensing.
LOWER AND UPPER EXPLOSIVE LIMITS (LEL/UEL) FOR FLAMMABLE GASES AND VAPOURS
Before a fire or explosion can occur, three conditions must be met simultaneously. A fuel (ie. combustible gas) and oxygen (air) must exist in certain proportions, along with an ignition source, such as a spark or flame. The ratio of fuel and oxygen that is required varies with each combustible gas or vapor.
The minimum concentration of a particular combustible gas or vapor necessary to support its combustion in air is defined as the Lower Explosive Limit (LEL) for that gas. Below this level, the mixture is too “lean” to burn. The maximum concentration of a gas or vapor that will burn in air is defined as the Upper Explosive Limit (UEL). Above this level, the mixture is too “rich” to burn. The range between the LEL and UEL is known as the flammable range for that gas or vapor.
The values shown in this table are valid only for the conditions under which they were determined (usually room temperature and atmospheric pressure using a 2 inch tube with spark ignition). The flammability range of most materials expands as temperature, pressure and container diameter increase.
All concentrations in percent by volume.
HAZARD AND OPERABILITY STUDIES
An important element of any system for the prevention of major accidents is conducting a hazard and operability study (HAZOP) at the detail design stage, of the plant in general and the operating and safety control systems in particular. HAZOPs seek to minimise the effect of an atypical situation in the operation/process by ensuring that control and other safety systems such as functional safety (e.g. emergency safe shutdown) are in place and work with a high level of reliability to achieve a safe outcome from a situation that could have resulted in a major accident.
The HAZOP process is used to identify potential hazards and operational problems in terms of plant design and human error. The technique is applied during final design of the process and plant items before commencement of construction.
HAZOPs have also proven to provide financial benefits to the plant owner/operator by minimising the time and money spent in installing add on control and safety systems, the need for which may become evident at the time of plant commissioning in the absence of a HAZOP. On the operability front benefits are gained by implementing at design stage, the remedial recommendations to operability issues identified during the HAZOP.
This advisory paper, published by the NSW government, aims to provide guidance to all persons associated with the design and operation of a facility to appreciate the need for a HAZOP and also the general procedure that is followed in carrying out a HAZOP and reporting the study results. It gives a broad indication of what is required in undertaking a HAZOP with a list of references for further study.
HOW TO REDUCE HUMAN FAILURE
There is a recurring number that despite magnificent developments in technical safety doesn’t seem to go away. That number is 80, and it relates to the percentage of incidents that in some way has been contributed to by a human. It is a recurring average across industry and in different settings. Looking on the bright side, it means that there is huge scope to make a significant improvement. So how can we reduce the rate at which people contribute to incidents? The first step is to recognise that human failure is not random, but systematically linked to the tasks that people perform, the equipment they use, and the characteristics of the work environment. By understanding these elements, it is possible to reduce the potential for errors to occur in the first instance, or at least to pre-empt them so that when they do occur, they can be safeguarded against.
This article, published by The Chemical Engineer, provides an overview of human failure, the factors that contribute to it and the process by which the risks can be reduced.
ECONOMIC PIPE DIAMETER
When designing a pipeline or a piping network for process plants, care should be taken to ensure the pipe and equipment are sized correctly. Specifying an oversized pump whilst under sizing the pipe diameter may increase pump purchase and operating costs. In contrast, specifying an undersized pump coupled with an oversized pipe diameter will reduce pump related costs, yet increase the purchase and installation costs of pipes. The correct combination of pump and pipe diameter need to be specified to present the most economical piping system.
This article aims to demonstrate a calculation method to determine with reasonable accuracy the most economical pipe diameter, taking into consideration the installation cost of pipe and pump operating cost throughout its design life. This article is complimentary can be downloaded by clicking the icon on the left and follow the instructions accordingly.