How HAZOPs Differ From Other PHAs
- Soter Software Team
- Oct 20
- 10 min read
A guide exploring how HAZOPs connect with HAZID, LOPA, FMEA, and Bowtie analyses across the project lifecycle
Hazard and Operability (HAZOP) studies are among the most widely used methods for identifying process risks in chemical, energy, and manufacturing facilities. Their structured approach enables engineers and safety professionals to systematically identify hazards, assess causes and consequences, and define safeguards to protect people, assets, and the environment.
However, HAZOPs are just one part of a broader toolkit of Process Hazard Analyses (PHAs), each of which should be applied at the appropriate stage of the design or operational lifecycle. Other PHA methods such as HAZID, SIL/LOPA, Bowtie, and FMEA studies each have distinct purposes, strengths, and applications. Understanding the differences between these analyses — and knowing when and how to apply them — is essential for effective risk management throughout a project.
This post explores how HAZOPs differ from other PHAs, when each type is typically used, and how integrating outputs across analyses can maximise their value for organisations.
Putting HAZOPs In Context
In earlier Knowledge Centre posts, we covered what a HAZOP (Hazard and Operability Study) is - a highly structured, systematic method for reviewing a process or system in detail.
HAZOPs are primarily used to:
Identify operational hazards and potential failures.
Verify that safeguards and controls are in place.
Generate actionable recommendations for design, operation, and maintenance.
The strength of HAZOP lies in its detailed, systematic approach, however, HAZOPs are just one type of Process Hazard Analysis (PHA). There are several other PHA methods, each suited to different stages of a project lifecycle.
To fully understand the context and purpose of HAZOPs, it’s helpful first to define what a PHA is and how the broader PHA toolkit supports risk management throughout a project lifecycle.
What is a Process Hazard Analysis
A Process Hazard Analysis (PHA) is a systematic approach used to identify, analyse, quantify (where appropriate) and manage risks associated with hazardous processes. A PHA takes a structured view of a designed system - such as a plant or facility - and examines potential hazards, their causes, consequences and safeguards needed to minimise risk to anyone or anything that could be harmed if the system does not operate as intended. Typically, the focus is on maintaining the core safety triad of people, environment and assets.
PHAs are essential for ensuring that safety is built into both the design and operation of a facility. They help organisations anticipate problems before they occur, reduce the likelihood of accidents, and maintain compliance with regulations and industry standards. Because facilities evolve over time - as equipment is modified, plants are expanded, and regulations or economic imperatives change - PHAs play a crucial role throughout the entire lifecycle, from concept design through to decommissioning. Revisiting and updating these analyses ensures that an organisations understanding of its process risks remain accurate and aligned with each facility’s current condition and operation.
There are several PHA methods, each with a different focus, level of detail, and stage in the facility lifecycle. Choosing the right analysis at the right stage is critical for effective risk management. While this is not an exhaustive list, the most commonly used PHA methods include:
HAZID (Hazard Identification): a high-level review that identifies potential hazards during the early stages of design.
HAZOP (Hazard and Operability Study): a detailed, systematic study of design intent, deviations, consequences and safeguards during detailed design stage
SIL/LOPA (Safety Integrity Level / Layer of Protection Analysis): a semi-quantitative method used to evaluate the adequacy of safeguards identified in a HAZOP and determine the required Safety Integrity Levels (SILs) for safety instrumented systems.
FMEA (Failure Modes and Effects Analysis): a component-level analysis that examines how individual equipment failures could affect overall system performance and safety.
Bowtie Analysis: Visualises risk scenarios, linking causes to consequences and control barriers in a single, easy-to-understand diagram.
Understanding how these analyses differ, and how the output from one can feed into another, is key to building a comprehensive safety strategy.
Let’s look at each of these in a bit more detail.
HAZID (Hazard Identification)
HAZID is a high-level, qualitative PHA method used to identify potential hazards in a process or system at the early stages of design. It takes a broad view of the overall process and its inherent risks and is typically conducted as a team-based brainstorming session facilitated by a safety expert.
A HAZID study is usually performed during the concept or feasibility phase of a project, when there is some level of information about the system’s purpose, its inputs and outputs, but the detailed design is still being developed. At this stage, the available information is often limited to high-level process flow diagrams, but the team generally needs to know:
Facility location and layout
Chemicals used and their properties
Expected operating conditions such as temperatures and pressures
Applicable regulatory and industry safety requirements
HAZIDs are particularly valuable when evaluating new processes, facility expansions, or modifications to existing systems. By identifying risks early, HAZID enables design teams to consider safety before significant investment is made in equipment or layout.
HAZID findings feed directly into the design as it develops and are then revisited in a HAZOP study once the design is mature. Essentially, HAZID sets the foundation for subsequent, more detailed analyses and ensures that potential hazards are considered from the early stages of the design process.
HAZOP (Hazard and Operability)
As covered in our Beginner’s Guide to HAZOP Studies, a HAZOP (Hazard and Operability Study) is a detailed, structured PHA method used to identify hazards and operational issues in a process or system once the design has matured. Unlike HAZIDs, which takes a high-level view, HAZOPs examine systems in depth, focusing on deviations from design intent and their potential consequences. HAZOPs may be qualitative or semi-quantitative, depending on whether risk assessment calculations are included in the study.
A HAZOP is typically performed at several stages of a project lifecycle. The first study usually takes place during the latter part of Front-End Engineering Design (FEED) or the Detailed Design phase, when process and operational parameters are well defined. Subsequent HAZOPs are conducted periodically throughout the facility’s lifecycle - to confirm that hazards and risks remain controlled as the plant ages, or during process modifications and expansion projects.
At this stage, building on the information available from HAZID, the team will typically have access to well-defined documentation and data, including:
Process and instrumentation diagrams (P&IDs)
Operating procedures and control philosophies
Chemical properties, process conditions,
Equipment specifications
Applicable regulatory and industry safety requirements
HAZOPs are particularly valuable for uncovering subtle or complex risks that may not have been apparent during earlier studies like HAZID. By examining the process in detail, the study ensures that credible hazards are identified, safeguards are validated, and actionable recommendations are generated for design, operation, and maintenance.
The outputs from a HAZOP study inform operational procedures, maintenance strategies, and follow on risk analyses, such as SIL/LOPA, feeding directly into ongoing safety management processes.
In essence, HAZOPs transform early hazard identification into detailed, actionable insights. They form the cornerstone of a robust, safe, and operable design, and helps crystallise the design in order to allow the next phase of procurement and construction to occur smoothly.
LOPA (Layer of Protection Analysis)
Layer of Protection Analysis (LOPA) is a semi-quantitative PHA method used to evaluate the adequacy of safeguards identified in earlier studies, such as HAZOP, and to determine the required Safety Integrity Levels (SILs) for safety instrumented systems.
LOPA sits between qualitative analyses (like HAZID and HAZOP) and full quantitative risk assessments (QRAs). It focuses on understanding whether existing layers of protection in a design or system are sufficient to reduce risk to a tolerable level.
LOPA is commonly applied when a HAZOP identifies a scenario where the estimated risk exceeds acceptable limits. For example, if a HAZOP identifies the potential for one or more fatalities following a failure of a critical item such as a safety valve (a Safety Instrumented Function (SIF)) a LOPA is used to determine how much additional reliability is required to reduce that risk. The level of reliability /performance needed is expressed as a Safety Integrity Level (SIL).
There are four SIL levels (SIL 1 to SIL 4), with SIL 4 representing the highest level of risk reduction and reliability, and SIL 1 the lowest. Each level corresponds to a target probability of failure on demand (PFD) and the degree of risk reduction the system must provide.
SILs are defined in technical standards which govern the functional safety of electrical, electronic, and programmable systems, such as:
In practice, determining a required SIL involves assessing:
The frequency and severity of potential hazardous events.
The independence and effectiveness of protection layers.
The tolerable risk criteria established for the facility.
A LOPA study provides the quantitative justification for assigning SILs, ensuring that each SIF performs reliably enough to reduce risk to an acceptable level.
In essence, LOPA connects risk analysis with engineering design, ensuring that safety systems protecting critical processes are correctly specified, maintained, and verified throughout the facility’s lifecycle.
FMEA (Failure Modes and Effects Analysis)
Failure Modes and Effects Analysis (FMEA) is a semi-quantitative, component-level PHA method used to identify how individual equipment or component failures could affect the performance, reliability, or safety of a system. Unlike process specific studies such as HAZOP or LOPA, which focus on operational hazards and deviations from the process design intent, FMEA examines failures at the equipment or component level, making it particularly valuable for understanding the mechanical or functional integrity of designs.
An FMEA study is typically carried out during the detailed design phase, when the design of critical equipment, control systems, or instrumentation is well defined. It can also be applied during operation or maintenance planning, especially for complex or safety-critical systems such as compressors, reactors, or control loops.
During an FMEA, each component or subsystem is reviewed to identify possible failure modes — ways in which it might fail to perform as intended — and the potential effects of those failures on the wider system.
During the study, the team evaluates:
The cause of each failure mode (e.g., mechanical fatigue, corrosion, control failure),
The effect on system operation or safety,
The likelihood of occurrence and detectability of the failure, and
The severity of its consequence.
In some cases, these factors are combined into a Risk Priority Number (RPN), allowing the team to prioritise which failures require corrective actions or additional safeguards.
FMEA is particularly useful for identifying vulnerabilities that may not emerge in process-level analyses, such as the impact of an instrument fault or valve malfunction on the safe operation of a plant. The findings from FMEA studies often feed into maintenance strategies, reliability-centred maintenance (RCM) programmes, or more complex analyses such as Fault Tree Analysis (FTA) and Quantitative Risk Assessment (QRA).
In essence, FMEA bridges the gap between design detail and operational reliability — ensuring that equipment integrity and functional safety are built into the system from the ground up, and that the design remains robust throughout its lifecycle.
Bowtie Analysis
Bowtie Analysis is a PHA that takes a different approach by focusing heavily on visuals which illustrate how hazards are managed within a process or system. It presents a diagram linking the potential causes of a hazardous event on the left, to its possible consequences on the right, with preventive and mitigative barriers displayed in between — forming the shape of a bowtie. This approach makes complex risk information accessible and easy to understand, helping stakeholders at all levels grasp how risks are controlled, where key safeguards exist, and where potential vulnerabilities may lie.
A Bowtie Analysis can be conducted at almost any stage of a project or operational lifecycle, but it is particularly valuable after detailed hazard identification such as HAZOP or LOPA has been completed. It takes the findings from these analyses and presents them in a simplified, visual format that highlights:
The initiating causes of major accident scenarios
The controls or safeguards in place to prevent those scenarios
The mitigations that limit the impact if an event does occur
Bowties are widely used for major accident hazard management, regulatory compliance, and safety leadership. They are especially useful in demonstrating to senior management, regulators, and external stakeholders how risks are being managed in a transparent and structured way. Bowties:
Provide a clear, visual summary of complex risk scenarios
Link causes, consequences, and barriers in a single view
Improve understanding and communication of risk management to non technical audiences
Help identify barrier weaknesses or gaps for follow-up action
In essence, Bowtie Analysis translates detailed technical study outputs — like those from HAZOP or LOPA — into a form that supports decision-making, operational awareness, and continuous improvement across all levels of an organisation.
Leveraging Outputs Across The Different Process Hazard Analyses
The value of PHAs increases significantly when the outputs from one analysis feed seamlessly into the next
HAZID → HAZOP: Early hazard identification from HAZID provides a foundation for the detailed examination carried out in HAZOP, ensuring that potential risks are explored thoroughly as the design matures.
HAZOP → LOPA: The detailed deviations, consequences, and safeguards identified during a HAZOP can be used to assess risk reduction quantitatively, supporting informed decisions about required Safety Integrity Levels (SILs).
HAZOP/LOPA → Bowtie: Recommendations and risk scenarios from HAZOP studies can be visualized in Bowtie diagrams, providing a clear, intuitive overview that communicates risk management strategies effectively to both technical and non-technical stakeholders.
By maintaining visibility across all PHA outputs, organisations gain a clear understanding of how hazards are identified, assessed, and controlled throughout the facility lifecycle. This integrated perspective helps to:
Reduce duplication of effort: Teams can build on existing analyses rather than repeating work at each stage.
Ensure data consistency: Accurate, aligned information across studies reduces gaps, miscommunication, and errors.
Enable proactive decision-making: Clear insights allow engineering, operations, and management teams to identify potential weaknesses early, prioritize mitigations, and prevent incidents before they occur.
Ultimately, connecting the outputs from each PHA creates a cohesive, transparent view of process risk, making it easier to manage hazards proactively, support continuous improvement, and maintain operational safety across the lifecycle.
Summary
Process Hazard Analyses (PHAs) are essential tools for understanding, managing, and mitigating risks in chemical, energy, and manufacturing facilities. Each method — from HAZID and HAZOP to LOPA, FMEA, and Bowtie — serves a distinct purpose, stage, and level of detail within the project lifecycle.
By understanding how these analyses differ and how their outputs can be leveraged together, organizations can create a cohesive, integrated view of process risk. This approach enhances visibility, improves decision-making, and supports proactive safety management.
Bringing it all together, these key takeaways show how each PHA method plays a distinct but complementary role in strengthening process safety and operational integrity.
There are several types of PHAs: Each type serves a different purpose and is best applied at specific stages of the facility lifecycle — from early concept (HAZID) to detailed design and operations (HAZOP, LOPA, FMEA, Bowtie).
Each PHA has a distinct purpose: HAZID identifies early hazards, HAZOP examines deviations in detail, LOPA quantifies safeguards, FMEA assesses equipment-level failures, and Bowtie visualises risk scenarios for clarity.
Integration amplifies value: Linking outputs between PHAs reduces duplication, ensures data consistency, and creates a connected, organisation-wide view of risk.
Visibility supports proactive safety: Accessible, traceable PHA insights enable teams to identify vulnerabilities early and take informed preventive action.
Communication strengthens understanding: Sharing PHA findings with the right people, in the right way, builds shared awareness and supports more proactive, informed risk management across the organisation.
Together, these methods form a connected framework for managing process safety - turning technical analysis into actionable insight that can help build safer, more resilient operations.