Taking safety to the next level: part one

The hazard and operability (HAZOP) study is recognised worldwide as a primary methodology for hazard identification conducted for the oil, petrochemical and chemical industries for satisfying two basic requirements. The first is checking a new design for safety and operability problems whilst it is at the process instrumentation diagram stage. The second is the identification of latent safety and operability problems in existing plants, which have yet to be revealed during operations.

A HAZOP study provides benefits to the owner and contractor as it:

• Identifies improvements for the safe operation of the process unit at an earlier stage in the project, making it easier and usually significantly less expensive to make those changes.
• Reduces the chances of unplanned shutdowns.
• Significantly reduces time and costs for future HAZOP studies due to changes made to the process unit during construction or later revalidations (a government requirement in some parts of the world) with the use of the electronic version of the HAZOP study report.
• Helps answer questions during the training of operators and maintenance personnel about deviations or unusual scenarios that may occur in the operation of the process unit.
• Provides information for developing process unit specific operating and maintenance procedures.
• Identifies links to process equipment 'outside battery limits'.
• Provides guidance for developing mechanical integrity programmes, including information required by the ANSI/ISA (US) or IEC 1508 (Europe) instrumentation standards.
• Demonstrates to the communities that potential hazards have been identified.

HAZOP study

Methodology

UOP HAZOP studies are well documented and include all of the supporting information used during the HAZOP sessions in a way that is useful to the owner and the contractor.

HAZOP studies of UOP process units are conducted with a team of experts, and specialists that can be consulted on specific points. HAZOP is carried out as a structured study. The method relies on using guidewords (such as no, more, less) combined with process parameters (e.g., temperature, flow, pressure) that aim to reveal deviations (such as less flow, more temperature and so on) of the process intention or normal operation. Ideally, HAZOPs should be carried out early in the project.

Consequences

The consequences are then risk rated based on their severity and likelihood. The risk ranking matrix used in all HAZOP reviews has four levels of risk:

• Risk ranking D: risk not acceptable.
• Risk ranking C: risk acceptable with additional administrative or engineering controls that lower the risk ranking to 'A' or 'B'.
• Risk ranking B: risk acceptable with additional administrative or engineering controls.
• Risk ranking A: risk acceptable even if no further action taken.

The application of the HAZOP technique to a detailed process unit design is a complex and lengthy task. UOP has created a model where the HAZOP worksheets are pre-populated by an experienced HAZOP leader who, during consultations with skilled experts, will allow the company to make the HAZOP reviews more efficient and less time consuming during the HAZOP meetings.

Using dynamic simulation in HAZOP reviews

In the past 30 years, much research effort has been dedicated to the development of the dynamic analysis. The dynamic analysis approach for a HAZOP of a process unit seems to be the most straightforward procedure that gives quantitative results.

Usually, the HAZOP analysis does not consider the duration and magnitude of the deviations generated during the operation. However, what exactly does the deviation ‘less flow’ mean: 80% or 10% of the usual operation value? Does the deviation occur as an immediate (step) decrease of the flow lasting five minutes or more, or is it only an impulse? Is this decrease continuous at some rate? Answers to these questions can be obtained using dynamic simulation, which is one of the newer features.

The dynamic analysis serves for analysing the time and direction of shifting from one steady state to another one due to a failure deviation. It is also very useful for the investigation of dynamic behaviour of the system with respect to the time and length of the failure. The results of safety analysis can be used in HAZOP studies. HAZOP, with dynamic simulation, has the potential to become a very practical and robust tool. This is illustrated through examples of dynamic simulation for conventional column on loss of reflux for UOP’s PhenolTM process operating pressure evaluation.

However, it is important to point out that the implementation of dynamic simulation for any chosen system is dependent on an adequate steady state model; this is dependent on: physical properties, kinetics, mass and heat transfer behaviours. During a HAZOP study, possible deviations are generated by rigorous questioning, prompted by a series of standard ‘guidewords’ applied to the intended design. After the guideword is matched with a parameter, a deviation is generated. The next step in the HAZOP study is to look for potential consequences. At this moment, the application of dynamic analysis is useful in order to find quantitative consequences.

The HAZOP study is traditionally a qualitative study. By definition, a dynamic simulation is the imitation of the operation of a real world process or system over time, which means that in principle it should be a realistic way of representing an actual process. Combining HAZOP with dynamic simulation does provide the means for investigating and demonstrating the consequences of deviations from normal operating conditions. Dynamic simulation now provides the UOP HAZOP team the ability to quickly investigate and test the effectiveness of various suggested strategies dealing with high risk situations (intolerable levels of risk).

By having a dynamic simulation of the process as a support tool, an extensive, easier and more complete study can be accomplished. That provides a systematic screening of process deviations associated with possible high risk events, determining the threshold values that may lead to such events and enabling the examination of a particular design for the adequate safe range of operation. Dynamic simulation should be seen as a tool that complements the traditional HAZOP procedure and does not replace it. There are still many processes that cannot be modelled accurately enough due to a lack of quantitative information, particularly in emergency situations.

Until now, process simulation has found very little or no use in safety-related studies such as HAZOP. In this article, a systematic framework is introduced based on a quantitative HAZOP (that is, HAZOP supported by dynamic simulation related to process malfunctions). The quantitative HAZOP differs from standard HAZOP in documenting results, classification of frequency and consequences of process deviations, and application of a risk potential matrix (risk ranking matrix).

Dynamic simulations can be used to support the HAZOP in the following ways:

• An important part of the HAZOP procedure is to think of the consequences of a certain deviation.
• For complex and nonlinear systems, it is not straightforward to assess the consequences. Hence, the use of dynamic simulation with the deterministic models can be helpful in assessing the effect of faults on the operations and dynamics of the process.
• The purpose of the methodology is to determine the effect of operational disturbances on the safety of the plant and devise ways to reduce the risk of the consequences.

HAZOP can provide qualitative answers regarding the magnitude of the deviations that will lead to severe consequences. It cannot provide the time it takes to reach a ‘no return’ stage of an accident after the deviation has occurred, or the action that can be taken in order to prevent the accident. Dynamic simulation can provide this information that a traditional HAZOP could not provide. Dynamic simulation in HAZOP can provide quantitative assessment of the consequences of abnormal operating conditions.

Read part two of this article here.

Written by Scott M. Wozniak and Bill Weide, UOP, a Honeywell Company, USA. This is an abridged article taken from Hydrocarbon Engineering’s January 2016 issue.

This article is based on a paper that was first presented at the American Institute of Chemical Engineers' 2015 Spring Meeting and 11^th Global Congress on Process Safety, held from 27 - 29 April 2015 in Austin, Texas.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/18012016/taking-safety-to-the-next-level-part-one-2180/