PT Notes
HAZOP Practices to Avoid - Use of Super Nodes
PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.
This PT Note is the second in a series to help you ensure your HAZOP studies follow best practices. Poorly conducted studies result in omitting hazard scenarios which may cost people’s lives and result in catastrophic property damage. It exposes companies to regulatory and legal liabilities that can cost millions of dollars.
HAZOP practitioners divide processes into nodes to focus studies and make them manageable. Generally, nodes are defined as pipe sections and vessels in which process chemicals are, or may be, present. This is known as “line-by-line” noding. However, some practitioners use combinations of lines and major vessels as nodes (called “super nodes”). Use of super nodes poses problems.
Super nodes are used in the belief that their use will speed up studies. However, it also complicates the analysis. While it provides a bigger picture of the process, which assists in identifying scenarios that involve multiple lines and/or vessels, it does so at the expense of missing scenarios because the complexity of the analysis interferes with scenario identification. If nodes with different intents are combined into a single node, practitioners face difficulties when developing deviations. Many deviations must be considered and multiple similar deviations are possible within the same node, for example, there will be multiple instances of the same deviation if there are different operating temperatures or different process materials in various parts of the super node. This is confusing for HAZOP teams making it more difficult to identify scenarios, increasing the chance that scenarios will be missed, and causing the team to become mired in the analysis. Also, if carried out to its limit, which is unlikely given the complexities faced by the team, it equates to the line-by-line approach which begs the question of why super nodes should be used at all. Line-by-line noding is the better approach, particularly when augmented with one or more global nodes to provide the big picture view of a process.
One exception to this best practice is in early design-stage HAZOP studies which may use super nodes when details of the design are not known and line-by-line noding is not feasible.
The key criterion for noding processes is that the design intent throughout a node should be more or less the same, normal variations excepted, e.g. temperature gradients. A new node should start whenever the design intent changes significantly, e.g. at specification breaks.
When multiple nodes have similar design intents, studies become repetitive. Consequently, lines or vessels with similar design intent can be represented by a single node to avoid repetition of hazard scenarios, e.g. parallel lines or a bank of heat exchangers. This is not the same as using super nodes. When using representative nodes, HAZOP practitioners must recognize that they are actually physically distinct lines or vessels; otherwise some hazard scenarios may not be identified, e.g. the simultaneous failure of identical but parallel lines due to a common cause. Also, scenario frequencies must be adjusted to accommodate the number of times the same scenario occurs in multiple lines or vessels.
The goal in noding a process for a HAZOP study is to choose the optimal number of nodes so that scenarios can be identified as completely as possible, the study can be performed within a reasonable amount of time, and the work of the team is as simple as possible. Line-by-line noding is the best way to accomplish these goals.
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