Helpful hints and in depth guidance on various process engineering issues

In a previous post I addressed the issue of not designing a relief valve for liquid overfill in relation with the requirements for instrumental protection. Taking this subject a little bit further, there is also a relation between liquid overfill and design pressure.

Not taking this relationship into account during early design stages may lead to surprises at later stages as safety reviews may require corrective action to safeguard the design. What could have been an easy solution (selecting a slightly higher design pressure) will then no longer be an option as it would usually mean massive re-work and schedule impact.
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waterAPI standard 521 provides a possibility to take credit for operator response to avert relief. It says that a commonly accepted time range for the response is between 10 min and 30 min, depending on the complexity of the plant. In practice this criterion is often used to rule out liquid overfill as relief contingency for large vessels like for instance distillation columns. A ‘nice’ fringe benefit of this action that we do not have to account for liquid discharges to the flare system (at least not on paper that is!).

Is Operator Intervention Safe?

An ‘interesting’ situation will occur if the shut-off pressure of a charge pump significantly exceeds the design pressure of the downstream vessel while the installed relief valve is not sized for the pump capacity (taking credit for operator intervention). This would then mean that catastrophic failure would occur in case the operator “fails on demand” to stop the overfill.

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Usually when designing a process unit, steam grid operating- and design conditions will be a given fact. This is because most projects will be executed on a brown-field site where the utilities are already available (save for sufficient capacity maybe). But what if you are executing a green-field study for a new facility that requires a heating medium and steam is one of the options to be considered (as it often is)? How do you define the grid operating and design consitions then?
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Does any of this sound familiar to you?

  • You are asked to update relief load calculations for an existing unit to under revamp and all of the relief cases seem to be listed randomly.
  • You have to input or update relief loads into a flare model and find duplicate scenario’s listed with loads and/or properties that differ for unknown reasons.
  • You have to verify relief loads and load scenario’s (either in a supervisors-  or  client/owners role) and you don’t find the cases you are looking for in the locations where you would expect them to be described.

Wouldn’t it be a good idea if everybody followed some kind of common standard in documenting failure scenarios? Whether or not this activity is aimed at producing a project deliverable (like a safeguarding memorandum) or “just” a project internal document, it will be obvious that it will be beneficial to follow some kind of standard. This will allow for effective verification (both on the supervisor as the client/owner side) as well as for easy retrieval of information and auditing later on. Read the rest of this entry »

In the EPC business a fair number of design mistakes, misconceptions or even simple goof-ups stand in the way of sound project execution (and profit!) each year.

As an engineer, you can play a pivotal role in the project team in case you are able to sniff out trouble the moment it’s heading towards you. This way the problem can be tackled before it takes on unmanageable proportions.

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