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What is Standard and Normal flow?

Regularly gases are measured based on its volume per hour and there are some types of devices to do it. Usually, measurement units in these cases are SCFH and Nm3/h. Most processes are designed and set based on these units.

As a chemical process engineer, you need to make surge that those measurement units fits your heat and material balance. I mean, volume flow depends on pressure and temperature. So any time you see “S” or “N” as a prefix to flow measurement confirm what is their basis.

For instance, Standard US is based on 101,325 kPa @ 15.55 C, Standard ISO 101,325 kPa @ 15C and Standard AGA 101,560 kPa @ 15.55 C. In the other hand, Standard (Normal) International is based on 101,325 kPa @ 0 C. Not to mention natural gas industry that uses 20 C as temperature basis.

Taking into account that PV=nRT, the wrong temperature assumption can result in approx 6% error in your flow measurement.

That may impact your process design and worse than that you may not guarantee a Customer requirement.

Some tips I would share with you to avoid this problem is to verify Customer’s contract definition, ask for Supplier clarification, inform instrumentation guy the correct P and T basis of the project.

Process Design

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