Thursday, August 20, 2009

Rule of Thumb

Rule of thumb basically means the experience on doing any particular job by applying the standard or reference. The variety of rule of thumb is varies among different people according to their experience. Different discipline engineers will have different rules of thumb that applicable to their work. For example, chemical engineers might have their rules of thumb when designing any process equipment. Piping engineers might have their own when routing the piping. The application of applying this rule of thumb mainly in engineering design work can shorten the time required to design.

This rules of thumb is mainly comes from research done by any institute such as American Petroleum Institute (API), Tubular Exchanger Manufacturer Associates (TEMA), etc. In addition to this, there are also from experience of the engineer him/her self all this while doing the design work. For example, pressure drop across the shell and tube heat exchanger is roughly about 0.7bar which typical value is allowed when designing it. This is purposely made so that the pumps that pumping upstream of the heat exchanger can cater the required pressure drop. There are no specific standards telling so, but from experience of people designing heat exchanger. Some people might say 1.0bar also can be considered. Some people might give more stringent value which is 0.3bar. So, if there is argument in deciding the value, it’s better to refer to the design basis that agreed by the team for that particular project.

The rule of thumb is basically would give some flavor of the rightness when getting the result. It is not only giving the standard to be used but when converting from one unit to another unit. For example, natural gas that having molecular weight 19-22kgmole/kg will give 1mmscfd equal to 1000kg/hr. Again this type of rule of thumb can be cross checked with simulation data to verify it. However, as we all doing the work with simulation interference, this can be on our finger tips.

Finally, this rule of thumb can be used to design any new equipment if you are referring very established sources such as TEMA, API, etc. That rule of thumb from your experience can be used to quick check of the calculation.

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