Just today I saw an example of how not to specify te elevation of equipment. A couple of years ago I saw another example of this and on both occasions HYSYS was blamed to produce the wrong results.
The relative elevation of the nozzles should ALWAYS be set as the are in reality. Do NOT use this relative elevation as a fudge for the total elevation! For example, you don't tell HYSYS that the base elevation of a valve is zero meters and then set the nozzle elevations of inlet and outlet to 12 meters to tell HYSYS that the valve is really located at an elevation of 12 meters.
The risk you run is wrong results. One of the causes of this is that HYSYS will do it's flash clculation at a pressure that corresponds with base elevation of the equipment. So if your valve carries a vapour that is close to the dew point and it comes out of tall equipment (a column for example), the flash calculation may get done at a pressure that is noticeably higher thant the column pressure and hence show some lquid drop out which isn't there in reality.
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Wednesday 9 January 2008
How NOT to specify equipment elevation in dynamic modelling
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Yes, when we specified this model, we made this error. It only came to light however, because we were working on the simulation in two different versions. Hysys 2004.2 patch 0 returned a pressure drop higher than our manual test calculation, while Hysys 2004.2 patch 5 returned an impossible negative pressure drop for flow througn the same pipe segment. We forwarded the model to Aspentech for advice, who replied that version 2004.2 patch 0 was flawed and should not be used for this model. They did not notice our error in specifying the elevations. This was not detected until the blogger himself reviewed our model. The model now works correctly , but not in version 2004.2 patch 0. With the elevations correctly specified, the patch 5 version calculates pressure drops in line with our manual calculation, and we are now satisfied that we have a working model.
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