Determining Design Flow Rates in General

[OilBoiler]

I have a question that I come across a lot when I'm designing any system in general: how to determine design flows. Determining design pressures and temperatures can be straight forward by evaluating possible upsets to the process or if a fire case is neglected, but design flows, at least for me, are trickier.

I will illustrate my confusion with a project I'm currently working on. For instance, right now I'm working on designing a vapor recovery system for three atmospheric tanks. I calculated that each tank will vent 75 SCFM of vapor at maximum pump fill rates. I also double checked it with some people in my group and here on the site and the flows seem to be right. Two other tanks that are exactly the same already have a vapor recovery system, but the design flows for these are 125 SCFM. Their design basis wasn't too clear, so I can't know how they came up with this number. Did they probably just calculated the 75 SCFM and left 50 SCFM for thermal venting or other upsets? These are residual oil tanks so vapor pressure is negligible and I'm not considering any thermal outbreathing in my flows. These tanks are also insulated and kept at around 385ºF all the time.

Do I design the system for 110% of the 75 SCFM? I guess my question is how much of turnup should I assign to this process? And in general, to any process? Are there rules of thumb??

Thanks again!

 

[TrevorP]

Check out thread798-113006 for some extremely useful pointers on dealing with historical systems with sparse design information.
If you have thoroughly considered all the scenarios and believe 75 SCFM is the maximum possible, then design to this. A 10% additional allowance seems reasonable. It may well be that someone else has allowed for an additional 50 SCFM above maximum (Possibly because they had less confidence in their calculations). Alternatively, they may have considered a scenario that you have not - so make sure you consider them all - and document them so that someone else coming to it later can see your thought processes.