Dew Point Determination - Vessels

[dogbertcountry2]

I have been accustomed to checking dew points on tanks (while air purging) to achieve a -40º dew point. I am unsure why we use this temperature - it is just what we have always done here as well as other plants I have worked. I did some quick internet checks and I see this same number show up frequently. Can someone explain to me why this might be or not be a standard?

 

[Homayun]

I think this might have something to do with the extend of dryness one wants to achieve. The gas at -40 DegrC is certainly dry.
That's my guess, maybe others know better.

 

[Montemayor]

dogbertcountry2:

The common value for the Celsius and Fahrenheit temperature scales just happens to be -40. But you fail to tell us if you are using the Kelvin or Rankine scale. We don't know which scale you are referring to unless you tell us.

Purging a process vessel to a -40 oF dewpoint does not, in my opinion, yield a very dry vessel. But that depends on the process and the requirements it imposes. You can check all you want or need to, but the -40 number is not magical nor is a universal solve-all. Different process or needs mandate different levels of "dryness" prior to being put into operation. You are better off reading and memorizing the scope and process description of your process and making sure that you are well beyond its requirements and needs.

It might also turn out that you only have -40 oF dewpoint instrument air available in your plant (which is one of the cheapest instrument air grades) and that is what is being used solely because it is available. I don't know; but that would be my guess. I always design for a nominal -60 to -75 oF instrument air grade because an adsorption dryer is required for this level of dryness and it is common sense to take what the adsorption process inherently gives you.

There are no "standards" in compressed air grades. You establish what you need for the purpose you have before you.