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Psv Fire Case Calculations

psv fire case simulation

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#1 Cleber Ronqui

Cleber Ronqui

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Posted 11 August 2015 - 08:36 AM

Good morning.

 

Using API 521 method for fire case, I´ld calculate a mass flow for my PSV, deppending on the wet area and heat of vaporization of one vessel content.

 

If I use the initial boiled vapor (say 1 to 10% vol vaporized), with the lower Mw I would expected the bigger orifice area. I tried to confirm this with dynamic simulation.

 

I simulated a vessel with a PSV with fixed area, oppening and closing in tree cycles. (keeping the heat, rising the temperature, and Mw of the vapor)

 

It oppens when pressure arrived at relief  pressure and closes again at set pressure. (these are arbitrary conditions).

 

Well, for my surprise, PSV opened time increased in cycles 2 and 3. Actually, cycle 3 should be the project case. 

 

Molecular weight and temperature raised, but the heat of vaporization dropped and also compressibility factor.

 

I checked some UOP designs for fire case (data sheets) and found temperatures about 300-450 ºC, high molecular weight (greater than feed molecular weight of that vessel), and compressibility around 0.7.

 

So, I suspect my "finding" is not new, but I´m not experienced in dimensioning these elements. Can you share your experience, gents?

 

Thank you.

 

 

 

 

 

 

 

 



#2 Bobby Strain

Bobby Strain

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Posted 11 August 2015 - 01:17 PM

The most practical method is to execute a series of flash calculations for the liquid phase, incrementing each by 5% or less, and calculating the orifice required. Select the maximum orifice. You can adjust the wetted area for each calculation if you want. Each flash stage takes the previous stage liquid. If you do this exercise often, you can build a good calculator with Excel vba to automate your process simulator and orifice sizing. With the automation you can reduce the step size to 1% or less to more closely simulate continuous vaporization. I don't trust the dynamic simulation unless you can confirm that it gives correct results.

 

Bobby






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