5 replies to this topic

[jentot]

Hi all,

 

I'm a new process engineer just recently graduated from university. I really like to read from this site as it really helps my day to day work. I'm stuck with my current work so I think posting the question in this site would help me.

 

I have a question regarding PSV calculation for pressure vessel in offshore platform. The normal operating pressure is about 100 barg, but the design pressure is 300 barg (designed to withstand wellhead shut-in pressure). In normal condition, there is a liquid level at the vessel. 

 

The only scenario considered is fire case. When the fire starts, the liquid will begin to boil. As the design pressure is so high, the liquid will continue to boil until it becomes supercritical. After that it still needs to build pressure before reaching the set pressure of the PSV. 

 

My questions are :

1. Due to high relieving pressure, the relieving temperature is also very high. In fact it exceeds API 521 recommended wall temperature. As such can we discard the PSV using this assumption? Note that our local code dictates every vessel for having a PSV. Is it imperative to follow the code regardless the relieving condition?

 

2. If the PSV cannot be omitted, which equation should we use to calculate the relief load and relief area? My understanding is that at supercritical condition, we shall use API 521 equation for vapor filled fire case to calculate both the relief load and relief area. But there are also different views of my colleague that because originally there is a liquid inside the vessel, we should use equation for fire case liquid filled using minimum value of latent heat (115 kJ/kg) to calculate the relief load and use HEM method to calculate relief area. Other comment is that we should use volume expansion method explained in API 521 6th edition to calculate relief load. Which one do you think is correct?

 

Sorry if it's too long, hope that I explained the problem clearly.

 

Thank you in advance!

[pdender]

question 1

your code dictates every vessel for having a PSV,

exceptions might be allowed in certain cases but these should be mentioned

 

question 2

you may try to estimate temperature (and state) inside the vessel at 300 Bar,

there are different procedures for that, see for example

http://prodesoftware...ns.blogspot.ca/

 

depending from these values you may decide which method to adopt for sizing the valve.

[fallah]

jentot,

 

1-No need to consider high relieving temperature in mechanical design of the vessel. Indeed, although the PSVs are generally incapable of protecting a vessel from fire exposure, one cannot disregard to apply.

 

2-Treat the vessel as it is filled with a very high density gas. This assumption will lead to an relative acceptable result in relief load calculation.

[jentot]

pdender, fallah

 

Thanks for your responses. We decided to retain the psv and size it with volume expansion method that is incorporated with API 521 6th edition.

[ChemEng01]

The equations from API for supercritical fluid can be used. 

 

Note the relief temperature calculated using the API equation is normally really conservative because is assumes ideal gas law. 

 

I normally use HYSYS to determine the relieving temperature. Take a copy of your stream at normal operating conditions. Change the pressure of the copied stream to relieving pressure. Then use an adjust to change temperature so that the volume is the same as the stream at normal operating conditions. 

 

This will give you a more realistic relief temperature for supercritical fluid. 

 

cheers

[Manan_Joshi]

I just want to add how we size psv for very high boiling point liquid.  I calculated the amount to heat that can be abosrbed based on API fire equation and then added the fire as thermal expansion.  Basically such system are liquid filled system so the overpressrisation is due to thermal expansion of liquid.  If there are no fire protection equipment avaiable I will suggest you provide fire proofing insulation as the temepratures can go very high in such heavy liquids.