7 replies to this topic

[Nicholas]

Good Day All

 

I would like to determine the fire relief rate for a PSV on a Condensate storage tank of nominal capacity 15,000 bbls. Now API 2000 gives a formula for an emergency fire case which requires the latent heat of vap of the stored liquid at the relieving temp and pressure.  It also gives a heat input of 14 090 000 Btu/hr based on the wetted surface area.

However, when I input this stream and the heat input (in a tank), the resulting stream is totally vaporised! So how then can I determine the latent heat at the relieving temp and pressure of the liquid?

 

Please advise. Thanks!

[Art Montemayor]

Nicholas:

 

Get a handle on the problem.  You can’t vaporize ALL of the tank’s contents instantly.  Go through the mental process of understanding what is physically happening in an assumed pool fire.  That’s right, I said “pool”.  The fire case for a tank as it applies to the API 2000 scenario is that there is a pool of combustible liquid near (or around) the tank and it ignites.  The heat generated by this pool fire is transmitted by radiation to the tank walls and into the tank’s liquid content where it adds SENSIBLE heat to the liquid until the liquid reaches its boiling point.  At that point, the heat from the fire starts to add LATENT heat of vaporization.  All of this heat transfer is done at a RATE and that takes time.  The heat transfer rate is defined by the empirical equation recommended in API 2000.

 

Submit your calculation(s) so that our members can review and see how you arrive at what you describe.  No engineer trusts another engineer’s calculation results without checking them himself/herself.  I would require my own sweet mother to submit her calculations (if she were still alive) before believing her results.

 

[fallah]

 

 

Nicholas,

 

You need to specify the type of the storage tank, at first. Fixed roof? IFR? or EFR?

 

HC condensate is normally stored in EFR tanks. Then if it is an EFR tank, basically no need to be equipped with an emergency vent means the relief load calculation of the emergency fire case isn't required...

Edited by Art Montemayor, 07 February 2014 - 09:52 AM.

[Nicholas]

Good Day gentlemen

 

Thanks for the update thus far. The attached is a calculation sheet that I've been working on.

 

 

 PSV Sizing Uploaded.xls   131.5KB   79 downloads

[fallah]

Nicholas,

 

Appears the tank to be fixed roof, but you hasn't specified the tank type yet. Please clarify...

[Nicholas]

Will verify this. However, the PSV already exists on the tank and I am determining the required relief rate using the methods in the s/sheet. Please see the drwg attached. Thanks.

 

 

Attached Files

•  Condensate Tank.docx   241.81KB   48 downloads

Edited by Nicholas, 06 February 2014 - 03:03 PM.

[fallah]

Nicholas,

 

It is a cone roof type storage tank. Now please submit the exact issue you are facing to calculate the PSV (or emergency vent) relief load...

[Nicholas]

Fallah

The issue is this:

I need to determine whether with the current operating conditons, the PSV installed is adequately sized to handle the flow. Three methods I am looking at:

1. Assume the tank contents is hexane and apply Table 6 of API 2000 for a wetted surface area greater than 2800 ft2. I'm assuming the tank is at it's maximum capacity. This gives me a relieving rate which is greater than the rated.

2. A hysys stream is used to define the condensate which is then heated in a tank at its maximum capacity with a heat input (14,090,000 Btu/hr). This method gives me a total vapour stream with no liquid flow out the bottom. Equation 12 of API 2000 is applied and the latent heat of vaporisation is taken from the inlet condensate stream. This results in an adequately sized PSV.

3. A constant heat interval method was used. Here, heat is added to a series of tanks in 20 min intervals until the maximum outlet vapour flow rate is achieved. The tank volume changes as vapour is lost and so the heat input then depends on the new wetted surface area. I see from using this method that the outlet flow rate is dependent on the flow rate of the initial inlet condensate stream. The relieving rate was above and below the rated for different inlet condensate flow rates.

Please see the general hysys layout attached.

Attached Files

•  Hysys Model.docx   38.4KB   32 downloads

Edited by Nicholas, 07 February 2014 - 08:32 AM.