8 replies to this topic

[Ariel]

Hello Everybody,

I want to ask you about de minimal restrictions orifices to use on blowdown calculation. As a guide I use Api 521 5th Edition (5.20 Vapour depressuring) . I am used to work with natural gas (PM-18-20) and pressures between 70-90 Kg/cm2g

Problem: Sometimes the volume to depressurize is to small and, to achieve 15 minutes of depressurizing, I have to mount orifices too small (at least too small for me, in the order of 3-4 mm). As a rule of thumb, in these cases I use minimum restriction orifice of 8 mm (of course the depressurization time in less that 15 minutes but in not as violent as it would be without restriction orifice). What do you think about that?. Anybody use 5 mm restrictions orifice sometimes?

Thanks in advance.
Ariel
Chemical Engineer

[Andrei]

Ariel,

I do not understand your "Problem".
API 521 refers to pressure relief and depressuring systems as protective devices. With a protective device you would want to relief the accumulated pressure asap, and not slowly over 15 minutes as you say. You would like to return to safe operation conditions without any delay. That's why in some cases valves are not quick enough and rupture disks and pilot operated valves are used.
Why would you install a restriction orifice on a PSV? Why would you not size the valve discharge orifice accordingly in the first place? I cannot understand this at all.
Maybe you want to use the arrangement for normal operation, and not as safety device. In that case you will not size your devices according to API 521. And of course you will have to install safety devices somewhere else.

Andrei

[shan]

Hi Arie,

The function of orifice on the blowdown line is to limited blowdown rate for satisfaction of flare capacity. You may have to run a Hysys Despressuring-Dynamics calculation to determine the peak rate. If the peak rate is over you flare capacity, you have to install a proper orifice to restrict the flow.

Regards

Shan

[Ariel]

Dear Shan/Andrei,

I agree with you. I already made a Depressuring-Dynamics calculation and the peak flow is more that my Flare Flow Design. So I need to install a Restriction Orifice to avoid this problem. Is there where I follow API 521 criteria to decide the orifice diameter.

Based on your words, I could choose a diameter small enough to have a peak flow smaller than Flare design flow, even though the time needed to depressurize is less than 15 minutes.

And follow Andrei word, the faster the better, and “15 minutes” API recommendation is just a maximum, if I can get a safe depressurization in less than 15 minutes I should do it.

Do you agree?

Thanks in advance!!!
Ariel

[shan]

Hi Ariel,

To prevent rapture of container under fire conditions, vessel shall be depressed to 100 psig or 50% operating pressure in 15 minutes. Surely, it is better if you are able to reach the goal in less than 15 minutes and keep release peak flow within flare capacity.

However, the concern is that there may be other blow-down relieves simultaneously under fire conditions when your blow-down valve is opening. The multi-valve flow may exceed the flare capacity. Therefore, you may have to reduce your restrict orifice and to extend relief time to 15 minutes for sharing the flare capacity with others.

Shan

[Ariel]

Dear shan,


Excellent recommendation. I’ll follow it.

I have 4 areas to blowdown. Two of them have important peak flows. The other two have very small peak flow where I have to install 5 mm orifice diameter to reach 15 minutes. I think I was doing things wrong. I will put a higher orifice diameter or not install one at all, always checking that I not exceeding Flare Capacity.

Thanks again for your advice.

Ariel
Process Engineer

[JoeWong]

All,
Good and healthy discussion here.
Some points to add...

As a guide I use Api 521 5th Edition (5.20 Vapour depressuring) .

Please take note that the latest API 521 is API STD 521 Ed 2007 with addendum. Read more in
API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section

Please verify if your project still sibject to earlier version.

The function of orifice on the blowdown line is to limited blowdown rate for satisfaction of flare capacity

The main purpose of depressuring (BDV/RO) is to evacuate the inventory from process system as fast as possible so that the reduced internal pressure stresses is kept below the rupture stress. Read more in "Don't misunderstood depressuring"

As Andrei mentioned, you just have to make sure the depressuring time is less than 15 minutes (if based on API RP 521 5th Edition). Neverthless, the 15 minutes is no longer spell out in API STD 521 2007 edition. Read more in Depressuring within 15 minutes no longer applicable ?

There are many ways to manage the total depressuring rate to flare system. When dealing with depressuring, simultaneous depressuring from several zones shall be considered. Nevertheless, zone segregation can also limit the depressuring rate. In addition, API has also limited the fire extend to 2500-5000 ft2. Read more in "Extend of Pool Fire... ". You may also consider stagger blowdown to spread the flow...

[Ariel]

Hi Joe,

Thanks for your comments. I´ve been reading your links and I was surprised by the “Depressuring within 15 minutes no longer applicable ?” file, where it says that a vessel with thickness less that 30 mm (most of the vessels I am used to work) may need to depressurize in less than 3 minutes.
Do you have more information about this or where I should look for? In my opinion it’s a bit too conservative. What do you think?


On the other hand the API 521 5th edition on page 37 gives the next example “an unwetted steel plate 25 mm (1 in) thick takes about 12 min to reach 593 °C (1 100 °F) and 17 min to reach 704 °C (1 300 °F) when the plate is exposed to an open fire” or see figure 1 on page 37.


Thanks!!!

[JoeWong]

Thanks for your comments. I´ve been reading your links and I was surprised by the “Depressuring within 15 minutes no longer applicable ?” file, where it says that a vessel with thickness less that 30 mm (most of the vessels I am used to work) may need to depressurize in less than 3 minutes.
Do you have more information about this or where I should look for? In my opinion it’s a bit too conservative. What do you think?

No suprise as the studies were based on jet fire heat flux.

It is always a problem with thin wall vessel. I think what you need to establish is the credibility of jet fire scenario.

On the other hand the API 521 5th edition on page 37 gives the next example “an unwetted steel plate 25 mm (1 in) thick takes about 12 min to reach 593 °C (1 100 °F) and 17 min to reach 704 °C (1 300 °F) when the plate is exposed to an open fire” or see figure 1 on page 37.

If not mistaken, it was based on pool fire. Have to check on the original article...