Blowdown of liquid filled segment

[bule99]

Hi,
after checking some treads on this very usefull site it is time to post one myself.
Here is my problem.
A condensate train, starting from 3rd stage separator @ 2 bar, including booster pumps @ 9 bar and export pumps @ 80 bar, needs to be depressurised in case of fire.
Upon confirmed fire or gas detection ESD valves are closed, one on suction side of booster pumps and one on discharge side of export pump. All pumps are stopped and blowdown valve, located downstream export pump, opens. The idea is that all this happens simultaneously.

What would be correct initial pressure for blowdown calculations?
Obviously there are 3 different pressure segments in this condensate train during normal operation. Using the discharge pressure from export pump would be too conservative because other two segments have much lower pressure. I know how to calculate settle-out pressure for compressor sections but don't think it is applicable here, since fluid is liquid (stabilised condensate).

Another question is what happens with pressure when pumps stop?
No pumps - no pressure increase, so another extreme is to use the lowest pressure in the system, i.e. pressure of 3rd stage separator. In this case there is no need for blowdown because pressure is only 2 bar.

Finnaly, in order to initate blowdown simulation I need to add some gas to my fluid, so calculated flow is based on gas, while in practise it is liquid that enters RO.

Hope someone has some (practical) experience about depressurisation of liquid segments (with or without heat input)

Thanks in advance!

 

[MortenA]

Im assuming that when you are talking about depressuring you dont mean draining btu only refer to reducing pressure to 0 barg.

If the segment is totally liquid fillid intially you should not need to blow anything down (almost - the compressibility from 9-0 barg must be close to 0). But if the liquid starts to flash (after depressuring and maybe heat input from fire) then gas/two phase relief will be required.

Why dont you calculate the max case (9 barg) and evalute if its possible to make such a smalle bore in the orifice? The look more carefully at flashing/fire load

I dont think that you can say whether its gas or liquids that will be released unless there is no gas flashing off but liquid would produce the largest orifice i guess.

The reason that your simulator (HYSYS?) wontcalculate anything if there is no gas present is because its so little volume that has to be removed.

Best Regards

Morten

 

[bule99]

Thanks MortenA,

yes, you are wright, I am talking about pressure reduction down to atm., not draining.

Max.case would be 80 bar, that is discharge pressure from export pumps and that is where the blowdown line is located. 9 bar is pressure out of booster pumps, and that is an intermediate pressure in the whole system.

My problem now is that when I use 80 bar as initial pressure and add some gas to enable the blowdown simulations, flow rate needs to be high in order to depressurise the segment quickly. That is in theory, in practice, since it is liquid filled segment, pressure reduction would be even faster and all that with smaller flow rates. At least that is how I see it but I have problems how to document it.

I could get smaller flowrates if I start blowdown from 9 bar, some kind of settle-out or average pressure for whole segment, but I am not sure if it is correct. So, back to my second question, what happens with pressure in the system when pumps are stopped? Shall I just pick up some kind of settle out pressure based on volumes of each section or use initial pressure, 2 bar - at inlet to booster pumps, and add 1 or 2 bar to have something to depressurize. Providing small orifice it would then be possible to simulate flashing of liquid due to fire and calculating required pressure profile for, lets say, 30 minutes.

best regards

 

[MortenA]

My point is: If your are sure that its only liquids - no flashing of then there is almost no volume even from 80 to 0 barg. Find the comressability of the liquid (or an equivalent) or use your simlator to determine the density of the liquid at 80 barg and atmospheric and then calculate the require valume to be released as (den_80-den_atm)*vol_piping/den_atm i still dont think it will be a lot.

This however changes if the liquid flashes.

Best Regards

Morten

 

[bule99]

Thanks again Morten!

Any idea or suggestions regarding heat input from fire and requirements for fire insulation? (apart from API)

best regards

 

[MortenA]

Use API - its the recognised standard! (and its very easy to use). HYSYS has an option for including added heat from fire to the blowdown utility.

Best regards

Morten

 

[Richler]

Hello,

thanks a lot for your help: I had the same problem, that is depressuring a dehydrator vessel containing 100% liquid from 8 barg to 4 barg in 15 mins (see API 521) in case of external fire.
I'm using HySys 3.1 and experienced a lot of problems just beacause the depressuring utylity there won't work well for these kind of systems since, I understood, it is intended for vapour and vapour/liquid systems only.
We're still trying to find out the best way to calculate the peak flow rate and then sizing the pipe from the vessel to the blowdown header.
Do you have any suggestion?

Regards,

Francesco

 

[WimVW]

Francesco,

You should be able to run this type of problem in HYSYS 3.1. Take the "Dynamics Depressuring". You do not need any additional license for this. I just tried it with an example inspired on your problem description.
As can be expected, the pressure drops very quickly. You will see some pretty noisy curves, the reason is that you get a two-phase mixture out of your valve once the liquid is hot enough. What I also noticed was that although the valve may be big enough to handle the liquid flow, you may find it to be too small once the liquid starts boiling because of the heating by fire. You then observe first a drop in pressure and after a while the pressure comes back up again and keeps going up!

 

[Richler]

WinmVW,

thanks for your answer. I used the utility you're talking about: problem is...are you confident with the results? If you are, how would you size the valve and the piping?
By the way: did you choose the "no flow" option for the liquid valve parameter? Do the results are still reliable in this case, considering that you get (obviously) liquid out of the valve?

Thanks again,

Francesco

 

[MortenA]

Richler

I havnt commented on your problem because im not sure i get it:

If you have a totally liquid filled container at the time of opening your BDV and if your temperature of the liquid is < boiling temp at atmospheric condition - then do as I have outlined earlier.

Im quite sure that even with a very small orifice (dont specify anything smaller then 11 mm unless you want your mechanical/piping guy's to laugh at you) then the pressure will be gone in an instant. HYSYS can not simulate this (if there is no gas). You have to do it by hand/spreadsheet. You can use HYSYS to determine the various densities. But since its a small change in pressure change in density is rather academic.

If you have heat input (from fire) its a little more tricky (but you still have to do it by hand if you are sure temp<boiling temp.

An expample:

Closed vessel 100% liquid filled at atm.

Fire around vessel

Fire heats vesse, temperature rises and due to change in density the pressure increases.

At 9 barg the temp is still < boiling temp@atm (very likely since the thermal expansion is much largen den the compressability of liquids) our blowdown valve opens and the pressure is releived instantaneously

problems might not be over: Fire continue to heat vessel. If BDV is mounted on top of vessel then sooner or later the liquid will be at the boiling point - and now pressure will start to increase if the orifice is not large enough to handle vapour formation at given heating rate at atm. pressure.

Best reagárds

Morten

 

[WimVW]

Francesco, Morten,

The new utility has TWO valves, one for the liquid and ond for the vapour. The liquid valve is at the bottom of the vessel. If you specify &quot;no flow&quot; for the liquid valve, that just means that the bottom valve stays closed.
You can get a correct result, even for the pure liquid depressuring, but you need to make sure that the time step you choose is small enough to model the phenomenon. If the depressuring of the liquid only takes 0.5 s, you need a 0.05 s timestep.
You currnetly can't model the piping after the valve.

 

[Richler]

Hi WimVW,

just like I wrote, I use the &quot;no-flow&quot; option for liquid since in reality we only have a BDV on top of the vessel.
So the problem, once again, is: if I model this way using Hysys, given that it seems obvious that you get L+V coming out of the valve (it is even proved with a &quot;real&quot; dynamic simulation using Hysys), do I get correct results?

Kind regards,

Francesco

 

[WimVW]

Eh, I could shortly say: yes.
In particular, if you say it was proven with real dynamic simulation in Hysys. The solver in the depressuring utility and the dynamic solver are the same.

 

[Richler]

You're right, but in real dynamic simulation I didn't specify ani &quot;no-flow&quot; option...I just have a valve and I see that I've mostly liquid coming out of it at first.

Anyway,

you've all been very helpful.

Thanks a lot for your kindness,

Best regards,
Francesco

 

[MortenA]

well we are still running 2.4.1

Actually i have just had my dongle replaced with a USB dongle and i dosnt work yet. But i think that may be related to using 2.4.1 - so i will talk to support re, thsi.

Best regards

Morten