Dear All,
Some of the designers consider some control valves whithin the plant which are intended to be routed to flare network.
For example 2 control valves with split range configuration from which one of them is routed to flare. Another example are control valves which should relieve any accumulated fluid to flare network due to shutdown of downstream units(for example one train of a compression train).
Is is a good practice to use flare connected control valves?
This may have some drawbacks:
1. Using control valves, in lots of the times we have loss of inventory to flare.
2. Control valves are expensive especially in mentioned cases high pressure drop is needed(because of a high difference between the upstream pressure and downstream back pressure)
3. Stingent leakage class may be needed.
4. Noise is a major problem.
Is not on/off valve a better option?
And my second question is that what should be the failure position of mentioned control valve?
Your valuable comments are appreciated.
Mojtaba
jprocessman@yahoo.com

1-For example, in the case of PV outlet routed to flare, using on/off valve instead PV causes pressure flactuation in operating pressure of Drum/Vessel on which PV is installed.
2-In cases which i have faced to, the failure position had been FC, supported by PSV installed on the same Drum/ Vessel.
Regards

I'm amateur in pressure safety valves area. can anyone give any links to download tutorials regarding pressure safety valve sizing

raajesh,
Welcome.

This post is related to Control valves connected to Flare. However, your query / response is related to PSV sizing.

You are encourage
i) to serve the forum for similar post. This issue has been discussed in many posts.
ii) to open your own new post

Please read this post (click here) . API RP 520 Part I will provide sufficient details to answer your query.

Your are encourage to conduct your manual calculation (at least twice) and compare your results with reliable PSV sizing program available free by manufacturer which listed in FREE & reliable Pressure Relief Valve Sizing Software (click here).

Jprocess,

Fallah has indicated one of the major problem with ON-OFF valve especially you have a large dumping line but small volume (vessel+ piping) attached to it. It create some level of instability. However, this instability can be dampened by slow opening, gap opening, etc. Infact, regardless "control valve" or ON-OFF valve, the stability of system shall be studied.



From operational stability perspective, a dumping valve is always a plus on achieving it.
From start-up/shutdown perspective, a dumping valve is a must in some configuration which you or your team shall study & define.



For all the points, i would say both are similar. If you use ON-OFF valve, you still experience inventory lose due to stability and leakage, on-off valve is not cheap either, stringent leakage will be required as well, you still experience large noise problem, etc.



Fallah has indicated FC base on inventory loss perspective. I have also seen some engineers put FO base on safety perspective. There is no fix rule for this. To define FC or FO, it need proper studies by considering all aspects e.g. operational, safety, availability, etc.

Hope this provide sufficient info and level of details to assist you.

Dear All,
Thanks a lot for all of your valuable comments.



Whys using on/off valves may cause instability? It will have a pre-specified set point and will not pop until reach the set point.(similar to depressuring valves)



"dumping" is a new word for me. by dumping you mean on/off?


On/off valve is cheaper and the other problems is more serious for control valve. For example in terms of inventory loss we will experience contineous loss of inventory!



This is exactly my mean from this question. Some of the engineers use FC and some of them FO!


Thnaks again.

ON-OFF valve open to its maximum without adjusting it position and internal pressure could drop drastically in short period. If you put RO (similar to depressuring valves) with "right size", may be it work. As mentioned earlier, regardless a ON-OFF or CONTROL valve, you have to study process stability.


Valve connected to flare for dumping extra inventory, sometime we call dumping valve. Just is term used like drain valve, vent valve, etc. It has no restriction on type of valve and how a valve work.


I don't really catch what your real meaning.
Why "the other problems is more serious for control valve" ? Don't you think both are experiencing same thing ?

Is this referring to ON-OFF option ? Why "inventory loss we will experience contineous loss of inventory" for a particular type ?

In fact a combination of on/off+RO should be used together.
But just one question which is related to your reply.
That "maximum" which you mentioned is in fact the "peak flow rate" across the on/off valve.Right?
If so, have this value any dependency to depressuring time?



No. I mean Contineous loss of inventory is a major concern for control valve.
Regarding to cost on/off+RO will be cheaper.
Regarding to leakage concern, because on/off have a pre-specified set point and will be closed until reach that point it may be a better option than control valve.

BDV/RO installed in depressurization line subjected to "peak flow rate" in the first moments of depressuring period,and the flow rate decreases within that period.
BDV (usually ball valve) has quick opening characteristic, and along with RO used in depressuring operation, while PV (usually globe valve) applied for regulating pressure,therefore ON/OFF valve , even though along with RO (for decreasing inventory loss),can not been applied for maintaining constant pressure.
Regards

I beleive that flare connected control valves have not control function. In fact they are included because of safety concerns. So we are not going to keep the operating pressure with the aid of flare conected control valves

Flare connected PVs (Such as those usually considered on Overhead line/Reflux Drum of Depropanisers or Ethane Treatment Units) at the first time used for column/vessel pressure control,but after that their release shall be routed to flare system because of safety concerns.Therefore in the cases of nonhydrocarbons/other hazardous materials, PV outlet could be routed to safe location other than flare system.

Depropanisers and Ethane Treament Units are good examples. Because these units are in fact produce final products that will be routed to storage area this is very important to keep the quality of product whithin stringent specifications.
Now a flare connected CV may cause the products to be infected by toxic species which exist whithin flare network. You should also think about a probable vaccum generation in reflux drum or other drums for which you have considered a flare connected CV.

Your concern about probable vaccum generation is right and must be considered, but as you know,mentioned PV usually is TSO type with FC failure position and opened with PIC when the pressure reaches the set value.Thus, in my opinion, there is no concern about reverse flow in the case of vaccum generation.

Happy New Year
Nowrooz Mobarak

What you all are talking about men?

Seems you are only trying to increase your posts number!

Hi,
What do you mean?!