I am looking at an existing design of an atmospheric storage tank which has a 2inch "emergency relief vent" (a conservation vent) which discharges to a scrubber and a 4 inch "normal process vent" which is an open vent that also ties into the same scrubber! I have calculated that the existing 2inch relief vent is not adequately sized for the various scenarios. Aside from my numerous queries on the strange design (does the conservation vent ever open?), my main question is, can I take credit for the relieving capacity of the normal process vent? Could this be "categorised" as an emergency vent since it is always open??
Thanks in advance for any pointers.

The use of an emergency vent is USUALLY because the process vent will NOT be large enough to handle an incident; usually fire. What you describe is backwards and highly suspect. I think someone got confused and put the emergency vent on the wrong nozzle.

And yes, you can take credit for all vent piping. Just think of the purpose of venting; to allow vapors to escape an enclosed area to prevent overpressure of that vessel. The use of conservation venting is to minimize vapor losses, e.g. those that might be valuable as a product. If I didn't care about loosing the vapor during normal operation, I would just design for an open vent.

I think we have to know the exact nature and use of the vessel being designed but I have seen a lot of vessels designed this way - and I can see no theoretical reason not to do it (perhaps someone can point something out to me )

Like Phil said - you can accredit for all vent lines when looking at emergency vent. I imagine the train of thought was to have a vent line (4") and then just incase this was incapable of venting in an emergency, 2" of additional area could be used. This is a common methodology, no calcs or anything. STOP RIGHT THERE PEOPLE.....I don't do this and it grates me that ppl do. Im just saying it happens.

What i'm wondering - do these lines both run to separately to the scrubber or do they join a common header. I would expect the latter and sized about 6".

Linus:

First of all, refer to my earlier comment on your other thread. Emergency Vents are just that; they are meant for emergencies – not for normal operating vent requirements. What you have described seems more like the conventional Conservation Vent – a device that is meant to relieve both for pressure and vacuum conditions and, in my experience, never used for emergency venting. Therefore, if that is the case then you have to respect the need for vacuum protection and relief.

I’ve done the application that you describe many, many times. We called the installation a vent “control” – why, I don’t know; it never “controlled” anything. It merely scrubbed the vapors positively vented from a storage tank, prior to the same vapors being vented to atmosphere.

The design I employed used a Conservation Vent (CV) to meter out the vented vapor going to the scrubber. I’ve used 2” and up to 4” sizes for this application. I agree with Phil; I also suspect the original designer/installer didn’t know what he/she was doing - especially if there is no documentation on the installation. The reason I employed a CV was that I maintained a positive pressure (an N2 blanket) on the stored liquid product and didn’t allow venting until a top set pressure was exceeded. If you are not blanketing, then there is no need for a CV. Now, there may be a reason for installing a CV: suppose there is a possibility of the scrubber getting plugged (we don’t know the fluids used or the chemistry). If that were a possibility, then the need for a CV is obvious – but not vented to the same scrubber!

Depending on your process and application you may need the vacuum relief. Perhaps that is the use that the CV is being put in for. It could be that there was no simple vacuum device available at the time and someone decided to put in the CV for that purpose – ignorance is everywhere. When a production plant or facility does not maintain detailed and accurate calculation and data files and documentation on their engineering installations that is what happens – and often! Don’t let that happen to you.

Unlike JEBradley, I can’t suggest any specific line size. We don’t know the vapor capacities, the tank size, the fluids, the application, etc.. You have to do the calculations – even if they are estimates. You can always estimate conservatively – and document the calculations and assumptions.

Thank you Art Montemayor, as usually your knowledge and experience make us understand the problems easily.
But related to this topic I have another question: Suppouse the case of an atmospheric vessel with a normal operation vent discharching to a vent collector where more vents discharche, and then going to a vent condenser. Then in case of an emergency (for example fire) the pipe vent size must be much bigger than the normal operation vent, and I wonder if the vent header wich goes to the condenser must be designed for emergency case, and thus the vents condenser ¿??? In that case the condenser will be over-dimensionated.
¿Is it a right solution to dimensionate the vent header and vent condenser for normal operation and place a PSV in the vent header covering all vessels the discharge there? In that case all the vessels will be pressurized in case one of them is in emergency.
Can you help me with this, please??
Thank you again

Mafalda:

I don’t want to stray from the subject of this thread, but in this case I agree with you that your question is somehow related to the title “Emergency Vent Versus Process Vent”.

Your example case has weaknesses in its basic description. You don’t cite whether the fluids involved are toxic, poisonous, flammable, or hazardous. The atmospheric vessel, in my opinion, should have an emergency vent if there is a possibility of a pool fire in the area. In a fire, the emergency vent normally opens and stays opened – during the fire if need be. That means that the vent line and condenser that you describe would be inoperative for those vapors. However, what I describe is not meant for toxic or poisonous vapors. Those type of life threatening vapors cannot be merely vented out to atmosphere and require another, independent methods of furnishing emergency pressure release – all designed according to the specific need and characteristics of the substance in question. For example, Chlorine is handled differently from Hydrogen Fluoride (HF), Phenol, and other similar, toxic chemicals. In those cases one would never have a common vent condenser.

It is very difficult to address a general, non-specific emergency case such as what you describe. You must be specific in order to generate a definite, specific response. There are just too many ways that one can respond to your general query and this serves no purpose, in my opinion. In order to definitely address your query – which I think is basically a good one – one should have a specific case and data in hand. I believe there is a good, practical, and well-engineered solution for every emergency vent case – when one has all the basic data at hand. If you can supply a specific case, we can all address that.

Just wanted to clear up a point.

Arthur - I think you misread an earlier post from me. I wasnt sizing the pipeline - I was suggesting what a third party (towhit the original designer) may have been thinking when they designed it.

I was actually trying to stress that you shouldn't design it that way (although a hell of a lot of people do).

Art Montemayor,

You are right that I did not specify the fluids to be relieved. In my case I have methanol vapours. As I told you during normal operation the vents are collected, condensed and returned back to the process, while the incondensables are sent to the atmosphere with a previous flame arrester. The rest of the vesssels with non dangerous vents, discharge, as you said, to the atmosphere.

Thank you again for your time and explanations .