Vent stack design

[kasiddiq]

Hi,

Does someone have the knowledge about how to design a vent stack for VCM vapour. API 521 has some information about it but is not much helpful in calculating the height of the stack. Can someone help?

 

[mbeychok]

I assume that you mean vinyl chloride monomer by your VCM. The first thing you should do is contact your local government's air quality control agency about your proposed venting.

You should also consider the fire hazard involved in venting VCM to the atmosphere.

In the United States, the local air quality agency as well as the national Environmental Protection Agency (EPA) would require compliance with their VCM venting regulations before they would grant a permit to do such venting.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[MikeClay]

Environmental conditions aside, the goal is to reduce the concentrations below hazardous levels. The height is set so that the exiting vapors are dispersed to acceptable levels before anyone is exposed to them. The US EPA has a free model dispersion model called ALOHA. It does not have the same features as the comercial packages, but the price is right.

--Mike--

 

[kasiddiq]

Hi Mike:

I know that environmental issues have to be taken into account for vinyl chloride venting. That is not what I want rather I need information on actual design method (model based or hand calculations)to actually size the vent stack. What information would that model require and would it be able to predict the height and dia of the stack? Thanks.

 

[mbeychok]

kasiddiq:

(1) You need to know the volume flow rate of vapor (ft³/second or m³/second) to be vented ... or the mass flow rate and the molecular weight of the vapor, from which you can calculate the volume flow rate.

(2) Then you calculate the diameter as you would any piping with a vapor flowing through it ... except you want a much lower pressure drop per unit of length than you would use for typical process piping. I would guess that will mean an estimated linear vapor velocity of about 30 feet/second (about 10 meters per second). From that velocity, and the volume flow rate, you can calculated the vent diameter. (The reason for low pressure drop requirement is to minimize the back-pressure on the VCM source vessel or pressure relief valve).

(3) You can't select the height until you resolve the enivronmental issues either by air dispersion modeling or by being given a mandatory height by your air quality regulators. Once that is done, you can do another pressure drop calculation and change the vent diameter if needed to maintain a low pressure drop.

Hope this helps.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[JLSeagull]

Consider flares or incinerators instead of vent stacks.

 

[kasiddiq]

Hi Milton:

Thank you very much and all other guys who tried to help. I just got the results of dispersion modeling of VCM from our contractor. I hope it will help to fix the stack height to ensure OSHA permissible limits. I would have considered incincerator but this system is for VCM storage and venting would only occassionaly be required for the maintenance purposes.

Regards