Emergency / Vacuum Vents Sizing ( Low Pressure Tanks)

[ReliefSystemEngineer]

Hi,
I'm working on Emergency / Vacuum Vents sizing on Low Pressure Tanks. I need help / guidance with calculations of relief rate for the following two cases:

1]Filling Hot Tank with Cold Liquid
2]Hot Feed to tank on startup with cooler material in tank.

How do I calculate the relief rate for the above two cases ? Is there any literature or writeup available for such cases ? Any kind of suggestions or ideas are most welcome.Thanking you in advance.
-PD.

 

[roker]

hi,

read API 2000, try companies like "Protectseal" .

regards,
roker

 

[Montemayor]

Relief:

Just precisely what is it that you are after as regards help or guidance? Please be specific. As a chemical engineer you know that you have to design for the worse case scenario that might arise out of the process operation that is taking place in your tank.

You are adding cold to hot and hot to cold; these are not extraordinary circumstances. We all do it all the time. What counts is the expected and predictable results of these operations. Are you flashing in the tank? Can you reasonably expect any over-pressurization scenario to develop? You haven't even gotten close to giving us a minimum of basic data for us to determine that. You should be in a solid position to determine that.

As roker correctly points out: Read and study API 2000 to orient yourself to the possible over-pressurization scenarios specific to your tank and identify the worst case.

Get serious. We can't recommend or suggest any method to "calculate the relief rate for the above two cases" without you thoroughly identifying the specific hazard and supplying the basic data. You should know better than to expect us to pull a rabbit out a hat. We're not magicians; we're engineers and we need factual, specific basic data and scope of work to specifically answer or address problems. Without that, all we can offer are generalities.

Good luck with API 2000. Let us know what you don't understand about it and perhaps we can go from there.

Art Montemayor
Spring, TX

 

[ReliefSystemEngineer]

Thanks Mr.Montemayor for all your input....you could have just written one sentence saying that I should be more specific & there was no need to write such long reply.
I agree that I was not specific & I should have given more details for my problem,it was my mistake & I apologize for that, but that dosen't give you the right to write anything. I'm a Chemical Engineer & I have read API 2000 & I have experience desigining vents,relief valves & rupture disks. If you want to share your knowledge its fine or else nobody is forcing you to reply.

Now, here are the details of my problem:
Data:-
1] Vertical Storage Tank built in accodance with ASME Sec. VIII with design press. 14.5 psig & 3" Vacuum & Design Temp. 0-650 ºF.
2] Tank Dimensions: 12ft O.D * 20ft.Tan/Tan Straight Ht.
3] Tank Capacity 18,000 gallons
4] Tank equipped with Bayonet Heater using 70#Steam.
5] Tank used for storage of HexamethyleneDiamine(HMD).
6] Tank has a 6" Emergency/Vacuum Vent set at 20" wc & 1.5" wc.

Case 1: Filling Hot Tank with Cold Liquid
Assuming tank contents are heated by 100 psig steam to the steam temperature(170ºC). Cooling casused by (50ºC) material to the tank. I'm trying to find the amount of vapors condensed ? (Required Vacuum Flowrate)

Case 2:Hot Feed to tank on startup with cooler material in tank.
The feed comes from a column & has a temperature of 180 ºC & assuming that at the startup the material in the tank is at ambient temperature (say 80ºF)(Assuming the bayonet heater is not on). What would be the amount of vapors generated ? ( Pressure Relief Flowrate).

Solution:
For Case 1:
Assuming that approx. 15 minutes are required for the temperature to equalize.
First calculate the vapor properties at 170 ºC,i.e., get the vapor pressure of HMD & N2, then get mol% HMD & N2 & multiply the mol% by volume of tank vapor space (ft³) & get the volume of vapors at 170 ºC & similarly get volume of vapors at 50ºC. Difference in volume of HMD vapors(ft³) / 0.25 hours = scfh air to be pulled in through vacuum vent.

For Case 2: Working backword of the above solution.

Now, thats what I have so far & I'm looking for suggestion & help from y'all. If you think diferent or if there is any other better way to do it,then please do let me know.

thanks,
PD

 

[Montemayor]

PD:

Thanks for the additional basic data and operational information. Now we can start dealing with specifics. However, there are some points that need clarification or confirmation from you:

You don’t say it, but I have to assume you are normally maintaining a N2 vapor blanket on top of the hot HMD stored in the storage tank. Am I correct?

14.5 psig = 401.35862 inch H2O. Therefore, the pressure relief set point on the 6” conservation vent on your storage tank can be set as high as 400 inches of WC instead of the indicated 20 inches of WC. Am I correct? If not, why is the storage tank an ASME Section VIII vessel instead of an API 650 design? This seems to be in conflict and may need clarification.

What you describe as possible scenarios involves an unsteady state heat and mass transfer operation – a process engineer’s nightmare of calculations. However, as I previously stated, what safety design guidelines dictate is that we define the worse-case scenario and design to relieve it – both on the pressure side as well as any possible partial vacuum situation. If we can accept this simplified criteria, then:

Case #1:
The worst-case scenario for in-breathing is that the tank is 100% full of only 170 oC (N2 + HMD) vapor –with no reasonable amount of hot liquid HMD present - and the maximum flowrate of cold feed is sprayed into the vapor space causing the HMD vapor to cool down to it’s dew point and causing a drop in the overall vapor space pressure due to the loss of the HMD’s partial pressure. However, you have to establish that the N2 supply would fail to operate and maintain the required N2. Additionally, there is a positive effect contributed by the maximum cold HMD flowrate into the tank: it increases the vapor space pressure by vapor displacement due to liquid addition.

In my opinion the possible failure of the N2 blanket feed supply is a credible possibility and this does not involve a double-jeopardy situation. Therefore you have a credible situation where you might pull a partial vacuum on the tank beyond the rated 1.5 inches of WC, and you must check to make sure you have enough inbreathing capacity through the vacuum relief. The solution of finding the HMD vapor condensation rate then is to establish the cold feed’s capacity to cool and condense the vapor and subtract the cold feed’s liquid flowrate. In order to condense the HMD vapor, the cold HMD liquid must sensibly cool the vapor to the dewpoint and then condense the vapor by removing the latent heat of vaporization. To obtain a complete solution, you strictly must apply simultaneous heat and mass transfer, but I would only employ the unsteady state heat transfer effect, assuming that the mass transfer rate is negligible. If not, then I would apply a conservative safety factor in the final vacuum flowrate requirement.


Case #2:

By working backwards, I assume you mean to say that you expect to be flashing the hot (180 oC) into the 80 oF storage tank. I would not take any credit for condensing any of the presumed flashing vapors. The amount of vapor generated by the flashing is determined by an adiabatic flash calculation of the hot HMD liquid (under presumed column pressure). However, one question arises here: isn’t this taking place 100% of the time, whether the storage tank is heated or not? If so, then the existing 6” relief valve should already be rated for the worse-case pressure scenario – which is the flashing taking place when the tank is operating hot, not cold. Clarification of this point may be needed to clearly understand the existing situation and potential for over-pressurization.

I believe I’ve interpreted your basic data and information correctly; if I haven’t please indicate so. I hope my comments are of some help.


Art Montemayor
Spring, TX

 

[tickle]

Relief System Engineer

Just be humble when a major (and well respected) contributor to this forum comments on the lack of information in your posting. You would not (well I would not) say the same face to face to superiors, so therefore somebody who is offering their experience gratis should be provided with far more respect.

Art was not necessarily attacking you personally, just reminding the rest of us mortals that the amount of effort that you put into starting a thread relates directly to the amount of effort people put into responding. And that the majority of engineers are professional (as I am sure you are) and that we spend enough of our time deaing with incompetents in our day to day activities that we get relief teaching others of our experiences.

I'll get down from my soapbox.


Art

Well done on your professional response to Relief System Engineer and thank you for your numerous postings that I eagerly read in the hope one day that I can be held in the same high regard as yourself. I certainly appreciate the amount of time and effort that you have contributed to this forum and others. Please continue.

Tony

 

[ReliefSystemEngineer]

Thanks a lot Mr. Art Montemayor. You made some valuable points & your comments are a great help.I will dicuss this further with my fellow-engineers & see what they have to say on this. Once again, I appreciate your help & thanks for your time.
Just to let you know, these vents are fabricated on-site & are not conventional Protectoseal or Sand & Jurs type vents. Its like a 6" double-pipe type structure with continous water supply to maintain a constant level of water(set pressure).The tanks were fabricated in the 60's,so not sure why ASME VIII code was used.

best regards,
PD