Emergency Venting for Fire Exposure for Double Wall Storage 1

[duaneloh]

I have a double wall storage tank. The inner tank (primary tank) has 2 vents installed (3" conservation and 8"emergency vents) for overpressure and vacuum protection, while the outer tank (secondary tank) has only a 3" conservation vent) installed.

Question 1: Does anyone know if there is any other standard convering overpressure / vacuum protection for a double wall storage tank besides API 2000?

Question 2: I'm trying to find the required emergency venting rate for the conservation vent protecting the outer tank when the tank is exposed to fire.

I browsed through API 2000 and NFPA 30 and cannot find any calculation method for emergency venting for the interstitial space.

API 2000 SEC 3.4.3.2 only says Emergency Venting For Fire Exposure For Double-Wall Refrigerated Storage Tanks ..... becos emergency venting for a double-wall refrigerated storage tank is complex, no calculation method is presented here. A thorough analysis of the fire relief for a double wall refriegerated storage tank should be conducted.

Is there any calculation method for determining the required relief flowrate for vapor expansion due to fire exposure?

I have tried using API 521 SEC 3.15.2.1 Equation (8) but the rate is too high.

 

[EGT01]

Duaneloh,

To start, you haven't indicated to what standard or code that your double wall storage tank was built. The fact that it is double wall design does not necessarily determine what relief protection guidelines you should follow and I would say follow the guidelines that relate to the code or standard to which the vessel was constructed.

My experience with relief design for double wall tanks is limited to ASME pressure vessels. In those cases I followed the guidelines in ASME Sec VIII and in the API recommended practices 520 and 521 for relief protection of the secondary system same as for the primary.

If your tank is built to API-650 or API-620, then the guidelines in API-2000 would be more appropriate but, as you noted, their direction regarding emergency venting for double wall tanks is less than complete. In other cases of abnormal heat input from external sources that had little effect on the liquid, I have estimated the effect of vapor expansion similar to the way outbreathing requirements are determined for an atmospheric tank.
Relief Rate = dV/dT * dT/dt, where
dV/dT = the change in volume per change in temperature
dT/dt = change in temperature per change in time and would be dependent on the rate of heat input.

Recently, I had to review the relief system on a storage tank that was built to UL-142. Though the tank I was looking at was not double wall construction, in reviewing UL-142, it has a section that specifically addresses relief protection for the annular space of secondary containment tanks. Interestingly enough, UL-142 indicates that the emergency venting requirements for the annular (or interstitial) space is derived from the same table as for the primary tank and is based on the wetted area. I would add that the emergency venting requirements table in UL-142 is taken from NFPA-30 which gets back to a liquid vaporizing event.

Now, after reading UL-142, it might be good to know the purpose for the secondary tank. Is it secondary containment? Then it might not be unreasonable to think that it may contain some amount of liquid at the time a fire occurs and vapor expansion may not be controlling.

 

[duaneloh]

EGT01

Thanks for the response.

As per the Equipment Spec, the double wall storage tank is made to the API 12F and UL-142 standards and I was told that the secondary tank does not contain any liquid.

When UL-142 specifies the emergency venting requirements for the annular space is the same as the one for primary tank, are they considering the fact that there might be some liquid contents in the secondary tank due to leakage when fire happens and so liquid vaporizing will be controlling?

I’m planning to use the method from I.Heitner, T. Trautmauis, and M. Morrissey, “Relieving Requirements for Gas filled Vessels Exposed to Fire” to determine the vapor expansion rate. Did you know where can I get that article?

Also when I evaluate the 8” emergency vent installed in the inner tank, I just use API2000 method to determine the heat input. I believe this approach is conservative. What do you think?

Thanks again.

 

[EGT01]

Duaneloh,

I don't have much experience with tanks built to these standards so I would suggest getting copies of both API-12F and UL-142 to be sure you are following their requirements. Each one includes their own table of venting requirements versus wetted area but they both seem to be based on NFPA 30.

http://api-ep.api.org/publications/index.cfm

http://ulstandardsinfonet.ul.com/

API-12F cost is not too bad but UL-142 is somewhat pricey, you may want to check your company or local library to see if you can find a copy otherwise.

Even though I would think that tanks constructed to either API-12F or UL-142 could be used in similar service, I find it unusual that there would be two different standards used in the construction of one tank. You may want to discuss this with a mechanical designer to know which one may be controlling.

Regarding the UL-142 emergency venting requirements, I would say the primary and secondary tank required vent capacities are only the same in that they are derived from the same table. The table is based on wetted area so you have to decide what wetted area of each section will be used to determine venting capacity. This is just a quick interpretation on my part and you should review UL-142 to make sure you understand. And, yes, I would say using the table of venting capacity versus wetted area for the interstitial space implies that liquid is present during a fire event.

Regarding using the API-2000 method to determine heat input, I don't see that either API-12F or UL-142 reference using API-2000 as a means of determining venting capacity. Both API-12F and UL-142 say to follow the tables included in their respective standards to determine venting requirements versus wetted area, there does not seem to be a heat input calculation method. I would be cautious about using a heat input method along with a latent heat value to determine venting capacity especially if it would result in a venting capacity less than what the tables would indicate for a given wetted area. Ultimately, I believe you must comply with the venting requirements as defined by the standards to which the tank is constructed. Now, since the venting capacity versus wetted area tables in API-12F and UL-142 are based on NFPA-30 and the table in API-2000 is based on NFPA-30, I think it would be reasonable to use the table in API-2000 and get the same result.

Regarding using the method from I.Heitner, T. Trautmauis, and M. Morrissey, “Relieving Requirements for Gas filled Vessels Exposed to Fire”. This method is referenced in API-521 related to pressure relief of an unwetted vessel. As referenced in API-521, the method was published as part of the 1983 Proceedings-Refining Department, Volume 62, American Petroleum Institute, Washington, D.C., pp. 112-122. In the past, I've made a modest attempt to locate this document by searching the above link to the API publications but was unable to find that API still offered the publication. I have not contacted API directly about this, so you may want to give that a try unless someone else might know where/how to obtain a copy. I would be interested to know myself.