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ERV SET PRESSURE

Saturday, August 05, 2017   By: ADI HAR-SHOSHAN [93] 4 Stars
To what pressure an ERV located on a Non-frangible Atmospheric Unanchored fixed roof tank should be set of :
1. The roof plates weight divided by the tank wetted area? OR
2. As per the formula of API-650 F.4.2?

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Tuesday, August 08, 2017   By: ITS [1678] 3 Stars
Okay, I think API-650 Appendix F should be used for guidance, but consider API Standard 2000 Venting Atmospheric and Low Pressure Storage Tanks as well as NFPA-30.

Do you have a photo of what is being used now?

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Tuesday, August 08, 2017   By: davidr@fast.net [39] 5 Stars
In general, your emergency pressure / vacuum settings need to prevent over stressing the tank and its components. Note that we are not sizing normal vents since your question is for ERV Emergency Relief Vents.
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The device should have pressure vs flow curves from the manufacturer.
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Size the vent so it flows the required CFM or CMH calculated per API 2000 at a pressure / vacuum that will not damage the tank.
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The set pressure is always less than this pressure because most vents dont flow much at the set pressure when they just start to open. You may need multiple vents.
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It can get complicated if the set pressure for these vents is close to another system such as Nitrogen blanketing. Sometimes you cant get a vent with a high enough set pressure to not affect the other systems yet flow enough at the design pressure. You may need pilot operated vents, etc.
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For a non-frangible tank, the shell-to-roof joint will likely and should be your weakest point. You use Annex F to calculate its failure pressure. Your vents should not exceed this pressure at the required flow rate. Your customer may want some factor of safety applied, or some head room to this failure pressure. Each tank is different, look for roof appurtenances or design details that may introduce a weaker area, or artificially strengthen the roof-to-shell joint piping, hand rails, etc .
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Note that these tanks are usually anchored and you dont want to exceed the capacity of the anchors.

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Wednesday, August 09, 2017   By: ADI HAR-SHOSHAN [93] 4 Stars
My question is to what pressure the emergecy roof vent should be set in the case that the roof in a non fringible type and in order to satified API-650 5.8.5 we have to install ERV.
Now, API-650 1.1.1 state that that the internal pressure should not exceed the roof plates in the case of an atmosphric tank basic design , on the other hand and api-650 5.8.5.3a state that relife pressure should not exceed the calculated pressure of F.4.1 & F.4.2.
I hope that clear my question and please correct me if Iam wrong

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Wednesday, August 09, 2017   By: ITS [1678] 3 Stars
The reason you are not getting a straight-forward-answer is there is a lot engineering judgement that has to be applied. The weight of the roof is a reasonable start, but having the tank un-anchored is very-troublesome, for in catastrophic event the bottom of the tank will separate from the shell while the shell and roof lift off together or intact. I think yo already know this... Without it anchored the tank at a minimum will get a rounded bottom like is wants to tip over. Ugh! The frangible roof is a lesser of 2 evils and will damage the tank, for sure, but prevent over pressurization from boiling-liquid with massive vapor production and minimize the calamity.

So, the basis of thought is as follows - 1- set pressure might be better understood by determining the strength of what the tank floor near the shell annular plate can resist before tearing -fillet welds of floor plates- at elevated temperatures. Then consider a number less than the weight of the roof and take the lessor of the two. Then give a margin of safety. 2- The relieving capacity of your emergency relief valve or ERV must be greater than the saturated vapor created on a full boil or rolling boil. You may have to think about superheated vapor if your ERV are marginal in size. I think you know how to do that too.

I am thinking of multiple large rupture -disk- square diaphragms. They will require maintenance.

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