7 replies to this topic

[Chemitofreak]

Hi,

I have some queries regarding storage tanks:

1. As per code, how much pressure accumulation is allowable in storage tanks (both LP (API-620) and ATM Storage Tanks(API-650)).

2. Is there any criteria for specifying the PVRV (Pressure Vacuum Relief Valve) and Blow-Off Hatch set pressure.

 

3. I have seen some engineers using the set pressure for PVRV less than the Design Pressure / MAWP for tanks designed by API-620. Is this a good engineering practice or is it case to case based on client's requirement, because as per code the set pressure can be equale to Design Pressure / MAWP is allowable.

 

Thanks in advance.

[fallah]

Hi,

 

1- For API 650 tanks no overpressure is allowed; For API 620 tanks the maximum overpressure of 10% is allowed for non-fire case and 20% overpressure is allowed for fire case...

 

2- There's not any specific criteria; but it has to be equal to or less design pressure...

 

3- Set pressure of PVRV in API 620 tanks can be equal to or less than tank design pressure...

[proinwv]

There has been much written on this forum about setting pressures for the valves and vents on a tank. I suggest that you search and read them.

 

But basically realize that besides a set pressure each device will have a reseat pressure and a max buildup pressure based upon the flow rate above set.

 

Devices need to have a dead band between them else it is possible to have two devices operating simultaneously which generally should be avoided.

 

MAWP and MAWV are what they say. I personally try not to operate up to that value.

[Manan_Joshi]

Will you still provide PVRV on API 650 tank ?

[proinwv]

If the vent valve did not lift, then I would look very closely at it. How do you know it did not lift?

[farhan.maqbool]

There are two PVRVs. Had they lifted,  the roof-to-shell connection would not failed due to excessive pressure.

[proinwv]

From what you have said, it seems to me that the first order of business is to inspect the valves and bench test them for set point. It seems unusual that two valves would have failed. My first thoughts are an environmental issue that has interfered with the action or simply the wrong set point.

[Art Montemayor]

farhan.maqbool:

 

I consider your posting of a failed frangible roof on an API 650 tank to be important as a topic.  However, this thread belongs to the OP, Chemitofreak.  By posting your topic here you are skyjacking his/her thread.  Please start a new thread within this Forum with your specific topic.  I will give you a couple of days before I delete the postings on this thread.  Please comply with our Forum guidelines.

 

Just like ProinWV, I have some queries and comments on the information you posted and would like for you to clarify these points:

 

Your Data Sheets are not complete.  You should be in possession of detailed design calculations as well as fabrication drawings.  The diameter and height of your tank are not even given.  The critical information for a frangible roof tank should be given in detailed calculation and data sheets.  Although I am not a mechanical engineer, I have enjoyed the priviledge of working directly with mechanical engineers in the design of storage tanks - both API 650 and 620 - and in my experience I was advised that for small diameter tanks, it is often not possible to have a frangible roof joint

 

Your Pumped inlet nozzle is given as 80 NB (3”) and your 2 PVSV nozzles on the roof of the tank are 80 NB (3”) each.  The 2 nitrogen “purging” nozzles on the roof are each 50 NB (2”).  Is this information correct?

 

What is the amount of inert nitrogen flowing through a possible failed open nitrogen control valve into the tank?  You must have this information available - as well as the tank’s design calculations as furnished by the fabricator.

 

In my opinion, I would never utilize a frangible roof tank in Dimethylformamide (DMF) service.  I frankly don’t see the reason for it.  Although DMF is not classifiable as a human carcinogen, it is thought to cause birth defects.  In some sectors of industry, women are banned from working with DMF.  In my opinion, to safe guard the tank and its contents in the event of an over pressure, it is far more practical, safe, and efficient to use an emergency vent.  It is very easy and cheap to install it in the roof manway, which API recommends to be 20" or larger (I always used 24” as minimum).  It gives lots of venting area for little money and keeps the tank sealed tight at normal operating conditions.

 

When you start your new thread, I will furnish you with design and other information.