Emergency Relief Venting 1

[Clamont]

Is there any recommended pressure above the storage tank (API 650) design pressure that an emergency vent should begin to open? I cannot find anything in API 2000 about this however I have seen recommendations for 33% above the design pressure.

I look forward to peoples replies!

 

[LittleInch]

I'm actually troubled by your term emergency vent. If its something that the tank can experience then it should be part of the design. Can you describe a bit more what design preassure you have and what the other vents lift off at and why you need a separate emergency vent. You should not normally exceed the design pressure except for short periods. Tanks I'm not sure about, but piping and other vessels is normally no more than 10 %.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Clamont]

The emergency vent is emergency venting in case of a fire as stipulated by API 650, API 2000 and NFPA 30. We have a PVRV set at 7.5mbar and I want to know what pressure the emergency vent should begin to open at over and above the design pressure which is determined by the opening pressure of the PVRV.

 

[TD2K]

My understanding is that API 650 design pressure is also the maximum pressure the tank should see. Ulike ASME Section 8, Div 1, there is no overpressure allowance for normal upsets, fire, multiple relief valves.

So if your design pressure is 1 psig, the relief vent device has to be set under that sufficiently so you achieve the required flow with no more than 1 psig tank pressure.

 

[IFRs]

For emergency venting, API allows quite a bit more than the design pressure. As long as the corner weld does not fail. If the roof-to-shell joint is frangible, you don't need emergency vents. Frangibility is defined in section 5.10.2.6. If this joint is not frangible, then the vents need to comply with API 2000 using the pressures stipulated in section 5.8.5.3 a), b) or c).

 

[Clamont]

IFRs....

What factor of safety below the pressures stipulated in Appendix F would you recommend as there is nothing stated in API other than it must be below?

 

[IFRs]

In general, where API is silent on a subject then that subject is not required. API 650 is a fairly mature standard and it has in general good technical basis for all sections. In section 5.8.5.3, API 650 specifies the maximum internal pressure and does not include a factor of safety, therefor no additional factor of safety is required. It is likely that such a factor is already in the calculations, or it was considered and deliberately not included. I would not add additional factor of safety on this pressure. I would consider the possibility of the vent being compromised due to wear, corrosion, environmental factors, maintenance, etc and the real-life precision of the settings, also consider corrosion in the allowable pressure calculations.

 

[MJCronin]

In my opinion, this thread borders on a related and very important topic.

Given the validity of the statements above, WHAT SHOULD BE THE MINIMUM DESIGN PRESSURE FOR AN API-650 TANK ?

This pressure must consider a reasonable actuation pressure an overpressure for Emergency venting devices commercially available.

My opinion is +10 - 16 IWC.....but I would like to hear the opinion of others.

Are there any company guidelines or NORSOK or other standards that someone could regerence ?

Best Regards

-MJC

 

[IFRs]

Baseline API 650 tanks are suitable for an internal pressure approximately atmospheric but not exceeding the weight of the roof plate. Higher pressures up to 2.5 psig are provided for in Appendix F. Tanks with a design pressure over 2.5 psig are by definition not API 650 tanks.

 

[LittleInch]

I think on re-reading some of these posts thatwe have all come to the same conclusion. I think Clamont that you are putting the cart before the horse in your statement "...above the design pressure which is determined by the opening pressure of the PVRV." This is the wrong way round. The design presusre is calculated according to Appendix F section 4.1, 4.2 and 4.3, with the lower of these calcualtions being the design pressure. According to section 5.8.5.3, as IFRs says, the emergency venting pressure shall not exceed the design pressure calcuated in either F4.1, 4.2 or 4.3. The only exception to this is if the tank has been designed to have a frangible roof joint (i.e. the roof lifts off in an emergency (!!)).

Once you've worked out your design pressure from 4.1 to 4.3 or use the stated (name plate) design pressure by the tank designer (if known), then you set your vent pressures accordingly to allow "normal" venting to occur, but with sufficient margin to allow activation of the emergency venting, but always less than or equal to the design pressure of the tank. This calcualted using the vendors accuracy limits. Your statement above seems to say that you believe the current PVRV is set at the design presusre or are you making an assumption? If the PVRV is set at the design presusre, then you will need to lower it so that the emergency vent can be set above the normal vent pressure so that the design pressure is not exceeded.

In respect of the 33% figure you mentioned, maybe someone applied the 33% over pressure allowed within the B31.3 piping code which is permitted for leass than 10 hours and no more than 100 h/yr? I've seen people mix and match design codes before, but this is not permitted.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[IFRs]

In reading this thread, the question seems to be "what is a good pressure setting for an emergency vent". An API 650 tank with a frangible joint does not need an emergency vent. Guidance on the failure pressure is given 5.10.2.6 and F.6. F.4.3 gives guidance on the ratio of design to failure pressure. Not sure if this helps at all...