ASME Vessels v API Sizing Method: 4

[mrtangent]

Dear Conserned Relief experts:

For various reasons we purchase ASME relief valve but we actually work in API on our site. I've no experience in ASME rating but our supply vendor uses ASME calculations.

I am curious: For ASME: Does a rating document exsit - eg.g that gives you a basis for how big the valve needs to be (e.g. like API521), and covers details of flaire, protection systems, fire height etc.. ?
or is it more a basis to determine a valve orifice size.?

how extensive is ASME on covering relief cases? is it as detailed as API521 ? Etc..

 

[don1980]

mrtangent,

Your post is very difficult to understand, but I think I know what you're asking.

Here is what I understand from reading your post. You are use the API standards as your "design manual" for pressure relief, and you're asking if ASME publishes detailed design guidance that is similar to that found in the API standards.

The answer to your question is no. The pressure relief section in ASME Sec VIII is relatively brief and it only contains high-level requirements. The API standards (521, 520 Parts 1&2) are complementary to ASME Sec VIII. In other words, the API standards work with ASME Sec VIII rather than being an alternative to it. The API standards function as a users manaual for how to comply with Sec VIII. They provide the user with detailed explanations of how to evaluate potential overpressure scenarios and how to then proceed with the actual sizing of relief valves. ASME Sec VIII is not at all a users manual for how to do this work. It simply states the rules. The API documents give engineers the practical guidance for applying those rules.

 

[mrtangent]

Sorry for the confusion Don,

API does have a method to size the capacity of valves.
So does ASME..

1. API 521 Gives a rating case
2. API 520 gives a valve capacity
3. ASME Sec XXX - Gives a capacity.

Or rather some relief valve vendors only use 3 not 2.

 

[don1980]

API 521 provides guidance in how to evaluate potential causes of overpressure, and how to evaluate the resulting risks.

API 520 provides guidance related to sizing and installation.

As I mentioned above, ASME Sec VIII only provides high-level requirements. It intentionally leaves all the risk evaluation and sizing decisions to the user. API fills that gap by providing practical advice to the end-user, including a procedure for sizing PSVs (API 520 Pt 1) that are compliant with ASME Sec VIII. API and ASME do not offer different methods for the final sizing of PSVs. Possibly you're confusing the API preliminary sizing procedure. That is simply a way to get a preliminary estimate of the PSV size. But, the final sizing must be done using the ASME orifice size and coefficient - API 520 Pt 2 clearly states that. So, understand that API and ASME do not have separate methodologies for the final sizing of a valve.

When a vendor sizes a valve for you, they are doing a final sizing calculation. That is, they're using their actual orifice size and their certified flow coefficient. When you size a valve using the API orifice sizes and API coefficient, you're simply doing a preliminary estimate of the valve size. That's a generic estimate that is not based on any vendor's specific size for that orifice, nor their specific coefficient. Obviously, once you go to a particular vendor, they're not going to use, for example, the generic API orifice size a G orifice. They'll use their specific size for that G oriffice and their certified flow coefficient. When you follow the API sizing procedure to its conclusion, you identify a specifc vendor and update your preliminary calculations by replacing the API generic values (orifice size and coefficient) with the specific valves for that particular vendor. The vendor's actual orifice size and actual coefficient values are refered to as ASME values, because they're certified according to ASME testing & certification protocol.

SUMMARY:
API Sizing = (1)estimate size using generic API values for orifice size and coeff, then (2)update the step-1 calc's by substititing the vendors values ("ASME values" for orifice size and coeff.) for the generic values used in step-1.

ASME Sizing = perform calcs using ASME orifice size & coeff -- this is exactly the same as the last part (step-2) in the API Sizing.

 

[Korichnevijgigant]

I have always used API 520 and NFPA 30 for firesizing

 

[don1980]

Korich..

It's perfectly OK to use NFPA-30 for fire sizing. There are two equally acceptable calculation methodologies for fire exposure to pressure vessels: NFPA30 and API521.

My observation is that most companies use NFPA30 for storage tank and API 521 for pressure vessels. But, there are many companies that NFPA30 for both vessels and tanks.

That's perfectly OK, but you can't do it the other way around, at least in the US. That is, you can't use the API521 heat input calculation for storage tanks. OSHA mandates the NFPA30 methodology for US storage tanks. That method, by the way, is the same as that found in API 2000 with one exception. If your tank is insulated, you must use the NFPA30 insulation credit factor rather than calculating the F value according to API 2000. The API F calculation will result in a greater reduction of the heat input than allowed by OSHA (NFPA30). Other than that, the API 2000 and NFPA30 calculations are identical.

 

[mrtangent]

a quick question for NPFA 30.. is the fire engulfment area formula different..

e.g. in APT 521 it is 25 feet and Q=F * Constant* A^0.82

 

[Korichnevijgigant]

I just looked into NFPA 30, and there seems to be no fire engulfment area formula

I searched the electronic copy, but nothing came up in the search.

I thought there was something, but I guess I was remembering API

 

[mrtangent]

I found the following summary useful

http://www.chemcad.co.uk/downloads/MNL036.pdf

 

[don1980]

The NFPA30 fire heat input equations, max elevation (30 ft), and surface area determinations are exactly the same as those you'll find in APi2000/ISO28300. The only difference is the credit factors. So, to find the difference between the fire heat flux equations from API521 and NFPA30, just compare the equations in API521 and API2000/ISO28300.

Be aware that NFPA30 has fixed values for heat reduction credits, while API521 and API2000 calculate the F factor based on insulation material and thickness.