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Smallest orifice D for relief valve and confusion with API and ASME sizes

Smallest orifice D for relief valve and confusion with API and ASME sizes

Smallest orifice D for relief valve and confusion with API and ASME sizes

Dear all

I have come across another thing which made me puzzled.

One of the existing relief valve is 3/4" X 1" with orifice area of 0.04in2. Supplier confirmed it is an ASME certified valve. If we compare this with the smallest orifice D of API, it hour be 0.11in2 and equivalent of ASME, it's 0.134in2.

My queries:
1) as the existing valve does not have API or ASME specified area, is it still ASME certified/compliant?

2) existing valve was found to be smaller, supplier says he can machine the orifice area to say 0.11in2 (API orifice D) and it will still be ASME certified. Does that sound ok? I meant is this normal practice?

3) agree, we use formula to estimate area and check against API orifice and also we need to multiply ASME Kd by 0.9 while using the API formula to calculate orifice area etc but why sometime relief valve has API orifice and sometime ASME orifice? And why both exist? Are we saying calculate all using API formula but then actual orifice area must be that given by ASME?

Thanks in advance

RE: Smallest orifice D for relief valve and confusion with API and ASME sizes

Thanks Dejan

I have gone through that thread but could not figure out much on question I have please.


RE: Smallest orifice D for relief valve and confusion with API and ASME sizes

Quote (chemks2012)

Supplier confirmed it is an ASME certified valve.

Do you not believe your vendor? Ask for the National Board certification or look it up in the "Red Book". If certified, the valve should have the NB stamp.

After looking through, NB-501 and NB-18, aka the "Red Book",I didn't see where the orifice size must be a certain size. ASME/National Board certifies the capacity ratings of pressure relief devices, not the orifice area. However, through time certain size orifice designations have become common knowledge and used by most vendors, but not all. Don't get hung up on the orifice designation too much. Just be sure the relief device is certified. Again, check the "Red Book".

Good luck,

Technically, the glass is always full - 1/2 air and 1/2 water.

RE: Smallest orifice D for relief valve and confusion with API and ASME sizes

Relief design terminology and lingo is a common source of confusion. API 526 valves (D-T orifice valves) are just one of many different types of safety valves. All API 526 valves are “ASME certified” valves, which means they’ve undergone the ASME testing procedure to determine their flow coefficient (they have a known tested flow capacity). There are thousands of other valves (non API 526 valves) whose manufacturers have paid for this same testing. Thus they are ASME certified valves but they’re a different type of valve from API526 valves. Many of these are tiny valves that you can fit into your shirt pocket. A “D” orifice is the smallest API 526 orifice, but there are tons of ASME certified valve that are smaller than a “D”. Any ASME certified valve satisfies ASME code requirements, but most companies prefer to use API 526 valves for medium to large vessels because they’re interchangeable (fixed face-to-face flange dimensions) and they’re easy to work on. API 526 valves come in a set of standard nominal sizes (D-T), however there’s no such thing as a “standard ASME orifices”. An ASME certified valve orifice can have any orifice size – it’s just an orifice that has been tested to determine its certified flow coefficient.
With that background, here are my comments to your questions:

1) This particular valve may or may not be built to the API 526 standard. Read the info from the nameplate, and then look it up in the vendor’s catalog to see if it’s identified as an API526 valve. This won’t be hard to figure out. If it’s an API526 valve, then it will be clearly labeled as such in the catalog, and it will have one of the API orifices (D-T) in it. There’s no such thing as “standard ASME orifice sizes”. An ASME certified valve can have any orifice of any size. There’s no set of discrete “ASME orifices”.

2) If this existing valve has the necessary capacity, then there’s no reason to bore out the orifice to make it the size of a D orifice. There are no countries or pressure vessel codes that require the use of API 526 valves. If you’re in an ASME jurisdiction, then you need to use an ASME certified valve, but it doesn’t have to be an API526 valve. If you’re in Europe, then it doesn’t have to be an ASME certified valve.

3) When you buy an API 526 valve, you’ll find that the actual orifice area is almost always a little larger than the nominal API orifice area, and the actual area will vary from one manufacturer to another. This stems from a change in ASME Sec VIII code back in 1962. We’re all familiar with the fact that ASME requires a 10% derating of the valve’s capacity. That requirement didn’t exist before 1962. Once this change was implemented, it reduced the rated capacity of all ASME certified orifices, including the orifices in API 526 valves which most people use. The manufacturers could have chosen to leave the size of their D-T orifice the same, and suffer capacity reductions, or they could make them larger to offset this reduction in effective capacity. They chose the latter. They increased the size of their API 526 orifices to avoid an effective reduction in capacity for this widely used type of PSV.

RE: Smallest orifice D for relief valve and confusion with API and ASME sizes

Thanks Latexman
Will check that NB stamp.


Thanks a ton for your valuable input.

I was referring to API vs ASME orifice area given in the link below.

When you say, if you are in Europe (yes, i am in Europe),valves do not have be ASME certified then which is the certifying body?


RE: Smallest orifice D for relief valve and confusion with API and ASME sizes

You'll have to pose that question to the author (Andrew) of that blog post. I don't know what he's referencing in that column "ASME area". It appears that he's listing orifice areas for a particular (unidentified) vendor. Regardless of where these nuumbers come from, they don't represent a set of standard ASME orifice area values - there is no such thing.

The current governing rules in Europe are those in the Pressure Equipment Directive. PSVs in Europe must be CE stamped. Whether or not the PSV is ASME certified isn't relevant in Europe.

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