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What is the Major Difference between API & ASME Standards?

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ASME (American Society of Mechanical Engineers) produces a series of documents. API (American Petroleum Industry) also produces a series of documents. I don’t know where in this small world of ours you are located but in the United States, all states except for two have adopted ASME as code or law. This means that the facilities located in these states must comply with what is in those ASME documents. These documents describe how a pressure vessel is to be constructed and what must be done to that vessel to prevent a catastrophic failure caused by over pressure. They also tell you the types of devices that are acceptable to use to accomplish this.

Being that ASME dictates what your facility must comply with, API documents tell you how to achieve compliance. The three most common of those documents are API RP520 Parts 1 and 2 (RP stands for Recommended Practices) and API Standard 521. Note that these API documents are not law but only packages of standards and recommendations (by the way, “RP” or “standard” makes no difference, they both carry the same weight as being nothing more than standards). So these (API) documents give you all the details you need (well, almost) to comply with the law (ASME). In other words, API provides those “rules of thumb” you are looking for.

Be aware that many of these “rules of thumb” may not be good for every relief device you have. For instance, the effective coefficient of discharge for a gas/vapor/steam relief valve is given as 0.975 as a “rule of thumb” but Farris PSVs use 0.953! So you can use API documents as a first approximation but must eventually check your design against the true values for the device you eventually purchase.

OK, even though the API documents are “just” standards and recommended practices, in the United States they are basically considered to be “good engineering practice” and you would be very foolish not to follow them as a minimum. You can be more conservative than what these documents say but you should not be any less.

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  1. It is true that these types of documents represent what we consider good engineering. People want to know that you have these types of thing and will look into it in order to ensure your credibility. I appreciate your description of how these things work and the professional advice you provided. Thank you!

  2. Hasan Siddiqui

    Both ASME & API produce series of documents. It depends on the facility owner. Every operator/owner has its own engineering specifications. These specs are prepared using local government laws and chosen codes & standards. Codes & standards will allow something; but local government law might not allow it; similarly, owner/operator might not allow it. The order of precedence shall be followed that is governed by how stringent the requirement is.

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