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Question:

API 17D recommends tightening to between 67% and 73% of yield on bolts for flanges, type 17SS and 17SV. API 6A recommends tightening to 50% of yield. If flanges specified to API 17D are tightened to the recommendations of API 6A then presumably this would result in an under stress condition and the connection would therefore be unacceptable? Are there any circumstances where the minimum of 67% in API 17D can be reduced? Second question, if an existing flange specified in API 6A is connected to a flange specified in API 17D, which bolt tension should be adopted? I can't find anything in the API Specs, but do you know of a requirement when using API 6A to increase the bolt tension from that specified (50% of yield in API 6A) when using on joints rated to 10,000 psi?

Some facts we know about flanges made to seal with BX ring gaskets may allow understanding of API 17D's specified bolt tightness equal to 67% to 73% of bolt yield strength.

Experience has shown, and API mentions in API Spec 6A, that due to dimensional tolerance, allowed for ring grooves in flanges and for external dimensions of ring gaskets, flanges sealed with BX ring gaskets may not meet with the raised face portions in direct contact, as intended by design, when bolts have a tension of 50% of yield strength.

A small gap between tightened flanges will seldom create a problem in surface well head installations, but because of bending forces and inaccessibility, subsea installations should have no remaining gap between flange contact faces after flange tightening. To assure closure of any gap remaining between subsea flange connections, API 17D specifies that "Closure bolting of all 6BX and 17SS and 17SV flanges shall be made up using a method that has been shown to result in a stress range of 67% to 73% of the bolt's nominal material yield strength."

In subsea applications, all API 17D flanged equipment join and seal with API BX ring gaskets (smaller 5,000 psi working pressure flanges, having BX ring grooves, being exclusive to API Spec 17D), these flanges intended to contact face to face, having inlayed corrosion resistant ring grooves as specified in API 17D. Bolt material used for subsea connection assembly shall have 105,000 psi minimum yield for diameters 2-1/2” and less, and 95,000 psi minimum yield for diameters greater than 2-1/2.” In subsea applications, bolts and nuts normally have Fluoropolymer coating, while surface applications may only use plated or un-plated bolts and nuts. Fluoropolymer coated bolt and nuts will have a lower coefficient of friction.    

We at WOODCO USA do not recommend tightening surface well head bolts to more than 50% of yield strength, but we don't know of any reason you should not, and we would not offer any advice for tightening subsea bolting that might conflict with API 17D specified tightness for subsea applications.

Click here to see a Bolt Torque Calculator that allows determining bolt torque required to achieve % of yield with different friction factors.

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