Why API prohivited the use of drilled and tapped hole on tubesheet ?

[elen_28]

Hi,
Why API prohivited the use of drilled and tapped hole on tubesheet since 2018 ?

(but TEMA 10th Edition added the clause about collar bolt and drilled hole since 2019.)

Did API discovered the something of bad things about drilled and tapped hole ?

Quote:
Supplementary Specification
to API Standard 660
Shell-and-Tube Heat Exchangers,december 2018

7.5.2 Tubesheets

7.5.2.1
Replace section with
For a vertical heat exchanger where the stationary tubesheet is at the bottom, a suitable means of holding the
tube bundle in place shall be provided for when the bonnet or channel is removed. If collar bolts are used, at
least four integral (one piece) bolts shall be provided with their location identified on the drawings and by
stamped markings on the outside diameter of the tubesheet. The use of drilled and tapped holes is prohibited.

7.5.2.4
Replace section with
A full diameter stationary tubesheet shall be provided for removable tube bundle exchangers with bonnets
(see TEMA, Figure N-1.2, Type B stationary head). The tubesheet shall be provided with integral (one piece)
collar bolts for a minimum of 25 % of the bolts (four minimum). The tubesheet design shall allow for hydrostatic
testing of the shell side with the bonnet removed and all bolting installed. The use of drilled and tapped holes
is prohibited.

 

[SnTMan]

elen_28, It is unclear to me from your post. are you quoting API 660 itself or a client specification?

Either way, the reason is to prevent the studs from becoming seized in the tapped holes thru corrosion or other means. The studs can then be extremely difficult to remove and doing so can damage the studs themselves or the tubesheet.

However in some classes of work tapped holes are acceptable.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Never use tapped hole in the tubesheet, this prevents the correct tensioning of the stud.
Also see PIP VEFV1126.

Regards

 

[elen_28]

SnTMan,
I confused “Supplementary Specification to API Standard 660 Shell-and-Tube Heat Exchangers,december 2018" to API regulation.
This supplementary specification added technical consideration to API660 is published by The International Association of Oil & Gas Producers (IOGP)
API and IOGP have strong partnership, but this supplementary specification is not mandatry with API660.


r6155,
I also think that tapped hole is problem method.

1. Possibility to damage on tapped hole
→ Tubesheet would have to be replaced.

2. Unstable axial bolt force
If tubesheet bolts to channel flange prior to shell flange for mistake of manufacturering staff, axial bolt force between tubesheet and shell flange may weak than channel side.
→ If channel cover is removed, leak of shell fluid may happen because shell flange and shell side gasket may free from axial bolt force in maintanance.

Regards

 

[r6155]

@ elen_28
Be careful
Collar studs are ONLY to maintain the gasket integrity and position of shell side.
Collar studs are not to maintain pressure in shell side.

Regards

 

[elen_28]

@r6155

Thank you for teaching me.
In Japan, some of users and manufacturers misunderstand the function of color bolt.
They believe that tubesheet with bolt holes is able to perform hydrostatic test without using a test ring.
I have seen some of heat exchangers that the tubesheet is not even considered for bolt load.
(thus the tubesheet thikness is thinner.)
Your information is very helpful for me.

Regards

 

[SnTMan]

elen_28, your experiences are not uncommon.

"If you could hydrotest with only 25% bolts installed, the other 75% wouldn't be there in the first place."

True conversation, unnamed engineer to client

The problem with sloppy work is that the supply FAR EXCEEDS the demand