LWN inside standard diameter in NPS higher than 12 INCH

[Marcsierra]

In LWN (Long Weldig Neck) NPS higher than 12" most of the datasheets propose the inside diameter as the outside diameter of the Pipes acc. ASME 36.1
Which is the reason? Is it to be used as Slip ON Flanges?? If so, why is not allow or recommended directly a SO Flange?

I will appreciate your collaboration to solve this doubt.

 

[Afox88]

For LWN flanges, the ID of the neck/flange is always equal to the nominal pipe size. This goes for any flange rating (150#, 300#, 600#, etc.) When the flange rating increases, the OD changes, not the ID.

Slip on flanges will not work on long weld neck flanges. Slip on flanges are only meant to be used with pipe. LWN flanges have their own flanges integral with the neck that has the same bolt circle as all of the other ANSI flanges.

 

[Marcsierra]

Afox88, I have seen LWN with equal inside diameter of SCH80 Pipe ID:

http://www.friulanaflange.it/eng/pdf/B16.5/B16.5-LWN.pdf

I would like to know why sometimes PIPE STD ID is used and why in other cases the OD of NPS is uses as bore diameter.
As far as I know, LWN are also used in pressure vessels. In this case, you should use WT=0 Flange to match the LWN with the same bore and it has not any sense.

 

[Afox88]

If the LWN ID matches the ID of Sch-80 pipe, it is just a coincidence. Do note, that LWN ID matches the nominal pipe size, not the nominal OD. For example, lets take 8" NPS Sch-80 pipe vs 8" NPS 150# LWN.

LWN
8" NPS
OD = 9.75 (per ANSI standards)
ID = NPS = 8"

Pipe
8" NPS
OD = 8.625
Sch-80 = 0.5" thk
ID = 8.625 - (0.5 * 2) = 7.625

This is how ID is calculated for both options. Any resemblence in ID is purely coincidence.

 

[Marcsierra]

Thank you Afox88

The last question. Which WN do you select to match a 14" LWN?

I think, the best one for LWN 8" (ID: 8") would be: WN 8" WT:0.312" (equal ID of the LWN)

But with a 14" LWN?¿ a WN with WT: 0?¿¿?

Thank you for your support,

 

[Afox88]

LWN flanges have their own flanges that come integral with the neck. You don't need to weld a flange to it, it already has one.

 

[SnTMan]

It was not made clear previously, the bores pipe-to-LWN will not match.

Regards,

Mike

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

 

[Marcsierra]

Does it have any sense? If I weld a 14" LWN to a pressure vessel as a connection, I understand that this LWN should be connected with a 14" WN and the WN should be welded with a 14" PIPE.
In this case just in the union of the LWN & Flange there should be a mismatch due to the different IDs
I can not understand this matter.

 

[SnTMan]

Way it is....

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

 

[Afox88]

If you are concerned about this, you don't have to use a LWN. Just weld a piece of pipe to your vessel and put a weld neck flange on it. That is a perfectly acceptable alternative.

 

[jte]

So what is the concern with the ID's not being perfectly matched? If you are concerned about flow turbulence, I suppose you could machine the ID of the mating WN to perhaps a 3:1 taper so you don't have the sudden transition.

Afox88 provides another viable alternative which is to use a nozzle configuration which is far more frequently used than LWN's.

LWN's are very good from a vessel perspective, primarily as they are much more robust than your typical pipe nozzle neck to WN flange construction.

 

[SnTMan]

If a heavy neck is need for opening reinforcement, it is common to use a custom forged neck and weld neck flange w/ bore to suit the piping, neck thickness to meet reinforcement. Similar to Fig UG-40 (d) & (e).

Regards,

Mike

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

 

[Marcsierra]

ASME B16.5_ 2013 says:
2.8: Straight Hub Welding Flanges (LWN)
2.8.3 BORE. The bore diameter shall be equal to B (Inside Diameter) of the welding neck flange. Other bores may be furnished by agreement between the end user and manufacturer. In no case shall the bore diameter exceed the bore of the same size and class lapped flang

 

[LittleInch]

Marcsierra,

You're asking questions we can't answer as the datasheets and purpose is unclear.

your earlier attachment from a supplier makes it clear (in small writing at the top) that dimension B (Bore) ad Y are "specified by client".

Hence if you want a slip on flange, then your B is the OD of the connecting pipe. If it isn't then you calculate and specify "B" to match your connecting pipes and be strong enough for the pressure.

LWN flanges are mostly seen when you have a small distance from flange to next weld and hence can save a weld and don't have this small spool piece.

Often seen on vessels to allow stand off from the vessel. What B is is up to the designer.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Marcsierra]

Thank you LittleInch

 

[Gilmiril]

As far as I'm aware, "Long Welding Neck" flanges are not actually covered by any standard. So when that catalog from Friulana mentions ASME B16.5, they are only meaning that the actual flange geometry meets that standard. ASME B16.5 is very explicit about what it covers, and it only covers what it says. That does not include LWN Flanges (which means it would be incorrect to stamp such flanges as B16.5). So if no standard covers these flanges, there can be no expectation that the dimensions follow any standard.

 

[LittleInch]

LWN would be covered by B 16.5 as a "Flanged Fitting" as this has no dimension for length, but dimensions for bore and minimum wall thickness of the neck for different ratings.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jte]

Funny... I thought that what we commonly call LWN flanges were incorporated into B16.5 a dozen years ago in the 2003 edition. Specifically, I thought that paragraph 2.8 was a reference to LWN. If not... then can somebody give me an example of what this paragraph is referring to?

 

[SnTMan]

My '03 Edition calls 'em Straight Hub Welding Flanges, Fig 15

Regards,

Mike

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

 

[jte]

Yep. Fig. 15 looks like a LWN to me, and paragraph 2.8 seems to describe a LWN.

Looks like a LWH, sounds like a LWN...