Straight Welding Crosses 1

[Southcoaster]

Hi All,
I am very new to pipe design and have the following query.
I am trying to purchase an 8" welding cross, but am being told this is on a long lead. I was wondering wether I could fabricate the cross from 8" pipe (API 5L). Inspecting this it would appear that an 8" to 8" joint was a bit of and ask as the land top and bottom would be small and the possibility of weld interfrence is high. Would I be correct in thinking that a cross of equal pipe size was not possible?
Also is there a good text on pipe design which would help in situations like this and similar?

 

[IVNJ]

Southcoaster,
Call a few more supply houses - we sell crosses every day. A234WPB - other materials may be more difficult to locate.

 

[BigInch]

My suggestion is to try to avoid crosses. I've personally never used one in 30 years.

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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[vesselfab]

Many specs specifically omit the use of crosses.

if you decide to fabricate one, I have seen the use of two tees, split and two longitudinal welds used.

 

[jistre]

Crosses certainly are a relative rarity. You can get the same function by running two tees in series: four piping legs joined together. The added bonus is that you can position the tees as close as possible to where the piping runs terminate and cut down on the pipe you need to use. A cross requires all four legs to terminate at the same point in space and may add unneeded piping.

F'rinstance, consider four legs that are connected. They are in pairs and each of the pairs is 100 feet apart, 2 here and 2 over there. If you use a cross, it will require at least 200 feet of pipe to connect everything up, no matter where you position the cross. Positioned in the middle, it's 4 runs of 50 feet. At either end it's 2 runs of 100 feet. To do the same thing with tees takes only 100 feet of pipe. You position one tee at one pair of legs, the other tee at the other pair of legs and run 100 feet of pipe between the two tees. It looks different than a cross, but it's still just four piping runs that all connect.

 

[jistre]

I just realized I neglected the part of your question asking for references.

I'm assuming that you're interested in standard buttweld fittings. If so, look at ASME B16.9-2003 Factory Made Wrought Buttweld Fittings, Mandatory Appendix I.

In that appendix, you'll find all the standard fitting types broken down by classes with dimensions. Crosses, like tees, have both straight and reducing standards.

You'll find that having copies of the ASME standards for any flanges, fittings, piping, etc. that you'll be working with on any project is going to be useful. Treasure trove of info.

 

[Southcoaster]

Thanks Guys,
I took the root of welding two tees together, cost a bundle but does the job a treat.

 

[zdas04]

I use crosses all the time. I've never had to wait more than a few days for either a straight or reducing cross smaller than 20 inches, and I have never seen any code that put any limitations on the use of a cross (vesselfab, do you have any references). They are worlds better than a weld-o-let on the backside of a tee (which I see pretty often).

I don't understand most of the comments above, nor do I understand why you would make an extra weld to put in two tees back to back (let alone the complex operation of cutting up a batch of tees to build your own; post-manufacturer longitudinal welds are mentioned unfavorably in the codes).

Crosses tend to be an elegant solution to several problems and a pipe designer that avoids them makes me wonder what other brute-force techniques he's employing.

Southcoaster, you should have hung in there, called every supplier in your area until you found one that had your interests at heart. You'd find it is worth the effort to get your design built the way you envisioned it instead of a pipe sculpture.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[KernOily]

Hi David. A lot of the old-line heavy designers, e.g. Rip Weaver, would pound it into your head that a cross was the absolute last resort because they were much more expensive relative to other fittings, especially a stub-in nozzle. According to him there is/was always a way to design around the use of a cross that would be cheaper to construct than the use of a cross. Not sure I totally agree with that, but he did this bidness a lot longer than I have thus far, so I figure he knew what he was talking about.

Mind you, his perspective was in a plant environment. Out in the oil patch, things are different to some degree. Our designers do not use them either though as a matter of course. Probably inertia, more than anything else.

Another important issue to keep in mind is that an SIF can't be quickly calculated for a cross, as far as I know, therefore you can't evaluate the stress level at a cross in accordance with the B31 codes in the way I can for a welding tee, a WOL, or a stub-in. In Kalifornia I have to demonstrate that any system I design can conform fully to the B31 codes. If I had FEM software I could calculate an SIF and defend it. Otherwise, I can't. Thanks! Pete

 

[vesselfab]

zdas04 (Mechanical)

I never said a code would not let you use them.

I said many specifications (users specs) that will not let you use them. Just like they won't let you use a bushing to reduce treaded pipe. Some companies just don't like to use them.

 

[zdas04]

vesselfab,
Sorry, I misunderstood the implied (user specs). Yeah, I had a boss once that said that company specs are like a cat on a stove: "If a cat jumps on a hot stove, he'll never jump on any stove ever again". The specs just keep growing to proscribe everything that ever caused anyone any problems for anyone in any operation. A lot of things are forbidden in Oil & Gas because they don't work with H2S so no one can use them on sweet service. I tell my clients that they should discard their specs and start over every decade. No one ever listens to that particular advice.

Pete,
I've heard that about crosses being expensive, but I've found them to be about 30% higher than a tee, so it's 35% cheaper than two tees, and the extra weld that you lose is just gravy.

I understand about the limitations of the stress programs. I thought Caeser had crosses now (they are easy to find in CadWorx). When I designed some pipe at Elk Hills with a cross a while back, the stress guy didn't kill me. I never heard if it went in or not.

David

 

[KernOily]

Hi David. CAESAR will give you a warning that 'you are framing four pipes into one intersection' (Pete paraphrase), that it can't calculate an SIF, and that it will use SIF=1.0 (which is a huge error). There is no Code SIF formula for a cross. In a critical application, then, where stress levels are expected to be high at a branch connection, one would be advised to avoid the cross simply because it can't be justified, whereas some other type of branch conection can be justified using Code rules. As I said, in Kalifornia, everything is governed by CAC Title 8 and the owner's insurance carrier. Many carriers have more stringent requirements than Title 8.

I'm not surprised about your stress guy not saying anything. A lot of them override the warnings in order to just get the job out. Hell, a lot of them don't even know what the code book says about anything, let alone SIFs. The use of CAESAR II and the like has had plenty of unintended consequences in regard to pipe flexibility analysis. CAESAR is the world's best poster child for GIGO (garbage in, garbage out). Especially nowadays, when the quality of engineering people we are getting in the oil patch is absolutely horrid. Anyone who's worth a crap is already working someplace and the ones that are floating around out there in the job shops barely qualify as warm bodies. I had to let a guy go last week, degreed ChE, process engineer, about 55, couldn't or wouldn't follow directions. It's a matter of time before there's an accident becuase someone misapplied a computer-aided stress program. But I digress. Sorry for the editorializing. Thanks! Pete