Stainless Steel Flanged Piping With Carbon Steel Bolts and Nuts 3

[regular]

I note that in my plant, carbon steel ( B7 /H2 bolts and nuts are normally used for stainless steel ( 304/316) flanges. I understand that the reason for this is that stainless steel bolts and nuts are harder and the tendency for the nut to loosen is high, hence the use of carbon steel bolts/nuts.
I personally find it difficult to accept this reasoning. Does anyone know of any other reason?

 

[prex]

Stainless steel is less resistant than many carbon steel bolts.
Also, if the choice of SS for flange is dictated by the type of fluid, there is no reason for the bolts to be SS also.

prex

http://www.xcalcs.com

Online tools for structural design

 

[regular]

Hi Prex,

Problem with the carbon steel bolts/nuts is that the pipe is a cold service and we get condensation on the bolts/nuts (where insulation have failed) and as a result they get corroded. Need to clarify what you mean by stainless steel is less resistant than carbon steel.

Regards

 

[prex]

About the strength of the bolts you could refer to the table in ASME B16.5 classifying the bolting specifications. Some of them, with B7 in the group, are classified as high strength and may be used with any flange material within the specified temperature limits, whilst others, and among them most stainless steel materials, are classified as intermediate strength, that need a check of their ability to sustain the gasket seating condition (separate from the check of the pressure rating of the flange), or as low strength that are limited to low pressure usage.
This doesn't imply that you can't replace your CS bolts with SS ones, but that a design check could be required.

prex

http://www.xcalcs.com

Online tools for structural design

 

[jay165]

prex is correct. SS bolts are not strong enough at the upper temp range of the flange rating. Hardness has nothing to do with it.

 

[arto]

Stainless will gall on it self- use Never Seez.
Do a flange analysis per ASME VIII D1 App 2 & see what you need for bolt strength vs. gasket seating vs. temperature.

A-193-B8 Class 2 is strain hardened [= high strength -up to 100ksi yield - watch your grade & size]if you need it. Class 1 is the usual stainless 30ksi yield.

Problems come in stocking parts/Murphy's Law/training/marketing types/bonehead buyers/quality assurance, etc., etc.: "It looks the same to me," "stainless is stainless," ....etc., etc. ....So use B7's if you can ;-) [plus they're probably a lot cheaper]

 

[regular]

Guys,

Thanks for the response. However, I need clarification viz. the pipe flanges are standard ANSI B16.5 sus flanges and the operating temperatures are low circa zero deg. C.
I would have thought that the stainless steel bolts and nuts should be able to take the required stresses for proper gasket seating and sealing under operation.
Oil and Gas consultant engineers have explained that carbon steel bolts and nuts are preferred (even when the pipe and flanges are sus.). Reason being that sus bolts and nuts tend to loosen; something which I find difficult to accept. I thought perhaps there may be other more plausible reasons.

 

[MJCronin]

regular,

I believe the problem may be due to "thermal cycling" of the system.

Stainless steel nuts/bolts/studs expand much more that carbon steel for a give temperature change. (the expansion coefficent is about ~40% larger) I believe it is this additional expansion that causes the loosening.

Carbon steel bolts/nuts do not expand as much, hence would not see this problem when installed on SS flanges.

MJC

 

[PAN]

regular,
For clear understanding, A193 Gr.B7 is low alloy steel stud bolt. This should not be called carbon steel.

 

[dcasto]

If you are concerned with the corrsion of the B7's, I have used a banding system that allows filling the bolting area with a wax. We have tried Teflon coated, but you must adhere to the torquing requirments else they will loosen like SS, but the B7 CrMo can take the higher stress.

 

[regular]

Guys,

Thanks for the feedback. From the comments and advice, I have managed to get an idea.
Dcasto's banding idea is interesting and I think I would like to consider that. My concern with wax is whether there is any risk of chloride contamination. Sure is worth looking into.

 

[Fizzhead]

Regular:

You don't mention if you have tried installing a dielectric kit on the flanges in question. These kits isolate the bolts from the flanges such that the galvanic cell is disrupted. The reason I have heard for the use of the carbon steel nuts and bolts are just as you have stated. I too used SS (in a past life) and found them to be a constant problem wrt loosening due to the cyclic service. We left the bolts and nuts open to the air and never had a problem with corrosion after that. But the application was in the southwest where the air is normally very dry. Anyway, was just curious if you had tried the dielectric approach.

 

[unclesyd]

See thread378-70790 for a previous discussion of this subject.

Stainless Steel Flanges in Corrosive Service
As stated in previous posts if you do an proper ASME flange calculation you should have no problems with the integrity of the flange connection itself. This is tempered with the fact the proper operating parameters have to be entered, that these parameters are not exceeded at anytime, and that the flange is made-up properly by the mechanic.

All the flange calculations in the world will be of no value if the flange is installed in a poorly designed piping system.
Careful selection of gasket type and material is extremely critical. The all important gasket seating stress and the physical properties under stress.
Never install a flange (SS) where it takes more than 1 chain fall (500 #) to make-up.
Use class 300 flanges especially in the 3"& 8" pipe sizes.
Have all SS studs/bolts 1" and under B8 Class 2 with B8 nuts.
Use the proper length of fastener for the flange. Use a lubricant and go slow.
Do not push the limits of SS flange connections as this makes every aspect of the piping system and installation a critical node.
If a flange connection has evolved and served well over the years be very careful in making any design changes without thoughtful consideration. This gasket is better than that gasket based on some basic information is not valid reason for a change.
Where one is designing large SS flanges use smaller bolts, only more of them. Everyone will love you for this.

As I’ve stated before I’ve never seen a situation where the use of CS fasteners instead of SS fasteners in a corrosive service has saved money over time. With few exceptions the value of this substitution, whether for integrity or cost, depreciates at an exponential rate and goes negative with limits set at the cost of shutdown and replacing the fasteners.


Where stainless flanges are used for purity or low corrosivity the use is CS fasteners is justified if it can be protected from the elements.

Flange design requires both an experienced engineer and artist, one being able to look at something and duplicate it with minor touch ups.

 

[regular]

Fizzhead / unclesyd,

The past threads were really informative. Thanks
Alas, someone who has experienced the sus bolts/nuts loosening. Because, I have not personally known of such incident though I have heard that it is a possibility. Then again, our pipes are not subjected to cyclic stresses so perhaps we could use sus B8 bolts and nuts and following practices and design as unclesyd has indicated.
Have not tried dieletric kit for the carbon steel (or low alloy bolts/nuts). Reason being that if the insulation had been good, there wouldn't have been any condensation (cold service and the humidity here is very high; >90 %). Besides, I think the corrosion could be due to the rather corrosive atmosphere we have in the plant ie. corrosion is more due to the corrosives than galvanic. We do have teh dielectric kit for our marine loading arms; which are basically to isolate the ship's potential from shore potential.

Regards.

 

[YQC]

Do you use SS washer to avoid SS contacting with CS?

 

[MickMc]

Our preference in the UK offshore industry is to use alloy steel A320 L7 for low temp and A193 B7 for high temp with most flange materials. These bolts and nuts would be coated eg hot dip galvanised to prevent galvanic corrosion.