Bolt material for Stainless Steel cryogenic high-pressure exchanger

[Peregrino7]

We have a high pressure Heat Exchanger in cryogenic service. Materials shall be SS-304.
Channel is design for 130 bar and for a MDT = -196ºC.
The closure of the channel shall be as per drawing attached.

For this case, bolts size define the thickness of the channel, so bolt diameter should be minimized.

Vendor is proposing Nickel-Chromium bolts: SB-637 N07718 (32 X 3” Nominal Diam.)

Since bolts are fit in the Stainless Steel forge, we are concern about the integrity of this joint, because there are important differences in properties between the two materials, mainly: thermal expansion, Hardness, and Allowable Stress.

We are considering bolts SA-320 or SA-453, because they have similar tehermal expansion to SS-304. On the other hand, manufacturer seems not to like to make big bolts on stainless steel.

Any advice regarding Bolt material and what would be the best option that allows a long life of the bolts and the threads, and reduce the possibilities of failure because of thermal expansion (or compression) differences, and maintenance is possible at any time during equipment life.

Thanks in advance,



PGh.

 

[metengr]

Peregrino7;
The selection of suitable bolt material is primarily based on allowable stress because size is the governing factor for your design. If you review ASME B7PV Code, Section II, Part D, Table 3, you can compare the allowable stress values at -20 deg F to 100 deg F between lets say

SA 320 Grade 304 bolt material @ 20-25 Ksi depending on diameter or
SB 637 (not 437) Grade N07718 @ 37 Ksi up to 6" in diameter

My point to the above is that in addition to linear coefficient of expansion, your bolt size will be determined by the required strength level. So, SA 320 Grade B8 bolts may not have adequate strength to achieve the necessary clamping force.

 

[TVP]

I agree with metengr that the allowable stress is very important, and probably more important than obtaining an exact match for the CTE. The CTE for Alloy 718 is about 25% lower than that of Type 304 (12.8 vs. 16.9 ?m/m•K).

 

[Peregrino7]

Mechanical calculations shows the following:

For bolts SB-637: 32 bolts x 3”
For bolts SA-320: 32 bolts x 3.5”
For bolts SA-453: 32 bolts x 3.75”

As you can see, all these bolts are feasible, and flange and cover can be adapted to any of these bolts (of course smaller bolts is always better).
But my question is regarding which bolt material gives the best performance for high pressure and cryogenic, and avoid problems because of differences in properties between channel and bolting.
On the other hand… Is it a good practice to specify Austenitic Steels bolts, even for 3” to 4” bolts ? Is there any disadvantage on this type of bolts form the manufacturing point of view?
Thanks again for your replies…!


PGh.

 

[Peregrino7]

...correction:

SA-320-B8 (304 type) = 32 bolts x 4"

PGh.

 

[metengr]

I would go with the UNS N07718 stud material.

 

[CoryPad]

The minimum temperature has been identified, but what is the maximum?

If bolt diameter is to be minimized, then A 320 B8 is the worst choice.

The other two choices will be much more expensive for materials cost, but may not be so expensive if you can use smaller/fewer fasteners.

There is nothing inherent about austenitic stainless steels that make them unsuitable for larger diameter parts. In fact, their lower flow stress and high ductility mean they can be formed into shape easier than the other alloys, and their simple heat treatment means there are fewer worries about quenching, distortion, etc.

A disadvantage of stainless fasteners is their tendency to gall, especially against other stainless surfaces, so lubrication is an important consideration.

At the sizes you are talking about, you are more likely to be working with bar-based processing rather than more conventional fastener manufacturing processes.

At the sizes you are talking about, preloading is a major issue. You may want to consider multi-jackbolt tensioners.

 

[doct9960]

Peregrino7,

You might also consider modifying the current seal design (diaphragm plate) to a self-energizing seal. It could probably reduce the bolt size. Have a look at Taper-Lok's items...

http://www.taper-lok.com/closure.htm

http://www.taper-lok.com/docs/_Diaphragm Elimination using Taper-Lok - NPRA.pdf

With the dimensions shown on your sketch, it looks like 32 x 3" bolts is the maximum you can fit in your 144mm thick channel. If you increase the bolt size or number of bolts, you've got no choice but to increase the channel thickness.

Your sketch also mentioned that the design temperature is 40°C. If your bolts are to be designed to this temperature, you can't use SA-320-B8 Class 1. They are only allowed up to 38°C.