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Design of end closure with split ring 1

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GD_P

GD_P

Structural
Apr 6, 2018
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I have been assigned with the task to design the closure (refer attachment) to comply with ASME Sec VIII Div 1. In this closure assembly, compression bolt engaged in split ring tapping along with split ring ensures the required sealing force.
Design conditions are
Material: SS316, temp: 300 deg C, Pressure: 10 MPa, Gasket: Grafoil, service: Non-lethal
Referring closure assembly parts, code doesn’t contain design rules for part other than shell. As per U-1(3) “The Code does not address all aspects of these activities, and those aspects which are not specifically addressed should not be considered prohibited. Engineering judgment must be consistent with the philosophy of this Division, and such judgments must never be used to overrule mandatory requirements or specific prohibitions of this Division.” So with this reference, I am proposing below procedure to comply the closure assembly to div 1 requirements. Please guide me if I am wrong.
Loose flange: As per Mand Appdx 2, 2-1(a) “The rules in Mandatory Appendix 2 apply specifically to the design of bolted flange connections with gaskets that are entirely within the circle enclosed by the bolt holes and with no contact outside this circle, and are to be used in conjunction with the applicable requirements in Subsections A, B, and C of this Division.” Loose flange doesn’t have any hole in this case. So by engineering judgement, considering PCD (pitch circle dia) at average dia of ‘Loose Flange to Split Ring Contact surface’, we can design loose flange as per Mand appdx 2 rules. Design conditions will be same as given in appdx 2 - Gasket seating & operating. (No doubt that operating condition will govern here).
Cover: Here bolt (compression bolt) PCD is different than the companion loose flange. Also bolt PCD is smaller than the gasket mean diameter. So to keep the design simple i.e., to use UG-34 rules, we can use the PCD same as considered for loose flange design. By engineering judgement this will result in conservative design.
Compression bolt: This will be standard bolt based on the load for “Operating condition” obtained in loose flange design. Here the bolt is in compression as against the mand appdx 2 bolts which are always in tension. So in such a scenario, should I use the same allowable stress for bolts (Sec II D, table3)? Or I can increase them upto yield strength?
Split ring: In div 1, there are no rules to design it. So as per U-2(g), ‘Mand Appdx 46 (46-4 DESIGN BY ANALYSIS) and Proof test are 2 available options. Generating 3.5 times design pressure to proof test is not possible for us considering our current capacity. So the only option is FEA. Also I was wondering that how do we fabricate this. This can not be fabricated from plate form, since this may result in tensile stresses in through thickness direction. Any suggestion.....


Regards,


GD_P
 
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Well I'll take a stab :)

1) Loose Flange: See Fig 2-4 Sketch 1. Take "G" as shown, "C" taken as mid radius (major) of the Split Ring, calculation of "W" is straightforward. See 2-3, definition of "A" and note that A < C presents no conceptual or computational problems. Ignore facing and bevel for calculation purposes. Proceed as per Apk 2.

2) Cover: If it is possible that your PCD can equal mean gasket diameter, no edge moment is applied to the cover, and UG-34 Sketch (p) could be applied. Otherwise Sketch (o) may be used. If dimensions are such that and edge moment is applied, Sketch (j) could perhaps be applied. It would seem that the applied moment would be opposed by pressure forces. I'd likely ignore this effect and simply apply Eqn (2).

3) Bolts: Required bolt load and bolt area is easily calculated. Proceed as per Apx 2-5.

4) Split Ring: Prior to 2019 Edition, I'd have just designed this per "good engineering practice", that is, a strength of materials approach. I am not familiar enough with 2019 Ed to know if this is still possible. If not, FEA may be the only option. Seems overkill to me, but if them's the rules...
Perhaps Apx 24 could be applied, I don't intend to study it enough to be able to say.

I'd make this from a ring forging, they are very common.

Anyway, for the most part it appears to me that straightforward Sec VIII, Div 1 rules could be applied. Offered for what is is worth.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand
 
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I am a bit concerned about this type of flange connections, and always look at them with the glasses of safety.

Everything depends on the pressure, temperature etc... can be managed by the design and analysis tools available or can be solved by hand. However codes are always look at the safety. I suggest you reconsider the application by reading the requirements of quick opening closures. I understand that you want to open the connection very often and quick, and don’t want to involve in the long process.

You need to think about the following in my mind right now;
1. How you are going to make sure bolts as they are not going to stay without loosening, and stay safe all the time? The temperature will rise in the vessel and the split rings will be cold and stiff, this will force the connections for loosening, or breaking the weak location in the system.
2. How you are going to make sure the connection is unbolted under no pressure in the system? I suggest a system or a gudget will empty pressure before unbolting.
3. How are you going to lift and engage the split flange with the available weight on each? I would split the split ring to the number of bolts in case you want to stay with this design.

To me the pressure, 10 MPa, is very high for special connection like this. I would suggest you to stay with welding neck flange connections, but you may play with the dimensions (reduce PCD if bolting permits) to reduce the weight slightly. In this case the flange shall be forged with the material selected. So this way you may apply the common code rules and calculation.

Hope it helps.
 
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@Mike & saplanti
I really appreciate for your time & detailed reply.
Mike,
From your reply, It seems like our perception of split ring is same, if not please refer link: It is simply semicircular ring of cross section as shown in the attachment of 1st post. 2 split rings along with cover, compression bolt & flange completes the assembly.
Could you please clarify below details:
Regarding ‘C’ dimensions,
Mike said:
"C" taken as mid radius (major) of the Split Ring, calculation of "W" is straightforward. See 2-3, definition of "A" and note that A < C presents no conceptual or computational problems.
Click to expand...
>>> In case of slots, A < C will be applicable. Exactly, where do you think C should be? Do you mean in between C1 & C2 from below figure? If yes, then A will be greater than C.
IMO, it should be avg of C1 & C2 or in the worst case C = C1. Since the contact between split ring & flange lies between C1 & C2.
Regarding split ring fabrication,
Do you mean ring forging of dimensions B1 & B2? If yes, I will be little concerned. Because we will have to remove some material from the ring forging to make it split ring shape. So is still allowed? I had perception that there no option other than close forged split ring shape.
Regarding bolt load / area calculations,
Sample calculations as per Apx 2-5 requires higher bolt area. Whereas when I saw few actual reactors (in working condition in lab) having same design conditions and closure type, the no. of bolts / bolt area provided is significantly less than required as per apdx-2 calculations. Am I missing some aspect?

saplanti,
You have made some valuable comments, i will definitely consider your suggestions.


GD_P
 
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GD_P, 1) yes, our perceptions of the split ring are the same.

2) For design of the Loose Flange I would take "C" (conservatively) as average of "C1" and "B2". This would tend to yield the maximum moment applied to the Loose flange, and give C > A. "G" is taken as average of "C1" & "C2". Again, see Fig 2-4 Sketch (1).

3) For fabrication, yes, purchase a ring forging with dimensions OD = B2, ID = B1 and calculated axial length, plus suitable machining allowances. This is a roll-forged ring of rectangular cross section. Machine to finished dimensions, drill & tap. Split the ring by any suitable means, saw, flame cut, etc. This is a very common way of making pressure vessel body flanges, flanges for heat exchanger floating head covers (see Apx 1-6) including their split backing rings.

4) Using Code allowable tensile stress to calculate required bolt area is no doubt overly conservative, however is it straightforward. I don't recall if Sec VIII, Div 1 provides guidance for allowable compressive stress or not. Guidance from strength of materials approach is certainly available, if it is desired to reduce required bolting, and the approach is acceptable to the involved parties.

A couple of other points: A) In my first reply I glossed over the high design pressure. If the vessel is not especially "large" my suggestions are likely adequate. Otherwise additional care may be required. B) TEMA has design rules for split baking rings for heat exchanger floating head covers. I don't think they can be easily adapted to your design however.

I don't believe you stated whether this vessel is to be stamped or not. If not, you may have a bit more flexibility as to design methods, etc.

The problem with sloppy work is that the supply FAR EXCEEDS the demand
 
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Mike, Once again thank you for the suggestions.
The vessel will not be stamped.
Vessel size will be DN 40 to DN 100 max.

Regards,

GD_P
 
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GD_P said:
The vessel will not be stamped.
Click to expand...

This would presumably excuse one from having to strictly follow current U-2(g).

Lab vessel?

EDIT: In that size range a roll-forged ring is perhaps not feasible or even necessary. Plate material could be suitable if free from laminations.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand
 
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I guess everythhing is related to the process in the vessel.
Is the vessel internally or externally insulated?
Is the vessel internally heated by using heating elementts or the process fluid comes into the vessel under the operating temperature?

I guess you have lots of things in your mind to solve before the flange decision.

Perhaps you may need to see the brochures for quick opeening closures before making decision on flange.

I have in my early engineering involved this kind of vessels. Flange decision depending on your budget is the last issue.
 
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Thank you all for your comment and suggestions.
Few doubts & comments as below:
Is my consideration that flange needs to pressed / clamped for min load as per Apdx 2-5 calculations is correct? OR it is overly conservative?
Is my consideration that COMPLETE flange clamping force calculated as per Apdx 2-5 has to be or will be transferred through compression bolts? OR Only fraction (depending on clamp geometry) of total load will be transferred through the bolt? It may not sound worth to think so much, instead of directly using the straightforward Apdx 2 caln. But for small vessels with high pressure, this may result in good space at top of closure for other arrangement.
@Mike,
Accept my apology if I am stretching the subject too much.
I really feel that my assumptions regarding the ‘C’ distance are more than sufficient i.e., avg of C1 & C2 or in the worst case C = C1. Just imagine that the distance B2 is very large as compared to C1, would you still consider C as avg of C1 & B2???
SnTMan said:
In that size range a roll-forged ring is perhaps not feasible or even necessary. Plate material could be suitable if free from laminations.
Click to expand...
Agree with you.
@r6155,
Considering the design data, standard flange rating of 1500 will be required for this vessel. Flange appears very big as compared to such a small vessel. By the way, changing the flange type i.e., split ring to standard flange is not in my scope. And currently my task is to design ‘Split ring end closure with gasket’.
Use of O ring could be a good option, but it is not possible to get an elastomeric O ring for des temp upto 300 deg C.
@saplanti
Actually, I am not aware whether it will be heated by process fluid or by exothermic reaction. My scope is limited to mechanical design. Also, I think we cannot directly use the quick opening closure, since we will need custom end closure with multiple connection openings.

Regards,


GD_P
 
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GD_P, I am having trouble interpreting your first few questions. I would just say that the loads applied to the Loose Flange must be calculated for the two conditions, operating and gasket seating, as per Apx 2.

Regarding the dimension "C", if I were designing this closure, as I stated previously, I would use the value average of "C1" and "B2" to maximize the moment applied to the Loose Flange. As the designer you may use whatever value you feel is appropriate.

Regarding use of an O-ring, might be nice, but it is not required.

I would not consider this a quick acting or quick opening closure. See UG-35.3(a)(1) language about swing bolts.

Finally, do not confuse this closure with B16.5 Class 1500 flanges, totally different animals.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand
 
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GD_P, you are saying that there are muitiple openings on the head. This makes everything interesting.

Another intersting thing is the split of tasks on the vessel between separate manufacture/designers. This is not wrong but these individuals do not know anything about the vessel purpose and its operation. This is the major mistake. I would expect these individuals come together and discuss what to do and how. Perhaps There is a person who knows nothing about the vessel design but probably follows the progress. This upsets me. Therefore I do not want to involve any further.

I wish you all good luck with the end result.
 
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Probably final post of the topic, to
Mike,
SnTMan said:
I am having trouble interpreting your first few questions. I would just say that the loads applied to the Loose Flange must be calculated for the two conditions, operating and gasket seating, as per Apx 2.
Click to expand...
Sorry for the inconvenience, I will restate again.
Regarding flange design loads, I agree with you. But i am concerned about compression bolt design i.e., in case of usual Apdx 2 flange, bolt load & flange load are same. Will it be applicable to 'compression bolt of split ring flange'? OR the bolt load will be less (considering the geometry of clamp) may be some fraction of flange design load.

r6155,
Thank you for the reference. It will be interesting to know its pressure capacity at 300 deg C. At high pressures, the O rings are used with backup rings (mostly PTFE, max allow temp 260 deg C).

saplanti,
saplanti said:
I would expect these individuals come together and discuss what to do and how.
Click to expand...
This will be done in later phases of project. I am currently working on preliminary stage of mech design.

GD_P
 
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GD_P, I suppose the bolt load could be less than the flange load, but how to determine this? I think in the interest of conservatism, I'd just take the two loads equal.

r6155 makes a good point about re-use of the O-ring. A grafoil gasket will not likely be reusable. An O-ring may be employed as either a radial seal or face seal, google up the Parker design guide if you don't have it already.

Best of luck.

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