PD5500 Reactor Vessel: A requirement for bolt loading calculations with an O-ring seal.

[David Browne]

[Please refer to attachment throughout]

Dear fellow users,

I'm validating a reactor vessel design as per the client's requested design code [PD5500]. Essentially I have three parts to this simple assembly - backing plate, shell and front plate - bolted together and sealed around the peripheral of the machined recesses (see O-Ring). Given the self-energising nature of rubber O-Rings (Du-Pont's Viton product in particular), the design establishes the sealing potential is independent of bolt torqueing - i.e. regardless how much the bolts are torqued, only the contact force between the backing plate contact face and shell face increases, ultimately unchanging the O-Ring sealing capacity.

Taking this information forward, and, applying PD5500 Section 3.5.5 for calculating minimum thickness for a bolted flat end, would it be conservative and correct to assume the required calculated bolt load for gasket compression is irrelevant given the position of the O-ring, and deformable nature of an O-ring? Would it be fair to assume there is very little bolt loading required for actual O-ring sealing and to purely satisfy the requirement of assembly?

I don't have a conventional spiral wound gasket or RTJ whereby a calculated radial distance (h_g) from the PCD to the reaction force mean diameter, G, is required for joint sealing.

THE ISSUE

The calculation is giving me thickness to be unrealistic = approx. 41mm minT.
DesP: 5.65MPa
DesT: 50degC
Shell O/D: 324mm
Shell I/D: 298mm
Shell nominal thk: 13mm
Calculated shell mint: 7.32mm
PCD: 362mm

Couldnyone offer their own experience of approach when considering O-ring sealing potential and requirement for bolt loading is pertinent to leak prevention.

Regards,

DB.

 

[SnTMan]

David Browne, I am not familiar with PD5500 at all, but under ASME Sec VIII, Div. 1, the loads required to seat and maintain a seal on a self-energizing are set to zero, m = 0, y = 0. Any moments associated with these loads are likewise zero, so essentially the bolting is required only to resist the hydrostatic end force.

There is a flange (term used loosely by me) moment imposed due to the difference between the vessel ID and the gasket mean diameter that would still need to be considered in the design of the flange itself. So to answer your two direct questions,

would it be conservative and correct to assume the required calculated bolt load for gasket compression is irrelevant given the position of the O-ring, and deformable nature of an O-ring? Would it be fair to assume there is very little bolt loading required for actual O-ring sealing and to purely satisfy the requirement of assembly?

I'd say the answer to both would be "yes".

Maybe a PD5500 expert can provide better information.

Regards,

Mike

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

 

[MrPDes]

ASME VIII Div 1 and PD 5500 O-ring flange design methods are essentialy identical.

The PD 5500 design method for O-Ring joints are identical to Gasket Joints. O-Rings have very low m and y values as specified in PD 5500.
The flange rotates about the centre of the O-Ring for sealing and bolt up conditions. This is conservative as the actual rotation point is about the OD of the raised face.
The flange thickness is minus the groove depth.