Considerations for Seal Selection
The selection of the most suitable seal for any service condition is often a delicate trade-off between load and spring back of the seal. The higher the requested load to compress the seal the better the tightness level, whereas the useful spring back of the seal will determine how well this tightness will be maintained with varying temperatures and pressures.
For a given seal cross section and seal type, it is generally true that with maximum load the spring back is lowest. And of course, a seal with minimum load will generate the highest spring back.
However, there are other parameters and circumstances that will influence the performance of a selected metal seal, e.g. type of flanges and fitting method, pressure and temperature cycles, bolt grade, bolt tension and method to stress the bolts.
Flange Rotation / Lift-off
In all cases, the goal should be to achieve a construction as rigid as possible to overcome movements (lateral and/or axial) caused by varying pressure and/or temperature changes and external loads.
Flanges, bolts and seal can all be considered "spring elements" within the system of which the seal is often a highly non-linear element in its load recovery behaviour.
Therefore, an assembly where metal-to-metal-contact between flanges occurs after bolting is the most rigid and stable one.
After the seal has been compressed in its groove, further tightening of the bolts against the system pressure does not have any negative effect on the seal.
Bolted Seal Assembly
The initial bolt load will generate the initial load (cfr seating stress) on the seal. Because of the system pressure the hydrostatic load tends to "unload" the seal, resulting in flange lift-off.
The degree of residual stress (energy) left, will determine the fi nal leak rate.
Stress relaxation can be caused by
- flange rotation / flange lift-off
- flow (creep of the seal material)
- bolt relaxation
- cycling application conditions
- external loads
To obtain the target leak rate, it is recommended to:
- design the assembly as stiff and rigid as possible
- Select the flange type
- Select bolts of suitable strength and quantity
- go for the seal requiring the best ratio load vs. elastic spring back
- use soft plating types if allowed and possible
- select the biggest possible cross section for the given diameter
- use metal seal grades having the best mechanical properties, even at high temperatures
The surface finish of the mating faces of the flanges is one of the most important parameters that will infl uence the performance of the metal seal.
The surface roughness and the method of machining of both flange surfaces will have a huge impact on the leak rate of metal seal. For flange surfaces, it is of major importance that the surface is machined by a lathe resulting in circular machining marks.
Methods of Machining
For flange surfaces it is of the highest importance that this is done by turning
- Surface finish will be noted as Ra-c - |μm|
- Radial machining marks must be avoided in all circumstances
- Be sure that flange faces are not 'damaged'
- Clean the flanges before fitting the metal seal
The final Ra-c to apply will be determined by a number of parameters like
- the requested leak rate
- type of the seal
- type of plating
- available bolt load
- material grades of flanges and bolts