DANIELE MOLARO CONSULTING
Oilfield Consultant in Surface and Subsea Wellheads Equipment,Christmas Trees,Flow Control Equipment,Seals(Elastomerics & Metal-to-Metal) Contact me NEW E-mail: email@example.com
3) SEAL ELEMENTS
Seal elements in wellheads design are critical for that reason one of it's functions is to maintain isolation between tubes and sections, making the assembly safe, and reliable, they have to deal with temperature, chemical agents, mechanical agents, and many others: like a proper installation, and design of the device where they have to work.
The potential for a seal to fail is generally high; aging process due to the wear condition, poor installation, are actually risky factors for the integrity of the wellheads working conditions. But in the early times due to the huge progress in plastics, material today are more dependable than in the 60', and the new progress in failure predictions (FEA) software, makes more reliable, and able to support extreme situations, like heat, and chemical agents.
We can divide the seal elements used in wellheads in two big groups, and sub-groups, like this ones:
-1)ELASTOMERIC SEAL ELEMENTS:
-1.1)SELF-ENERGIZED (O-RINGS & COMPRESSION SEALS)
-1.2) INJECTION ENERGIZED ( PLASTIC INJECTION)
Next drawing: a single completion system, showing seals elastomeric type.
Figure of the left is part of the magnification of the Tubing Hanger's boby showing the annulus Seal Compression type Element. The figure of the right is a Secondary Seal Element used to isolate the Bottom Flange in many oil completions, inner ring makes contact with the casing string below, allowing the change in the next casing or tubing dimension.
O-RINGS & COMPRESSION SEALS TYPE:
This seals styles are probably the most popular and widely used in the Oil Industry in general because they are effective, cheap, inexpensive,and reliable. There is to consider differences in the way that an O-Ring and a Compression Seal work, and that is: an O-ring needs the pressure to work, so it seals thanks to the pressure of the fluid retained, that pushes the round surface of the O-Ring to make contact with the three basic points: one the surface to seal with, and the two points of contact inside the Groove that retains the O-ring. By the other hand talking now of Compression Seals the name says all, they need the action of a mechanical device installed in between (generally), that permits the initial push to accommodate the elastomer to the surface to seal with; so they in that way expands mechanically, and makes enough contact with the surfaces to seal that retains the fluid's pressure; so they only: stops it, and they don't need the pressure to work, or be energized.
Left drawing is a: Seal compression type notice the action of the lower flange in contact with the metal lower ring of the pack-off forcing it to seal against the tube (in yellow color), and the retainer cavity.The elastomer is squeezed due to the compression of the mechanical devices in between, and due to that it seals.
PLASTIC INJECTION TYPE:
The plastic injection seals are another kind of seals elements which by the injection of a special mix of plastic compound ( Teflon plus additives, and plastificants) through the plastic injection ports, they reach the elastomer, pushing it to make contact with the surface to seal, achieving a positive seal, that separates, the zones of the wellhead, from deferent pressures . They are generally used in high pressure service 10,000 PSI and above 15,000 PSI ; in low pressures below 5,000 PSI it depends on clients specifications. Plastic injected seals uses a variety of versions from: plastic strips, wrap-around, up to chevron packing, or double tandem of seals energized by a lantern ring, where plastic is distributed uniformly, and exert pressure between the seals and the casing string sealing the all contained surfaces. The image of the left is a Secondary Seal Pack-Off System where an elastomeric element in a strip and by means the pressure of plastic injected it wraps-around the casing or tubing string; there is a detail of the lantern injection port in the image of the right.This is a very effective pack-off seal system and also has the advantage of the short casing cut-off.
Metal -to- Metal are the most sophisticated seals, and comes in wide sizes, and designs, probably the most important and widely used are those who uses a tapered surface in contact with the other same shape but with a degree or more of differential taper contact. They work extremely good, and are superb reliable, and the use is: high pressure service, with high concentrations of CO2, and H2S, we are talking of 10,000 PSI and up. The disadvantage of this kinds of seals are the potential to fail under an inadequate installation, scratched seal surface, and a unproper storage, and handling.
Updated on:01/28/04Copyright @2003 Daniele Molaro Consulting
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