Design Considerations for Subsea Valve Stem Seals
Valve design in general is a very broad subject, so in order to offer design considerations it is necessary to set some limits to the type of valves covered in this article. The subsea valve applications discussed herewith are Ball Valves, Gate Valves, Regulating Valves, Check Valves and Choke Valves.
There are typically three places in valves where seals are required; stem seals, seat ring seals and static seals (for covers and bonnets etc). The stem seal is located on the valve stem going out of the valve, and is used for operating the valve. The stem seal is to be considered as dynamic or semi-dynamic, with turning, reciprocating or helical movement.
The movement ranges from continually dynamic in a regulating valve to static for months or even years in a subsea shut-off valve. Each application requires special abilities from the seal. The seal in the regulating valve needs to have low hystersis and a long wear life, and the seal in the shut off valve must have a reliable function once the valve stem is moved.
Material choice for stem seals within subsea ball valves is critical to operational performance. Trelleborg Sealing Solutions recommend Turcon® M79 – a high grade composition of virgin PTFE resin compounded with aromatic polymer and lubricant, for high integrity valve stem sealing.
Aromatic polymers are extremely soft filler materials that are added to PTFE, with the major advantage being the creation of a very smooth and tight dynamic seal surface after a short run‐in. This makes it suitable for sealing gasses and low viscosity fluids, and the materials are primarily used for both linear and rotary “low” pressure applications with the requirement of a high sealing effect. At low pressure where the system pressure is not adding much power to the activation of the seal, seals in polymer filled Turcon® have shown good results.
NORSOK M-710 Qualification
NORSOK M-710 – Qualification of non-metallic sealing materials and manufacturers (Rev. 2, Oct. 2001) – requires that all sub- components of oilfield equipment must be approved to stated specifications. Specifically, individual seal materials are rigorously tested and approved based on numerous criteria such as Rapid Gas Decompression (RGD), sour and sweet gas aging, compression set tests, and material property tests.
Trelleborg Sealing Solutions has one of the largest and most comprehensive selections of fully approved NORSOK M-710 materials on the market. The range includes best-in-class formulae over all material types, designed specifically to meet demanding oilfield conditions.
Differential Pressure Considerations
The stem seal is typically exposed to pressure coming from one side only, inside the valve. However, there are occasions (particularly within subsea use) where the differential pressure outside the valve could be as high or higher than the media pressure inside the valve. In situations like this it is important to consider the seal position when designing the valve stem arrangement. This can be done with one or more seals facing each direction, possibly with support rings in the spring grooves, to prevent seal damage in case of accidental back pressure.
If operating conditions permit it, bidirectional seals such as Turcon® Double Delta® can be used. Turcon® Double Delta® is the original slipper seal developed to improve the performance of O-Rings and Back-up Rings. It is a double-acting seal energized by an elastomer O-Ring. Demonstrating good friction properties, Turcon® Double Delta® provides stick-slip-free starting and excellent dry running.
|Turcon® Double Delta®|
5,000 psi / 35 MPa
(15,000 psi / 103 MPa with Back-up Ring)
|Speed:||Up to 50 ft/s / 15.0 m/s|
Depending on elastomer material
-49°F to +500°F
-45°C to +260°C
As per AS568 and ISO 3601 seal sizes.
Larger clearance possible with Back-up Ring
Mineral oil-based hydraulic fluids, flame-retardant hydraulic fluids, environmentally safe hydraulic fluids (bio-oils)
Downhole fluids, water and others depending on the elastomer material selected
Hydrocarbons, Sour Gas (including high concentrations), amines, acids & corrosion inhibitors, methanol
High Integrity Sealing Requirements
Trelleborg Sealing Solutions recommend the Multi-Contact Turcon® Variseal® when operational conditions require a subsea ball valve stem seal to perform with high resistance to chemical media, and under extreme temperatures whilst maintaining good extrusion or compression characteristics.
The Multi-Contact Turcon® Variseal® has been developed to replace multiple chevron seals or V-stack type products in subsea valve stems requiring a high-integrity sealing capability. It includes a series of single-acting spring-energized seals housed within a U-shaped Turcon® PTFE based material seal profile.
The unique single-piece design significantly improves sealing integrity and leak tightness in high-pressure applications by providing higher contact force across multiple seal hardware interfaces. It has been designed to provide integrated redundancy, helping to extend the flex life of seals in high-pressure applications while providing better performance and lower operator costs. With the addition of PEEK anti-extrusion devices, the Multi-Contact Turcon® Variseal® can withstand pressures of up to 207 MPa/ 30,000 psi.
|Multi-Contact Turcon® Variseal®|
|Operational Pressure:||30,000 psi / 207 MPa|
|Speed:||Up to 32 ft/s / 10 m/s|
-320°F to +509°F
-196°C to +265°C
|Clearance:||Typically up to 0.3mm at 2 MPa|
|Media:||Almost universal media compatability|
The information in this article is based on many decades of experience in the manufacture and application of subsea sealing systems. However, unknown parameters and conditions may restrict general statements. For a complete design consultation on stem seals for subsea valve applications, contact your local Trelleborg Sealing Solutions Marketing Company.