Case study: SPETORING® RTJ SRX special gaskets - an effective solution for 'BALTIC PIPE' underwater joints
Jan Kasprzyk, Chairman of the Management Board of SPETECH®
Radosław Sieczkowski, PhD Eng., Head of SPETECH® Technical Department
The RTJ (Ring Type Joint) metallic gaskets commonly used today, were patented and introduced into industrial applications as early as the middle of the last century. Their inventor and author of the patent was Arnold Beckman - an American scientist, inventor and philanthropist known for many other solutions found to this day in industry and technology. It was the same RTJ ring gasket concept that was used for the critical connections of the underwater sections of the 'Baltic Pipe' pipeline which was built in 2022.
In their early days, RTJ-type gaskets were intended for use in industries related to oil extraction and processing and the petrochemical industry. Due to high pressures and temperatures, as well as the hazards posed by the release of a hazardous medium, the gaskets used in these industries must have very high mechanical strength, temperature resistance and reliability. The unique properties of RTJ-type rings have been recognized and further popularized by the American Petroleum Institute (API), the industry's trade association for oil producers. With the proliferation of nickel alloys, the greater availability of titanium and other special alloys, RTJ gaskets have also become widespread in the gas and chemical industries.
The described case of using a special gasket type RTJ SRX relates to the construction of the <<Baltic Pipe>> pipeline which was implemented in 2019-2022. What needs special emphasis is that the final stage of the pipeline's construction took place during the deep energy crisis caused by Russia. The commissioning of the <<Baltic Pipe>> project acquired additional importance in view of the need to ensure gas supplies for the winter of 2022/23 for Eastern Europe customers.
Figure 1. 'Baltic Pipe' pipeline route. SRX R105 gaskets were installed in the underwater section No. 4 of the project.
At the beginning of July 2022 SPETECH® was given the task of sealing sixteen flanged-screw joints with RTJ "SRX 105" rings with size adequate for the 900 mm diameter of the pipeline. Additionally, the fastest possible delivery time was expected. The case described here concerns an order for "SRX 105" rings in Alloy 625 material.
Figure 2. Octagonal cross-section of Spetoring®RTJ gasket
Problems during implementation of the project
The first problem is that the "SRX 105" ring, for which the flange grooves have been prepared, is not included in the standards. Indeed, a groove with such a number exists and is intended for RTJ type R (the basic type, for applications up to class 2500-ASME 16.5, occasionally up to class 5000-API 6B), but this is not sufficient for the detailed design documentation of the SRX type gasket. In this situation, a ring with a pitch diameter corresponding to the R 105 ring was designed, while the other geometric features were selected while maintaining the desired elastic recovery and radial susceptibility of the gasket.
The second problem is the difficulty of machining alloy 625, which is considered hard to machine. Particular difficulties are faced when attempting to drill narrow and relatively long holes. These holes are necessary in subsea SRX profile due to the underwater nature of the gasket application - the openings provide a connection between the dead spaces of the flange (between the bottom of the flange and the top of the gasket) and the pipeline pass. This prevents compression of the water enclosed in these spaces at the expense of contact pressures between the face and the gasket.
Figure 3. Spetoring® SRX R105 gasket design with underwater application - specific holes to prevent 'dead space' between the gasket and the bottom of the flange face
The third challenge was to procure Alloy 625 forgings, with a set of non-destructive testing, with 3.2 type delivery document acc. to EN 10204 and in strict timeframe acceptable to the customer, who, in turn, was faced with a tough deadline due to the political and economic importance of the timely commissioning of <<Baltic Pipe>>.
It is worth noting that the implementation of the project coincided with the crisis in the nickel market, which caused the suspension of trading in the metal on the London Metal Exchange at the beginning March 2022.
An additional condition for the base material used was the hardness of the gasket, which typically should not be higher than the hardness of the gasket face.
What exactly are RTJ gaskets and what are their special properties based on?
RTJ gaskets are made by turning solid metal rings to a specific cross-sectional geometry and diameter dimension. These gaskets are intended for high-pressure and/or high-temperature applications. They require the use of special flanges (usually called API flanges), which have seats with a trapezoidal cross-section. Installed in such a seat, the RTJ gasket contacts the walls with an inclination of 23° and creates two contact surfaces in each seat - two sealing lines. Thanks to the geometry of the seat, the axial force from the bolts is transmitted into contact pressures with a x 2.5 multiplication factor.
Figure 4. Cross-section through Spetoring® RTJ gasket of octagonal type - view in 'API flange'
Thus, we have relatively small contact areas (in the case of the oval RTJ, it is a ‘Herz contact’) and very high contact pressures. During the installation of the RTJ ring, plastic deformation of the gasket's contact zone with the seat walls occurs. As a result, the gasket's contact surface is deeply deformed and hardened. The result of this phenomenon is excellent tightness of the connection to the RTJ gasket. On the other hand, under no circumstances should such a gasket be reused.
Testing confirms the high dependence of seal tightness on manufacturing technology and on the surface quality of both the faces and the gasket itself. Literature data repeatedly determine this tightness to be better than 10-4 mbar*l/(m*s). In the SPETECH® test laboratory for both oval and octagonal RTJ gaskets, the determined tightness is 10-7 mbar*l/(m*s) (material grade A182 F5, pressure 40 bar, medium Helium).
It is engineering practice to make the gasket softer than the flange and this effect can be obtained either by material selection of the "flange + gasket" pair, or by heat treatment (softening) of the gasket in the case of the same flange and gasket material.
Another feature about the RTJ gasket is that it is absolutely blow-out resistant. There are no known cases of this type of gasket being blown out (even when it breaks) and, also importantly, this property does not change with increasing the joint diameter. The RTJ gasket, on the other hand, does not have the ability to compensate for the misalignment of the flanges being joined, unlike the lenticular gasket. It is self-centering and, like the lenticular gasket - requires a high smoothness of the contact surface.
Figure 5. Spetoring® SRX R105 gaskets during the final production stage at Spetech® factory
The "SRX 105" special gaskets constructed for the Baltic Pipe project described above were delivered by SPETECH® on schedule and in the required quality. The problems that arose during the project (procurement, timing and quality problems concerning special alloys) were solved thanks to close cooperation and understanding between the semi-product (forgings) supplier - the gasket manufacturer - the customer. The Baltic Pipe pipeline was opened in conformity with the planned schedule.