The approval process for industrial and commercial plant and equipment in Germany requires compliance with the ‘Technical Instructions on Air Quality Control’ (TA Luft). This regulation sets forth respective emission limits for all relevant air pollutants.
Valves and fittings and components of industrial plant and equipment in chemical and process engineering are designed and developed in line with these prerequisites. The demands made on seals in valves and fittings are of central importance in this context. In addition to the geometry, the selection of a suitable material is crucial to comply with the relevant criteria when designing the seal.
For spindle transmissions in control and shut-off valves and fittings low-friction, self-adjusting seal ring packings from PTFE have proven their viability. The sealing system consists of V-shaped packings, the number of which is adapted to the respective operating conditions, and which are enclosed by one neck and one end ring, respectively. The design and flexibility of the V-shaped packings also depends on the operating conditions. The neck and end rings are made of metallic materials in the event of gap extrusion. The complete sealing set is loaded with a continuous spring force so that a uniform sealing effect occurs.
The material, PTFE, is particularly well-suited for this type of application due to its wide temperature range and very good sliding properties. No stick-slip effect and low break-away forces even after longer downtimes are further pluses of PTFE.
Special fillers in the PTFE provide good wear resistance and thus long seal life. Another important advantage of this PTFE packing is its very good chemical resistance. Hence these valves and fittings are suitable for a wide spectrum of applications:
- Chemical and petrochemical industries
- District heating and utilities
- Food and aseptic sector
- Production of industrial gases
- Facility automation
- Laboratory applications and industrial research
- Cryogenic engineering
In addition to their numerous advantages, hard filler particles in the PTFE structure also have the disadvantageous effect that the mating surface may be damaged depending on the operating conditions. To guarantee sealing performance across long service life it is important to ensure a respective quality of the mating surface. The main aspect in this regard is to assure that the PTFE packing is mated with a hardened surface. Additionally, a ground and subsequently rolled spindle surface with a surface quality of Rz = 1µm minimises wear.
Material is decisive for TA Luft acceptance
The sealing area of the spindle in control valves and fittings is considered a critical location in the spirit of TA Luft.
Under certain circumstances emissions may occur here that exceed the maximum values which are permissible today. For this reason a special focus is now put on the seal when developing valves and fittings. The requirements to be met by shut-off and control devices are clearly specified by DIN EN ISO 15848 or TA Luft/VDI 2440 (November 2000). According to these specifications, the leakage rates when using PTFE seal ring packings must not surpass a limit value of 1×10⁻⁴ mg/(s-m) after a 24hr test at temperatures up to 250°C. Compliance with these requirements must be demonstrated by a special design test. This test is performed using helium as a test gas in operating conditions. The test certifies that the valves and fittings can be expected to deliver the intended function in operating conditions according to the requirements of TA Luft.
Emission performance of the control valves and fittings is not solely affected by the design and geometry of the packing. Besides the interface problem (sealing effect between two surfaces being moved relative to each other), permeation through the seal materials is a crucial parameter to be observed. The discussion about upper emission limits quickly makes the correlation between leakage rate and material density evident.
Measured results of a test series with packings made of various types of PTFE: in addition to some unfilled materials, designated as Virginal PTFE (standard PTFE), Moldflon (thermoplastic material) and HS 17022 (modified PTFE), one type containing an organic and an anorganic filler, respectively, were tested as part of the TA Luft certification process. The leakage across the packings during the duration of the measurement is marked by a sigma-shaped pattern. This behavior is known and its cause attributed to the material properties of PTFE. In numerous laboratory investigations carried out by the material manufacturers this pattern was proven as a characteristic feature of the permeability of PTFE.
A significant improvement of wear resistance is achieved by mineral or metallic fillers such as carbon, graphite glass and carbon fibers and bronze, or by organic fillers. The filler enrichment, though, also contributes to loosening the material composite.
As the filler content increases the material density of the PTFE compound decreases. The increased pore volume between the molecules in the PTFE structure increases the permeability of the material and the medium, which has penetrated the material spreads into all directions.
This raises the question about available means to increase the barrier effect of the seal without having to accept significant losses in wear resistance. The following measures may be considered in this regard.
- Use of modified PTFE instead of standard PTFE
- Increase of layer thickness: when doubling the component thickness by approximately 50% permeation may be reduced by approximately 50%. Therefore, the question needs to be considered whether the available assembly space and the friction conditions allow the inclusion of additional V-packings to reduce the permeability of the packing
- The crystalline areas in the PTFE structure are considered impermeable. The amorphous content in the structure, which is responsible for permeation, is influenced by the cool-down speed during the sintering process. A reduction of the cool-down speed by approximately 40°C/hr results in a decrease of helium permeability by 20% – 25%
- Reduction of operating temperature: by decreasing the temperature by 12°C to 15°C the permeation rate can be cut in half
Another option to increase the barrier effect of the seal is to use Moldflon® as the seal material. Moldflon® is a new type of thermoplastic material which largely equates to conventional modified PTFE in terms of its composition. Unlike modified PTFE, this thermoplastic material can be processed from the melt and thus has almost no pore volume. Compared to PTFE Moldflon is a material with a significantly higher barrier effect. This has also been clearly revealed by the helium leakage investigations conducted so far in conjunction with the TA Luft requirements.
By adding suitable fillers the strength values of the material, which are good to begin with, can be enhanced even further. This is a major benefit particularly for components which must withstand high pressure loads and simultaneously occurring high temperatures. In addition, this improves abrasion behavior. The abrasion-reducing effect of the filler is clearly higher than that of respective PTFE-based compounds. In view of these advantages Moldflon can be recommended as a suitable material alternative for control and shut-off valves and fittings.
Author: Miroslav Dimitrov, technical sales.