Constantly increasing requirements of the sight glasses and their sealing technology for a secure and efficient operation

Due to new regulations, amendments of the law or customers’ request, the expectations towards the armature branch keep increasing. This does also concern all types of sight glass armatures, no matter if it is flow sight glasses, level indicators, container sight glasses or pipe sight glasses.

This article offers an insight into the selection criteria of sight glass components and their sealing technology as well as into two examples of sight glass armatures for special applications.

The industrial and legal requirements for plant manufacturers and operators are constantly increasing. The effects of these regulatory adjustments and requirements are thus also noticeable in the valve industry. For example, innovations in the TA-Luft flange connections, sealing technology and even sight glass fittings. Generally, the requirements are covered by good standardisation, such as vessel sight glasses, according to DIN 28120 (ACI Type 320), DIN 28121 (ACI Type 321) or flow sight glasses, according to DIN 3237 (ACI Type 550). But what if the requirements of the plant constructor or operator for efficient and above all safe monitoring of the process exceed the possibilities of the standard sight glasses?

Generally, a huge variety of options for the process monitoring is given. That might be vessel sight glasses, (e.g. ACI Type 320 or 321), flow sight glasses (e.g. ACI type 550 or 530), level indicators (e.g. ACI type 330 or 410) or pipe sight glasses (e.g. ACI type 620 or 622) for flange mounting, for screwing on/off or even for welding in/on. 

By doing so, these standard sight glasses occasionally reach their limits. Especially when it comes to the high-temperature and high-pressure range, and particularly the combination of high pressure and high temperature, most standards will not do. It is especially problematic when an armature needs to be used with a highly aggressive, explosive, or dangerous medium. By using high-quality materials, we ensure the compliance with the requirements of the sight glass armatures, even when it is special sight glass fittings.

For the production of quite versatile valves and fittings, it is very important to provide a great variety of choice regarding components and materials. Glass, sealings and screws for instance are important elements.

To cover a wide compatibility of media, it highly depends on the chemical composition of the material. For example, stainless steel does not automatically equal stainless steel and not every material is universally resistant to media. However, those materials are by far not everything. It is the expertise that is necessary to build such special valves and fittings in accordance with the rules. Furthermore, a good quality management as well as extensive tests need to be done as most of the pressure-maintaining equipment, such as flow sight glasses (e.g. ACI type 550 or 530), must be designed and extensively tested for quality and conformity in accordance with the Pressure Equipment Directive 2014/68/EU, in conjunction with the AD2000 and the appropriate harmonised standards.

In the following, we would like to demonstrate that there are possibilities even for more complex application areas by some examples:

Application example: Medium 

As body we used a further development of our ACI type 590 high-pressure body DN150 PN250, which was adapted for three welding neck flanges DN150 PN63 according to DIN 1092-1 type 11. In this design, the welding neck flanges are the weakest pressure-bearing component of the sight glass body. In order to achieve a full 40 bar at a temperature of 280°C, it was important to use PN63 flanges. They are permissible and designed for 43,5 bar, according to the pressure-temperature-rating of the DIN EN 1092-1. For a quick and independent determination of the permissible pressures in relation to temperature, pressure class (PN / Class) and material, you can find our free pressure-temperature assignment, according to DIN EN 1092-1 and ASME B16.5 on our website under the category “Tools”.

For the housing and flange material we decided for the stainless austenitic steel 1.4539 (X1NiCrMoCu25-20-5 / 904L) because of its particularly useful asset with regards to the medium. The welding process was conducted by audited and certified staff. When it comes to stainless austenitic steel, subsequent pickling and passivation of the weld seams is mandatory for maintaining corrosion resistance.

In order to meet the safety requirements of our customer, all weld seams were subjected to a 100% test with X-rays and the completely assembled sight glass was subjected to a TÜV inspection with a 3.2 certificate. Unless requested otherwise, the extent of documentation is always based on the valid regulations of the AD2000 and the Pressure Equipment Directive. We replaced our standard borosilicate sight glasses with metal-fused borosilicate sight glasses. That was the only way for us to enable an effective sealing connection against phosgene gas between glass and housing in a more uniform and higher surface pressure besides the extended bolt circle.

As sealing material, novaphit® VS was chosen. It is a graphite gasket which consists of a pre-compressed graphite foil made of high purity expanded graphite (purity > 99%). Due to the temperature range of the application that needs to be covered (-20°C bis +280°C), condensate formation as well as formation of hydrochloric acid is possible. For this reason, the stainless-steel reinforcement insert made of 1.4404, which is otherwise commonly used for novaphit® seals, was dispensed with.

Application example: Pressure

We were contacted by a major aircraft and helicopter manufacturer regarding several high-pressure flow sight glasses with internal threads for a high-pressure oil application. For this use case, we were able to use one of our standard products: The ACI type 522 – a high pressure flow sight glass with internal thread.

The ACI Type 522 was designed as 400 bar version for a maximum temperature of 280°C and could thus easily meet the customer’s requirements. That way, we were able to deliver the high-pressure flow sight glasses to the customer according to his wishes and specifications at short notice and without a long waiting period. For these sight glass fittings, high-quality borosilicate glasses particularly made for ACI were used. Special glasses, which are not listed in the respective norm due to their high requirements, are generally designed and calculated by us in accordance with that standard for the required field of application. For the design of such glasses, our free glass advisor for a variety of glass materials and shapes is also available on our website in the “Tools” category.

For the housing material, 1.4571 (X6CrNiMoTi17-12-2 / A316Ti) stainless steel (made in Germany) with the best features for high-pressure applications was used, being characterised by its durability and corrosion resistance. For this reason, no additional protective paint layer is necessary. Looking at the seal, we decided for the novaphit® MST with a standard XP technology. In retrospect, this has shown the
best results with our sight glass fittings in the past due to its very high-quality properties. The following illustration shows damaged sight glass plates that were installed with a commercially available graphite gasket.

Figure 5:
Damaged glass plates with standard graphite gasket

Novaphit® MST currently displays the highest development stage of graphite gaskets. The multi-layer construction with 0.5 mm high-purity graphite (purity > 99.5%), followed by a stainless-steel inlay made of 1.4404, ensures maximum mechanical stability. For a sealing thickness of 2 mm for example, three inserts are used, whereby the outer reinforcing inserts are each designed as expanded metal. They ensure an optimised distribution of the surface pressure, among other things, which constantly increases the performance of the graphite seal. With novaphit® MST, high surface pressures can be applied even to gasket dimensions with very narrow webs without destroying the material. Hence, novaphit® MST is predestined also for the use with the highest internal pressures.

Furthermore, the so-called XP technology is used as standard which is an inorganic deep passivation of the graphite. The resulting effect concerns two important areas. On the one hand, the oxidation stability of the graphite is increased, so that significantly better long-term features can be achieved, especially in applications with higher temperatures (> 300°C). On the other hand, the tendency of the graphite to adhere to the sealing surface is drastically reduced or even completely avoided. This does not only save time during the change of sealings but also avoids the almost unavoidable damage of sealing surfaces during the cleaning process. The residues of novaphit® MST with XP technology (picture on the right) can be easily removed with a moistened cleaning cloth without leaving any residue.

Figure 6:
(left) Sight glass plate incl. pressed novaphit® MST gasket with XP technology, (right) Sight glass plate with minimal residue from the removed novaphit® MST gasket.

We received an inquiry from a petroleum refinery client in Norway, asking for a high-pressure sight glass for low temperatures being seaworthy. The following requirements must be met:

Especially the saltwater protection and low temperature resistance at -60°C were important to our customer. In this case, we could use our ACI Type 520A, a high-pressure flow sight glass with ANSI flanges.

For the housing of the type 520A high-pressure flow sight glass, we used the 1.4462 stainless steel as material. It is saltwater-resistant due to its outstanding features and also very resistant to CO2, oil, and methane gas. The ASME B16.5 RF 3“Class 900 connection flanges were made of this material too. For this material, we needed a 3.2 certificate with proof of suitability. Furthermore, we upgraded the sight glass from stress group I to stress group II, according to AD2000 data sheet W10 (materials for low temperatures). Thanks to this step, the chosen material (1.4462) could reach the required -60°C, instead of the approved -40°C. Due to this regrouping, the welding process of the main body required a 100% x-ray inspection of both weld seams. The glass we used was specially manufactured borosilicate glass similar to DIN 7080. It is resistant to the media mentioned above as a standard and saltwater worthy. We used the novaphit® MST graphite sealing as sealing material as this is perfectly suitable for high-pressure applications due to its special structure in the low-temperature range.

We successfully carried out a pressure test with water at room temperature for 60 seconds at PN 150 bar x 1.5 (225 bar) on our own pressure test rig.