Stainless steel fittings: regulations, techniques and shapes

In plant engineering it is necessary to create conditions of durability and safety even in the case of use with particularly aggressive fluids or processes. Even more so when it is not only the integrity of the system itself that is called into question, but also the safety of people.
The best implementation is through socket welded, butt-connected or flanged connections, capable of ensuring tightness and durability over time, also aided by the use of stainless steels suitable for the purpose.
However, there is also a range of threaded fittings that can be used in less critical situations.
The choice takes place in the design phase and must be consistent with the use to be made of it, and for this purpose it must be agreed with the supplier, in this case with our sales office also the type of necessary certifications that must accompany the products when they leave our warehouse.
In fact, every item that leaves our company is accompanied by a certificate drawn up as required by the EN 10204 standard and whose level of accuracy depends on the final use. Accuracy means the level of testing to which products must be subjected before being put on the market.

Stainless steel fittings: available shapes

In the INTERTUBI catalogue, fittings cover the majority of the offer and are made of both stainless steel and carbon-alloy steel. The catalog is then completed by stainless steel valves and a section dedicated to seamless pipes.
- The range of fittings is decidedly wide and includes different shapes made of stainless steel, the reference standards are also both European and American, in order to be able to serve the market completely.
- Elbows, tees, concentric and eccentric reducers, end caps made in compliance with the EN10253-3 standard, are generally not compliant with the PED directive.
- Some shapes such as metric concentric reducers and hydroformed tees comply with the ISO 5251 standard.
- Assortment of tees, elbows, eccentric and concentric reducers as well as end caps that follow the indications and measurements of the American standard ASME B16.9 and can also be butt welded.
- Complete range of socket welding fittings that refer to ANSI B16.11.
- Threaded fittings made according to both the European or international ISO 4144 or EN10241 standard and the American ANSI B1.20.1 standard.
- Complete series of forged flanges EN 1092-1 or ASME B16.5.

The choice of legislation is based on the requests of the designer and certainly of the reference market in which the installation will be carried out.

Stainless steel fittings: materials and construction techniques

The production techniques for stainless steel fittings are varied, and each is indicated for the specific type of fittings, mainly hot plastic deformation is used, bringing the material to a temperature suitable for its deformation.
For elbows we can use the mandrel deformation method which ensures the correct shape without losing the circularity of the product and without giving significant variations in thickness. For tees, however, hydroforming is used, which gives the best results, or welding is used for particularly large items. For end caps and reducers, both hot and cold forging techniques are used.
The materials commonly used to make these fittings are from the austenitic steel family and in particular AISI 316/316L and AISI 304/304L.
AISI 316 steel is also identified by the acronym X 5 Cr Ni 18 10 and it is a highly alloyed steel with high chromium and nickel contents which give it the characteristics of stainlessness, in slang it is also called "18 10 stainless steel" and has always been used for the creation of aesthetic products such as pots and cutlery sets. This is because it has excellent cold deformability which allows it to be processed by drawing, but it also has excellent workability on machine tools.
It is particularly suitable in the food, chemical and pharmaceutical industries, and in the petrochemical sector, and is appreciated for its good resistance to hot corrosion which allows it to maintain good characteristics during continuous operation up to approximately 800 °C.
It also has excellent resistance to the action of saline environments, but also optimally withstands the action of acids, salts and other chemical reagents of various types. This makes it the ideal material for use in various fields including the nautical and food industry where resistance to often aggressive cleaning products is required.
AISI 304 steel, on the other hand, designates the material X8CrNi188, also commonly known as INOX 18/8, an alloy containing approximately 8% of Nikel and approximately 18% of chromium, an element that gives the characteristics of resistance to corrosion by creating a surface film of oxides.
It is a non-magnetic material which is used thanks to its very high resistance in situations where almost eternal durability is required, such as in the case of nuclear or particularly critical plants.
For both acronyms there is a version with the suffix L called "low carbon" and which is characterized by a lower carbon content. This serves to increase the resistance to intergranular corrosion and to also partially improve the weldability of these materials.
It is a non-magnetic material which is used thanks to its very high resistance in situations where almost eternal durability is required, such as in the case of nuclear or particularly critical plants.
For both acronyms there is a version with the suffix L called "low carbon" and which is characterized by a lower carbon content. This serves to increase the resistance to intergranular corrosion and to also partially improve the weldability of these materials.

The comparison between stainless steel and brass fittings

The world of fittings is characterized by the difference between various types of models, materials, production methods and manufacturers. An important difference is the choice, at a manufacturing level, between stainless steel and brass. 
Stainless steel, as described, has a series of advantages that make it highly suitable for this type of application. Brass, which is an alloy between copper and zinc, is also widely used in the field. But what are the differences and motivations in choosing one over the other?
An important aspect is resistance to corrosion: both have characteristics suitable for this type of application as they are highly resistant to corrosion, even if they suffer from particular types of highly corrosive environments. From the point of view of workability, brass is better and allows you to obtain components and details of significantly smaller dimensions than those obtainable with steel. On the other hand, the latter has a longer life and also greater resistance: in certain applications the ability to withstand stress and loads is a fundamental requirement. From a cost point of view, stainless steel has a slightly lower price than brass and this represents another favorable characteristic.
Therefore, on the basis of this comparison, it is possible to state that, based on the individual application, it is possible to choose one or the other material, guaranteeing in any case excellent characteristics of the final product.

The role of simulation in the design of stainless steel fittings

The advantages and features of stainless steel fittings have been described in the previous paragraphs. What is not always considered when using a fitting is the design process underlying these products: in particular, the role of structural, fluid-dynamic and technological simulation is not always clear to those downstream in the production chain and use of fittings.

First of all, the simulation, regardless of the type, is the virtual representation of the component in different phases of its production and operational life. For this to be as similar as possible to reality, some precautions are necessary:

- correct choice of material and its characteristics: all simulation software has a more or less updated database with numerous materials. Understanding which material to consider represents the necessary but not sufficient starting point to obtain a realistic representation of the component.

- correct choice of loads and conditions of transformation and use: only in this way is it possible to simulate the actual situation that the stainless steel fitting will face during the transformation and use phases.

- careful evaluation of the results: understanding the outputs and evaluating whether these can be considered acceptable is the heart of the simulation process. The same result, applied to two different contexts, may be acceptable or unacceptable.

- comparison with practical tests and prototyping: validating the simulation is not only a practical precaution but also constitutes an opportunity to improve company and design know-how, understanding the gaps in the simulation process and improving them.


Conclusions

It is not easy to choose all the parameters necessary for the purchase of a fitting to be installed in the safest possible way, the advantage of stainless steel is that it resists aggressive environments even at high temperatures, which are usually present concomitantly in industrial but also food environments.
The regulations also give the right parameters useful for the best design, which guarantees the required resistance.