Material selections

For the large majority of applications involving thin wall bellows, the question : Which type of material shall I select? Is readilly answered. The 18/9 titanium stablised (BS 1449:pART 2:83 321931) type of stainless steel is perfectly adequate for almost every expansion situation, and it is only in exceptional circumstances that an alternative need be considered.However, where exceptions do occur they must never be overlooked, as the effects of both media and external environmental conditions can cause a bellows unit to fall in operation if the material is not sufficiently resistant. The following notes refer to the most common service conditions where care in the selection of bellows material must be exercised.

Steam

For the majority of steam applications the use of 18/8 stainless steel gives satisfactory service life. In some applicatons Chlorides may be present in such quantity that there is risk of failure of stainless steel bellows due to stress corrosion. Similarly, in somehigh temperature steam services, where conditions are highly alkaline, there may be risk of failure due to Caustic stress corrosion. In these cases the use of Incoloy 825 or other high Nickel alloy may be necessary.

Marine services

The 17/11/2,25 Mo (Bs.1449.316S31) stainless steel has been shown to give satisfactory service in general marine use, including pipelines carrying sea water (for example tankwashing aboard oil tankers), where pipework is exposed to sea water spray, and in general where temperatures do not exceed 80C. However, under ambient conditions, where for example crude oil or sea water remains static in the convolutions for prolonged periods. Type 316 materials may sometimes fail. Also, where the pipe-work operates at temperatures in excess of 80C-for example steam services- and where there is prolonged contact with sea water either inside or outside the bellows, there could be used incoloy 825 material.

Crude oil lines

For pipelines carrying crude oil-for example, discharge and suction lines to crude oil storage tanks- consideration must be given to the Sulphur or seawater content in the oil. In many cases, where the oil is reasonably pure, Type 316 stainless steel will give satisfactory service, but if the above impurities are present, Incolory 825 provides better resistance towards pitting corrosion and is to be preferred.

Flue gases

Where flue gases contain such constituents as Sulphur Dioxide- for example, from burning oil containing Sulphur-problemscan arise if the temperature falls bellow the Sulphur Acid dew point, and lagging should be used to prevent this where possible. If there is any uncertainty on this point it is preferable to use Incoloy 825.

Disel engine exhaust mainfolds

Stainless steel is satisfactory for many Hydrocarbon lines, for some of the more arduous applications it is sometimes necessary to use an appropriate high Nickel alloy such as incoloy 825.

Other applications

There are very few companies in the world which can rival Đuro Đaković Kompenzatori d.o.o. experience of different bellows materials used for different applications, and advice on new or problematical applications is freely available from Đuro Đaković Kompenzatori metallurgical staff.

End fittings

It is often the case that bellows manufactured from one material need to be welded to end fittings of another material Đuro Đaković Kompenzatori d.o.o. have evolved tehniques for producing welds of necessary high quality, and many years of service experience have proved that these welds are fully reliable.