Air receivers: Focus on facts

In the past few months BCAS has received many calls from members and end-users asking for clarification on compressed air receiver construction and testing as they are hearing conflicting information. Here Chris Dee, executive director of BCAS, puts to rest the rumours and misinformation and presents the facts

Compressed air receivers traditionally manufactured in the UK (pre-EU law requirements) were always made more than thick enough since all air receivers standards back to the 1930s were based on steam receivers. Over time air receivers standards were refined a little to reflect compressed air use until the last purely UK standard BS5169 resulted in air receivers that even at small sizes, e.g.

75 to 100L, were less than the fictitious 'half an inch thick' and were about a quarter of an inch thick on the main shell.

When EU law came in to the air receiver market with the introduction of the Simple Pressure Vessels Directive (SPVD) continental practices and a re-think of vessel construction to reflect that they were for compressed air use and not steam was introduced. The other issue is that if a calculation is done for a vessel constructed in steel to retain pressure for any given size it is in fact much thinner than you would expect.

Taking this into consideration the SPVD identifies that the minimum wall thickness for a steel vessel shall not be less than 2mm not including any corrosion allowance which shall be in addition and not less than 0.5mm according to EN286-1.

BS EN286-1 is the standard produced for the SPVD and replaced BS5169. For a wall thickness of 2mm that would give a maximum vessel size of about 75L based on the energy stored which would be the product of the pressure and volume of the air receiver. For vessels larger than this, the wall thickness would increase due to the increased stored energy. EN286-1 gives a formula for calculating wall thickness based on a number of factors all of which contribute to the stored energy.

Another practice adopted from the continent is the issue of corrosion protection and allowed by the SPVD. This can either be the traditional UK approach to add a thickness of metal for sacrificing as corrosion or the continental approach where there is no additional metal thickness but rather an approved coating for the vessel. In the early days 1994/5 the 'approved' coating could be a splash of machine oil which then resulted in very few failures of receivers if they had compressors mounted on them.With that and rumours spread by others that because the vessel were only 2mm thick, compressor mounted receivers were failing all over the place. This was not true, in our experience.

We have only heard of three failures since 1994 and these were closer to that date than now. Most vessels coming into the UK from the continent (generally from Italy) are now painted, galvanised, epoxy coated or vitreous lined.

Another UK practice not generally used on the continent and not specifically required by the SPVD is the use of 'bearer' plates welded to the shell of an air receiver that is to be used to take a saddle for a compressor. The SPVD requires that the shell of the receiver shall be capable of withstanding any load imposed upon it. So where an air receiver is to be used to support a compressor then its design shall take this use into consideration. In the early days small receiver mounted compressors up to about the 75L mark with the thinnest walls and no corrosion allowance were those that were suspect to failure due to the fact that resilient mounts were not correctly specified.

There was also a need to identify the actual use of the complete compressor/receiver package since the EU includes the hot southern countries and the cold northern countries and therefore the temperature consideration comes into play. The SPVD requires as a minimum a vessel to operate to temperatures down to -10°C. Again in the early days the small cheap thin walled vessels did fail in Sweden due to winter temperatures being below -10°C.

In conclusion, the issue that seems to raise its head most with modern air receivers is that 'they are too thin'! This is not the case; they are designed and constructed for maximum performance and when used under identified conditions and correctly serviced and in-service inspection is carried out correctly at specified intervals there is no reason to think that the air receiver will not have a predictable and useful service life.

With old air receivers built in the days of steam, yes, they were constructed much thicker and appeared to 'last for years'! This is not necessarily a good thing industrially when you have an air receiver constructed in 1950 stamped at 100 psi maximum working pressure now having an unknown wall thickness due to corrosion and all that the owner can see is the edge of the plate where the end meets the main shell and it still looks as if it is 'half an inch' thick! There should be little if any difference in service life for a standalone air receiver or its counterpart used to support a compressor as both will have been designed for their expected use.

If you have any questions on compressed air receivers or any part of the system please contact: [email protected] for advice.