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.