Tradition is one of
the strongest influences in the design of many stainless steel tanks and
vessels. Stainless is generally used where there are
concerns regarding hygiene or corrosion. Since both occur at a microscopic level
and may not be fully understood, most who require stainless vessels to hold or
process their valuable products, may therefore favour a proven design and construction
technique, and that is a
compellingly reasonable approach. Unfortunately however, carbon steel
tradition often remains
unnecessarily residual in many stainless tank
Carbon steel is probably the most successful material of construction ever
developed. If, when carrying out a design, any doubts arise about the long-term adequacy of any
particular size or thickness, using a heavier gauge just in case, is a
reasonably cost effective approach. Carbon steel's major drawback however
is corrosion, and standard codes that must ensure adequate long-term
integrity, often include significant allowances for corrosion in their thickness
When designing a tank in stainless however, this is not appropriate. If
significant corrosion risk persists, either another
grade must be used or stainless steel is not appropriate. It would be
better to use carbon steel so that the corrosion rate will be
obvious, and in some at-risk cases (such as pitting corrosion) it could even last longer. An example where observable deterioration is
an advantage is the rigging wire of cruising class yachts. Stainless steel
may look more desirable but evidencial rusting will encourage replacement
of the much cheaper, galvanised steel counterpart, in time to avoid a
potentially disastrous failure.
The base cost of stainless is several times that of carbon steel. The
materials in stainless steel tanks contribute between 30 and 60% to the
ex-works price. Therefore, if a design procedure derives say 3.1mm
thickness, the next commercially available size of 4mm is nearly 30%
inefficient. Available standard thicknesses
impose a commercial reality. This is particularly important if an
'exotic' grade of stainless is envisaged since an entry level duplex grade for
instance, costs nearly ten times as much as mild steel.
Tanks, as opposed to pressure vessels, should be considered of
'thin wall' construction where buckling is the predominant design
constraint. Like carbon steel, stainless is relatively strong in
tension but when 'thin' is vulnerable to buckling. Therefore any tank not designed to
withstand vacuum, is likely to be at its most vulnerable during
construction, transportation and/or installation. Only experience of the expected conditions can foretell
adequacy to resist the rigours of that! Once in service
and filled, the contents actually contribute to its stability.
An excellent example of this is spacecraft fuel cells. If left empty and unsupported,
they are so thin-walled that they would buckle under their own weight.
Consequently, during construction and making ready for lift-off, they are
filled with inert liquid, which is only pumped off at the last moment,
during charging. Isn't it great that stainless steel tanks aren't rocket
science. Imagine needing to be an astronaut to be able to afford a
beer...or a glass of milk if you prefer.
If a tank is designed according to common codes with seismic acceleration
of zero, its ability to resist damage before service tends to be marginal. If
on the other hand it is designed to a minimum acceleration of 0.2 it is in
my experience more up to it.
Once the required volume is set, the shape
of the tank depends on any restrictions to its diameter or height, the type of ends (dome, cone or flat) and
the efficient use of material.
When specifying the volume, consider how much extra volume needs to
be included. This is sometimes referred to as ullage or head space, and
may include allowance for possible future needs, process events such as
thermal expansion or foaming and also filling control, particularly when
high speed pumping will be used. Not only is it embarrasing to have
precious product spraying out through a vent or overflow but the hammer
effect when the contents suddenly hit the vent could cause severe damage
to the tank or filling system. It is worthwhile mentioning that the actual
volume should include the bottom, seldom the top, and be measured only up
to the lowest point of any overflow.
Restrictions to height or diamter are obvious. Suffice to say that you
should check to ensure that any internal agitator components and the like
can be withdrawn for maintenance when the tank is in service.
Whether dome, cone or flat ends are used requires a little thought and is
also integrated into the cost factor. This with regard to their cost
relative to each other and their cost relative to the barrel wall.
Appearance is also surprisingly inherent in the choice.
Dome ends are probably favoured when it comes to appearance, but
may not be appropriate. When custom built, they are usually more
expensive due to their labour content, particularly if of "duplex"
grade which is prone to work hardening. Perhaps surprisingly, they may not
be the best structurally either. They are the strongest option when
subject to uniform internal pressure but their shape makes them less able
to withstand external point loading at the centre, such as when a
relatively heavy agitatotor is mounted there or even when an also
relatively heavy maintenance engineer alights. Therefore, they will be better suited to a suspended bottom than a top. Yet how often
do we see a dome top, resplendant with agitator and a cone bottom
cone bottom will probably have been correctly chosen for its superior
draining quality, but why the dome top? Likely it just looks better.
Flat ends should be constrained to bottoms so they can be supported by a
sand or insulation blanket, on a plinth of sufficient height to
facilitate outlet fittings and drainage. They are neither aesthetic nor
cost effective as tops except on very small tanks, because they need
thickness to attain even reasonable rigidity in service. This extra thickness
and their usual need of a supporting bridge often makes them more costly
even than domes. If height restriction forces you to opt for a flat top, I
would suggest a minimum thickness in millimetres equal to the tank diameter in
meters, plus a bridge if the diameter exceeds about 1.5 meters. If you
also plan a heavy agitator, the bridge will need to be quite
substantial and should perhaps be rigorously analysed.
With a good (minimum 35mm radius) knuckled edge, cone tops remain my
personal preference. They are relatively easy to fabricate,
structuraly sound and of acceptable appearance. Depending on the relative
material to labour price ratio, you can expect them to cost about a
third less than domes. Do not however be tempted to make them with less
than 15 degrees of slope angle. There is a rapid loss of inherent
strength, in fact the API code does not allow it.
Since we are aiming to contain volume efficiently, we might normaly expect
a shape of least surface area to be the best. That of course would be a
sphere but for most containment applications spheres
are not as yet, cost effective, due to the difficulty in making them. The next best choice would be a cylinder
with an aspect ratio of 1, where height = diameter. However, even cones
are, at best, about 25% more costly per unit area than the barrel wall.
Taking required and practical thicknesses for each into account, the most cost
effective aspect ratio is around 2:1 for a cone topped tank on a flat base
and around 3:1 for a dome topped one.
If you are involved in the design, specification or procurement of
stainless steel tanks to any significant degree, you will benefit by
looking into the features of TankGenii which
performs both the fundamental design and facilitates analysing their
cost, from the basic information. It allows the options to be compared, almost instantaneously.
Finally, for designing any significant tank, some experience of
stainless steel and its construction techniques is a recommended
qualification, but needn't cost the best part of a rocket science
most common, in-service, failures occur through:
1. Over pressurizing, caused by rapid filling against a restricted
vent condition. Sometimes an otherwise perfectly adequate vent can be
restricted by a later, in-service event. Even the odd bird's nest has
caused problems, so a grating should be fitted to any ventilation duct.
Typically, compressive buckling failure will occur at the barrel to roof
joint. The best solution is a knuckled or rolled joint, and depending on
the tank size, this should never be less than 25mm or 1-inch radius, more
on larger tanks.
2. Barrel wall buckling
due to inappropriate design,
particularly if hot-cold, clean-in-place, occurs against inadequate
ventilation. Since steam condensation under cold spray is almost
instantaneous, it is virtually impossible to design an adequate vent. A
common safeguard is the inclusion of a fail stop device that requires a
full sized manway to be open, before CIP can proceed. Another cause is to
underestimate the effects of applied heavy roof loads, such as when a
heavy agitator is fitted and inadvertently started when the tank is empty.
Also consider the possibility of a plastic bag being sucked onto the vent
outlet during discharge. The barrel wall must always be checked for
buckling and since fabrication techniques will never produce a perfect
cylinder, the application of the classic Euler formula should produce
stresses not more than 60% of allowable.
3. Leg attachment failure.
On tanks of any size, legs should
always be designed so that their neutral axis meets the material of the
barrel wall, as in the sketch above. The upper end of each leg thus needs
to be profiled to match the bottom joint. Some designers insist on doubler
plates to help spread the load. If they are fitted, they should include a
small 'weep' hole. It is probably more economic, at least for the common
austenitic grades say less than 6mm thick, to use extra thicknesses in the
bottom and barrel wall.
It is worthwhile, for both structural and in-service hygiene reasons to avoid
any 'hard' corner joints such as at tank wall to end joints. In the
case of square tanks (rarely economic) a simple folded edge and a butt
weld is better. For cylindrical tanks, the ends are best knuckled, even if
'knocked up' by hand.
Surface finishing may be desirable for cosmetic
reasons, in which case some microscopic blemishes will be acceptable, or
for targeting hygienic product processing, in
which case they will certainly not be. Absolute sterility cannot be
achieved but is more probable with appropriate clean-in-place regimes than
highly finished vessel surfaces that are very labour intensive to produce.
All surface treatments, including electropolishing are
sacrificial, thus exposing the sub-surface, and sometimes, not only the
weld but also the parent metal may have pits, crevices or laminations
hidden below the surface. For
hygienic construction, polishing only the welds and using pre-finished parent material
if available, is
worth considering from a cost perspective.
A simple 2000 liter tank out of 2B material might
require 100 man-hours to fabricate with basically passivated welds, 120
man-hours with the welds dressed to 180 grit both sides and 200 hours with
the inner welds finished to 600 grit, which would be an appropriate spec.
prior to electropolishing the inner surface. With the entire inner surface
mechanically mirror polished, the man-hours become 350 provided no
subsurface imperfections are encountered. 180 grit is a similar surface
roughness to No. 4 polish but the same uniform lustre is almost impossible
to achieve. With the inner and outer surfaces mirror polished, the
man-hours leap to over 550, unforeseen imperfections not withstanding.
That is an expensive look and if a firm quote is required to produce it,
how can the unknowable but possible problems be allowed for? The reality
is they cannot with certainty. If it's of any help, we have a highly
developed stainless steel tank estimating
program that operates as stand-alone software. It asks for the surface
finish required and calculates the necessary labour cost, so it
can be used for comparitive purposes. A fully functional, time limited demo
version is available here►TankGenii
If you can
use the metric version, it is more highly developed than the inches and lbs.
version at this stage.
to visit the Tanks
Page the Tank Design Software the Stainless Notes or Stainless
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