Too often refractory linings at petrochemical plants are designed
and the anchoring system is merely an afterthought, therefore anchor design should
be executed with the overall refractory lining in mind says Jan Theron, Refractories
Specialist at CERAM.
The majority of high temperature units in the petrochemical industry
have an interior which features a refractory lining. These refractory linings are
kept in place against the shell using a combination of steel and ceramic anchors.
It has often been found that a great deal of care is given to
the choice of refractory materials, but anchor design is seen as an afterthought,
especially where steel anchors are considered. During failure investigations carried out by CERAM it has become apparent that many of the failures are
caused by incompatible anchor and refractory lining combinations.
When specifying steel anchors there are a plethora of factors
to consider, including: steel type, the shape, the length, the thickness, the form
– round or flat, corrugation depth, notches, the manufacturing process, shell fixing
method, expansion allowance, spacing, etc. Each anchor design has an individual
reason for existence and some refractory designers firmly believe flat anchors are
superior. However, anchor design should be considered independently for each individual
high temperature unit and/or refractory lining, according to the merits/features
of both; it is therefore unwise to be guided by the generic opinions of others or
by the general consensus.
The plant maintenance manager or the project manager responsible
for a new unit are unlikely to be a refractory experts, hence they rely on the guidance
from reputable refractory manufacturers or refractory installers. This approach proves to be effective in the majority
of cases. However, there are incidences when unsuitable anchors fail. These failures result in economic implications
through the downtime of units and repair costs.
In order for end users to ensure they make educated choices,
the interaction between all parties involved must be understood. Furthermore, end
users must have a clear understanding of material selection (or at least be mindful
when making design changes) and acknowledge the engineering boundaries of the prescribed
materials. It is vital to appreciate the
role that refractory linings play. As linings
account for a very small proportion of the overall plant, end users tend to have
little knowledge of refractory related issues. A huge consideration is usually to try and drive
costs down; refractory costs are tangible and frequently seen as an area in which
costs can be reduced. With this in mind, cheaper options are often selected without
considering the detrimental future risks.
Refractory manufacturers and design engineers are mostly involved
with the design of the refractory linings and try to obtain the best material for
that specific application. Once the material selection has been made, the anchor
design may be chosen with little or no guidance, or with the assistance of a manufacturer
or installer.
Manufacturers of anchors stand alone; they have a good knowledge
of steel types and forming methods, but often have limited refractory knowledge.
For economic reasons, manufacturers are able to devise cheaper forming methods which
can somewhat limit performance. Alternatively,
they try to distinguish themselves as leaders through the invention of different
anchor types for specific applications such as floating anchors, stud weld anchors
and multiple branch anchors.
The installers are mostly responsible for installing the anchors
together with the refractory lining and often advise on anchor types or suppliers
to use. In some instances, the anchor design is left for the installation company
to organise; this can prove to be helpful. Installers frequently have more experience
of difficulties encountered during the installation of refractory materials and
some anchor systems which ease the installation, thus reducing the risk of improper
refractory material installation.
All those involved with the attaching or welding of anchors to
the steel shell have varying capabilities. There are three ways in which welding
can be conducted: the first is to use an individual with masonry expertise and train
them on a welding machine; the next is to appoint a qualified welder and train them
as a bricklayer; and, finally, anchor welding is done by a third party. Each of
these alternatives has advantages and disadvantages.
Case study
The design of the refractory lining was changed very slightly
(the thickness of insulation and dense refractory layers were changed), which consequently
had a very dramatic impact upon the unit. Although the modification was very small in essence,
some major implications were created as a result. Both the original design and the modification to
this design had very similar heat losses and the outside shell temperature was comparable. Upon first glance, this would suggest that
everything in the unit was in order. The
overall lining thicknesses were kept the same in both designs; however, in the modified
design a much more insulating material was used. The main driver behind the material change was
a reduced installation time, saving on downtime for the unit.
Three months into the campaign the roof section of the unit collapsed,
leading to very serious health safety issues surrounding the subsequent repair work,
plus significant financial implications because of the downtime.
The original design had a fairly uniform temperature gradient
throughout the refractory lining. With the
modified design, a much larger ‘hot zone’ was created within the lining due to a
combination of the diminished thermal conductivity within the outer part of the
lining and the increased thermal conductivity of the inner section. In addition, the thickness of the working lining
(the denser part of the refractory) was increased to compensate for the thinner
insulation section that was used (due to its lower thermal conductivity); consequently
this added weight to the lining, imposing a much higher load on the anchor system. The higher temperatures experienced in the
steel anchors significantly reduced their tensile strength, and, coupled with the
extra weight, made the anchor yield. This
ultimately led to catastrophic failure of the roof section.
If a design modification is made by the end user, it is advisable
that alteration implications to the overall system are clearly understood. This
will result in less obvious implications being explored and the engineering pitfalls
of material weaknesses avoided.
Follow us on
