1. Cost-Effective Corrosion Solutions
Through Design & Engineering
By
Dennis Lamberth – Tricor Metals
In today’s chemical plants, you will find a variety of metallurgy, ranging
from your basic carbon steel, to titanium, zirconium and tantalum. While
each material has it’s own specific quirks and characteristics, there are
common sense approaches to working with each.
Many design engineers are comfortable working with carbon and stainless
steels, but when corrosive processes are encountered, more corrosion
resistant materials must be considered. Of course, the resident metallurgist
will specify a particular metal for a given process, but it is left up to the
design engineer to “design” a piece of process equipment, that will not only
meet the requirements of the ASME Code, but also be cost effective.
Some design engineers (not all) have come up through the ranks and have
seen a variety of metallurgy and diverse applications, but still consider
corrosion resistant materials such as Titanium, Zirconium and Tantalum as
“exotic” alloys. Whether out of inexperience or simply a fear of the
unknown, they generally have a tendency to migrate to a very conservative
design, which is usually over designed and more costly.
As processes change over time, metallurgy of the equipment must also be
considered, and be re-designed with corrosion resistance in mind. An
example would be where there is HCl service requiring titanium Gr-2. If
there is an increase in the temperature and concentration level, perhaps Gr-7
will now be required.
Due to the increased cost of Gr-7 over Gr-2, it is imperative to redesign this
equipment as cost effectively as possible. Additionally, the equipment
should be designed for that particular application, not designed for a
multitude of possible scenarios. This is not only true with towers, columns,
heat exchangers, vessels but piping systems as well.
This information regarding cost effective corrosion solutions, discusses how
a properly designed piece of equipment can save the end-user money, but
also save time in the fabrication process.
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2. As a fabricator, we see ideas drawn up on everything from napkins to high
quality Cad drawings. However, we also see designs that are complete in
every way, but many times we will receive a specification data sheet, where
the engineer has simply crossed out the note specifying stainless steel and
merely penciled in the word titanium, never giving any consideration to
material stress/yield values at temperature, corrosion allowance or give any
consideration to material densities.
As with any project, the scope of what is truly required is the first of many
steps in the design process. What type of service will this equipment
encounter? Accurate readings of operating pressures and temperatures.
These are just a few of the questions that the design engineer needs to
consider before beginning the design.
Obviously, a process engineer or a metallurgist will determine the
metallurgy of the equipment based on a need for the use of a corrosion
resistant material such as titanium, zirconium or tantalum.
When performing a mechanical design of equipment, the design pressure
and the design temperature need to be set as low as possible, to avoid
excessive material thickness due to over design. Considering the price per
pound of reactive alloys, a reduction in wall thickness will equate to cost
savings.
Corrosion allowance is also a key factor in designing equipment. One
simple rule is, if corrosion allowance is not required, then don’t use it.
Additionally, with respect to wind loadings and seismic calculations use
braces or bumpers to prevent deflection.
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3. Example
Vessel: SB-265-2 (60” ID X 144” long)
Operating Pressure: 20 psig
Operating Temperature: 750 F
Corrosion: None
Using the above operating conditions; design the vessel using 30 psig, using
a relief valve, a design temperature of 1000 F, and zero corrosion allowance;
the calculated thickness of the shell component is 0.0629”. Therefore, you
can manufacture the vessel using 3/16” thick material.
Using the same design temperature of 1000 F, but increasing the design
pressure to 285 psig and adding a corrosion allowance of 1/16”, the
calculated thickness increases to 0.6689”. Not only will this increase the
cost of materials, but also add to the fabrication time, since the fabricator is
now welding thicker materials.
However, if we maintain the pressure at 30 psig, but increase the design
temperature to 2500 F, external pressures mandate that the material thickness
needs to be increase. Add in corrosion allowance of only 1/16” and it only
compounds the problem.
If you increase the design pressure to 60 psig and increase the design
temperature to 4000 F and leave the corrosion allowance at zero, the
calculated thickness increases to 0.2054”. Now 1/4" thick material is
required.
The point here is that all variables come into play, with regards to design,
and they all need to be given proper consideration.
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4. Loadings
As defined by ASME Code, Section VIII, Division 1, Section UG-22,
loadings to be considered while designing a vessel shall include the
following:
1. Internal and external pressure, as defined in UG-21
2. Weight of the vessel
3. Weight of attached equipment such as motors or other vessels
4. Attachments
a. Both internal and external
b. Supports such as legs, skirts, saddles
5. Wind, snow and seismic
6. Temperature gradients and thermal expansion
When designing a vessel, it is imperative that all of these factors are taken
into consideration. However, never use an arbitrary minimum thickness just
because the “company design specification” calls for it.
The design of nozzles and manways is equally as important as the vessel
design itself. When designing nozzles, use lap joint flanges with a stub end
and a lap joint flange, instead of using solid flanges.
If ASME Code will allow it’s use (UG-45), use Sch 10S for nozzles, instead
of using a arbitrary “standard” Sch nozzle. Additionally, use carbon or
stainless steel with reactive metal liners in place of solid blind flanges.
When specifying structural attachments, use common sense. Use carbon
steel with reactive metal clips or use a lower grade of reactive metal to make
those attachments. Try not to use reactive metal attachments, or at the very
least, use a lower grade metal as an attachment.
An example would be, if a vessel were fabricated from titanium grade 7, use
titanium grade 2 for the reinforcement pads or clips.
Never use an arbitrary weld size. Always calculate what weld size is
required and use “skip” welding if possible.
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5. When designing “large” diameter horizontal vessels, use reactive metal for
wear plates. Use either carbon or stainless steel for structures and either bolt
together or strap the saddle to the vessel. Do Not use reactive metal for
100% of the support structure.
Reactive metal clip & wear plate, Reactive metal wear plate, CS web
CS web plate, Gusset & base plate plate, gusset, base plate & strap
Fig 1 Fig 2
Equally as important as saddle design, is the design of the skirts for towers
and columns.
REACTIVE / C.S. ALL REACTIVE METAL
Fig 3 Fig 4
Figure 3 shows the majority of the skirt being fabricated from carbon steel,
while figure 4, shows the entire skirt be manufactured from reactive metal.
Considering that the equipment is manufactured from titanium grade 2, the
cost savings here would be in excess of $110,000.
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6. 3600 RING SUPPORT
The preferred method of attaching ring supports as well as typical supports
is to use as little as possible reactive metal, instead using as much carbon or
stainless steel, to reduce cost. Figure 5 shows the ring support, with a thick
and narrow top ring, with short gussets, and a thin reactive metal base ring.
The thicker portion of the load bearing support is fabricated from carbon
steel.
Inexperienced fabricators will use the design as shown in figure 6, not
realizing that the design shown in figure 5 would save approximately $5,000
(cost), which is a savings that could be passed along to the end user.
Obviously, the larger the vessel, the greater the savings.
THICK C.S. BASE RING THICK / SOLID BASE RING
Fig 5 Fig 6
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7. Dealing with external pressure
As discussed earlier, equally as important as internal temperatures and
pressures, is it’s relationship with temperature and external pressure. For the
purpose of this example, we will use the following data as a reference point.
Design Pressure: 10 psi and FV
Design Temperature: 1000 F
Dimensions: 96” ID x 40’ long
Corrosion Allowance: Zero
Material of Construction: Titanium Gr-2
Ideally, when you begin your design, you start with a common material
thickness, run your calculations and then “tweak” your design, considering
all factors, until you achieve a “working” design, that is both cost effective
as well as functional.
Using the data from above, but only considering internal pressure, the
required material thickness is only 0.0625” and 3/16” material could be
utilized on this project. However, when you consider external pressure,
even 3/8” thick material is only good for 2.55 psig, and the required material
thickness is 0.7615”, requiring 1” thick solid titanium or clad construction.
The data also indicates that the maximum length of the shell at this
temperature and pressure is on 83” long.
Working towards a proficient design and considering only material
thickness, unless you find some 3/4" plate which is running very heavy, 1”
thick material must be used to construct this equipment. But when you
consider the cost of reactive metals, it is imperative for the design engineer
to look at all cost factors and design a piece of equipment that is cost
effective, as well as within ASME Code.
With the addition of stiffener rings, the design engineer can design the piece
of equipment using thinner materials and add “rings” of like material (or a
lesser grade) to the outer diameter of the equipment, approved by ASME
Code and more cost effective.
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8. As with the example above, using 1” thick titanium Gr-2, solid construction,
raw material cost alone, will be in excess of $650,000, not including labor or
external cost, such as rolling or head forming.
Using stiffener rings for external pressure, the cost of the shell material can
be reduced dramatically. Raw material cost for 3/8” plate on the same
equipment would only be approximately $250,000. Of course, you need to
calculate the size and number of stiffener rings needed. In this example, you
will need to add five (5) rings, measuring 3/8” thick and 4-1/2” wide.
The raw material cost of the stiffener rings would only add an additional cost
of $14,000, a total savings of approximately $400,000. These rings could be
either stitch welded or continuous welded, but obviously, stitch welding
would be less costly in terms of labor hours added.
Now, consider the same design parameters as previously, but now the
equipment is to be manufactured from titanium Gr-7 material.
Raw material cost for the shell sections would be in excess of $1,100,000,
using 1” thick solid titanium Gr-7. Using stiffener rings made of Gr-7
material, you can reduce the cost of material by over $650,000. However,
we are trying to save our customers money, and remembering from above,
we need to use a “lower” grade of titanium. Therefore, we should utilize
titanium Gr-2 for the stiffener rings. This would equate to an additional
savings of $9,000.
Proper consideration should also be given to which manufacturer is going to
construct your equipment. When welding reactive or refractory metals, the
end-user should understand that they should always utilize a specialized
fabrication shop, which has experience in working with these metals.
The fabricator should have a “clean-room” type of environment, which does
not necessarily mean NASA clean, but it also means that carbon steel should
not be fabricated anywhere in the area, as this will produce airborne particles
which will contaminate the welds. Proper shielding of the weld area is
critical, and as mentioned earlier, since this material is expensive, it must be
welded properly to avoid costly repairs, at the fabricators expense.
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9. The design engineer should also give consideration to explosive bonded
alloys or even roll-bonded materials. Since reactive and refractory metals
are expensive, another means of saving money is clad metals. Instead of
having a vessel that is two inches thick in titanium or zirconium, you can
have your carbon steel or stainless steel backer that thickness, and only use
perhaps 3/8” reactive metals, as your less expensive material will be used for
pressure calculations.
Loose lining of stainless steel or carbon steel with only 0.020” or 0.030” of
tantalum can save a considerable amount of money, as opposed to a cost
prohibitive piece of equipment, were it to be manufactured from solid
tantalum.
Another KEY factor is fabrication is QUALITY. One definition of quality is
consistent performance of a uniform product, meeting the customer’s needs
for economy and function.
But what is the cost of quality? Research shows that the cost of poor quality
can range from 15% - 40% of business cost, in terms of rework, reduced
service levels and lost revenue. Many companies don’t know that their
quality cost is because they do not keep reliable statistics.
Typically, the cost to eliminate a failure in the customer phase is five (5)
times greater than when the equipment is in the development or
manufacturing phase. Ideally, effective quality management decreased
production cost because the sooner a defect is found and corrected, the less
costly it will be.
Costs that would be eliminated if quality were perfect which often include:
incoming raw material inspection, corrective engineering change orders,
scrap, in-process control systems, downtime, material and labor rework
charges, quality personnel labor costs, field service repair personnel,
returned goods processing, customer warranty claims and many others.
While most of these examples are purely “common sense”, it proves that if
you use common sense during the design process, you will not only provide
the end user a well-designed piece of equipment, designed to ASME Code,
but also, save the end user a considerable amount of money. Ultimately,
your goal is to provide a “Cost-Effective Corrosion Solution Through
Design & Engineering”.
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