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 its 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.

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?  What are 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.

For 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.

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.

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.

 

Equally as important as saddle design, is the design of the skirts for towers and columns.

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.

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 calculations and then “tweak” the 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.

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.

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”.

Tricor Metals

---

CANADIAN PROCESS eNEWS