Spherical Pressure Vessel

The Spherical Pressure Vessel article provides you with information about this vessels characteristics and related points for the pressure vessel inspection.

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What is construction code for spherical pressure vessel? What is In-Service code for spherical vessel? What are the applications? What are the advantages?

This article either provides you the answers of your questions or refers you to the specific sources.

What is the Difference Between the Design Formula of a Cylindrical Shell and a Spherical Shell?

In the same design condition with the same design pressure, design temperature and material, the thickness you obtain from the internal pressure formula for spherical pressure vessel will be half that of the cylindrical pressure vessel.

The design formula for the cylindrical shell is t = PR/ (SE-0.6P) and for the spherical shell is t = PR/ (2SE-0.2P)

When the “t” is represent Thickness, “R” the Inside Radius,” S “the Allowable Stress, “P” the Design Pressure and “E” the Joint Efficiency;

For example, if your design pressure is 250 psi, inside radius 20 inch. , allowable stress 20,000 psi and joint efficiency 1.

Your thickness for cylindrical shell will be 0.24 inch. or 6.10 mm, and for the spherical shell, it will be 0.125 inch. or 3.175 mm.

What is the Application and Advantage of a Spherical Pressure Vessel?

When you need to make storage for a great amount of pressurized liquid or gas, meaning you need a big volume, then you need a big pressure vessel, so a spherical shell pressure vessel would be more economical.

As you see above, the thickness is half that for a similar design condition. Also placing a long cylindrical pressure vessel might not be suitable in regards to the regarding plant layout.

What is the Disadvantage of a Spherical Pressure Vessel?

Spherical shell pressure vessels are more expensive than cylindrical pressure vessels to fabricate, and this higher price is only justifiable for large vessels.

What is Difference in Fabrication Method Between Cylindrical and Spherical Shell Pressure Vessel?

Cylindrical shell pressure vessels generally are built in the shop and then transferred to the plant field except for long process towers, which might be built in two pieces and completed in the field by one circumferential weld.

This is only because of the facilitating shipping process.

But spherical shell pressure vessels are assembled in the field. The plates generally are formed in the rolling shop and then carefully transferred to the field for assembly.

What is the Difference in the Post Weld Heat Treatment (PWHT) Process?

If PWHT is a code or process requirement, the PWHT for cylindrical shell pressure vessels generally are done in the furnace, and if the vessel is too long, it is done in two heating process with a 5 ft. overlap.

But PWHT for spherical shell pressure vessels are done by one or more high velocity burners that are fired into the vessel using the top or bottom manways (or both) as burner entry and exhaust positions.

This is done because using a PWHT furnace is not possible.

What Kinds of Materials are Stored in a Spherical Pressure Vessel?

Most of the LNG (Liquid Natural Gas) and LPG (Liquid Petroleum Gas) tanks are Spherical Vessels.

Butane, Propane, Ammonia, oxygen, hydrogen and nitrogen also are stored in spherical vessels.

What are the In-Service Inspection Requirements for Spherical Shell Pressure Vessels?

In-service inspection requirements are similar to those for the cylindrical pressure vessel and are based on the requirement of the API STD 510 Standard.

Some important points about in-service inspection are:

The inspection plan needs to be developed for the vessels, and generally, internal inspection is a mandatory requirement.

You need to do internal inspection in intervals not exceeding one half of the remaining life, or 10 years, whichever is less.

You can calculate the remaining life with the following simple formula:

RL = (Tc-Tmin)/CR ;

When Tc is Actual Thickness, and Tmin is Minimum Required Thickness, it is can be obtained by the deduction of nominal thickness from corrosion allowance, and CR is corrosion rate.

Example: you have a spherical shell storage vessel with an actual thickness of 1.121 in., and the minimum required thickness is 1.0 and the corrosion rate is 0.005 in./year, so the reaming life will be:

(1.121-1.0)/0.005 = 24.2 years;

So the internal inspection interval will be:

Half-life = 24.2/2 = 12.1, so 10<12.1, then the internal inspection interval will be 10 years.

But in most plants, evacuation of storage vessels is very difficult and costly, so integrity engineers try to find a new technique to replace internal inspection.

API STD 510 allows the On-Stream inspection to be replaced with the internal inspection with some conditions.

This is the definition of On-Stream Inspection per the API 510 Standard:

“An inspection performed from the outside of a pressure vessel while it is on-stream using NDE procedures to establish the suitability of the pressure boundary for continued operation.”

But only you may replace this technique if all of following conditions are met with your vessel:

The general corrosion rate is less than 0.005 inch. per year, remaining life is greater than 10 years, there is no corrosive character in the contents, no questionable condition is observed in the External inspection and the vessel is not subject to SCC cracking.

What is the Best Technique to Use as On-Stream Inspection?

The best technique, which is widely used, is acoustic emission testing. In this technique, the spherical vessel is completely filled by content fluid to maximize stresses.

Meanwhile highly-sensitive proprietary acoustic sensors are attached to the vessel wall to detect the fracture of corrosion products during the monitoring period.

The suspect areas are identified and located. If the suspect shows a high risk issue, then the vessel would be evacuated for internal inspection and necessary repairs.

What are the Repair Methods for a Spherical Shell Pressure Vessel?

There are three sources for repair of a spherical pressure vessel; API STD 510 has provided some repair requirements and instructions.

Based the API 510, only repair organizations are allowed to do repairs, and the definition and condition for being a repair organization has been provided in the API 510.

The other source is the ASME-PCC-2, which provides you detailed instructions and requirements.

The other source is the NBIC Part 3 Code book, which similarly provides requirements and instructions.

Please note if your spherical pressure vessel is a “U” or “U2” stamped pressure vessel, only repair organizations that are ”R” stamp holders from NBIC are allowed to do repair on your vessel.

For more detail on the stamped pressure vessel, review the Pressure Vessel Certification article.

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