Storage tank design trends 1

[KrugerJR]

I am working for a company that is operating in the petrochemicals and chemicals environment. I am responsible for specifying storage tanks (AST’s i.e API 620) for new plants. These tanks are typically between 10m and 30m in diameter. Also, some shop fabricated tanks. I am currently investigating trends in storage tank design and fabrication. Many of the tanks that we have been dealing with lately contain products with a high vapor pressure or that are hazardous substances.

I am trying to find out if there are any trends over the last couple of years in the fabrication details of storage tanks that are aimed at preventing potential spillages due to equipment failures. More specifically, I want to know if there is a trend towards full penetration bottom corner welds, and if there is a trend away from single lap welded bottoms to full penetration butt welded bottom plates. Does anyone have statistics on how often tank bottom lap welds tear and cause significant spillages, or is general corrosion the only cause for tank bottom leaks

 

[unclesyd]

I been associated with S/S tanks that are in your size range storing very hazardous materials. We have been requiring full penetration welds overall for 25 years. We dye penetrant test welds as they are being made. All welding is SMAW. The tank fabricator says full penetration welding is becoming common practice with S/S or alloy tanks holding hazardous materials. In fact he says that all the tanks he has built recently have full penetration welds. All our tanks built recently (20 yrs) have butt welded floors, welded from both sides. Another thing we have done is to put all S/S tanks on 6' grillage bars due to past tank floor/bottom problems. We also modify the head to shell design depending on the service. We try to minimize all weld to the tank shell, even the ground. All required reinforcing pads are integral pads. All pipe nozzles are Sch 80 with weldneck flanges. All anchor bolts are S/S. We also used manway flanges designed with many small studs insted of the few large ones. We used studs instead of bolts, cheaper. All studs were S/S. We had strict requirements on materials and quanitatevly checked them prior to construction.

We were losing S/S tank bottoms due to MIC and chloride indueced SSC. C/S tank bottoms were lost due MIC and general corrosion usually in slag pocket that slipped by the vacuum box. We lost one due the contents causing problems with hydrogen.

One word of caution is make sure you have the customers' current specifications. Tank specification are changing fast.

 

[MJCronin]

This topic was also discussed on another engineering forum devoted to Aboveground Storage Tanks.

See

http://www.ast-forum.com

MJC