IN-HOUSE FABRICATION OF LARGE SODIUM STORAGE TANK

On 30th  March 2000, the sodium storage tank, a critical component of Steam Generator Test Facility (SGTF) was rolled out of Central WorkShop (CWS), for  installation at SGTF building. This tank with a volumetric capacity of 21 cubic metres is for storing the entire quantity ( 18 Tonnes) of sodium to be used in this facility. The tank has a vertical shell closed with torispherical dished ends at the top and bottom. The tank is 3 metres in diameter with the total height of 4.02 metres. It rests on a flanged skirt which is bolted to the concrete base at the lower end.
 

Fit up of shell with dished end.

Inside view of top dished end with nozzles

The principal material of construction is AISI 316 conforming to SA 240 for the plates and SA 312 for the pipes. The skirt flange, a load bearing member, is of carbon steel IS: 2062, St 42W. The tank was fabricated conforming to   ASME  SEC VIII DIV 1 (code for unfired  pressure vessels).

At CWS, the three metre diameter shell was rolled from plates of standard size.  The total length of butt weld is   54.0 metres and that of fillet weld is 45 metres. Single V butt joints were accomplished with two passes by GTAW and five passes with SMAW. Double V butt joints were carried out with two passes of GTAW and two passes of SMAW on both sides. The total defective length was only 25mm, over 34 meters of radiographic quality weld. This works out  to 0.074%, a near zero-defect quality. The defects were later corrected. The ovality of shell is within 0.5%. Accurate location of nozzles on curved top dished end and its vertical alignment with inside bottom dished end was a difficult area. The same was successfully accomplished with the skill and experience of the technicians of CWS.Another intricate fabrication job was welding of heaters to the  heater sleeves. The heater flange with relief grooves for welding heater sleeves was made using the special fixture shown in sketch. 1.

Strict surveillance at every stage of manufacturing  ensured the quality standard of the product. The tank was subjected to hydraulic and pneumatic tests. Extensive care was taken for handling the tank safely inside the shop and also during transportation to SGTF using low bed trailer. The tank was helium leak tested under vacuum at room temperature and certified, satisfying, the acceptable leak limit of less than 1x10-8  torr. l/s.

 The successful completion of fabrication and quality assurance was possible due close co-ordination between various groups and is one more example of what can be achieved in-house.

(K.Balachander and CWS Team)