Mark Mor Pin hole leaks : Are these from the lap welds ? Are these comingfrom the tank plate? Problem could have come in 2 years time - i believe- due to original construction lap welds- not tested and inspected to API code stringency. If bottom plate is corroded, then it should have affected both tanks equallly. Bottom plate Lap welds - pin hole leaks are known to develop in many tank bottom plates - world over. If this is established, simple vacuum box test to API standards will reveal all locations susceptible for pin hole leaks. Vacuum test is the most sensitive and quick and reliable test method to establish leaks in lap welds and if required in entire bottom plate (if you suspect the plate had developed leaks at some random locations). From your description of the bottom foundation insulation asphalt dissolved by leaky kersonene, you have a probable risk the plate lap welds will see more stress and the risks of further pin hole leaks develop are more with further operation. Replacing the bottom plate in segments would be the best with prior insulation barrier of same asphalt or consdier new concept of FRP / Polyvinyl type insulation layer between the bottom plate and foundation. (already suggested by other users' views Gopal/Chakra and Debasis). If you suspect only segmental areas in a big tank had been affected , you can think / do segmental replacements of bottom plate. Segmental replacements are done in petroleum storage tanks world over with reliable repair approach and reliable testing and inspection with good success. With no support between foundation (asphalt being dissolved in kersonee) risks of pin hole leaks in bottom plate lap welds are more and these have to be recognized to empty tank and do thorough vacuum / D.P. or FLMPT of lap welds and segements of bottom plates immediately. The more you delay, i believe , you will encounter more problems due to addtional stress load on lap welds (due to no cushion available with dissolved asphalt). For Ex: an unsupported bottom plate and bottom plate welds may fail faster if you delay the inspection & testing. After evaluation of the condition of the enire bottom plates and bottom plate segments (affected segemnts first) you can decide whether to go for complete replacement or segmental replacements of bottom plates. Trust this is of help to you. C.V.Srinivasan Nishi Engineers Pvt Ltd India August 15,2005 E-mail: nishi@vsnl.com >Hi >Kerosene tank had been leaked after two years in services. >Inspection was done and revealed that three pin hole leaks >under by side corrosion .The tank foundation is asphalt and >had been dissolved by keorsene. Tank history showed during >T&I 2003 two pin hole leaks were patched and kerosene >product already contaminated under the tank foundation and >it is the main cause for the failure at 2005. Now the tank >foundation is dissolved and the tank bottom had metal loss >40-50% at all plates also, three pin hole leaks. In regard >to that it recommended to reproflie the asphalt foundation >and patch all plates that had metal losses or replace all >the bottom plate major replace . Your input will be valued.

Dear All, For detection of leaks thru' bottom plate lap joints, vacuum box testing is very useful. However, the system is time consuming & only used for lap joints. To check bottom plates for suspected leakages, if any, pneumatic testing is preferable. As a matter of fact, in our refineries, it is a part of any tank M&I (maintenance & inspection or say T&I) programme. The system consists of pressurizing the tank bottom underside with air and putting soap solution or water on bottom plates. Any leak will be easily detected. To carry out such test, weld a nipple on bottom plate at central location & put an air hose. Weld another 3-4 nipples at different location and install U-tube manometers. Build a clay dam around the tank, approx 10" ht (from outside) and put 6" of water. Now pressurize the underside of bottom plates with air to approx 75-80 mm WC and spray soap solution on tank bottom plates. All small leaks will be evident within 15 minutes time. The same test can be done with putting 6" of water on tank bottom plates. However, in that case, approx. 12" of water will be put in the clay dams. In that case, approx. 225 mmwc air pressure is required under the tank. Our experience indicate that for a 35000 Cu. M tank, approx 8kg/cm2 air pressure is required for soap solution test and approx 15 kg/cm2 pressure reqd for water test. Please note that soap solution/water will be sparyed on the bottom plates before pressurizing the underside of the tank. During testing, some hammering on the bottom plates will help any leak clogged with dirt to open up and get detected. For more details, please refer to the "API guide to the inspection of refinery eqpt" Chapter XIII clause 1304.046. To the best of my knowledge, this is the best method for detection of bottom plate leaks and the method takes approx 24 for installation and 30 minutes to check all the leaks. And testing with water is better than testing with soap solution. And if time permits, go for simple replacement of bottom plates rather than overlaying of patch plates. Overlaying will only increase the underside corrosion rate fewfold. 'Hope it helps. Chakra