Debasis Roof corrosion profile can be monitored / profile will be known fairly well on a selective basis of choice of insulation pockets removal / replacements by providing well distributed ways of removal of locations of insulation pockets which will given enough indication of the roof plate thickness whenever monitored. If the initial monitoring of cut insulated pockets do not show varied thickness profile patterns on the roof (assuming the removable insulation pockets are immediately put back after thickness monitoring completion), then additional insulation pocket removal may not be necessary. If the original tank roof insulation has problems of not properly laid insulation with voids / air gaps and moisture entry at the top, then heat loss would definitely occur to cause extra steam consumption . This is not avoidable. If the original insulation is quite tight and quite adhering with good overlaps provisions and with no moisture or rain water entry, then removal of insulation pockets during thickness monitoring and replacing the same insulation caps at the locations will not dissipate too much energy loss or evaporation to bring down temp by 1or 2 degrees at best from 90 deg cent. If the original insulation is loose with not good over lap provisions on roof with already sufficient air and rain water (moisture) entry at several locations, then insulation pocket removal and replacing after thickness check will contribute to some additional heat loss in the tank. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Feb 06, 2010 E-mail: nishi@vsnl.com >Dear Experts, > >I have a technical question as below. > >Question : You have internally heated ( through steam coil ) >cone roof tank which is externally well insulated and the >purpose is to keep the tank containment hot above 90C. The >containment crude is waxy and if the temeparture falls below >70C there is definite chance of formation of wax and if once >formed there will be a hazard and mess. > >Now so far as External CUI Inspection of tank shell & Roof >are concerned do you recommend to create multiple inspection >pocket by cutting insulation ? I am asking this question >because a QA/QC inspector probably will say "YES I WANT >INSPECTION POCKET IN ALL SHELL COURSE " but from Process >point of view do you think that due to workmanship problem >or otherwise all these pockets can become potential site for >continuous heat loss which can increase steam demand and >also will create susceptibility for wax formation by local >temperature drop ? > >Need your Expert View . > >regards

Debasis Cut insulation pockets for process pipelines, tanks and in some cases for pressure vessels is a well known practice by industries for monitoring thickness and corrosion at selected places where pre-determined monitoring of locations had been finalized by plant inspection and process engineering groups. For tanks with hot insulation at the top (with steam coils at the top to prevent wax formation from crude oil), cutting insulation pocket is the only option left for monitoring roof corrosion by thickness checks. Some companies take extra precaution to provide additional over lap from strengthening the insulation pocket (voids or air gaps from entry of moisture or rain water) so that additional over lap will prevent any ingress of moisture or rain water entry thru the insulation (cut insulation pockets) The only possibility - as i know of - if cut insulation pocket windows are done properly and put back properly: During thickness check if rain comes, then insulation open areas are susceptible for water entry at that time. If the plant is located in torrential rain prone area this problem will eventually affect insulation quality even if you do not cut insulation pockets for corrosion profile monitoring of selected areas of top roof plates. From monitoring of corrosion profile, on stream or even during shut down time of crude oil tank (with top steam coil to prevent waxing) there is no other way other than providing good cut insulation windows at selected locations all over the roof. Additional over lap provision should certainly take care of even if there is some slight loose fixing while replacement after thickness monitoring. Otherwise you have to wait for tank to be emptied and see the roof place corrosion profile with elaborate scaffolding, removal of wax coating and take accurate thickness checks to know corrosion profile. Choice of providing cut window pockets would certainly assure a plant user (both on stream and without necessity to wait for a full tank empty condition for only thickness monitoring) but with extra care to provide additional strengthening over lap (at the areas windows are made in roof insulation) apart from cut window sizes would be the best choice for monitoring roof plates. Hope this helps you to discuss with plant and insulation people C.V.Srinivasan Nishi Engineers Pvt Ltd India Feb 19, 2010 E-mail: nishi@vsnl.com >Dear Mr. Srnivasan, >My question is at the construction completion stage is it >worth creating multiple Inspection Pocket at corrosion >susceptible locations on a HOT INSULATED WAXY CRUDE OIL >TANK ? There are 2 groups of opinion. One group encourages >it for QC for CUI purpse but another group strongly >discourages creation of even a single pocket at corrosion >susceptible considering the risk of moisture/water ingresss >from the pockets causing more harm on the metal/insulation >and rather the 2nd group encourages for a better & full >proof tight insulation. I align with the 2nd group. >What is your opinion, Sir ? > >>Roof corrosion profile can be monitored / profile will be >>known fairly well on a selective basis of choice of >>insulation pockets removal / replacements by providing well >>distributed ways of removal of locations of insulation >>pockets which will given enough indication of the roof plate >>thickness whenever monitored. >> >>If the initial monitoring of cut insulated pockets do not >>show varied thickness profile patterns on the roof (assuming >>the removable insulation pockets are immediately put back >>after thickness monitoring completion), then additional >>insulation pocket removal may not be necessary. >> >>If the original tank roof insulation has problems of not >>properly laid insulation with voids / air gaps and moisture >>entry at the top, then heat loss would definitely occur to >>cause extra steam consumption . This is not avoidable. >> >>If the original insulation is quite tight and quite adhering >>with good overlaps provisions and with no moisture or rain >>water entry, then removal of insulation pockets during >>thickness monitoring and replacing the same insulation caps >>at the locations will not dissipate too much energy loss or >>evaporation to bring down temp by 1or 2 degrees at best from >>90 deg cent. >> >>If the original insulation is loose with not good over lap >>provisions on roof with already sufficient air and rain >>water (moisture) entry at several locations, then insulation >>pocket removal and replacing after thickness check will >>contribute to some additional heat loss in the tank. >>Hope this helps >>C.V.Srinivasan >>Nishi Engineers Pvt Ltd India Feb 06, 2010 >>E-mail: nishi@vsnl.com >> >>>Dear Experts, >>> >>>I have a technical question as below. >>> >>>Question : You have internally heated ( through steam coil ) >>>cone roof tank which is externally well insulated and the >>>purpose is to keep the tank containment hot above 90C. The >>>containment crude is waxy and if the temeparture falls below >>>70C there is definite chance of formation of wax and if once >>>formed there will be a hazard and mess. >>> >>>Now so far as External CUI Inspection of tank shell & Roof >>>are concerned do you recommend to create multiple inspection >>>pocket by cutting insulation ? I am asking this question >>>because a QA/QC inspector probably will say "YES I WANT >>>INSPECTION POCKET IN ALL SHELL COURSE " but from Process >>>point of view do you think that due to workmanship problem >>>or otherwise all these pockets can become potential site for >>>continuous heat loss which can increase steam demand and >>>also will create susceptibility for wax formation by local >>>temperature drop ? >>> >>>Need your Expert View . >>> >>>regards