Ashish It is not clear from your question whether the fabrication is intending to to provide cladding of 2 mm SS 304 on C.S for your K.O.D is with lap weld joints of 300 mm spacing with plug weld (staggered to suit your ID - L X D) or with strip cladding. I presume that your fabricator is proposing to provide strip cladding at 300 mm spacing due to ID being 1500 mm from facilitation of welding from inside and from good cladding support (from corrosion prevention) for the back up C.S shell. If strip cladding is intended for your KOD by the fabricator, it is quite normal to provide plug welds to vent out the entrapped gases between the C.S and SS liner and finally seal weld the plug. Plug welds (final seal welding after initial pneumatic test of all strip liner welds (both longitudinal and circumferntial welds) to ensure that each joint is leak tight and free from surface defects by DP testing. Final plug weld and DP will ensure that all entrapped gases have been removed and the SS liner is more or less an integral part of the C>S Shell. For your KOD pressure and temp 250 deg cent, i guess, that you may not face any serious problem of differential expansion of joints as the welds are sound and more or less integral to take both pressure and temperature load variations. Your question raises on hypothetical assumptions of differential expansion of SS and C.S. This question will not arise if strip cladding is done well and well tested before final seal welding is done to plug the plus (10 mm plug hole areas - staggered). Your design pressure is too low and design temperature is just 250 deg cent that it may not affect the performacne of welds in strip cladding or even by lap joint design with plug welding done in a staggered way at 300 mm pitch distance (rotated staggering). It is difficult to assess the load UW 17 (d) formula can be taken and if it satisfies both shear or tension, you need not worry about thermal expansion effects at just 250 deg cent for S.S 304. If you are particular to assess you have to take simulation welding and carry out XRD of the plug welds to know the qualtitative and quantitative tension and compressive stress values. To my mind, it appears, that it is enough if you do shot peening at local plug weld areas to ensure the plug welds have less of shear or tension stresses but more of compressive stresses. This will adequately cover all problems in liner welds - even if the weld is made by lap joint method. I am sure that the fabricator would have intended in his design to maek strip weld lining only for your KOD due to limitation of space to work in a 1500 mm dia vessel. Fabricator design if strip cladding technique is proposed for your KOD is quite adequate with final seal plug welding at spacing of 300 mm (roating staggering of plug weld). To my mind, there is no designer desparation for such a low pressure design vessel. Trust this helps you to interpret ASME code usefully and use of fabrication design practices to suit vessel geometry carefullly. C.V.Srinivasan Nishi Engineers Pvt Ltd India Aug 09, 2006 E-mail: nishi@vsnl.com >Hi! > >I am currently reviewing a fabrication drawing for a 6.0 mm >thk. carbon steel KO drum with 2.0 mm thk. SS 304 cladding. >The base plate is SA 516 Gr. 60 and the SS plate liner is SA >240 Gr. 304. The vessel design Pr. is 0.26 MPa @ 250 deg. >C. Vessel dimensions are 1500 mm O.D x 6000 mm T/L. The >vendor proposes to attach the SS liner by plug welding (10 >mm plugs) at 300 mm rotated sqare pitch. > >Considering that SS has a higher co-efficient of thermal >expansion than CS, would the proposed attachment method be >adequate? > >Is their a method where by the pitch of plug welds can be >ascertained for such cladding methods based on the design >temperature / pressure? > > > >Ashish

Ashish Based on the information given, work load on a plug weld either in shear or tension as per UW 17 (d) comes approx: 700 MPa. Though it is difficult to calculate the differential thermal expansion stresses in plug welds, i guess this will be relatively less compared to the shear or tension work load for this pitch distance 300 mm rotated square pitch 10 mm plug welds in your KOD. Roughly you may have to make around 300 plug welds of 10 mm dia. I believe that the lap weld at the perimeter would be subject to higher shear or tension stresses and relatively more differential thermal expansion stresses at your design temp of 250 deg cent. Again thermal stress will be predominantly tensile stress in nature at the perimeter lap weld. If there is a possibility of a review to have rolled plates to make 3 or 4 quadrants to suit the 1500 mm ID x 6000 mm long and make both longitudinal and cirumferential welding (without lap joints) butt welding done in a staggered way to reduce heat inputs, your hypothetical asumptions and likely problem of distortion of 6 mm C.S can be minimized to a larger extent. Staggered welding will certainly reduce heat input and also reduce distortion effects and also reduce both shear and tensile stresses in butt welds. Prior buttressing the C.S with S.S over lay weld at the quadrant sections to be butt welded may alleviate most of your problems. This is a common approach when thin sections are welded. All said and done, you have to accept a fact that a lap weld is inferior and will have only 60% of allowable stresses compared to 3 or 4 quadrant welded (3 or 4 segment buttressed butt weld between 6 mm C.S and 2 mm SS 304 plates) butt welded liners. To me it appears that shear or tension load on plug welds will be more than your hyptothetical fear or assumed differential thermal expansion stresses. If you cannot review the current design proposal, given by the fabricator, at this stage but still agree to have the concept of lap joint welds at the perimeter with 300 numbers of 10 mm dia plug welds for this thin 2 mm, i believe that this will create more stresses than the imagined thermal expansion stresses in such thin walled vessel. 300 numbers of plug welds and lap weld joints in 2 mm thicknesss in SS 304 will contribute to more tensile stresses than differential expansion stresses, try to perform a simulation trial weld and perform XRD (X ray diffraction study) to quantiatively evaluate the shear or tension related stresses in a 2 mm SS plate. Comparatively a staggered butt welded S.S strip lining would be trouble free from long term usage. If i am the designer i would go for a strip welded liner inspite of a 6 mm back up C.S shell but limit distortion effects to the minimum by reducing heat input in a judicious and careful heat control welding sequence operation than go for 300 number plug welds with a weak lap joint welds. You should remember codes are based on experience of various design and fabrication criteria and well accepted industrial practices and outputs on problems faced over years. You could expect only general guidelines from code practices and not on specific problem related issues. Trust this helps you to evaluate the comparative advantages and disadvantages before final review on design acceptance. C.V.Srinivasan Nishi Engineers Pvt Ltd India Dec 09, 2006 E-mail: nishi@vsnl.com >Thank You Mr Srinivasan for sharing the detailed >information. > >Allow me to clarify the situation. > >The 6.0 mm SA 516 Gr. 60 shell is to be lined with 2.0 mm >thk. SA 240 Gr. 304 liner. The methodology proposed is to >use standard plate width (SS 304), rolled to suit the vessel >ID and attaching the liner by means of 10 mm dia plug weld >on a ROTATED SQUARE pitch of 300 mm (applied liner). The >perimeter of the liner plate is to be lap welded to the base >plate. Strip cladding was not preffered due to the >extensive welding involved and the subsequent distortion of >the 6.0 mm thk. base (shell) plate. This is NOT a strip >cladded vessel. Hence, the bulk of the loads imposed due >to differential thermal expansion are to be countered by the >plug welds. > >As I understand, the design temperature plays a crucial role >in determining the pitch of plug welds rather than the >design internal pressure. The differential thermal expanion >of SS, imposes substantial stresses on the plug welds and >250 C is can definitely not be ignored. The requirements of >the CODE are inviolable. > >Shoot peening of plug welds is definitely not an option that >can be considered. > >Being a designer, theoretical justification is not only a >habit but also a job prerequisite and hence the desperation. >Information given in Brownell & Young, Jawaad & Faar, >Bednar, and the codes are rather sketchy on this issue. > >Hope the above information would clarify the issue. Please >give us your valuable advice in light of the above. > >Thanking you in advane

Thank You Krish. Unfortunately, Process requirements do not permit any Corrosion Allowance and hence SS 304 it shall be. ashish