JLAB-TN /15/2007. JLab Cryomodule Thermal Shield Circuit Piping Design per ASME B31.3 Edward F. Daly, Gary G. Cheng and John Hogan

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1 Introduction JLab Cryomodule Thermal Shield Circuit Piping Design per ASME B31.3 Edward F. Daly, Gary G. Cheng and John Hogan This technical note analyzes the JLab cryomodule thermal shield circuit piping design in accordance with the American Society of Mechanical Engineers (ASME) code B31.3 for process piping. The thermal shield circuit designs of the original CEBAF and upgrade cryomodules are very similar. Despite the fact that thermal shield designs have been built and performed properly and safely, this note serves as an in-depth review of the pressurized piping systems between the end cans in terms of provisions from ASME standards. The shield circuit cools the nominal 50K thermal shield. The design pressure for the shield circuit is 10 atm (see drawing D-0080). The gas/fluid circuit consists of pipes, tubes, fittings, and valves, most of which reside in the end cans and were analyzed and documented elsewhere using ASME B31.3 piping code [1]. The remaining components, which consist of copper-stainless steel solder joints, copper tubing, and stainless steel bellows, are analyzed and documented in this technical note. As described in [1], the ASME B31.3 code comprehensively defines design requirements in various aspects: material selection, component pressure and temperature ratings, fabrication, assembly, erection, etc. In this note, thermal shield piping and components are examined in reference to pertinent code sections. This analysis is applicable for pressure systems that have been identified and tracked by the Pressure Systems Committee in accordance with JLab s intent to comply with 10CFR851. In particular, the following pressure systems have been identified: PS Injector Cryounit Thermal Shield Circuit, and PS Cryomodule Thermal Shield Circuit. Further, the analysis can be applied to thermal shield circuits on both C50 refurbished cryomodules and 12 GeV Upgrade cryomodules since the design features analyzed herein are identical. 1/6

2 Thermal Shield Circuit Piping Design Thermal Shield Bellows Figure 1. Section View of Bridging Area Cryounits 2 & 3 Figure 1 shows a section view of the bridging area between cryounits 2 & 3 in an existing CEBAF cryomodule. Note the bellows assembly that strain-relieves the thermal shield piping. This is a typical joint in all CEBAF thermal shield designs (see for example E-0001). Thermal Shield Flex Hose Figure 2. Section View of Bridging Area Cryounits 1 & 2, 3 & 4 2/6

3 Figure 2 shows a section view of the bridging area between cryounits 1 & 2 and also 3 & 4 in an existing CEBAF cryomodule. Note the upper braided flex hose assembly that strain-relieves the shield piping. This is a typical joint in all CEBAF thermal shield designs (see for example E-0001). Some salient features of the thermal shield circuit are shown in the table below. References to sections in the B31.3 Process Piping code are given where applicable. Operating Temperature 2 K 300 K Operating Pressure 29.4 psig (3 atm) Design Pressure psig (10 atm) Fluid Service Normal per Figure M300 Tubing Description 304L Stainless Steel; 0.75 OD x wall Copper, Type L, Hard-drawn; 0.88 OD x wall Material Allowables 304L Stainless Steel ksi allowable per A-1 Copper, ASTM B88-12 ksi allowable per A-1 Joining Methods GTAW, Silver solder Table 1. Thermal Shield Piping Circuit Requirements and Features Straight Pipe Minimum Wall Thickness The design pressure for shield circuit is 10 atm. Equation (3a) in B31.3 para (a) is applied to calculate the minimum required wall thickness for straight tubes or pipes in this circuit: t = 2 P OD ( S E W + P Y ) where t = Minimum required wall thickness, P = Design pressure, (P = 10*14.7 psi), OD = Outer diameter of tube, S = Material allowable stress from B31.3 Table A-1, E = Quality factor for longitudinal weld from Table A-1B, herein E = 1.0, W = Weld strength reduction factor, herein W = 1.0, and Y = From Table , Y = 0.4 for steels & other ductile mat ls at low temp. 3/6

4 Component Reference drawing # OD of main run (in.) Wall of main run (in.) B31.3 required wall (in.) Safety factor Stainless Tubing E Copper Tubing E Table 2. Thermal Shield Piping Circuit Minimum Wall Thicknesses The copper safety factor increases by a factor of two when using cold properties. The safety factors for the copper and stainless piping are acceptable. Tubing Solder Fittings Figure 3. Detail of Solder Joint (reference drawing E-0008) There are soldered joints in the circuit which consist of an inner sleeve of stainless steel and an outer sleeve of copper (Figure 3). The filler metal is Easy-Flo 45, a silver solder. There is no axial stress in the tubing because the ends are balanced. The safety factors calculated for the straight tubing segments can be applied directly. Pressure Relieving As described in [1], a single pressure relief valve is installed in parallel with a pressure gauge for monitoring pressure levels in the shield circuit. The set pressure of the relief valve is decided per BPV code provisions. There are axial restraints on each bellows to ensure that the bellows convolutions do not over-extend during pressurization. 4/6

5 Flexibility, Stress, and Support Flexible Ligaments (~ 20 per segment) 7/8 OD Copper Tubing Figure 4. Shield Segment Details (reference drawing E-0008) The copper tubing segments between bellows are strain-relieved axially by a series of ligaments that support it on the circular shield (Figure 4). The flexible bellows and braided flex hoses between shield segments reduces the transverse stresses to negligible levels. Figure 5. Detail of Solder Joint (reference drawing E-0008) The bellows assembly (Figure 5 taken from D-0092 P4) provides axial strain relief during cooldown, normal operations and upset conditions (Figure 1). Per (a), there exists extensive, successful service experience under comparable conditions with similarly proportioned components of the same or like material. For maximum operating conditions, the bellows minimum required thickness is compared with an actual thickness of /6

6 Under maximum operating conditions, the peak stress in the bellows does not exceed the maximum allowable compare 11.7 ksi vs ksi (h) 1, t P( d + w) 4w S q 1 [ d /( d w) ] 2 t = tm + where t t m P d D S w q = nominal thickness of bellows after forming, in, = miminum thickness of bellows sheet mat l before forming, in, = Design pressure, (P = 10*14.7 psi), = inside diameter of bellows, 0.97 in, = outside diameter of bellows, 1.43 in, = Material allowable stress from B31.3 Table A-1, = convolution depth, (D d)/2 = 0.23 in, = convolution pitch, 0.4 in, Based on the operational experience, the axial restraints and the low probability of an upset occurrence, this bellows thickness is deemed acceptable. Fabrication and Assembly The B31.3 Chapter V Fabrication, Assembly, and Erection extensively listed the basic qualifications for welders and welding procedures. Jefferson Lab has established its welding requirements and standards per ASME Section IX. The two are compatible. Welding of all circuit components are specified and qualified in accordance with ASME Section IX. Because of the thin wall thickness, according to ASME B16.25, the butt welding of the tubes can be done with square or slightly chamfered tube end preparations. Summary The cryomodule thermal shield circuit design has been reviewed per ASME B31.3 code for process piping. It is found that the tubing and bellows in the thermal shield meets the design requirements of ASME B31.3. The tubing has wall thicknesses which yield suitable safety factors. The bellows convolutions have been analyzed and determined acceptable. Weld preparations have been specified in accordance with ASME B31.3. REFERENCES [1]. JLab Technote TN , C100 Cryomodule End Can Piping Design per ASME B /6