1. Why is Jacketed Piping used?
Jacketed Piping is a type of piping used to convey very viscous process fluids. It consists of an inner pipe with a jacket surrounding it, which is then heated by a medium such as steam, hot water, hot oil, or other heating media. This creates a thermal barrier between the core pipe and the jacket, allowing the process fluid to flow more freely. Vacuum jacketing is also used as an insulator for cryogenic fluids, and can be analyzed using a similar calculation method as that used for heated jacketed piping.
2. What is the density value to be entered into the Caesar spreadsheet when water (density=1000Kg/m3) is flowing through the jacket?
The equivalent density of the jacket fluid can be calculated by using the following formula: Actual jacket fluid equivalent density = [(rj2 – Rc2)/ rj2 ] x dj. This equation takes into account the inner radius of the core (rj) and the outer radius of the pipe (Rc), as well as the density of the heating medium (dj). For this example, we are assuming that the heating medium is water, with a density of 1000 kg/m3. Therefore, the equivalent density of the jacket fluid is also 1000 kg/m3.
What are the key stress checks that need to be performed when analyzing a Jacketed Piping system?
The stress limits due to sustained and expansion loads should be separately evaluated for the core and jacket pipe according to ASME B31.3 Clause 302.3.5. Buckling load should also be checked manually, as this is not a feature of Caesar-II. The critical force, Pcr, is calculated using the equation Pcr = (4π2 *Ec*Ic)/L2 for the core and Pcr = (4π2 *Ej*Ij)/L2 for the jacket, where Ec and Ej are the modulus of elasticity of the core and jacket material, Ic and Ij are the moment of inertia of the core and jacket, and L is the length of the pipe between the junction of the core and jacket. If the force calculated by the computer program, P, is less than or equal to Pcr, then no buckling failure has occurred.
The weld strength check between the jacket and core pipe involves calculating the force P at the junction point between the core and jacket pipe, and comparing it to the allowable load at the weld point. The allowable load is calculated by multiplying the area of the weld by 80% of the hot allowable stress of the material. The area of the weld is calculated using the diameter of the core pipe and the root of the weld, which is 0.707 multiplied by the weld size. If the calculated force is less than the allowable load, then the system is deemed safe.
The deflection of the jacket should also be checked, assuming that no spider/spacer is used between the core and jacket. The maximum allowed deflection of the jacket is T/2, where T is the thickness of the jacket. In addition, the jacket may be subjected to partial vacuum conditions due to steam condensation. The jacket should be checked for vacuum conditions in this case. If the core is at a pressure of 30 PSIG and the jacket is at a pressure of 180 psig, then the core is subjected to an external pressure of 150 PSIG, and should be investigated for collapse or local buckling from the external pressure load.
The axial stress should also be checked. The calculated displacement stress range (expansion case stress) is SE, and should be corrected by adding axial stresses for the local analysis of the junction point. The axial force can be obtained from CAESAR output or can be calculated using the equation Faxial = (E x ΔL x Area) / L. CAESAR also calculates the value of axial stresses for operating cases, and the axial stress due to thermal differential should be added to the calculated expansion stress and compared with the allowable loads as per ASME B31.3.
4. What is the allowable value used for evaluating the welding at core jacket interconnections?
There are two methods commonly used to check the welding at core jacket interconnections. The first method is to consider 0.6 times the tensile strength of the electrode, as required by the American Institute of Steel Construction (AISC) code. The second method is to consider the sum of 1.25 times the electrode’s tensile strength (Sc) and 0.25 times the electrode’s shear strength (Sh), as per secondary stress generated theory. Both methods ensure that the welding is done safely and is within the allowable limits.