DAMAGE ANALYSIS AND FATIGUE EVALUATION OF AN ALUMINIUM BRAZED PLATE FIN HEAT EXCHANGER

Transcription

1 Proceedings of the ASME 2014 Pressure Vessels & Piping Conference PVP2014 July 20-24, 2014, Anaheim, California, USA PVP DAMAGE ANALYSIS AND FATIGUE EVALUATION OF AN ALUMINIUM BRAZED PLATE FIN HEAT EXCHANGER Reinhold Hölzl Thomas Hecht Pascal Freko ABSTRACT During operation of an air separation unit (ASU) a plate fin heat exchanger developed a leak. This apparently simple heat exchanger, exchanging heat between two gaseous streams of air versus nitrogen, was not expected to suffer significant stress levels for producing a damage. During the analysis of the damage which occurred at the connection welds of three adjacent modules module construction is widely used in the process industry an operating situation was discovered leading to high thermal gradients inside the exchanger. It occurred during a stop of the plants nitrogen cycle compressor. The warm flow continued and heated up the exchanger in a very fast way. This led to unacceptably high stresses in the top part of the exchanger. But a more severe situation occurred when the cycle compressor has been restarted. Then cold nitrogen entered the lower end of the exchanger which had been warming up in the meantime. A finite element analysis based on a modeling technique [4] for brazed plate fin heat exchangers was performed for this dynamic incident showing extreme stress levels at the location of the damage. A fatigue evaluation of the calculated stresses showed that only a few cycles were acceptable for this scenario. As a consequence operational measures have been implemented to protect the exchanger against this severe condition. INTRODUCTION Aluminum brazed plate fin heat exchangers (PFHE) are typically used for transferring heat in cryogenic air separation units. These exchangers combine large heat transfer surfaces with a small volume. This can result in significant thermal stresses. Certain operational situations called scenarios which occur several times and produce significant stress levels may develop a fatigue damage and lead to a leak. To detect and repair a leak often an unplanned shutdown is necessary with high impact on costs and availability 1 Copyright 2014 by ASME

2 This paper describes a heat exchanger which suffered very high thermal stresses and consequently developed a leak. That heat exchanger is used as case study for the development of a leak, the simulation of the damage mechanism and the improvement measures. For the stress analysis of PFHE's a modeling and calculation procedure described in [4], [7], [12], based on finite element method, was applied. DESCRIPTION OF THE DAMAGED PFHE PFHE's seem to be very simple due to the layer-bylayer construction but may become very complex with a rising number of streams. They are manufactured [10] by joining of layers of fins, partition plates and side bars, thus building up a compact block with a huge heat transfer capacity. Three modules add up to the stacking height. They are welded together at the top and bottom section, see Fig. 4. Inside the core there is an alternating sequence of two stream chambers A and B. Stream A is gaseous air, entering at the top nozzle N01 with a temperature of +19 C and exiting at the bottom nozzle N02 at -173 C where partial liquefaction occurs. The second stream B is gaseous nitrogen entering at the bottom nozzle N12 at -178 C and exiting at the top nozzle N13 at +15 C. The countercurrent flow pattern is typical for PFHE's. In total there are 270 layers, 135 for each stream. The flow rate is about Nm³/h at approximately 5.5 bar pressure for both streams. OPERATING CONDITION - DAMAGE DESCRIPTION The main task of this exchanger is to cool down dry process air and warm up a pressurized nitrogen flow from the cycle compressor. Due to some incidents, for example when the electric power fails, the cycle compressor stops and interrupts the cold flow to the PFHE. The flow of air continues and warms up the block. When the cycle compressor is restarted again, cold nitrogen enters the block from the lower section which has been warmed up in the meantime. After nine month of operation a pressure increase in the coldbox was detected, indicating a leak inside the coldbox. In the following the ASU was stopped and warmed up to prepare for leak detection. After warming up the perlite had to be removed out of the coldbox which is very expensive and time consuming. During the leak search a leakage of the PFHE was detected. It is unknown from experience that a two stream gas-gas PFHE tends to leak. This made a detailed analysis of the root cause necessary. Location of the leak Perlite (white) Weld connection between modules Fig. 1 Geometrical information of the damaged exchanger The PFHE in focus has a width of 1200mm, a length of 3675mm and a stacking height of 2963mm see Fig. 1 above. Fig. 2 Detected leak at the lower module layer connection weld Fig. 2 shows a section of the PFHE which is surrounded by perlite for insulation purposes. At the leaking region the perlite was removed for leak detection, visual inspection and repair, if 2 Copyright 2014 by ASME

3 applicable. The leak was found at the module layer connection weld at the lower end of the PFHE. At that time the question arose which loads were high enough to have the potential for damaging the exchanger. Thus the process was analyzed very thoroughly which is described in the following sections. THERMO-HYDRAULIC DYNAMIC ANALYSIS OF THE DAMAGE The equation based Linde in-house process simulator OPTISIM has been used to perform a thermo-hydraulic simulation of the PFHE. Within OPTISIM, heat exchangers are set up using the declarative modeling language of the OPTISIM equation block for differential-algebraic equation systems [13]. This process has been modularized by setup of a model library. Customized heat exchanger modules can be used like any other process unit operation within a process calculation. The failure of the nitrogen cycle compressor leads to a loss of nitrogen flow within 5 minutes in two steps. The first step is a reduction from 100 % flow down to 16 % within 60 sec. After 180 sec the flow drops again within 60 sec down to 0 %. In parallel the air flow reduces from 100 % flow within 11½ min only to 12 % flow (instead of 0 % flow). The beginning of the air flow reduction coincidences with the reduction of the nitrogen flow. After a downtime of half an hour (without nitrogen flow and 12 % air flow), both flows are ramped up again to 100 % flow. This has been performed with an intermediate unbalance. The cold nitrogen flow was ramped up quicker than the air flow. The described scenario has been set-up using the PFHE geometry. Heat transfer coefficient correlations which have been implemented into the thermal-hydraulic model account for the different mass flows. the cold end of the PFEH is again cooled down very quickly. Due to the excess supply of cold duty, the heat exchanger profile bends and the warm end also cools down to some amount. At the end, the temperature profile normalizes again and the heat exchanger returns to normal operation conditions. DESCRIPTION OF THE FINITE ELEMENT MODEL OF THE PFHE A detailed finite element modeling principle for PFHE's, already described in [4] and [12] has been developed several years ago. This principle was applied to the given exchanger. The model considers the complete stacking sequence of the real exchanger with three modules and includes all relevant parts like different fin types, parting sheets, dummy passages, side bars and headers, see Fig. 4. Three modules Module layer weld connection Fig. 4 Finite Element Model of the PFHE Fig. 3 Metal temperature distribution of the PFHE for scenario loss of nitrogen Fig. 3 shows the dynamic metal temperature distribution of the PFHE. In the beginning, the heat exchanger is warmed up quickly due to the loss of cold duty. After restart of the flows, With that modeling principle and the created model various process situations, steady state or dynamic, can be analyzed. The model is loaded with a pressure of 5.5 bar in each chamber and a longitudinal distribution of heat transfer coefficient and stream temperature for air and nitrogen flow. The input was given by the OPTISIM program for 88 time steps with variable step sizes of 15 to 600 seconds, depending on the temperature gradient, and a total calculation time of 4800 seconds. On that basis the time dependent metal temperature distribution already shown in Fig. 3 and out of that the thermal stress in the whole exchanger has been calculated. 3 Copyright 2014 by ASME

4 Fig. 5 shows the metal temperature distribution during the restart phase of the cycle compressor where the deformations have reached a maximum. For better visualization the deformations are scaled by a factor of 100. It can be seen very clearly, that most of the PFHE has been warmed up. The yellow color represents temperatures higher than -4.5 C. The lower section, roughly one fourth of the exchanger length, suffers a steep metal temperature gradient down to -180 C. Due to thermal elongation effects high deformation differences are induced in that lower section. This results in high stress gradients at the tip of the module layer weld connection shown in the next section. because of the steep temperature gradient and because of the change in stiffness at the end of the weld connection. As a result stresses of 250MPa after a time of 2550 seconds are calculated. Extremely high stresses at the weld tip where the leaks occurred. Tip of the module layer weld Fig. 6 Equiv. v. Mises Stress distribution [MPa] with a maximum at the location of the leaks. CALCULATION OF LOAD CYCLE NUMBERS Fig. 5 Most extreme metal temperature distribution [ C] during restart of compressor STRESS ANALYSIS OF THE DAMAGE Based on the given pressure and temperature distribution the stress distribution is calculated. Fig. 6 shows extremely high stress gradients especially in the lower part of the exchanger at the upper end of the module layer weld due to the high thermal gradients. Stresses far above the allowable limits are reached which result in a limited number of load cycles. The reason for that is, that the inner fin sections, which have low mass and heat capacity compared to the massive border areas, the sidebars, react very quickly to temperature changes. Due to thermal elongation effects strain differences between inner section and border section develop. This inequality of the strain field leads to stresses which are very high in this case The linear elastic stress analysis gives extremely high values of up to 250 MPa. For a steady state case approximately 60 MPa would be the allowable stress level for an upset scenario. Therefore the allowable value is exceeded by nearly a factor of four. The stresses come out of temperature differences which are secondary stresses. This allows an evaluation against fatigue. Based on calculations and experimental investigations [7], [12] we can correlate the calculated stresses to a fatigue curve of the German pressure vessel code AD-2000 Merkblatt S2, which has an appendix for fatigue evaluation of aluminum structures. The fatigue evaluation for a stress level of 250 MPa leads to 32 load cycles. No code safety factors are included because we do not consider a design case but an actual damage. The records of the control system of the plant showed that this scenario occurred 40 times. This has to be compared to 32 calculated cycles. Because fatigue tests show a wide spread range of cycles at a given stress level the correlation here between 32 calculated load cycles and 40 counted load cycles is really good and so the reason for the damage was confirmed. 4 Copyright 2014 by ASME

5 IMPROVEMENT MEASURES FOR OPERATION The thermo hydraulic analysis demonstrated that an unbalance of the warm and cold flow rates produces great temperature gradients already after a short time. Therefore all scenarios in which an unbalance of the warm and cold stream can occur were investigated. Control loops were installed to prevent critical temperature profiles during normal operation and load change. Additionally the heat exchanger is isolated in case of a cycle compressor trip. To improve the restart of the cycle compressor a ramp-up function to balance the warm and the cold stream were implemented into the control system. CONCLUSION An unexpected damage in a PFHE of an air separation unit occurred due to extremely high stresses during the stop and restart of the cycle compressor. A very detailed thermo hydraulic analysis followed by a finite element stress analysis discovered stresses of about 250 MPa which is far beyond allowable limits. Fatigue considerations give 32 load cycles until a damage a leak has to be expected. Counting of the number of events for the stop of the cycle compressor gives 40 cycles. This is in good accordance with the calculated results. An improvement measure for the future is the implementation of a valve to stop the air flow if the cycle compressor fails. So both streams fail and are restarted at the same time leading to significantly lower stresses. Therefore this upset scenario is driven back into the safe range. Beside that the capability of dynamic process simulation, stress analysis, stress and fatigue evaluation with the given tools [4], [13] has been proven again. As a consequence this methodology described in [14] can be applied to any PFHE and may be used to support other companies for consulting and fault diagnostics. 5. Dib, J., et al, "Application des techniques d'homogeneisation pour l'analyse tridimensionnelle d'echangeurs de chaleur a plaques et a ondes", Actes du 7eme Colloque National en Ca1cul des Structures, May Noishiki, et al., "Development of High Pressure Large ALEX for Gas Processing Plants", Kobe Steel Engineering Reports, Sep. 2003, vol. 53, No Hölzl, et al "Experimental fatigue evaluation of aluminum brazed plate fin heat exchangers", ASME Pressure Vessel and Piping, Mizoguchi, et al., "Fatigue Strength Corrugated Fin Type Heat Exchanger", ASME, Pressure Vessel & Piping Nakagama, et al, "Three Dimensional Structural Analysis of the Plate Fin Heat Exchanger", ASME Pressure Vessel and Piping English translation of JP-A The standards of the brazed aluminum plate-fin heat exchanger manufacturers' association, ALPEMA, Third edition, ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, par FATIGUE ASSESMENT, Edition 2011a 12. Hölzl, R., "Lifetime estimation of aluminum plate fin heat exchangers", ASME Pressure Vessel and Piping, Thomas, I., A process unit modeling framework within a heterogeneous simulation environment, Proceedings of the 21st European Symposium on Computer Aided Process Engineering (ESCAPE-21), pp , (2011). 14. LIBAS, Brochure from Linde Engineering on brazed aluminum heat exchangers REFERENCES 1. Carter, et al., "Failure Analysis and Life Prediction of a Large Complex Plate Fin Heat Exchanger", Engineering Failure Analysis, 1996, pp ,vol. 3, No Nakaoka, et al, "Evaluation of Fatigue Strength of Plate Fin Heat Exchanger Under Thermal Loading", International Conference on Pressure Vessel Technology, ASME 1996, vol Inomata, et al., "Thermal stress analysis of plate-fin heat exchanger", 15th Computational Mechanics Conference of Japan Society of Mechanical Engineers, pp , United States Patent US B2, Aug. 31, Copyright 2014 by ASME