22 March The Institute of Petroleum’s “Guidelines for the Design and Safe Operation of Shell and Tube Heat Exchangers to Withstand the Impact of Tube Failure” Published in 2000 The history presented by Colin Weil

22 March Design and Safe Operation of Shell and Tube Heat Exchangers The problem: Compressor inter and after coolers high pressure gas at (say) 200 bar, often cooled by water at (say) 10 bar. Shell and Tube Heat Exchangers can, in rare cases, suffer catastrophic tube failure. If a tube rupture occurs rather than a pin hole leak then large volumes of high pressure gas will be expelled into the low pressure utility. The solution: Safe protection of the pressure envelope and safe release of any expelled fluids

22 March Where we were in the last century: Overpressure protection was rarely mounted on the shell being protected Relief valves were normally mounted in the piping Relief valves were mounted at a convenient location That location could be many metres from the shell supposedly being protected from an overpressure incident Design and Safe Operation of Shell and Tube Heat Exchangers

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Basis of need: BE EN ISO aka API 521 says that tubes can fail catastrophically and as such the system must ensure pressure envelope integrity and safe removal of the pressurised fluid. What Happens ? 1. Pressure Waves 2. Reflections 3. Pressurised balloon of gas overcomes water momentum

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Choices: Design the LP side’s hydrostatic test pressure to be equal to the highest operating pressure of the HP fluid – ensure LP piping is also adequate or Protect the LP side against risk of tube failure by one or more bursting disks or relief valves: on the shell in the utility piping

22 March Design and Safe Operation of Shell and Tube Heat Exchangers What the industry was doing for extreme units: Using a one dimensional computer simulation Which was fast and cheap to run Questions: Widely used and validated for piping transient flow analysis Not validated for complex geometries ? There was no certainty on its accuracy or the sensitivity to numbers used in the calculation (e.g. speeds of response)?

22 March Design and Safe Operation of Shell and Tube Heat Exchangers IP managed JIP Health & Safety Laboratory and Sheffield University Experiment using real shell and tube geometry High pressure gas Tube failure within a bundle protected by a bursting disk Monitor pressures against time inside the tube bundle and in the piping Record strains in shell Compare experimental and analytical results (1-D model) Analyse results to determine actual stress and strain resulting in the pressure envelope Use a shock-tube to assist in defining shell failure criteria when subjected to short, high amplitude pressure transients

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Experimental Shell with Instrument Connections

22 March Design and Safe Operation of Shell and Tube Heat Exchangers ID Dimensions of Test Shell 13 mm plate (typ)

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Schematic Elevation of 6 inch Bursting Disc Line

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Strengthening work on 6 inch bursting disk line to protect against severe forces generated

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Instrumentation Aim was to record both long and short-term pressures and strains, using: 14 two-axis strain gauges up to 10 pressure transducers on shell 2 pressure transducers inside bundle near failure point 6 pressure transducers on discharge pipework using over 3000 ft cable Simultaneous logging of 50 channels

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Investigations into reproducible tube failure

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Bursting disc details 6” Graphite disc 10 bar(g) + 5 % rating Sized on assumption that air entering the shell displaces an equal volume of water Tests undertaken 1) 71 bar 2) 102 bar 3) 146 bar 4) 130 bar

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Physical processes during relief Compressible gas flow will choke at tube exit site Transient stage: –high pressure bubble of gas generated –liquid’s initial momentum must be overcome –liquid is forced from the LP side –maximum pressure in the LP side of exchanger likely to occur during the transient stage Steady-state stage: –pseudo steady-state depends on the flow resistance Sizing based on flow from two open ends of tube

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Pressure trace following tube rupture showing transient and pseudo-steady stages

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Short term pressure trace for internal transducer bar test Long term pressure trace for transducer on 6” discharge pipe bar test

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Summary of shell-side pressures with two large exit routes and little back-pressure

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Dynamic Models One dimensional Assume plug flow Liquid is compressible Uses average “friction factor” for exchanger shell Need to make assumptions about disk (or valve) rupture speed Blind comparisons were undertaken to compare the experimental data against the analytical results

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Comparison of experimental results and the model predictions for peak shell pressure

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Interesting Experimental Data Tube burst speed 0.7 ms with 90% flow by 0.23 ms (material CS) Disk rupture speeds ms Baffle movement minimal ? No secondary tubes damaged ? Piping water velocity ~ 20 m/s = force of 12 Tonne

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Illustration of forces involved

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Validation of FE Analysis

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Vessel failure criteria Work done by Sheffield University suggested that finite element modelling could be used to determine dynamic failure pressures, but modelling is specific to particular design Recommended approach to compare dynamic stresses with those allowed in vessel design code, not to specify dynamic failure pressure.

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Summary of results from the JIP Large scale experiments undertaken to provide model validation data Data supported use of 1-D fluid dynamic models Experimental data also supported use of finite element models to account for very fast transients The results were used to develop the design guideline by The Institute of Petroleum Presented to the API Pressure Relieving Sub-committee and subsequently incorporated into API (RP) 521 Following the JIP the approach was as follows:

22 March Design and Safe Operation of Shell and Tube Heat Exchangers When was dynamic simulation recommended? Large operating pressure differentials (say 150 bar) Long shell with baffles on tight pitch For existing units where the over pressure protection was mounted on the utility piping For existing units with high differential pressures where a relief valve was used Use One Dimensional (pipe) Model

22 March Design and Safe Operation of Shell and Tube Heat Exchangers The Guideline recommended for critical new heat exchangers Disks for fast reaction Mounted directly on the shell (LP Side) Use 2 disks when long or very high differential operating pressures Minimise contamination by use of NRV and ESD valves in the utility piping (Note that this has an effect on pressures generated)?

22 March Design and Safe Operation of Shell and Tube Heat Exchangers Findings and conclusions published by the Institute of Petroleum (now the Energy Institute: “Guidelines for the Design and Safe Operation of Shell and Tube Heat Exchangers to Withstand the Impact of Tube Failure”

22 March Design and Safe Operation of Shell and Tube Heat Exchangers

22 March Design and Safe Operation of Shell and Tube Heat Exchangers