Hydraulic Analysis Limited have undertaken a large number of heat exchanger tube rupture projects. In exchangers containing high-pressure gas the designer is advised in API Std 521 to take account of the risk of serious tube failure. The consequential impact onto the utility inlet and outlet lines, any local instrumentation and the need and position of any overpressure protection devices should then be considered.
Shell and tube heat exchangers can be designed such that the LP (utility) side of the heat exchanger can be hydrostatically tested to a pressure equal to or greater than the operating pressure of the HP gas. This can have serious weight penalties, be expensive and can also be very difficult to manufacture. The alternative is to design the low pressure side to its own system pressure. In these cases it is necessary to protect the LP side from the risk of over-pressure by installing one or more pressure protection relief devices. Where the integrity of the LP side of the exchanger is proven by undertaking a hydrostatic test at a pressure greater than the operating pressure of the HP side, the designer should still ensure the associated LP piping system can withstand the static / dynamic pressures and forces generated in the piping following tube failure.
The results of our study will enable the client to understand the over pressurisation levels in the system and to determine whether rapid acting relief valves are required on the exchanger headers. It is also possible to determine the distance which the process side product will extend into the cooling water system before the pressures in both systems equalize.
Our software shows a high degree of correlation when compared to laboratory testing (within 1-2%) of the rupture of a tube and can handle either liquids or gases on the tube side. i.e. we can model the effects of gas entering the liquid cooling system.
For the purpose of the analysis, we would normally assume that a complete shear rupture of the tube occurs at the weld on the plate end of the tube. Experience has shown that this is the most likely location for a tube rupture due to the load placed on the weld generated from tube vibration or strain. This also represents the worst case scenario from a surge pressure perspective as the process fluid pressure will be at a maximum pressures at the plate end as there are no headlosses associated with the fluid passing through the tube (unlike a failure at the mid point of the tube). It is also possible for us to analyse a split in the tube or a pinhole leak to make a direct comparison between different failure modes.
Scenarios are normally agreed with the client before the analysis commences and any assumptions can be discussed at this stage of the project (such as the capacity of the process side reservoir).
In 2013, HAL became a panel member of a Joint Industry Project (JIP), managed by the Energy Institute (EI) on behalf of the JIP sponsors. Our scope of work is the mathematical modelling of the impact of fast transient tube ruptures on various STHE design geometries, and assessing the impact of the pressure wave in the adjoining pipework. This ensures consideration for overpressure of the low pressure side, connecting piping and effects upstream and downstream of the exchanger in the low pressure piping system where the tube failure occurred.
The nature of an instantaneous guillotine failure of a single tube in a shell and tube heat exchanger allows high pressure fluid to pass into the low pressure side of the exchanger leading to the generation of very rapidly applied surge pressures (and pipe loads), as the high pressure gas quickly overcomes the liquid momentum on the low pressure side. The initial surge pressure rise usually occurs so rapidly that no relief path can be established and excessive peak pressures can easily be generated. It is therefore essential that a dynamic analysis is undertaken to estimate the magnitude of surge pressures in the low pressure side of the heat exchangers and determine if differential pressure limits can be established below which transient effects from a tube rupture can be ignored.
HAL have always been at the forefront of industry developments and worked on the original Joint Industry Project (set up in the late 1990’s) which generated experimental data on tube ruptures in a full scale shell and tube heat exchanger in order to validate current computer modelling techniques.
This scope of the recent Hydraulic Analysis Limited work package is aimed at:
- Determining when the design of the overpressure protection needs to consider dynamic (transient) effects in addition to the steady-state analysis.
- Establishing design criteria for connected piping if fast-acting pressure relief devices are not warranted on the heat exchanger.
- Determine the impact of transient loads on the piping systems if bursting disks are not applied for overpressure and develop appropriate design guidelines to ensure that the piping design is robust but not overly conservative.
Our modelling results have been used to determine when pressure safety valves (conventional spring relief valves) can be used for high pressure protection instead of burst discs which will often burst when not required due to secondary factors. The use of re-closing pressure safety valves will minimise any loss of inventory into the flare header. Our results have shown that the response (opening) time of spring relief valves is comparable to burst discs and we have calibrated the spring relief valve module in our software against physical modelling tests which were undertaken by the University of Sheffield on behalf of the JIP. This ensures we have accounted for secondary dynamic factors (such as compression spring surge) in the opening time of the relief valves. As such we can now determine with a high level of confidence whether re-closing spring relief valves can be used for high pressure protection on a shell and tube heat exchanger. This will significantly reduce operational problems and process shutdowns due to unwanted rupture of burst discs.
