A. Background

Safety of high pressure heal exchangers is an important topic area. By designing such heat exchangers to national and international Standards, safe designs can normally be achieved; the topic of mechanical design is dealt with in depth in Part 4 of HEDH. However, even exchangers designed to the (conservative) Standards may experience failure due to untoward events such as fire, impact by falling objects etc.

Considering the most common case of a shell-and-tube heat exchanger, it is self-evident that the shell and the tubes respectively should be designed so as to avoid failure. Often, the pressure of the fluid on one side of a heat exchanger is higher than that on the other. In shell-and-tube exchangers, the common practice (where there is a significant difference in these pressures and it is feasible) is to have the higher pressure fluid on the tube side. A problem arises if tube failure occurs, thus exposing the shell side to the high pressure fluid. Failure of the shell side can, of course, be avoided by designing the shell to withstand the tube side pressure, but this is often not economic. Alternatively, relief systems can be installed to limit pressurisation of the shell side in these circumstances. A further option is to choose an alternative form of heat exchanger.

The material in this Section and Section 496 is derived mainly from a set of Guidelines published by the Institute of Petroleum in 2000 Institute of Petroleum (2000). These Guidelines were produced in the context of the oil and gas industry, but they have far wider implications. The Guidelines were based on a range of assessment and research studies (Ewan, 1995, 1999; Goyder, 1997; Thyer and Wilday, 1998; Trident Consultants Ltd and Foster Wheeler Energy, 1993; Wilday and Thyer, 1999; Wilday et al., 1996; Weil, 1994).

The oil and gas industry frequently requires to heat or cool high-pressure (HP) gas. The most common method used has been in shell and tube heat exchangers and there are a large number used offshore on production platforms in the North Sea. The low-pressure (LP) side of the exchanger, which contains a utility fluid such as sea water, is therefore at risk in the event of any leakage from the HP side of the exchanger. Such units may have greatly differing operating pressures between the two fluids and the designer has to consider several variables when choosing the optimum exchanger type, including selecting suitable materials, which fluid should be within the tubes and the design pressure and temperature for each side of the exchanger. It lias become common practice for the LP side to be designed to withstand a pressure just above the operating or flow lock-in pressure of the utility fluid, but well below the HP side's operating pressure. There is a risk that tube failure could lead to failure of the LP pressure envelope and the release of large quantities of flammable gas. The LP side therefore needs to be protected against tube failure by either fitting bursting discs or safety valves. The adequacy of the methodology used to design the LP side to withstand the sudden release of high pressure gas through tube rupture was not proven. The consequences of such a failure can range from (at the worst) catastrophic rupture of the shell with considerable financial loss and risk to personnel, to satisfactory release through the over-pressure protection system.