The Basic Design Basis records reference values for the design of the facility, its main technical options, along with rules to be applied for sizing its equipment, pipe-work etc. The latter, called design criteria, either specified by the Client, or otherwise the Engineer’s standards and, in any case, within the limits permitted by the applicable codes.
Process design criteria include that for the design pressure of equipment and piping. The latter is set a few % above their maximum operating pressure. Such allowance is required for proper staging of overpressure protection system, avoiding its spurious operation. Setting the design pressure at 110% of the maximum operating pressure (MOP), for instance, will allow to set the pressure safety automation at 105% of the MOP and the pressure safety valve at 110%. This will avoid spurious trips due to drift in calibration of the high pressure safety sensor as well as spurious opening of the pressure safety valve.
The wall thickness of pressure vessels and lines is directly calculated from the design pressure, as per the formula given by the applicable design code. The design pressure criteria has therefore a large impact, not only in terms of material cost but also in terms of construction cost (increased pipe thickness will lead to increased welding time).
The impact of the design pressure on piping flanges is not as direct as on other piping items (straight pipes, elbows, tees), as flanges come in classes of admissible pressures (up to 20 bars, from 20 to 50 bars, from 50 to 100 bars, from 100 to 250 bars, etc.).
Other Process design criteria are defined, the most important being that defining the design temperature of equipment and lines, and the diameter of lines.
Both high and low design temperatures are specified. An allowance (10-20 degrees) is provided between the high design temperature and the maximum operating temperature, allowing some tolerance to process upset without the need to shutdown. The low design temperature is normally the one that is achieved during emergency depressurisation.
Both high and low design temperature dictate the selection of the material of construction. A low design temperature below -29C will, for instance require a low temperature carbon steel rather than a regular one.
The sizing criteria of lines come from the requirement to prevent line erosion, vibration and noise. It translates into a maximum fluid velocity, or kinetic energy, or both. It results in the selection of the diameter of each line, as a function of its flow. The line sizing cirteria has a large impact on the cost of a facility’s pipework, as it impacts both material and installation costs.
