The Term “PS = maximum allowable pressure” is specified in PED. “PS” is the pressure at a specific point on the pressure equipment for which the pressure equipment as a whole was designed. The specified location is usually the highest point on the pressure equipment, where the safety devices for pressure limitation are usually also installed.
The maximum allowable pressure PS plays a very important role in the PED:
- is the important pressure value in bar or MPa (gauge pressure) for which the pressure equipment as a whole is designed
- is a mandatory value on the name plate
- must be used to determine the category of a pressure equipment
- must be specified by the manufacturer for a specific point on the pressure equipment
- is to be used for the calculation of the test pressure PT as part of the final inspection (see link below).
EN 13445 requirements with regard to the maximum allowable pressure PS
For internal pressure, the maximum allowable pressure “PS” shall not be less than:
- the differential pressure which will exist at the same specified location in the vessel (or chamber) when the pressure relieving device starts to relieve;
- the maximum differential pressure which can be attained in service at the same specified location where this pressure is not limited by a relieving device;
For external pressure, the absolute value of the maximum allowable pressure “PS” shall not be less than:
- the absolute value of the differential pressure which will exist at the same specified location in the vessel (or chamber) when the pressure relieving device starts to relieve;
- the largest absolute value of the differential pressure which can be attained in service at the same specified location where this pressure is not limited by a relieving device.
Other symbols and abbreviations for design calculation
The table bellow show the typical symbols and abbreviation for using at design calculations of whole pressure equipment or pressure parts several load cases e.g. normal operation and/or testing load cases.
| Symbol and abbreviations | Definition 1 | Definition 2 |
|---|---|---|
| PS = maximum allowable pressure PED 2014/68/EU | The maximum pressure for which the equipment is designed, as specified by the manufacturer, and defined at a location specified by him, being either the connection of protective and/or limiting devices, or the top of equipment or, if not appropriate, any point specified; | The maximum allowable pressure PS of a vessel (or a chamber), for normal operating load cases, shall be defined at a specified location. Note: This shall be the location of connection of protective and/or limiting devices or the top of the vessel (or chamber) or, if not appropriate, any point specified. |
| PS = maximum allowable pressure EN 13445-3 Section 5.3.4 | The maximum allowable pressure PS of a vessel (or a chamber), for normal operating load cases, shall be defined at a specified location. Normal operation load cases: Are those acting on the pressure vessel during normal operation, including start-up and shutdown. | Specific location: a) connection of protective and/or limiting devices or the top of the vessel (or chamber) or, if not appropriate, any point specified. |
| Pmax = maximum permissible pressure EN 13445-3 Section 3.16 | a) maximum pressure obtained from the design by formulae or b) relevant procedures of EN 13445-3 for a given component in a given load case or c) for the whole pressure vessel the minimum of these maximum permissible pressures of all components; | Note 1: The differences of the nominal design stress ƒ, the analysis thickness ea (= e*min – c**) and the joint coefficient z for the calculation of the maximum permissible pressure in different load cases are specified in 5.3.2. Note 2: If no explicit formula is given for the maximum permissible pressure Pmax then Pmax may be calculated as pressure which gives the required thickness equal to the analysis thickness. Note 3: The maximum permissible pressure Pmax used for the simplified assessment of fatigue life in Clause 17 and for the calculation of the equivalent full pressure in 5.4.2 is calculated for normal operating load cases. **c = corrosion allowance *emin = minimum possible fabrication thickness |
| Pd = design pressure EN 13445-3 Section 3.10 | Pressure at the top of each chamber of the pressure equipment chosen for the derivation of the calculation pressure of each component | Pd ≥ PS (for normal operating case) Note 1: Any other location may be specified. |
| P = calculation pressure EN 13445-3 Section 3.4 | Differential pressure used for the purpose of the design calculations for a component; P = Pdiff + Pstatic | The calculation pressure P shall be based on the most severe condition of coincident differential pressure and temperature. It shall include the static and dynamic head e.g. at column where applicable, and shall be based on the maximum possible differential pressure in absolute value between the inside and outside of the vessel (or between the two adjacent chambers). |
| Pdiff = differential pressure | Pressure which algebraic value is equal to the pressure difference on either side of a separation wall |
