The prevention of impermissibly high pressures in process plants in the event of malfunctions is an important safety aspect and an essential part of the design. Purely mechanical pressure relief devices, and especially safety valves and/or rupture disks, are often used as the ultimate safety measure.
For the design of these overpressure safeguards, precise knowledge is required of the processes to be secured, the assumed malfunctions and blow-off cases and the regulations and calculation rules to be complied with.
The range of services offered by the weyer group in the context of the overpressure protection design incorporates the entire path from determination of the blow-off cases and mass flows to selecting the suitable overpressure protection. The cases that lead to a maximum permissible overpressure being exceeded in parts of process engineering production plants can be very diverse. These can be excessive exothermic reactions, but also unintentional shut-off of system parts or a classic fire, to name just a few. All possible malfunctions and, not to be forgotten, also malfunctions during commissioning or start-up and shut-down of the plant must be recorded. A structured procedure is available for this purpose.
Following the analysis of the possible malfunctions or non-unusual operating conditions, the cases that cause the highest input of energy into the system are then examined in more detail. This determines which flow rates are to be discharged via the safety valve when the maximum permissible overpressure is reached.
It is possible that several differently dimensioned safety valves for different response pressures must be installed in order to be able to intercept different faults. If a safety valve is overdimensioned for faults where a lower flow rate must be discharged, undesirable effects such as valve flutter may occur.
After identifying and calculating the respective required flow rates, the actual design and selection of the safety valve(s) takes place. Different designs then become available as a result. Not only do the individual safety valve has to be considered, but also the supply and discharge pipes. Here, the pressure loss that occurs when blowing off via a safety valve then plays a significant role.
Furthermore, it must also be checked whether the discharge of the released substance into the atmosphere is permissible at all. If this is not the case, additional safety measures such as separators or collection devices must be installed. The necessity can be checked, for example, by means of a dispersion calculation, which can be used to estimate whether, in the event of a blow-off, accident limit values will be exceeded in areas requiring protection.
In summary, the sole calculation of a safety valve only takes up a small part in the prevention of impermissible overpressures in systems. Rather, the actual activity is to record all operational faults of a plant that lead to overpressure of a plant part and to evaluate them qualitatively as well as quantitatively.
Depending on the application, corresponding calculation rules, which are set out in standards, are used. Furthermore, thermodynamic system states can only be represented quantitatively with sufficient accuracy with the help of process simulation.
If a flammable gas or vapour is released when the overpressure protection is triggered, explosive mixtures can also form around the overpressure protection, so that in these cases a potentially explosive atmosphere must be defined. The relevant regulations for defining potentially explosive atmospheres provide for case-by-case consideration when determining the dimensions of such areas. After evaluating the individual case at hand, the weyer gruppe can assist by performing a dispersion calculation and determine the extent of the potentially explosive atmosphere.
The consideration and assessment of potentially explosive atmospheres is always a case-by-case consideration and is only offered for single-phase blowdown cases.
- Determination of the relevant blow-off cases
- Determination of all relevant substance data
- Selection, design and recalculation of the overpressure safeguards
- Design/recalculation of the downstream systems
- Assessment of whether and under what circumstances a free discharge into the atmosphere is permissible
- If necessary, determine the spread of flammable vapours and define a potentially explosive atmosphere around the point of release of the overpressure protection into the atmosphere.
- Manufacturer-independent documentation
weyer special: Safety Valves
A central safety aspect in the planning of process engineering plants is the effective prevention of impermissibly high overpressures in the process plants in case of operational disturbances. In many cases this can only be guaranteed by the installation of safety valves – or more generally – mechanical overpressure protection…Download weyer special here