Separate sizing and selection of safety valve or bursting disc take place by means of tables in the manufacturer’s catalogues or using special computer software. If both components are combined, several aspects have to be taken into account to ensure safe operation.
Erhard Stork
Safety valves and bursting discs are normally used for pressure protection in containers and pipelines. Both systems are advantageous compared to other pres-sure protection devices, as they are independent of external energy, and they are also allowed to be used without back-up components.
A combination of both systems by aligning the bursting disc in front of the safety valve (Fig.) unites the advantages of both systems: total sealing plus the high media resistance of the bursting disc and the resealing properties of the safety valve. Systems can continue to work even after the safety device has operated. The technical rules in the AD-A1 code of practice (1/95) describe the following requirements for this type of combination:
• insertion of non-fragmenting rupture discs (non-shattering),
• the distance between the bursting disc and the safety valve must be large enough to ensure correct operation of the bursting disc,
• possibility of purging any condensate that emerges between the bursting disc and the safety valve,
• the pressure drop in the inlet lines and between the bursting disc and the safety valve must not exceed 3% of the difference between the permitted operating pressure and the back pressure of the safety valve,
• facility for recognising a damaged bursting disc (pressure gauge, bursting disc sensor, etc.). This simultaneously ventilates the interspace, so that no back pressure which might alter the burst pressure of the bursting disc can develop. Compared to separate sizing of the bursting disc and the safety valve, the following aspects have to be taken into account. Sizing takes place in two steps: First, the safety valve is sized on the basis of the data below. It is a good idea to use Safe, the computer sizing software:
• set pressure,
• back pressure,
• capacity (mass flow/volume flow),
• medium (liquid/gaseous),
• temperature,
• working characteristics (full lift/normal),
• material,
• approvals.
The subsequent choice of bursting disc depends on the following criteria:
• bursting pressure of the bursting disc = set pressure of the safety valve,
• nominal size of the bursting disc = nominal size of the safety valve inlet,
• medium (iquid/gaseous),
• temperature,
• tolerance of 65% for the combination with full lift safety valves or 610% for the combination with standard safety valves,
• pressure rating of holder = inlet pressure rating of the safety valve,
• materials of the bursting disc and holder,
• approvals,
• safeguard device (reference pressure gauge or disc sensor).
The required pressure drop of 3% is guaranteed if the bursting disc is arranged directly in front of the safety valve and the cross-section of the holder fulfils the following condition:
Ageom • a > 1,5 • A0 • aw
where Ageom = geometric area of the bursting disc and holder, a= coefficient of discharge of the bursting disc and holder, A0= narrowest flow area of the safety valve and aw = coefficient of discharge of the safety valve.
ARI Armaturen
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Further information cpp-204
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