What should be noted when installing a safety valve and a burst disc together?

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In industrial production, excessively high pressure in pressure vessels and piping systems can lead to serious safety accidents. To prevent this from happening, we usually install safety valves and rupture discs on them; these are sometimes installed separately, while at other times they need to be installed together. This paper discusses the precautions for combined installation. I. Combination Forms and Installation Requirements 1. In parallel combination, the burst discs and safety valves are installed separately to handle different overpressure conditions. The calibrated burst pressure of the rupture disc must not exceed the design pressure of the vessel, and the opening pressure of the safety valve should be slightly lower than the calibrated burst pressure of the rupture disc. 2. For the series configuration on the inlet side (with the rupture disc placed before the safety valve), it is suitable for situations where the medium is highly corrosive or where leakage of the process medium must be prevented. In such cases, the discharge area of the rupture disc must be larger than the cross-sectional area at the inlet of the safety valve; the distance between the two devices should not exceed five times the pipe diameter. Additionally, a pressure monitoring and venting device must be installed in the intermediate chamber. Series connection on the outlet side (with the safety valve first) is used to protect the safety valve from corrosion or external pressure influences; it is necessary to ensure that the bursting pressure of the burst disc together with the pressure in the discharge pipe does not exceed the set pressure of the safety valve or the design pressure of the equipment. 3. Installation location and piping (1) The safety relief device shall be installed vertically in the gas phase space of the container or on the connected piping, and the diameter of the connecting pipe shall not be smaller than the inlet area of the safety valve. (2) The medium discharge path must be led to a safe location (such as a flare system); toxic or flammable media must not be discharged directly into the atmosphere. (3) The inlet pipe of the safety valve should be short and straight to prevent pocket-like fluid accumulation ； The outlet pipe must be equipped with counterforce supports, and the discharge opening should not face the operating surface. II. Requirements for pressure setting and discharge capacity 1. The set pressure of the safety valve should generally not be higher than the design pressure of the container, nor lower than the operating pressure ； The burst pressure of the rupture disc must also fall within this range. In parallel operation, the opening pressure of the safety valve should be slightly lower than the set burst pressure of the rupture disc (usually by no more than 10%), to ensure that the safety valve acts first. 2. The combined discharge capacity of the relief valve and the burst disc must be greater than or equal to the container’s safe discharge capacity. When connected in series, the discharge area of the burst disc must not be smaller than the inlet cross-sectional area of the safety valve, to prevent obstruction in discharge. III. Material Requirements
1. Medium requirements: When installed in series (especially on the outlet side), the medium must be clean and free of sticky or clogging substances, so as to prevent any interference with the operation of safety valves or rupture discs. 2. Material requirements: Both materials must be resistant to corrosion by the medium (e.g., Hastelloy for acidic media), and the type of rupture disc (positive arch or negative arch) must be suitable for the phase state of the medium (gas/liquid). IV. Intermediate Chamber and Emission Requirements 1. The intermediate chamber, in which pressure monitoring and emission requirements are combined in series, must be equipped with a pressure gauge, an alarm device, as well as exhaust/waste discharge pipes to prevent pressure buildup (for example, ASME requires that the chamber pressure not exceed 10% of the set pressure of the safety valve). 2. In the design to prevent backpressure: When a rupture disc is installed in series on the outlet side of a safety valve, it is necessary to ensure that the backpressure does not affect the valve’s opening performance; if required, a support structure should be added. V. Other Requirements 1. Maintenance requirements: Rupture discs must be regularly inspected for signs of corrosion or aging, and replaced immediately after rupture ； In harsh operating conditions (such as high temperatures and highly toxic environments), replacement is required annually. The safety valve should have its opening pressure and sealing performance checked regularly (at least once a year). 2. Standard specifications: International standards: ASME BPVC Section VIII (pressure vessels), API 520/521 (design of pressure relief systems). Domestic standards: GB/T150 (pressure vessels), TSG21 \"Safety Technical Inspection Regulations for Fixed Pressure Vessels\". Industry standards: Chemical industry (HG/T20570), oil and gas industry (SY/T10044), etc. VI. Requirements for Special Scenarios 1. The outlet of the safety valves for flammable/toxic media should be connected to a flare system or a closed treatment facility; anti-clogging measures must be installed in liquefied hydrocarbon pipelines (such as a purging device in front of the rupture disc). For high-risk media such as ethylene oxide, a nitrogen-sealed rupture disc must be installed in front of the safety valve to provide nitrogen protection during discharge.