What should be done when a spherical tank needs to store multiple types of media? What are the key points?

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For the following reasons, more and more customers want the same tank to be able to store different media at different times: 1. Reducing initial investment and operating costs； 2. Avoid equipment idling due to fluctuations in the demand for a single medium ； 3. It is feasible from a technical and regulatory perspective. However, when constructing and using such spherical tanks, there are certainly significant risks and challenges without the appropriate experience and thorough consideration. Below, I will share some of my own experiences and insights on the design of such spherical tanks. I. Medium compatibility and material selection: First, it is necessary to analyze whether there are significant differences in the operating parameters of various types of media. For those media with very different physicochemical properties, it is absolutely not advisable to attempt to use the same storage tank for them. 1. Material versatility: Prioritize materials that are compatible with multiple media, and select the most suitable base material based on the harshest operating conditions. For example, among several common media, liquid ammonia and liquefied gases with high hydrogen sulfide content impose special constraints on materials. Materials with excellent resistance to stress corrosion are safe for use in conventional media such as butane, propane, and propylene. 2. Corrosion margin: The corrosion margin is determined based on the most corrosive media, and regular monitoring of the corrosion rate is carried out; if necessary, the wall thickness margin is re-evaluated. For large spherical tanks, a corrosion margin of 3 mm is generally required for liquid ammonia media, whereas propylene media are less corrosive, and a margin of 1 mm is sufficient. If liquid ammonia and propylene are used together, the corrosion margin should be determined based on liquid ammonia. II. Design redundancy: When designing for redundancy, it is necessary to take into account the pressure and temperature requirements of all types of media present. The design pressure should be determined based on the medium with the highest saturated vapor pressure, while the design temperature should be determined based on the medium with the lowest operating temperature. The medium density should also be considered as the highest possible value. For example, when butane, propane, and propylene are used together, pressure and temperature should be considered based on propylene. III. Special design of safety relief devices: The biggest risk associated with multi-media spherical tanks during use is the need to have safety relief devices with different setting pressures (or burst pressures) corresponding to the various media involved. Of course, the specifications of the safety vent must meet the requirements for the maximum discharge volume. For example, when butane, propane, and propylene are used together, when filling with butane, the opening pressure of the safety valve needs to be adjusted to a pressure suitable for butane ； When filling with propylene, the opening pressure of the safety valve needs to be increased. Ensure that the safety valve does not activate prematurely, nor does it activate too late. Delayed initiation of venting can pose a significant risk of overpressure in the tank; as the liquid expands when heated, there is no more any gas space left in the spherical tank, and in such a situation even if the safety valve opens, it will not be possible to achieve a sufficient amount of safe discharge. IV. Cleaning and medium replacement: The most important aspect of using such spherical tanks is to carry out cleaning and medium replacement as required. 1. Replaceable design: Provision of cleaning interfaces and an inert gas replacement system, along with a detailed medium replacement plan (such as nitrogen purging and steam cleaning). 2. Residue detection: It is necessary to verify the cleanliness inside the tank before switching media, in order to prevent residues from reacting with the new medium (such as acid-base neutralization or gas mixing explosions). 3. Time interval control: A sufficient cleaning cycle is required between corrosive media and high-purity media to prevent cross-contamination. V. Upgrade of monitoring and control systems 1. Multi-parameter monitoring: Install various types of sensors (pressure, temperature, liquid level, gas composition analysis) to monitor changes in the medium and its compatibility in real time. 2. Automatic switching logic: The control system must incorporate a medium identification function to automatically adjust the safety thresholds (for example, the pressure limit is adjusted based on the saturated vapor pressure of the medium). 3. Remote alarm: A remote monitoring system is configured to promptly respond to abnormal fluctuations in parameters that occur during medium switching. VI. Safety Assessment and Operating Procedures 1. Risk assessment report: Engage a third party to conduct a risk assessment of the coexistence of multiple media, in order to identify potential risks (such as sudden pressure changes, chemical reactions). 2. Detailed operating procedures: Develop specialized operation manuals for each type of medium, including loading and unloading sequences, temperature control, and emergency measures. 3. Staff training: Provide regular training for operators to ensure they understand the complexities of multi-media storage and the emergency response procedures. VII. Other matters 1. Regular review: Before switching to a new medium, pressure tests and leak detections must be conducted again, and permission from the local regulatory authorities must be obtained. 2. Record tracking: Maintain a history archive of media storage, recording data on each cleaning, testing, and maintenance session to facilitate subsequent compliance audits. VIII. In summary, storing multiple media in the same spherical tank requires risk assessment as the core approach; through material compatibility design, redundant safety systems, strict replacement procedures, and dynamic monitoring, safety under various operating conditions can be ensured. It is recommended to consult professional agencies during the design phase for a customized assessment, and to continuously improve operating and maintenance procedures in order to prevent accidents caused by medium changes. Although multi-media compatible spherical tanks can overcome the limitations of devices designed for a single purpose, the complexities arising from differences in medium properties must be taken into account during design and management, with practical and reliable measures put in place to ensure safety throughout the operation process.