The issue of pressure relief in hydrocracking

There’s no need to release it all the way; as long as it meets the process requirements and can be controlled, that’s sufficient.

For emergency pressure relief, it must be reduced below the nitrogen pressure, and then nitrogen should be used for protection!

1. Startup conditions for emergency venting at 0.7 MPa/min: (1) When the cycle hydrogen compressor fails and stops operating, pressure is released automatically at 0.7 MPa. (2) In the event of an accident when the temperature and pressure in the reactor are normal and prompt shutdown is required, manual depressurization at a rate of 0.7 MPa/min can be employed. (3) In the event of a fire or a potential fire in the device, manual pressure reduction can be used. 2. Startup conditions for emergency venting at 2.1 MPa/min: (1) The temperature at any point in the reactor bed exceeds the normal temperature by 25°C, or the temperature at any point in the bed exceeds 420°C. (2) A major explosion accident occurs at the facility. 3. Conditions for shutting down the emergency vent at 0.7 MPa/min: (1) When the automatic 0.7 MPa/min pressure reduction system is activated (the cycle hydrogen compressor stops operating), the pressure relief valve can be shut down only after it has been confirmed that the cycle hydrogen compressor is operating reliably and normally for at least 10 minutes, and that the temperature of all reactors is 30°C lower than the normal operating temperature. The pressure relief valve can be closed only after the pressure of the device has dropped below 0.07 MPa (gauge). (2) When the manual 0.7 MPa/min pressure reduction system is activated (with the cyclohexane compressor running), the vent valve can be closed only when the temperature in each reactor is at least 30°C lower than the normal operating temperature. 4. Conditions for shutting down the emergency vent at 2.1 MPa/min: (1) When the manual 2.1 MPa/min pressure reduction system is activated, the device must be reduced in pressure to below 0.07 MPa (gauge) before the vent valve can be closed.

The principle of pressure relief is also the purpose of such relief, which is to keep the device under controlled conditions. Generally, it is necessary to allow the pressure to be released completely; this minimum pressure level is usually above the low-pressure threshold but below the nitrogen pressure. Accidents caused by stopping the pressure relief process midway often lead to excessive temperature rises

Unloading to the end is done to protect the catalyst; if pressure release is stopped halfway, it’s also okay to continue releasing pressure if an rapid increase in temperature is detected, as long as you can keep the temperature from rising too much. If you’re not sure, it’s better to unload to the end. This is especially true when the plant has just started operating and there are many beginners around – it’s not worth taking risks.

0.7 MPa/min and 2.1 MPa/min are both standard values commonly used for pressure relief in industrial installations; they are based on experience, and this standard is adopted for combined oils at pressures above 10 MPa, without changing depending on the total pressure. 0.7 MPa/min is a safety mechanism designed to release the pressure in the device quickly; using 0.5 MPa/min or 1.0 MPa/min is not necessarily unacceptable either. 2.1 MPa/min is a protective mechanism used when the reaction bed experiences severe overheating; the maximum rate at which the emergency pressure relief system can release pressure is typically limited to about 2.0 MPa in the first minute of pressure release. The pressure release rate in the first minute must not exceed 2.1 Mpa; if the release rate is too fast, the pressure difference across the entire high-pressure system increases suddenly, exerting significant pressure on the equipment, especially on the reaction bed layer, which can cause damage to the internal structural supports of the reactor. It should also be noted that when designing the internal structure supports, the weight of the catalyst and solid deposits, as well as the pressure exerted on the bed at a startup rate of 2.1 MPa/min, must all be taken into account. The design of the pressure relief pipeline for installing the torch was carried out by specialists from the engineering contracting company.