Emergency pressure relief function of control valve

There are many application examples of emergency pressure relief of reaction system by using shut-off valve and flow limiting orifice in series, but there is a disadvantage of unstable relief flow rate in practical operation. At the initial stage of pressure relief, since the pressure in front of the flow limiting orifice is equal to the working pressure of the reaction system before the pressure relief start, the pressure value is relatively large, and the relief flow rate through the flow limiting orifice is relatively large. With the passage of time, after the pressure of the reaction system decreases, the discharge flow rate through the flow limiting orifice will be relatively small. In order to depressurize to the target value within the specified time, the peak discharge flow rate at the initial stage of depressurization is large. Excessive discharge flow rate has a relatively small impact on the fixed bed reactor, but it will bring many disadvantages to the fluidized bed reactor, such as catalyst loss, reactor internals damage, etc. The use of regulating valve for sectional pressure relief according to several different predetermined openings has some improvement, but there are still deficiencies. This problem can be effectively solved by adjusting the continuous opening of regulating valve for emergency pressure relief of reaction system, that is, the same relief flow rate shall be maintained as far as possible during the whole pressure relief process.

The opening of regulating valve is small in the initial stage, and with the decrease of reaction system pressure, The opening of the regulating valve is gradually increased to compensate for the decrease of the actual circulation CV value caused by the decrease of the pressure in front of the valve, so as to achieve the purpose of stable pressure relief. The relatively stable discharge flow rate plays a very important role in protecting reactor internals and preventing catalyst loss.

Since the emergency pressure relief adopts the split range control with the reaction system pressure as the target value, the measured value (PV) of the pressure control circuit is very important. Three independent pressure transmitters are set and the algorithm of taking the median (equivalent to two out of three) is adopted to ensure the reliability of the PV value of the pressure control circuit.

Put forward the expected stable discharge flow rate from the process, calculate the corresponding CV value according to the flow rate and differential pressure value of 9.6mpa (the target pressure of system pressure relief in the first step is 10MPa minus the pressure of pipe network behind the valve, and then leave a certain margin to obtain the selected CV value of regulating valve. Use this CV value to check whether the regulating valve hv-101a / B has sufficient flow capacity to complete the pressure relief in the second step, that is, further reduce the system pressure from 10MPa to 3Mpa according to the pressure relief rate of 0.6mpa/min. Check method: as long as twice the flow capacity (CV value) when the differential pressure is 2.6MPa can meet the requirements of discharge flow rate and have appropriate margin. The instrument discipline shall provide the final CV value of the regulating valve to the process discipline as the design basis of emergency pressure relief supporting pipe network. As the pipeline of the emergency pressure relief system is relatively single and there is no heat exchanger and other equipment in the middle, the pressure before and after the regulating valve is basically equal to the pressure difference between the reaction system pressure and the pipe network pressure. When selecting the CV value of the regulating valve, only a small margin can be left, which can reduce the design cost of the system pipe network. It should be noted that hv-101a / B may be in the fully open position due to the control failure or the failure of the regulating valve itself during the design of the system pipe network; When the pressure in front of the valve is the normal pressure of the reaction system, the discharge flow rate will be relatively large, and the pipe network design shall be able to withstand this discharge flow rate.

The pressure relief scheme of the reaction system of the unit leaves room for future changes. The regulating valve hv-101a / B can carry out split range control of the pressure circuit PID according to the reaction system pressure PT-101. At the same time, it can realize manual two position valve opening or closing operation in the control room or on site according to the actual operation needs. The regulating valve adopts double cylinder regulating valve, At the same time, it has the function of continuous regulation / two position valve opening / two position valve closing. The gas circuit connection taking the 2-position 3-way solenoid valve as an example is shown in Figure 5. In practical application, the gas circuit connection can be simplified according to the different requirements of specific functions.