Instrument control description of large ethylene cryogenic full capacity tank

1. Pressure measurement and control measures of large ethylene cryogenic full capacity tank

Doer  to heat transfer, ethylene evaporation causes the tank pressure to rise. In addition, the gas displaced when the storage tank is filled with ethylene will also cause the pressure of the storage tank to rise. In order to maintain the pressure in the tank within the normal operating range, the ethylene tank is equipped with a pressure measurement, control and regulation system. Two exhalation relief valves and three vacuum suction valves are installed on the top of each tank as a measure to protect the tank from damage.

For the tank pressure deviates from the controllable range, the pressure is too high or too low. The tank is designed with multi-layer safety protection measures to prevent tank overpressure: 1) recover the gas evaporated from the tank through bog pipeline; 2) Discharge the gas from the storage tank into the low-pressure flare through the vent pipeline; 3) The exhalation relief valve jumps to discharge the gas from the storage tank to the low pressure torch; 4) Emergency safety shutdown;

Safety protection sequence to prevent low tank pressure: 1) reduce the amount of gas recovered by bog pipeline; 2) Filling the tank with ethylene or nitrogen; 3) The suction valve of the tank supplies air to the tank.

In order to ensure the reliability of pressure detection, the storage tank is equipped with 4 sets of pressure transmitters to output 4 ~ 20mA signals to the user's DCS and SIS system, with HART protocol. The storage tank pressure is displayed and recorded in the control room. Local pressure indication is set on the tank top, and the tank pressure is automatically controlled by DCS system.

2. Liquid level measurement and control measures

The storage tank is equipped with two servo level meters and one radar level meter to continuously measure the liquid level in the tank. The servo level meter and radar level meter (SiL2 certification) output 4 ~ 20mA signal to the SIS system of the user in the control room. At the same time, the high and low switches are provided for the ultra-high or ultra-low liquid level interlock of the storage tank. At the same time, the signal is output to DCS for liquid level display.

The design, installation and replacement of the liquid level gauge shall not affect the operation of the storage tank, and the full-diameter ball valve shall be used to connect with the storage tank body. Prevent liquid overflow caused by too high liquid level or empty tank produced by starting pump or compressor when too low liquid level. One servo liquid level meter can detect the density and measure the medium density at different liquid levels in the tank.

3. Temperature measurement

Temperature measurement facilities are set on the inner tank, tank wall and tank bottom of the full capacity tank.

The tank wall and bottom are equipped with multi-point armored thermal resistance (the thermal resistance and extended shielded cable are integrated and encapsulated in stainless steel protective tube（ Φ The protective tube is filled with MgO.

A set of average thermometer with multi-point temperature transmitter is set in the inner tank liquid. The multi-point temperature transmitter and average thermometer output modbus-rs485 signals to the DCS system in the control room for display, which can ensure accurate temperature measurement at different positions of the storage tank during start-up precooling and normal operation, and meet the production operation needs of uniform cooling, precooling and flexible adjustment.

A temperature measuring point is set at the bottom of the insulation foundation of the inner tank to detect whether there is liquid leakage.

In addition, an armored thermal resistor is respectively set on the tank vent pipeline, and the RTD signal is output to the DCS system for display and alarm.