Control scheme of typical chemical unit

The classification of explosion danger zones for chemical production plants and tank farms shall be carried out in accordance with the "Code for Design of Electric Devices in Explosion and Fire Dangerous Environment" (GB50058-92).

The anti-lightning and anti-static systems in chemical plant, tank farm and oil loading and unloading area shall be implemented in accordance with the Design Code for Lightning Protection of Building Materials (GB50057-92)2000 edition;

The fire protection design of various buildings in the plant shall be implemented in accordance with the Code for Fire Protection in Building Design (GBJ16-87), 2001;

The treatment process design of toxic materials should be implemented in accordance with the "Health Standard for Industrial Enterprise Design" (GBZ1-2002).

If the user has requirements, as long as it does not violate the above main standards, it can also be according to the user's requirements. In order to secure and reliable design, the engineering design should refer to the applicable standard specifications according to the specific conditions of the contract and the project, and when the basis is not found in the standard specifications, the corresponding design manual and empirical data are supplemented.

4.2 Common Security Risks

▲ Equipment and pumps and other exits are blocked

The outlet of almost all containers, pumps, compressors, flame heaters or other equipment can be blocked due to mechanical failure or human accident. In this case, the bleed load usually refers to the maximum flow rate produced by the pump, compressor, or other flow source under pressure relief conditions.

▲ Fire

Fire is one of the most difficult accidents to predict in a chemical plant, but this situation can be a condition that produces the greatest drainage requirements. If a plant-wide fire is possible, then the size of the entire drainage system can be determined on this condition. However, because the equipment can be distributed in zones, the impact of fire on the safety system is limited to a certain area. In the area of germanium tank with a large number of uninsulated germanium tanks, more steam will be generated. The various empirical formulas described above can be used to determine the discharge rate of a vessel exposed to a flame, depending on the system and fluid under consideration. A fire may overpressure a vapor-filled, liquid-filled, or mixed phase system.

▲ Pipeline rupture

When there is a large difference between the design pressure on the inside of the shell of the heat exchanger and the design pressure on the side of the tube (usually the ratio is 1.5 to 1 or greater), pressure relief measures should be taken on the low pressure side. Usually only one pipe break is considered in the design. The leakage volume of a broken pipe can be calculated by proper formula. When the cold medium is in contact with the heat flow, the effect of rapid evaporation should be taken into account, as should the possibility of instantaneous overpressure caused by the sudden release of vapor into the all-liquid phase system.

▲ Control valve failure

The possible failure of the instrument and control valve should be carefully estimated. In fact, the failure of the control valve may not be in the expected position. Valves can get stuck or there is a problem with the control loop, so safety protection should also be provided for these factors. In this case, the size design of the safety valve should be based on the flow coefficient (manufacturer's data) and pressure difference of the specific control valve and the device involved.

▲ Thermal expansion

If the process line is isolated on the cold side of the heat exchanger, the pressure relief valve protection should be installed on the line or on the cold side of the heat exchanger because the heat input from the hot side will cause overpressure.

If equipment or lines are isolated when filled with liquid, a safety valve should be installed to prevent expansion of the liquid in them. Solar radiation or atmospheric temperature changes require thermal expansion protection, and flash through the safety valve also needs to be considered.

▲ Utility failures

Sometimes regional or plant-wide cooling water leaks can occur, affecting fractionators and other water-cooling equipment. When determining the size of the flare system, the cooling water failure is usually the decisive condition.

As with cooling water system failures, power failures can occur in one area or plant wide, resulting in multiple effects. Since electric pumps and motor driven air coolers are often used in each process device, a power supply accident will immediately cause the simultaneous shutdown of electrical equipment. Power supply failure will be the main factor affecting the bleed capacity.

Failure of the instrument air system, whether or not related to a power failure, should be considered in determining the size of the system, because the pneumatic control circuit will be interrupted by failure.