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1. "Petrochemical Enterprise Occupational Safety and Health Design Code" SH3047-93, 2.2.7 stipulates: Equipment pipelines where material backflow may cause danger should be equipped with automatic shut-off valves, check valves or intermediate containers according to specific circumstances.

2. "Petrochemical Enterprise Fire Protection Design Code" GB50160-2008, 7.2.7 stipulates as follows: When public engineering pipelines are connected to pipelines or equipment for combustible gases, liquefied hydrocarbons and combustible liquids, the following provisions shall be met:

a) Check Valve should be installed on the public engineering pipelines used continuously, and a shut-off valve should be installed at its root;

b) Check valves and one shut-off valve or two shut-off valves should be installed on the public engineering pipelines used intermittently, and a check valve should be installed between the two shut-off valves;

c) Public engineering pipelines used only when the equipment is out of service should be equipped with blind plates or disconnected.

3. "Petrochemical Enterprise Fire Protection Design Code" GB50160-2008, 7.2.11, stipulates as follows: Centrifugal flammable gas compressors and flammable liquid pumps are equipped with check valves on their outlet pipes.

4 Summary of design to prevent backflow

By reading SH3047-93 and GB50160-2008, it is found that the design institute can completely design a check valve to prevent the possibility of major process safety accidents caused by material backflow. The check valve can be used as a protection measure, but according to the requirements of my country's "Guidelines for the Application of Layer of Protection Analysis (LOPA) Method" AQ/T3054-2015, the check valve is not an independent protection layer (IPL), and there is no data to support its average required failure probability (PFD), which means that it cannot effectively reduce the frequency of accidents.

5 Internal design regulations to prevent backflow

The author collected the internal design regulations of a domestic engineering company, but did not find specific national or industry design standards. They are summarized as follows: The following settings should be considered at the pump outlet to prevent backflow, when the pump outlet pressure:

a: P≦4.0MPa: one check valve;

b: 4.0 MPa<p≦8.0 mpa: two check valves of different types; <="" span="">

c: 8.0 MPa<p: two check valves of different types, add a low-low flow automatic shut-off valve on the main pipe. <="" span="">

6 Good design for backflow prevention in a gasoline hydrogenation unit

The gasoline hydrogenation reaction pressure is 2.3~2.4 MPa. In order to prevent the high-pressure hydrogen from backflowing into the low-pressure catalytic cracking gasoline system when the catalytic cracking gasoline pump is shut down, the design institute has two check valves at the outlet of the hydrogenation feed pump, but adds the pump P101A/B shutdown signal to automatically close the catalytic cracking gasoline flow control valve FV00102.

Figure 3: Backflow prevention design for hydrogenation feed pump

7 Summary

When the inlet and outlet pressures of pumps, compressors, etc. differ too much, or when the continuously used public works are connected to the hazardous material system, the check valve cannot effectively prevent the material from backflowing, and even two check valves cannot be regarded as an effective check design. When doing HAZOP analysis, it is recommended to use the backflow deviation for hazard analysis. To prevent high-pressure materials from flowing back into the low-pressure system, which may cause major process safety accidents, one of the following three options can be added on the basis of a check valve design:

1. The pump or compressor is stopped and the discharge valve is closed by interlock;

2. The discharge flow is low and the discharge valve is closed by interlock;

3. The pressure difference between the two ends of the discharge valve is low and the discharge valve is closed by interlock.