RTO system piping design and how to prevent freezing in common freeze-prone locations

Winter is also a cold winter for the environmental protection equipment industry. Many equipment cannot be used normally due to the weather, especially RTO equipment. RTO generally includes four parts: pre-treatment, RTO, post-treatment and emergency bypass. Pre-treatment equipment is mainly used to remove waste gas. Particulate matter, corrosive medium, vapor mist, etc.; post-processing equipment is mainly used to cool down the flue gas at the RTO outlet, remove corrosive media (sulfur dioxide, nitrogen oxides, inorganic acids, etc.) and particulate matter; when the concentration of exhaust gas in the system is too high, it is necessary to In emergency direct discharge, the emergency bypass pipeline can prevent the occurrence of potential safety hazards. The four parts of the equipment in the RTO system are prone to freezing in cold environments:

(1) The easy-to-freeze positions of the RTO body are mainly distributed in the low temperature area of ​​the furnace body and the vertical pipeline area between the RTO outlet and the post-processing equipment;

(2) The easy-to-freeze positions of pre-processing and post-processing equipment are mainly distributed in the circulating water pipeline, the standby circulating water pump, and the vertical pipeline containing water vapor distributed between the outlet of the purification tower and the dehumidification box;

(3) The emergency bypass is a discontinuous section, the two ends of the emergency valve are dead ends, and the pipeline and valve are easy to swell;

(4) In some connected auxiliary equipment, the location where the effusion occurs is also prone to freezing; * It is worth noting that when the equipment that is prone to effusion is not drained regularly, it will freeze in winter. The volatile organic compounds in the liquid are volatilized in large quantities, and the local concentration is too high, causing potential safety hazards.

(5) The precise parts of the instruments (temperature, pressure, flowmeter, concentration detector, etc.) in the entire system are frozen, etc.

Therefore, the parts that are easy to freeze in the RTO system are mainly concentrated on the pipelines and instrument valves. Today, I will learn about the pipeline design in the RTO system and the common easy-to-freeze locations and how to deal with them.

First of all, what is a low temperature environment, and what does the lowest temperature mean?

The design definition of the minimum temperature: it is generally determined according to the minimum metal temperature that may occur under various possible conditions expected by the equipment during operation.

However, my country has a vast land and abundant resources, and the temperature difference between north and south is also very large. The minimum temperature value should be selected according to the local value. Then in cold regions, when the medium temperature and ambient temperature are met, at the beginning of the design of the RTO system, it is necessary to select the appropriate minimum design temperature and suitable materials to meet the actual operating conditions and ambient temperature requirements.

1. Pipeline design in RTO system in low temperature environment

When designing the RTO system, it is necessary to predict the possible low temperature area first, and then make the corresponding selection of values ​​and materials for the area.

(1) The value of the minimum design temperature

In the actual operation process, because the metal wall temperature is not easy to accurately predict, the minimum design temperature of equipment and pipelines is usually confirmed according to the minimum operating temperature of the medium, and the minimum design temperature should correspond to the design pressure of the equipment and pipelines, and its value is listed below. The minimum value of each value.

①Minimum temperature of continuous operation of medium minus 5~10℃;

②Minimum ambient temperature (when there is no pipeline heat tracing);

③ Calculate the metal wall temperature for the minimum medium temperature under discontinuous operating conditions.

(2) Material selection and low temperature toughness requirements for pipes and fittings

Under low temperature conditions, the pipelines, equipment and corresponding welding materials in the RTO system should choose materials with sufficient crack initiation toughness to avoid crack initiation. In order to ensure that the material can be used safely and reliably under low temperature conditions, the current common practice to verify whether the material has qualified crack initiation toughness is to use the Charpy impact test (under the test temperature conditions specified in the project, the Charpy impact absorption energy meets the can only be used when specified).

In the VOCs treatment industry, the commonly used metal pipes are carbon steel and austenitic stainless steel. Based on the difference in their chemical composition and manufacturing process, the minimum service temperature corresponding to different grades of steel is also different. GB/T20801.1 and AEME B31.3 specify the minimum service temperature of the material and the minimum requirements for Charpy impact test. Usually, the minimum operating temperature of the national standard carbon steel is -20°C, and the American standard is -29°C; the minimum operating temperature of the national standard low-temperature carbon steel is -40°C, and the American standard is -45°C; austenitic stainless steel basically has a relatively high temperature. Good low temperature performance, the minimum operating temperature of the national standard material is -196 ℃, and the American standard is -198 ℃.

Therefore, in the northern region, the key lies in the selection of pipelines, equipment and valves. In addition to considering whether the medium is corrosive, it is also necessary to comprehensively consider the calculation/estimation of the minimum design temperature, and the selection of low-temperature carbon steel or stainless steel.

In the cold regions of northern my country, the temperature is low in winter, and the equipment and pipelines in the RTO system have large heat dissipation losses. When the external ambient temperature is lower than the freezing point (freezing point) of the medium in the device, the medium in the process pipeline and equipment will condense or freeze. Here are some common freezer-prone locations and how to handle them.

2. Common freezing positions and treatment methods of pipelines in RTO systems

(1) For pipelines that have been frozen, the processing methods are as follows:

① After the equipment is frozen, steam or hot water should be used for freezing, and carbon steel/stainless steel pipelines and equipment should be thawed slowly to prevent bursting;

② It is forbidden to heat the pipeline/equipment that is frozen and cracked in a sealed state. The open horizontal pipe should be thawed from low to high first, and the vertical pipe should be thawed from top to bottom;

③For the metal valve that has been frozen, use warm water or a small amount of steam to slowly heat it to prevent sudden heat damage;

Plant operators should also increase inspection efforts in winter, find problems in time, and repair them in time.

(2) Antifreeze measures for bypass pipeline:

When the RTO system is running normally, if the bypass pipeline is too long, the bypass is closed, and the two ends of the valve are dead ends. In this case, heat tracing is not economical. In winter, give the bypass valve a small opening, or connect a DN15/DN20 valve on both sides of the bypass valve to ensure stable medium flow.

(3) Antifreeze measures for circulating water pipelines:

The diameter of the circulating water pipeline is relatively large, and the heat tracing needs to consume a lot of energy. In this case, only a small diameter bypass line can be connected between the inlet and outlet of the circulating water to make the circulating water flow and prevent freezing. A little energy can prevent freezing.

(4) Antifreeze measures for vertical pipes containing water vapor:

For some vertically installed gas pipelines, if the gas contains water, the water is mixed with the gas medium in the form of steam under normal working conditions. When the external temperature is lower than the dew point corresponding to the gas, the water will condense into a liquid state, and when the temperature is lower than the freezing point will freeze. This phenomenon of water freezing mostly occurs near the riser and the connecting elbow, tee, and valve below it. The freezing will block the pipeline and even swell the pipeline. In such a case, the 90° elbow of the riser and the horizontal pipe can be changed to a 45° elbow to prevent liquid from accumulating at the elbow, and the horizontal pipe can be inclined at a 45° angle at the connection between the riser and the horizontal pipe, which is convenient for operation. Insert the standpipe, connect the drain valve vertically downward, and drain regularly.

(5) Antifreeze measures for tower dosing pump and circulating water pump when they are arranged in the open air:

①For pumps that need to be started automatically, the inlet and outlet valves of the pump need to be opened.

②Prevent the standby pump from freezing → set a warm pump line at the outlet of the standby pump, and lead a thin pipeline from the back of the check valve at the outlet of the running pump to the front of the check valve. The medium in the pipeline flows back from the standby pump, and there is a The flow of the medium prevents the pump from freezing.

③Drain the residual liquid in the pump during shutdown. A bypass is set in the middle of the pump cooling water inlet and outlet pipelines, so that the circulating water can circulate between the bypasses to prevent the pipeline from freezing.

④ The metering pump itself has a small pipe diameter and low flow rate, which is easy to freeze in cold areas. It is necessary to heat the water at the outlet of the metering pump. The outer wall of the dosing tank storage tank is heated with water and heated.

In summary, the current solution is to sacrifice energy to ensure the safe operation of the RTO system. There is no systematically perfect equipment, anti-condensation and anti-freezing criteria for pipelines in the current effective design standards. It is necessary to selectively carry out anti-freezing and thermal insulation design according to the actual situation. In the design of the entire RTO system, it is necessary to pre-consider the location of the easy-to-freeze location. The rationality of the process design and structural design of anti-freezing measures. While the process engineers comprehensively consider the working conditions, medium and ambient temperature for the entire system, the grasp of material selection and the advanced design of anti-freezing measures, the mechanical designers are in the overall equipment layout. , will take into account the smooth design process, shorten the length of the pipeline as much as possible, the layout is reasonable, and the influence of pipeline resistance will be minimized, so as to bring better design and after-sales service to our customers as much as possible.