Problems in the design and installation of chemical pipelines analyzed
The article mainly discusses the design and installation of pipelines in the field of chemical engineering, analyzes the problems existing therein, and puts forward corresponding solution measures.
With the continuous development of China’s social and economic development and the increasing progress of science and technology, the chemical industry development space is also larger and larger, and pipeline engineering is an important field in the chemical industry; pipeline design and installation directly affect the quality of chemical engineering. This paper briefly analyzes the common problems in the design or installation site and puts forward corresponding measures for these problems.
1.1 Problems in the pipeline design process
In the pipeline design stage, if the design pressure is low, it may cause the actual production of pipeline pressure and lead to pipeline damage. If the material selection is unreasonable, it may lead to pipeline corrosion by the transport medium and leakage; if the design temperature is unreasonable, it may lead to pipeline brittleness or toughness changes and fracture. Suppose the design stage does not consider the special conditions (such as parking maintenance). In that case, it may also lead to pipeline medium due to environmental changes in the pipeline medium phase, temperature, or pressure changes, resulting in damage to the valve pressure at both ends of the pipeline.
1.2 Problems in Pipeline Installation
Pipeline installation is unreasonable; not only may it cause production difficulties, but it may also cause serious safety accidents. If the pipeline is installed in the evacuation channel, may cause the operation of workers in an emergency escape failure; if the upper pipeline installation is too low or the lower pipeline installation is too high, may cause the operation of workers touching the head or feet; if the valve installation is too high or too low, can cause production workers to operate the difficult; if the root of the safety valves are not sealed, may be mistakenly shut off by the operator so that the safety valve loses its protective effect; if the filter installation is too low, may lead to filter cannot be replaced in time and then the safety valve is damaged, the filter cannot be replaced in time. If the filter is installed too low, it may lead to the strainer not being replaced in time and damage the pump. Suppose the regulating valve group is installed in the vertical installation. In that case, the regulating valve group is installed in the bypass valve above or the regulating valve is installed in the vertical pipeline, it may lead to cracking of the weld seam of the pipeline in the valve group and the leakage of the material; if the pipeline of high-temperature media is not thermally compensated, it may lead to the deformation of the pipe rack or the pipeline is slipped from the rack and fractured.
2. Chemical process piping design points research
2.1 Pay attention to the selection of chemical piping materials and grade requirements
Before the design stage of the chemical pipeline, the material characteristics of the transport medium of each pipeline, production conditions, and pipeline installation environment should be clarified. Then, the relevant design selection should be made according to the actual situation.
- (1) Select qualified pipeline design pressure. If the design pressure of the pipeline is low, the actual production of the pipeline to withstand the pressure is large and causes damage to the pipeline; if the design pressure is high, it may cause the pipe procurement costs and maintenance costs to increase.
- (2) The material characteristics of the conveying medium should be clarified. If the conveying medium temperature is high, it should use high temperature resistant metal materials and flexible analysis of the pipeline; if the pipeline conveying medium is corrosive or the production stage requires frequent use of corrosive materials to clean the pipeline, it should be based on the corrosion characteristics of the selection of the appropriate corrosion-resistant materials; when the pipeline conveying a high degree of hazardous or extremely hazardous media, even if the production conditions for the atmospheric pressure or vacuum, the design of metal pipeline or steel lined pipeline Pressure should not be less than 2.0MPa.
- (3) Select the appropriate pipe diameter. Pipe diameter can be determined by the pipeline volume flow and flow rate; conveying flammable and toxic media materials should be appropriately enlarged pipe diameter so that the medium is in the safe flow rate range.
- (4) Pay attention to the impact of the environment on the pipeline. For example, often subject to sunlight on the design of a small-diameter safety valve on the liquid transportation pipeline to prevent the pipeline from thermal expansion of high pressure generated by the use of valves at both ends of the pipeline leakage; also, such as the arrangement of outdoor process water pipe additional insulation layer to prevent the pipeline from freezing in the winter.
2.2 Tower and container piping design and installation
- (1) Tower top oil and gas pipeline medium is generally gas-phase, the pipe diameter is large, the pipe as short as possible, to “step by step low”, there shall be no bag-shaped pipe, and has a certain degree of flexibility;
- (2) The fractionation tower top oil and gas pipeline is generally not insulated, only anti-scald; the pipeline to several parallel cold exchange equipment is appropriate to symmetrical arrangement to avoid bias flow;
- (3) When the top of the tower is two-stage condensation, its piping arrangement should make the condensate self-flowing stage by stage, and the oil and gas main pipe and the condenser inlet branch pipe should be symmetrically arranged to make the flow uniform;
- (4) When the tower top pressure control with thermal bypass, the thermal bypass pipe should be insulated and ensure that the pipeline is as short as possible, its regulating valve is installed in the upper part of the reflux tank, and the pipeline to avoid the appearance of the “bag shape”;
- (5) Special requirements of the pipeline (such as the top of the decompression tower oil and gas pipeline) and tower openings directly welded instead of flange connection to reduce leakage.
- (6) Tower top vent pipe is generally installed at the top of the tower top oil and gas piping at the highest level of the top of the horizontal pipe section and should meet the requirements of the fire code;
- (7) The tower side piping generally has a feed pipe, a return pipe, a sideline extraction pipe, and a reboiler extraction and return pipe. To make the valve closed without accumulation of liquid, the valve is suitable for direct connection with the tower body orifice;
- (8) Inlet (outlet) piping at the same angle when there are more than two inlet (outlet) openings, the piping should be flexibly analyzed;
- (9) If there is a regulating valve on the pipeline from the sideline of the fractionating tower to the stripping tower, its installation position should be close to the stripping tower to ensure that there is a section of the liquid column in front of the regulating valve, and the height of its liquid column should meet the process requirements;
- (10) The distillation tower bottom temperature is generally high, so the pipeline should be stress analyzed and meet the requirements of the relevant standards; the bottom of the tower and the pump connected to the pipeline should be short and have fewer bends to reduce resistance to prevent pump cavitation;
- (11) Horizontal container on the feed pipe and discharge pipe mouth should be as far away as possible; the pipeline arrangement should meet the requirements of the axial displacement of the equipment; the pipeline low-point discharge net put net mouth from the ground or floor net distance should not be less than 150mm;
- (12) The pipeline of the vertical kettle reactor should avoid the installation and testing area of the stirrer.
2.3 Pump piping design and installation
- (1) The inlet pipe of the pump should be as short as possible, with fewer bends; there should be no “air pockets,” and the pipeline should meet the requirements of the pump’s “cavitation margin”;
- (2) The pump before the installation of the filter, the filter should have a filter element extraction space;
- (3) For the horizontal suction centrifugal transfer pump, the pump inlet diameter should be used eccentric reducer. When the pipe is fed into the pump from the bottom upwards, the reducer shall be installed with a flat top, and when the pipe is fed into the pump from the top downwards, the reducer shall be installed with a flat top and a net discharge at the low point, but when the conveying medium contains solids or slurry, the eccentric pipe shall be installed with a flat bottom;
- (4) To prevent the pump outlet liquid backflow caused by the pump impeller reversal, the pump outlet should be equipped with a check valve;
- (5) The pump outlet pipe generally needs to be reducer; when the pump outlet is in the upper part of the pump body, the outlet pipe should be installed with a concentric reducer; when the pump outlet is on the side of the pump body, refer to the setup form of eccentric reducer for pump inlet;
- (6) The pump outlet pressure gauge should be installed on the pipeline between the pump outlet and the first shut-off valve, with the pressure head facing the operation surface for easy observation;
- (7) When the temperature of the conveying medium is greater than 200 ℃, the pump should be set up before and after the pump outlet valve (group) of small flow back to the warm pump pipeline.
- (8) Conveying liquefied hydrocarbons, liquid ammonia, liquid chlorine, or liquids in bubble point state, a balancing line should be set to prevent the inlet liquid from producing vapors or bubbles causing pump cavitation;
- (9) Ambient temperature is lower than the condensation point of the conveyed material, the import and export of its standby pump should be set up to prevent condensation bypass pipe, anti-condensation piping set up to accompany heat insulation;
- (10) When the conveying pump is equipped with refrigerant, the refrigerant piping of each circuit of the pump should be connected in parallel and should not be connected in series.
2.4 Installation of valves and piping accessories
- (1) Extremely hazardous media, strong corrosive media piping, and equipment on the valve should not be arranged in the range of human head height;
- (2) Vertical pipeline valve handwheel center from the operating surface of the distance is appropriate for 1.2 m; the maximum distance should not exceed 2 m. When more than 2 m, the handwheel should be set up or will be installed in the valve set and additional operating platform;
- (3) The direction of the stem of the valve on the horizontal pipeline shall not be vertically downward;
- (4) The safety valve should be installed vertically and close to the protected equipment or pipeline. When the outlet pipeline of the safety valve is discharged into the atmosphere, the outlet should not face the equipment, platform, cable, etc. A tear hole should be opened at the lowest point of the elbow at the lower end of the safety valve outlet. If necessary, the condensate should be connected to the collection tank. When the outlet pipeline of the safety valve (DN ≥ 50) is discharged into a closed system, it should be inclined at the top of the discharge main pipe 45 ° along the medium flow direction;
- (5) The breathing valve should be installed at the highest point of the gas phase space of the tank; when the tank is equipped with an inert gas protection system, the inert gas inlet should be away from the breathing valve;
- (6) Check valves have two types: lift type and swing type. Lift type should be installed on the horizontal pipeline, and swing type is preferred to be installed on the horizontal pipe; when vertical lift type or swing type is installed on the vertical pipeline, it should be ensured that the medium in the pipeline is flowing from the bottom to the top;
- (7) The trap installation location should not be higher than the trap point; float trap, thermodynamic trap, and inverted bucket trap should be installed horizontally, pulse trap should be installed on the horizontal pipeline, and bimetal trap can be installed horizontally or vertically; more than one trap at the same time when the installation must be connected in parallel;
- (8) When the regulating valve group is installed upright, the regulating valve should be installed below the bypass, and the low point of the pipeline between the inlet side of the regulating valve and the upstream shut-off valve should be equipped with a clean-out valve;
- (9) Flame arresters shall be installed at both the tail gas main pipe for conveying combustible medium into and out of the production device; flame arresters for combustible material storage tanks shall be installed directly on the pipe opening at the top of the tank, or flame arresting breathing valves can be installed directly;
- (10) Filter forms are different, and the installation requirements are also different: Angle T-type should be installed at the occasion of 90-degree bends in the pipeline, straight-through T-type must be installed on the straight pipe of the pipeline, Y-type should be installed on the horizontal pipeline when the cartridge extraction direction should be downward, and the reverse-flow Y-type should be installed in the vertical pipeline of the medium from the bottom up;
- (11) In the design conditions and conveying medium allows, give priority to pi type compensator; ∏ type compensator is best set in the middle of the two fixed points; when the construction site is restricted can be adjusted appropriately, but should not be less than one-third of the distance between the two fixed points;
- (12) Cut-off valves and blind plates should be installed at the entry and exit of pipelines conveying combustible or toxic media.
2.5 Other considerations
- (1) According to the process needs to set the temperature, pressure, flow, and other detection instrumentation, timely monitoring of pipeline operating conditions;
- (2) According to the material temperature and installation environment, set up reasonable adiabatic materials;
- (3) According to the design conditions of high temperature pipeline flexible design, and according to the results of flexibility analysis to set the phase compensator, guide frame, sliding frame, and fixed frame;
- (4) According to the diameter of the pipeline, the transport medium, the pipeline, and its insulation material maximum span combined with the construction site conditions to ensure that the appropriate pipeline support hangers are set up; outside the plant pipeline should also ensure that the fire rescue, emergency maintenance vehicle access;
- (5) Pipeline installation should be completed by the requirements of the pipeline weld detection, blowing and pressure test, anticorrosion treatment, and pressure pipeline should also be reported to the local quality inspection department for the record.
The quality of chemical piping design and installation has an important impact on chemical safety. This paper discusses the design and installation of tower and vessel piping, pump piping design and installation, valves, piping accessories installation, etc., to improve the safety of pipeline operation and reduce the chances of safety accidents.
Author: Liu Xiaojuan, Yao Wen, Wang Kingland Nan