The pipeline and instrument flow chart is an important part of chemical process design. During the design process, all equipment, instruments, pipes, valves and other related public works systems of the system should be represented in detail according to the requirements of the entire process flow chart. The content of this issue mainly introduces the typical design examples of pressure, temperature, pump and vessel in the pipeline and instrument flow chart
Design pressure and design temperature
1. Design pressure
The design pressure of equipment and pipelines is the main basis for the strength design of equipment and pipelines. When the process system major determines the design pressure of the equipment, it should be economical and reasonable on the basis of satisfying the long-term safety production of the equipment. The maximum working pressure of each equipment cannot be considered statically and individually. The equipment should be placed in the process system for analysis, and the maximum pressure that may occur under various working conditions and additional system conditions (such as system pressure changes, safety valves in the system) should be considered. relative position of the pump, the relative position of the pump outlet valve, etc.) on the maximum pressure. At the same time, the characteristics of the medium in the equipment should be analyzed, such as flammable, explosive, toxic and harmful, freezing point, saturated vapor pressure, precious materials, etc.
The determination of equipment design pressure is based on certain principles, and the preliminary design pressure of each equipment is determined, and then the preliminary design pressure is adjusted according to the system analysis method, and the final design pressure of the equipment is obtained.
1. Terminology
(1) Pressure
Sub-gauge pressure and absolute pressure, respectively, add G or A after the pressure unit. When unspecified, it is usually referred to as gauge pressure.
(2) Maximum working pressure
The maximum pressure that can be reached at the top of the container under normal operating conditions. This value is proposed by the chemical technology professional.
â‘ For internal pressure vessels, it refers to the maximum pressure that may appear at the top of the vessel under normal working conditions.
â‘¡ For a vacuum container, it refers to the maximum vacuum degree that may appear at the top of the container under normal working conditions.
â‘¢ External pressure vessel refers to the maximum internal and external pressure difference that may appear at the top of the vessel under normal working conditions.
(3) The closing pressure of the pump (compressor)
The shut-off pressure of the pump (compressor) refers to the maximum discharge pressure of the pump (compressor) when the outlet flow is limited.
(4) Opening pressure of safety valve
The moment the safety valve disc starts to rise and the medium is continuously discharged, the static pressure at the safety valve inlet.
(5) Maximum pressure
The base pressure used to determine the design pressure of the vessel. It is the pressure that may be reached at the top of the container after the maximum working pressure of the container plus the additional conditions of the system in the process flow. This value is determined by process calculations.
(6) Design pressure
It refers to the set maximum pressure at the top of the vessel (including the additional system conditions of the technological process), which together with the corresponding design temperature is used as a condition for equipment design, and its value is not lower than the maximum pressure. The design pressure is determined according to the maximum pressure of the equipment and the relevant design specifications.
(7) Maximum (minimum) working temperature
The maximum (minimum) working temperature refers to the maximum (minimum) metal temperature that the component metal may appear during the normal operation of the container.
(8) Design temperature
The metal temperature of the element at which the vessel is set under normal operating conditions. This temperature, together with the design pressure, is used as the vessel design condition, and its value shall be determined according to the maximum (minimum) working temperature of the equipment and the corresponding specification calculation.
2. Design specifications
(1) Those that meet one of the following conditions shall be determined according to GB150 "Steel Pressure Vessels":
1) 0.1MPa(G)≤design pressure≤35MPa(G)
2) The degree of vacuum is greater than 2kPa (200mmH2O)
(2) Those that meet one of the following conditions shall be determined according to GB/T4735 "Steel Welded Atmospheric Pressure Vessels":
1) The design pressure is lower than 0.1MPa (G)
2) The degree of vacuum is less than or equal to 2kPa (200mmH2O)
(3) If the design pressure is greater than 35MPa (G), it should be determined according to the relevant specifications.
3. Determination principle of equipment design pressure
Table 1 Determination principle of equipment design pressure
4. Determination of the highest pressure of equipment in various systems
(1) Equipment
Equipment that withstands a variety of different working conditions In chemical production, when the same equipment needs to withstand a variety of different working conditions, for example, some reactors need to adapt to various chemical processes such as purging, pressure testing, temperature reduction, chemical reaction, catalyst regeneration, etc. If a variety of working conditions change, the principles for determining the design pressure of this type of equipment are shown in Table 1, and the time and medium changes of the corresponding changes of working pressure and working temperature at each stage should be explained to the equipment professional.
(2) Equipment for special media
â‘ The leakage of highly toxic medium directly affects personal and environmental safety. Generally, the design pressure of such equipment is set to be higher than the value specified in Table 1 to ensure safety.
â‘¡ For some media with high freezing point, such as asphalt, paraffin, phthalic anhydride, etc., because it is easier to block the system or block the safety device and the discharge system during discharge, causing the system pressure to rise, so in addition to considering heat tracing measures, it is possible to Appropriately increase the design pressure of such equipment.
â‘¢For some valuable materials, once leakage will cause certain economic losses, after making economic trade-offs, the design pressure can be increased.
â‘£ For some media, due to chemical reaction or physical process, the working pressure may change sharply, such as chemical reaction or liquid phase evaporation causing a sharp rise in pressure, and the condensation process of condenser under low pressure will cause vacuum operation, etc., should be based on specific circumstances. to determine the design pressure.
(3) Centrifugal pump system
â‘ The maximum pressure of the equipment upstream of the last shut-off valve on the pump output side
a. If the design pressure of the suction side container is selected according to Table 1, the maximum pressure of the pump output side equipment should be equal to the maximum pressure of the pump suction side container plus the pump outlet closing pressure difference plus (or minus) the static head.
b. If there are special requirements, the maximum pressure on the output side of the pump should be jointly agreed by the process system professional in conjunction with the relevant professional.
â‘¡ The maximum pressure of the equipment downstream of the last shut-off valve on the output side of the pump should be the maximum working pressure given by the chemical technology profession plus the additional conditions of the system.
(4) Positive displacement pump system
The output pressure of the pump is mainly limited by the strength of the pump casing and the torque of the driver, so the word "closing pressure" is usually not used for positive displacement pumps, but the word "stop pressure" is used, and its value is equal to the value required to stop the driver. differential pressure.
The "stop pressure" is usually much higher than the normal working pressure of the positive displacement pump. Therefore, the equipment on the output pipeline of the positive displacement pump should not be designed according to the "stop pressure". The maximum pressure of the equipment on the output pipeline of the positive displacement pump is the maximum equipment proposed by the chemical technology profession Additional conditions for working pressure plus empty system.
(5) Refrigeration system
The chemical process profession usually provides the maximum working pressure that the refrigeration system is expected to achieve during operation. But after stopping, the pressure on the high pressure side will decrease a, 0 and the pressure on the low pressure side will increase until the pressures on both sides of the system are equal, and the pressure at this time is the "stop pressure".
The maximum working pressure of the high pressure side is usually the value specified by the process, which is higher than the "shutdown pressure".
The maximum working pressure on the low pressure side is the "shutdown pressure" plus a margin that depends on the heat input during system shutdown and the thermodynamic properties of the refrigerant. The maximum working pressure of the low-pressure side during long-term shutdown shall be taken as the equilibrium pressure of the refrigerant at the highest expected ambient temperature, or selected with reference to the following (8).
"Stop pressure" is calculated according to the constant enthalpy throttling from the high pressure side to the low pressure side plus the additional conditions of the system, that is, as the maximum pressure of the refrigeration system, the high pressure side and the low pressure side are determined separately.
(6) Compressor system
When the compressor system handling steam and steam mixture and other various equipments are connected in series, the maximum pressure of the equipment should be selected according to a group of equipment (between two shut-off valves) under the same overpressure source; and the following points should be paid attention to.
â‘ The safety valve should be set as far as possible in the part where the working temperature of the group is closest to the normal temperature.
â‘¡The maximum working pressure of the equipment immediately upstream of the safety valve is the benchmark for determining the maximum pressure of the rest of the equipment in the system.
â‘¢ The opening pressure of the safety valve is equal to the design pressure of the upstream equipment minus the pressure drop from the equipment to the maximum normal flow of the safety valve.
(7) Tower system
The column system includes column equipment, reboiler, overhead condenser and reflux tank. The maximum pressure of the tower equipment should be determined according to the maximum working pressure at the top of the tower specified by the chemical technology profession and the additional conditions of the system.
(8) Containers for liquefied gas
①For pressure vessels containing liquefied gas whose critical temperature is higher than 50°C, when a reliable cold-insulation facility is designed, the maximum pressure is the saturated vapor pressure of the liquefied gas that may reach the highest working temperature; if there is no cold-insulation facility, the maximum pressure The pressure shall not be lower than the saturated vapor pressure of the liquefied gas at 50°C.
② For pressure vessels containing liquefied gas whose critical temperature is lower than 50℃, when it is designed with reliable cold insulation facilities and can ensure low temperature storage, the maximum pressure shall not be lower than the saturated vapor pressure at the highest temperature measured; there is no measured data or no The maximum pressure of the pressure vessel of the cold-insulation facility shall not be lower than the gas pressure at a temperature of 50°C when the liquefied gas contained is in the specified maximum filling capacity.
③ The pressure vessel containing mixed liquefied petroleum gas at normal temperature should be designed with 50℃. When its saturated vapor pressure at 50°C is lower than the saturated vapor pressure of isobutane at 50°C, take the saturated vapor pressure of isobutane at 50°C as the highest pressure; when it is higher than the saturated vapor pressure of isobutane at 50°C, Take the saturated vapor pressure of propane at 50°C as the highest pressure; if it is higher than the saturated vapor pressure of propane at 50°C, take the saturated vapor pressure of propylene at 50°C as the highest pressure.
(9) Safety valve
For systems with safety valves, the final design pressure should be determined according to the relative position of the equipment and the safety valve.
â‘ The design pressure of equipment with safety relief device can be determined according to the principles in Table 1.
â‘¡ The design pressure of the equipment downstream of the safety relief point is equal to the opening pressure of the safety relief device plus the static pressure head that may exist from the safety net to the downstream equipment. If a safety valve is installed in the top line of the tower, and after the condenser returns to the flow tank =F, the design pressure of the return tank should be equal to the opening pressure of the safety valve plus the static head of the condenser to the return tank.
â‘¢ The design pressure of the upstream equipment of the safety relief valve is equal to the opening pressure of the safety relief device plus the pressure loss and static head between the equipment and the safety relief device. If the safety valve is set in the reflux tank, the design pressure at the top of the tower is equal to the opening pressure of the safety valve plus the pressure loss and static head between the top of the tower and the safety valve of the reflux tank.
5. Selection of pipeline design pressure
(1) Scope of application
It is suitable for pipelines whose design pressure p is in the following working range.
①Pressure pipelines in the range of 0MPa(G)≤p≤35MPa(G).
â‘¡ The vacuum pipeline p < 0MPa (G) pipeline.
â‘¢Applicable to all fluid pipelines including fluidized solids.
(2) Determination principle of pipeline design pressure
â‘ The design pressure of the pipeline shall not be lower than the maximum working pressure.
â‘¡ The design pressure of pipelines equipped with safety relief devices shall not be lower than the opening pressure (or burst pressure) of the safety relief devices.
â‘¢ The design pressure of all pipelines connected to the equipment should not be less than the design pressure of the connected equipment.
â‘£ For pipelines transporting refrigerants, liquefied gases and other low-boiling mediums, the design pressure shall be the maximum saturated vapor pressure that the medium may reach when the valve is closed or the medium does not flow.
⑤ When the passage between the pipeline or pipeline components and the overpressure relief device may be blocked or cut off, the design pressure shall be determined not lower than the maximum possible working pressure.
â‘¥ For the pipeline whose wall thickness needs to be calculated according to the engineering design, the calculated pressure listed in the pipe wall thickness data sheet is the design pressure of the pipeline, and the working temperature corresponding to the calculated pressure is the design temperature of the pipeline.
(2) Selection of pipeline design pressure
â‘ Pressure pipeline with safety valve: p is greater than or equal to the opening pressure of the safety valve.
â‘¡Pressure pipeline connected to equipment without safety valve: p is greater than or equal to the design pressure of the equipment.
â‘¢ Centrifugal pump outlet pipeline: p is greater than or equal to the closing pressure of the pump.
â‘£ The outlet pipeline of the reciprocating pump: p is greater than or equal to the opening pressure of the pump outlet safety valve
⑤ Compressor discharge pipeline: P is greater than the opening pressure of the equal-dry safety valve + the pressure drop of the compressor to the maximum normal flow along the safe route.
â‘¥Vacuum pipeline: p is equal to full vacuum.
⑦ Any pipeline p that does not belong to the above range is greater than or equal to the maximum value in the variation of working pressure.
2. Design temperature
1. Determination of equipment design temperature
The design temperature of the equipment refers to the temperature corresponding to the material used in the equipment under the highest design pressure during normal operation. When the design temperature of the equipment (as a design condition) needs to be proposed by the process system professional, it can be determined with reference to this clause.
The process system major takes the highest (or lowest) working temperature of the medium in the normal three-work process or the wall temperature at the highest working temperature of the medium (this wall temperature is calculated or measured by heat transfer) proposed by the chemical technology major as the design temperature.
When the process system major cannot perform heat transfer calculation or actual measurement, the normal working temperature of the medium in the normal working process proposed by the chemical process major plus (or minus) a certain margin is used as the design temperature.
â‘ When the wall of the equipment is in direct contact with the medium and there is external heat preservation (or cold preservation), the design temperature shall be determined according to I or II in the table below.
Table 2 Design temperature selection
â‘¡When the medium in the equipment is directly heated by steam or indirectly heated by built-in heating elements (such as heating coils, electric heating elements, etc.), the design temperature shall be the highest temperature of the medium during normal operation.
③ When both sides of the equipment wall are in direct contact with different temperature media, and there may be only a single medium in contact, the design temperature should be determined according to the higher medium temperature; but when the temperature of any medium is lower than -20 ℃, it should be The lower medium temperature determines the minimum design temperature.
â‘£ The material temperature of the equipment shell is only determined by the atmospheric ambient temperature conditions: when the minimum design temperature can be determined according to the meteorological data of the region, the lowest value of the "monthly average minimum temperature" in the past years can be taken.
a. "Monthly average minimum temperature" refers to the sum of the minimum temperatures on each day of the month and divided by the number of days in the month. The minimum value of "monthly average minimum temperature" is the minimum value in the monthly average minimum temperature data measured by the National Meteorological Administration.
b. For areas lower than or equal to -20℃, the minimum design temperature shall be -20.
c. The minimum design temperature in the area lower than, equal to -10℃ and higher than -20℃ shall be -10℃.
⑤ In the following cases, the temperature of the equipment material should be obtained as the design temperature through heat transfer calculation:
a. The inner wall has a reliable thermal insulation layer;
b. Both sides of the wall are in direct contact with different temperature media, and there is no contact with a single medium.
â‘¥ When different parts of the equipment may have different temperatures during the working process, the corresponding design temperatures of the components should be selected according to different temperatures.
⑦ When the maximum (or minimum) working temperature of the equipment is close to the allowable temperature limit of the selected material, the design temperature should be carefully selected according to the specific situation, so as not to increase the investment or reduce the safety.
2. Determination of piping design temperature
The pipeline design temperature T refers to the temperature of the pipeline material that may be reached under the corresponding design pressure during the normal operation of the pipeline. According to the working temperature of various working conditions in the normal working process provided by the chemical technology professional, the professional of the process system selects the design temperature of the pipeline according to the "combination of pressure and temperature under the most severe conditions". The design temperature of the pipeline proposed by the process system specialty (this section refers to the maximum working temperature of the medium in the pipeline) can be determined by referring to the following principles.
â‘ Take the pipe wall temperature at the highest working temperature of the medium in the normal working process calculated by heat transfer or measured as the design temperature.
â‘¡ In the case that it is not convenient to calculate the heat transfer or measure the temperature of the pipe wall, the highest (or lowest) working temperature in the normal operation process is used as the design temperature of the pipe.
a. Metal pipes
ⅰ. For uninsulated pipes with medium temperature less than 38℃, T = maximum temperature of medium.
ⅱ. For pipes whose medium temperature is not less than 38℃, T=95% of the maximum medium temperature.
â…². External insulation pipeline, T = maximum temperature of medium
iv. Internal insulation pipeline (lined with thermal insulation material), T = heat transfer calculated tube wall temperature or experimentally measured tube wall temperature.
V: The temperature of the medium is not greater than 0°C, and T= the lowest temperature of the medium.
b. Non-metallic pipes and metal pipes with non-metallic linings
â…°. For pipes without the influence of ambient temperature, T = the maximum temperature of the medium.
ii. Pipes installed in an environment where the ambient temperature is higher than the maximum temperature of the medium (except those who have taken protective measures), T = ambient temperature.
â‘¢ In the normal working process proposed by the chemical technology major, the normal temperature of the medium plus (or minus) a certain margin is used as the design temperature, and the design temperature can be determined as follows:
a. When the normal working temperature of the medium is 0～300℃, T≥the normal working temperature of the medium+30℃.
b. When the normal working temperature of the medium is greater than 300℃, T≥the normal working temperature of the medium +15℃.
④When the temperature of the fluid medium is close to the allowable temperature limit of the selected material, the design temperature should be carefully selected according to the specific situation, so as not to increase the investment or reduce the safety. From an economic point of view, it is allowed to reduce the additional amount of 15 °C according to the engineering design requirements, but the process must have measures to prevent over-temperature during operation.
⑤ When the working pressure and the corresponding working temperature have various conditions or periodic changes, the process system designer should list the various working conditions data proposed by the chemical technology specialty, and explain to the pipeline material specialty.
Design of piping instrumentation and flow chart of equipment
The chemical production process is ever-changing, which is manifested in the design of pipelines and instrumentation flow charts. But the same is true, all pipeline and instrument flow charts are composed of various basic units. The pipeline and instrument flow charts of the basic units in different production processes may not be completely consistent, but there will not be much difference, and there are certain typical designs. This chapter mainly discusses its common problems, and introduces some common and typical design schemes for design reference.
1. Design of the pipeline instrumentation and flow chart of the pump
1. The structure of the pump
According to the way the pump transmits energy to the liquid to be sent, it can be divided into vane pump and positive displacement pump. The main structural types are as follows:
This section uses centrifugal pumps and reciprocating pumps as examples to illustrate the typical design of their piping and instrumentation flow diagrams.
Figure 1 Flow chart of the pump's piping and instrumentation
2. Typical design of centrifugal pump
Figure 2 shows the flow chart of the pipeline and instrumentation of the centrifugal pump, and the design points are described as follows.
â‘ The inlet and outlet of the pump should be equipped with a shut-off valve, generally a gate valve. The valve diameter is generally the same as the pipe diameter.
â‘¡The pump body and the inlet and outlet pipes of the pump should be equipped with exhaust and purge pipes with valves. For check valves above DN50, it is also possible to drill holes in the valve cover to install purge valves, and the discharge should be connected to a suitable discharge. system.
â‘¢The size of the inlet and outlet pipes of the pump should generally be one level or more larger than the nozzle of the pump.
â‘£The suction port of the pump should be equipped with a filter
⑤A check valve should be installed at the pump outlet.
â‘¥The outlet of the pump must at least be equipped with a pressure gauge.
⑦ When the medium may be vaporized at the pump inlet, a balance pipe and a shut-off valve should be installed between the pump inlet and the inlet shut-off valve. The balance pipe leads to the suction side container or discharges into the corresponding exhaust pipe nearby, and cannot form a bag shape. as shown in picture 2.
Figure 2 Balance piping of centrifugal pump
⑧ If the pump is likely to run for a long time below the minimum flow of the pump, a minimum flow pipe should be installed. A restrictive orifice and a shut-off valve should be set on the minimum flow pipe, as shown in Figure 4.
Figure 3 Minimum flow tube for centrifugal pump
⑨ If the pump is used to transport medium above 200℃ and is equipped with a spare pump, a warm pump pipeline with a restrictive orifice should be installed. If the ambient temperature is lower than the pour point or freezing point of the material, an anti-condensation pipeline should also be set up. As shown in Figure 4.
Fig. 4 Heating pump piping and anti-condensation piping of centrifugal pump
â‘©Bypass pipes should be installed before and after the outlet shut-off valve of the high-lift pump. As shown in Figure 5.
Figure 5 High lift bypass piping of centrifugal pump
3. Typical design of reciprocating pump
Figure 6 shows the flow chart of the pipeline instrumentation of the reciprocating pump. The design points are as follows:
â‘ Compared with centrifugal pump, the outlet of reciprocating pump may not have check valve.
â‘¡A safety valve should be installed between the outlet pipe of the reciprocating pump and the outlet shut-off valve. The discharge pipe of the safety valve can be connected to the downstream of the suction filter or to the suction container.
Fig. 6 Flow chart of pipeline instrumentation of reciprocating pump
Design of pipeline and instrument flow chart of container
1. Design points of the P&ID diagram of the container
â‘ The top and bottom of the container are generally provided with a vent valve and a purge valve, and a utility interface with a door should be provided near the bottom of the container. The valve should be directly connected to the vessel nozzle.
â‘¡ There is not necessarily a shut-off valve at the mouth of the material population of the container. Under normal circumstances, only the shut-off valve is set at the liquid phase outlet of the container. If there is another shut-off valve within 15m of the horizontal distance of the nozzle, the shut-off valve may not be set at the outlet of the container. The utility pipeline connected to the container shall be provided with a shut-off valve near the mouth of the container. The shut-off valve between the container and the connecting pipeline should be installed directly at the mouth of the container as far as possible.
â‘¢ The on-site liquid level gauge, liquid level transmitter, liquid level alarm or pressure gauge connection on the container can be set in the container iiif. The gas phase and the liquid phase are connected to the riser.
â‘£ When the container needs to be set with a safety valve, the safety valve can be set on the gas phase part or the gas phase pipeline at the top of the container.
⑤ When the container has specific requirements for the installation elevation, the minimum elevation shall be marked.
2. Typical design example
Figure 7 is a flow chart of the piping and instrumentation of the horizontal vessel.
Figure 7 Flow chart of piping and instrumentation of horizontal vessel
Figure 8 Flow chart of piping and instrumentation of vertical vessel
Figure 8 is a flow chart of the pipeline and instrumentation of the vertical container with nitrogen sealing. The tank body and the material pipeline are equipped with steam tracing. Because it is an inter-song operation, the safety valve may not be provided with a bypass and an upstream shut-off valve.
Figure 9 Atmospheric pressure horizontal container with nitrogen seal
Fig. 9 is the flow chart of the pipeline instrumentation of the atmospheric pressure horizontal container provided with nitrogen seal. The upper part of the tank is provided with a breathing valve and a flame arrester. The material is led by the pipeline to below the liquid level of the tank. The liquid phase outlet at the bottom of the tank is provided with an anti-vortex plate, which is connected to the suction port of the pump. The container has installation elevation requirements.
Figure 10 Atmospheric vertical vessel with stirring device
Figure 10 is a flow chart of the piping and instrumentation of the atmospheric vertical vessel with a stirring device. Because the tank is equipped with a stirring device, the eddy current plate can be omitted.
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