Discussion on heat tracing design of chemical pipeline
This paper briefly introduces the heat tracing methods of process pipes in chemical plants, and studies the steam heat tracing design of process pipes in chemical plants based on various heat tracing methods in chemical production process.
With the increasing energy crisis in the world, how to save energy has become one of the highly important research areas in the world. Insulation of equipment and pipelines is one of the important energy saving measures. In order to prevent the equipment and pipeline from emitting or absorbing heat to the surrounding environment during the production process, thermal insulation engineering has become an indispensable part of the chemical plant. Process pipeline commonly used with the heat of the way there are four kinds: internal with the heat of the tube with heat, external with the heat of the tube with heat, jacket with heat, electric with heat. Process pipeline commonly used with heat medium generally have four kinds: hot water, steam, heat carrier and electric heat. In this paper, mainly for the chemical process pipeline with heat design to discuss.
Chemical process pipeline with heat design
1. Should be used with the pipe or jacket with the heat of the process pipeline.
- 1) Need to compensate from the outside for the loss of medium in the pipe, in order to maintain the temperature of the medium being transported pipeline.
- 2) In the process of conveying, the gas pipeline which generates condensate due to heat loss and may lead to corrosion or affect normal operation.
- 3) Pipelines that are self-cooling due to a sudden drop in medium pressure during operation and may freeze resulting in blockage.
- 4) During switching operation or indirect stoppage of transmission, the temperature of the medium in the pipe drops due to heat loss, and the medium cannot be released to purge the pipeline that may be solidified.
- 5) Pipeline that may precipitate crystallization due to heat loss during transmission.
- 6) Conveying medium due to heat loss viscosity lake;
- 7) conveying medium solidification point is equal to or higher than the ambient temperature of the pipeline.
2. The selection principle of the accompanying pipe accompanying heat.
- 1) The terminal temperature of the conveying medium or the ambient temperature is close to or lower than its solidification point of the pipeline.
- a) Medium solidification point is lower than 50 ℃, should choose to accompany the tube with heat.
- b) Medium solidification point of 50 ℃ to 100 ℃, should choose to accompany the tube with heat.
- 2) Conveying gas medium dew point is higher than the ambient temperature of the pipeline to be accompanied by heat, should be used with the tube accompanied by heat;.
- 3) Medium temperature requirements of the lower process pipeline, transport medium temperature or ambient temperature close to or lower than its freezing point of the pipeline, should use hot water with the pipe with heat;.
- 4) Liquid medium solidification point lower than 40 ℃ pipeline, gas medium dew higher than the ambient temperature lower than 40 ℃ pipeline and heat-sensitive medium pipeline, suitable hot water accompanying pipe accompanying heat;.
- 5) Often in the gravity of the self-retaining or stagnant state of easy to condense medium pipeline, should choose the jacket casing accompanying heat or with heat-conducting mud steam accompanying pipe accompanying heat.
3. Process pipeline with the pipe commonly used with the heat medium.
- 1) Hot water: applicable to the operating temperature is not high or can not use high temperature accompanying heat medium conditions, as the heat source of accompanying heat.
- 2) Steam: generally used for the operation temperature of the medium in the pipe is less than 150℃.
4. Steam accompanying pipe heat insulation scope of application.
Equipment, pipeline similarity detection quality of the solidification point, viscosity is larger, the process medium need to maintain the temperature is higher, or equipment, pipeline is located in the area of explosion-proof level is higher, the medium of corrosive, heat sensitivity is stronger, should choose the steam tracing heat insulation form.
5. With the pipe design should pay attention to the problem.
- (1) The steam of the accompanying pipe should be led out from the top of the steam pipe in charge, and set the cut-off valve near the lead-out, and the cut-off valve should be set on the horizontal pipe;
- (2) Each companion pipe should be set up separately trap, not combined with other companion pipe traps; through the trap after the non-recovery of condensate, should be discharged centrally;
- (3) To prevent steam from fleeing into the condensate network so that the system back pressure rises, interfering with the normal operation of the condensate system, the trap set should not be set up bypass valve;
- (4) Steam should be introduced from the high point of the accompanying pipe, along the pipeline to be accompanied by heat from high to low laying, condensate should be discharged from the low point, should minimize the U-shaped bend, in order to prevent the generation of gas resistance and liquid resistance;
- (5) When the companion tube is laid horizontally, the companion tube should be installed under the companion tube or both sides, when laid vertically, the companion tube is equal to or more than 3, should be laid evenly around the companion tube;
- (6) When the companion pipe passes through the valve and the pipe fitting, the companion pipe should be laid along its shape, and it is advisable to avoid or reduce the “U” shape;
- (7) When the main tube is accompanied by heat, the branch tube is not accompanied by heat, the first cut-off valve on the branch tube should be accompanied by heat;
- (8) The sampling valve, liquid discharge valve, venting valve and sweep line valve on the pipeline should be accompanied by heat;
- (9) The accompanying pipe connection should adopt welding, in the valve, flange, etc. after being accompanied by the pipe can adopt flange or live joint connection. φ12, φ10 copper pipe or stainless steel accompanying pipe should adopt ferrule type joint connection.
6. The selection principle of accompanying heat way.
Steam with the tube often to 0.3-1.0MPa saturated steam as heating medium, with the tube diameter is generally 15-76mm, but commonly used 18-25mm. conveying solidification point lower than 50 ℃ of the material, can use the pressure 0.3MPa steam with the tube insulation; conveying solidification point higher than 50 ℃ of the material, can use the pressure of 0.3-1.0MPa single or more with the tube insulation; conveying The solidification point is higher than 150 ℃ of the material, should use steam jacket heating.
Accompanying heat equipment technology comparison
- 1. The steam accompanying heat steam accompanying heat system and heating system is independent of each other two levels of the system, its by the point source of heat, after the steam accompanying heat pipe to achieve the function of heat transport to the process pipeline. Because the steam itself in the process of transmission will inevitably produce certain heat loss, then the process pipeline and the heat source point of the distance is different, get the heat compensation will be correspondingly different, near the point will appear overheating compensation and far point may appear compensation insufficient. However, because the electric accompaniment heat is limited by certain environmental factors, the application range of steam accompaniment heat is still relatively wide.
- 2. The electric accompanying heat and steam accompanying heat system is different, the electric accompanying heat heat dissipation system and heating system is combined with each other as a whole, its for line heat source at the same time supply heat. Although in the actual application of its compensation value and the system required heat can not do strictly equal, there is a certain gap, but the electric accompanying heat system does not have the heat transmission problem, so the pipeline heating is relatively uniform, will not appear in the above steam system of the pipeline far and near the problem. Usually there are about three kinds of electric tracing heat equipment as follows: one is mineral insulated heating cable, which is made of insulating material of magnesium oxide and the combination of core wire and metal armor. The mineral insulated heating cable in this electric tracing heat equipment belongs to the electric resistance element, which will produce Joule heat when using the electric current passing through the core wire resistance. But the product can’t change its length at will, so in the application process is relatively inconvenient; Another kind of constant power electric tropical, its and mineral insulation cable principle also belongs to a resistance element, and its power is also not adjustable. But its product structure and mineral insulation heating cable compared to a certain improvement, that is, according to the actual want to cut short. But another problem is the resistance wire winding interval a certain pitch will appear a welding point, if cut off in the pitch, then the rest of the circuit will be lost because of the circuit is not working, which for the short pipeline with the heat of the impact is relatively large. The last is the self-limiting electric heat tape, its choice of polymer PTC material made, both product structure or principle of action are different from the above two. Self-limiting electric heat tape can track the actual temperature of the process pipeline to make adjustments to its own output power, segmentation of independent also eliminates the impact of seasonal and diurnal temperature changes. The equipment of the self-limiting heating method makes the electric heat tape more adaptable to the heating system, but the above two methods are to let the heating system adapt to the electric heat tape, this is the most fundamental difference between the two. For flanges, valves and shaped pipes, using self-limiting electric heat tape is very suitable, but because the production of self-limiting electric heat tape material for polymer PTC, its for the outside temperature is very limited, so in some special occasions can not be used.
With heat equipment performance comparison
Electrical equipment used in flammable and explosive and other dangerous areas, its safety is the first important factor to consider. The design and application of electric tracing heat tape usually have to consider two places of safety, one is the power supply joints and switches and other joints, the other is the electric heat tape surface temperature. These two problems, the safety of the junction problem is relatively easy to solve, only need to be equipped with special explosion-proof accessories can be, for mineral insulated cable and constant power electric tropical, its power is designed by the voltage and electric resistance to determine the electric tropical, and electric heat transfer rate by its power and decide, then in the case of heat transfer coefficient is certain, constant power electric tropical and mineral insulated cable surface temperature by the heat transfer environment and its power to decide; if the heat transfer effect is not good, then the surface temperature of the electric heat tape will rise accordingly. Therefore, in comparison, steam heat tracing and self-limiting electric heat tape can automatically control the heating temperature, so the safety factor is higher. The steam pressure determines the maximum surface temperature of the steam tracing equipment, while the maximum surface temperature of the self-limiting electric heat tape is determined by its material. If the insulation is too good or poor heat transfer and over-current over-pressure, the surface temperature of mineral insulated cable and constant power electric tapes may exceed the melting point of its material, resulting in a short circuit, leading to combustion, serious than the explosion temperature of the surrounding gas may be higher, leading to explosive accidents. But the self-limiting electric heat tape and steam heat tracing device at most reach its highest surface temperature, and not because of the external conditions of change or misuse lead to overheating, these two kinds of heat tracing device in its highest surface temperature of the safety limit, the other fullness can be guaranteed.
Through the heat accompanying design of chemical process pipeline, the technology comparison of heat accompanying equipment and heat accompanying performance comparison, can well analyze the heat accompanying design of chemical process pipeline.
Source: China Chemical Pipeline Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
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