Sampling and sample transfer

The sample processing system is required when the sensor elements of the on line analyzer are not directly installed in the process pipeline or equipment. The sample processing system is a system which connects the source fluid and the discharge point of one or more on line analytical instruments. Its function is to ensure that the analytical instrument can get representative sample in the shortest time lag. The state of the sample (temperature, pressure, flow rate and cleanliness) is suitable for the operating conditions of the analytical instrument.

The sample processing system can achieve the following basic functions: sample extraction, sample transmission, sample processing, sample discharge. These basic functions are also the main components of the sample system and the basic process of the sample in the system.

Whether the online analytical instrument can be used well is often not in the analyzer itself, but depends on the completeness and reliability of the sample processing system. Because the analyzer is complex and accurate, the accuracy of analysis is limited by the sample's representativeness, real-time performance and physical state. In fact, the problems of fainting in the sample processing system are often more than the analysis, and the maintenance of the sample processing system is often more than the analyzer itself. Therefore, we should attach importance to the role of the sample processing system, at least put it in the same position as the analyzer to consider.

The basic requirements of sample processing system can be summarized as follows:

1.The sample obtained by the analyzer is consistent with the composition and content of the source fluid in the pipeline or equipment.

2.Sample with minimal number

3.Easy to operate and maintain

4.Long-term and reliable work

5.The system structure is as simple as possible

6.Fast circuits to reduce sample transport latency

Sampling and sampling probe

Selection of sampling points

The following principles should be followed when selecting the position of sampling point of the analyzer on the process line. The best position may be the trade-off and compromise of some points in each point:

1.The sampling points should be located on the sensitive points which can reflect the changes of the properties and composition of the process fluid.

2.The sampling point should be in the most suitable position for process control to avoid unnecessary process lag

3.The sampling point should be at the position where the available process pressure difference forms a fast circulation loop

4.The sampling point should be selected at the sample temperature, pressure, cleanliness, dryness and other conditions as close as possible to the position required by the analyzer, in order to minimize the number of sample processing components

5.The location of the sampling point should be easily accessible from the escalator or fixed platform

6.The sampling points of the online analyzer shall be set separately from those of the laboratory analysis

It is generally believed that sampling from the turbulent locations where good mixing occurs in most gas and liquid pipelines ensures that the sample is truly representative. Because a gas or liquid mixture is not easily fully mixed unless there is turbulence. The sampling point may be selected at a downstream position of the last bend immediately after one or more 90 DEG bends, or at a relatively calm position downstream of the throttling element (do not rest closely with the throttling element).

Avoid the following as much as possible:

1.Do not sample downstream of a fairly long and straight pipe, because the flow of fluid at this location tends to be laminar, and the concentration gradient on the cross section of the pipe results in a non-representative composition of the sample.

2.Avoid sampling at locations where contamination may be present or dead volumes where gases, vapors, liquid hydrocarbons, water, dust and dirt may be present.

3.Do not drill directly on the pipe wall. If the sample is directly sampled on the pipe wall, one is unable to guarantee the representative of the sample, not only the fluid is in laminar or turbulent state, but also in turbulent state, it is difficult to guarantee the representative of the sample; Second, because of the absorption or adsorption of the inner wall of the pipeline will cause the memory effect, when the actual concentration of the fluid is reduced, desorption will occur, the composition of the sample changes, especially for the analysis of trace components (such as trace water, oxygen, carbon monoxide, acetylene, etc.), the effect is particularly significant. Therefore, the sample should be taken out by the insertion type sampling probe.

Selection of sampling probe type

1.For gas samples with dust content less than 10mg/m3 and clean liquid samples, a straight-through (open-type) probe can be used for sampling. The straight-through sampling probe is usually a rod-type probe with a 45° angle of the plane, the opening is installed in the direction of fluid flow, and the particles around the probe are separated from the fluid by using the principle of inertia separation, but the particles with smaller particle size can not be separated. Most of the sampling probes used in on line analysis are such probes.

2.When the liquid sample contains a small amount of particulate matter, viscous matter, polymer and crystal, it is easy to cause blockage, and it can be sampled by a plug-in probe without stopping pressure. The probe can also be used for gas samples containing a small amount of easily occluded matter (condensate, viscous matter).

The sampling probe as shown in the figure 15-1 is a non-stop, pressure-inserted and pull-out sampling probe, also known as a detachable, probe-type sampling probe, which can take out the sampling tube from the pressure-charged tube for cleaning under the condition that the process is not stopped. The invention is composed of a sealing joint and a gate valve (or ball valve) which are arranged on the straight-through probe.

Figure 15 - 1 Detachable probe type sampling probe structure

The structure of the sealing joint is shown in Figure 15-2. The structure can be divided into two parts, one is the clamping and fixing part of the sampling tube, and it adopts the clamping and pressing structure; The second part is the connecting part with the flange of the gate valve, which adopts the screw connection mode and realizes the sealing between the two parts by the sealing element. Be careful to align the groove orientation of the sampling tube with the arrow orientation (direction of fluid flow) on the flange at installation. In order to facilitate plug-in operation and ensure safety, the front end of the sampling tube is welded with a boss, so as to prevent the sampling tube from being blown out by the pressure in the tube during the pulling-out process, thus causing a safety accident; when the boss reaches the end of the blind flange disk, the gate valve can be closed, then the sealing joint is rotated to take out the sampling tube.

For gas samples with higher dust content (>10mg/m3), a filter probe can be used for sampling

Figure 15-2 Structure of sealing joint

The so-called filter type sampling probe is a probe with a filter, the filter element adopts sintered metal or ceramics (<800°c), silicon="" carbide="">800°C), and corundum Al2O3 (>1000°C) according to the sample temperature. The design of the probe should consider the use of fluid erosion to achieve the purpose of self-cleaning.

The filter mounted on the probe head (inside the process pipe) is called the built-in filter probe, and the filter mounted on the tail of the probe (outside the process pipe) is called the built-in filter probe. The disadvantage of the built-in filter probe is that the filter is not easy to be taken out and cleaned, only the blowing can be carried out by the reverse blowing mode, and the aperture of the filter can not be too small, thus preventing the dust from being frequently blocked. The probe is suitable for primary coarse filtering of samples. The external filter probe is commonly used, and the probe can conveniently remove the filter for cleaning. When the filter is used for sampling the flue, because the filter is arranged outside the flue, in order to prevent the moisture condensation in the high-temperature flue gas from blocking the travel, the filter part should adopt the electric heating or steam heating mode to keep the temperature of the sampling flue gas above the dew point temperature. The probe is widely used in flue gas sampling of boilers, heating furnaces and incinerators.

No filter probe should be used for the dirty liquid sample, because the wet dirt has strong adhesive force, and it is difficult to achieve the self-cleaning purpose by the washing of the fluid. Generally, a straight-through probe with a larger diameter is used to remove the liquid and remove the dirt.

For the sampling of ethylene cracking gas, catalytic cracking regeneration flue gas, sulfur recovery tail gas, coal or heavy oil and gas, cement rotary kiln tail gas and other complex conditions, a special design sampling device shall be adopted.

Selection of Probe Specifications, Insertion Length and Orientation

The 316 stainless steel tube is usually used as the sampling probe. The volume of the probe should be limited to reduce its size as much as possible.

The specifications of the probe are as follows:

6mm or 1/4"OD Tube - for gas samples.

10mm or 3/8"OD Tube - for liquid samples.

3mm or 1/8"OD Tube - Liquid samples for gasification and transport.

12mm or 1/2"OD Tube - For fast circulation loops, higher dust-laden gas samples and liquid samples called dirty.

The length of the probe is mainly determined by the length of the insertion. In order to ensure the representative of the sample, it is generally considered that the length of the insertion is at least 1/3 of the inner diameter of the pipe. EEMVARecommended insertion length for the No.138 standard is:

Minimum length: 30mm.

Maximum length: (0.56d+10) mm (d is the inner diameter of the pipe).

The insertion position of the samplingHorizontal pipe: gas sampling, the probe should be inserted from the top of the pipe to avoid possible liquids or droplets; The liquid sample, the probe should be inserted from the side wall of the pipe to avoid the vapor and bubbles that may exist at the upper part of the pipe, and the residue and sediment that may exist at the bottom of the pipe.

Vertical pipe: When the liquid is inserted from the side wall of the pipe, the liquid is taken out from the pipe section flowing from the bottom to the top, so as to avoid the gas mixing when the flowing liquid is abnormal.

Considerations for designing and making probes

The following issues should be noted.

Probe should be considered as follows:

1.The sampling probe should be fixed by a T-shaped short pipe joint with flange.

2.The material used, part of the T-shaped joint assembly are considered, and the stop valve is preferably a gate valve or a ball valve. When the sample is high pressure gas, the double stop valve system can be considered, which is an additional protective measure of double isolation.

3.The sampling stop valve should be considered as part of the probe assembly, and the stop valve should be a gate valve or a ball valve. When the sample is high pressure gas, the double stop valve system can be considered, which is an additional protective measure of double isolation.

4.The sampling probe should have enough mechanical strength to maintain a rigid fixation in the process fluid. When the fluid speed is fast and the flow force is large, if the probe is thin, the reinforcing tube can be sleeved to protect the probe.

5.The position of the probe and the direction of flow of the pipeline should be marked on the flange.

6.In designing the probe, it should be noted that the rupture due to resonance effect is prevented.

sample transmission

Basic requirements for sample transmission:

1.The transmission delay time should not exceed 60 s, which requires the distance between the analyzer and the sampling point as short as possible, the volume of the transmission system as small as possible, and the sample flow rate as fast as possible (1.5~35m/s is appropriate).

2.If the time is more than 60s after the flow allowed by the analyzer, a fast loop system should be used

3.The transmission line is preferably straight to the analyzer, with only a minimum number of bends and corners

4.No dead branch and dead volume

5.For the gas samples containing condensate, the transmission line should keep a certain slope downward, the lowest point should be close to the analyzer and equipped with the condensate collecting tank. The slope break is 1:12, and the viscosity condensate can be increased to 1:5.

6.The phase change is prevented, that is, during the transmission process, the gas sample is completely kept in the gaseous state and the liquid sample is completely kept in the liquid state.

7.The sample pipeline should avoid passing through the extreme temperature change area, which will cause the sample condition to change without control

8.The sample transmission system shall not be leaked, so as to avoid the leakage of samples or the invasion of environmental air.

The fast circuit is a pipeline that accelerates the sample flow to shorten the sample transmission delay time. The rapid circuit is usually composed of two types, namely, a rapid circulation circuit returning to the device and a rapid bypass circuit leading to waste.

Quick loop back to device

The fast circulation loop of the tool returning device is called fast circulation loop, which utilizes the pressure difference in the process line, and connects a pipeline between the upper and the lower reaches, the sample is drawn from the process and returned to the process circulation system, the sample needed by the analyzer is drawn from the loop close to a point of the analyzer, see Figure 15-3.

Fast bypass circuits are typically used in the following situations:

1.When the sample discharge does not cause environmental hazards and pollution.

2.When the process of returning the sample is not realistic, such as the gas after decompression the vapor after liquid gasification, etc.

3.When the recovery cost of the sample is higher than its value, the process of returning the sample is not economical.

4.Returning samples to a process that may lead to contamination or degradation, such as mixed samples measured by multiple flow paths, etc.

Sample transmission line

Pipes and fittings

The pipes and fittings used for sample transmission pipelines shall meet the following requirements:

The 316 stainless steel seamless tube pipe should be preferred in the sample transmission line. The pipe should be annealed. Advantage is:

316 stainless steel will not react with the components in the sample flow path, and has excellent corrosion resistance.

The results show that the inner wall of the seamless steel tube is smooth, the adsorption on the sample is little, and the pressure-resistant grade is high.

The tube is connected by press-joint, and has good sealing performance and small dead volume.

The tube of annealing treatment has high flexibility, which is convenient for bending construction and press connection.

The connection of the pipe should adopt the way of press connection, the double-card sleeve type press connection joint should be used, the material and specification of the pipe fittings (joints and valves) should be the same as and matched with the pipe.

Avoid the use of non-metallic tubes and fittings unless their physical and chemical properties have a clear advantage and are allowed by the user 

The copper tubes and fittings can only be used in pneumatic and heat-accompanying systems, and not for sample transmission.

Determination of Pipe Diameter Size

As the flow rate of the sample system is very small compared with the process logistics, due to the limitation of transmission delay time, the pipe diameter of the cherry well may be reduced. The diameter of the pipe can be determined according to experience.

6 mm or 1/4"OD Tube for gas sample

The liquid sample is 10mm or 3/8"OD tube

The quick circulation loop or dirty sample adopts 12mm or 1/2"OD tube.

Determination of Wall Thickness

The pressure capacity of tube is related to the wall thickness and is restricted by temperature. The requirements of the thickness of the sample pipeline wall in the general engineering design are:

∮3×0.7 or 1/8"×0.028

∮6×1.0 or 1/4"×0.035

∮10×1.0 or 3/8"×0.035

∮12×1.5 or 1/2"×0.049

Equipment for washing facilities

In the following cases. The sample pipelines and components should be equipped with washing facilities:

1.When the kinematic viscosity of the sample is higher than 500cSt(1cSt=1mm2/s)(at 38°C)

2.Possible solidification or crystallization of samples

3.Corrosive or toxic samples

4.Other occasions for users

The flushing medium may be nitrogen or steam, which should be introduced from the downstream adjacent to the sampling point, with particular attention to flushing the additional independent components of the system (e.g., parallel double filters, etc.).

Tube pipe and fitting

Differences between Pipe and Tube 

The Pipe and Tube tubes are two types of tubes with different diameters, connecting methods, representation methods and application range.

1.The Pipe tube is a tube with large diameter. The tube diameter is between 15~1500mm(1/2~60in). There are also Pipe tubes that are less than or larger than this range, but use little. The Tube tube is a small diameter tube, the diameter of which is between 1/8~1/2in(3~12mm).

2.The Pipe has three kinds of connection modes: flange connection, thread connection and welding connection. In most cases, flange connection is used, and thread connection is allowed in low pressure. However, the Tube wall is very thin, the thread is not allowed to cover on it, after annealing treatment, using the way of clamp connection, also known as pressure connection.

3.The Pipe tube represents the tube diameter specification of the tube with a nominal diameter DN. The nominal diameter is not equal to the outer diameter of the pipe or the inner diameter of the pipe, which is a size number commonly used for all components (including pipes, flanges, valves, joints, etc.) in the pipe system, and the pipes, flanges, valves, joints with the same nominal diameter can be connected with each other, regardless of whether other dimensions (outer diameter, inner diameter, wall thickness, etc.) are the same. Simply put, the nominal diameter allows the connection between the tube and the tube to be simplified and unified, which is why the Pipe tube uses DN to represent the tube diameter.

The Tube tube represents the tube diameter specification of the tube with an outer diameter OD, such as 1/4in OD Tube for a Tube tube with an outer diameter of 1/4 inches. Because the tube is connected by the way of the sleeve, this connection way is concerned with the outer diameter, the tube with the same outer diameter and the tube piece can be connected by the sleeve, which is the reason that the tube uses the OD to express the tube diameter.

4.The wall thickness of Pipe tube is standard. It is usually expressed by the serial number of wall thickness (Sch.NO.—Schedule Number for short), Sch.No. is also called the pressure level number, from Sch.No.5 to Sch.No.160. Tubes of different diameters or materials have their standard wall thickness series. Or, Sch.No. The actual wall thickness of a tube of the same diameter or material is different.

The wall thickness of the tube is represented by the actual thickness size (in inches or mm)

5.The Pipe is widely used, and the pipe is used in both the process pipe and the public engineering pipe. The tube is only used in the measurement pipeline of instrument system, pneumatic signal pipeline and sample of on line analyzer.

Types, specifications, and related parameters of common tube

There are several commonly used tubes: According to the material, there are mainly 316 stainless steel and 304 stainless steel. According to the forming process, there are two kinds of seamless steel pipes (hot rolled before cold drawing) and welded steel pipes (welded by strip steel). There are two kinds of inch Tube tube-meter tube in the metering unit system according to the outer diameter and wall thickness.

The outer diameter and wall thickness of commonly used tubes, the maximum allowable working pressure and their temperature degradation coefficients are shown in Tables 15-1 to 15-5.

Table 15-1 Specifications and maximum allowable working pressure (bar) of common rice-made tubes (material 316SS or 6Mo)

Note: 1. The working pressure system ASTM A-269 measured in the table, the safety factor is 4:1 [safety factor = expansion (rupture) pressure: Work pressure]

2. The working pressure in the table is effective in the temperature range of -20 to +100°C of the tube. If the temperature increases, the temperature degradation coefficient should be multiplied. See table 15-2.

Table 15-2 Temperature degradation coefficient of tube meter 

NOTE: For example, a seamless 316SS tube with an outer diameter of 12mm × 1.00 wall thickness has an operating pressure of 245bar at room temperature (see Table 15-1). If operating at 800°F (427°C) with a temperature degradation factor of 0.80 (see table 15-2), the maximum allowable operating pressure at that temperature is 245bar×0.80=196bar.

Table 15-3 Common in-inch Tube Pipe Specification Maximum Allowable Operating Pressure (psi, lbs/in2) (316 or 304 seamless steel pipe)

Table 15-4 Specifications and Maximum Allowable Operating Pressure (psi) for Common Inch Tube (316 or 304 Welded Steel Tubes)

NOTE: 1. Data in tables 15-3 and 15-4 are in line with ASME/ANSI B31.3 Chemical plant and refinery piping standards (1987 version)

2.The operating pressure values are the pressure values at ambient temperature (72°F or 22°C), and the temperature degradation coefficients are shown in Table 15-5.

3.pressure safety factor is 4:1

4.Unit Conversion lin=25.4mm, 1psi=6.89kPa≈0.07bar.

Table 15-5 temperature degradation coefficient of inch tube 

Note:For example, a seamless 316SS tube with 1/2" outer diameter x 0.049 wall thickness (about 12.7mm outer diameter x 1.25mm wall thickness) has a working pressure of 3500psi (about 245bar) at room temperature. If operated at 800°F (427°C) temperature, its temperature degradation coefficient is 0.80, at which temperature the maximum allowable working pressure is 3500psi x 0.80=2800psi (about 196bar).

Fittings for Tube

There are many types of fittings used by tubes, but these can be summarized as the following.

A middle joint (Union) is used for the connection between the Tube pipe and the Tube pipe, or a joint with both sides that are connected by a sleeve. There are mainly the following types:

Straight-through middle connector  Union

Three-way middle joint  Union Tee

Four-way intermediate connector  Union Cross

Bent middle joint  Union Elbow

(90° and 45° bend)

Through plate connector  Bulkhead Union

The invention is used for connecting the tube pipes with different pipe diameters, which is commonly called the big head and is also a middle joint.

A terminal connector is used for connection of tubes and meters, auxiliary devices, etc. The connector is connected with the Tube pipe by a clamp sleeve, so that the connector is connected with the meter, the auxiliary equipment, etc., and is a connector at the terminal of the Tube pipe, so the connector is called a terminal connector. There are only one of the following:

Pass-through terminal connector   Connector

Three-way terminal connector  Connector Tee

Bent terminal connector  Connector Elbow

Through plate terminal joint  Bulkhead Connector

The Gage Connector is used for connection between the tube and the gage, and is also a terminal connector. There are two main types: Pass Connect and Pass Connect Te.

Others, such as short fittings (Adapter), pipe plugs (Plug), pipe caps (Cap), etc., are not unnecessary or unnecessary.

If you are separated from the attachment, the fitting used by the tube pipe has two attachment modes.

Socket connection

The sleeve type connection is used for the connection of the joint and the Tube pipe, which is connected and sealed by the pressing force of the circular hoop, so the sleeve type connection is also called the pressure connection. The circular hoop has two kinds of hoop (single hoop, Single Ferrule) and double hoop (double hoop, Twin Ferrule).

threaded connection

The thread connection is used for the connection of the joint, the instrument, the auxiliary equipment and so on. There are two kinds of common threads.

1.Tapered pipe thread There are two types of NPT threads (60° tooth angle) and BSPT threads (55° tooth angle). The taper angle of taper is 1°47'. The more tightly the taper, the use of its deformation can play a sealing role, so it is also called the "thread of pipe sealed with thread". In practical use, sealing agent is usually added, such as PTFE tape, sealing agent of compound tube, etc., to prevent leakage.

2.Cylindrical pipe thread. There are Straight threads (60° angle) and BSPT threads (55° angle). Cylindrical pipe thread without taper, is a straight pipe thread, it has no sealing effect, so it is also called "non-threaded sealed pipe thread". The gasket (gasket) is used to seal the connection.

In addition, the thread on the outer surface of the joint is called positive thread, and is labeled with M(Mel); The thread on the inner surface of the joint is called a female thread, which is labeled with F(File). The screw thread which is rotated clockwise is called the right screw thread, the screw thread which is rotated counterclockwise is called the left screw thread, the model of the left screw thread is labeled LH, the right screw thread is not labeled.

Most of the threads used in tube pipe fittings are NPT conical pipe threads, some of the air cylinders are left-handed threads, and in other cases, they are right-handed threads.

Because of the variety of pipe fittings used in Tube pipe, and the inconsistent methods of model and specification of pipe fittings manufacturer, this manual no longer provides the information in this respect. In fact, according to the size, type and connection mode of the fitting, the fitting can be selected conveniently according to the product sample.

Sleeve type pipe joint

The Tube Fittings is a fitting for connecting Tube pipes (as can be seen from its English name). It is connected and sealed by the pressing force of the circular hoop, so it is also called the pressing joint. There are two types of the sleeve fitting: Single Ferrule and Twin Ferrule. Figure 15-5 is the structure and working principle of the double sleeve fitting.

Figure 15.5 Structure and Working Principle of Double Card Sleeve Pipe Joint

The two clamps are driven to advance towards the joint body by the thrust generated by the clockwise rotation of the nut; under the mutual extrusion action of the taper port of the body, the front clamp and the back clamp, the conical surface of the tube is pressed for two hours, and the connection and the sealing are realized by the pressing force between the two conical surfaces of the front clamp and the back clamp and the Tube tube.

The following points should be noted when connecting with a sleeve fitting:

1.Before connection, the tube must be round, the tube end has no burr, the surface has no obvious defect

2.Insert the tube pipe into the connector and ensure that the pipe in the cage is inserted in place and tighten the nut by hand. It is recommended that you draw a line between the nut hexagon and the joint body as the baseline of the starting point of the nut rotation.

3.It is not necessary to use the vice to clamp the pipe into the joint, the vice will leave a mark or scratch on the pipe, even make the pipe into an ellipse, which is easy to leak.

4.Using the wrench to tighten the nut along the clockwise direction, the joint of ≥1/4in(6mm) need to rotate 11/4 times;<1/4in(6mm) connector requires 3/4 rotation as shown in Figure 15-6.

5.If you need to disconnect and reconnect, note the original tightening position and use the wrench to disconnect the connection. When reassembled, tighten the nut to its original position, then gently tighten the wrench until the torque increases slightly

Steam heat conduction

Heat tracing and thermal insulation

Heat tracing refers to the use of steam heat pipe and electric heat pipe to heat the sample pipeline to supplement the heat loss in the transmission process, in order to maintain the sample temperature in a certain range. Thermal insulation refers to the coating measures taken on the outer surface of the sample pipeline in order to reduce the heat dissipation to the surrounding environment or absorb heat from the surrounding environment during the transmission process, also can be said to be the isolation measures taken to ensure that the samples are not affected by the surrounding temperature during the transmission process.

The sample transmission line often needs heat or heat insulation to ensure that the phase state and composition of the sample are not changed by temperature change. A significant source of temperature change in the process of sample transmission is the change of weather, China is in the continental monsoon belt, the difference between winter and summer extreme temperatures is often more than 60°C. In addition, the heating effect of direct solar radiation must be taken into account, and the surface temperature of the sample pipeline can sometimes reach 80~90°C under the sun exposure in summer. Therefore, the influence of the ambient temperature on the phase state and composition of the sample should be considered in the design of sample transmission.

The gas sample contains the components which are easy to condense, and should be accompanied by heat to keep the temperature above its dew point; The liquid sample contains components which are easy to gasify, and the liquid sample should be insulated and insulated below the evaporation temperature or keep the pressure above the vapor pressure. Trace analysis samples (especially trace water and trace oxygen) must be transported with heat, because the adsorption effect of the tube wall increases with the decrease of temperature, while the desorption effect is opposite. The samples which are easy to condense and crystallize must also be accompanied by heat transfer. In short, according to the conditions and composition of the sample, according to the change of the environment temperature, choose the reasonable insulation way, determine the insulation temperature.

There are two kinds of heat-preservation methods: steam heat-preservation and electric heat-preservation.

The advantages and disadvantages of steam heating

The advantages of steam heat-accompanying are: The temperature is high and the heat is large, so the sample can be heated quickly and kept at a higher temperature. The disadvantages are as follows:

1.Because of the thin diameter of the steam pipe, the air pressure can not be too high and the height of the vertical pipe changes, the effective length of heat conduction is greatly limited, so that when the sample pipeline is long or heavy load heat conduction, the method of sectional heat conduction must be adopted. According to the foreign data, the maximum effective heat conduction length of steam is 100ft(30.48m). Therefore, for the 60m long sample pipeline, it is usually divided into two stages.

2.The fluctuation of steam pressure will lead to a large change of temperature, and the insufficient supply of gas or even short-term interruption of gas is sometimes occurred. It is difficult to meet the requirements of equilibrium and stability of the temperature associated with the heat of the sample pipeline.

3.It is very difficult to control the associated temperature when the sample pipeline is heated by steam, or it is not controllable (the sample processing box can be controlled by temperature control valve).

Thermal vapor and thermal insulation material

There are two kinds of steam accompanied with heat, i.e. low-pressure superheated steam and low-pressure saturated steam.

Table 15-6 Main physical properties of saturated steam(SH 3126—2001)

Aluminum silicate insulation rope, silicate products and so on are commonly used as insulation materials for sample pipelines. The thermal insulation materials commonly used in the sample processing box or the analysis thermal insulation box are polyurethane foam, polystyrene foam, etc. The selection of the associated steam pressure and the thickness of the insulation layer can be found in Table 15-7.

Table 15-7 Thickness of thermal insulation layer at different atmospheric temperatures (SH 3126-2001)

Aluminum silicate insulation rope, silicate products and so on are commonly used as insulation materials for sample pipelines. The thermal insulation materials commonly used in the sample processing box or the analysis thermal insulation box are polyurethane foam, polystyrene foam, etc. The selection of the associated steam pressure and the thickness of the insulation layer can be found in Table 15-7.

Table 15-7 Thickness of thermal insulation layer at different atmospheric temperatures (SH 3126-2001)

Figure 15-7 Structure of heavy and light heat tracing

When the sample is easy to condense, freeze and crystallize, heavy heat may be used; When the heavy heat accompanying the sample may cause polymerization, decomposition reaction or gasification of the liquid sample, the light heat accompanying the sample should be used.

Water trap for steam heat-treatment system

The hydrophobic device is also called a hydrophobic valve, and its function is to regularly discharge condensate in the steam heat-accompanying system, prevent the leakage of steam, and save energy. A water trap should be installed separately in each of the heat-associated systems.

According to its working principle and structure, the water repeller has many kinds. Currently, the commonly used water repeller in the instrument thermal insulation system is a thermal power type water repeller, and also a temperature-regulated water repeller which utilizes the principle of thermal expansion and cold contraction of temperature sensitive elements to drain water automatically. and a combination of temperature-regulated and thermo-dynamic water repellents. The water repeller is not in the work scope of online analytical instrument maintenance, and this book does not introduce.

Electrical Companion

Advantages and Disadvantages of Electric Heating

At present, most domestic industrial enterprises use steam-assisted heat treatment, the main reason is that the steam boiler already existed in the plant can be used, but the heat-assisted efficiency and the maintenance and consumption in the future operation are far less than the use of electric-assisted heat treatment economy. In addition, the material of the steam supply pipe network and the water return pipeline, the heat preservation installation and the future maintenance cost, and the purification cost of the steam water are also considerable.

Compared with steam heat-accompanying, electric heat-accompanying has the following advantages:

1.The electric heating system is a relatively simple heating system. It does not need a complex steam pipe network and water return pipeline as the steam heating system, and the required power supply and distribution facilities can be shared with other electrical lines.

2.The scope of heat loss and the operating and maintenance expenses of the electric heat accompanying the heat shall be limited to the heat accompanying the line

3.Electric heat is a very easy-to-control heat-accompanying system, its temperature control can be very accurate, this is the steam heat-accompanying system can not reach

4.No noise, no pollution, steam tracing has "run, run, drip, leak" phenomenon, electric heat tracing does not

5.The electric heat belt has a service life of 25 years or more, which is difficult to achieve with steam

6.Easy installation, use and maintenance

Many developed countries have widely adopted electric heating technology in the industrial field. At present, the electric heating has been adopted in the instrument system of large-scale petrochemical projects. Compared with the steam heat, the main disadvantage of electric heat is low temperature and low heat. The temperature range of the electric heating is usually lower than 250°C, and the steam heating range is up to 450°C. Some liquid samples still need to be gasified by steam heating.

Electric heating cable

There are several kinds of electric heating cable in the electric associated heat system:(1) Self-regulation of the electric heating cable; (2) Constant power electric heating cable; (3) power-limited electri heating cable; (4) Series-connected electric heating cable

The first three are all parallel type electric heating cable, which are composed of parallel electric heating elements between two parallel power supply. At present, most of the electrical heat of the sample transmission line is selected as self-regulation electrical heating cable, and generally does not need temperature controller. When the sample temperature is higher (such as the high temperature flue gas sample of CEMS system), the power-limited electric belt can be adopted.

The advantages of the constant power electric belt are low cost, and the disadvantage is that the electric belt has no self-temperature adjusting function, and is easy to overheat. The invention is mainly used for the heating of the process pipelines and equipment, and a temperature control system must be arranged when the sample pipelines are used for the heating.

The series-type electric associated belt is a associated belt which takes the cable core line as the heating body, namely, the core line with certain resistance is connected with current, the core line generates heat, the heating core line has two types, namely single core and multi-core, which are mainly used for the heat associated with long-distance pipelines.

Fig. 15-8 Structure of self-regulating and electric belt 1-nickel-plated copper power supply bus; 2 - Conductive plastics; 3-Fluoropolymer insulation; 4-tin-plated copper wire braided layer; 5-polyolefin sheath (suitable for general environment); 6-Fluoropolymer Sheath (for corrosive environments)

self-regulating electric belt

Self-regulating Electric Accompanying Band, also known as Power Self-regulating Electric Accompanying Band, is a kind of parallel electric accompanying band with positive temperature characteristic and self-regulating. Figure 15-8 is the structure of the self-regulating electric belt.

The self-regulation electric heating belt consists of two power supply and conductive plastic connected in parallel between the two power supply. The so-called conductive plastic is made by introducing a cross-linked semiconductor matrix into the plastic, which is a heating element in an electric heating belt. When the temperature of the heated material increases, the conductive plastic expands, the resistance increases and the output power decreases. When the temperature of the material is lowered, the conductive plastic contracts, the resistance is reduced, and the output power is increased, that is, different heat will be generated at different ambient temperatures, and the conductive plastic has the function of self-regulating the temperature. It can be cut or lengthened arbitrarily, and it is very convenient to use.

The electric heating belt is suitable for the situation of low maintenance temperature, especially the situation of difficult calculation of heat loss. Its output power (10°C) is 10W/m, 16W/m, 26W/m, 33W/m, 39W/m and so on, and its maximum maintaining temperature is 65°C and 121°C. The so-called highest maintenance temperature means that the electric heating system can continuously maintain the highest temperature of the object.

Most of the electrical heat associated with the sample transmission line in on line analysis are self-regulated electrical heating belt. In general, there is no need for temperature controller, and the starting current is about 3-5 times of the normal value. The selection of components and wires in the power supply circuit should meet the requirements of starting current.

Limited power electric companion

Power-limited electric heating cable is also a parallel type of electric heating belt, its structure is the same as the constant power electric heating belt, see figure 15-9, the difference is that it uses resistance alloy heating wire, this kind of heating element has the positive temperature coefficient characteristic, when the temperature of the heating material increases, can reduce the power output of the heating belt. Compared with the self-regulation electric belt, the regulation range is small, the main function is to limit the output power in a certain range to prevent overheating.

Figure 15-9 Limited power supply with electric heating belt 1-Copper Power Supply Bus Bar; 2,4-Fluoropolymer insulation; 3-resistance alloy electric heating wire; 5-Tin-plated copper wire braided layer; 6-Fluoropolymer sheath

This kind of electric heating belt is suitable for the situation of high maintaining temperature, its output power (10°C) has several kinds such as 16W/m, 33W/m, 49W/m, 66W/m, etc., the highest maintaining temperature has two kinds of 149°C and 204°C. The invention is mainly used for the sampling pipeline of the CEMS system, which is used for heat preservation of the high-temperature flue gas samples, so as to prevent the moisture in the flue gas from condensing and separating out during the transmission process.

Electric Trace Tubing

Electrical Trace Tubing is a combination of a sample transport tube, an electrical trace tropical, a moisture retention layer, and a sheath layer.

Figure 15-10 is the structure of self-regulating electric heat pipe cable. The cable is suitable for the situation of low maintenance temperature, the highest maintenance temperature is 65°C and 121°C, and the number of the sample tubes is single and double following.

Figure 15-10 Self-regulating electric heat pipe cable structure

Left—single sample pipe cable; right—double sample tube cables; Structure (from outside to inside): Sheath - Black PVC Plastics

moisture retention layer-non-hygroscopic glass fiber; Thermal reflection belt—aluminum copper polyester belt; Electric heating belt—self-regulation type;

Sample tube—Tube of various sizes and materials

In addition to the electric heat pipe cable, there is also a steam trace tube cable, which is the same structure as the electric heat pipe cable, except the steam heat pipe replaced the electric heat pipe. It has two types of heavy and light heat accompanying, and the number of single and double heat accompanying sample tubes. The heat pipe cable is convenient to use, which saves the trouble of on-site coating and heat preservation construction. The invention has good water proof, moisture proof and corrosion resistance, and is reliable and durable, which is worthy of recommendation.

The cable can be selected according to the type selection sample provided by the manufacturer, and it also needs to be verified and confirmed through calculation. Figure 15-11 The working curve of the self-regulation electric heat pipe cable. The sample tube is a single 1/4in Tube tube, the left longitudinal coordinate is electric heat power, unit W/ft; The vertical coordinate on the right is ambient temperature, unit°F; The lower horizontal coordinate is the temperature of the sample tube, unit°F. The required thermal power can be identified by the intersection of temperature and ambient temperature that the sample tube needs to maintain. The rough line in the middle of the figure is the working curve of different specifications of electric heating belt, for example, the rough line is the working curve of self-regulation electric heating belt with power 3W/ft (10W/m at 10°C), according to the change of the curve, we can find out the change of the temperature of the sample tube under different environmental temperature when using the adjoint thermal.