How to choose the insertion depth of thermocouple and thermal resistance thermometer

The insertion depth of temperature instruments needs to be considered from the aspects of design and temperature elements.

The insertion depth of temperature elements that need to be calculated in the design:
The insertion depth of thermocouples, thermal resistors and bimetallic thermometers is based on the half of the pipe + the size of the temperature straight connector.
Thermocouples and thermal resistors are externally sealed, so the entire temperature straight connector size is added (normal 60, insulation 120); bimetallic thermometers are internally sealed, so the entire temperature straight connector size is added (normal 80, insulation 140) minus 20.

How to measure the insertion depth after getting the temperature element:
The insertion depth of thermocouples and thermal resistors is also counted with the thread.
The insertion depth of bimetallic thermometers is calculated from the bottom of the thread.
The flange method is calculated after the flange.
The movable thread or flange is adjusted by itself.

The insertion depth of the thermometer is different for different measuring media.

In order to ensure the measurement accuracy, the sensitive section of the thermometer must be inserted into the measured medium. The required immersion length of the thermometer is usually:

When measuring gas, the thermocouple should be greater than 95mm, and the thermal resistor and bimetallic thermometer should be greater than 115mm.
When measuring liquid, the thermocouple, thermal resistor, and bimetallic thermometer should be greater than 46mm.
The total insertion depth of the thermometer is: connector + pipe wall thickness + immersion length.

Different equipment has different insertion depths due to different installation positions.

In design, we often take the center line of the pipeline as the standard to calculate the insertion depth of the thermometer. This calculation is applicable to most applications, but it may not be applicable for temperature measurement of large pipe diameters, tower equipment, furnaces, etc. First, it may cause waste, and second, it cannot be installed and fixed.

For the temperature measurement of general tower equipment, when the thermometer is installed horizontally, the insertion depth is 300~400mm. For the temperature measurement of the boiler furnace, the thermometer is mostly installed horizontally. Excluding the thickness of the furnace refractory bricks, the thermocouple only needs to be inserted into the furnace 150mm.

Different pipe diameters have different insertion depths of thermometers due to different flow rates.

The flow of fluid in a pipe will form two states, laminar flow and turbulent flow, due to different flow rates. The flow rate of the fluid is the largest at the center of the pipe and the smallest near the pipe wall. When the flow rate is low, the fluid flows in layers without mixing, which is called laminar flow. Gradually increase the flow rate, the streamlines of the fluid begin to swing in a wave-like manner, and the frequency and amplitude of the swing increase with the increase in flow rate. This kind of streamline is called transitional flow; when the flow rate increases to a large value, the streamlines are no longer clearly discernible, there are many small vortices in the flow field, the laminar flow is destroyed, and there is not only sliding but also mixing between adjacent flow layers. At this time, the fluid moves irregularly, and a component velocity perpendicular to the axis of the flow tube is generated. This movement is called turbulence.

In the turbulent state, the thermometer can measure a relatively uniform medium temperature change as long as it passes through a very thin laminar layer at the pipe wall. In laminar flow, the change of upstream temperature is first sensed at the center of the pipe, and then gradually sensed at the pipe wall. The temperature change sensed at 1/4 of the pipe is only a very short delay compared with the center. For this reason, the insertion depth of the thermometer is not necessarily calculated from the center of the pipe, especially for large-diameter pipes, otherwise the thermometer is very long, which increases investment and is easy to damage.

The insertion depth of the thermometer is calculated from the bottom of the mounting thread of the thermometer. There is a mounting nut seat on the pipe. The height of the nut seat is generally 40mm. No matter how large the pipe diameter is, this cannot be less.

The length of the temperature sensing part of the thermometer is about 30mm. This part must be on the center line of the pipe. Therefore, assuming that the pipe diameter is 500mm, the insertion depth of the thermometer is: pipe radius 250 + mounting nut seat 40 + probe part 30 = 320mm.

If the pipe is in a full pipe state for a long time in the process and the pipe diameter is large, it is not necessary to be bound by this regulation. The length of the insertion into the pipe is equal to or greater than 10 times the diameter of the protective pipe.

If some pipes have a thicker insulation layer, the height of the installation nut seat may be higher.

If the diameter of the process pipeline is too small (less than 80mm in diameter), when installing the thermometer, an expansion tube should be made. This is a diameter change made on small-diameter pipes to meet the temperature measurement needs.

For the installation of thermal resistors, attention should be paid to accurate temperature measurement, safety, reliability and convenient maintenance, and not affecting equipment operation and production operations. To meet the above requirements, the following points should be noted when selecting the installation location and insertion depth of the thermal resistor:

In order to ensure sufficient heat exchange between the measuring end of the thermal resistor and the measured medium, the measurement point position should be reasonably selected, and try to avoid installing the thermal resistor near valves, elbows and dead corners of pipelines and equipment; thermal resistors with protective sleeves have heat transfer and heat dissipation losses. In order to reduce measurement errors, thermocouples and thermal resistors should have sufficient insertion depth:

(1) For thermal resistors that measure the temperature of the fluid in the center of the pipeline, the measuring end should generally be inserted into the To the center of the pipeline (vertical installation or inclined installation), if the pipeline diameter of the measured fluid is 200 mm, the insertion depth of the thermal resistor should be 100 mm;
(2) For the temperature measurement of high-temperature, high-pressure and high-speed fluids (such as main steam temperature), in order to reduce the resistance of the protective sleeve to the fluid and prevent the protective sleeve from breaking under the action of the fluid, the protective tube can be shallowly inserted or a heat sleeve thermal resistor can be used. The shallow insertion thermal resistor protective sleeve should be inserted into the main steam pipeline to a depth of not less than 75 mm; the standard insertion depth of the heat sleeve thermal resistor is 100 mm;
(3) If the temperature of the flue gas in the flue needs to be measured, even if the flue diameter is 4 m, the insertion depth of the thermal resistor is 1 m;
(4) When the insertion depth of the measuring element exceeds 1 m, it should be installed vertically as much as possible, or a support frame and protective sleeve should be installed.

In short, as one of the four major industrial measuring instruments, the temperature meter is also essential for standardized installation operations when its own quality is qualified. If you want to minimize the measurement results and errors, you must grasp the problem of the insertion depth of the thermometer.