Introduction: Instrument Reliability - The "Life Line" for the Stable Operation of Sulfuric Acid Plants 

In the production facilities of sulfuric acid, the accurate measurement of parameters such as temperature, pressure, liquid level, and flow rate is a crucial prerequisite for ensuring the safe operation of the facilities, optimizing process control, and achieving energy conservation and emission reduction. However, for a long time, the failure rate of on-site instruments in domestic sulfuric acid production facilities has remained high, and the problem of inaccurate measurement has been frequently encountered. This has seriously restricted the automation level and operational efficiency of the facilities. 

According to statistics, approximately half of the temperature instruments in the sulfuric acid plant are used to measure the temperature of the converter, the steam temperature, and the temperature of the sulfuric acid medium. These temperature instruments are precisely the most prominent aspect where measurement errors and difficulties in later maintenance occur in the current sulfuric acid plant. At the same time, measurement inaccuracies in analytical instruments such as sulfuric acid concentration analyzers and sulfur dioxide gas analyzers are also extremely common. The root causes of these problems are often not the quality defects of the instruments themselves, but rather stem from two core factors: improper selection and improper installation. 

This article will systematically explore the key methods for enhancing the measurement reliability of on-site instruments in the sulfuric acid production plant from three aspects: instrument selection, installation standards, and operation and maintenance. It aims to provide practical reference for sulfuric acid manufacturing enterprises and instrument technicians. 

I. Analyzer Selection and Installation: The "First Step" for Accurate Measurement 

1.1 Analyzer - The "hotspot" where the measurement of the sulfuric acid device is inaccurate 

Among all the measuring instruments in the sulfuric acid plant, the failure rate and measurement inaccuracy rate of the analyzers are the highest. Among them, the acid concentration analyzer and the sulfur dioxide gas analyzer are the two types of instruments with the most prominent problems. 

The acid concentration analyzer is used to measure the concentration of sulfuric acid and is a key instrument for controlling the acid concentration in the dry absorption section and ensuring product quality. The sulfur dioxide gas analyzer is used to monitor the SO₂ concentration at the inlet and outlet of the converter, and is the core basis for determining the conversion rate and optimizing the air supply. If the measurements of these two types of instruments are inaccurate, it may lead to process fluctuations and substandard product quality, or even cause environmental pollution over-limitation and corrosion accidents. 

1.2 The root cause of measurement errors: Dual mistakes in selection and installation 

The core reasons for the inaccurate measurement of the analyzer can be summarized into two points: 

First, the selection of equipment did not fully consider the operating conditions. The medium of the sulfuric acid plant is highly corrosive, has high temperatures, and contains fuming acid crystals, presenting complex characteristics. The chemical properties, temperature, pressure, and operating status of the medium at different measurement points vary greatly. If the selection is not made in accordance with the specific operating conditions, the instrument will soon experience corrosion, blockage, drift, and other problems, resulting in inaccurate measurements. 

Second, the installation location does not meet the technical requirements of the instrument. Even if the selection is correct, if the installation location is improperly chosen - such as insufficient representativeness of the sampling point, excessively long sampling pipe, or incomplete pre-treatment system - it will also lead to deviations in the measured results from the actual values. 

1.3 Three Key Points for Enhancing the Reliability of Analyzers 

(1) Select the equipment based strictly on the operating conditions parameters. 

Before making a selection, it is essential to fully grasp the working conditions of the measurement points: 

Chemical properties of the medium: concentration of sulfuric acid, presence of impurities, corrosiveness, crystallization, dew point grade 

Medium temperature: Normal temperature, Maximum temperature, Temperature fluctuation range 

Medium pressure: Normal pressure, pressure fluctuation range 

Operating status: Is it in continuous operation or intermittent operation? Are there any extreme conditions during the start-up and shutdown processes? 

Environmental conditions: Installation area temperature, humidity, vibration, electromagnetic interference, etc. 

Take the acid concentration analyzer as an example. For the high-temperature concentrated sulfuric acid medium, a sensor material that can withstand high temperatures and corrosion should be selected; for the medium containing a large amount of impurities, the filtering capacity of the pre-treatment system should be considered. For the SO₂ gas analyzer, an appropriate sampling and pre-treatment scheme should be selected based on the gas temperature, the content of SO3, and the concentration. 

(2) Select the installation location scientifically 

The installation points must meet the following requirements: 

Sampling representativeness: The sampling points should be located in the area where the medium is uniformly mixed and the flow rate is stable, avoiding dead corners and vortex zones. 

Easy maintenance: The installation location should be equipped with sufficient space for maintenance, facilitating regular calibration, cleaning and replacement. 

Environmental adaptability: Avoid installation in areas with high temperature, strong vibration, or severe electromagnetic interference. 

Pipeline optimization: The sampling pipeline should be as short and straight as possible, with fewer bends and dead ends, and measures should be taken to prevent condensation and blockages. 

(3) Improve the pre-treatment system 

For gas analyzers, the pre-treatment system is crucial for ensuring accurate measurements. It should be configured based on the characteristics of the medium: 

Filtering device: Removes SO3, acid mist, and other impurities 

Condensation/Dew Point Removal Device: Removes moisture to prevent corrosion by condensation acid. 

Voltage stabilizing device: Stabilizes the sampling pressure to prevent pressure fluctuations from affecting the measurement. 

Heating device: Prevents condensation and blockage in the sampling pipeline 

II. Selection and Installation of Control Valves: The "Key Hub" for Achieving Automatic Control 

2.1 Incorrect selection of control valves - The "invisible killer" that makes automatic control impossible to achieve 

In the sulfuric acid production plant, the control valve is the core executive component for achieving automatic control of process parameters. However, many sulfuric acid plants have serious misunderstandings in the selection of control valves, which makes automatic control difficult to achieve and even leads to process fluctuations. 

The most common mistake is: The position of the regulating valve is chosen as a butterfly valve or ball valve for regulation, without paying attention to the flow characteristics of this regulating position. The flow characteristics of butterfly valves and ball valves are quick-opening characteristics, which are not suitable for continuous regulation; while the regulating valve should be selected based on process requirements with flow characteristics such as equal percentage, linear or logarithmic. Incorrect selection directly leads to poor regulation accuracy, system oscillation, and makes automatic control impossible. 

2.2 Five Core Parameters for Selecting Control Valves 

The selection of scientific control valves must be based on the following process parameters: 

(Maximum Flow Rate, Qmax) 

The maximum flow rate refers to the maximum operating flow rate required by the process. During the selection process, the maximum flow capacity of the regulating valve should be slightly greater than the maximum flow rate of the process, typically leaving a 10% - 20% margin. 

(2) Minimum Flow (Qmin) 

The minimum flow rate refers to the minimum operating flow rate required by the process. The control valve should still be able to regulate stably at the minimum flow rate without experiencing flow fluctuations or improper closure. 

(3) Common flow rate (Qnor) 

The normal flow rate refers to the typical operating flow rate of the equipment when it is in normal operation. The optimal working range of the regulating valve should be close to the normal flow rate to ensure the sensitivity and accuracy of regulation. 

(4) Selection of Flow Characteristics 

Select the appropriate flow characteristic based on the process characteristics: 

Constant percentage characteristic: Applicable to situations with significant changes in load and pressure difference, such as acid concentration control in sulfuric acid plants, boiler liquid level, and acid production control, etc. 

Linear characteristic: Suitable for applications with stable pressure difference and small load variations. 

Quick opening feature: Only applicable to two-position control (switch control), not suitable for continuous adjustment. 

(5) Fault Safety Position Selection (FO/FC) 

According to the process safety requirements, select the fault open (FO) or fault close (FC): 

Fault Closure (FC): The valve closes during a fault, suitable for situations where the medium needs to be cut off, such as the acid production regulating valve. 

Fault Open (FO): The valve opens during a fault condition. It is suitable for applications where the flow of the medium needs to be maintained, such as in cooling water control valves. 

2.3 Technical Requirements for Installation of Control Valves 

(1) Installation location 

The regulating valve should be installed at a location that is convenient for operation, maintenance and repair. 

There should be sufficient straight pipe sections before and after the valve to ensure the stability of the fluid flow. 

Avoid installation in areas with high temperature radiation and strong vibration. 

Install strictly according to the flow direction indicated on the valve body. Do not install it in reverse. 

(3) Bypass Design 

Important regulating valves should be designed with bypasses, facilitating on-site maintenance and manual operation. 

The bypass valve should be a stop valve to avoid using gate valves or ball valves.

III. Temperature Instrument Selection and Installation: The "Temperature Guardian" of the Sulfuric Acid Plant 

3.1 Temperature Instruments - The "Typical Component" Addressing Measurement Challenges in Sulfuric Acid Plants 

Temperature measurement holds an extremely important position in the sulfuric acid production process. According to statistics, approximately half of the temperature instruments in the sulfuric acid plant are used to measure the temperature of the converter, the steam temperature, and the temperature of the sulfuric acid medium. Temperature instruments are currently the most prominent aspect in the sulfuric acid plant where measurement inaccuracies and subsequent maintenance difficulties are most prominent. 

Among them, the measurement of the converter catalyst temperature is the most challenging part in temperature measurement. The converter is the core equipment of the sulfuric acid plant, and the temperature of the catalyst bed directly reflects the reaction state and the activity of the catalyst. It is a key basis for determining the conversion rate and optimizing the operating parameters. However, due to the large diameter, high temperature, and strong corrosiveness of the medium of the converter, the measurement of the catalyst temperature has long-term problems of poor accuracy and difficult maintenance. 

3.2 Technical Key Points for Measuring the Temperature of the Converter Catalyst 

(1) Installation location of the thermometer: Avoid embedding it in the catalyst. 

When installing thermometers in the catalyst bed of the converter, it is essential to be careful that the thermometers at the inlet and outlet should not be inserted into the catalyst itself. 

If the sensing element of the thermometer is buried with the catalyst, it will result in: 

Measurement cannot promptly reflect the changes in gas temperature. 

The difference in heat conduction of the catalyst and convective heat transfer of gases leads to measurement errors. 

It is difficult to remove during maintenance and poses challenges in upkeep. 

The correct approach is: The sensing element of the thermometer should be placed 150mm above the catalyst interface, or within the gas flow area, to ensure that the measured temperature is that of the gas rather than that of the catalyst particles. 

(2) Temperature gauge placement: Arranged at the four corners of the box, with the gauges installed crosswise to each other. 

In order to comprehensively reflect the temperature distribution of the catalyst bed, due to the large diameter of the converter, if the installation method of installing around the four directions (east, south, west, and north) is adopted, when there is no ladder on the converter, it will be impossible to carry out maintenance on the faulty thermometer. Therefore, it is recommended to adopt the cross-installation method: 

Thermometers on the same plane are inserted from opposite sides, forming an intersection. 

This not only ensures the representativeness of the temperature distribution, but also makes it convenient for maintenance from both sides. 

Avoided the inspection blind spots caused by single-sided layout 

Temperature uniformity verification: It is recommended to install multiple thermocouples at the inlet and outlet of each catalyst bed (for example, one every 120°) to verify the temperature uniformity across the reactor section. 

(3) Thermometer selection: High-temperature resistant, corrosion resistant, long lifespan 

The measurement environment for the converter catalyst temperature is extremely harsh: 

Temperature range: 420℃ - 620℃ 

Medium: High-temperature corrosive gas containing SO₂, SO₃ and O₂ 

Operating condition: Long-term continuous operation, sudden temperature change upon shutdown 

Therefore, the selection of the thermometer must meet the following requirements: 

Thermocouple type: It is recommended to choose K-type or N-type thermocouples. Their long-term operating temperature can reach above 1000℃. 

Material of the protective sleeve: It should be made of alloy materials that are resistant to high temperatures and corrosion, such as Inconel 600, 310S stainless steel, etc. 

Pipe structure: A thick-walled pipe should be used to prevent perforation caused by high-temperature corrosion; a heat-conductive material should be filled between the pipe and the thermocouple to ensure the speed of thermal response. 

Sealing method: Utilize metal sealing or ceramic sealing to prevent the leakage of high-temperature gases. 

3.3 Precautions for Temperature Measurement in Sulfuric Acid Medium 

For the measurement of medium temperatures at locations such as sulfuric acid storage tanks and acid circulation pipelines, the following points should be noted: 

Corrosion protection: Select the appropriate protective tube material based on the concentration and temperature of the sulfuric acid. For low-temperature concentrated sulfuric acid, 316L stainless steel can be used; for dilute sulfuric acid, Hastelloy alloy or tantalum material should be selected. 

Insertion depth: Ensure that the temperature sensing element is located in the area where the medium is fully mixed, and avoid being close to the tank wall or pipeline wall. 

Thermal insulation measures: For high-temperature sulfuric acid media, the thermometer sleeve should be well insulated to prevent heat loss and ensure accurate measurement.

IV. Pressure Measurement: The "Dilemma" of Stripped Pressure Lines and Diaphragm Transmitters 

4.1 Converter Pressure Measurement - A Typical Challenge in Sulfuric Acid Plant 

The pressure measurement of the converter is the most representative challenge in the pressure measurement of the sulfuric acid plant. The medium inside the converter is high-temperature flue gas containing SO₂, SO₃, and O₂. The accuracy of pressure measurement directly affects the judgment of the catalyst bed resistance, the assessment of the fan operation status, and the analysis of the plant's energy efficiency. 

However, many enterprises have long faced the problem of inaccurate pressure measurement in their converters, mainly manifested as: 

The pressure guiding tube frequently gets clogged, causing the pressure signal to drift or be interrupted. 

After using the diaphragm pressure transmitter, the diaphragm was damaged and the accuracy decreased. 

The pressure gauge tube froze in winter, causing abnormal measurement values. 

4.2 Blockage of the pressure guiding pipe: "The main cause of sulfur trioxide condensation" 

The medium used to measure the flue gas on the converter contains a large amount of sulfur trioxide (SO₃). When the temperature of sulfur trioxide is lower than the dew point temperature, it will condense into liquid sulfuric acid or solid sulfuric acid mist in the pressure guiding tube, gradually blocking the pressure guiding tube and causing inaccurate pressure measurement. 

This is the main problem that causes inaccurate pressure measurement in many conversion devices. Once the pressure guiding pipe gets clogged, the signal received by the pressure transmitter cannot reflect the actual pressure. This can lead to incorrect process judgments in the mild cases, and in severe cases, it can even cause safety accidents. 

4.3 Diaphragm Pressure Transmitter: "Constrained Solution" under High Temperature Conditions 

To solve the problem of blocked pressure lead, some users choose to use diaphragm pressure transmitters on the converter for measurement. The diaphragm pressure transmitters directly contact the medium through the diaphragm, without the need for a pressure lead, thus fundamentally avoiding the problem of blockage. 

However, the operating temperature of the converter can reach 420℃ to 620℃. In such a high-temperature environment, using a diaphragm pressure transmitter for measurement would result in: 

Pressure measurement accuracy decreases: High temperature causes changes in the elastic modulus of the diaphragm, resulting in a deterioration in linearity. 

Diaphragm damage: Long-term operation under high temperatures accelerates diaphragm fatigue, resulting in diaphragm rupture or permanent deformation. 

Filling oil vaporization: High temperature causes the filling oil to vaporize, resulting in inaccurate measurement or even damage to the transmitter. 

Therefore, the diaphragm pressure transmitter is not an ideal solution for converting and measuring pressure. 

4.4 Correct solution: Short pressure lead pipe + insulation 

For the special working conditions of the converter pressure measurement, the correct solution is: 

(1) Use the pressure guiding tube method and strictly control the length of the pressure guiding tube. 

According to the principle of nearest piping connection, the distance between the pressure guiding pipe from the converter to the pressure transmitter should not exceed 1 meter, and the number of elbows should not exceed three. 

Advantages of the short pressure relief pipe: 

Reduce the residence time of sulfur trioxide in the pressure guiding tube and decrease the probability of condensation blockage. 

Reduce pressure transmission lag and increase measurement response speed 

Easy for inspection and drainage 

(2) Pressure guiding tube material and diameter 

Material: The material should be selected from alloys that are resistant to high temperatures and corrosion, such as 316L stainless steel, Inconel, etc. 

Pipe diameter: It is recommended to choose stainless steel pipes with a diameter of Φ14×2 or Φ18×3. This will ensure strength while reducing volume lag. 

Slope: The pressure guiding tube should be inclined towards the transmitter to facilitate the return of condensate and prevent liquid accumulation. 

(3) Insulation of the pressure guiding tube - A necessary procedure for winter measurements 

In areas with lower temperatures, such as regions where the temperature is below 10℃ in winter, the pressure guiding pipe must be insulated. 

The purpose of insulation: 

Keep the temperature of the pressure guiding pipe higher than the dew point temperature of SO₃ to prevent condensation from blocking. 

Ensure that the pressure transmitter operates within the normal temperature range to avoid the impact of low temperatures on accuracy. 

Reduce the impact of temperature fluctuations on measurements 

Thermal insulation measures: 

Use electric or steam heating to maintain the temperature of the pressure guiding pipe above the SO₃ dew point. 

The insulation materials should be selected from those that are resistant to high temperatures and corrosion, such as silicon dioxide-alumina fibers and rock wool. 

The insulation layer should be tightly wrapped to prevent heat loss. 

(4) Regular cleaning and maintenance 

Regularly check the unobstructed condition of the pressure guiding pipe. If any blockage is found, clear it promptly. 

Regularly calibrate the pressure transducer to ensure measurement accuracy

V. The "Three-Step Strategy" for Enhancing the Reliability of the System's Instruments 

5.1 Step 1: Scientific Selection - "Tailoring to Fit" 

The first step to enhance the reliability of on-site instrument measurements is to make scientific selections based on process parameters. 

When making a selection, one must have a comprehensive understanding of: 

Medium properties (composition, concentration, corrosiveness, chemical properties, etc.) 

Medium temperature (normal, maximum, minimum, fluctuation range) 

Medium pressure (normal, maximum, minimum, fluctuation range) 

Flow range (maximum, minimum, average) 

Operating status (continuous, intermittent, frequency of start-stop) 

Environmental conditions (installation area temperature, humidity, vibration, electromagnetic interference, etc.)

Selection principles: 

Applicability priority: Select the most suitable instrument for the specific working conditions, rather than the most expensive or the most advanced one. 

Redundant design: Key measurement points should consider backup instruments or redundant measurements. 

Standardization: Try to select mature and standardized products to facilitate maintenance and spare parts management. 

Maintainability: Consider the ease of calibration, cleaning and replacement of the instrument. 

5.2 Step 2: Standard Installation - "Proper Positioning" 

After the instrument selection is completed, the installation quality directly determines the measurement effect. During the installation process, the following must be met: 

(The installation location complies with the technical requirements of the instrument.) 

The sampling points are representative, avoiding dead corners and vortex areas. 

Convenient for daily maintenance, inspection and calibration 

Avoid areas with high temperature radiation, strong vibration, and strong electromagnetic interference. 

Leave sufficient room for operation 

(2) The installation method complies with the specifications. 

Temperature instrument: The insertion depth is sufficient, and the temperature sensing element is located in the area where the medium is fully mixed. 

Pressure gauge: The connecting pipe is short, straight, and has the correct slope, avoiding liquid accumulation and blockage. 

Flow meter: The straight pipe sections before and after the instrument meet the requirements, avoiding the influence of vortex and pulsating flow. 

Level gauge: Avoid the feed impact zone and the stirring vortex zone 

Valve: The flow direction is correct, the straight pipe sections before and after are sufficient, and the bypass design is reasonable. 

(3) Pipeline System Optimization 

The sampling pipeline should be as short and straight as possible, and minimize the number of bends and dead corners. 

The pipeline material is corrosion-resistant and heat-resistant, and the pipe diameter is appropriate. 

The pre-treatment system is complete (including filtration, condensation, dehumidification, pressure stabilization, etc.) 

Thermal insulation and heating measures are in place to prevent condensation and blockages. 

5.3 Step 3: Operation and Maintenance - "Continuous Care" 

After the instrument is installed and put into operation, the operation and maintenance are the key to ensuring long-term reliability. 

(1) Thermal insulation measures 

For measurement points exposed to high temperatures, low temperatures, or where the medium is prone to condensation, proper insulation must be ensured: 

The pressure-tapping pipe and sampling pipe are heated and insulated to maintain the temperature above the dew point of the medium. 

The instrument body is insulated to prevent the influence of environmental temperature on the measurement accuracy. 

Regularly inspect and replace the insulation materials to ensure the insulation effect. 

(2) Regular calibration 

Establish a regular calibration system for instruments: 

Analyzer: Calibrate once a month or once a quarter, and verify using standard samples. 

Temperature instrument: Calibrated once a year and compared with the standard thermometer. 

Pressure gauge: Calibrated once a year using a standard pressure source. 

Flow meters: Calibrate them regularly based on usage conditions, or use online comparison methods. 

(3) Preventive Maintenance 

Regularly check the unobstructed condition of the pressure guiding tubes and sampling tubes, and promptly clear any blockages. 

Regularly check the sealing performance of the instruments to prevent leakage and corrosion. 

Regularly inspect the electrical connections to prevent loosening and oxidation. 

Regularly inspect the integrity of the insulation layer and promptly repair any damages. 

(4) Fault Analysis and Improvement 

Establish an instrument failure file and analyze the causes of the failures: 

Is it a problem with the selection? Or an installation issue? Or a maintenance problem? 

Have similar faults occurred repeatedly? Are there any systemic issues? 

Based on the analysis of the faults, optimize the selection, improve the installation, and enhance the maintenance. 

VI. Conclusion: Enhancement of Instrument Reliability - The Foundation for the Intelligent Transformation of Sulfuric Acid Plants 

The intelligent and automated transformation of the sulfuric acid plant cannot be achieved without reliable and accurate on-site instrument measurements. From analyzers to control valves, from temperature instruments to pressure transmitters, the selection, installation, and maintenance of each type of instrument directly affect the accuracy of the measurement results and the automation level of the plant. 

In summary, to enhance the reliability of on-site instrument measurements in the sulfuric acid plant, three core aspects must be firmly grasped: 

First, scientific selection. Based on the operating conditions' parameters, tailor the type, specification and material of the instrument to match the specific conditions, avoiding the approach of "one-size-fits-all" and "blind pursuit of high-end". 

Second, standard installation. Select the installation locations strictly in accordance with the instrument technical requirements, optimize the pipeline design, improve the pre-treatment system, and ensure proper insulation and heating. This will guarantee the measurement conditions from the very beginning. 

Thirdly, meticulous maintenance. Establish a closed-loop management mechanism for regular calibration, preventive maintenance, and fault analysis and improvement, to continuously maintain the optimal working condition of the instruments. 

Incorporating these steps into regular management will help continuously improve the issues of inaccurate measurements and high failure rates, providing a more solid data foundation for the automatic control and stable operation of the equipment. 

Under the general trend of digital and intelligent transformation in the sulfuric acid industry, the improvement of instrument measurement reliability is not only a technical issue but also a management issue. Enterprises need to start from the top-level design, establish an instrument full life cycle management system, and incorporate selection, installation, and maintenance into standardized and institutionalized management, so as to truly achieve the goal of "data-driven decision-making and intelligent optimized operation". 

Li Zhenglai Technology (Chongqing) Co., Ltd. focuses on the sulfuric acid industry and can provide the industry with a complete set of technical services including scientific selection of instruments, guidance on installation, meticulous maintenance, personnel training, and standardized management. Welcome to contact us. Let's start from each measurement point and each instrument, to lay a solid foundation for the intelligent transformation of the sulfuric acid plant, and jointly promote the high-quality development of the sulfuric acid industry!