Online Sampling Systems for Process Analyzers

A fast loop in a sampling system is a design element used primarily in process industries, such as chemical, petrochemical, oil and gas, and refining industries, to ensure that the sample presented to the analyzer is fresh and representative of the current process conditions. The fast loop is part of the online sampling system and plays a critical role in maintaining the integrity and timeliness of the sample.

Function and Purpose

The main function of a fast loop is to rapidly circulate a portion of the process fluid (gas or liquid) from the process pipeline to the analyzer and back to the process or to a vent/flare system. This continuous circulation at a high flow rate serves several purposes:

1. Timeliness: It ensures that the sample reaching the analyzer is fresh and accurately represents the current process conditions.
2. Cleanliness: The high flow rate helps keep the sample line clean by preventing sedimentation or the accumulation of particulates, which can clog the system or lead to sample contamination.
3. Conditioning: It helps in maintaining the sample in its native state, preventing phase changes (like condensation in gas samples or vaporization in liquid samples) that might occur due to pressure or temperature changes.

Components of a Fast Loop

A typical fast loop system includes the following components:

• Sample Tap: Where the fast loop is connected to the main process line.
• Fast Loop Pump (if required): Ensures high flow rate for liquids; for gases, the process pressure might be sufficient to maintain flow.
• Fast Loop Tubing: Larger diameter tubing than used in the final sample line to the analyzer to minimize pressure drop and ensure adequate flow rate.
• Filter/Strainer: Removes particulates to protect the analyzer and ensure sample purity.
• Flow Control Valve: Regulates the flow rate in the fast loop.
• Bypass Loop: Allows the sample to circulate back to the process or to a safe disposal point if not drawn into the analyzer.
• Sample Cooler/Heater (if necessary): Conditions the sample to the required temperature before analysis.
• Pressure Regulator: Adjusts the sample pressure to suitable levels for the analyzer.

Operational Considerations

• Flow Rate: The flow rate in the fast loop must be sufficiently high to ensure sample freshness but balanced against the need to minimize process fluid wastage.
• Design: The loop should be designed to minimize dead volumes where the sample might stagnate, ensuring that the entire sample path is swept by the fast loop flow.
• Maintenance: Regular maintenance is crucial to ensure the fast loop and associated components like filters and valves function correctly and do not introduce sample bias or errors.

Fast loops are essential in scenarios where the sample’s representativeness, purity, and condition directly impact the reliability of the analysis and subsequent process decisions. They are a critical component in ensuring the accuracy and effectiveness of online sampling and analysis systems in continuous processing environments.

A transport line in the context of sampling systems is the conduit through which a sample (gas, liquid, or sometimes slurry) is moved from the point of extraction (the process stream) to the analyzer or collection container. The design, installation, and maintenance of the transport line are crucial for the integrity and representativeness of the sample, ensuring that the analysis reflects the actual conditions of the process stream from which the sample was taken.

Key Aspects of Transport Lines

• Material: The material of the transport line must be compatible with the sample to prevent chemical reactions, adsorption, or any other interactions that might alter the sample composition. Common materials include stainless steel, Teflon, and other polymers, chosen based on the sample’s chemical properties and the required durability.
• Diameter and Length: The internal diameter and the total length of the transport line affect the speed and efficiency of sample delivery. Smaller diameters and shorter lengths reduce the volume of the sample in transit, leading to faster response times but may increase the risk of clogging or pressure drops, especially for liquid samples with particulates or high viscosity.
• Temperature Control: Temperature control, either heating or cooling, may be necessary to maintain the sample in a stable state. For example, heating lines prevent condensation in gas samples, while cooling might be necessary for volatile liquid samples to prevent phase changes or losses due to evaporation.
• Pressure Control: Maintaining the appropriate pressure is crucial to prevent phase changes in the sample and to ensure that the transport is efficient. Pressure regulators and pumps may be part of the transport line system, especially in long lines or when transporting liquids.
• Purge and Cleaning Systems: Incorporating purge and cleaning capabilities into the transport line design can significantly enhance system reliability and longevity, especially for samples that may deposit residues or corrode the line.

Challenges and Solutions

• Adsorption and Absorption: Certain sample components might stick to the walls of the transport line (adsorption) or dissolve into the material of the line itself (absorption), leading to sample loss. Using inert materials and conditioning the line can mitigate these issues.
• Dead Volume: Areas in the transport line where the sample might stagnate (dead volume) can lead to contamination or dilution of the sample. Proper design aims to minimize or eliminate dead volumes.
• Time Delay: The time it takes for the sample to travel from the process stream to the analyzer is a critical factor, especially for processes that require rapid adjustments based on sample analysis. Minimizing the length of the transport line and optimizing flow rates are common strategies to reduce time delays.
• Pressure and Temperature Effects: Uncontrolled pressure and temperature can lead to phase changes in the sample, altering its composition. Transport lines often include pressure and temperature control elements to maintain sample integrity.

A well-designed and properly maintained transport line is essential for the accuracy and reliability of the sampling system, ensuring that the analysis truly reflects the conditions of the process stream.

A sampling conditioning system is an integral part of a process sampling system, designed to prepare a sample taken from a process stream (gas, liquid, or slurry) for analysis. The conditioning system modifies the sample to a state that is suitable for the analyzer, ensuring accurate and representative measurements. The specific components and configuration of a sampling conditioning system can vary widely depending on the nature of the sample and the requirements of the analysis.

Functions of a Sampling Conditioning System

• Temperature Control: Adjusts the sample temperature to optimal levels for analysis. For gases, this might mean heating to prevent condensation; for liquids, cooling might be necessary to stabilize volatile components.
• Pressure Regulation: Reduces or stabilizes the sample pressure to levels suitable for the analytical equipment, preventing damage to sensitive instruments.
• Filtration: Removes particulate matter from the sample to prevent clogging and damage to the analyzer and to ensure that the analysis reflects the soluble or gaseous components of interest.
• Phase Separation: Separates different phases (e.g., gas from liquid, liquid from solids) to ensure that only the phase of interest is analyzed, which is crucial for multiphase samples.
• Stream Selection: In systems where multiple process streams are monitored, a stream selection component directs the desired sample to the analyzer.
• Flow Control: Manages the flow rate of the sample to the analyzer, ensuring a consistent and suitable volume of sample is analyzed.
• Sample Loop: In systems utilizing techniques like gas chromatography, a sample loop precisely measures the volume of sample introduced to the analyzer.

Components of a Sampling Conditioning System

• Coolers/Heaters: Devices to adjust the temperature of the sample.
• Pressure Regulators: Components that reduce the pressure of the sample to manageable levels.
• Filters and Strainers: Remove particulates from the sample.
• Membranes or Coalescers: Used for phase separation, they remove droplets of liquid from gas samples or separate immiscible liquids.
• Flow Meters and Controllers: Devices to monitor and adjust the sample flow rate.
• Valves: Used for directing the flow, isolating the sample, or stopping the flow entirely for maintenance.
• Sample Cylinders: Containers used for collecting and transporting samples, especially in offline systems.

Considerations and Challenges

• Compatibility: The materials used in the conditioning system must be compatible with the sample to prevent chemical reactions or adsorption, which could alter the sample composition.
• Maintenance: Components like filters and membranes may require regular maintenance or replacement to ensure consistent operation.
• Dead Volume: Minimizing dead volume (areas where the sample might stagnate) is essential to maintain the integrity of the sample.
• Sample Integrity: The conditioning system must preserve the sample’s representativeness, meaning it should not significantly alter the sample composition beyond making it suitable for analysis.

A well-designed sampling conditioning system is crucial for obtaining accurate, reliable analytical results, ensuring that the sample analyzed reflects the true conditions of the process stream.

4. Online Analyzers in Sampling Systems

What is an online analyzer?

An online analyzer in the context of process analyzers refers to an analytical device that is integrated into a process stream to provide continuous, real-time analysis of the material being processed. These analyzers are crucial in various industries, including chemical, petrochemical, pharmaceutical, oil and gas, and environmental monitoring, enabling immediate feedback and control over the production process. Online analyzers help optimize process efficiency, ensure product quality, maintain safety standards, and comply with environmental regulations.

Key Features of Online Analyzers

• Real-time Analysis: Online analyzers provide immediate data on the process stream, allowing for quick adjustments to maintain optimal process conditions and product quality.
• Continuous Monitoring: They monitor the process continuously, 24/7, providing a constant stream of data that can be used for process control and decision-making.
• Integration: Online analyzers are directly integrated into the production process, often with minimal or no need for manual intervention, making them an integral part of process control systems.
• Automation: Many online analyzers are fully automated, from sample extraction and conditioning to analysis and data reporting, reducing the potential for human error.

Types of Online Analyzers

Online analyzers can vary widely in their technology and application, depending on the specific components or characteristics they are designed to measure. Some common types include:

• Gas Chromatographs: Used for separating and analyzing compounds that can be vaporized without decomposition. They are widely used in the oil and gas industry.
• Mass Spectrometers: Offer detailed analysis of the chemical composition of a process stream by measuring the mass-to-charge ratio of charged particles.
• pH Meters: Measure the acidity or alkalinity of liquid process streams, crucial in many chemical and pharmaceutical processes.
• Conductivity Analyzers: Used to measure the electrical conductivity in a solution, which is directly related to the concentration of ions, important in water treatment processes.
• Infrared (IR) and Near-Infrared (NIR) Spectroscopy: Used for analyzing organic compounds and functional groups in a process stream by measuring the absorption of infrared light.
• Density and Viscosity Meters: Measure the density and viscosity of fluids, important for quality control in the food and beverage, chemical, and oil industries.

Considerations for Implementing Online Analyzers

• Sample Conditioning: Even though analyzers are online, the sample might still require conditioning (temperature, pressure adjustments, filtration) to ensure accurate analysis.
• Calibration and Maintenance: Regular calibration and maintenance are essential to ensure the accuracy and reliability of the measurements.
• Integration with Control Systems: Online analyzers are often connected to process control systems, allowing for automated adjustments based on the analyzer’s data.
• Safety and Reliability: Given their integration into the process, the safety and reliability of online analyzers are critical, especially in hazardous environments.

Online analyzers are key components in modern industrial processes, providing the data and insights needed to optimize operations, enhance safety, and ensure product quality in a wide range of industries.