Gas and Process Sample Transport Systems for Analyzer Applications















"As many as 50% of the process analyzers installed today are either not in service or fail to provide reliable results."

 

Improperly selected tubing bundles are one of the major contributors to this complaint. 

Tubing Bundles
Specifying the right tubing bundle for analyzer applications involves specific information about:

Design Considerations

O’Brien products are designed and manufactured to provide predictable and repeatable results. O’Brien’s design team focuses on four criteria:  

  1. Finite element analysis 

  2. Arithmetic heat transfer modeling 

  3. Chamber testing and 

  4. Failure mode analysis per IEEE 515 standard for electric traced bundles.

Combining the results of these studies provides what the industry regards as the best sample transport system available.

O’Brien  enjoys the industry reputation as  the ultimate solutions provider in analyzer sample transport systems. TRACEPAK® sample transport bundles are in service in thousands of applications around the world and our experience with sample transport tubing and conditioning is second to none. The primary considerations of sample transport tubing would fall under the following:

  • Pressure

  • Temperature

  • Flow

  • Corrosion / Metallurgy

Sample Composition
Know what the composition is for the sample you will extract and transport to the sample conditioning cabinet. It is not enough to know the theoretical norm, you must also know what the composition will be under all conditions that can reasonably be expected. Those include shut-down, start-up and upset conditions during operation. 

Sample composition is not limited to the chemical composition but also the physical state: pressure, phase (or phases), and temperature.

Acceptable Delay
What is an acceptable delay in the sample cycle time? Often the cycle time is often narrowly calculated as the time lapse between extraction and completing the analysis. If the analyzer is part of our process doesn't it follow that we should also include the time to affect the process fluid based on the results of the sample analysis in any cycle time calculations? 

Beyond the selection of where the sample will be taken and the location of the analyzer shelter there are several variables that affect the total cycle time or delay. Establish an acceptable delay for the system. Sample size, process phase, pressure drop and run lengths will  become part of the decision process in specifying the sample transport tubing.

Dew and Freeze Point
You will need to know the minimum temperatures allowable consistent with maintaining an accurate and representative sample. Determining the lowest dew point for any component in a gas sample is just as critical as determining the freeze point of a liquid sample. 

In both cases (dew point and freeze point) hotter is not always better, nor does it always lead to an economic or a simpler lower maintenance system. Generally the hotter the process must be maintained, the more complex the installation and control scheme becomes.

You should set window of allowable operating temperature as wide as possible determined by the process fluid itself not the accuracy of system components. Deal with component issues separately otherwise we end up adding safety factor on top of safety factor and creating a system more complex and expensive than really necessary to achieve reliable results.

Sample Temperature
Determine the maximum temperature of the sample during normal and upset conditions. This will influence the tubing, heater, process connections, bundle configuration and installation details.

Ambient Temperature
What are the minimum and maximum ambient temperatures. Consider the point at which the sample is taken not just the climatic average for the region. The ambient temperature around an ethylene furnace is likely to be 15C higher than the rest of the plant. 

Heating Media
Electric is the most widely used but you must confirm that sufficient power is available for the installation. Determine what voltage is available also. Steam may be readily available and if the supply is trouble free and reliable steam can be a very good choice for heating sample lines. 

Location, Location, Location
How long a sample line is required to go from the sample point to the sample conditioning cabinet? Obviously this affects the cycle/delay time. The length must include detours for routing along existing pipe racks and supports. Determine the area classification at each end of the sample run. It is permissible to route a Division 2 approved assembly through a Division 1 area as long as there are no tees, splices, or other joints in the assembly and as long as T-ratings of the heater are appropriate for the Division 1 area being traversed.

Sample System Design
Is this a fast loop? Is the return point near the extraction location? Is the waste sample going to be routed to a flair? These determine the number of tubes needed in the sample bundle.

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