Barry Landaiche| Oct 12, 2011

Do you want to find the elusive holy grail of analyzer reliability?

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Finding answers may be easier than you think!

Often, the root cause of analyzer unreliability is inadequate sample system design, not mechanical failure on an analyzer.   Many times a “cookie cutter” approach is taken on sample systems.  Many people will say, “What works for one application, will work for similar ones across the board.”  I put this in the same category with, “The check is in the mail.”  You can believe what you want, but I can show scars of battling numerous sample systems that the cookie cutter approach was not the right approach!

Several years ago, I had the pleasure of coordinating the installation of a continuous emission monitor system (click here for more information on CEMS).  The engineers on the project wanted to use the design of an identical system installed on a similar incinerator stack in another part of the refinery.  The existing system was operating at nearly a 99% reliability rate.

Since both incinerators were similar and the existing system was operating well beyond expectations, their conclusion was to use the exact design specs and parameters on the second unit – the cookie cutter approach.

The new system was installed and commissioned.  After several months of operation, the reliability was near zero and regulatory fines were imminent due to non-compliance.  The engineering department could not understand how two identical systems on two similar stacks could operate so differently!

After several weeks of investigation, it was determined that the sample transport line was operating at or slightly below the dew point of the stack gas.  The sulfur compounds in the gas were solidifying in the sample line, causing a loss of sample.   It was also determined, that the stack gases of the two incinerators were not similar at all.  The process gas components and their concentrations were overlooked in the design phase of the project.  The “cookie cutter” approach did not work.

The costs for not considering the new installation specifications on its own merit were: regulatory fines due to non-compliance, several hundred man-hours of maintenance and engineering work, and replacement of a 200 ft sample transport bundle.

The total monetary costs amounted to over $150,000!  The low reliability was not caused by the analyzer but caused by the sample system.

The key to a reliable analyzer is to include a sample system to deliver a compatible sample. I’ve adopted this definition: a reliable sample system is one that will not harm the analyzer nor prevent it from producing a reliable analytical result.  I would agree that the definition seems to be simple; however, putting this theory into practice is not quite that easy.

A sampling system must meet all of the following to be reliable:

  • Compatible – A compatible sample system will not harm the analyzer, nor prevent it from producing a reliable analytical result.
  • Timely – A timely sample is one that provides an analytical result with an acceptable delay from real time.  Some delay is inevitable but should be minimized.  Note:  Delay is calculated from the time the process sample is refreshed (including the volume of process piping, vessels, drums, etc.) to the time the analysis appears within one analysis cycle.
  • Representative – A representative sample is one that provides a meaningful analytical result useful for its intended purpose.
  • Reliable – Keep the system simple, easy to repair and prioritize maintenance by $$ return.  Most importantly, is the analysis believable?
  • Cost-Effective – Know the cost benefits of the system.  Costs include maintenance, spare parts, consumables, utilities, wasted or recycled process, lost opportunities and process down time.
  • Safe – ALL SAFETY PROCEDURES APPLY…THEN SOME!  Consider chemicals, support utilities and environment used to support the “system”.

 

Remove any one point and there would be no sense in moving forward with the design or installation!

To be clear, a sample conditioning system is a subassembly of an analyzer system that changes the sample in a particular way, such as removing solids or liquids, reducing the pressure and temperature, vaporizing a liquid sample, or condensing a vapor sample.  The sample conditioning system is usually designed and fabricated offsite.  Often, it is assembled on a plate or into an enclosure.

The purpose of sample conditioning is to modify and control the condition of the sample to ensure that it is always compatible with the analyzer.

Compatibility is achieved by carefully:

  • Controlling the sample pressure, flow, and temperature to stay within acceptable limits.
  • Using a filter, kinetic separator or cyclone to remove solid particles large enough or numerous enough to damage the analyzer or block the sample flow path.
  • Using a coalescer, kinetic separator, knock-out pot, or membrane filter to remove entrained liquids from a gas or liquid sample.
  • Keeping the sample all in the gas phase or all in the liquid phase (never a mixture of both!) by maintaining appropriate temperature or pressure.
  • Operating all components within their specified constraints.
  • Selecting durable materials that are not damaged by sample or the working environment; metals or plastics that resist corrosion and elastomers that maintain tight seals.

 

The absolute #1 goal of any analyzer project is to include the correct design of a compatible sample conditioning system.  Unfortunately, this is where the least attention is given.

So, if your analyzer system is not reliable, can the root of the problem be traced to mechanical failure or is the sample conditioning system causing the problems?


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