Switching valves

• Switching Valves


  Valves that play a very important part in the proper operation of a Process Gas Chromatograph (PGC)
• A faulty valve could result in erroneous results from the PGC
• Gas chromatograph valves


  Also known as analysis valves or analyzer valves

  
• Typical functions of valves in PGC

  • Sample Injection
  • Column Switching
  • Atmospheric Referencing (or Atmospheric Balance)
• Atmospheric Referencing Valve


  1. Stop the flow of vapor sample just prior to sample injection using a sample shut off valve (SSO)

  2. Actuate the atmospheric reference valve (ARV) to allow the trapped sample in the sample loop to equilibrate with atmospheric pressure

  3. Timing of actuation is between 3 to 5 sec
• Ideal Gas Law


  PV = nRT, where n = # of mole, R = real gas constant
• In a PGC, with a fixed sample loop and isothermal oven, the Ideal Gas Equation becomes P α n, hence the number of moles that are trapped is solely a function of the pressure in the loop
• Process pressure


  Less stable than desired, subjected to process upset
• Using higher process pressure


  Will result in costlier PGC components
• Atmospheric referencing


  Referencing the sample loop to atmosphere pressure prior to sample injection to ensure a constant, lower and repeatable injection pressure
• Sample Injection Valves


  Function is to inject and deliver to the head of the column a constant volume of sample of the process stream to be analyzed
• Types of Sampling Valves

  • Rotary Valves
  • Slider Valves
  • Plunger Diaphragm Valves
• Sample Injection - Rotary Valve

  1. Deenergized position: Carrier gas flows into the column, process sample flows through the sample loop

  2. Energized position: Valve rotates through 60 degrees, carrier gas sweeps the metered volume of sample into the column

  3. After a few seconds, valve is deenergized to refill the sample volume ready for the next injection
• Rotary Valves


  • Typically used to inject very small amounts of liquid sample (less than 1μL)
• Rotary Valves


  • Deenergized position: Not an analysis
  • Energized position: An analysis
• Sample Injection - Slider Valve


  Physically transfer a metered volume of the process fluid from the sample flow into the carrier gas flow by means of a moving plate or slide
• Slider Valves


  • Slides for liquid samples may have a "straight-through" configuration or a "cavity" type sample volume
• Sample Injection - Plunger Diaphragm Valve

  1. Valve consists of six ports and six plungers, arranged in a circular pattern

  2. Plungers are operated in alternate sets of three

  3. Mechanical interlock ensures normally closed passages are opened during valve actuation
• Plunger Diaphragm Valve


  Small size allows very high sealing pressure on the diaphragm
• Potential problems with Plunger Diaphragm Valves are diaphragm leakages of minute quantities of liquid samples in the diaphragm material or at the points where the diaphragm is pinched
• Column Switching - Rotary Valve

  Rotary valves are almost exclusive choice for column switching
• Column Switching - C2 Application

  1. At sample injection, V1 is switched to allow the Carrier 1 to push the sample gas in the sample loop into Col. #1

  2. After sample injection, V1 switches back to the original position and the carrier gas continues to push the sample gas into the column

  3. After the sample passes through Col. #1 and the separated components flow to Col. #3, before all the separated components can pass through V2, both V2 valves switch to push the backend from Col. #1 back to Col. #1 and the frontend into Col. #3

  4. After the frontend passed through Col. #3, both V2 valves switch back to original position
• Column Switching - Methly-acetylene/Propadiene Application

  Creative use of rotary valves and columns have reduced the time required to measure a component of interest from hours to minutes
• Common Column Switching Techniques

  • Backflush
  • Heart-cut
  • Trap and bypass
• Backflush Technique


  To reduce the total analysis time by flushing unwanted heavier components to vent, to remove components that would interfere with the required separation, to remove components that would damage one of the columns, to regroup a number of components for measurement, and to maintain good performance by ensuring that the columns do not accumulate impurities from the sample
• Backflush Technique


  Normal operation and backflush operation
• Heart-Cut Technique


  To remove most of a major component, to reduce the amount of a major component reaching the second column down to a concentration level that does not tail, to increase the amount of the measured component relative to the major component to 1 in 20 or better, and to remove other components that may interfere with the analysis
• Heart-Cut Technique


  Normal operation and energized operation
• Trap and Bypass Technique


  Used to capture the lighter components that are harder to separate and to bypass the heavier components that are easier to separate
• Trap and Bypass Technique


  The lighter components that elute from Column 1 are allowed to flow into Column 2 and trapped, while the heavier components pass via Column 2 Simulator
• Rotary Valve


  • Field proven high Reliability, 4 port / 6 port, single side/double side action
• Rotary Valve Usage


  Sample Valve (Gas/Liquid), Back Flush, Column Switching, Stream Switching, Atmospheric balancing
• Rotary Valve Sample Volume

  Min 360 micro litter (Gas), Min 0.33 micro litter (Liquid)
• Rotary Valve - Parts


  Mounting screw, Spring coil, Valve Sheet, Valve Grand with pipe, Washer, Ring Screw, Valve gland
• Rotary Valve - Operations

  Injection Mechanism, Pinion for sample injection, Carrier gas flow in and out
• The study material is adapted from Yokogawa instruction manuals