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400A THC analyzer sample pressure & flow control

The Rosemount 400A hydrocarbon analyzer has the ability to maintain control of its sample pressure at a given value by way of three major components:

  1. An internally mounted Siemens-Moore, 0-7 psig, backpressure regulator (BPR)
  2. A sample ‘frit’
    • Rosemount uses frits for particulate control and to control bypass rates; in essence, it is merely a restrictor that also has filtration capability
    • According to the Mott Corporation, “These frits protect HPLC (high pressure/performance liquid chromatography) columns from particulate contamination and distribute dissolved samples evenly for optimal column performance.”
  3. An unrestricted bypass vent line

To be perfectly stable and precise, everything has to be a constant. That is, the BPR has to maintain its characteristics, the frit has to exhibit the same pressure/flow characteristics, the exhaust must remain open and ‘appear’ as near atmospheric pressure, and the sample pressure must remain constant. Let’s look at these in a little more detail.

The BPR:

  • Maintaining a constant pressure at the head of the capillary is critcal to ensure a consitant sample flow rate to the burn and is the primary function of the BPR.
  • As upstream sample pressure increases, the BPR valve will open and allow more flow to the bypass vent while maintaining the setpoint pressure. 
  • As upstream sample pressure decrease, the BPR valve will close, causing less flow to the bypass vent whie maintaining the setpoint pressure.
  • Therefore, the bypass rate for a normally operating system should be set high enough to accomidate a reduction of sample supply pressure, but low enough to allow for increases in pressure without flooding the bypass vent.  Rosemount recommends .3 to 3L/min of bypass which should be set by controlling the sample pressure to the analyzer after the the desired BPR setpoint is acchived.

The Frit:

  • A normal frit will allow the Rosemount 400A to create 1500 cc/min of bypass with the BPR set to 5 psig and the sample delivery pressure set to about 8 psig. 
  • A fouled frit will require a higher sample source pressure in order to maintain the same bypass rate. You could also lower the BPR setpoint but in our application the internal sample pressure must be held at a constant value in order for the sample capillary to deliver the correct sample flow to the burner.
  • Removal of the sample frit will cause internal pressure control to be almost impossible and will result in either low unstable pressure control when sample source pressure is below the 5 psig of the BPR, or unpredictable pressures when sample source pressure is above the 5 psig setpoint of the BPR.  When  the flow limitations of the BPR are exceeded the BPR will  start “singing” as it attempts to shunt the excess pressure by dumping significant volumes of sample to the bypass vent line.

The Bypass:

  • The bypass vent line should be unrestricted, however, it is not as critical to pressure control as the first two items. 
  • Restrictions in the vent line will result in increased pressure at the capillary head which will increase the display reading.