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Catching Samples

July 15, 2014

Sample-Valve-1
The material in this post is extracted from Chapter 3 of the book Plant Design and Operations.

Drawing liquid hydrocarbon samples from process equipment will expose small quantities of the material to air, thus potentially creating a flammable vapor mixture. Similarly, draining water from the bottom of a hydrocarbon tank to an open drain is likely to allow some hydrocarbon to be exposed to air before the drain valve can be closed. In cases such as these, the following precautions should be considered:

  • Operators should stand where they can immediately shut off the flow of liquid if a fire or large spill should occur.
  • If self-closing valves have been provided these should never be blocked or tied open.
  • An open valve should never be left unat­tended.
  • All open-end connections should be plugged when not in use.
  • The amount of sample flush should be minimized and all flush should be routed to an appropriate safe collection system or location.

If the distance between the body of the material to be sampled and the sample point itself is long, thus requiring a long flush time, a circulation loop should be set up as shown in Figure 3.1. Normally valves A and B and the sample valve are closed. When a sample is to be taken, A and B are opened for a sufficient period of time to allow fresh material to flow through the sample circuit. The sample valve is then opened and the sample caught (in a bottle for liquids and a sample bomb for gases). Then all three valves are returned to the closed position. It is important not to leave A and B open because otherwise the control valve will not be able to fully stop the flow of fluid during normal operations. Also, if they are closed and the sample valve is opened by mistake, not much liquid will escape.

Figure 3.1
Sample Valve Flush

Sample-Valve-2

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