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Calculate drag force on a temperature probe inserted in a water pipe

  1. Oct 3, 2014 #1
    I'd like to add a temperature probe to a 4" water pipe downstream of a centrifugal fill pump, but I am afraid of the probe shearing from being subjected to a constant force from the upstream pump (which pumps at about 400 gal/min). Naturally, my thought was to use the drag force equation, but I am not sure if I am misusing it. Bassically, I am using a Cd of 1, a reference area of 0.75 sq.in (assuming a thin rectangular plate of 1/8" by 3"), the density of water, and the velocity of water moving through a 4" pipe at 400 gal/min (which I calculated as 2.82 m/s). The answer I got was around 0.5 lbf, but this seems low to me. If it is in fact correct, how can I verify how much stress fatigue the probe is capable of undergoing until it is at risk of breaking?

  2. jcsd
  3. Oct 3, 2014 #2
    To find an allowable stress, what temperature is the probe going to be operating at and what material is it made out of? Then you can use the ASME Section II-D to find an allowable stress value.

    Also, you need to be cautious of flow induced vibrations causing the probe to fail.
  4. Oct 3, 2014 #3
    Thanks for the input. I expect 40F and it is a 304SS. Is there an easy way to look up ASME sections, or do you recommend being a member to acquire access to the full database of codes, etc.? And yes, good call on the flow-induced vibrations. I'll make sure to be conservative in my approach.
  5. Oct 3, 2014 #4
    For that temperature and material ( I am assuming is SA-312 SA-304) the basic allowable is 20,000 PSI and 1.5 times that value for membrane plus bending stress. If the pressure in the probe is low then the membrane stress is low. What is the external pressure of the pipeline that the probe is being inserted into?
  6. Oct 3, 2014 #5
    The pipe is located outside, so just atmospheric conditions. The internal flow rate of the water is about 400 gpm at a discharge pressure of up to 120 psig.
  7. Oct 3, 2014 #6
    No, what is the pressure in the pipeline? That is pressure that will become the external pressure on the probe.
  8. Oct 3, 2014 #7
    My apologies. As I stated above, the maximum internal pressure of the pipeline under flow will be 120 psig.
  9. Oct 3, 2014 #8
    Ok, as long as the probe is rated for that external pressure and the stress on it is less than the allowables, and provided it doesn't vibrate, it should work.
  10. Oct 3, 2014 #9
    Thanks for the input
  11. Oct 6, 2014 #10
    How could one calculate the effects of any flow-induced vibrations?
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