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

In summary, the conversation discussed the concern of adding a temperature probe to a 4" water pipe downstream of a centrifugal fill pump. The individual considered using the drag force equation, but was unsure if it was being used correctly. They then asked for suggestions on how to determine the allowable stress for the probe and how to verify its ability to withstand potential breaking. It was recommended to consult ASME Section II-D for allowable stress values and to be cautious of flow-induced vibrations. The temperature and material of the probe were also mentioned, with the basic allowable stress value for SA-304 being 20,000 PSI. The external pressure of the pipeline and the internal flow rate of the water were also discussed. Finally, it was advised to ensure
  • #1
steves1080
65
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?

Thanks,
Mike
 
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  • #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.
 
  • #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.
 
  • #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?
 
  • #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.
 
  • #6
No, what is the pressure in the pipeline? That is pressure that will become the external pressure on the probe.
 
  • #7
My apologies. As I stated above, the maximum internal pressure of the pipeline under flow will be 120 psig.
 
  • #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.
 
  • #9
Thanks for the input
 
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Likes CFDFEAGURU
  • #10
How could one calculate the effects of any flow-induced vibrations?
 

1. How do you calculate the drag force on a temperature probe inserted in a water pipe?

The drag force on a temperature probe can be calculated using the formula Fd = 0.5 * p * Cd * A * V^2, where p is the density of the fluid, Cd is the drag coefficient, A is the cross-sectional area of the probe, and V is the velocity of the fluid.

2. What is the significance of the drag coefficient in calculating the drag force?

The drag coefficient is a dimensionless value that represents the level of resistance encountered by an object moving through a fluid. It takes into account factors such as the shape and size of the object, as well as the properties of the fluid, and is essential in accurately calculating the drag force on a temperature probe.

3. Can the drag force on a temperature probe be affected by the fluid's temperature?

Yes, the drag force can be affected by the temperature of the fluid. As the temperature increases, the density of the fluid decreases, resulting in a decrease in the drag force on the probe. Additionally, changes in temperature can also affect the viscosity of the fluid, which can impact the drag coefficient and the resulting drag force.

4. How does the velocity of the fluid impact the drag force on a temperature probe?

The drag force is directly proportional to the square of the velocity of the fluid. This means that as the velocity increases, the drag force also increases. Therefore, the faster the fluid is flowing, the greater the drag force on the temperature probe will be.

5. Are there any other factors that can affect the drag force on a temperature probe?

In addition to the fluid's temperature and velocity, other factors that can impact the drag force on a temperature probe include the roughness of the probe's surface, the angle at which it is inserted into the fluid, and the presence of any obstacles or obstructions in the pipe that may cause turbulence.

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