# How do I qualify the pressure in this engineering situation? **URL:** \ **Category:** Factual Questions\ **Created:** [December 7, 2017, 10:49pm UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847 "2017-12-07T22:49:04Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [December 7, 2017, 10:49pm UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847/1 "2017-12-07T22:49:04Z")
I have a Pitot tube sealed into the side of a long, straight duct. I’ve got a packing gland with nylon ferrule that fits nicely to the 1" OD tube and holds it in place. Inside the pipe is flowing, industrial process gas headed toward an exhaust vent. Let’s back-up, and assume I have 15psig at the 1”FNPT sampling port before I remove the gauge and insert the pitot. Does the ferrule connection only have to deal with 15psi, or is the pressure affected by the pitot’s surface area? Have I explained the situation clearly and/or given enough info? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 7, 2017, 11:14pm UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847/2 "2017-12-07T23:14:31Z")
You only have to worry about the 15 psi.If you are concerned about the Pitot having a “piston” effect, don’t be - it’s only the pressure normal to the area of the opening that matters. ---
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [December 7, 2017, 11:17pm UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847/3 "2017-12-07T23:17:36Z")
Yes sir, that was my concern exactly. Thank you. Hypothetically, if I had both tail ends of the Pitot open to atmosphere, would that affect anything? ---
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [December 7, 2017, 11:40pm UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847/4 "2017-12-07T23:40:04Z")
Though the question’s been answered, [I’ve found and linked an image for those of you playing along at home.](http://www.dwyer-inst.com/images/AirVelocity-fig_pic3.gif) It’s not exactly what we’re doing, but’s it’s pretty close. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 7, 2017, 11:40pm UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847/5 "2017-12-07T23:40:31Z")
I don’t think so, but I can’t visualize this. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [December 8, 2017, 12:49am UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847/6 "2017-12-08T00:49:43Z")
As a process engineer, you’d need to think about these additional questions : 1. Is the gas being vented hazardous ? And is the connection (the ferrule you talk about) rated for the temperature / corrosivity of the gas ? If the duct is large and the flow high, have you considered flow induced vibrations in the pitot tube that may require reinforcing the connection ? 2. Is a pitot tube the best instrument in this application considering : a. Pitot tubes require a straight run of duct (a few times the duct diameter). b. Pitot tubes are calibrated for Gas molecular weight (density in this case). Is the vent gas molecular weight fairly constant ? (Typically vent gases vary in composition I.e. density) c. Pitot tubes are sensitive to entrained liquids in gases. Vent gases can at times carry entrained liquids that may disturb readings/plug up. d. Vent gases can be corrosive and if the pitot is not of the right material or the connection presents a low point on the duct for liquids to accumulate, you will have reliability issues. Best is to have the pitot connection at the top of the duct. An ultrasonic flow meter is the industrial norm for these applications Thanks ---
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [December 8, 2017, 2:33am UTC](https://boards.straightdope.com/t/how-do-i-qualify-the-pressure-in-this-engineering-situation/803847/7 "2017-12-08T02:33:04Z")
Excellent questions. Even though (in this case) I’m only concerned with the packing gland and ferrule, these are all valid considerations. The Pitot is our go-to instrument in most of these cases. We’re just looking for a _snapshot_ of the flow during the same period that we sample for various pollutants. I’m assuming it’s harmless. The client has confirmed that hazardous/explosive/corrosivity is of no concern, but I didn’t ask @ the specific process. Valid measurements are required to be at least 8 diameters downstream and 2 diameters upstream from a flow disturbance. If those conditions aren’t met, we’re to collect more points of data for the same area. We’re assuming dry conditions, or at least dry enough to get us through an hour with minimal headache. Many of these are combustion sources where we can assume a MW based on the fuel source, etc. Depending on the ultimate use of the data (e.g. permitting, engineering), the flow numbers from the pitot are compared to the plant’s ultrasonic, or other instrumental, flow apparatus.