Thermocouple Pressure Drop in Pipe
Thermocouple Pressure Drop in Pipe
(OP)
How do you calculate the pressure drop in a pipe due to a thermocouple in the middle of the fluid stream?
I have a 4" pipe with water flowing through it and a thermocouple will be permanently inserted into the stream. I see thermocouples can be inserted vertically or at an angle along the direction of flow.
I have a 4" pipe with water flowing through it and a thermocouple will be permanently inserted into the stream. I see thermocouples can be inserted vertically or at an angle along the direction of flow.





RE: Thermocouple Pressure Drop in Pipe
If I was doing this I would probably just neglect it.
Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com
RE: Thermocouple Pressure Drop in Pipe
Anyway, I have never heard of anyone doing this calculation, but I have left word with one of the vendors. I'll let you know what they have...
RE: Thermocouple Pressure Drop in Pipe
Granted, this is not an orifice plate but I still doubt the dP will be that high. Interesting question, I've seen vibration calculations routinely done for thermowells but never a pressure drop. Sometimes I've seen them swedge up the line to insert the thermowell or alternatively, put it in a elbow facing into the flow.
RE: Thermocouple Pressure Drop in Pipe
Man that is a BIG TW. I've never seen one that large - a typical process plant standard TW is, like, 1/2" OD, no?
Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com
RE: Thermocouple Pressure Drop in Pipe
A BIG corrosion allowance.
RE: Thermocouple Pressure Drop in Pipe
K EQUIVALENT - THERMOWELL
K = [A (sub P) * A (sub TW)] / {[A (sub P) - A (sub TW)]^2}
A (sub P) = OPEN AREA OF PIPE (SQ. IN.)
L (subTWe) = ROJECTION LENGTH OF THERMOWELL END INTO PIPE (IN.)
W (subTWe) = PROJECTION WIDTH OF THERMOWELL END INTO PIPE (IN.)
L (subTWs) = PROJECTION LENGTH OF THERMOWELL SHANK INTO PIPE (IN.)
W (subTWs) = PROJECTION WIDTH OF THERMOWELL SHANK INTO PIPE (IN.)
A (subTW) = PROJECTION AREA OF THERMOWELL INTO PIPE (SQ. IN.)
The projection will vary depending on whether a straight -olet or latrolet is used.
RE: Thermocouple Pressure Drop in Pipe
RE: Thermocouple Pressure Drop in Pipe
RE: Thermocouple Pressure Drop in Pipe
The area is calculated for the end and shank separately as they were different for the thermocouples we used. The two areas combined give the total area used for the calculation.
RE: Thermocouple Pressure Drop in Pipe
Patricia Lougheed
RE: Thermocouple Pressure Drop in Pipe
Also, what do you mean by "THERMOWELL?" Is this the K for the Thermowell?
RE: Thermocouple Pressure Drop in Pipe
In my pursuit of accuracy, I constantly have to remind myself that the various friction loss equations out there (Darcy, BBM, etc., etc., etc.) are only close approximations to the 'real' friction loss. You will never be able to match your dP calcs to field-measured dPs. The best I've gotten is within about 8%, but that is plenty close enough, even on pump suctions, because you're always going to add 2' to the manufacturer's published NPSHr before you put your design to bed, right?
I guess my point is - I don't see that the loss across a TW is worth determining, given the big picture. Enter engineering judgment...
SuperG, just curious, why do you need to account for the dP across this device?
Thanks!
Pete
pjchandl@prou.com
RE: Thermocouple Pressure Drop in Pipe
RE: Thermocouple Pressure Drop in Pipe
The pressure drop across the thermowell is not the only problem.
Sounds like the customer is having problems with that measurement, hence the overly heavy design being used.
1. Calulate the pressure drop (only an estimate) of a sudden restriction imposed by the thermowell. Crane or Camron's hydraulic handbook will work.
2. Determine why such a heavy walled thermowell is being used.
3. None of the thermowell manufacturers can offer much help in such applications. Seems like there are related articles on thermowells in Hydrocarbon processing (nov. 2001)and in the oil & gas journal(april 2002).
What is your flow rate? I'll run a quick calc using some FEA thermowell software we use for identifying the flow induced stresses.
Measurement errors (response time)will be a problem if the well is as thick as you've stated.
RE: Thermocouple Pressure Drop in Pipe
Flow induced vibrations are not a problem in condensate, as long as you are keeping the fluid velocity with in reason.
However, wells of this size are typically intended for super-critical service.
You don't want the thermowell to block more than 10-15% of the flow area, if it does then you must resize the pipe to to keep the superficial velocity within the ratings established for that class of piping.
good luck
RE: Thermocouple Pressure Drop in Pipe
Perhaps these websites can help you a bit further:
http://www.pressure-drop-calculator.com/ (freeware)
or, even better:
http://www.pipeflow.co.uk/ (30-day trial)
For the thermowell I would say: use a smaller one or use a temperature sensor which is submerged in the pipe-wall. I know Yokogawa has them. Then you avoid any pressure drop problems!
Good luck & Happy New Year!