Lag Length VS Extension Length for Thermowell

No joy with Emerson over the phone, then?

Our rule of thumb is that the thermowell must be at least 10 diameters long.

Dear CarlWhat i am asking is the difference between a simple lagging extension and a nipple union based extension.

For the lagging extension in your paseted diagram i have found different purpose stated by different manufacturers. Mostly they say the purpose for lagging extension is to install thermowells on insulated lines. this extra length gets the transmiiter side of the well out of insulation.but this configration suggested by ashcroft (page 2 point 3) for example will keep the process connection trapped in the insulation.

http://www.ashcroft.com/tech/upload/AshThermWell8Steps.pdf

But emerson says that this length helps the temperature to drop on the transmitter side. so i am confused on the actual purpose of lag length??

The normal practice at where i work is in order to get the thermowell out of the insulation we increase the U-length(insertion length) this gets the flange out of the insulation. We dont rely on the lag length for this purpose.we keep a fixed 50mm lag length and a varying nipple union extension for the temperature drop.

P.S i hope the attachment can be read, if not let me know. Also Can you please explain what does emerson mean by the term "uninsulated distance from the process".

thanks

I don't quite understand your statement, 'getting the flange out of the insulation'.

Isn't your thermowell flange mating to a tank nozzle flange, where the tank nozzle is welded to the tank and fixed at some distance from the tank?

If you're not mating your thermowell flange to a tank nozzle flange, why use a flange?

If your thermowell flange is mating to the tank nozzle flange, the U length only determines how far into the tank the well extends. So I'm confused.

Most installations I've seen do what (I think) you do - the thermowell has a fixed lag length (relatively short, like yours) and nipples/pipe and a union are used to bring the wiring and the mounting point for the transmitter head out beyond the insulation. The reason I've heard is that the extension distance can be custom adjusted for whatever the insulation thickness is.

I assume Emerson's statement "uninsulated distance from the process" means how far the nipples and union extend the head/transmitter beyond the insulation, which allows for cooling.

may be i didnt say it right..

lets say i have a line of 14", i take out a nozzle from the line with length about 6" and then weld flange to that nozzle. the insulation on the line is 4".

Now what i will do is keeping an immersion length of about 6"(length of thermowellthat goes in the pipe) i will go for a U-length of about 12"(immersion length + nozzle length). And depending upon the high temperature i can select a nipple union extension or lag length that will keep my transmitter at a cooler temperature.

I think you agree to most of what i will do.

But have a look at what ashcrost says page 2 point 3

http://www.ashcroft.com/tech/upload/AshThermWell8Steps.pdf

"LAGGING EXTENSION & INSTALLATION WRENCH FLATS

is shown as "T" on engineering drawings when applicable. This is used when the

vessel or pipe into which the thermowell is inserted is insulated. This is the extra

length between the process connection and the instrument connection of a thermowell,

which is determined by the insulation thickness."

Ashcroft is using this lag length for a different purpose from the above one for the above line what they can do is keep a 6" immersion length (part that goes in the pipe) they can reduce the nozzle length to say 2" (even if the insulation is 4"). So with Ashcroft i can select a thermowell with U-length of 8"(immersion length +nozzle length) [note that the flange will be covered by the insulation]

and in order to get the other end out of insulation for mounting a remote transmitter i will select a lag length of 4" this will bring the transmitter at the same level as the 1st method but whats the use of doing this..i mean i will have to cut open the insulation for dismounting the thermowell..The only plus i can see is that by this scheme Ashcroft thermowell requires a reduced U-LENGTH. My scheme required 12" of U-lenght while the second scheme requires only 8".

Any thoughts on this.