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Dear friends,

We have an application wherin process temperature is 1500 deg cen and process pressure to be measured is 2 Bar abs. But here the problem is the PT we are using cann' withstand temp more than 85 deg cen.

So, we planned to use ss 304 inpulse tube of ID = 10 mm & OD = 12 mm,in the form of coil to thermally insulate our PT.

But the problem is we dont hav the exact calculation which can tell us about the length of such impulse tube to get desired drop in temperature. Please suggest.

I don't have a calculation but the length would be < 1 meter. Radiant heat is probably more of a problem. You also need to think about thermal expansion, allow enough bends in the tubing so that it doesn't put any strain on the fittings with temperature change.

Roy

Dear Roy,

We have made a spiral with that tube with fine curves. As we are using ss 316, i feel there is not going to be problem as far as thermal expansion is concerned.

However, how did you arrived at the length viz is less than 1 m. As we have to bring the temperature from 1500 Deg Cen to 50 deg cen (drop of 1450 deg Cen.

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Siemens SITRANS P, Series DSIII Transmitters USER'S MANUAL, pg 58 (pdf); pg 4-5 (paper)

4.3.3 Impulse Piping for Absolute and Differential Models

When using impulse lines on a high temperature process, locate the transmitter far enough away from the heat source to keep it within temperature specifications [28°C (50ºF) per foot cooling to a normal ambient is assumed for non-insulated impulse lines].

UMSITRPDS3-1; Rev. 9; August 2008; Supersedes Rev. 8

http://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csi nfo&lang=en&objid=10806940&subtype=133300&caller=view

Hello carl,

Actually your statement confused me somehow. As per your statement we 1 foot of impulse tube for a drop of 28 Deg cen but these thumb rules vary a lot.

As par Emerson process engineer we need 1 m impulse tube of ss for a drop of 50 deg cen.!!!

I would be thankful if someone can help with concrete calculation or if someone has actually done this & if can share his/her experience. Also,link give by you is restricted!!

Carl, I'll see your 50°F/foot and raise you 50°F/foot:

Manual No. 85165000, Rev. 01, NUFLOTM Differential Pressure Cone Meter User Manual, page 13:

As a general guideline when planning tubing lengths for temperature control, run tubing horizontally where possible, and allow for a temperature drop of 100°F (38°C) per foot (305 mm) of tubing. This is merely a guideline, however; the operator is still responsible for ensuring that the temperature at the transmitter does not exceed the transmitter's rating for the environmental conditions present.

http://www.c-a-m.com/cam/search/showdocw.cfm?DOCUMENT_ID=52013

We call this 'shopping for answers' at work.

David

Hi david,

What i found till my search is that for the desired impulse tubing calculation there are many thumb rules which vary from vendor to vendor but still no concrete calculation.

As a thermodynamic laws,all of us knw that to get a drop for high gradient we need less energy compared to lesser gradient.

for instance - if we want to drop from 1500 to 1400 dg cen we might require say length of 1 feet but getting a drop from 150 to 50 we need much length for thermal insulation.

SOS,any concrete formula or practical experience!
as par thumb rules i need approx 25 meters of impulse tube!

Sushant,

It seems like a difficult question but my gut feeling is 3ft (1 meter) of tubing is enough. I have measured the pressure in reacters running almost red hot with less tubing.

It also depends the process for a condensable fluid you need to prevent it entering the transmitter as a vapour then condensing.

Since you seem to be getting such a wide range of answers from my 1 meter to 25 meters why not do an experiment. Take a length of SS tubing capped off at the end, connect to your process and then run your hand along the tubing (from the cold end) untill you find it too hot. My guess is < 1 meter.

Let us know
Thanks
Roy

Hi Roy,

That can be done practically but since temperature is on higher side and process pressure is critical parameter,we want to play safe.

We have already made 3.5 m impulse tube in spiral coil form and even tested it up to 880 deg cen temperature and we are amazed to see that heat is not even crossed half of meter.But in the actual process there will be hot gas which will be flowing through the tube which will constantly pass the heat which it contains,thereby increasing the required impulse tube.

In our application process media is producer gas which doesn't contains vapors.

Even i hav some formula's, one for measuring the tubing length wher in there is no flow in the impulse tube and once equilibrium is reached there will be constant heat transfer across the walls of tube.That calculation give me length of .9 m of ss tubing However by using normal thermal calculations under constant flow conditions,length comes to be around 3 m.

In both cases iam taking abt the required rop of 1450 Deg cen.!

Then on what basis these famous thum rules are frames if these calculation are equally valid in real world!

Sushant,

> we are amazed to see that heat is not even crossed half of meter. But in the actual process there will be hot gas which will be flowing through the tube <


How do you figure the gas will be flowing? If the tapping point is connected to the pressure transmitter it cannot flow, just pass the pressure. (unless you blow it down through the vent valve).

Is plugging a problem?

If you send me a sketch of your instalation I may be able to help further.

roy_matson@yahoo.ca
Regards
Roy

Hi Roy,

I agree with you that the gas is not going to flw and once in equilibrium it will pass over its heat by convection and at that time it will be in steady state.

But considering the steady state, what actual length wll be required?? iam also mailing you the calculation iam using to calculate the length on your id. Please review it and give your valuable feedback.

Sushant

hi,

May i know that which formulae you're used to infer this 3M length?

You can ask the Pressure transmitter manufacturer regarding the tubing details, generally they provide such details..

Dear Swapnil

We are again talking about thumbrule , it is about 15 deg c per ft of impulse tubing , but providing 20 mtrs impulse tube is not very simple option.

Presently we have similar application [700 DEG C ]. we have done following

1] Provide a condensate pot [ 2" DIA ], well supported [ 3 mtr ] away from sensing point.

2] We have provided purge rotameter with DP regulator [ flow 5 % ] of total flow with NRV. very close to sensing point.

We have just installed the system & probably would be in position to give feedback by First week of June 2009

I would also request all the readers also to give me your opnion on this.

Thanks in advance

Jari
iconcnl@vsnl.net

Note : Earlier Taylor Instruments [ABB] used to give NAK filled Transmitter for very high temp may be 700-800 deg c [not sure] for application in fertilizer industry, check up with ABB about this fill fluid , this would considerably ease up your measurment

Our Company is going to build some pressure transmitters with NaK fill for a customer. We are alkali metal specialist and build other equipment for use with these metals. Our em pumps actually pump liquid metals up to 1500 deg F and our flow meters are good for 1500 as well.

Rich

Dear Swapnil: Just to inform you that, installation used and mentioned in this thread works very well & temperature at pressure transmitter end is less than the permissible limits of transmitters. Even the temp is 1500 DEG C, should not matter much with this limits, unless Catch pot can not withstand this high temp. use the same MOC of your reactor.


Dear Richard: Just to know one point for NAK filled transmitters. what is solidification point for this fill? what additional arrangement is required to avoid solidification of this fill fluid?

I would really appreciate your feedback , we tried to locate this datasheets of transmitter[taylor Instruments], but this was long ago.

Regards
Jari
iconcnl [at] vsnl.net

A word of warning. Having worked on systems all the way up to 3000 C, I am sure there are a number of factors not mentioned in your description. For example:

a.) Will the connection point see direct radiation?

b.) Is there water cooling?

c.) What is surface temperature of the equipment?

d.) Is the connection point above the unit or on the side?

e.) Will there be good air flow?

f.) What will be the maximum ambient temperature?

etc.

You man not be providing the needed information and people aren't asking for it.

Regards,
James Fountas

Hello James,

Thanks for the your keen interest and giving a new perspective to the discussion. Here goes the answers to your question -

a.) Will the connection point see direct radiation?

Yes,the connection point will see direction radiation and we are using nipple of ss 304 with chemical insulation to enable it to withstand high temperature.

b.) Is there water cooling?

As of now if didn't done any water cooling but yes,its in our plan in case we didn't get desired drop.We are planning for thermo syphon.

c.) What is surface temperature of the equipment?
We have a thick isolation.surface temperature would be maximum 100 deg cen.

d.) Is the connection point above the unit or on the side?

We will install the impulse line horizontally to the connection point and on other end there will be out PT.

e.) Will there be good air flow?

Yes,its in open area.

f.) What will be the maximum ambient temperature?

It would be 45 deg maximum.

In case of any more information is required,please let me know. Looking forward for suggestion from your expertise. In case you give your email id, i will also send the calculations & offline experiment snaps.

have a nice day
sushant chanana

Sushant,

The application note AN 84/01 you sent me from Adam Kane of Kulite confirmed my suspicion that 1 meter is adequate, in particular the graphs and last paragraph:

"As a rule of thumb, one foot of steel tubing, any diameter, will isolate a transducer from any temperature"

The main transfer of heat is through the tubing, not the contents.

Regards
Roy

That's a great white paper by Kulite. Beats the oral tradition of 50° or 100°F/ft.

http://www.kulite.com/reference/AN8401HeatTube.pdf

Carl

Hi Roy,

Please see the thread by James as he raised some equally valid concerned which were overlooked by us.

Warm regards
Sushant Chanana

To measure the desired pressure through PT we must keep the Tx enough away in atmosphere (ambient temp) with a condensate pot with cooling water jacket. U shall get the desired pressure.

The Kulite paper cited above, that provides calculations for using impulse tubing to isolate pressure transmitters has a new URL:
http://www.kulite.com/docs/technical_papers/AN8401HeatTube.pdf
(take out any spaces the forum inserts into the URL)

The Kulite paper is a good start for a conservative estimate for external fittings.

If you include radiation effects, etc. and require that the measured fluid contain no condensibles, the 0.50" od / 0.035" wall, stainless tubing with a 2000F source will drop the temperature to 120F in 6", but that is impractically short, would recommend at least twice the Kulite rec for safety...

The more important issue is the temperature of the connection where it penetrates the equipment wall. Typically the shell temp is a lot less that the interior of the furnace.

Can we go in for alternate method like measuring back pressure of an inert fluid. Say instead of measuring the vessel pressure directly, we inject air at specified pressure. With the increase in vessel pressure back pressure will also increase proportionately thus giving the true pressure. Its just a raw idea. We can refine it and implement it.