7 replies to this topic

[VISHAL KHADKE]

Dear Sirs,
I am a process engineer working with a Polyester Manufacturing Company. We are working on a project of POY transfer line. In one of the jacketted pipes of size 4" x 6" the dimensions are as follows (Core: Polyester Melt & Jacket : HTM Liquid 'Dowtherm')

Inner Pipe(Not a standard XXS pipe) : SS pipe : Hollow Bar SS 404,DIN-17458,EN-10216-5
Tolerances : Inner Diameter = 0 / -2% (min -1mm)
Outer Diameter = 0 / +2% ( max +1mm)
Wall Thickness = (c1)-0mm
ID= 100 mm / Thickness= 20mm / Outside Diameter = 140 mm
Outer Pipe : Pipe Seamless Carbon Steel ASTM 106 Gr B Butt Weld Ends, Schedule 40
Dimensions As per ASME B 36.10
ID= 154.1 mm / Thickness= 7.1mm / Outside Diameter = 168.3 mm

If we see, the annular space(gap) between inner pipe and outer pipe is 7.01 mm. I have following concerns regarding this
1) Is this gap okay for carrying out fabrication & putting spacers between jacket & core pipe?
2) At present the Cross Sectional area available for the HTM Liquid flow is equavalent to cross section area of DN 50 pipe. This area is good enough for the HTM flow rate of 3500 Kg/hr from velocity and pressure drop considerations. Am I right ?
3) Is is possible to use the 4" x 8" combination instead of present 4" x 6 "? In this case the cross sectional area available for HTM flow is equal to DN 150 pipe and my concern is if we use this the HTM flow velocity through jacket will be very low & it will also get channelized.
4) I could not find the guidelines for design of jacketted pipelines in any of standard piping reference book. Can somebody suggest where can I get this information ?

Thanks in advance.
VISHAL KHADKE

[Art Montemayor]

Vishal:

Please refer to the attached Excel Workbook. You seem to mixing pipe dimensional codes. As a result, you are closer to using what, in the USA standard, would be a 5 inch inner pipe and a 6 inch outer pipe (in nominal sizes). This, in my personal opinion, is too close a sizing. A 4 inch x 6 inch or a 5 inch x 8 inch (although I consider a 5 inch pipe as a “bastard” size – i.e., a non-standard size that is difficult to find and to work with) would be preferable. It all depends on the basic dimensioning of your process pipe – which in this case seems to be the 5 inch nominal pipe size.

I consider the annular gap between inner pipe and outer pipe of 7.01 mm (1/4”) as too small for efficient construction and fluid flow. Since you have already selected the basic 5 inch size, that leaves you with only the option of using the 8 inch outer pipe (which is the next higher standard size. I would have selected the 6 inch x 8 inch sizes instead, which gives you a gap of 8.6 mm which gives you a gap increase of 22.8%.

I am not available to make the hydraulic calculations to tell you if the recommended gap is OK for your Heat Transfer Medium (HTM) flow rate of 3500 Kg/hr. You should do this yourself and find out if the pressure drop is OK for you.

You can also use the 4 inch x 8 inch sizes; but again, that is up to your finding out if the pressure drop is OK.

You normally select the size of the pipe jacket by selecting the appropriate and acceptable pressure drop for the HTM

I hope this helps you out.

Attached Files

•  Pipe Jacketing.xls   43.5KB   211 downloads

[ankur2061]

Vishal,

The purpose of a jacketted line for polymer melt is to prevent heat loss from the polymer melt.

Circulating thermic fluid in the annular space of the jacketted pipe can easily be circulated with velocities of 3-3.5 m/s as long as your puming system can cater to the pressure drop for the given flow rate & the aforementioned velocities. Velocities lower than 2 m/s are generally not recommended due to the fact that solid particles formed due to thermal degradation of the thermic fluid can cause deposit/layer formation in the annular space of the jacketted piping & thus lead to cold spots for the polymer melt. However, it is important to note that cold spot formation in jacketted polymer lines is contributed by several other factors also, such as quality of insulation, change in melt pipe routing, incorrect location of static mixers etc.

Hope this helps.

Regards,
Ankur.

[VISHAL KHADKE]

Dear "Art Montemayor" & "Mr Ankur",
Thnak you very much. You helped me faster and better than my own piping department. After reading your reply I could draw following conclusions.
1] My 4" pipe is actually a 5" pipe as per ASME 36.10 dimensions. So I feel we can use 4" (Hollow bar) x 8" CS 40sch pipe combination.
2] In my earlier experience I have seen a core & jacket combination of 5" x 8" working very well with liqid dowtherm jacket. However I will also pay attention to haydraulic considerations before finalizing the design.
3] To avoid chanellizing or settling of solids I am also thinking of adding "spiral fins" (pardonn me I dont know the exact word for that) on the outer surface of core pipe to avoid channelizing of HTM liquid and thus avoiding accumulation of solids. I have seen such kind of arrangment in jacketted pipes having core size > DN 250.

Thanks and regards,
VISHAL KHADKE

[Art Montemayor]

Vishal:

Based on your latest statements, I would offer the additional comments:

Your idea of employing a spiral(s) baffle within the annulus of the two pipes is a very good method to increase the heat transfer within the length of the process pipe. However, bear in mind that the basic scope of work of the HTM jacket is to compensate for the heat loss of the process fluid due to its exposure (mostly conductance through the flanges, valves, fittings, and pipe wall. This, I believe is the spirit of Ankur's excellent comments. The HTM jacket is not normally designed to be a double pipe heat exchanger intended to introduce more heat into the process fluid than it started out with. If that is your case, then heat transfer exchanger efficiency, as such, is not the priority of your design and the spirals may be an overkill - as well as an added investment. I have used spiral baffles in double pipe exchangers and have discovered an operational weakness: you must fill the HTM fluid into the annulus slowly and with care at the initial startup. If you subject the annulus to a quick, sudden filling of HTM you could collapse the spirals because these are normally only tack welded (& not seal welded) on the outer surface of the inner pipe. These weak "baffles" have a tendency to come loose if subjected to a hydraulic hammer.

Pay particular attention to the annular fluid velocity recommended by Ankur. This is important in determining the annular cross-sectional area and, consequently, the sizing of the outer pipe. Again, if you employ spirals you will need to carefully identify the REAL cross-sectional flow area through the spiral path in order to ensure that you control the Ankur values. Identifying this spiral cross-section can be a math challenge.

Good luck and happy fluid flowing.

[ankur2061]

Vishal,

I think you are having the idea that by providing spiral fins as you call them, you are probably looking to increase turbulence of the liquid dowtherm in the jacket. However, I am not sure that is a good idea, since the fins or extensions can themselves be a cause of fouling by picking up degradation solids on their surface. In my 19 years of career in the polymer industry I have never seen the arrangement that you are proposing.

It is important to note that the circulating thermic fluid (Dowtherm) in the annular space of the jacketted line serves no other purpose than preventing heat loss from the melt pipe. In other words, the function of the circulating thermic fluid is NOT heat transfer to the melt pipe but just to maintain the temperature of the polymer melt. The only heat transfer function in polymer melt lines that I know is of polymer cooler where the melt is actually cooled by transferring heat to the jacket media (the temperature of the return thermic fluid is higher than the supply temperature).

Hope this helps.

Regards,
Ankur.

[VISHAL KHADKE]

Dear Sirs,
Thank you once again. Now I will first calculate the velocity and pressure drops and see how it works.
Regards,
VISHAL KHADKE

[Dazzler]

Hello Vishal
My feeling is that 7mm gap is ok - can be constructed, and maintained/cleaned. Fouling would not be that bad as to possibly block that significantly.

If you make the gap bigger then the flow rate needed for a selected optimum velocity in the gap (optimum for heat transfer and preventing deposition/fouling) will increase quickly. That will increase the HTF pump size.

The bigger gap will also increase the HTF system volume and therefore the tank size and fill costs. Larger equipment and larger inventories of HTF may also then increase the hazard scenario severities for that area of the facility.
Dazzler