11 replies to this topic

[Shakil]

Dear All, I have a Ethylene Cylinder @70 bar pressure. I need to use Ethylene in downstream reactor for polymerization at different pressure say in the range of minimum 1bar to 50 bar . If i throtle the Pressure using Pressure regulator the temp. will go as low as -104 0C if throtled to 1bar & 10.5 0C if throtled to 50bar (from cylinder pressure 70 bar). (Calculated using ASPEN Plus Simulation) Limiting condition for temp. is Minimum design Metal Temperature(MDMT) for SS-316 tubing & fittings used for transfer of fluid (say 5.5 0C) What can be done to deliver the Ethylene above MDMT & pressure range 1-50bar? Plz. suggest!

[kkala]

Stainless steel 316 (<http://www.azom.com/...x?ArticleID=863>, background) can be used even in cryogenic applications. Can you check connecting pipe MDMT (mentioned ~5,5 oC)? It should be much lower, suitable for ethylene service at 1 bara, unless pipe contains some part (fitting, flexible hose, gasket, etc) not made of 316. In the latter case replacement of this part is a way out.

Let us go to Perry, Transport and storage of fluids, Process-Plant  Piping, Table of "Allowable Stresses in Tension for Materials" (Table 10-49 in Perry's 7th edition). Minimum temperature (*) for stainless steel pipe and tubing is between -425 and -325 oF (-254 and -198 oC). Probably 316 stainless steel is represented by A312 TP316, minimum temperature (*) -325 oF (-198 oC).

 

(*) Design minimum temperature, normally not requiring impact test. However at temperatures below -20 oF (-29 oC) the (piping?) code may require impact test (note 18 of Table 10-49, Perry 7th ed). To be certain, find written confirmation on MDMT  of this connecting pipe.

Edited by kkala, 05 March 2013 - 08:30 AM.

[meetyourmaker]

Why does your polymerization pressure vary so much(1-50 bar)? If you plan on going to as low as 1barg then you must ensure all downstream equipment can withstand -101deg C. I am quite sure MMDT of ss316 is much lower than 5.5deg C.

[Art Montemayor]

Shakil:

 

Where did you find that the minimum temperature for 316 SS was only 5.5 ^oC?

 

It is a well-known field fact that the stainless steels (and the "soft" metals, like copper, brass, lead, silver) have good tensile properties at the cryogenic temperatures (below -150 ^oF).   In fact, the pioneering work and invention of Carl Linde in liquefying and distilling atmospheric air was done in 1890 using nothing more that a copper disillation tower and heat exchangers soldered together with lead.  I used such equipment commercially in 1960 when I started my career and later employed stainless steel equipment in cryogenic service in the 1970s - in air liquefaction and LNG production equipment.  Therefore, I can tell you from personal knowldege and experience that you have nothing to worry about 316 stainless being able to sustain cryogenic temperatures.  Today, most (if not all) cryogenic equipment is made from such material.

[Shakil]

Thanks kkala & Art Montemayor, meetyourmaker.

 

as we are designing this for R&D department where they need to test Polymerization performance of catalyst at different pressure & temp. condition the pressure range varies so much.

 

I still have Four queries:

1. Where can i find data for Allowable stress V/s cryogenic temp. for SS-316?  Because even at 5 bar the temp. is as low as -70 ^oC, & i am afraid that when we will reduce pressure it should not result in tube rupture.
2. Is there any other method to avoid such drop in temp?
3. I had checked in ASPEN Plus if I start with 70 bar & 70 ^oC instead of 30 ^oC the outlet temp.at 1 bar is 10 ^oC. As rightly indicate by Kkala allowable Pressure loads value are available above -20 ^oF or -29 ^oC & we are falling above this limit.  So just by selecting right thickness we can sustain at given pressure.  So, if i use Steam Trace immediately at D/s of Press.  Regulator we can raise its temp. to 70 ^oC.  But again how to calculate the required Steam trace length & heat load?  And whether this is feasible?
4. In Pressure Relief Valves, where relieving pressure is very high & discharged to Atm. same situation persist where temp. of vent will go very low?  What precaution we take over there?

 

Plz. reply.  It will clear major of my concepts!

 

[kkala]

1. Data for pipes of 316 stainless steel can be found in Perry. I understand allowable stress can be either 20000 or 16700 psi from -325 to +200 oF. No mention about impact test, except note 18 (see post no 5).

2/3.  You need ethylene at low pressure. Increase pipe wall temperature (e.g. by electrical tracing) would make things more complicated. I would avoid it. 

316 steel can  withstand temperatures below -29 oC, the question is whether code requires impact test on piping for these conditions, and whether this has been implemented.

4. PSVs and their piping material shall be suitable for low temperatures developed during discharge.

Note: Storage of C2H4 in the cylinder is at supercritical conditions (tc=9.5 oC, Pc=50.76 bara); for unknown reason I cannot find enthalpies to rougly verify -104 oC.

In conclusion: I would try to contact the supplier of this transfer pipe and have MDMT in writing. This pipe should be also covered with cryogenic insulation for personnel protection.

If confirmation of MDMT is not possible, I would seek advise from an expert knowing more than me on piping material and codes, also presenting the issue of note 18 of Perry's Table 10-49.

Having not  real experience on ethylene handling, other views would be useful.

.

Edited by kkala, 05 March 2013 - 11:35 AM.

[Art Montemayor]

Shakil:

 

I have already advised you on what I personally know as factual information on the stainless steel application in the cryogenic zone.  I don’t know why you doubt what is factual truth, but the real, authoritative source for allowable stresses is not Perry’s Handbook for Chemical Engineers.  The source to use is none other than ASME Section VIII.  There, you will find the allowable stress for 316 Stainless Steel.

 

Why are you trying to heat up the cold expansion product?  The temperature of -100 ^oC is certainly well within the accepted range for stainless steel.

 

To shake your lack of faith in the use of stainless steel in the cryogenic zone, I further add that everyone and his brother within such companies as Linde, Air Liquide, Air Products, Praxair, etc., etc. are using stainless steels in their cryogenic safety relief valves and not worrying about  it.  We have done it for the past 50 years and have yet to suffer any criticism for the choice of materials.

[S.AHMAD]

1. I am not an expert in material science but from process engineering knowledge, my suggestion for your research project I strongly believe you have pilot plant facilities.

2. I believe that operating temperature is also one of the variable of interest in you polymerization reactor. Therefore, my suggestion is that you install two pressure letdown valve. The first valve is to letdown the pressure to 50 barg and followed by a heater (using hot water, as an example) to a required temperature so that  after the second letdown valve, the temperature is the temperature than is needed for the polymerization reactor. The second valve (can be used as flow control valve) to let down to the required reactor pressure. Of course you need another pressure control valve for your reactor.

3. Hope the above is practical enough for you to continue without worrying too much about the material of construction.

4. Good luck. Hope you can share the findings of you research for our common interest.

Edited by S.AHMAD, 05 March 2013 - 07:19 PM.

[kkala]

But no need for the control scheme per post no 8,  if MDMT of existing piping (rather short)  is provenly -104 oC (or lower). This is probably true needing just verification. In my non expert opinion, contacting vendor of this connecting piping can be a convenient way to get relevant information. It seems that exact grade of material is not known either.

Perry indicates  ASTM A312  as specification of piping material (316 steel); grade may be TP316, TP316H, TP316L; note 18 says that use at temperatures below -29 oC is established by rules elsewhere in the code, including impact tests. More grades are reported in <http://www.steeltube...M/ASTM A312.pdf>, with much less information.

Now look at <http://www.bestpdfdo...dar-pdf.html>, mainly para 1.1(*). Some A312 grades should be impact tested for temperatures below -50 oF (-45 oC), is any 316 steel grade in this category?  An expert can reply, I cannot. Besides, if impact test is required for the specific piping material, I do not know what has  been done. Fame of 5.5 oC is false or hiding some truth? Vendor could hopefully clarify all these matters and advise MDMT of the piping package. This includes valves, probably gaskets, etc, apart from the stainless steel parts.

Above (see post no 6) may seem meticulous and probably can be simplified by a specific expert proposal. It is known that cryogenic piping can be of 316 steel. Question now is somehow inversed; is the specific piping package  suitable for -104 oC? how can this be certified?

 

Evidently mentioned piping can safely bear 70 barg at normal ambient temperature (assuming no PSV). If MDMT=-104 oC is proven, can it safely bear 70 barg at -104 oC? Advice would be welcomed, even if this is considered too conservative for the present case. It would also 'help" similar matter touched in <http://www.cheresour...-cold-blow-down>, posts no 17, 19, 22.

 

(*) Please consider negative values in mentioned temperatures, understood as typos.

 

Edited by kkala, 06 March 2013 - 05:35 AM.

[Shakil]

Thanks KKALA, Montemayor & S.Ahmed for such detailed explanation. I will get in to details & will share the findings with all. You too plz. post any findings or info. on this topic.

[Shakil]

Dear All

Attached is "AtLas Technical Handbook" giving some info on Cryo temp. data for SS-316 & 304.

As rightly touched by KKala we need to check the Charpy V-Notch Impact Energy (J) at cryo temp.

As the energy required will be reduced if the material turns Brittle at low temp. & loses its Ductility prop.
This temperature is called DBTT i.e. Ductile To Brittle Transformation Temp.

From graph on Page 19. it is indicated that Charpy V-Notch Impact Energy (J) is 180 J over entire range from
-200 to 80 0C.

This indicates that SS-316 can be safely used with required thickness as per ASME at cryo temp.
For addition safety we can take 10 % marging on thicness.

I dont know about the authenticity of the data provided, but it serves the purpose.

Correct me if i am wrong & plz give info on any such data as it is turning interesting.

[kkala]

Impact test 20 J is mentioned in 2nd link of post no 9, but this may not be important.

Atlas Technical Handbook has not been attached. Does it indicate that all possible grades of 316 steel pipe can be safely used without impact test?

If so, this would clarify first part;  second part would be to know about "valves, probably gaskets, etc" of the specific piping package and their MDMT (minimum design metal temperature).