14 replies to this topic

[mohammad reza]

Please submit some method to calculat the steam consumption for jacketed line(for example sulfur line).

[ankur2061]

Dear Mohammed,

Please see the following post for calculating the heat loss through an insulated pipe. Naturally the sulfur pipe is jacketed and also insulated:

http://www.cheresour...?showtopic=6248

Once you have calculated q as per the equation given in the above post, calculate the steam requirement as:

m = q/H

where m = steam flow rate in lb/h

q = heat loss in Btu/h

H = Enthalpy @saturation pressure in Btu/lb (e.g. H= 1179.3 Btu/lb @ 50 psig)

Note: For your case the Di for the pipe will be the OD of the jacket pipe and the Do will be Di + 2*insulation thikness.

Hope this helps.

Regards,
Ankur

[djack77494]

I'm not sure it's obvious, but you treat the pipe jacket (or steam piping) as if it is "the line", and you calculate heat losses from it. Essentially, you have no heat loss or gain to/from the inner (sulfur) line. The steam in the jacket maintains the inner temperature by eliminating heat losses. Steam consumption is then based on replacing the ambient heat loss.

[ankur2061]

Daer Doug,

I worked in a company where jacketed piping was the norm rather than an exception. I must have done dozens of calculations for heat loss from jacketed piping and equipment. This is the only method I know and it is based on Fourier's law of conductivity. Also, this calculation method was approved by the process licensor.

I would love to know if there is any other method for calculating the mass flow of a heating medium whose sole function is to replenish heat lost to its surroundings due to a temperature gradient beween the process and the surroundings.

Anticipating your reply.

With Regards,
Ankur.

[djack77494]

My apologies, Ankur. for not being clear. I am not suggesting a new method for calculating the heat losses from a jacketed pipe. The point I was attempting to make was that the transfer of heat from the jacket fluid to/from the (innermost) process fluid should be minimal (negligable). My experience has the process fluid entering the piping at a good temperature. From that point on, you are not attempting to heat or cool the process fluid; you merely want to avoid heat losses. Since the pipe containing the process fluid is totally contained within the jacket, it quickly reaches the jacket fluid's temperature. Using a condensible fluid like steam, the temperature throughout the jacketed pipe is constant. Steam consumption would then be utilized to balance the heat lost to the environment. Sorry for contributing to any confusion.
Doug

[misskarleone]

Hello to everyone!
I'm also calculating steam flowrate for steam jacket sulfur pipeline and I'm little bit confused with heat losses.

For heat losses I also used this formula
Q=2*3.14*insulation conductivity*delta T/ln(DO/DI)*pipeline length

my first confusion was with diameters: normally we have 8 inch steamjacket line and 6 inch sulfur line inside it, and finally 60 mm of rock wool insulation.
Firstly I did heat loss calculation for 6 inch sulfur line including insulation. But then I came to thought that I have to calculate heat losses for 8 inch jacket pipe (including insulation).

second confusion with delta T. At our weather conditions lowest ambient is -40degC, for all equipment of our plant this is a lowest design temperature.
Sulfur should be maintained at 138-140 deg C. For steam jacket we are using low pressure steam (500 kPa, Toper=150-152 deg C, Hfg=2108.5 KJ/kg). I'm not sure which temperature difference we should include to heat losses calculation:
140-(-40) = 180 deg C
or
152-(-40) = 192 deg C
Can You please advise, maybe You struggled with that before.
Thanks

[ankur2061]

Hi,

Jacketed pipes with steam in jacket are not only provided insulation but also with aluminum sheet metal cladding to strenghten the mechanical integrity of the provided insulation. The surface of the cladding is the one which is exposed to ambient air. Generally, the idea is to maintain a surface temperature of the cladding not more than 60 deg C for the obvious reason that above 60 deg C it can cause a burn. A more practical temperature range could be 50-55 deg C. If that is the case then your delta T would be as follows according to me:

delta T = saturation temperature of steam i.e 152 deg C - surface temp. of cladding 50 deg C = 102 deg C.

Considering ambient temperature of air of -40 deg C will give you large values of heat loss which may be quite a lot of overdesign. However, if there are no issues of overdesigning the system and sufficient steam is available, I guess you can consider -40 deg C. When designing systems for heating of jacketed pipe it is also normal practice to consider a safety factor of 1.5 to 2 on calculated heat loss due to uncertainities in pipe routing and bad/damaged/wet/improperly applied insulation.

Hope this helps.

Regards,
Ankur.

[Srikrishna Chaitanya]

Dear Ankur ,

Do we have any codes /specifications for doing heat load/Loss calculation for steam jacketing of pipelines , One doubt Why always we have to use Saturated steam , Why not Super heated steam for this jacketing ?


Regards ,
Satish

[Srikrishna Chaitanya]

Dear Ankur,
One more question to you , why you have not considered the heat transfer between Jacketed fluid & Inner core fluid because this is going to be significant & substantial . If you do not consider that heat transfer then there is no difference between, normal insulated pipe and a jacketed pipe. This question is already raised by DOUG , I am asking again to avoid my confusion . Any body having dealt with Jacketed piping can post a answer for this

Thanks in advance


Regards,

Satish

[Qalander (Chem)]

Dear Ankur ,

Do we have any codes /specifications for doing heat load/Loss calculation for steam jacketing of pipelines , One doubt Why always we have to use Saturated steam , Why not Super heated steam for this jacketing ?


Regards ,
Satish

Dear Satish Hello/Good afternoon;
Although right now I don't recall any Code/Standard(s),

But basically it is more to do with the mode of physical(actual) heat transfer rather than anything else

sulfur is in either solid lumps or granules of irregular shape and size and therefore; obviously conduction is the phenomenon controlling the heat transfer rate.

Now the steam (saturated) has latent heat with slight or Nil sensible heat to input into the surrounding environment through conduction for much longer periods keeping water molecule in much larger %ages 'in contact the conductive heat transfer area'.

Whereas for superheated steam;

Firstly the gaseous phase provides much lesser %ages of water molecules 'in contact the conductive heat transfer area' and even that for a shorter period of time although with higher sensible heat.

Secondly with highly superheated.dry steam the localized overheating of conductive heat transfer surface to such an extent that sulfur may get oxidized resulting in corrosion and chances for other undesirable reactions to occur can not be ruled out. Therefore saturated steam is the logical preferred choice,I believe.Further explanation may follow from other learned/ experienced colleagues.

Hope this helps to understand what was rightly pointed out by ankur and doug and other in the above post already.

[ankur2061]

Dear Ankur ,

Do we have any codes /specifications for doing heat load/Loss calculation for steam jacketing of pipelines , One doubt Why always we have to use Saturated steam , Why not Super heated steam for this jacketing ?


Regards ,
Satish

Dear Satish,

As of now there are no international codes (ISO/API) that I know off regarding heat loss calculation from jacketed pipes. In the company I used to work in India we used to follow the licensors (DuPont) method of calculating heat loss which is nothing but a simplified form of 'Fouriers law of conductivity'. This method has served me well in doing heat loss calculations for jacketed pipe.

As regards using saturated steam instead of superheated steam, the heat transfer coefficients of saturated steam are better predictable and well defined compared to superheated steam. If you do some digging you will find out that majority process heating or heat conservation applications in the chemical process industry when using steam as a heating media are done using saturated steam and not superheated steam.

See the link below to understand why saturated steam is preferred over superheated steam:

http://www.spiraxsar...eated-steam.asp

Hope this helps.

Regards,
Ankur.

[ankur2061]

Dear Ankur,
One more question to you , why you have not considered the heat transfer between Jacketed fluid & Inner core fluid because this is going to be significant & substantial . If you do not consider that heat transfer then there is no difference between, normal insulated pipe and a jacketed pipe. This question is already raised by DOUG , I am asking again to avoid my confusion . Any body having dealt with Jacketed piping can post a answer for this

Thanks in advance


Regards,

Satish

Dear Satish,

I think you have still not understood the concept of heated jacketed pipe. This is not a heating application. The application is to prevent heat loss from a fluid which if gets cooled by even a couple of degrees below its freezing point will solidify, the most common applications being molten sulfur and molten Dimethyl Terephthalate (DMT). Economical insulation thickness is insufficient to prevent the heat loss from the pipes carrying molten sulfur/DMT below its freezing point. If you try to maintain the molten liquid temperature just by insulation the insulation thickness arrived at would be impractically and ridiculously large. It just cannot be done. This is the reason that jacketed pipes are provided where a medium which is slightly hotter than the process fluid is introduced in the annulus of the jacketed pipe. The heat loss to the surroundings is compensated by the heat steam transfers to the process fluid which would just be the latent heat of saturated steam in ideal conditions. In other words:

Heat loss to ambient = Heat transferred by steam to process fluid.

So you see that a jacketed pipe steam consumption has to be calculated based on the above principle and the calculations have been outlined in the earlier posts.

Providing perfect and flawless insulation is a very tricky business. Insulation is also prone to damage and weathering. Most people just forget about insulation integrity once it is installed. These factors need to be taken into account when doing heat loss calculations. But how to account for bad or poorly installed insulation? The practice that I have successfully followed over the years is to provide a safety factor of 1.5-2 on the calculated heat loss based on the pipe length. I have never encountered a problem when utilizing such a safety factor.

I hope I have been able to provide you clarity that jacketed pipe transporting molten liquids just involves heat loss calculations and there is no net heat transfer from the so-called heating medium to the molten process fluid. The application is JUST TO MAINTAIN THE TEMPERATURE OF THE PROCESS FLUID AT A NEARLY CONSTANT VALUE ABOVE ITS FREEZING POINT.

Regards,
Ankur.

[Srikrishna Chaitanya]

Dear Ankur & Qalander ,
Thank you very much for throwing good amount of light on the topic, Ankur Sir , link provided by you is Interesting & Informative . I am already aware that using super heated steam in jacket is going to be weird , Even though I given some thought to it because here we don’t really require some heating with Jacketing (To keep the Inner fluid temperature constant ) , I felt Super heated /Saturated steam any thing can do this small job . Here my application is bit different and I will explain that first

We have a Tubular reformer to Produce syn gas using Naphtha & Steam , Fuel to reformer is Vaporized naphtha at 235 degree Centigrade and 3 bara pressure , Depending on its composition & Pressure The dew point of Naphtha is around 120 degree Centigrade which is pretty low compared to the operating temperature , Still Naphtha ling got jacketing with a HP saturated stream of 275 centigrade , This jacket is approximately 300 m long starts from some where and continues till the burners , Now my doubt is , will Vaporized Naphtha be at 235 centigrade or it get heated to 275 that of steam temperature in the jacket by the time it reach burners ? (Delta T between steam & Naphtha is significant and I feel It may lead to some Heat transfer to Naphtha as well , So I thought designers might take care of this heat transfer too while designing ) , While I was doing some searching I found this interesting Slufur line jacketing and thought that can be applied to my problem as well . Correct me if my thinking is not correct



Regards ,

Satish

[ankur2061]

Dear Satish,

I am really not familiar with the application you have mentioned of having a jacketed line for vaporized naphtha. However, one thing is sure, the heat transfer coefficients for vapor phase are going to be pretty small. Essentially your application looks the same as preventing the vaporized naptha to liquefy or form liquid droplets in the line and the jacketing with steam does the trick of preventing formation of liquid droplets in the line. In my opinion, there should be no significant temperature rise of the vaporized naphtha in the line due to steam jacketing.

If the intention is to prevent even the slightest amount of condensation then I suggest that you provide a condensate pot with a mist eliminating device at the terminal end of the vaporized naphtha pipe on the reformer side to remove any liquid droplets that are formed.

Hope this helps.

Regards,
Ankur.

[Srikrishna Chaitanya]

Thank you very much for the reply