6 replies to this topic

[Boeid]

I am currently doing a feasibility study (with some basic engineering) on transporting 40 ton/hr of superheated steam (at around 40 barg and 400 degC) of which the distance between the supply point (a new boiler) and the landing point is quite significant (about 1 to 2 km).

The landing pressure and temperature are fixed while I can still play around a bit with that of the supply (as we have yet to finalize the outlet pressure and temperature of the boiler). The criteria that I use in specifying the steam pipeline are the pressure drop and heat loss.

I already arrived at an acceptable solution (it is not perfect as eventhough the selected size of pipeline will give an acceptable pressure drop, the insulation thickness is much higher than usual – 270 mm – in order to obtain the desired landing temperature. Anyhow I believe a change of insulation material with lower thermal conductivity can address this which we will do in later stage). However, the issue now is on the steam velocity which is lower than the recommended value (15 m/s vs. 30 to 50 m/s).

I believe in order to satisfy this steam velocity requirement I can:

• Vary the pipe size

While reducing the pipe size can increase the steam velocity (and improve the heat loss of which can solve the issue of thick insulation) it will also increase the pressure drop (hence not meeting the desired landing pressure). If I then increase the supply pressure to compensate for this additional pressure drop, it will reduce the steam velocity (Higher pressure = lower specific volume = lower volumetric flow rate = lower velocity). Increasing the pipe size is not an option as it will reduce the steam velocity (and higher heat loss) eventhough it will help me in term of lower pressure drop.

• Vary the supply pressure (using my initially acceptable pipe size)

If I reduce the supply pressure, the steam velocity will increase but I will not get the desired landing pressure even if the pressure drop is more or less the same. Obviously I cannot increase the pressure as it will reduce the steam velocity.

Suffice to say that I am going nowhere – just going around and around – in trying to have a solution that satisfy all 3 criteria i.e. pressure drop, heat loss and steam velocity.

Is there any other option that I can try? Or:

• Is velocity of 10-15 m/s will pose serious condensation issue? (If not, I will stick to my initial solution which meet the pressure drop and heat loss requirement)
• Will having additional steam traps solve/manage the condensation issue? (If yes, again I will stick to my initial solution)

[breizh]

http://www.spiraxsar...pipe-sizing.asp

This resource may be useful.
Breizh

[joesteam]

Maybe you can design with more superheat and if your calculations show you do not go into the saturated steam level you will not develop any condensate (will need steam traps just for start-up an upset conditions). Then you will be able to tolerate slower velocities because the heat loss will be a drop in temperature only.

[Anup Paul]

Dear Boeid,

Higher steam velocity is generally preferred in steam lines for improving heat loss but doesn’t mean that we can’t go for lower velocities. Lower velocities are not preferred because of more heat losses.
I don’t see any reason for condensation if the velocity is 10-15 m/s, until your steam is in superheated condition.
To address pressure drop,

• Try to go for shortest route with minimum fittings.
• Increase the supply pressure and temperature. Only increasing pressure will reduce Specific Volume of Steam so you need to increase temperature correspondingly but this may add additional cost to the project.

[S.Chittibabu]

Steam velocity of 45 to 50 m/sec is preferred.
Temperature drop of 8-10 deg at the landing point is to be considered. Pressure drop should not exceed 1-2 bar.You have to keep these figures and arrive the line size and superheat required at the supply point.

[Boeid]

Thank you all for the inputs.

I have not have a full relook at my case but there is an example of (almost) similar case to mine in the link given by breizh (Case 10.2.2) where the steam velocity is relatively low, about 10 m/s but a smaller pipe is not selected as this will not meet the required landing pressure (i.e. too high pressure drop).

Anyhow I will try suggestion (2) in Anup's post by increasing both the pressure and temperature and will share with you guys on the outcome.

Thanks again!

[Sunkara]

While sizing any pipeline,
1. Pressure drop will decide the size
2. High velocity limit will decide the size.

Normally low pressure operation cases, velocity limit will be the constraint. your case heat loss also includes, but you hav to stick to the Delta P while sizing even though you could see lower end velocity. Actual condition higher velocities will cause higher pressure loss and hence temperature drop. So always pressure drop and (or) higher velocity will decide pipe size.


Your sizing approach is correct.


Nagesh
Pipeline Enginner