Low saturated steam velocity

[sefmp]

Hi,

I have a question regarding a new steam distribution line we are designing. We need to design a steam distribution line that will work both ways: during start up of the plant we need to guarantee a steam import flowrate of about 45t/h but then, under normal operation, it will drop to a steam export flowrate of about 3 t/h (desuperheated steam from a process boiler). The plant supplier will use the same line for both purposes. Due to pressure drop issues the line will be something between 8" and 10". Under normal operating conditions steam velocity will then be between 1 and 1.7 m/s. This is way below the recommended saturated steam velocity. What problems can arise from this situation? Higher losses due to excessive condesation? Will it help if we consider a higher degree of superheating at the outlet of the desuperheater? Does it justify creating a different line for normal operating conditions?

Regards,

sefmp

 

[rmw]

If your export flow is very steady state, then you can desuperheat less and transport the steam with more superheat in it. In that very few desuperheaters are designed to nor capable of producing steam right at saturation, (there are a few out there but I don't think you want one for this application) your DSH outlet temperature does have 5-10-20 deg (F) of superheat anyway.

If your steam user can take more superheat, you can raise the outlet temperature controller to give higher temperature.

Not knowing the distances involved nor the type of equipment that you will be using, you might put your temperature controller sensing unit at the end of the run, but that would only work if the flow is steady state and constant. Too much lag time if the flow is variable with this suggestion.

You could also calculate (or determine empirically) your temperature losses along the export route and put just enough superheat at your DSH outlet to enable the steam to arrive at the user approaching saturation. Seasonal temperature variations would have to enter into the design of that approach.

rmw

 

[sefmp]

Thank you for your prompt answer.

You are right, the DSH will work within 5 to 10 ºC of the steam stauration temperature. I was just wondering if it would be a good idea to use its adjustment to overcome the line size issue under export conditions. This is obviously easier than considering an independent line for steam export. After commissioning of the pant I will check the line losses and adjust the DSH accordingly. Maybe 5 to 10ºC is already enough.

sefmp

 

[unclesyd]

We use rmw's approach on several internal steam distribution systems to compensate for a batch wise operation. On a 5 unit system the superheat is set to be the lowest when all 5 units are on line and if we drop to 1 unit the superheat is relatively high. The only concern I've heard is that on 1 unit operation the batch time is slightly longer. In this system we are always above saturation as the range of superheat is 35F-100F degrees at the DSH. We reduce from 650 steam + 100F degree superheat to 300 steam + 35F-100F degrees of superheat.
We at the same time take the 300 pound steam and reduce it to 175 steam + 50F-35F superheat for a 16 unit operation.