Steam-to-water HX pressure relief, vacuum breaking, and air removal

[MinnesotaSlinger]

I am installing a steam-to-water HX with 450 F, 40-60 psig superheated steam. (Our plant has no saturated steam distribution system, so we're stuck with using superheated steam. Our HX is selected and sized for this at our design conditions.) I'm wondering about what components I need on the unit for pressure relief, vacuum breaking, and air removal. The first is essential, of course. I'd say the second is as well. As for air removal, an F&T steam trap on the condensate line can remove air, but wouldn't I also want an automatic air vent on the top to remove air especially during S/U? Also, the HX has two 3/4" connections, one on the top of the shell and one on the elliptical head near the steam inlet, which is near the top of the shell but not quite at the top? Could I put all three off of the top connection, or can/should I use the connection on the elliptical head? Thanks.

 

[dougarthur42]

Sounds like you have what you need, if the Hx is rated for SH steam. You should have a pressure relief valve on the inlet or shell side to prevent over pressure, that part is true. automatic air vent is not needed. upon initial start up you will need to bleed or drain out any excess air from both sides( shell and tube). and this can be done manually with a hand valve. when drain flows steady with no air then close the drain and put into service. The steam side can be done the same way. introduce steam slowly with vents and drains open when the blow pure steam then air has been removed,close the drains and vents and let the flow go through the steam trap. This should work fine. Note. Flows should go in the opposite directions, drains and vent work best when they are on the outlet side of the flow. these drain can be blown manually after the unit has been in service for 15 minutes or so.

 

[MinnesotaSlinger]

Thanks, but is there a way to start up a shell-and-tube steam-to-water HX without manual valve operation? The plant has few operators and would want to be able to start water and steam flow to HX remotely without need for manual operation. Can that be done?

 

[TBP]

You very much DO NOT want superheated steam in your HX. You won't get any measureable heat exchange until the steam gives up it's latent heat - and it can't do that until the sensible heat that is superheat - is gone.

Until you get past the superheat, your heat exchanger will behave as if it is airbound.

 

[MinnesotaSlinger]

I understand about the superheat, but we don't have a saturated distribution system at the plant. I looked at putting in a desuperheater ahead of the HX (which would have been smaller) and getting a bigger HX sized by the manufacturer for the superheated inlet condition, and I went with the latter, since the cost and complexity of the desuperheater and length of demineralized water piping to the desuperheater made the desuperheater option worse. Given that we aren't getting some gargantuan HX, the additional cost of the bigger HX wasn't that significant compared to the cost of desuperheater equipment.

 

[GenB]

So no latent heat no steam trap to shoot condensates your only fix is to use orifice trap,which are not actually traps, it will cost you 20% of the steam used or needed but it will be heat exchange. Genblr'

 

[MinnesotaSlinger]

Thanks although I'm not quite sure what you mean by no latent heat. The HX is sized and has materials selected so that the portion near the steam inlet desuperheats the incoming steam and can accommodate the large delta-T. (Ideally, I would have had access to saturated steam, but I don't, so I have to use what the plant has available and select equipment accordingly.) After that, latent heat transfer occurs as it would in a normal saturated steam case, with all incoming steam condensing. If incoming steam didn't desuperheat and condense (e.g., no water flow through the tubes), that would be bad, but a lot of bad things can happen if one doesn't use their mechanical equipment appropriately.

By orifice trap, are you referring to the trap from the condensate nozzle (i.e., shell outlet flow)? I am intending to use a F&T trap since the condition there should be no different than if a shell-and-tube HX with saturated steam coming in and saturated condensate leaving. I don't understand why I would want to use an orifice trap unless you're referring to a different location on the HX.

 

[novakoni]

Hi Minnesota,

I am from Europe - and maybe that's why I don't get all your expressions correct but this are my comments:

- usually you vent and drain at start up with manual valves - if you want you can make this valves with actuator (electrical or pneumatic) - but that's expensive solution as needed only for start-up.
- during operation is good idea to have continuous air venting from steam space (as removal of non-condensable gasses). This is usually done at steam-condensate surface inside heat exchanger - as this inert gasses will be released exactly at this point.
- Vacuum breaker standard place is highest point of steam inlet pipe - after control valve.

 

[GenB]

"I am intending to use a F&T trap"

Go ahead, use the F&T trap, if it does not work, go back to orifice.
And please let us know the results.
Good luck and goodbye. GenBlr.