31 replies to this topic

[katmar]

It is very difficult to predict how much you will need to vent.  In your case I would guess the vent lines will be in the 6 to 10 mm range, with needle or globe valves installed for regulating.  You close the valves as much as possible and open them further when you see that there is a problem with heat transfer.  Eventually you will find the right amount of opening to leave them at, but the loss of steam is negligible.

 

Perhaps I was being a bit pedantic wanting the vents to bypass the condenser.  It is very important to keep incondensibles out of total condensers, but you will have a small through flow to the ejector anyway so the incondensibles will not build up.

 

I agree with Bobby that the vents should be taken from near the condensate outlet at the bottom of the exchanger.  Practical experience has shown this to be the best location.  I am not convinced that it is because the incondensibles are heavier than steam.  In my mind, there is so much turbulence inside the steam chest that the small differences on a molecular level are not going to cause separation.  But there is a net flow of steam and condensate towards the condensate outlet and it will entrain the non condensibles in that direction.  Whatever the reason, we agree on the location!

[curious_cat]

But there is a net flow of steam and condensate towards the condensate outlet and it will entrain the non condensibles in that direction.  Whatever the reason, we agree on the location!

 

This calendria has a design where steam enters at the center. In hindsight I think this one was designed to be vented at both ends simultaneously. i.e. In addition to the top nozzle that I indicated using for venting I see a spare nozzle at the bottom just above the condensate nozzle. 

 

I could modify the piping to simultaneously vent from both ends. If steam is indeed entering at the center that arrangement might make sense as the NCG would be swept towards both ends. 

 

With a bottom vent isn't the vent line located pretty close to the condensate level? That was another reason I hadn't used the bottom vent originally. If condensate level wasn't tightly controlled it might back up into the vent line. 

[curious_cat]

It is very difficult to predict how much you will need to vent.  In your case I would guess the vent lines will be in the 6 to 10 mm range, with needle or globe valves installed for regulating.  

 

Thanks! Right now the scheme has globe valves on a 1" vent line. 

[breizh]

Curious_cat ,

To support your work , an example of triple effects I've installed and operated . You should notice the yellow line for non condensable material and the connection to the vent condenser prior to vacuum pump (liquid ring )

 

Hope this helps 

 

 

Breizh

[curious_cat]

@Breizh! 

 

Thanks very much for that document. That helps very much! It validates some of my decisions but some questions:

 

1. I see twin vent lines on each HEX. Are the actual locations on the HEX body as indicated in the P&ID? What vent line size did you have. Did you adjust vents manually to prevent steam blow through?

 

2. I see all your yellow vacuum vents are tied in upstream of the condenser. I presume the vacuum pump connection is taken from the shell side of HE104? Basically is your vacuum entirely floating? I see no control on the vacuum side. You just let the pump pull the best vacuum it can on the last effect and the other effects adjust themselves? 

 

3. Why were there twin circulation pumps on each effect? e.g. P102 seems typical but what was the function of P105?

 

4. What is the vessel VN107? Is that to catch wetness of incoming steam? 

 

5. For the steam control valve PCV1 what was the controlled variable? Was it just a steam pressure set point? 

 

6. What does HE107 achieve? It looked like it was heating the exiting stream with steam? Is it? 

 

Sorry if I asked too many questions! But that P&ID looked so interesting to me!  

 

I note another very interesting point on your scheme that wasn't brought up so far: You seem to be flashing condensate from one effect into the higher vacuum of the next to get some additional flash steam advantage? 

Edited by curious_cat, 19 July 2015 - 02:35 AM.

[breizh]

Curious_cat ,

 

Answers to your question .

 

1) yes ; 25 mm throttling valve ; yes

2) yes ; no the vacuum is controlled on the 3rd effect and the others adjust themselves

3)  1 pump is a circulator ( bottom of the effect) , the other is for feeding  the next effect ....easier control for operators 

4) yes , to catch some water during the expansion

5) Pressure is set and maintained

6) Additional heat exchanger to maintain (increase) the temperature to mitigate the risk of crystallization in the pipe, some distance from the outlet of the 3rd effect to the crystallizer .

 

Hope this helps

 

Breizh

[montie3]

Thank you for the great discussion. I'm designing 4-effect caustic soda concentration system.

The discussion was really informative I've found some answers to my questions and felt the obligation to express my pleasure.