7 replies to this topic

[AndyChemEng]

Hi all,

 

I've got a vessel with the following pressure rating - (-64/+64mbarg).

 

There is a desire to clean this vessel with hot water via a sprayball as hot water has proven to be more effective at cleaning the material we process (non-flammable formulation work).

 

My concern in reagrds to utlising hot water for this system is specifically the inbreathing rate associated with vapour collapse - hot water cleaning suddenly changing to cold i.e. 60oC saturated vapour space converting to 10oC saturated space.

 

Info:   10m3 vessel     -64/+64mbarg

Hot water: 60oC

Cold water: 10oC

Sprayball: 7.2m3/h total flow

Relief stream: 4" swan neck - 2300m3/h relief capacity

 

Calculating the relief volume is relatively straightforward but the rate is hard to quantify. Is it 1 second? 5 seconds? 10 seconds? 6.5 seconds is the maximum rate that this relief stream could handle under these conditions.

Is there any guidance / good practise in regards to these operations? Do i do an enery balance approach i.e. entalpy balance to work to relate time vs temp assuming instant equilibrium conditions?

Or is good practise to avoid hot cip cleaning of atmospheric vessels in general to eliminate the risk and accept the poorer cleaning efficiency - water usage/waste incineration/time etc?

 

Any advice is much appreciated.

 

Andrew

[breizh]

Hi ,

Can you describe your reactor ? Do you have a manhole ?  If yes keep it open during the Cleaning process . 

Right now I don't see the problem using hot water # 60 C to clean the wall , make sure the temperature is well managed .

note : I've been using hot water to clean latex reactors, surfactants reactors . 

 

I may have missed your point.

Breizh 

[AndyChemEng]

Hi breizh, there is man-way door but that could be left closed and ideally i would prefer that having this open isn't a critical operation and would prefer to know that the 4'' open vent swan neck is enough to cope with the sudden inbreathing duty.

 

My query is to do with what happens if you have a 60oC saturated water vapour space and suddenly spray cold water throughout the vessel.

My worry is water vapour collapse coupled with the air density change will cause a sudden negative pressure which could lead to implosion i.e. a negative pressure less than -64mbarg.

 

An extreme example being steam collapse. 

 

Andrew

[latexman]

Is the vessel sheltered from the weather?  Can it see a cold rain, snow, sleet, hail?  Is it insulated?

 

Could the hot and cold water be mixed to a controlled temperature?  Is the vessel pressure monitored?  Such that one could systematically creep up to a hotter and hotter temperature while monitoring the pressure?  Kind of a testing phase, if you will.

 

After a hot water water spray, could the vessel be purged with plant air before cooling with cold water?  That would reduce the risk of vacuum and collapse.

[breizh]

Hi ,

Sorry I don't understand your process and I'm still puzzle by the fact you are using cold water @ 10C not hot water ,you mentioned earlier to be more effective .

To me you should clean your reactor using tempered hot water having the manhole open to visualize the cleanliness of the reactor prior to rinse , if necessary with cold water (????) .

 

note : based on my experience with latex , I've seen inlet PSV clogged with material . Pay attention to that too.

 

I cannot offer more, good luck

Breizh 

[AndyChemEng]

I'll try and explain better.

 

We currently clean the vessels with cold water via a spray-ball. 

Operations would like to use hot water but the risk of underpressurising the vessel due to vapour collapse has been raised in the past.

 

If we used hot water (60oC) to clean the vessel and this was suddenly changed to cold water, lets say the thermostatic valve failed,then it would be expected that you would see a significant underpressure occur inside the vessel. The vessel's relief stream would have to protect against this.

 

My question relates to vapour collapse for relief. If you spray cold water into a hot vessel then a significant under pressure would occur.

[AndyChemEng]

Hi latexman, 

 

I would like to calculate if my relief stream can handle the event of vapour collapse.

 

I've attempted to model what would happen if cold water was suddenly sprayed into a hot vessel for the conditions I specified above.

This is for 3.2kg/s of 10oC water being added to 10m3 60oC vessel which seems to suggest that the vessel should be ok just with its swan neck 4" relief in preventing an underpressure below the design.

 

 

[AndyChemEng]

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