12 replies to this topic

[Butterfly]

I have come across two different datasheets, with different design conditions in similar service (different engineering companies).

 

The equipment is a vessel full of LPG liquid (no gas phase). One is designed for vacuum condition ant other isn't. My thought on this was, if the vessel is isolated, liquid full, and temperature falls, with density change liquid will vaporize (LPG equilibrium), so threre won't be vacuum condition. And if, in another situation, I have the vessel isolated and I open a drain valve (or a vent valve). I don't think I will have vacuum either.

 

The vessel is located at pump discharge line. There are no pumps at vessel outlet line.

 

What is your opinion on this? Is vacuum condition possible? Maybe if I open a vent/drain valve too quickly/too much?

 

Thanks in advance

[Bobby Strain]

You should not do engineering by opinions. Especially those of persons you don't know. That could lead to problems. You should rely on your skills, or the skills of others in your organization. LPG can be lots of things, too. You should provide more information so anyone who responds will have a basis for their response. There may be good reason for the different design conditions. Or, maybe not.

 

 

Bobby

[Pilesar]

When I specify a pressure vessel, I don't try to determine a worst case vacuum scenario -- I specify that it be suitable for full vacuum. Perhaps the designer of one of your vessels had a similar philosophy.

[fallah]

I have come across two different datasheets, with different design conditions in similar service (different engineering companies).

 

The equipment is a vessel full of LPG liquid (no gas phase). One is designed for vacuum condition ant other isn't. My thought on this was, if the vessel is isolated, liquid full, and temperature falls, with density change liquid will vaporize (LPG equilibrium), so threre won't be vacuum condition. And if, in another situation, I have the vessel isolated and I open a drain valve (or a vent valve). I don't think I will have vacuum either.

 

The vessel is located at pump discharge line. There are no pumps at vessel outlet line.

 

What is your opinion on this? Is vacuum condition possible? Maybe if I open a vent/drain valve too quickly/too much?

 

 

Hi,

 

To get a proper and reliable response, please provide a simple sketch of the system you described along with a brief description of relevant operating conditions...

[latexman]

Sometimes, and I'm not saying this is or isn't the case here, the pressure rating of a vessel controls the metal thicknesses.  If this is so, the FV rating is basically free.  But, you have to ask for it!  Maybe, that is what you are seeing.  Why would you ask for a FV rating if you really don't need it?  # 1 because it was free, and # 2 pressure vessels may serve several purposes/processes before they are old, corroded, and not fit for service.  That FV rating may come in handy in the future.  I've seen pressure vessels about 100 years old in their 3rd or 4th service.  I would be thankful to a wise, old (experienced) engineer who had the vision to ask for a FV rating.

Edited by latexman, 25 January 2021 - 08:57 AM.

[Butterfly]

Thanks so much for all your answers! You are totally right. Ok, my thinking (maybe wrong) is that, from a process point of view, I don't need a full vacuum rating. I attach a simple sketch (there is a filter upstream of the vessel. Vessel has a thermal relief valve). Operating conditions downstream of PCV are 17 barg and temperature is ambient conditions.

 

I will try to ask the engineering company in order to understand the case. Of course I won't have a vacuum if, after shut down with everything full of liquid the vent is open (it connects with a low pressure system) and I don't think that it will happen if I open the drain either. And if density increases during a cold night, volume shrinkage will be compensated by fluid vaporization.

 

 

[Butterfly]

Forgot the sketch

Attached Files

•  Sketch.PNG   1.16MB   2 downloads

[Pilesar]

Even if you won't have a vacuum in normal operation, it is difficult to account for all situations the equipment may encounter. What if the interior gets fouled and during a shut down in the future someone decides to steam clean the inside? And then when they think it is all clean, they turn the steam off and close the vessel up to keep it clean? The remaining steam condenses and pulls a strong vacuum! The designer is then asked about it and says 'who can think of all the crazy things people are going to do to a vessel in the future?' And he would be right! So I specify pressure vessels for full vacuum just in case. I've done process optimization projects for old plants where I've had to replace equipment with new equipment the same dimensions as the old equipment and the only difference being in the vacuum rating. If the original equipment had been spec'd for full vacuum, the project would have been more profitable. Processes change and often equipment pressure ratings are real constraints.

Edited by Pilesar, 30 January 2021 - 09:35 AM.

[Bobby Strain]

Pilesar sounds like a client, not an engineering contractor. And, the contractor needs to follow the client's specification, not add unnecessary cost to a project. I loved fixed price contracts as a contractor. Always opportunities for client changes, like asking for full vacuum rating after the project is started with the contract signed.

 

Bobby

[Noah Tibasiima]

Personally I always  specify an FV rating when considering that my system will meet any of the requirements:

1. the system normally operates under vacuum

2. the system is subject to vacuum during start-up, shut-down or regeneration

3. the system normally operates full of liquid and can be blocked in and cooled down

4. the system can undergo vacuum through the loss of heat input though this is on a case by case basis

 

So to answer your question I would support the design at vacuum conditions since the system would satisfy option 3.

[breizh]

Hi,

In case of vacuum (hypothetical) why not adding  a rupture disk on the top to mitigate the risk ?

Should be based on risk analysis .

My 2 cents

Breizh 

[MAHSANOORI]

You can refer to specific volume/pressure diagram for mixtures. Since your vessel has not any outlet therefore, mass and vessel volume are constant. If liquid temperature falls (refer to specific volume/pressure diagram), pressure also will fall but specific volume (specific volume is equal to volume when mass is constant) can not increase (because your vessel volume is constant and vessel is full of liquid). Therefore volume will decrease with density increasing (specific volume decreasing) and a part of vessel will be empty and vacuum condition will be happened.

 

[fallah]

Full of liquid LPG vessel is normally designed for full vacuum; especially if the LPG is well sub-cooled and is subject to the ambient temperature falling...or LPG drainage from the vessel...

If the minimum ambient temperature is lower than the corresponding temperature of the saturated vapor pressure equal to the atmospheric pressure, saturated LPG containing vessel could be designed for partial vacuum corresponding to the vapor pressure of the LPG at such minimum ambient temperature...

Edited by fallah, 02 March 2021 - 02:43 AM.