8 replies to this topic

[felderosfelder101021]

Hi.

 

So I am dealing with a 2-phase flashing through the PSV. I am tasked to design or size the outlet piping but one of the leads told me I need to size the vapor and liquid lines at the outlet. Where the outlet line eventually splits into two; separate line for liquid and vapor. I am not sure the approach on this sizing method but can you help shed some light on this? See a depiction of diagram of my system below.

I am having doubts that vapor and liquid will fully-separate as it passes through the tee designated. Is it really possible since I am trying to contest that we cannot fully-separate the 2-phases at the tee. 

 

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[breizh]

Hi,

You are correct with your statement, no means to separate the phases like KO drum.

Edit:

Before going to ground you may install a catch pot to separate the gas and liquid.

what is the proportion gas to liquid +gas in your stream?

Depending on the nature and quantity of the product, you may release it to atmosphere.

Consider the chapter safety devices in Perry's chemical engineers' handbook, page 26-31 fig 26-16 

Blowdown drum.

This is a typical set up I had on a polymerization reactor of surfactants, instead of rupture disk it was a PSV.

 

Breizh   

[felderosfelder101021]

Hi,
You are correct with your statement, no means to separate the phases like KO drum .
Breizh

Thank you. I was having second thoughts on this as I was advised to size it this way. If the relief is going to be two phase, it is better to relieve it to the atmosphere? The PSV is located inside a building. The catch is it will relieve a two phase flow inside a building if we decide to relieve it on the ground or grade. Can you confirm if relieving it to the atmosphere make sense? (e.g roof of a building)

[breizh]

Hi,

Yes, definitely to atmosphere, make sure nobody will be hurt during release. 

Pay attention to walkway.

EDIT :  more Information needed about the materials and the Operating conditions

Breizh

[latexman]

Impossible for me to answer; we do not know the components in the fluids, the flow rates, or the conditions (T & P).  We need a better feel for the safety aspect.

[Pilesar]

You can separate the two phases at the tee with a large enough vertical pipe. A standpipe is a legitimate way to separate phases if designed where the Froude number is low enough. When you relieve two-phase fluid upward into the atmosphere, liquid will rain back down. Better to use the standpipe to route the bulk of the liquid downward. I would calculate the required standpipe diameter and then oversize it further. Depending on your relief cases, a hydraulic restriction in the downward section could be included so that the vapor will preferentially travel upward. Hydraulics should be calculated for the entire range of relief situations for the best standpipe design. All destinations of the relief fluid need to be in a safe location.

[felderosfelder101021]

Basically, I just have water relieving being heated due to blocked in of a heat exchanger. The fluid flashes through the PSV as it is being relieved in the outlet line and I'm tasked to size the outlet lines as what I have outlined above. Liquid and Vapor should be relieved to atmoshperic conditions.

Hi,

Yes, definitely to atmosphere, make sure nobody will be hurt during release. 

Pay attention to walkway.

EDIT :  more Information needed about the materials and the Operating conditions

Breizh

 

Impossible for me to answer; we do not know the components in the fluids, the flow rates, or the conditions (T & P).  We need a better feel for the safety aspect.

[felderosfelder101021]

You can separate the two phases at the tee with a large enough vertical pipe. A standpipe is a legitimate way to separate phases if designed where the Froude number is low enough. When you relieve two-phase fluid upward into the atmosphere, liquid will rain back down. Better to use the standpipe to route the bulk of the liquid downward. I would calculate the required standpipe diameter and then oversize it further. Depending on your relief cases, a hydraulic restriction in the downward section could be included so that the vapor will preferentially travel upward. Hydraulics should be calculated for the entire range of relief situations for the best standpipe design. All destinations of the relief fluid need to be in a safe location.

Hi Pilesar, I understand where you're headed at. I believe maintaining a Fruode no. <0.3 will allow vapors to be vented upwards. Should I consider the liquid full-pipe @outlet conditions as the velocity to be used in Froude number calculation (not the two-phase velocity)?
Also, should it be the downward piping only from Tee? Or the vapor line should be same size?

[katmar]

 

Basically, I just have water relieving being heated due to blocked in of a heat exchanger.

 

For the first time in this thread we have some relevant and useful information.  But we still can't answer in any meaningful way.

 

What are the quantities involved?  What are the operating temperatures and pressures?  What is the set pressure of the PSV?

 

We could be dealing with a 1/2 inch screw in psv that can safely vent into a 2" line, or we could have a massive valve that requires very detailed hydraulic and mechanical design for the piping and the knock out drum.

 

This is partly why you have received contradictory advice so far.  Some people (myself included) imagined that we could be dealing with a 50 tonne reactor of poisonous organics and some might have imagined the water system that you have.  Engineering has to deal with specifics and generalized advice should never be relied upon.