18 replies to this topic

[farid.k]

Hye all.

Normally in process design, we have gas liquid separator (to remove liquid from gas). Anyone has experience to do liquid gas separator (to remove trapped air in liquid)? Normally we simply use vent valve but client refuse because that valve working philosophy is by mechanical and cannot detect when that valve failed. Thus client request to design a vessel in order to separate trapped air in liquid. Draft idea as per below:

 

Fluid: cooling water return and supply (CWR & CWS)

Equipment/item:

1 x vessel separator (located at highest point to accumulate trapped air),

1 x pump (to pump back in cooling water loop). Flow rate pump will be as per inlet of the drum (to maintain the level)

1 x level alarm transmitter (when liquid level at low point, pump will off and venting valve will open at the top to release air) and when low low, it will give alarm to indicate trapped air not release to safe loc.

1 X nitrogen pressure regulator (set at 1 ATM) in order to avoid vacuum inside the vessel incase inlet valve accidentally closed

 

(refer picture for sketch)

 

Personally I don’t have experience to design that kind of system in order to eliminate trapped air in liquid except simply plug in vent valve at high location. Thus I need some other opinion/idea in case anyone of you has experience in this.

Thanks

Attached Files

•  Untitled.jpg   67.64KB   3 downloads

[Bobby Strain]

What is the purpose of this separator? It sounds a bit squirrely to me.

 

Bobby

[farid.k]

What is the purpose of this separator? It sounds a bit squirrely to me.

 

Bobby

dear bobby,

 

to remove trapped air in liquid. 

[Bobby Strain]

I have never seen the need for this function in a cooling water system if a cooling tower is used. If it is a closed system then there will be an elevated expansion tank that is vented to atmosphere. The expansion tank is on the return, from which the circulating pumps take their suction. So, maybe you asked the wrong question.

[farid.k]

I have never seen the need for this function in a cooling water system if a cooling tower is used. If it is a closed system then there will be an elevated expansion tank that is vented to atmosphere. The expansion tank is on the return, from which the circulating pumps take their suction. So, maybe you asked the wrong question.

yes, it is for closed system.

currently they have expansion vessel but on the ground (not elevated). thus i assumed trapped air at high point cannot be vented. so they need a new equipment such as a small drum to trap the air and release it to atm/flare.   

[latexman]

In my experience, this is unusual for a cooling water system.  Is there a reason so much air is present?  Is there proof that air is present?  Or, are they trying to fix something that is not broke?  I don't understand why a high point vent valve cannot be made to work.  A high point vertical section of piping (1 m?) with top vent valve and a dP cell (level instrument) would do just as good and cost so much less.  And, why use nitrogen on a non-combustible liquid?  Just be sure the piping (or vessel, if you really must) is good for FV.

 

farid, I know the client is King, but, with what little I've seen, this is messed up!  Of course, there may be a valid explanation I don't see yet.

[Art Montemayor]

Messed up is a proper description if one considers that air is called the entrained (or dissolved) non-condensable in the circulated water - because this raises the question of why the sketch shows it being vented to a flare.  Air is not a flammable gas, so if there are hazardous flammable gases involved with the circulated water this should be mentioned as to how and why before attacking the effects and not the cause(s).

[farid.k]

Messed up is a proper description if one considers that air is called the entrained (or dissolved) non-condensable in the circulated water - because this raises the question of why the sketch shows it being vented to a flare.  Air is not a flammable gas, so if there are hazardous flammable gases involved with the circulated water this should be mentioned as to how and why before attacking the effects and not the cause(s).

Dear Mr. Art, glad to have your reply here.

 

Why vented to flare? Client said, this liquid-air separator is dual purpose. 1^st is to release trapped gas (client also not sure what kind of gas. Could be dissolve air, inerts or hydrocarbon gas) and 2^nd, the cooling water also supplying to few heat exchangers (mostly water at shell side & hydrocarbon at tube side) and if tube leak happened, the hydrocarbon gas will go through the cooling water pipe and client expected the leak gas to be vented at this vessel since this vessel to be located at the highest point. That why it vented to flare.

[farid.k]

In my experience, this is unusual for a cooling water system.  Is there a reason so much air is present?  Is there proof that air is present?  Or, are they trying to fix something that is not broke?  I don't understand why a high point vent valve cannot be made to work.  A high point vertical section of piping (1 m?) with top vent valve and a dP cell (level instrument) would do just as good and cost so much less.  And, why use nitrogen on a non-combustible liquid?  Just be sure the piping (or vessel, if you really must) is good for FV.

 

farid, I know the client is King, but, with what little I've seen, this is messed up!  Of course, there may be a valid explanation I don't see yet.

 

Dear Mr. Latexmen,

thanks for the reply,

 

“Is there a reason so much air is present?  Is there proof that air is present?”

Last year, their main reactor is trip and no production for quite few days. A study has been conducted by licensor and their final result is, there is trapped gas in the cooling water. When they quantity of accumulated trap gas is significant, trapped gas can goes to the reactor and cause pressure to deviate from normal operating pressure and cause to trip.

In order to operate back, they open some of the cooling water piping at high point and drain to the plant drain continuously in order to avoid the trapped gas accumulate again. And after the draining, the reactor can be run as per normal again until now. But they loss quite significant amount of cooling water. That’s why they want to recover back and make a liquid and gas separator.

 

Why a high point vent valve cannot be made to work

Because, when tube leak happened, they don’t want the hydrocarbon gas leak to atmosphere. Refer answer to Mr. Art.

 

A high point vertical section of piping (1 m?) with top vent valve and a dP cell (level instrument) would do just as good and cost so much less

Yes, actual planning is to have a vertical piping 6” diameter with 3 meters high. Top has small control valve with link to the level transmitter and at the bottom has a small pump. But it found that, that piping volume is too small as the total cooling water that currently drain is quite big (flow rate around 20 m3/h)

 

And, why use nitrogen on a non-combustible liquid? 

The only available sources of gas is nitrogen at utility station at the top of platform. That nitrogen is just to maintain the piping/vessel pressure in case the pressure drop below 1 atm. This is just to protect pump.

 

hope i answering to your question.

[latexman]

farid,

 

Thank you.  You have been very thorough.  The problem is becoming clearer.  They are trying to deal with the gas in a safer, more productive manner, but this does not stop the problem.  I hope they have tried, or are trying, to find the root cause of the gas and preventing it from entering the CW.  That would be a much better solution

[farid.k]

farid,

 

Thank you.  You have been very thorough.  The problem is becoming clearer.  They are trying to deal with the gas in a safer, more productive manner, but this does not stop the problem.  I hope they have tried, or are trying, to find the root cause of the gas and preventing it from entering the CW.  That would be a much better solution

dear mr latexman,

 

i did read some article from vendor. it says:

 

How air enters the water network

Water contains 2% - 3% soluble air. As water temperature rises and/or pressure in the line changes, this soluble air is released from the water. These bubbles grow and rise to the top of the pipe and accumulate at elbows and high points in the system. If not released, air pockets are formed, reducing the effective diameter of the pipe. Velocities higher than 5 ft/s move the air bubbles towards the end of the pipe. Note: The friction of the water along the air layer can be much higher than the friction along the walls of the pipe, especially when the air moves in the opposite direction to the flow of water.

[Bayo Alabi]

The functions of the tank are as follows:

1. Accommodate expansion due to temperature fluctuations and/or process gas leak-in;
2. Disengage entrained liquid from gas before venting - liquid is displaced from the system by process gas in-leak;
3. Remove scale and other solids from the system;
4. Ensure the cooling water system is liquid-filled;therefore the tank must be located at the highest point of the system. 

The problem is that point number 4 above was not taken into account in the original design. The solution, in my opinion, is to relocate the tank to the highest point of he system. 

[manojkaila]

Dear Farid,

 

I have operated near to same type of system. Following are my observation.

 

1 We have one common tank (Little big size) which will behave as a reservoir during system trip.

2 Air / any non soluble gas is released in tank.

3 We are using DM water with chemical addition to avoid soluble air(O2) & corrosion.

4 We have the N2 Padding in the tank with breathing valve. But vent is in atmosphere. ( You can put one online analyzer which will give indication for any hydrocarbon leak , We have one such but in other system)

5 Regular water analysis for the quality.

6 We are using this water as a cooling & heating media for energy saving.

 

I hope this observation will help you.

 

Regards,

[farid.k]

Dear Farid,

 

I have operated near to same type of system. Following are my observation.

 

1 We have one common tank (Little big size) which will behave as a reservoir during system trip.

2 Air / any non soluble gas is released in tank.

3 We are using DM water with chemical addition to avoid soluble air(O2) & corrosion.

4 We have the N2 Padding in the tank with breathing valve. But vent is in atmosphere. ( You can put one online analyzer which will give indication for any hydrocarbon leak , We have one such but in other system)

5 Regular water analysis for the quality.

6 We are using this water as a cooling & heating media for energy saving.

 

I hope this observation will help you.

 

Regards,

Dear manojkaila, thanks so much for the reply.

How about the location? is it at the highest point? 

 

The functions of the tank are as follows:

1. Accommodate expansion due to temperature fluctuations and/or process gas leak-in;
2. Disengage entrained liquid from gas before venting - liquid is displaced from the system by process gas in-leak;
3. Remove scale and other solids from the system;
4. Ensure the cooling water system is liquid-filled;therefore the tank must be located at the highest point of the system. 

The problem is that point number 4 above was not taken into account in the original design. The solution, in my opinion, is to relocate the tank to the highest point of he system. 

Dear Bayo Alabi

Massive work needed if we elevate existing expansion drum because the size is quite big (dia:3m), tan-tan 6.8m) and highest point also very high which is around 23m from the ground. That’s why we are trying to create a new one at highest elevation but a very small scale (might be using 6” piping as a drum). What say you?

[Bobby Strain]

You have the right idea. Put a standpipe to connect to the drum and the water return. Connect the return at the high point in the system.

 

Bobby

[manojkaila]

How about the location? is it at the highest point? 

 

Our location is on ground floor because we have pump suction from the tank & after circulation it will come again to the same tank.

 

Regards,

[manojkaila]

Dear Farid,

 

Small part I am attaching it may help you.

 

Regards,

 

Attached Files

•  Hot Water Tank.docx   1.2MB   16 downloads

[Bayo Alabi]

 

Dear Bayo Alabi

Massive work needed if we elevate existing expansion drum because the size is quite big (dia:3m), tan-tan 6.8m) and highest point also very high which is around 23m from the ground. That’s why we are trying to create a new one at highest elevation but a very small scale (might be using 6” piping as a drum). What say you?

 

 

farid.k,

 

I assume you have compared the costs of relocating the old tank and installing a completely new one. Is that right?

 

I am not in position to say if a 6" piping used as a drum is right or not - I simply don't have the data you have.

 

In one of my previous projects, we considered the following with respect to the volume of tank required and alarm conditions:

1. As a minimum, the tank must be capable of accommodating the change in volume of the CM due to thermal expansion. Normally this results in a very small tank and a more practical size is selected. In my case ±1m3 was allowed for this between normal level and high level alarm and low level alarm, respectively;
2. In addition you might want to consider provision of a reservoir for filling individual users or parts of the system. In my case, 5m3 of cooling medium inventory remains below low level alarm;
3. In addition 8m3 of space was provided above high level alarm to allow for cooling medium displacement from the system by a major gas in-leak. A high high level alarm will annunciate at 80%, indicating the risk of cooling medium overflow.

Regards

 

Bayo

[farid.k]

For sure it very costly to relocate existing expansion drum to high point.
To make it short, I am going to design a new small expansion drum at highest location. Anyone who has go by or design guide or good reference?