9 replies to this topic

[Marco78]

Dear all,

first of all I would like to introduce myself. I'm a chemical engineer and after many years of working as project engineer I have to do some hydraulic calculations...they are very simple but I have some doubts, sorry if they look dumbs.

The attached scheme describes the system: liquid with flow rate of 2,8 m3/h comes from a system (column+condenser)to a gravity separator (E-1). The scheme simplifies the situation in which the bottom valve is closed and the liquid exits from the top of the separator.
The plant manager asked me to verify that liquid doesn't go to vent line. My idea is calculating
pressure drop along the discharge line to the second vessel (E-2) and demonstrate (converting it to mcl from bar) that is lower than the distance DH (that is the vent height). Is it correct?
From my calculation the pressure drop will be negative (due mainly to the difference in height between the two vessels).
Thanks in advance for your help
Marco

Attached Files

•  system DP.pdf   18.44KB   136 downloads

[Art Montemayor]

Marco:

Your scheme possibly is not as simple as you might think. Your schematic is very general and doesn’t show the details of what you call “the bottom valve is closed and the liquid exits from the top of the separator”. If what you are describing is a type of liquid gravity flow from one vessel to another, then you have a system that should be analyzed much more in detail as to how it is actually proposed to be piped-up. You could potentially create a bad installation if you don’t take some critical fluid flow factors into consideration.

If you are “overflowing” from one vessel into another by gravity, then you have to analyze whether you are trying to flow the liquid in the overflow pipe as 100% liquid-full – something that I don’t think you want to do.

You can also make the overflow line self-venting – which is what I would normally recommend.

But you have to be more detailed as to what you are proposing (or have already in-place). Additionally, what is “mcl”? Why are you having a DN 40 pipe as a vent? Are you separating any non-condensables or other vapors in vessel E-1?

I recommend you furnish a detail sketch of how the piping is going to be connected – to the vessels and in the field. The hydraulics involved can change dramatically, depending on how you install your piping. Also define your fluids and compositions as well as all pressures involved.

You should also employ our SEARCH machine and look up gravity flow. You should find my copy of P.D. Hills’ classic article on gravity flow and you should study it.

Await your reply.

[Marco78]

Dear Art Montemayor,

first of all many thanks for your attention.
Please find the attached sketch (a bit clearer I hope even if I have made it by hand...I don't have MS Visio at home).
Normally the vessel (it is a gravity separator) works in continuous and I think the overflow of the light phase is self-venting. What do they asked to me is to check the overflowing line in case of wrong action (that is closing the bottom valve): what they don't want is flowing of the liquid through the vent line. I think that the vent line is an equilibrium line (between the two phases discharge lines) in order to avoid siphoning.
mcl stated for meters of liquid column (sorry I made confusion with my language) and the fluids are water and an hydrocarbon (lighter than water), the pressure is atmospheric
I'll search your article on gravity and I'll study it.
Thank you very much in advance,
Marco

Attached Files

•  sketch0001.pdf   265.95KB   108 downloads

[Zauberberg]

This doesn't look as a promising setup as you might pressurize the overflow vessel and prevent the light liquid phase from draining through the DN 25 line into it - that will certainly lead to overflowing the vent line as well. A siphon breaker is required.

Similar rules apply for draining Heavy liquid phase from the collection vessel - make sure that you have properly sized outlet nozzle/piping for gravity flow.

In general, my comment is that you have very unreliable and difficult-to-handle design/system. The light liquid phase drain line should be separated from the vent line, and equipped with a siphon breaker device. If nothing else is feasible, generously size the light phase outlet line going into the vessel shown at bottom of your sketch.

[Marco78]

This doesn't look as a promising setup as you might pressurize the overflow vessel and prevent the light liquid phase from draining through the DN 25 line into it - that will certainly lead to overflowing the vent line as well. A siphon breaker is required.

how pressurize it? This was my former question: if the bottom valve is closed (consider it as possible human error) the overflow (by gravity) through the dn 25 line will generate pressure drops and consequently a pressure that has to be smaller than the height of the vent line, is it correct? My calculation leaded to a negative pressure drop (due to the height difference) avoding the flow through the vent (1 ,5 meters higher than overflow line)
which kind of siphon breaker you suggest? from my experience in my company the siphon breakers are made as you have seen in the sketch.

Similar rules apply for draining Heavy liquid phase from the collection vessel - make sure that you have properly sized outlet nozzle/piping for gravity flow.

I'll check it

In general, my comment is that you have very unreliable and difficult-to-handle design/system. The light liquid phase drain line should be separated from the vent line, and equipped with a siphon breaker device. If nothing else is feasible, generously size the light phase outlet line going into the vessel shown at bottom of your sketch.

You are perfectly right and I agree with you: for such an application I would design a coalescer working under pressure (with a pump) and a dLC for the discharge of the heavy phase, the problem is the purchasing of such equipment and instrumentations...

Edited by Marco78, 24 September 2010 - 03:12 PM.

[Zauberberg]

If there are no safety concerns with respect to introducing air into the overflow line, a common vent line open to atmosphere should be sufficient. A common practice is to have the vent line size at least 50% of the drain line - having it bigger doesn't harm as well. You can follow the existing company practice as well, and connect the overflow line in the same way as it was done for the Heavy liquid line.

The lowest vessel (and the drain/overflow line) can be easily pressurized by vapors evolved from the liquid phase - it is a common phenomenon in gravity drain systems. In such case the flow will stop, and level in the main vent line will rise in order to overcome the pressure difference. If this difference is too high, you will end up with liquids in the vent main.

Gravity Flow by Art Montemayor, and another article on the subject, re-attached below.

Attached Files

•  Gravity Flow by Art Montemayor.zip   1.03MB   224 downloads
•  Designing Piping for Gravity Flow.pdf   1.02MB   270 downloads

[Marco78]

If there are no safety concerns with respect to introducing air into the overflow line, a common vent line open to atmosphere should be sufficient. A common practice is to have the vent line size at least 50% of the drain line - having it bigger doesn't harm as well. You can follow the existing company practice as well, and connect the overflow line in the same way as it was done for the Heavy liquid line.

Thanks, we have a very few atmospheric vents because of the fluid's nature. In this case it is flammable and toxic.

The lowest vessel (and the drain/overflow line) can be easily pressurized by vapors evolved from the liquid phase - it is a common phenomenon in gravity drain systems. In such case the flow will stop, and level in the main vent line will rise in order to overcome the pressure difference. If this difference is too high, you will end up with liquids in the vent main.

Ok, I didn't understand you referred to that tank. Anyway it is vented and the inlet pipe is actually a siphon (I will consider its heigth in pressure drops calculation).
So, considering atm pressure for that tank is it correct my idea to calculate the pressure drops in the overflow line (they are slightly negative because of the difference in heigth = -1 meter) to veryfing that they are lower than the vent line height (1,5 m above the overflow line)

Gravity Flow by Art Montemayor, and another article on the subject, re-attached below.

Thank you very much. I'll download them and read them very carefully.
You are very kind to respond to my (even dumb) questions

[Zauberberg]

Pressure drop calculation - it is correct, but only in the case if there are no vapors that may evolve from the liquid phase. If anything flashes out from the liquid, flow through the overflow line will choke, and the main vent line start to overfill with liquid.

[Marco78]

Pressure drop calculation - it is correct, but only in the case if there are no vapors that may evolve from the liquid phase. If anything flashes out from the liquid, flow through the overflow line will choke, and the main vent line start to overfill with liquid.

The temperature of the fluid will be 20-25 °C (more or less 80-90 °C less than atmospheric bp). From the system I described during overflow do you think can be any (substantial) pressure reducing?

Edited by Marco78, 24 September 2010 - 04:17 PM.

[Zauberberg]

It seems not, but I like sleeping quiet at nights.