11 replies to this topic

[halkeshhulk]

Can someone explain why even very small quantities of non condensables in steam affects its condensing ability very much?

[latexman]

Since it will not condense, and by it's physical presence, non-condensable gases near the steam/water interface disrupts the condensing heat transfer process.  This adds an extra resistance to the heat transfer.

[Pilesar]

Can someone explain why even very small quantities of non condensables in steam affects its condensing ability very much?

 

What effect are you seeing? Small quantities of inerts should be not much problem unless the molecules accumulate in the condensing equipment. The dew point temperature of steam is affected by its partial pressure. When inerts take up the vapor space, the steam partial pressure is reduced and the condensing temperature is lowered. In a heat exchanger, the temperature difference between the hot and cold side would be smaller which affects the exchanger capacity.

[halkeshhulk]

Can someone explain why even very small quantities of non condensables in steam affects its condensing ability very much?

What effect are you seeing? Small quantities of inerts should be not much problem unless the molecules accumulate in the condensing equipment. The dew point temperature of steam is affected by its partial pressure. When inerts take up the vapor space, the steam partial pressure is reduced and the condensing temperature is lowered. In a heat exchanger, the temperature difference between the hot and cold side would be smaller which affects the exchanger capacity.

It is observed in our lean amine pre heater with LP steam, huge fluctuation of steam control valve opening and its flow to maintain Delta Temperature of just 5degC and then later we noticed, steam venting through vent line after maintenance is improper.

Control valve opening fluctuated from 15 to 30% valve opening.

Once it is done everything went smooth. Later on control valve opening of Lp steam stabilized at just 2% to make Delta T of 5degC.

[Pilesar]

It is observed in our lean amine pre heater with LP steam, huge fluctuation of steam control valve opening and its flow to maintain Delta Temperature of just 5degC and then later we noticed, steam venting through vent line after maintenance is improper.

Control valve opening fluctuated from 15 to 30% valve opening.

 

A very curious event! I would not have suspected inerts causing this. Many times it is difficult (for me at least) to find a cause that fits the evidence. But that is the fun part... if it were too simple, there would be no need for my company to pay me!

[halkeshhulk]

It is really fun sir, no doubt.

Being production engineers, we neglect many important things given on equipment, either it could be vent valve or balancing line etc.

Everything has got its own importance.

We struggled for 3 to 4 hrs to normalize the fluctuation, by checking desuperheating valve passing, steam traps checking and its bypass opening etc.

Finally just, opening vent valve for 5minutes and made everything normal.

Looks like, this event lesson is, every big problem has a simple solution hiding within the equipment, just utilize and go ahead.

Edited by halkeshhulk, 27 February 2023 - 08:29 AM.

[latexman]

Please describe your lean amine pre heater. Vertical/horizontal? Which side is steam? S&T?

[breizh]

Hi,

This seems to me a very common problem with Thermosyphon reboiler where you need to vent prior to stabilize the equipment.

Based on my experience,

Breizh 

[halkeshhulk]

Please describe your lean amine pre heater. Vertical/horizontal? Which side is steam? S&T?

Horizontal Heat Exchanger

Steam is flowing through tube side.

[katmar]

The reason why even very small amounts of non-condensables have a large impact on the heat transfer is that in a heat exchanger that is condensing steam there is a net flow of steam towards the tube wall where the steam is converted to water and is removed by gravity.  This movement of steam entrains the inert molecules with it so there is also a net movement of inerts towards the tube wall but when it gets to the tube wall it simply remains there and is not removed the way the steam is. So the natural operation of the heat exchanger causes the non-condensables to accumulate at the tube wall and create an insulating barrier.

 

In evaporators it is normal to vent the inerts continuously, and not only at start up.

[Pilesar]

... steam entrains the inert molecules with it so there is also a net movement of inerts towards the tube wall but when it gets to the tube wall it simply remains there...

Thanks for the clear explanation! I tend to think in homogenous vapor mixtures but this makes sense to me that there is a reduced surface area available to the water molecule. I know that venting a condensing steam exchanger effectively is troublesome. When I put inert vents in vertical steam condensing exchangers, I try to put the vents about two-thirds of the way down the shell since air molecules are higher molecular weight than steam. I don't know that it makes much difference after thinking about your explanation of the phenomenon. I wish I could track the phase composition profile inside a steam condenser. A web search suggests this has probably already been done with computational fluid dynamics software but it is not obvious to me how to narrow the search hits to something useful.

[latexman]

So the natural operation of the heat exchanger causes the non-condensables to accumulate at the tube wall and create an insulating barrier.

IIRC, I’ve heard this described as “the tubes are blinded”. Anyone else hear that?