17 replies to this topic

[gimenz]

Good day.

 

Please excuse my ignorance but I would like to seek for advise/opinion.

We  have this exchanger that uses HP steam and the condensate is received in the steam condensate pot.

 

In the model review, licensor commented that the elevation of the exchanger maybe decreased. Consequently, we have changed the configuration and attached the condensate from exchanger to the outlet piping of condensate pot. (below drawing of attached file).

 

i am not experienced, but i find the configuration is new to me. The way i see it, it may not cause problems because there is an equalizing line on the overhead of the condensate pot.

 

I really appreciate your views on this matter about this new scheme.

 

Thank you so much.

 

Attached Files

•  condensate pot.JPG   22.33KB   36 downloads

Edited by gimenz, 10 June 2013 - 01:42 AM.

[fallah]

gimenz,

 

The problem might be raised due to lack of adequate height of the pot...

 

The licensor comment in model review regarding the vertical situation of the exchanger respect to the pot should has been submitted in relevant PID finalization, because normally the top flange of the pot should be installed above the top of the exchanger's tube bundle in the amount of, let say, 300-400 mm and it could be easily reflected on the PID then to be followed by piping in 3D Model preparation.

 

However, if in modified configuration the above criteria has been met, appears there would be no problem in operational point of view...

[gimenz]

Mr. Fallah sir, thank you for your quick feedback!

 

If I may add sir, what about the liquid level settings. will it be affected with this new set-up?

[fallah]

gimenz,

 

It depends, and it might or might not be possible...

 

In fact, it might be possible provided that: after modification the top flange of the pot would still be above the top of tube bundle and the level transmitter range would still cover the condensate level build up inside the exchanger...

[Art Montemayor]

Gimenz:

 

The top of your condensate pot should be no higher than the bottom of your exchanger.  Additionally, your contractor is trying to save pennies and potentially causing a condensate level error by connecting the equalization line to the steam inlet line.  The pressure in the condensate pot vapor space should be equal to is the vapor space pressure at the top of the exchanger shell - NOT TO THE STEAM LINE.

 

Too many engineeing contractors today are making this very ignorant mistake because their engineering organization at the process engineering level is not organized.  The process engineer on this part of the project obviously forgot about the need to use a condensate pot (or he was not aware of it) and left out a simple 3/4" coupling at the top of the exchanger shell.  When the P&ID was finalized it was discovered that there was no place to equalize on the shell and a "quicky" decision was made to connect to the steam inlet line - which is a lot cheaper and faster than having to cut and weld a coupling on the exchanger shell after it has been built.  I have personally seen this goof up many, many times recently as a Process Consultant and Advisor.  The pressure in the steam line is different (and varying) while the steam is flowing and this causes erratic operation in the condensate pot level.

[latexman]

In the new arrangement, the liquid level settings will affect the capacity of the heat exchanger.  The higher the liquid level, the more tubes are submerged, the less the capacity.  Not a good situation!

[Zubair Exclaim]

Gimenz .... the concept of second type condensate pot is to avoid fluctuation in control. These condensate pots match their level with the level inside the exchanger (increasing level in pot has flooding in exchnager to achieve turn down and decreasing level is vice versa).

 

 

 

The second configuration is exactly the right one, for this condensate pot you need to make sure of following

 

1-working Level setting .... Height working heigh (NLL to LLL shall consist of two parts)

                             a) dia of exchanger x turn down (fraction)

                             b)line losses + exchanger pressure drop

2- High level ....................Height from NLL to HLL 

                             a) Dia of exchanger x (1-turn down)

 

and size the outlet line from exchanger to condensate pot with a velocity of 3 ft/s for smooth operation.

 

the configuration should something like this

 

 Also search for article on condensate pot by Saed Rahimi it gives  aexcellent approach

Attached Files

•  untitled.bmp   667.93KB   117 downloads

[latexman]

A drawing to scale would result in better and more consistent replies.  The first drawing leaves too much detail up to interpretation.

[Bobby Strain]

Art gives the designer a lot of credit by calling them engineers. But this is typical of what you should expect from your engineering contractors now. There is much room for improvement and cost reduction besides the elevation of the exchanger and pot.

 

Bobby

[gimenz]

I'm not sure if this will be appropriate.. but I am overwhelmed with the responses and really appreciate these views/opinion on this thread.

I want to express my sincere appreciation.

[Zubair Exclaim]

Gimenz:

 

 What are you unsure of ?  

 

see the first configuration calls for a bigger condensate pot with a retention capability since only way for condensate out is through condensate pot.

however the second configuration results in a smaller pot since you only have it for a smooth turn up and tun down control for heat exchnager by matching heights, insead of putting a  retention time and vap liq separation as the governing criteria.

[Bobby Strain]

Some might have missed a significant item in the sketch provided by gimenz. The steam inlet has a control valve labeled as TV. I assume that this valve is used to control steam flow to the exchanger based on the cold fluid outlet temperature, but I can only guess since gimenez shared minimal information. So, the function of the pot is simply as a substitute for a steam trap. It has nothing to do with flooding the bundle to vary the exchanger area available for heat transfer. If this were the case, there would be no need for the expensive control valve on the steam intlet line. It's not just the control valve; it's the whole control valve station complete with isolation valves and bypass. To function as a drainer, as the sketch implies, the pot must be located below the heat exchanger so that the exchanger is always drained. All the control is achieved by the inlet steam valve.

 

Bobby

Edited by Bobby Strain, 11 June 2013 - 09:07 PM.

[Flyou]

Gimenz, 

 

I totally agree with Bobby.

There are 2 main kinds of steam heater control:

 

1/ Control on the condensate level

In this case, the level in the pot must be controlled between the lower and higher TL of the exchanger and there is no control valve on steam inlet to the exchanger. 

The effective area in contact with vapor is controlled by flooding more or less the exchanger with liquid water. 

The pressure (and consequently temperature) is constant on steam side.

 

In the equation Q=Ft*U*A*LMTD, you are acting on A.

To have more duty, the level goes down and A is increased and to have less duty, the level goes up and A is decreased.

 

2/ Control on the steam flowrate to the exchanger

In this case, the level of the pot must be controlled below the lower TL of the exchanger and there is a control valve on steam inlet to the exchanger.

The effective area in contact with vapor is always the same, equal to the total area of the exchanger.

The pressure (and consequenltly temperature) varies on steam side according to the control valve opening on the steam inlet.

 

In the equation Q=Ft*U*A*LMTD, you are acting on LMTD, A being constant.

To have more duty, the pressure increases (and temperature also) and LMTD is increased and to have less duty, the pressure decreases (and temperature also) and LMTD is decreased.

 

 

As Bobby said, it seems that for your particular case, option 2/ has been chosen since there is a TV on the steam inlet. 

In this case, the level of the pot must be controlled below the lower TL of the exchanger and not between lower and higher TL as it seems to be the case with your second scheme.

 

Your scheme 1 seems to be more appropriate for control option 2 and your scheme 2 seems to be more appropriate for control option 1.

Nevertheless, scheme 2 could be appropriate for control option 2 as long as the level can be controlled below the lower TL.

 

In your case, the fact that the HP condensate are going directly into the pot or are connected to the bottom line of the pot is not a problem as long as the level is controlled below the lower TL.

 

Flyou

[Flyou]

To be more precise, when i said "Your scheme 2 seems to be more appropriate for control option 1/", i meant your scheme 2 but without the TV on the steam inlet line (and with the TC on the outlet process line cascading the LC of the condensate pot).

 

Flyou

[kabtik]

I wonder whether we will even have some condensate in the second scheme even when the level control valve  is closed. Does the condensate drum extend below the exchanger condensate outlet nozzle?

[Bobby Strain]

Never use condensate level to control exchanger duty. Just connect the temperature controller directly to the condensate valve. You can use pot low level override to prevent steam blowing into the condensate header. If you design to use level for control, it will just get reconnected to operate properly during or shortly after startup.

 

Bobby

[Zubair Exclaim]

 

Bobby Strain:

 

Ok, i missed some action here ...

 

In a conventional set up when you use a inlet valve on the steam:

 

→ steam flows in the exchanger… when heat duty is achieved the inlet steam vale tends to close…. Pressure inside exchanger falls to a level when condensate stops flowing out of the exchanger to a condensate header etc. this condition is called stall

→ after stalling the process side temperature falls again and steam inlet valve opens up again and condensate is dumped out. so in general you have :

                → process side temperature fluctuation between- steam valve closure- condensate stall- condensate dump

                →water hammering due to hot steam and stalled condensate contact.

                → Condition is much worse if you go for a turn down, conditions get worse

                →Thermal stress and stress induced corrosion for tubes

 

Scheme 2 avoids that by :

 

→ There is no process fluctuation since a constant level is maintained in the exchanger to match process duty. And steam/condensate flows in/out of the exchanger continuously

→There is no water hammering since equilibrium is established when a constant level is maintained, steam valve doesnot closes instead it minimized to a flow rate and excess surface area in covered by condensate level.

→Turn downs are well maintained

                          →No thermal shocks and stresses.

 

Sorry for late reply ... topic is probably dead already

[Zubair Exclaim]

 Also to add ...

 

In both schemes no temperature control valve shall be installed at the steam inlet you cannot control flow in and out simultaneously ..

 

→Just use temperature controller on the process side and cascade with level on the condensate side (at outlet of pot)

Also scheme 2 is more advanced. Scheme 1 requires a much bigger diameter for the pot for the retention time required, scheme 2 just controls level by matching it                             

 

I have experience with these systems in refineries associated with reboilers and scheme 2 is the one that is more cost effective and efficient.