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Column Overhead Air Cooled Condenser Outlet Piping Hammering


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#1 Raj@1983

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Posted 02 December 2018 - 07:21 AM

In one of our unit, we are facing the issue of heavy line vibration in column overhead condenser outlet line.

Over head condenser is a air cooled condenser of having 12 bay and each bay consists of two number of fans.

The design is as per UOP.

The run down line from the outlet header pipe is 18" line.

As per UOP design, there is an equalization line (6") from the same header connected to receiver.

As per the UOP design basis, the equalization line to be exactly on top of run don line. Refer attached P&ID.

At present, it is deviating the design. Equalization line is 3 meters far from the run down point. Is it the cause of line hammering and vibration?

UOP mentioned in their design basis, that Equalization line has been given to keep the outlet stream in two phase. Subcooling liquid may creates more problems.

One more observation is the outlets from all the connected to header and rundown line is not at its center. i.e it is not symmetric. What are the impacts of non symmetry in condenser outlet?

 

Now the line supports have been verified for two phase flow and found adequate as per Mechanical.

 

Please provide your valuable inputs.

 

Thanks!

 

Rajesh

Attached Files



#2 fallah

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Posted 02 December 2018 - 09:00 AM

Raj,

 

With limited info in hand, non symmetry might be affected...

 

One more thing is that the equalization line is connected from the same header to receiver while normally it should be connected to the receiver through the dedicated nozzle...it could also might be a cause of heavy vibration...



#3 Bobby Strain

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Posted 02 December 2018 - 10:13 AM

You might get better response if you provide an isometric sketch.



#4 Raj@1983

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Posted 03 December 2018 - 12:27 AM

Pipe routing sketch has attached.

 

It is observed that out of total length, 70% is horizontal in nature with 0.1° slope to make it free drain line. Is it the sufficient one? Do we need to have self-venting design here?

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#5 gegio1960

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Posted 03 December 2018 - 02:12 AM

from a process point of view, the system is highly non-symmetrical and this is not acceptable.

each couple of the outlet lines should be symmetrically joined in a common sub-header and this step shall be repeated down to a single line.

all the pipe segments shall be carefully sized following the rules of the 2-phase flow.

a larger sub-header shall be provided where the symmetry can't be exactly respected (eg if you start with 12 outlet lines, you go symmetrically to 6 and then to 3 but at this point you shall provide the larger subheader to mitigate the loss of symmetry).

the non symmetrical pipes will provide preferential ways to the the fluid and so high velocities and the consequent vibrations in the overloaded sections.

moreover, the operating conditions/performances shall be compared to the design. how much flow? which is the vapour fraction at the cooler outlet?

less generic answers could be obtained with all the details available.

the 2nd drawing you provided is not an isometric and does not show the equalization like. anyway, the outlet from the header seems not in accordance with the good engineering rules of 2-phase flow (ie absence of sharp deviations). 

good luck!



#6 PingPong

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Posted 03 December 2018 - 11:26 AM

UOP mentioned in their design basis, that Equalization line has been given to keep the outlet stream in two phase. Subcooling liquid may creates more problems.
This sounds very strange to me.

Does this mean that the outlet of the aircooler is liquid only (total condensation) ?

Can you post a process datasheet of the aircooler as provided by UOP?

 

How is the pressure in the reflux drum C-19051 controlled?



#7 katmar

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Posted 04 December 2018 - 01:04 AM

Pipe routing sketch has attached.

 

It is observed that out of total length, 70% is horizontal in nature with 0.1° slope to make it free drain line. Is it the sufficient one? Do we need to have self-venting design here?

 

My experience has been that 2-phase flow in horizontal pipes is often problematic.  I have always designed condenser outlet piping for self venting flow in any section where 2-phase flow is possible.  My rule of thumb (self imposed) has been a minimum slope of 45 degrees, but that could be overkill.  What is the liquid volumetric flow rate in the 18" section?



#8 Sharma Varun

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Posted 04 December 2018 - 10:58 PM

In one of our unit, we are facing the issue of heavy line vibration in column overhead condenser outlet line.

Over head condenser is a air cooled condenser of having 12 bay and each bay consists of two number of fans.

The design is as per UOP.

The run down line from the outlet header pipe is 18" line.

As per UOP design, there is an equalization line (6") from the same header connected to receiver.

As per the UOP design basis, the equalization line to be exactly on top of run don line. Refer attached P&ID.

At present, it is deviating the design. Equalization line is 3 meters far from the run down point. Is it the cause of line hammering and vibration?

UOP mentioned in their design basis, that Equalization line has been given to keep the outlet stream in two phase. Subcooling liquid may creates more problems.

One more observation is the outlets from all the connected to header and rundown line is not at its center. i.e it is not symmetric. What are the impacts of non symmetry in condenser outlet?

 

Now the line supports have been verified for two phase flow and found adequate as per Mechanical.

 

Please provide your valuable inputs.

 

Thanks!

 

Rajesh

As per my understanding this equalizing line is provided to float air cooler outlet directly with reflux drum, maintaining same pressure at cooler outlet (thereby avoiding any back pressure due to liquid head between the cooler outlet (cooler being total condenser) & reflux drum). Further even if the air cooler is designed as total condenser in case some non condensables exist they can directly escape to the reflux drum.
 
For better understanding & clarity please specify relevant P&ID notes, say 6 to 8.

Specify operating pressures of overhead circuit and condensate density at actual conditions.

And please confirm the elevation difference (Actual as per construction) between the air cooler top (vent flange) & top of equalizing line.

 

 

UOP mentioned in their design basis, that Equalization line has been given to keep the outlet stream in two phase. Subcooling liquid may creates more problems.
This sounds very strange to me.

Does this mean that the outlet of the aircooler is liquid only (total condensation) ?

Can you post a process datasheet of the aircooler as provided by UOP?

 

How is the pressure in the reflux drum C-19051 controlled?

 

This must be a total condenser and pressure balancing may be by push pull (split range) arrangement on the reflux drum, Rajesh can confirm.

 



#9 PingPong

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Posted 05 December 2018 - 02:13 AM

I also think it is a total condenser. However the oulet pressure of the aircooler (tie-in point of the equalizing line) will then be lower than that in the reflux drum due to the static head of the liquid column in the 18" line. The gas flow in the equalizing line will then be towards the aircooler outlet, thereby introducing gas into the liquid and creating two-phase flow, and so creating the vibration problem as liquid slugs hit the bends in the 18" line.

Even if there were non-condensible gas pockets at the aircooler outlet they would not be able to leave through the equalizing line as the drum pressure is higher for reason already described.



#10 Sharma Varun

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Posted 06 December 2018 - 12:18 AM

I also think it is a total condenser. However the oulet pressure of the aircooler (tie-in point of the equalizing line) will then be lower than that in the reflux drum due to the static head of the liquid column in the 18" line. The gas flow in the equalizing line will then be towards the aircooler outlet, thereby introducing gas into the liquid and creating two-phase flow, and so creating the vibration problem as liquid slugs hit the bends in the 18" line.

Even if there were non-condensible gas pockets at the aircooler outlet they would not be able to leave through the equalizing line as the drum pressure is higher for reason already described.

In my opinion if we consider the system without equalizing line than the air cooler outlet pressure will be higher than the reflux drum due to back pressure created by liquid head in outlet line.

However once cooler outlet is equalized with vapor space of reflux drum there will be same pressure at both points. Considering drum pressure is maintained by some control & air cooler outlet pressure is dependent on back pressure upto drum.

Thus any un-condensables if present at air cooler outlet, shall have least resistant path through equalising line rather than liquid filled return header.

This is why equalizing line elevation from cooler outlet is important, it should not be filled with liquid due to available head.


Edited by Sharma Varun, 06 December 2018 - 12:19 AM.


#11 Raj@1983

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Posted 06 December 2018 - 12:45 AM

It is not a total condenser. Refer attached PFD. There is a vent cooler and receiver.

 

Operating pressure of column is 0.26 Kg/cm²g and the receiver is at 0.12 Kg/cm²g.

Two phase flow is there. How to mitigate the vibration as firm supports with respect to two phase flow already provided by static stress analysis.

 

Question is similar setup provided to other column overhead systems. here is the only problem of vibration. The difference from other systems is only Equivalization line is not located on top of run down line. How is it causes the line vibration.

 

Attached Thumbnails

  • PFD.jpg


#12 Sharma Varun

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Posted 06 December 2018 - 01:35 AM

It is not a total condenser. Refer attached PFD. There is a vent cooler and receiver.

 

Operating pressure of column is 0.26 Kg/cm²g and the receiver is at 0.12 Kg/cm²g.

Two phase flow is there. How to mitigate the vibration as firm supports with respect to two phase flow already provided by static stress analysis.

 

Question is similar setup provided to other column overhead systems. here is the only problem of vibration. The difference from other systems is only Equivalization line is not located on top of run down line. How is it causes the line vibration.

So all our assumptions of column pressure control go down the line. In this case just check the relative elevation as I have commented earlier.



#13 Raj@1983

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Posted 06 December 2018 - 02:09 AM

Equivalization line is elevated than air cooler. Liquid cant enter in this line.



#14 katmar

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Posted 06 December 2018 - 02:17 AM

If the 18" line is running full, or nearly full, then having the equalization line in the wrong place could well be the cause of the vibration.  Because of the multiple steps in the rundown line it would be best to connect the equalization line to the top of each of the vertical sections.  With your liquid being close to the bubble point you do not need to develop much negative pressure to induce boiling and vibration. 



#15 PingPong

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Posted 06 December 2018 - 05:24 AM

In my opinion if we consider the system without equalizing line than the air cooler outlet pressure will be higher than the reflux drum due to back pressure created by liquid head in outlet line.

No, pressure at aircooler outlet is lower due to the liquid head in 18" pipe, irrespective whether aircooler is total condenser or partly. Aircooler outlet is always located higher than drum so pressure on top of liquid head is lower than in drum..

 

I have never seen a design (not from UOP or any other licenser or design company, my employer included) with an equalizing line between condenser outlet and reflux drum, not when total condenser, not when partial condenser.

I cannot think of any reason why such equalizing line would be a good idea.

It is nonsens if the condenser would be total, it is even bigger nonsens when the condenser is partly.

All it does is recycle vapor from the reflux drum to the condenser outlet and add it to the conderser outlet flow, thereby increasing its vapor content and consequently increasing the velocity of liquid slugs.

 

In the PFD that line is not shown so it seems that somebody added it later during detailed engineering.

My advice: get rid of it, or put a valve in it so that you can close it partly or completely.

And while the unit is down anyway, modify if possible the piping connected to the aircooler outlet header. Now the first part runs horizontal, but ideally it should first run vertical down before running horizontal, as indicated in red in drawing below:

 

Modified Condenser Outlet Piping.jpg



#16 gegio1960

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Posted 06 December 2018 - 08:00 AM

the arrangement with the equalization line is typical of the refrigeration cycles. if I remember well it is described in some "bibles" of the refrigeration.

i agree with the modifications proposed by pingpong about the vertical pipe and the valve on the equalization line.

moreover, I would suggest to apply a positive slope to the remainder horizontal sections and 45° elbows where possible.

finally, nowadays critical lines like this one can be studied by detailed fluidodynamic s/w (CFD). 

good luck!



#17 Bobby Strain

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Posted 06 December 2018 - 10:15 AM

What does UOP recommend? What you have installed now is a total mess!

 

Bobby






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