re: Tube Plugging Margin

[TurbineBlade]

Hi,

I would like to gather information about factors to consider when plugging the HX tube. What are the factors that I should considered? The HX is a low pressure and low temperature (<70C) application.

Thanks

 

[rmw]

Please give more information about your application; tubesheet material, tube material, service (e.g. steam, water, oil, other) accessibility and anything else you can think to tell about it that would help us give you some 'tips'.

rmw

 

[TurbineBlade]

HI rmw,

Tube Side
P=0.7 Mpa(g),
Tinlet = 66C,
Flow 525 kg/s
Secondary side
P=1.03 MPa(g),
Tinlet=22C,
Flow : 630 L/s

The tube O.D. is 0.625in,in a triangular pitch arrangement
Tube Material: Stainless Steel

 

[gr2vessels]

blade,
There are many type of tube plugs, just google the "exchanger tube plug". For various service and applications, materials and pressure / temperature conditions, you will find a suitable plug. Some fancy explosive applications, some rammed up the end of the tube, some welded, etc..
However, the plugging is not a simple issue, you need to understand that the plug could fail also and you must take that in account. The reply from rmw tried to point you towards the need for you to understand ALL of the issues associated with plugging a HEX tube. Do you have a full understanding of why did fail the tube joint first place? One tube leak might herald the catastrophic failure of the whole equipment, hence rmw needs to make sure he's asking all the relevant questions and you answer to all of them, before committing to an answer suited to your problem.
Please read again rmw's reply and try to answer correctly all the items he listed. Send as many as possible (or allowed) details, in order to get the best answer, so you and others in your shoes may learn from the topic.
cheers,
gr2vessels

 

[TXiceman]

IS the tube failure in the tube wall or is it at the tube/tube sheet joint?

As noted above, there are several types of tube plugs. The most simpleis the tapered plug that is driven into the tube ends to isolate the "Bad" tube. Problem with this approach is how hard do you drive the plug. If you drive it too ahrd, it will distort the tubes and effect the ajoining tube seals, especially if it is a less than TEMA tube pitch and thin tube sheets.

Then you start to chase a leak in an ever expanding circle.

Next is how many tubes to plug before you adversly effect the heat transfer. If you have a HE rating program, you can check the selection based on gropping some of the surface area. Generally you can go to about 5% of the tubes before you notice a difference. If all of the tubes are plugged in one pass, watch the tube velicity to make sure you do not creat a errosion problem.

A plugged tube should not be considered a permenate repair, although some run for years. You need to get to the root of the failure, especially if you have more tubes fail.

Ken

Ken
KE5DFR

 

[TurbineBlade]

Hi all,

This HX was designed 30 years ago. The HX is for water cooling application. About 3% of the tubes are to be plugged to fix a FIV problem. During commissioning dated back 30 yrs, it was found the HX has a FIV problem when the secondary side flow reaches to 100%.

The uncertainty that I am facing is that I do not know the condition of the HX. The HX was inspected 15 years ago and eddy current inspection was performed. Everything looked fine. However, things may be different when we inspect the HX now. Therefore, I am trying find out about industry practise when it comes to the tube plugging. So far, 5% seems to be the magic number without doing any in-depth analysis. If it is >5%, what are the things that one should be aware.


Tube sheet material: SA 240 TP304

thanks

 

[TXiceman]

As you plug tubes, you loose heat transfer surface and you also will increase tube velocity. Generally you wwant to keep tube velocity under 7 to 8 fps with 5 to 6 being preferred.

You can calculate the heat transfer surface loss from the number of tunes dropped. In your heat transfer equation
Q=U*A*LMTD, assume U is constant and assume your LMTD is constant. With the reduced area, you can calculate the reduce heat transfer duty.

With fewer tubes, and a reduction in surface, the LMTD will drop a bit as well. If you take more tubes out in one pass than another, becare of the tube velocity in the pass wit the most dropped tubes. In order to keep water velocity within range, you may have to drop the flow on the chiller a bit.

Like I said, if you have access to a program line HTRI or B-JAC, you cam run the exchange with different tube counts to see how it is effected.

You can predict the out come but it is hard to hit it 100% on the money.

Ken
KE5DFR

 

[rmw]

Your question is more understandable now. As a general rule of thumb 5% is the plugging margin. However, 30 years ago some HX's (this sounds like a FWH or a ACW to me) were designed with plenty of excess margin on the pressure drop sizing. I'd get the mfgr or a competitor of the mfgr to run the calculations if you can't.

However in either case, TXiceman has hit on the limiting factor and that is the velocity limit of the SS tubing which is 7-8 fps. Plug until you reach that limit. Make sure you don't have to account for any high flow scenarios.

Material wise, you will be safe with tapered SS plugs at this low pressure. No use to go to heroics like explosive or hydraulic; way too over kill.

For the tubes that you are plugging for the FIV reasons, are you planning to put a rod in the plugged tubes (at least in the FIV zone) or stake the tubes in the zone that is experiencing the FIV?

If you are plugging otherwise good tubes, you should consider puncturing the tubes before plugging.

rmw

 

[MJCronin]

Turbineblade...

Plugging of S&T HXs is sometimes more of an art than a science. You can easily make more problems than you currently have....

It may be advisable to call in an expert and get his opinion.

There are two individuals who have a lot of experience in this area:

Carl F Andreone & Stanley Yokell ( The MGT Consultin network)

http://www.mgt-inc.com

Consider the purchase of this book:

http://www.amazon.com/Tubular-Heat-Exchanger-Inspection-Maintenance/dp/0070017786

-MJC

 

[Stanman1]

Thanks to the individual who suggested you contact Carl Andreone or me (Stanley Yokell). As one writer suggested, tube plugging is not always a simple matter. If they ever get around to it, Chemical Engineering will publish a recently written article on Tube Plugging and Sleeving. Look for it.

In the meantime, if you would like to talk over your problem give me or Carl a holler.

Stan Yokell