Shell and Tube Heat Exchanger Tube Failure

[Sloot]

We have two shell and tube heat exchangers that were just put into service in the last month. They are for process and make-up air heating with 45 psig steam in the shell and 40% propylene glycol through the tubes. The heat exchangers are simple horizontal u-tube with ¾” x 0.049” stainless steel tubes. The tubes were rolled into the stainless steel tube sheet and seal welded. One heat exchanger is about 18” diameter x 4 ft long and the other is 12” diameter x ~ 4 ft long.

From what I’ve read both have suffered classic water hammer damage resulting in ruptured tubes. The tubes that were damaged were on top of the tube bundle with a number of tubes appearing to be crushed. These tubes were not under the steam inlet but about half way down the length of the shell.

Google helped me find quite a bit of information about how to prevent water hammer with nearly all saying that the problem is poor condensate drainage. I only found one reference that attempted to explain what actually is happening in the heat exchanger when it is experiencing water hammer. It was on the Armstrong web site at:

http://www.armstrongpumps.com/Data/ioguides/Links/01_04_003/138.65_SHELL_AND_TUBE_I&O.pdf

From this reference:

“In a water heater using steam in the shell, when the demand for hot water ends the steam control valve closes, but there is a good supply of steam left in the shell of the exchanger. As this steam condenses, the pressure drops, often below atmospheric or even practically to full vacuum. This prevents condensate from leaving the shell and sometimes even syphons in condensate from the line beyond the trap. Now, when the steam valve opens again and admits steam to the shell, the rapid condensation, as it strikes the cold condensate, causes streams of water to rise, hitting the top of the shell and bouncing onto the top tubes. Sometimes the breaks in the tubes look as though a 4” spike had been driven through the topside. Other times the tubes may be crushed as if with a blunt chisel over lengths of a few inches or up to two feet.”

We experienced this exact type of damage so I am sure that the problem is water hammer but I am having a problem understanding how this could have happened in our arrangement. The big difference is that the capacity control of our heat exchangers is by varying the condensate flow rather the steam flow, i.e. in our arrangement our heat exchangers were designed to run flooded. There is an automated on/off valve on the steam inlet which is programmed to stay open during the heating season. We should therefore have constant steam pressure on the shell with the condensate level varying with the demand.

A good description of the various ways to control the capacity of steam heat exchangers is in an article found at:

http://www.driedger.ca/ce4_sh/CE4_SH.html

So if the automated on/off steam valve is staying open and we have constant pressure on the shell of the heat exchanger I don’t see how we could experience the rapid condensation that seems to cause the water hammer. I want to check the tuning of the condensate valve but I don’t see how even a big change in the condensate valve position could cause a large swing in steam load or condensation rate. The condensate control valve is 1 ½” (or less?) and the steam inlet is 6” I believe. The load on the glycol side should not change quickly as it is for heating outside air.

I’ve only read about this type of flooded heat exchanger control and have not used it before. I realize that many people will immediately say that you will always have water hammer when your heat exchanger is flooded like ours normally will be but I know that this type of control, although maybe not common, is used and used with success.

So where did we go wrong? Is this type of control prone to water hammer? Can it be avoided? Does this water hammer typically happen when the condensate level is low? Or high? We did not experience water hammer when these two heat exchangers were commissioned but it did happen later.

I can’t seem to come up with an explanation for the water hammering other than maybe, for some reason, the automated on/off valve on the steam inlet is opening and closing when it isn’t supposed to. This valve is programmed to open very slowly over 5 minutes but it is a ball valve so even if it is a crack open there will be a large flow.

Any comments, experience or suggestions would be appreciated.

 

[unclesyd]

Ball valves are notorious for causing problems with water hammer when use for flow control. I think your first order of business is change out the ball valve to globe or steam control valve.

Steam control valves are readily available. There are several companies like Leslie that have package system that work extremely well and last for ever.

http://www.lesliecontrols.com/Products/TempRegulators.htm

It doesn't take but one event to do considerable damage. The only problem with your picture is the broken tube as I never seen a broken tube in a hammer damaged heat exchanger unless it had been previously plugged.

 

[Unotec]

But wouldn't the water hammer affect the shell mostly? If the steam is in the shell and you have the hammer, it would hit the ends of the shell. The bundles collapsing in the middle is pretty weird.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[sterl]

What else is consuming steam from your circuit?

As banged up as these tubes are over a considerable extent: Something has got to be dropping the steam pressure, which causes the condensate to lift as it tries to re-establish saturation; and in worst cases your heat exchanger starts generating steam for deliver to the alternate sink...

The local control isn't at fault, though the inventory of condensate is certainly a player: If the Glycol temp is fairly consistent and doesn't have much of a range, there's go tto be something elsse diverting your stema flow.

 

[TBP]

Systems arranged like this CAN work properly. I've seen them installed, and installed them myself. I seriously doubt that the steam inlet valves and their operating speed have anything to do with your problems.

I'd start with the condensate control valves. Does each HX have it's own? (If not, they should.) Are these valves on/off or throttling? (They should be throttling.) How were these valves sized? Do you have the flow rates on the glycol pumps, and the delta-T's?

My experience with steam systems is that a great many components - HXs, control valves, traps, piping, etc - are WAY oversized, and that oversizing causes FAR more problems than everything else put together. My guess is that the condensate valves are too big, and are "hunting" at low loads. (If you're in the northern hemisphere, last month would typically be the start of heating season - "low load" conditions.) A steam bubble is getting trapped by sub-cooled condensate rising in the HX shell when the condensate valve drives shut, collapsing instantly. The in-rush of condensate to fill the vacuum created by this collapsing steam bubble will generate a BIG pressure spike, which would cause the collapse of the tubes. Wayne Kirsner has some excellent articles on his website kirsner.org on water hammer events.

 

[Sloot]

We removed the damaged tubes and welded plugs into the tubesheet early last week. We then restarted these two systems. The start-up went very smoothly and neither of the heat exchangers made a noise .... but we started with empty heat exchangers, not flooded.

We disabled the two steam inlet valves and now have them held 100% open.

These two heat exchangers aren't in normal building heating service as the primary source of heat is from a process energy recovery system (ERS). This results in the steam being used to top-up the building heating system likely resulting in a number of start/stops in one week.

I wasn't there but there were a couple of occasions on the day after start-up when there was hammering and looking at the trends on the PLC, it coincided with the process ERS being down and the steam HX being called on to provide heat.

We had steam traps installed right above the heat exchangers on the steam inlet so when the steam was called for the heat exchanger would have been fully flooded up to the trap just above the heat exchanger.

I've talked to a few more equipment suppliers and I have now pretty much concluded that hammering cannot be avoided in the start-up of a "horizontal" flooded heat exchanger. I have also learned that others have been able to successfully operate flooded shell and tube heat but only if the heat exchanger is installed "vertically" with steam inlet on the very top of the shell with the bonnet on the bottom. See the attached sketch.

 

[steamdog]

Personally, I do not like flooded heat exchangers. One idea for you (maybe) is to slow down the start-up or the initial in-rush of steam. Hot steam hitting cold water (or vice versa) will lead to water hammer. If you can stage the in-rush this may help. Another idea is to reduce the steam pressure, if your heat exchanger and condensate system can handle this.

 

[phoenix911]

from your reiterattion from Armstrong;

"In a water heater using steam in the shell, when the demand for hot water ends the steam control valve closes, but there is a good supply of steam left in the shell of the exchanger. As this steam condenses, the pressure drops, often below atmospheric or even practically to full vacuum. This prevents condensate from leaving the shell and sometimes even syphons in condensate from the line beyond the trap."

Is there a vacuum breaker installed on the unit?

phoenix911