Hi All,
Here's an example of a heat exchanger behaving oddly. The personnel present were unable to manage the problem, and only when the exchanger was stripped down and examined could the trouble be diagnosed. This probably doesn't give the answer to the problem that started an earlier thread about odd HX behaviour, but it shows how obscured the basic fault can be. I think it contains good lessons about troubleshooting.
This was a large oil/oil shell and tube exchanger:- 2 passes on tubeside, 1 pass on shell side (cross flow). Unit had bolted end flanges. The "cold" oil entered tubeside at 85C and exited at 140C. The heating oil entered at 285C. The automatic temp control loop manipulated a three-way valve on the inlet of the heating oil, but its sensor was situated in the cold oil stream at another exchanger. This valve split the flow of hot oil - part would go through the exchanger, part would bypass the exchanger.
Please view diagram at :-
An upset in other equipment caused the incoming cold oil to drop to well below zero C. The exchanger started leaking from one of its end-flanges. The personnel correctly deduced this was a result of warping of the flange due to a severe temp differential. Retorquing the bolts had no effect on the leak. Other events overtook their efforts to fix this leak.
When the unit was taken apart it was found to have holes in tubes near the hot end (the end opposite the leaking end).
The reason for the leak was that when the incoming cold oil dropped in temp, the controller closed off the 3-way valve, so that the heating oil bypassed the exchanger and went to another unit so as to preheat the cold oil. Confused? It was a complex arrangement. But - the incoming oil dropped in temp. and the flow of heating oil bypassed the exchanger.
And the end flange started leaking. Now that is odd. The flow to the exchanger is shut of and the end flange warps and leaks.
The reason for this was the holes in the tubes. With the valve directing flow passed the exchanger, the hot oil was able to flow into the shell, through the broken tubes and into the cold oil (reverse flow). This meant that hot oil at 285C contacted tubes that contained the cold oil - a severe temp differential, and the end flange warped and leaked.
Not a simple scenario, is it? Close off the flow of hot oil to the exchanger and the hot oil starts to flow thru the unit in reverse! Counter-intuitive. Who would figure that out when under such pressures?
What do you think? If you were present would you have diagnosed the problem? I feel the people on the ground at the time had no hope of solving this. You have to deal with the parameters and factors you can see. There often isn't time to think about what might be going on in hidden places. Cheers,
John.
Here's an example of a heat exchanger behaving oddly. The personnel present were unable to manage the problem, and only when the exchanger was stripped down and examined could the trouble be diagnosed. This probably doesn't give the answer to the problem that started an earlier thread about odd HX behaviour, but it shows how obscured the basic fault can be. I think it contains good lessons about troubleshooting.
This was a large oil/oil shell and tube exchanger:- 2 passes on tubeside, 1 pass on shell side (cross flow). Unit had bolted end flanges. The "cold" oil entered tubeside at 85C and exited at 140C. The heating oil entered at 285C. The automatic temp control loop manipulated a three-way valve on the inlet of the heating oil, but its sensor was situated in the cold oil stream at another exchanger. This valve split the flow of hot oil - part would go through the exchanger, part would bypass the exchanger.
Please view diagram at :-
An upset in other equipment caused the incoming cold oil to drop to well below zero C. The exchanger started leaking from one of its end-flanges. The personnel correctly deduced this was a result of warping of the flange due to a severe temp differential. Retorquing the bolts had no effect on the leak. Other events overtook their efforts to fix this leak.
When the unit was taken apart it was found to have holes in tubes near the hot end (the end opposite the leaking end).
The reason for the leak was that when the incoming cold oil dropped in temp, the controller closed off the 3-way valve, so that the heating oil bypassed the exchanger and went to another unit so as to preheat the cold oil. Confused? It was a complex arrangement. But - the incoming oil dropped in temp. and the flow of heating oil bypassed the exchanger.
And the end flange started leaking. Now that is odd. The flow to the exchanger is shut of and the end flange warps and leaks.
The reason for this was the holes in the tubes. With the valve directing flow passed the exchanger, the hot oil was able to flow into the shell, through the broken tubes and into the cold oil (reverse flow). This meant that hot oil at 285C contacted tubes that contained the cold oil - a severe temp differential, and the end flange warped and leaked.
Not a simple scenario, is it? Close off the flow of hot oil to the exchanger and the hot oil starts to flow thru the unit in reverse! Counter-intuitive. Who would figure that out when under such pressures?
What do you think? If you were present would you have diagnosed the problem? I feel the people on the ground at the time had no hope of solving this. You have to deal with the parameters and factors you can see. There often isn't time to think about what might be going on in hidden places. Cheers,
John.
