Sagging in HRSG tubes

[Muhammad Shoaib Anwar]

Hi,
Sagging found in HRSG Unit in vertical fin tubes. What type of rectification should apply. Unit is only 4 years old. Sagging in first row of tubes of economiser in gas flow direction.
Please advise for possible rectification. Note: braces for tubes are perfect, cause of sag, it seems some operational reasons.
Apparently there is no damage or crack except tube bent. Finned area of tubes visually checked and tube to header weld by NDT.

BR

 

Are the tubes sagged or bent?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[metengr]

Most likely thermal distortion. Leave it and monitor over time. You may want to install some TC to monitor start up conditions.

 

[Muhammad Shoaib Anwar]

It is sagging. (To explain tube physical condition used bend instead of bowing)

@metengr thank you for advise.

 

[Dhurjati Sen]

Dear Muhammad Shoaib Anwar,

If the sagging is more than 4 x diameter of tube, you need to cut those tubes and plug both the headers.

You can live with the other sagged tubes.

Operations should be careful of low water level in the steam drum. Instrumentation regarding low drum level alarm should be accurate.

Regards.

DHURJATI SEN

 

[XL83NL]

Im in no way an expert in this field, but could thermal bowing be a factor?

 

[chicopee]

Sagging tubes indicate low water condition. One boiler that I was involved inspecting back in the 80's had a low water condition and had to be retubed entirely. The reason for retubing entirely because some tubes that were slightly bent( and initially not deemed to be replaced) made it impossible to do a partial job due to interference. In your case it may be necessary to entirely retube your HRSG unit. Carefully inspect all tubes if possible before ordering replacement tubes otherwise you may find yourself making more than one order and that is delay which cost money.

 

[Muhammad Shoaib Anwar]

Dear Dhurjati Sen,
Regarding 4 x diameter criteria for sagging tube, could you please advise reference standard for acceptance/ rejection criteria.
Thnx

 

[Dhurjati Sen]

Dear Muhammad Shoaib Anwar,

Unfortunately the world as well as this forum relies too much on codes.

Which code mentions that you can live with sagged tubes? Had the manufacturer supplied the HRSG with sagged tubes, would you have accepted it at the time of commissioning, or even at the project stage?

As per me, it is short term overheating although it rarely affects economizers. So the sagging of economizers is a far lesser problem than the water walls (the evaporators in case of HRSG).

Carry out a thorough examination of the Evaporators for any bowing or sagging (how many do you have, two or three?.

The 4 x dia criteria was given by a HRSG manufacturer. You should contact your manufacturer and seek their opinion regarding the permissible sagging.

Regards

DHURJATI SEN

 

[chicopee]

Have you determine the cause for the tubes to sag?

 

[davefitz]

the hrsg vertical tubes have a very low resistance to columnar compressive collapse/bowing, due to the high L/d ratio and thin wall . In the case of a circuit with 200 + tubes operating in parallel, a thermal hydraulic flow unbalance that causes a few tubes to flow all of the water and the remaining tubes to flow zero water can imply the colder tubes are more than 150 F colder than the hotter tubes, perhaps only for a short time period ( typically during hrsg startup ops) . This high a temperature difference causes the colder tubes to pull up on the lower header causing the remaining tubes to bow out of plane in compression. Only a small compressive load can cause such bowing, and if the stresses are below yield stress then the bowing should be temporary. A permanent out of plane bowing may indicate some structural support may have yielded, or there could a permanent temperature difference caused by a peculiar circuit design.

The typical hrsg designed in the 1995-2005 bubble period had assumed few startups and high capacity factor as with a base load operation, so the designer may have ignored ( or been ignorant of) economizer flow instability at low loads or during fast load transients ( as may occur with steam drum level upsets causing massive changes in feedwater flow demand) . Such designs had heated up-down circuits connected to a common header ( with internal diaphrams separating circuits) and may have had the feedwater control valve upstream of the economizer. Such designs are susceptible to flow reversal , flow instability, and tube bowing for every operating case except the design case of steady full load flow.

Placing the feedwater control valve downstream of the economizer prevents economizer steaming and prevents 2 phase flow instability in up-down circuits. Redesigning the circuit co it only has heated up flow provides a "positive thermal hydraulic charcteristic" so flow reversal and flow instability will not occur. See papers by Ledeneg ( 1930s in german) or test data on recent hrsgs published by Bob Anderson ( combined cycle journal).

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[davefitz]

I thought about it a little longer, and the likely cause for the sagging tubes is that those particular tubes had run at least 150 f cooler than the remaining tubes during startup operations ( due to the flow instability described above). This led to tensile yield / ratcheting, so those tubes "grew" an inch or two longer than the remaining tubes. They will then have a permanent bow to them when normal flow distriutions thru the harp ensue at normal loads, and appear to have columnar buckling/ collapse.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick