Guys we have a shell and tube heat exchanger whose data is as follows:

Type: AES
No of tubes: 402
Material: A-213 T5

Operating Pressure:
Shell: 149.3Psig Tube:127Psig

Operating temperature:
Shell: 352-361 deg.C Tube: 514-626 deg.C

Service:
Shell: Naptha + H2S Tube: Fuel oil + S

We have countering problem of tubes sagging. Kindly tell me why the exchanger tubes sagging
Is any one knows the possible reasons of tube sagging?
I will be waiting fir your response

Regards;
Hammad