We have a reboiler with no control valve in steam (25 psig), and control valve in the condensate drainage, so the duty is controlled by flooding and unflooding the shell, so the exposed area.
I our case we experience hammering in the shell, mainly at intermediate capacity, so the operators often run the reboiler at maximum capacity, even when it is not necessary.
I have read about similar problems, but I have not found any solution in the consulted bibliography.
The only certain thing is that the hammering has to do with the condensate level and therefore with the amount of subcooled condensate in the shell, which is on the other hand the control scheme for the reboiler duty. Every time I hear "bump" inside the shell, I imagine the waves of subcooled condensate lifted up by steam flow and making it collapse.
Has anyone any idea to avoid this problem, even at intermediate and low duties?

Lower the condensate level at lower duty.  Maintain a level which maintains the same retention time and you're temperatures should be equal.  Or get rid of level control and use a steam trap.

check your control loop. if it is not working properly or the valve is sticking, you'll get a bang when the valve opens due to flashing. Lower the controller gain to start with.

You HEX works best at constant pressure, if the condensate valve is not modulating properly but swinging wide open: bang!

Preume that when the level valve is wide open or on manual every thing is fine but the bill from the steam plant

Do you actually measure the liquid level in the reboiler shell or the flow of condensate leaving the reboiler? These steam pressure and control valve stability could be important, especially if the control is steam flow controller working a condnsate control valve directly. Hopefully steam flow is at least cascaded to a reboiler condensate level

Usually these cold condnsate hammers start in the condensate return system as a result of cold condensate collapsing flash steam, even though the shock waves may propgate and make it sound like the reboiler. You should make a test by routing the condensate to grade after your control valve, reduce load, and see if the hammers still exist.

Otherwise to reduce your subcooling and reduce the reboiler liquid level, try raising the tower pressure if that is an option.

If you cannot raise the tower pressure as suggested by sshep, is it possible to lower the steam pressure instead?  It sounds as though your reboiler is oversized, and either of these measures will counteract that by lowering the temperature difference.

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