All the reboilers I have worked with have had the flow control valve in the steam line feeding the reboiler, and a steam trap in the condensate exit line.

However, I recently saw a P&ID which showed the flow control valve in the condensate line, but with the flow element still in the steam line. I suppose this has the advantage of making the valve smaller and saving a steam trapping station, but I would be interested to know if this works without blowing steam into the condensate line?Screw pumps

This is a situation of expecting many things to be done without enough measured or manipulated variables.  You are correct that there is no mechanism to prevent steam from blowing through to the condensate line.Self-priming pumps  I suggest an external liquid leg with steam trap be placed on the condensate outlet or a level instrument to measure the liquid level as TDK2 suggested.  Even if the control valve fails open, the steam trap will prevent blow through.  You may also want to relocate the control valve, albeit larger, back to the reboiler inlet.  This method hase some weaknesses, but nothing as bad as what you'd get with this scheme.

This scheme has a potential failure built into it that could be very aggravating.  The tower will respond to a change by increasing the heat input and steam flow rate.  The condensate drain valve will open and reboiler could lose the liquid leg and get steam blowing through. At this point the steam is providing sensible and no latent heat to the tower.  The controls will ask for more steam flow and increase the amount of blow through.Chemical pumps

Also maintaining a water level in the reboiler promotes corrosion and after a while you'll get a leak in the reboiler and something is going where it's not wanted.Pneumatic and electric diaphragm pumps

By not regulating the steam supply one provides the reboiler with the maximum pressure (and saturation temperature) available. So instead of changing the delta T, one changes the surface. Changes in HE surface would be "linear" if the the reboiler is a vertical cylinder.

A demand for the "highest" and "constant" steam temperature for the particular duty could serve to explain the reboiler's controlling procedure.Sweage pumps &Sludge pumps

Besides, by not regulating down the steam pressure, one may obviate the need of using a special pump/trap for the removal of condensate.

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