Q & A > Question Details
In a hydrotreater I have normally seen shutdown valve provided on HC line exiting the HP separator. The shutdown valve will close upon sensing low level in the HP separator. Shutdown valve is provided between HP to LP interface. Level indication failure of HP separator will lead to gas break through from separator to stripper and stripper is not designed for such high pressure conditions.
But in a kerosene hydrotreater unit I have seen the shutdown valve tripping logic on high-high pressure in HC line exiting the HP separator instead of low level in HP separator ( this low level tripping is not present ). The tapping for pressure ( 2 out of 3 tripping logic ) is taken from downstream of angle valve (pressure reducing valve ). What can be the reason of changing the shutdown valve closing logic from low-low level in HP separator to high -high pressure in the HC line exiting the HP separator?
08/09/2014 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The advantage of using high pressure trip logic it is will respond as soon as the pressure goes high (from a fail-open level control ?) instead of waiting for a low low level. Is the potential time difference long enough to cause over-pressure in the LP separator?. Or is this likely to be a 'close the door after' the liquid level is already lost?
The disadvantage is it requires substantial additional instrumentation installed on piping, DSC logic etc.
The LP separator WILL still have to be protected with a PSV.
Personally, I prefer using the HP separator liquid level signal and the pressure signal from the LP separator in an 'either will trip' configuration. The HP Separator level control valve should be a 'fail close' to preclude a loss of containment issue. The activation of this event should also trigger a full shutdown --cut feed, cut hydrogen, cut furnaces, cut recycle compressor. A kerosene hydrotreater catalyst will not 'hydrocrack' and cause a temperature excursion as will hydro-cracking operations.
08/09/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Actually, the codes and recommended practices with which I am familiar would require that the stripper relief valve handle the loss of liquid seal case, and the shutdown systems are to avoid upsetting column internals should such an event occur.
08/09/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
In some designs, the pressure relief valve on the stripper overhead is sized for the case where the level in the HP Separator is lost. This is apparently not so in either of the two cases you describe. In your first case, the loss of level itself actuates stopping the flow to the stripper. In your second case, the backpressure increase resulting from the loss of level is sensed and actuates the shutdown. In all three cases, the intention is to avoid exceeding the pressure rating of the stripper column and associated equipment.