Relief Scenarios for Atmospheric Crude Distiller

[fdomin]

I´m checking PSV capacities for atmospheric crude distillation.
I have some doubts about some scenarios.
1- When reflux pump fails reflux is lost and no liquid enters on top. Pumparounds do still have liquid to run? If so, it will diminish the effect of lost reflux, won´t it?

2- Is it acceptable to consider complete flooding of a large tower (150 ft aprox) due to bottom pump failure?

 

[TD2K]

On #2, with a bottom pump failure, how long would it take to fill up the bottom of the column to the vapor inlet? Many companies will allow you to take credit for operator intervention if there is a reasonable time (I've seen 15 to 20 minutes used as a reasonable estimate, sometimes a lot less).

For a reflux failure, would the pumparound automatically partially compensate for the loss of reflux or are you depending on a control loop responding? Many companies won't take credit for instrumentation (and ASME does not allow it) for protection against overpressure except under very specific circumstances (HIP system or in some cases, where you have multiple control loops that 'should' respond to reduce the relief event).

 

[fdomin]

1- Pumparounds are all flow controlled with return temperature control in last water cooler. There is no sign of reflux compensation, at least in P&I diagrams.
Nevertheless, I cannot take credit for instrumentation response due to company practices.

1.Filling time to feed inlet nozzle is 30 min (I suppose you are refering to this inlet, not steam inlet). Company practice allows 1.5 times design pressure in case that overfilling takes more than 30 min and independent LSH is provided. Even this is acomplished in this case, I´m aware of complete overfilling because when PSV´s discharge hidrostatic pressure may exceed 1.5 Dp at tower bottom head.
I don´t know if I should consider this case because complete liquid overfilling sounds weird (on a pure liquid basis it would take more than two other, less if foam is considered but in this case a relative density will reduce hidrostatic pressure, doesn´t it?)

As you see this item is taking me a lot of thinking time because is a major tower and I don´t to make mistakes.