PSV & Tank Hydraulics Question 1

[rose55]

Consider a tank rated at 50 psig MAWP and containing a hydrocarbon at 50% level and heated externally such that the vapor pressure in the tank is about 10 psig. Not exactly a flash tank, but similar. The supply source (60 psig from pump) delivers hydrocarbon to the tank at elevated temperature, with some flashing of liquid into the tank. Heat is applied to generate more vaporization.

Question: If the vapor valve to the column is accidentally closed and tank pressure is 10 psig, one PSV scenario is obviously continued heat input into the tank causing pressure to exceed PSV set pressure. Ignoring the external heat input into the tank for the moment, is there a second case associated with hydraulics from the supply source being 60 psig, or will liquid flow stop when the vapor valve is closed? Is it possible to get any additional liquid into the tank due to the supply pressure being 60 psig, thereby compressing the vapor space until reaching the supply pressure?

Rose55

 

[JohnGP]

The way I see it is this.

If the pump can supply 60 psig liquid to the tank, then it will continue to pump against that pressure when the vapor line is shut. If the pump is pumping to the bottom of the tank then the pressure that the PSV sees will be reduced by the liquid head in the tank.

Possible pressure depends on the type of pump - i.e. as pressure increases in the tank, a centrifugal pump will typically move back on its curve, with flow reducing until it dead heads (at a higher pressure than it outputs under normal operation).

But then again it may sart to cavitate, but I would still expect the PSV to lift (unless this is a very tall tank).

Cheers,
John

 

[rose55]

JohnGP,

Thanks for your thoughts. Pump is centrifugal and liquid flow enters the top via a pump recirculation line from bottom of the tank. I agree that it seems liquid will enter tank until vapor compresses to the dead-head pump pressure, which is about 60 psig. Thus, looks like there will be a vapor release when the RV lifts, followed by liquid release after the tank is completely filled with liquid.

Regards,
rose55