Column Depressurization 3

[ghamsa]

please help with the following

There is a fractionation column which has multiple safety relief valves installation on top. Also, the column is equipped with depressurization pressure control valve. My questions are the following:
1-What is the function of the depressurization pressure control valve and how is it sized?

2-How should the depressurization pressure control valve be set i.e. below or above the safety relief valves?

Thank you very much

 

[ash9144]

I would think the depressure control valve is there to allow the column to vent under high pressure conditions that are not emergencies. Such as starting up and the column is full of nitrogen gas. It most likely vents to a condenser or some other safe in process location.

The relief valves are for emergencies. Such as an external fire. The relief valves probably vent to a flare or even atmosphere or both since you say there are multiple RVs.

You definitely want the control valve to vent first. General rule of thumb is you never want a relief valve to need to relieve.

 

[agf94510]

Without viewing a P&ID, relief valves are required to protect the distillation column as a pressure vessel. Multiple relif valves (likely staged) would allow full flow at 116% (two valves) or at 121% (three or more valves) overpressure.

Relief valves are set at or below column design pressure. Relief valves require a blow-down to reseat properly so that column operating pressure needs to be some 20+% below the relief valve setting.

Distillation column operating pressure can be controlled by a depressuring control valve. Control valve sizing depends on function of the control valve. Need to look at the type of condenser, fin-fan or water cooler or both, location of condenser (above or below receiver), and if hot vapor by-pass (HVBP) is installed or not. Control valve sizing depends on what function the valve performs. To answer this part an experienced process control applications engineer should be consulted.

 

[JLSeagull]

The depressuring valve could be for rapid blowdown. Multiple process conditions (besides pressure) could initiate depressurizing.

Sometimes a split ranged pressure control scheme controls fluid inlet, heat or whatever affects the vessel pressure at one pressure setpoint; and a higher setpoint to release excess vapor to the flare header. The higher setting can be calculated to be some fixed pressure over the operating setpoint. In any event the setting would be well below the PSV setpoint. It would be unusual for a pressure relieving control to be set closer than 10% of the PSV setting.

 

[lizking]

Refer to API 521 for guidance on depressuring systems.

agf94510: the limit on overpressure is 116% for multiple valve installations (non-fire case); it doesn't matter how many RV's you have, it's still 116% of MAWP.

 

[agf94510]

lizking

Thanks for correcting my error. The 116% MAWP is the max overpressure for multiple RV's.

JLSeagull

Most distillation columns run at a fixed pressure. (Floating pressure control is unusual.) Providing an elevated (above normal) pressure control setpoint and activating a depressuring valve would be considered an upset condition. Product quality would be affected. Depending on consequence, the upset condition should have alarms and the valve may need position feedback. Flow measurement may be required to log material loss.

 

[ghamsa]

Thank you all for your help

Now, I can understand that depressurization control valve must be set below PSV setting. But, considering the fact the depressurization control valve normally comes in large size, the PSV may not open. How the PSV could open while the depressurization control valve is lower?

thank you again

 

[GL431]

The PSVs are supposed to never open as said by the posters above. The depressurisation valve normally operates at a pressure below the set point of the PSVs so that disturbances can be controlled away and the PSVs do not need to open. If a PSV actually opens, one would have the hassle of having to take it down and having to service it in the shop, which could be quite a large effort.

One of the rapid depressuring cases mentioned above could be the fire case. For this case the depressuring valve is designed to bring the system pressure down to 7 barg within 15 minutes (API 521 ...). This could result in quite a large depressuring valve. Such valves are more common on high pressure systems and are rare on fractionator towers which run at moderate (not high) pressures (the PSVs on a fractionator might actually be set for lower than 7 barg - in which case please forget the rapid depressuring design). Perhaps your depressuring valve is sized large for other reasons? The plant design books might say something about the sizing philosophy.