ON/ OFF valve on Stripper bottom rundown line in case of emergency 3

[nRGee]

Greetings,

I came across this design in one of the refineries handling Naphtha. Kindly refer to the schematic attached herewith.

Naphtha stripper column is provided with an ON/OFF valve on stripper bottom product rundown line. The purpose of this ON/OFF valve is to stop sending the OFF-SPEC product to storage tank in case of unit shutdown. On the other hand, there is one more line to OFF-SPEC PRODUCT TANK.

In case of Manual shutdown of Naphtha treating unit, this ON/OFF valve gets closed as per logic. Manual shutdown is used for emergency scenarios like fire.

Stripper column, usually, will have an inventory of 20m3 and the entire stripper bottom product system has a hold up volume of roughly 30 m3.

In case of fire, this valve will be closed and the entire system will be exposed to fire. There are several flanges and other instruments that get exposed to fire. Failure of flanges, instruments may dump all the Naphtha in that area and add FUEL TO THE FIRE.

My proposal is to provide a SOFT RESET button to the ON / OFF valve on DCS. So that operator can RESET the valve to OPEN it and push the Naphtha content to OFF-SPEC product tank.

Is it okay to push Naphtha out of the system so that fuel to the fire can be reduced? is it a recommended and good engineering practice? is there any specific guideline on this?

Thanks in advance. Don't miss to see the schematic for better understanding.

 

[shvet]

just for cases like this HAZOP was invented

Note that DCS is intended for routine operations while for emergency ones ESD system is intended.

 

[MJCronin]

Ragu, ...

Was there a HAZOP (or similar) analysis performed for this system ?..... Why not ?

Please post details about the make/model and configuration of the valve in question ...

Does the suspect valve have a "fire rated" design as per API and other standards ? ..... Why not ?

https://whatispiping.com/fire-safe-valve-api-607-vs-api-6fa/

MJCronin
Sr. Process Engineer

 

[LittleInch]

nRGee,

you're making some huge assumptions here:
"and the entire system will be exposed to fire. There are several flanges and other instruments that get exposed to fire."

How?
For how long?
Why is it "exposed to fire"?

These are the things that are considered during the design and in general the best thing to do is isolate all inventories into relatively small sections and provide pressure relief for fire cases to prevent vessels rupturing if exposed to fire.

For all you know the off spec tank could be on fire or the leak could be in the line to the off spec tank so you end up feeding a fire that way.

In a large fire scenario bad things happen, but the key is trying to prevent sudden large scale ruptures of vessel or tanks holding hazardous substances. Operators will be on total overload in a fire scenario and adding to their workload or having them potentially send hot naptha into tanks is an even worse scenario.

T=you can't just look at one vessel and decide to change things - doesn't work that way for good reasons.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Its a good idea to depressure and drain out flammable inventories to some remote tank in the event of a fire. But some things may stand in the way of implementing this idea:
a) Do you have sufficient pressure in the naphtha stripper at the time of fire to push this product out? i.e. will the naphtha stripper be depressured through some overhead line ?
b) What if the cause of fire is a large body flange failure at one of these 5 product-feed cross heat exchangers? By the way, it sure looks like the shutdown valve marked ON/OFF valve on your sketch should be upstream of these 5 heat exchangers. Your description also does not match this sketch re the location and number of on-off valves. Pls correct sketch.

c) Are you sure the product off spec tank can handle hot product rundown from this stripper during this emergency ?

 

[nRGee]

@LittleInch

the assumption of "entire system on fire" is limited to the system shown in the schematic. I did not assume the off-spec storage tank which is far way from this unit is on fire.

why?
there are Naphtha surge drums and a reflux drum located above the exchangers shown in the schematic. if there is any Naphtha leak, if forms a pool of Naphtha leading to pool fire.

For how long?
It depends on the amount of Naphtha leaked.

Why is it "exposed to fire"?
Because these exchangers and stripper are classified under the fire hazardous area.

The unit is still under design stage, so we are thinking of a minor modification as mentioned below:

Upon emergency shutdown, A pulse signal will be generated from ESD to close the ON/OFF valve. So that, operator can RESET (OPEN) the valve, if required.

 

[nRGee]

@georgeverghese

Please find below the response your questions:
a) Yes there is enough pressure to push the content to downstream tank.

b) the ON/OFF valve is on downstream of the exchangers.

c) there are coolers on downstream of this ON/OFF valve (I have not marked those coolers in the schematic). that will cool the product before routing to storage tank/off spec tank.

 

[georgeverghese]

Well the, the on/off valve should be upstream of these exchangers, since exchangers pose a significant fire risk in most plants. If it is possible to confirm that the source of fire is not from these exchangers (preferably with additional permissive from some local fire detectors), then operators may choose to enable this soft reset to open it and drain out the stripper bottoms.