19 replies to this topic

[Shirley2012]

I need to review the relief scenarios for a horizontal flare knockout drum with boot. Iknow this is an unsusal case as I've never seen a PSV on KOD before and it's understandable becaue the system is fully open to the flare risers but I've come across a couple of scenarios where a PSV is already installed and I need to evaluate the reason for these.

I appreciate your kind responses and experiences on below questions:

 

1- Is it a regular practice to have PSVs on flare K.O. drums?

 

2- What will be the major scenarios? I think it'll be the external fire but what about area of connected pipings? They are very long fully open pipes to up/downstream systems and the area would be really big, so do we have to take them into account?

 

Many thanks for your time.

 

 

[demank]

Dear Shirley 2012,

Here I quote a sentence from API 14C.

 

" A PSV need not be provided on a vessel if the vessel is the final scrubber in a flare, relief, or vent system; and is designed so that back pressure, including inertial forces, developed at maximum instantaneous flow conditions will not exceed the working pressure of the lowest pressure rated element; and has no internal or external obstructions, such as mist extractors, back pressure valves, or flame arrestors. If obstructions exist, a PSV, or, as an alternative, a PSE should be installed to bypass the restriction."

 

^{Then I assume that your Flare KOD have high back pressure / obstruction, so that the PSV is needed.}

[Bobby Strain]

A relief device on a flare knockout drum makes no sense to me. But I only have 45 years experience in design and operations. Internals make no sense, either.

 

Bobby

[fallah]

Shirely2012,

 

Normally no need to consider a PSV on a KOD, but in rare cases as mentioned in quote by demank from API 14C, it might you have to consider.

Edited by fallah, 03 July 2013 - 02:26 AM.

[Shirley2012]

Many thanks everyone. I appreciate your advices and they confirm my initial thought. This is an old project and I'm thinking that maybe at the time the gas outlet valves at two parallel knockout drum have not been LO (they are now) so these PSVs were designed for blocked outlet scenario or any other similar reason which existed then but not now. Thanks again.  

[The Wolf]

The answer is 'it depends'.  API 14C is for off-shore top-sides platforms.  And it is very specific about when a relief device is warranted on a flare KO Pot.  If you have internals installed in the KO, in 14C there is a requirement to install a relief bypass around the KO Pot.  The very first question you should ask anytime you read a Code or Standard is "why this statement in there".  Typically, the Codes are written in blood or something really bad happened.  This is the case here. 

 

There was an incident about 4 years ago on a Statoil Platform in the North Sea (Specifically the Visund Platform) were the internals of the flare knock out failed and plugged the outlet during a blowdown.  The plug was in the form of a sheet metal placed near the outlet that failed.  This caused the vessel to experience very high pressure and this pinned the sheet to the outlet until the sheet failed like a rupture disc.  The outlet piping to the flare blew out (the incident report is debatable) and created an enormous vapor cloud that engulfed the platform.  Fortunately, the cloud did not ignite.  The incident report stated that the fragment from the internals blew thru the piping, however, it appears from the incident pictures that the cause was flare stagnation or some form of liquid hammer.

 

 

[hoyar]

hi

I want to size a K.O.Drum and its safety valve installed in natural gas path enter a fire heater(boiler). would you please me which procedure i have to follow. also is it necessary to consider any redundant safety valve for it or not. working pressure of drum is around 7 barg.

[fallah]

hoyar,

 

A simple sketch can help much more than a description...

 

Anyway, at first you should specify credible scenarios for over pressure; then find the governing scenario and finally size the PSV based on that scenario. Considering redundant for PSV isn't mandatory, but some engineering practices recommend to consider a spare for each PSV set...

[hoyar]

thank you for your reply

 

i have attached a sketch for your more information

Attached Files

•  sketch1.pdf   43.23KB   81 downloads
•  sketch1.pdf   43.23KB   46 downloads

Edited by hoyar, 09 August 2014 - 05:38 AM.

[fallah]

hoyar,

 

Lack of info as follows:

 

Design pressure of KO Drum?

PSVs set pressure?

Any valving between KO Drum and boiler? Is blocked outlet a credible scenario in the line from KO Drum toward boiler?

Is fire case applicable for KO Drum? Means: Is it installed in fire zone?

 

Some strange points in your sketch:

 

Having check valve in PSV discharge line as a pressure relieving path...!

Taking branch of PSV inlet line from demister pad downstream instead its upstream...!

 

Please at first clarify a.m. points...

[hoyar]

Thank you Mr. Fallah for your warm reply

 

Design pressure of KO Drum: 6 barg

PSVs set pressure: 5.5 barg

There are C.V and also some Shut off valves to boiler

Drum is in fire zone

 

 check valve in PSV discharge line has considered to prevent flare back pressure

 for last item you mean S.V conection shall be in upstream of pad?

[fallah]

hoyar,

 

In your first post; no. #7 you mentioned the working pressure of drum is 7 barg while now you say its DP is 6 barg...! Please clarify...

 

As I well understood the credible scenarios for that drum are fire and blocked outlet...Am I right?

 

Normally the PSV inlet connection is taken from upstream of pad due to possibility of its plugging; but here because a strainer is available at the drum inlet it might that practice be ignored...

[hoyar]

i said working pressure is around 7 . i made a mistak 6 barg is correct. and mentioned scenarios are correct

[fallah]

hoyar,

 

It might the PCV along the NG supply line to be failed in wide open position and create another credible scenario to over pressure the KO Drum...

[hoyar]

Yes This scenario is applicable too, 

 

I have some unclear items more, the shown arrangement for S.V. in sketch is correct or each S.V shall have its nozzle on drum, also check valve on S.V. outlet is correct or not.

[fallah]

hoyar,

 

Check valve isn't allowed to be used in PSV discharge line (and inlet line also) due to safety issue.

 

As per the sketch you did upload, appears a three way changeover valve has been used to change the working PSV between 2*100% PSVs setting. If so, no need each PSV has its own nozzle on drum and it's adequate the flow area of common inlet piping at least to be equal to flow area of individual inlet line to each PSV.

[hoyar]

So if we are not allowed to install CHK valve after S.V. , is any more consideration is necessary to prevent back pressure of flare(Flare back pressure 1.5 barg)?

[fallah]

hoyar,

 

If the valve is conventional or balanced bellows type there is no possibility of back flow through the PSV due to any back pressure; if the valve is pilot operated type it's adequate the PSV to be equipped with a back flow preventer. Then in no case it's needed to consider a check valve either in the inlet line or in the outlet line of a PSV...

[hoyar]

thank you

[Root]

hoyar,

it is not Flare KOD but Fuel gas KOD and PSV,s on Fuel gas KOD is justified, rest and critical things already explained by other forum members.

Cheer

Toor