14 replies to this topic

[jprocess]

Dear All,
Which parameters should be taken into account when setting the failure position of control and ESD valves?
Is there rule of thumbs to select FC or FO position?For example anti surge valves are FO.
Thanks in advance.
Cheer.

[Padmakar Katre]

Dear,
I think there won't be thumb rules for to decide the control valve action in case of the instrument air failure.Mainly it is decided on the basis of the plant safety i.e. nothing but based on HAZOP.

What should be the position of the control valve is mainly decided on the basis of the safety/hazards analysis of the perticular loop and according the control valve postion is decided i.e. FC(Air Fail to Clsoe the Valve),FO(Air Fail to Open the Valve) and FL (Air Fail to keep the Valve in its last position i.e. it remains open with the % opening what it was prior of instrument air failure).
Its mainly based on the experience and the thorough analysis of the consequences/hazards to the plant at the safety level of the control valve position in this event.
May be my experience is not adequate to comment in detail.As well there may be some common guidelines for the control valves which are in certain loops likewise you stated that the antisurge valve is FO or steam valves are FC.This is quite specific with the certain loops and systems.You have to analyse the event i.e. if instrument air fails so what should be the safe position of your control valve and if not like this then what are the hazards.

I hope it can give a rough idea regarding the query.I expect some more comments from seniors.

[jprocess]

Dear Padmakar:
Thanks a lot for your valuable comments.May I ask you that why steam valves as you stated are FC?
"May be my experience is not adequate to comment in detail.As well there may be some common guidelines for the control valves which are in certain loops likewise you stated that the antisurge valve is FO or steam valves are FC."
Cheers.

[pleckner]

In most cases it's a safety issue and not an operational issue. If a control valve fails (hardware or software controller isssues or loss of instrument air) you want it to fail such that the system goes into a safe state. Steam adds heat (energy) to a system and that usually translates into an unsafe condition if it is not controlled. The safest way to guard against these type of problems is to have the steam control valve close if there is a problem.

In a compressor circuit, it is not usually a good idea to starve a very expensive piece of equipment such as a compressor so we fail the surge control valve open to keep a supply of gas to the equipment so it can be shut down in an orderly fashion.

Sometimes you want to keep your process going even if there is problem with instrumentation. You might want to fail these "last position". Also, in continuing with the "safe state" approach, sometimes you want to fail a valve open so that you can remove energy from the system . This may be done for vent systems that control non-condensbles and where you prefer not to allow the relief valve to handle the pressure build-up.

So one guideline for you is to fail valves closed that are on streams that add energy to a system and fail those opened when you can remove energy from the system. Remember, this is not hard and fast as every system is different.

[Padmakar Katre]

Dear,
I think Mr.Pleckner replied your query so no need to say the same thing once again.There may be some common guidelines which are normally followed but you are the one who have to analyse the event and decide the logic (FC/FO/FL) accordingly.
Like I have a system which is at @ 20 Kg/cm2g pressure (Scrubber) and second one at lower pressure at @ 0.6 kg/cm2g and there are 2 control valves (HV-ESD valve which will act on LLL and LV for level control) in the connecting line from HP to LP system now these 2 valves will have a logic of FC in order to protect my LP system (Stripper).
I hope it will make you to understand how the logic (FC/FO/FL) is decided?

[Satyajit]

Hi,
Steam valves are not always FC. It depends on process and safety requirement. For Example, you want to send steam to process ( reactors, heating coil,purge line etc) for protecting the unit from overheating or back flow ( to avoid hazardous conditions) during plant trip,start up/shut down ,you need your steam CV to open in absence of air and it must be designed as FO. Some times it is FLFO ( Fail locked position and Fail to Open ); otherwise, you have to use manual handjack operation.
The general consideration for FO and FC depends on following apart from process safety issue. If it is a direct control ( e.g. Level CV located on an outlet line from a tank is a direct control), normally the valve is FC ( Fail to Close). If the LCV is on an inlet line to the tank, it is an inderect control and control valve is FO ( fail to Open) type.

Again, as an operation engineer or a design engineer , you need to look into emergency situations requirement while reviewing your process control loops, P&ID etc.
Hope this will help little bit.
Kind regards,
Satyajit

Dear Padmakar:
Thanks a lot for your valuable comments.May I ask you that why steam valves as you stated are FC?
"May be my experience is not adequate to comment in detail.As well there may be some common guidelines for the control valves which are in certain loops likewise you stated that the antisurge valve is FO or steam valves are FC."
Cheers.

[JoeWong]

Satyajit,
Good example of FO steam control valve...

...send steam to process ( reactors, heating coil,purge line etc) for protecting the unit from overheating or back flow ( to avoid hazardous conditions) during plant trip,start up/shut down ,you need your steam CV to open in absence of air and it must be designed as FO.

protecting unit from Overheating...
Are you saying you use steam as coolant ? What kind of process is this ? Do mind to share with us ?

protecting unit from Backflow...
External cooling may condense steam. Is it a good medium for purge ?

Some times it is FLFO ( Fail locked position and Fail to Open ); otherwise, you have to use manual handjack operation.

Do you mind to share more how FLFO works ? Any recommended supplier and vendor ?

Thanks in advance.

JoeWong

[jprocess]

To Phil:
You stated that:
"If a control valve fails (hardware or software controller isssues or loss of instrument air) you want it to fail such that the system goes into a safe state. "
As I know the FC/FO/FL discussion refers only to position of control valve due to loss of instrument air(Or any other fluid which is used for CV actuator like nitrogen) and not due to hardware or software controller isssues.In this case we usually protect the system with PSVs considering the control valve failure scenario.Am I right?
And about this:
"Sometimes you want to keep your process going even if there is problem with instrumentation. You might want to fail these "last position"
Could you please explain more about it with some example?

To Padmakar:
You stated that:
"I have a system which is at @ 20 Kg/cm2g pressure (Scrubber) and second one at lower pressure at @ 0.6 kg/cm2g and there are 2 control valves (HV-ESD valve which will act on LLL and LV for level control) in the connecting line from HP to LP system now these 2 valves will have a logic of FC in order to protect my LP system (Stripper)."
Could you please include a figure of your system?
I believe that FC control valve will not be enough to protect your LP system.In order to complete your protection you should consider control valve failure scenario when sizing your LP system PSV.

To Satyajit:
I am also interested of knowing about the questions which are made by JoeWong.

Thanks in advance.

[pleckner]

You stated that:
"If a control valve fails (hardware or software controller isssues or loss of instrument air) you want it to fail such that the system goes into a safe state. "
As I know the FC/FO/FL discussion refers only to position of control valve due to loss of instrument air(Or any other fluid which is used for CV actuator like nitrogen) and not due to hardware or software controller isssues.In this case we usually protect the system with PSVs considering the control valve failure scenario.Am I right?

What I basically am saying is that we want the control valve to always fail in a safe state, whatever safe state for that particular process is, but there are times we cannot rely on devices to allow this so yes, we might have to protect the system with a relief device. And yes, fail positions are referred to loss of instrument fluid.

And about this:
"Sometimes you want to keep your process going even if there is problem with instrumentation. You might want to fail these "last position"
Could you please explain more about it with some example?

@Satyajit gives an example.

[Padmakar Katre]

[quote name='jprocess' date='Nov 21 2007, 11:06 AM' post='14327']

To Padmakar:
You stated that:
"I have a system which is at @ 20 Kg/cm2g pressure (Scrubber) and second one at lower pressure at @ 0.6 kg/cm2g and there are 2 control valves (HV-ESD valve which will act on LLL and LV for level control) in the connecting line from HP to LP system now these 2 valves will have a logic of FC in order to protect my LP system (Stripper)."
Could you please include a figure of your system?
I believe that FC control valve will not be enough to protect your LP system.In order to complete your protection you should consider control valve failure scenario when sizing your LP system PSV.

Dear,
I am so sorry to say that the system which I am talking about is in one of my project and its licensor's document so I cann't upload here.But for your understanding say that I have lean solvent circulation in scrubber(HP System) which absorbs one of the component in the gas mixture and becomes rich solent which I am sending it to the stripper via an exchanger (Lean & Rich Solvent-Just Energy Conservation i.e. to heat up the lean solvent stream by rich solvent stream coming from stripper bottom pumps) and I am keeping certain liquid level in the scrubber to have a liquid seal.Now in the event of LLL my HV i.e.ESD valve will close t protect my LP system from overpressurization.
Now as you told to design the PSV on my LP system in the event of my HV and LV remains/sticks open that care is taken.That's not at all an issue.We are discussing about the logic behing the position/status of control valves in the event of instrument air fail and for the sake of your understanding I gave this example.
Now as Satayjit stated that the Steam valves may be FO but I am doubtfull(It doesn't mean I disagree with him please don't take in other way) but for the reason to maintain the tempearture of the reactor or purge any system I think this doesn't convince me as I think that if all the control valves in the inlet streams coming to that perticular system are FC then I think no need of having your steam valve FO.
But for the sake f purging of certain lines in the event of shutdown caused by instrument air failure it can be FO.

[gunjan]

To JProcess,

You have given one example of Compressor Surge control valve where you always kept CV of FO.

I can cite one more example where you have your firm rules of either having FO/FC/FL position.
CV of the process pipelines to Fired Heater will be always FO. Incase you have FC then that particular tube will starve and get overheated.

Regards,

Gunjan

[Padmakar Katre]

Dear,
Would you please justify your statement i.e. FLow Control Valve in Process stream to Heater is always 'FO' if the fuel oil or fuel gas valve is already set to Air fail to close i.e. FC then why it is needed to have flow through the process stream line.You don't think its credibale to consider the effect of the stream which will exit at lower tempearture from the heater as your heater is not running in this case.There would be the consequences to keep the flow through this process stream.Culd you give more emphasis on this by giving any of the industrial example where this sort of application is there.I will be thankful because I have never seen the Inlet flow control Valves of process streams as FO in the event of Air failure event.
There would be the case I don't deny but I am quite curious to know it.Thanks in advance.

[gunjan]

Dear Mr. Katre,

I agree with you, Fuel Gas/Fuel Oil to heater is always kept as FC. I was actually talking about the process fluid. I would like to give example of VGO HT heater in UOP refinery. The heater is Vertical bottom fired cylindrical type. Process stream there by divides into many sub streams to feed different vertical tubes in fired heater. Each such stream has a Flow Control Valve in the inlet to Fired heater. So in case there is some air failure for only this control valve (say some leakage) then the tube should not starve which otherwise cause overheating and tube failure in due course.

Answer to your question is in case of air failure, whole unit will go into shutdown (due to all trip logics) only if there is instrument air failure from source. But say if there is air failure for only that particular control valve then why whole unit to be taken into shutdown i.e. your fired heater will be in operation.

Hope I have clarified the things.

Cheers,

Gunjan

[Dharmesh]

Good Discussion going on.
Everybody shared there experience very well.

I think the outcome of all the reply as well as my personal experience may be like this.

All the control vallves designed FO/FC/FL only based on the process safety condition. At the time of ESD all the process conditions should be in controlled manner. So heating fluid control valves may be FC. Pressure control valve will release pressure to safe condition i.e. It may be in FO condition.
Beside this many other parameters like Minimum level, Temp., Pressure/Vacuum etc. are to be considered to keep process at safe operating limit in ESD situation.

[Padmakar Katre]

Dear Gunjan,

I am partly agree with your justification but let me clear you one thing

1.There can't be or very rare chance of air failure for a control valve.Because the mechanical construction of the instrument air tubes is such that it won't or will never leak/burst.Only thing is that if by mistake someone close the air supply valve to the perticular valve which is even not usual or logical.

2.The example you given is of VGO heaters to which I am new as my experience is in petrochemicals and not in refinery still I have one doubt in case of air failure your fuel supply valves will get close subsequently your burners will fail so there is no heat/energy input so why to run these VGO pumps unnecesserily as the purpose of heating that stream is not going to be achieved.

3.There would be some materail inside the Tubes in case of the Process Stream flow control valves will closed (As I said that they should be closed i.e. FC) so that will take care of the tubes from dry heating.

4.As your burners are failed so even if there is dry heating of the tubes which will not be for longer period. And as per my knowledge the heater tubes are always designed for dry heating. Now I will tell you how to predict the that the tubes are getting dry heated is by checking the convetion/radiation bank temepratures.

5.If we keep the VGO flow through the heater in this case you will not get the required temeparture for the heater outlet stream so where you put this outlet stream (If you are cracking VGO here then in this case you will not be able to crack it then what happens to this material? where it is sent? if you will send it back to the reciever-VGO feed tank then do you consider this case while deciding the design temperature of the VGO feed tank to heater?

I hope you will reply my questions.Or anyone who is well known with this process I request him/her to give his comment.
Thanks in Advance.......