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Power-User

Join Date: Mar 2008
Location: Mumbai
Posts: 250
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Valve Failure Position

12/03/2008 4:10 AM

Dear Frnds,

We are having a tank with (Crude /Water),-Level transmitter- Water disposal pump( 08 Nos Parallel ) -downstream of pump Level control valve being controlled by LT in the tank - final water deing disposed to water disposal well.

We are having argu about the valve fail position ,

Ideally wat should be the failure position Should it be FO or FC,

Plz note thers no licensor &the above has not been covered in PHA.

Regards

Jose

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Power-User
South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
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#1

Re: Valve Failure Position

12/03/2008 5:40 AM

Hi there,

It is difficuilt to say. You would need to know whether the tank is under pressure or not. I.e. is there vacuum protection or is it open.

As a rule of thumb to guide you from petrochem.

All inlets to the vessel would be fail closed. All outlets from the vessel, especially those going to a flare would be fail open. This is because in petrochem the gas/liquid is dangerous to the environment and man. So we stop feeding all the vessels and flare off/incinerate the gas/liquid. This gives us good clean vessels when we start up and we do not have any offspec to contaminate.

Our product tanks are different. All inlet and outlet valves fail close - including push pull purge systems - this is to keep a gaseous blanket (nitrogen) over the liquid.

Taking the above rule of thumb as your guide you would also have to look at vacuum proctection. You have to look if the process fluid is hazrdous to the environment and where the pump pumps to (open well?). If hazardous why not make the valve fail close to keep the dangerous liquid in the tank. Even if it is no hazardous - why waste the liquid?

It is very difficuilt to decide unless you have a HAZOP where all the different diciplines can provide their input.

I would do the following: All inlet and outlet valves in your case to fail closed - preventing more liquid entering and overflowing the tank and preventing losing the liquid I do have from spilling. You are obviously controling the level for a reason - hence why I would make the outlet fail closed too.

Also one other thing - you could add a flow transmitter downstream of your control valve and cacade the flow to the level - this will give you much tighter control.

Please let us know the outcome of the argument?

Regards

Craig

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Power-User

Join Date: Mar 2008
Location: Mumbai
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#2

Re: Valve Failure Position

12/03/2008 5:58 AM

Its a low pressure tank with design pressure 10pressure /2 VACcum " WC & Its got BReather valve

regards

Jose

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Guru
Engineering Fields - Instrumentation Engineering - EE from the the Wilds of Pa.

Join Date: Feb 2006
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#3

Re: Valve Failure Position

12/04/2008 8:12 AM

Can you put the level control on the inlet side? If so, have it fail closed. Let the level control detect low levels also (you say control not switch so I assume a continual measurement) and shut the pump down with an interlock should the level drop to low. Have the shutting of the control valve trigger an alarm. (again without details I assume some kind of BMS/DCS/PLC) This gives a short while for reaction to a failure where the tank can not be over filled as it would with the valve on the outlet, and the pump shutting down due to low levels. Of course, we do not know the consequences of stopping the flow to the tank, so I am assuming this is not catastrophic.

If you must never shut off flow to the tank, then with the valve in the outlet side the fail open allows you the most time to react before the tank over fills, again assuming some flow can occur without the pump.

Basically this is all conjecture since so many details are not known.

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Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
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#4

Re: Valve Failure Position

08/14/2023 6:34 AM

There shouldn't be an <...argu...>.

There should be a HazOp instead.

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