7 replies to this topic

[seeker_of_knowledge]

Hi,

I am reviewing a pump system and noticed that there is an emergency shutdown valve (Fail-close) placed on the pump minimum flow recycle line downstream of minimum flow control valve (fail-open valve). The purpose of the shutdown valve is to isolate the pump building (building in which this pump is housed) from hydrocarbon tank by preventing back flow from tank through this min flow recycle line in the event of fire in this building.

My question is: Is it acceptable to have a fail-close shutdown valve as stated above on the min flow line downstream of min flow control valve which is a fail-open valve ? There is also a low low flow shutdown interlock at the discharge of this pump as an added safe guard to protect the pump.

Any thoughts would be appreciated.

Thanks

Fez

[ankur2061]

Fez,

Emergency Shutdown Valves logic is generally based on the perception of safety of personnnel and equipment by the experienced engineer who perceives what ESD logic will help provide safe operation of the plant. Often ESD valves get introduced in a new plant due to experience of operation by the operations engineer who has been first-hand witness to some safety issues while operating a similar plant. Some of the good engineering and operating companies have provided guidelines where ESD valve should be provided for a given type of installation. Shell for example has a design engineering practice for provision of ESD valves which has very precise guidelines where to provide ESD valves and the logic behinfd it.

Having said all the above, I personally do not agree to provide a ESD valve in the minimum flow recycle line of a centrifugal pump. The action of the minimum flow recycle control valve is Fail-Open whereas the ESD valve functions to stop the minimum recycle flow. The actions of the FCV and the ESD are contradictiory. If I have to provide a means to prevent reverse flow I would provide a check-valve on the minimum flow recycle line close to the minimum flow recycle connnection on the hydrocarbon tank. If I have doubts about the effectiveness of a single check valve I would provide two check valves of dissimilar type after performing a risk analysis which will provide me the probability of failure for a single check valve and allow me to make a judgement whether I need to have two dissimilar check valves on the minimum recycle flow line.

Hope this helps.

Regards,
Ankur

Edited by ankur2061, 04 October 2012 - 12:45 AM.

[Dacs]

It's really hard to answer that without a more detailed risk analysis. I presume you'd need to conduct SIL analysis to mitigate the risk of inadvertent closure of this shutoff valve.

That said, I think it's better to design the system to mitigate the reverse flow in the first place (by adding check valves or liquid seal, etc) without the need for this SOV.

[fallah]

Fez,

All SDV's have FC characteristic as their safe positions and i had never seen such application of a SDV in minimum flow loop of a pump. Of course you didn't specify if minimum flow line is terminated to pump suction line or pump suction drum and also about the location of the SDV ( inside or outside of the building). Anyway appears you think the backflow from the tank toward the pump in the building through the minimum flow line to be an credible scenario even though in fire case the pump is tripped and there would be a check valve in discharge line to prevent such backflow.
The main possible risk of considering SDV in minimum flow line is its malfunction to close during normal operation and because the minimum flow control valve is normally closed at a glance seems there would be no problem but at least an alarm should be considered to announce the operator on loss of open condition for the SDV.

Anyway as other members mentioned there is hard to submit a proper and comprehensive answer without a sketch of the system along with additional information to support the system understanding.

Fallah

[seeker_of_knowledge]

Ankur, Dacs and Fallah

Thank you for your replies.

Regards

Fez

[Bobby Strain]

Usually one relies on a manual activation in the field for such a shutdown valve. And valve actuator can be electric.

Bobby

[jr31]

It is not clear if a shutdown valve is placed on pump sunction line. The Recirculation line after control valve dischage can be possibly connected at upstream of an ESD valve in pump sunction line. In case of fire the Pump trips also the ESD valve closes thus islolating the pump from tank. The check valve in recycle line should provide additional protection.

Another option in case the cofiguration cannot be changed is to consider a Smart ESD valve with dual inputs(24 V DC & 4-20 mA) with a reverse acting relay, This can provide partial solution to Failsafe problem.

Sil assement shoyld be done for any soultion,

[tarafdar]

Hi,

Your concern is at the event of fire.In case of fire ESD system should take the plant in a safe mode.The mentioned pump also to be stopped.In that case the signal for activation of the SDV may be taken from stoppage signal for the respective driver(motor or turbine) of the pump.

Thanks.