13 replies to this topic

[Christiano]

We have an estimated the limiting CV for gas blow by as 440; However the selected valve has an available CV of 650; a mechanical stop is being proposed. Could some one share their experience how reliable are mechanical stops? 

 

Regards

 

Raj

Edited by Raj India, 26 April 2013 - 02:53 AM.

[fallah]

Raj,

 

The mechanical stop is almost reliable, but anyway you have to consider a PSV on the downstream separator with set pressure equal to its design pressure (here 10 bara) and a relief load corresponding to CV equal to 440.

[Atttyub194]

Dear Raj

 

Good day!

 

One can suggest more precisely if you can share a brief process description, what you intend and a P&I Diagram

 

Based on my experience, Please note the following.

1. Control Valve CV limiter are not a substitute or don't fore go any safety requirement despite the fact there reliability is not in question
2. If a limiter is installed, In my personal opinion safety requirements should be studied without considering such limiter as during maintenance some one can inadvertently  remove or readjust the CV or torque limiter 
3. Just to give you an idea, please note that CV limiter just limit the lift of the valve and instead of opening to full lift the actuator can not open fully the valve due to mechanical restriction. Please refer attached picture
4. The relief load to be calculated based on selected CV of 650 and not on restricted CV unless the CV is limited by using restricted Port trim , however, restricted ports are available for limiting CV i.e. may be  in the range of 40 % or 60 % of the CV (fixed) and only when the CV requirements are beyond these limits only then mechanical limiters are proposed
5. While restricting the CV please do consider the suitability of valve with respect to start up condition

 

Best regards and God Bless you

 

 

 

Attached Files

•  PIc-1.docx   112.75KB   163 downloads

Edited by Attyub194, 21 April 2013 - 07:41 AM.

[rxnarang]

First I agree with attyub. A mechanical stop is not a preferred solution to this problem. Ideal solution is to limit the cv to 440. So why can't we? Calculated cv is 380, which is 86% of 440. Normally we would like to keep it 80%, but coming to a pinch I am sure you can work with 86%.

[ankur2061]

Raj,

 

In one of the modification projects that we did recently for a reputed client where we replaced the control valves on a fuel gas line for increased flow for a combination of fuel gas consumers downstream of the control valve, the PSVs downstream of the control valve provided for control valve stuck open case have not been replaced. Instead mechanical stoppers have been proposed for limiting the opening of the control valve. The mechanical stoppers will allow for the increased gas flow for the process users but will not result in the flow exceeding the rated capacity of the PSVs. The control valve CVs were selected with due deliberation for both the process requirement and the valve stuck open case. However, even with the optimum selection, the flow for the control valve stuck open case was exceeding the PSV rated flow and it was ultimately decided to provide mechanical stoppers along with engineering design measures such as mentioning as note in the relevant P&ID, updating the safeguarding memorandum and the safeguarding philosophy to indicate the provision of mechanical stoppers and updating the Plant Operating & Maintenance manual to state the requirement of mechanical stoppers for the control valves and the precautions that needed to be taken during maintenance of these control valves.

 

This adoption of mechanical stoppers was done after a formal HAZOP review exercise which included representation from the client, the engineering consultant and the representative of the control valve manufacturer.

 

From the aforementioned description, to me it was clear that credit can be taken for mechanical stoppers for control valves to restrict flow. To conclude, these days reliability of design of instruments (in this case control valve) has helped in discarding old ideas about not taking credit for them in safeguarding functions.

 

Regards,

Ankur.

[Bobby Strain]

You are asking for trouble if you proceed with a mechanical stop. Many operators also require downstream protection for flow through the bypass with the control valve wide open. Hazop teams should only identify problems, not solve them. And just because you can get a group to agree doesn't make it safe or prudent. Good luck.

 

Bobby

[ankur2061]

Bobby,

 

I would certainly give more weightage to the collective opinion of a group who have deliberated on the issue in a formal review meeting rather than the opinion of an individual who reads something and comes with an instantaneous answer in two minutes.

 

Regards,

Ankur.

[fallah]

Raj,

 

A mechanical stop isn't a preferrable solution but in any case if it has to be applied, IMO, credit can be taken for a mechanical stopper in a control valve to limit the passing flow because it inherently acts mechanically, provided that the relevant operating and maintenance manual to be updated accordingly as Ankur mentioned before...

[Christiano]

Bobby,

 

As mentioned this is an old facility and we have certain limitations to modifications. Having said that I don't want to simply overlook any major safety issue also. We have a manual LCV bypass valve which is Locked Closed and also an RO downstream in the bypass line. The bypass line is only used when the LCV goes for a short maintenance or else the train is shut. The failing of LCV (by default Fail Close) when bypass valve is open therefore becomes less probable.

[Zauberberg]

Have you considered to install an orifice upstream of the valve, to limit the flow in the gas blow-by case?

[Christiano]

Hi Zauberberg, I cant install RO in the main line as it will hinder the throughput in LP operating mode.The challenge is to operate the LCV to deliver the flow capacity in HP & LP operational modes.

[Atttyub194]

Dear All

 

Good day!

 

Please do share the mechanical stopper proposed by the vendor. This would be a good learning for all of us

 

I do agree that my knowledge is limited to mechanical stoppers installed on Fisher, Masonelian, Flowserve, GE, Dresser Becker valves, Euro valves etc and just to share with you that the mechanical stoppers are not very different for all these type of valves and  are either limiting the torque or lift and you remove it, or readjust it in your ordinary workshop very easily

 

Had your safety system not limiting, I would simply comment to just install it.

 

I would suggest  to please do consider 02 small valves ( 50 % capacity or one big one small ) in split range to limit CV to desired level so that your safety system should not be jeopardized in any case.

 

I personally do not agree that updating operating instructions or manual will help as this would  not eliminate the actual risk due to human error unless otherwise the severity is mitigated some other means which probably you are the best judge as we are not fully aware of your process

 

Hope you may understand

 

Best regards and God Bless you

[Atttyub194]

Dear All

 

Good day!

 

Please find attached details of limit stops used with Fisher Control valves

Hope this will further clarify and gives a better understanding.

 

Best regards and God Bless you

 

 

Attached Files

•  d100087.pdf   375.83KB   196 downloads

[Atttyub194]

Dear Raj

 

Good day!

 

 Could you please share P&I diagram so that all members can give you valuable suggestion

 

Regarding split control , please refer attached sketch

 

Hope this will give you a better understanding

 

Best regards and God Bless You

 

Ahmed Attyub

 

Edited by Attyub194, 25 April 2013 - 11:23 AM.