5 replies to this topic

[sagm]

Hello,

 

 Data Sheet.JPG   54.03KB   59 downloads

 Dibujo.JPG   42.26KB   61 downloads

 

My concern is There is a 8" valve installed that controls AGO and return it to the AtmosphericTower...To have it out of service means a lost in production, so it is a very critical valve. 

This valve is a 8" Fisher ET with CAVIII trim and has a bypass line for maintenance purposes.

 

Even though it has a bypass line, this control valve can't be removed for its regular maintenance routine because the bypass valve fails all the time, I mean, the vibration in the bypass line raises as the bypass valve opens and the control valve closes, and most of the time big and unstoppable leakages start from the packing flange, having then a hazardous and operational situation. It is not a spare part issue, the bypass valve has been well packed all the time.

 

I´m attaching the control valve data sheet  and a P&ID extract to see the arrangement.

 

I want to know how can I approach this problem ir order to have a reliable bypass valve.

 

I have thought about reducing velocity upstream in the bypass before the valve, or to have a bypass valve with the same characteristic of the control valve. But of course these are thoughts, I want to support my choice based in good practices and analysis.

 

Is there a ISA or API standard to help designing a bypass line? Any literature? 

 

Thank You

[ColinR33]

The control valve is a CAVIII trim which indicates to me this is a severe service with potential flashing (cavitation) issues requiring the special trim.  You have a pretty large relative pressure drop through the valve (78% of inlet P) and there may be some cavitation, which could explain the vibration issues with your bypass valve.   You may have to go with a special bypass valve designed to handle cavitation.  Just my $0.02.

[Propacket]

Looking at the control valve datasheet, your fluid is pretty subcooled both at inlet and outlet as its vapor pressure is less than operating pressure. I disagree with Colin that it is a flashing service. However I noted another thing which is high operating temperature (441 degF). I am not sure if your bypass valve trim is adequate for such high temperature. The high temperature may have damaged the packing.

[jrtailor09]

Sagm,

 

Your concern is bypss valve. See my reply in one of the thread for bypass valve size.

 

http://www.cheresour...ge-2#entry69163

 

May i know which kind of valve is it metal to metal seated or soft seated ?

 

You should go for metal to metal seated. you may facing this problem because of softseated.

 

Check about the gasket material.

 

Can i get fluid density ?

 

Regards,

JAtin

Edited by jrtailor09, 19 May 2013 - 09:11 AM.

[Art Montemayor]

Silvia:

 

Have you resorted to contacting your Fisher local representative?  In past years, I always had Fisher field service technicians assistance in Venezuela.  Is this still a practice?  That would be the ideal and surest way of resolving what I would call an intolerable situation.  This size of control valve should not give you any problems - much less vibration problems.  You are doing the right thing by giving it your attention.

[sagm]

Hello,

 

Thank you for the advices, 

 

Propacket and jrtailor09, I took the suggestion and checked the bypass valve specs, it is a API 600 Globe valve, metal to metal seated, and is packed with graphite, which resists until 550°C @500 bar, so it was well design for the fluid characteristics. The AGO is 19,1API @ 60°F.

Art Montemayor, I asked Fisher but the problem is not the control valve so they didn’t have any answer about it. 

I’m still with doubts about the trim in the bypass valve…