Control Valve Bypass Arrangement 2

[DGrayPPD]

Good morning everyone,

Just had a quick question. It is pretty basic, but I would still like the opinions of some of the experts on here.

Please refer to the image below. I have given two examples of a control valve station from a plan view. Station #1 would seem to allow easier access to the bypass handwheel and it reduces cuts and welds which saves $$$. Station #2 however shows a more balanced, even arrangement between the bypass and control valve.

My question is, which do you feel is the more ideal arrangement? Is there a benefit to having the bypass even with the control valve in Station #2? If so, what is it? I know what my opinion is, but it is always nice to have validation or for someone to give you legitimate reasons why you may be wrong.

Thanks for the tips.

 

[WPetroChem]

DGrayPPD,

A rule of thumb I believe I was told was to have the bypass control valve as close to the upstream side as possible, which is option #1 if flow is from left to right. Exactly why, I'm not sure. It may have to do with keeping as short of a dead leg as possible.

Hopefully someone on here will have a better answer.

One thing I thought of is if you have a large actuator on your control valve, moving the by-pass control valve will help to keep them from interfering. Just a thought.

 

[pennpiper]

DGrayPPD,
Just my opinion.
These configurations are commonly found as a schematic form on a typical P&ID, but nobody should ever do this on a real physical Control Valve Station in any type of real plant.

The real problem here is the inability to remove the CV for maintenance or replacement
The Bypass will prevent flexibility to spread the flange joints to remove the old gasket, then the CV. Then you will need to reinstall the CV and new Gaskets. Good Luck! And that is with common Raised Face Flanges. Wait until some idiot tries to use one of these two configurations with RTJ Flanged components.

 

[DGrayPPD]

Thank you WPetroChem.

So pennpiper, in your opinion, what is the ideal bypass valve arrangement?

 

[pennpiper]

DGrayPPD,
I am trying to find a photo I can post here, but I have not been successful so far.

The basic configuration is a "U" shape with a cross pipe.
The CV is in the bottom of the "U" shape. The up stream and down stream block valves are
in the vertical legs on each side of the CV station. The Bypass valve is in the cross pipe above the CV (with clearance to access the CV top works).

For Maintenance you remove one of the elbow spool on either side of the CV (Up stream or down stream) and that frees the access for CV removal.

I can find a picture but I cant copy and paste. Sorry!

Sometimes its possible to do all the right things and still get bad results

 

[DGrayPPD]

Pennpiper,

I know the arrangement you are referring to. The U-shape is in the vertical plane. I know that my images show the arrangement in the horizontal plane, this is just one of the ways we have had to design CV stations on platforms. My question could apply to either arrangement.

I guess the ultimate thing I would like to determine is if the bypass valve NEEDS to be even with the CV valve for any reason that I am unaware of. If it is up to me, no matter the vertical or horizontal arrangement, I would butt the bypass valve up to the tee and save the extra cut and welds.

 

[pennpiper]

DGray,
It does not matter where the BP Valve is located in the BP itself.

Is your piping RTJ?
If so then get one of the Block Valves around the corner to allow a removable spool piece.

Sometimes its possible to do all the right things and still get bad results

 

[DGrayPPD]

Pennpiper,
No, this is just a hypothetical arrangement.

Thank you for the advice.

 

[DGrayPPD]

Pennpiper,

Side topic, but I would just like your opinion on this as well. For a vertical CV station like you had suggested, what would be your ideal CL elevation from control valve to grade? 24"? 30"?. I have heard that the ideal distance from piping to grade is 30" for ease of operation and accessibility, but would just like to know if this differs from your opinion and expertise.

Thank you.

 

[LittleInch]

Ideal arrangement - No bypass valve.... See recent post in another forum.

If you really want on then two tees and two parallel streams. Gives you the option of putting in a second control valve if you ever want one.

Why reduced size bypass line?

a lot will depend on the incoming and outgoing pipe orientation, height, space product ( minimize dead legs or not).

The drop down off the rack is probably as good as any if you have the vertical space.





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DGrayPPD]

LittleInch,

So you think the arrangement in Station #2 is more ideal? What instances might arise that might require a second CV where the bypass valve is?

And normally the CV's are a line size smaller than the actual piping, and the bypass size generally matches the CV sizing on the projects I've been a part of. So the smaller bypass line is just matching the bypass valve sizing. Just a means to save on fittings, welds, and overall cost.

Most times if the option is available, yes I try to go with the vertical bypass arrangement out of the pipe rack.

 

[DGrayPPD]

LittleInch,

Never mind, I went back and read your responses on the recent post regarding bypass valves. I forgot the many good points you had made. Although I tend to agree with your thinking on the matter, I foresee a very difficult time trying to persuade almost anyone where I am at to follow this logic. They are very "old-fashioned" in their thinking.

 

[Gator]

Is there a specific reason why the control station is horizontal?

 

[DGrayPPD]

No, like I said it was just a hypothetical question. We've had some arrangements on top of vessel platforms that were horizontal so that is what I referred to for my example.

 

[pennpiper]

DGrayPPD,
Re: your "Side Topic"

Think about the whole Maintenance and Operation Issue.

During start-up and ongoing over the next 25 years what will people be working on the most. Not the CV Valve Body, they will be working most of the time on the Positioner on the side of the Top Works. That may be between the chest and eye level.

Regardless of the Control Valve size I would set all CV valve CL at 24". With 24" (for all sizes) this will give me a consistent Base Support height. Yes, I might adjust that for some odd and as yet undefined reason.




Sometimes its possible to do all the right things and still get bad results

 

[MickMc]

I would prefer station 2 configuration as it makes it easier to spring the flanges if needed although station 1 is correct regarding dead leg. I would also prefer the vertical configuration for space constraints but offset the bypass to allow maintenance/lifting of CV. A common compaint I see from maintenance guys is how remove CV when bypass is directly overhead.

 

[LittleInch]

Dgray,

For your old fashioned colleagues, there are usually a few questions which might cause them to think again or you can wathc them wrap themselves in knots.

1) Does this system need the control valve to work (pressure, flow temperature control, whatever) Yes or No
2) If Yes ( otherwise why it is there?) will the process still work / be safe / not trip if it isn't there?
3) If answer to 2 is No, then the follow up is "Why then is there an installed function that allows you do that"? (i.e. the bypass valve)

Hopefully this causes enough of a thought process to take place....

The only way any of that can work is if you can show that there is a flow restriction that prevents flow or pressure exceeding what the control valve would do at max flow. Even then there is complete loss of "control"

As soon as you introduce a bypass valve, in any HAZOP or similar LOPA etc you cannot take any credit for the fact that your first line of defence is control of the fluid by the control valve because you've installed something which can disable that protection in one easy movement. Forget the argument over locked closed, CSC etc - the valve is there, someone can open it. Your careful design and control systems just got thrown out of the window.

/rant over



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DGrayPPD]

LI,

I like the logic and thought process. Will get back to you after I hear what they have to say.

 

[DGrayPPD]

LittleInch,

Just following up on our conversation. Spoke to one of the Lead Process Engineers at our firm and they agreed with your logic, however they stated that "We still put the bypasses in for maintenance because a lot of the times operations/maintenance is able to throttle the valve enough to get them in the ballpark of where they need to be."

So, like I figured, I doubt too much will change with their thinking.

 

[LittleInch]

Not exactly what I would call "control".....

If they are going to do that then you really want an isolation valve and globe valve / manual control valve and park someone there.

Isolation valves are notoriously CRAP at control.

To me that means you don't really need a control valve if that type of "control" is tolerated.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.