4 replies to this topic

[hades_lear]

Hi!

 

As a junior process engineer I am struggle with some problems lately. When I am studing some P&ID I usually see a on/off main control valve which is bypassed by using other on/off control valve but with a smaller diameter. I know why this structure is used; when we want fluid to pass through this system the small valve is opened and once the pressure before and after the systems are even, the big valve can be opened avoiding many problems like water hammer.  Other times I don´t see this system, many times because (I think) the difference of pressure might be not really high, others because there is a control valve before... the thing is that I don´t know when you have to design in one way or in other....

 

My question is,

 

Are there any standards or reference that any of you know in order to design these kind of systems in a proper way?

 

Thanks in advance,

 

 

[shan]

You need a by-pass branch for the control valve maintenance also. 

[fallah]

hades_lear,

 

As far as i know,there is no registered standard for your query and the engineering practices are usually used in this regard.

 

In general, by pass line/around a control valve might be used for various functions as below:

 

1. For maintenance

2. For Start up

3. For control

Edited by fallah, 25 March 2013 - 04:01 PM.

[Dazzler]

_Hades-lear,

_{Take care not to put a manual bypass valve around the main actuated valve if it is a safety critical trip valve, because in this case the bypass can defeat the intention of the trip.}

_Dazzler

[Dacs]

Just to add on the discussion, this is really what we're suppose to do (as Process Engineers): to justify the need of placing bypass lines for valves.

 

Some projects would have guidelines on whether a bypass valve is needed. You can start checking on those.

 

It helps if you place yourself in an operator's position and you were in charge to operate the plant you're designing. For instance, there's this Flow Control Valve that feed a fluid to a column. You'd probably want to ask these questions:

1. Do you foresee taking the FCV offline during normal operation?
2. What's the impact when you do this?

 

If you answer NO on #1 (which is very unlikely BTW), then you'd probably don't need a bypass line. If you answer "the plant can operate normally even if the FCV is taken offline" to #2, then you'd also probably don't need a bypass line.

 

I'm grossly oversimplifying things for you (since there are other factors as mentioned by fallah), but this what we normally have in mind when dealing with this.

 

Don't worry though, you'd get your chance to evaluate those when you do your P&ID review.

 

BTW, I think the reason why you have a smaller XV (on-off valve) as a bypass for the larger XV is because of torque issue on the larger XV. You'd want to have a smaller pressure delta when you operate the larger XV and the smaller XV (which would require smaller torque due to its size) would do the job.

 

Hope this helps