20 replies to this topic

[rohollah]

Dear experts

 

We are going to install a by pass line for a control valve according to attached sketch.

what size should be selected for by pass line?

 

Regards

Rohollah

 

 

 

 

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[gegio1960]

It's a little bit difficult to quickly understand the reason why, for a 6" PCV, you have very very different 14"x20" in/out pipes.

Assuming a short length of the bypass, I would start with 6" pipe and valve and then check the pressure balance.

[Bobby Strain]

You must consider that with the bypass, you have potential for much greater flow to the system downstream. So you must revisit overpressure protection downstream. Check the max Cv for the control valve and provide a bypass valve with similar Cv.

 

Bobby

[gegio1960]

Good point Bobby.

In order to limit/avoid that trouble, you shall specify your bypass valve as CSC (Car Seal Closed) or LC (Locked Closed).

[thorium90]

Is the bypass line simply for pressure equalization just for startup or you intend to use it to permanently increase the flow to the downstream pipe so you do not need to buy a new bigger control valve?

Edited by thorium90, 11 February 2013 - 01:43 AM.

[fallah]

Dear Rohollah,

 

At first you should specify mentioned by-pass is to be considered for which purpose among following ones:

 

1-Control by-pass

2-Maintenance by-pass

3-Start-up by-pass

 

In fact, at first you need to define the purpose of the by-pass line/valve and then going to size it.

Edited by fallah, 11 February 2013 - 02:09 AM.

[kkala]

Similar matters are discussed in following links, probably useful.
http://www.cheresour...ve-bypass-size/
http://www.cheresour...ss-valve-sizing
Can you also clarify whether flashing occurs downstream of control valve?

[joesteam]

For our steam stations in NYC we are to use a bypass valve that has a Cv of equal or slightly lower than the Cv of the control valve, never higher.

[rohollah]

Dear Fallah

 

 

In this case maintenance by-pass is required.

I searched previous posts and found that by pass valve
should have 100%<Cv<200% of control valve Cv.

Is there any reference or standard for that?

Regards

Rohollah

 

[rohollah]

Dear kkala

 

Sorry for lack of information.

 

Fluid : steam

operating temperature:410c

max flow:75000kg/h

inlet pressure:40barg

outlet pressure:32barg

control valve Cv:390

 

 

Regards

Rohollah

[SSWBoy]

20/14" lines either side of a 6" control valve doesn't sit right with me!

[fallah]

Dear Rohollah,

 

Generally, when a maintenance manual by pass valve is installed, it shall have a CV not higher than that of the relevant control valve in full open position.

 

 

 

[rohollah]

Dear Fallah

 

What about Control by-pass & start-up by-pass?

 

Regards

Rohollah

[fallah]

Dear Rohollah,

 

For start up by pass, normally:

 

- As per line diameter up to 8": A globe valve with 1" size

- As per line diameter more than 8": A globe valve with 1"1/2-2"

 

For control by pass:

 

-Per case as required

[kkala]

Rohollah in post no 9 (Feb 10th) said:
.......I searched previous posts and found that by pass valve should have 100%<Cv<200% of control valve Cv. Is there any reference or standard for that?......

 

Roholah, if you mean post no 23 (by kkala) in the link <http://www.cheresour...ve-bypass-size/>, it is clarified that 100% - 200% of control valve Cv concerns sum of bypass and control valve Cvs during operation.
Each of the two valves has same (maximum) Cv in above mentioned post; for sizing of downstream PSV, the operating cases of fully open control valve+half open bypass valve (i.e. operating at 0.5Cv) or of fully open control valve+fully open bypass valve are considered, as indicated in post no 12 (by kkala) of above link.

Of course you can also consider the option of single fully open valve, either control or bypass, if bypass valve is CSC or LC during normal operation, as clarified in post no 4 (by gegio1960). I think this represents the clear maintenance bypass case (manual operation only when control valve is removed for maintenance)

 

 

 

Edited by kkala, 12 February 2013 - 06:35 AM.

[jr31]

Number of members have already commented on size of Base control Valve. General thumb rule for control valve size is two size reduction. Even with 14" as Inlet size 6" valve means 4 size reductions! ( 12", 10", 8" & then 6") and suggests a review of process data or selection.  CV shown in the post looks OK. In case this is an existing installation , the Valve must be very noisy  even with sch 80 inlet & outlet piping. In case it is a new installation Suggest to look at Noise level & outlet velocity , You probabally need a 10" size control valve with noise reducing with reduced trim to take care of CV.  Most companies have limit of 85 or Max 90 dB noise limit at Control Valve outlet.

Some one needs to review the selection.

Coming to Bypass  valve , Looking at process parameters it looks more to be maint. Bypass rather than a control Bypass. For Maint. bypass criteria suggested is fine ( CV equal to 100 % but less than twice). In case the process is critical and a bypass is envisaged for Control , you may even consider a parallel Control valve or a Control valve with Handwheel,(Obviously more initial Costs!). For Maint. bypass a manual valve should be fine.

 

Hope this helps!

[kkala]

At least according to the case mentioned in post no 15 (by kkala) bypass valve had same Cv as the control valve. Sizing of downstream PSV was based on total 1.5Cv (could be also 2Cv), since CSC or LC of bypass valve during normal operation was not provided.
Hope this clarification is useful. If Cvs of control and bypass valve are not identical, most conservative case has to be considered for any PSV affected.

[rohollah]

Is it acceptable to install a maintenance manual by-pass on a control by-pass valve?

 

Regards

Rohollah

[kkala]

1. In the case of flash drums referred in post no 15 (by kkala), maintenance by pass valves were manual.

2. Case of two identical control valves, operating simultaneously in parallel;  one manual maintenance bypass could be installed here (the two control valves would not fail at same time).

"Control by-pass valve" may mean this. A specific practice recommendation is not known.

3. Case that "control by-pass valve" is a maintenance valve, that is a  spare control valve installed on the line.

You could install a common maintenance bypass valve, but is it necessary? Failure of both control valves at same time is improbable. Besides, manual valve would not result in satisfactory control in the specific case, that is why automatic valve was installed.

[fallah]

Is it acceptable to install a maintenance manual by-pass on a control by-pass valve?

 

Regards

Rohollah

 

 

Dear rohollah,

 

Would you please clarify your query a bit more...

[ecovarrubias]

Hi rohollah,

 

As was said by gegio1960, SSWboy and jr31 you should review you piping design because you have a 20" pipe and you are reducing it to 6" for your control valve.

I would like to add another rule of thumb for the one explained by jr31, the size of the control valve must never be less than the half of the pipe. You should consider these two rules when you determine the size of the control valve.  

If you size the valve and the cv calculated fits to a 6" valve then you must probably check if the calculations done to determine the pipe size of 20" are correct.