10 replies to this topic

[farid.k]

Dear team,

 

how does open the bypass piping (i.e. bypass of control valve) can increase the flowrate across piping? how does this happened? i.e. during 100% of control valve opening can deliver 100T/H, but during control valve open 100% and bypass open it goes 120 T/H. 

 

 

[fallah]

 

how does open the bypass piping (i.e. bypass of control valve) can increase the flowrate across piping? how does this happened? i.e. during 100% of control valve opening can deliver 100T/H, but during control valve open 100% and bypass open it goes 120 T/H. 

 

Hi Farid,

 

What exactly do you mean?

 

It's obvious if a by pass is considered around a CV for flow control compensation, if to be opened while the CV is 100% open can increase the passing flow rate...hence please specify your exact query...

[farid.k]

how does open the bypass piping (i.e. bypass of control valve) can increase the flowrate across piping? how does this happened? i.e. during 100% of control valve opening can deliver 100T/H, but during control valve open 100% and bypass open it goes 120 T/H.

My friend (operation guy) said they have to open the control valve bypass in order to get more flow so that they can increase more yield. I am thinking off how does this happened because even you open the bypass, actually upstream of the bypass comes from 1 single piping. Open the bypass will make the flow split into two. How does it can increase the flow?

Hi Farid,

What exactly do you mean?

It's obvious if a by pass is considered around a CV for flow control compensation, if to be opened while the CV is 100% open can increase the passing flow rate...hence please specify your exact query...

[fallah]

 

My friend (operation guy) said they have to open the control valve bypass in order to get more flow so that they can increase more yield. I am thinking off how does this happened because even you open the bypass, actually upstream of the bypass comes from 1 single piping. Open the bypass will make the flow split into two. How does it can increase the flow?

 

 

Hi Farid,

 

Control valve size is normally smaller than line size. Now even for a control valve size same as line size and without the by pass line the Cv value of the valve in full open position, say 85% open, is such that the valve will throttle the flow passing the main line. Hence an open parallel by pass line will compensate the reduced cross section area due to in-line control valve and allow to create a combined Cv in the control valve and by pass line not smaller than that of the main line which lead to getting more flow.

Edited by fallah, 15 October 2017 - 11:23 AM.

[farid.k]

My friend (operation guy) said they have to open the control valve bypass in order to get more flow so that they can increase more yield. I am thinking off how does this happened because even you open the bypass, actually upstream of the bypass comes from 1 single piping. Open the bypass will make the flow split into two. How does it can increase the flow?

Hi Farid,

Control valve size is normally smaller than line size. Now even for a control valve size same as line size and without the by pass line the Cv value of the valve in full open position, say 85% open, is such that the valve will throttle the flow passing the main line. Hence an open parallel by pass line will compensate the reduced cross section area due to in-line control valve and allow to create a combined Cv in the control valve and by pass line not smaller than that of the main line which lead to getting more flow.

Thanks fallah. Now i get it for the control valve. No wonder sizing the relief valve for control valve failure with and withouy bypass is different. What if tjere is a heat exchanger that has a bypass? Does opening the bypass will increase the flow (assuming the heat exchanger is insignificant and can be bypassed)?
I dont think so isn't?

[Saml]

Opening a heat exchanger bypass will increase the flow as long as the pressure drop of the exchanger is a measureable part of the total system pressure drop. If you have a flow control valve in the same line, It will probably cause the control valve to close a little to compensate.

[farid.k]

Opening a heat exchanger bypass will increase the flow as long as the pressure drop of the exchanger is a measureable part of the total system pressure drop. If you have a flow control valve in the same line, It will probably cause the control valve to close a little to compensate.

I would say insignificant figure as delta p across hex normally minimum less the 10kpa isnt?

[Saml]

Not always. Think about a cooling water system where delta P in the exchangers is significant part of the total system pressure drop.

So may be, may be not. You have to do the numbers case by case.

[canhhienfamily]

Dear all

This is the first time I joint this forum. I would like to say hello to everyone.

I 've just been mobilized from Technology group to engineering group since June 2017. Actually, my experiences in engineering have limited. Therefore I need your helps.

I have some concerns about the bypass line/valve of control valve as below:

- Main line size and control valve size: 16"; Cv 2250

- Bypass line and gate valve" 6" only (instead of globe valve as i known);

- Fluid: crude oil, flowrate 1070m3/h at 50oC.

Last time, due to the control valve has problem and need to be isolated for maintenance. However, we did not want to reduce throughput of the refinery. The bypass valve was fully opened while control valve was isolated.

The velocity through the bypass line 6" at that time is quite high 17.7 m3/s against 3.66m/s (as refinery standard) or 4.5m/s as other standards.

Please advice if the above mentioned control valve configuration is correct? Do we need to resize the bypass line and replace a bigger bypass valve which are sufficient to handle the high flow-rate?

If modifications are not allowed, Do we need to reduce flowrate to ensure the crude velocity in the normal range?

Thanks

[Saml]

Welcome to the forum.

 

Probably you won't have to change the design if you can maintain the throughput.

Velocity that high is a problem on the long term, but probably not in the short term required to repair a valve.

 

But don't take this as advice. Just an oppinion with the limited data you provided. Talk to your refinery inspection/mechanical integrity people about this. It may deserve a thikness measurement or X-.ray.  May be that section of pipe has no been identified as deserving a particular inspection plan. Or it may happen that they have already identified it, and everything is OK.

 

And please. Next time start a new discussion. It is better to have one topic per thread.

Edited by Saml, 17 October 2017 - 09:22 PM.

[AZIZ_MN]

My friend (operation guy) said they have to open the control valve bypass in order to get more flow so that they can increase more yield. I am thinking off how does this happened because even you open the bypass, actually upstream of the bypass comes from 1 single piping. Open the bypass will make the flow split into two. How does it can increase the flow?

Hi Farid,

Control valve size is normally smaller than line size. Now even for a control valve size same as line size and without the by pass line the Cv value of the valve in full open position, say 85% open, is such that the valve will throttle the flow passing the main line. Hence an open parallel by pass line will compensate the reduced cross section area due to in-line control valve and allow to create a combined Cv in the control valve and by pass line not smaller than that of the main line which lead to getting more flow.

Thanks fallah. Now i get it for the control valve. No wonder sizing the relief valve for control valve failure with and withouy bypass is different. What if tjere is a heat exchanger that has a bypass? Does opening the bypass will increase the flow (assuming the heat exchanger is insignificant and can be bypassed)?
I dont think so isn't?