31 replies to this topic

[ajayd]

Dear All,

Please explain me why we are putting RO in the Bypass Shutdown Valve Line...........?

Regards,

Ajay

[fallah]

An isolation valve followed by RO to be installed in by-pass SDV line (usually with smaller size than that of main line) for pressurizing the downstream vessel in start-up condition to prevent SDV and vessel internals to be subjected to high torque due to high differential pressure.

[ajayd]

Dear Fallah,

Thanks for your reply.

Would you please expalin me about the differential pressure means where does it takes place & because of what condition it is.?

[fallah]

As i mentioned, during start-up if pressurization of a pressure vessel would be decided to be done by main valve (SDV,or any isolation valve...with large size) high differential pressure between its upstream/downstream along with high flowrate cause the internal parts of the valve and also downstrean vessel internals would be subject to damage.

Thus, providing smaller size valve followed by a RO in by pass line prevents the main valve from any damage during start-up (while the main vave is closed). After pressure equalization the main valve would be opened and by pass line would be closed.

In normal operation,e.g. when liquid is flowing from a high pressure vessel to a low pressure one a RO that may provided downstream of the existed LCV has minimum impact on operation of LCV.
But level controller failure caused LCV to be WIDE OPEN, and large amount of liquid follow by gas blow by will pass through the LCV and RO will limit the flow rate and prevent overpressuring of downstream vessel.

[diego]

Generally on big shut down valves you have the bypass because, if you start from a condition where you have a high pressure difference between upstream and downstream parts of the plant (with respect of the valve we are talking about), when you start opening the big valve without using the bypass you obtain an high flux across the slit between seat and obturator of fluid.
The flowing of fluid at high velocity could damage the valve creating for example leakages in the future.

In smallest bypass valves the duration of the phenomena of high velocity flowing is reduced due to the shortest opening times.

Bye

Diego (the very small contributor)

[fallah]

In smallest bypass valves the duration of the phenomena of high velocity flowing is reduced due to the shortest opening times.

Seems would be vice versa,the smaller bypass valve (lower CV) result in longer duration of high velocity flowing and also longer opening time for pressure equalization of upstream and downstream of the valve!

Please kindly clarify.

[diego]

Sorry .......I try something better.....

In smallest valve you obtain the condition of fully open in a shorter period that is the optimum working condition (design working condition of the valve) and so no damages for the valve.

If also in this case you may damage the bypass valve you have to install the orifice to concentrate the pressure loss on the orifice and not on the valve.

On the biggest on the main line you have a transient state longer

It is all a matter of bad english

Sorry

Diego

[fallah]

If for example,by pass valve to be a SDV (ball valve type- full bore), by-pass line sizing would be done based on maximum expected flowrate through the line considering allowable mach number, vibration,....

Downstream RO(s) on which approx. the whole pressure drop (difference of initial upstream and downstream pressure) would be concentrated, being sized based on max flowrate and whole pressure drop.

Therefore, i think we haven't so many options for sizing the by pass line and valve due to process limitations.

[shan]

If a SDV is designed for a certain differential pressure value, it shall be able to operate at the specified differential pressure without an equalizer.

Bypass line RO is for reducing bypass Cv to match with main line Cv.

[fallah]

High differential pressure across a big shutdown valve will result in high torque against its actuator and it may not be cost effective to provide an large actuator just to open the Shutdown valve in start up.

Thus,the differential pressure value in start up would be too much higher than that specified for shutdown valve operation and by pass line as equalizer is needed.

By pass line RO/globe valve is for throttling to avoid high pressure concentration across small shutdown valve upstream of RO/globe valve in by pass line.

[shan]

If a RO is involved in the bypass line, the main line diameter should not be that large. Bypass Cv = Main Line Cv is necessary to maintain downstream process condition when main valve is closed and bypass valve is in charge.

[Qalander (Chem)]

Dear ajayd, Shan is right here since the very purpose of almost all of the control valve bypass piping line-up circuit provision is

1) to ensure work as and when necessitated on the isolated control valve without affecting/ jeopardizing the process operation itself.

2) Additionally the total flow rate via bypass pipeline has be to equal/ matching the 'Main' pipeline's flow.

Hope this helps in the way forward.

[fallah]

If a RO is involved in the bypass line, the main line diameter should not be that large. Bypass Cv = Main Line Cv is necessary to maintain downstream process condition when main valve is closed and bypass valve is in charge.

Occasionally,Main line diameter is large with respect to by pass line.In normal operation by pass line would be closed and to be used in start up condition for pressure equalization.

There is no concern to maintain down stream process condition in start up and no need to have the same CV for main and by pass lines.

[ajayd]

Please can anybody explain me what is the floe condition at the downstream of orifice whether it is maximum or minmum with respect to the upstream flowrate.?

[fallah]

Please can anybody explain me what is the floe condition at the downstream of orifice whether it is maximum or minmum with respect to the upstream flowrate.?

No clear question.

Gas or Liquid service?

Mass or Volumetric Flowrate?

[ajayd]

Dear Fallah,

If You Can Then Please explain me for the both services.

thanks,

[shan]

An orifice provides a differential of pressure not a differential of flow in a piping system. Therefore, the orifice upstream pressure value is higher and downstream pressure value is lower, while its upstream flow value is identical with its downstream flow value.

However, the variations of pressure lead to variations of flow in the flow circuit as shown on pump/piping system curve charts.

Orifice downstream process conditions are defined by the certain process. An orifice is just a mean to achieve the desired process conditions by reducing pressure and restricting flow.

[fallah]

Dear Fallah,

If You Can Then Please explain me for the both services.

thanks,

In liquid service : Volumetric flowrate/Mass flowrate in Downstream of the RO would be the same as Upstream

In gas service :Mass flowrate in Downstream of the RO is the same as Upstream,but Volumetric flowrate in Downstream of the RO increases due to higher gas velocity

[ajayd]

Dear Fallah,

Then how we are controlling the flowrate through RO which is required for optimum sizing of the flare.
Dose it means that if we will not install RO then the size/capacity of the flare will increase?

If RO Cant reduce the flow then how we are controlling the flowrate through flare?

Please expalin.

[fallah]

RO isn't a flowrate control device, but if installed after BDV connected to flare network reduces the flowrate with respect to the manner that not to be installed.

Yes, if RO isn't installed size/capacity of flare network will increase.

RO can reduce the flow to flare.

[ajayd]

RO isn't a flowrate control device, but if installed after BDV connected to flare network reduces the flowrate with respect to the manner that not to be installed.

Yes, if RO isn't installed size/capacity of flare network will increase.

RO can reduce the flow to flare.

Dear Fallah.

But earlier you said that In gas service :Mass flowrate in Downstream of the RO is the same as Upstream,but Volumetric flowrate in Downstream of the RO increases due to higher gas velocity.

Then how RO Will reduce the flow.

[Qalander (Chem)]

Dear ajayd Hello,

May I enter discussion here with a comment that what 'fallah' had earlier mentioned was most probably a "qualitative" statement rather than a "quantitative" one.

Hope this clears away the ambiguity/vagueness if any.

[fallah]

Dear Fallah.

But earlier you said that In gas service :Mass flowrate in Downstream of the RO is the same as Upstream,but Volumetric flowrate in Downstream of the RO increases due to higher gas velocity.

Then how RO Will reduce the flow.

Please try to read my statements accurately.I mentioned:

"if installed after BDV connected to flare network reduces the flowrate with respect to the manner that not to be installed."

Thus RO reduces the flow in a line in comparison with the flow in that line without RO.

But to prevent increasing of volumetric flowrate of a gas passing through RO, normally size of the line after RO would be increased by using an expander.

[herrani]

Hi all

I hope you don't mind me adding a few lines to the discussion here.

In the case of a compressible fluid (gas) the density of the fluid changes with pressure. It is because of this reason that the volumetric flowrate before and after an orifice will be different (the mass flowrate will be the same, according to a mass balance).

Regarding the comment on the line sizing of a flare header, this is usually done based on the fluid velocity, which depends on the actual volumetric flow, and so using a restriction orifice will reduce the required flare header capacity.

In addition to this, there is the issue of critical flow. In the case of compressibe fluids, there is a physical limit on how much volumetric flow can pass through an orifice. This is the critical flow. This means, that above a certain pressure difference before and after the orifice, the flow that can pass through the orifice is limited. This property is the one used to size restriction orifices for blowdown lines. Typically they are used to ensure that the volumetric flow from the blowdown valve will not be higher than the flare capacity or similar applications.

In the case of a non-compressible fluid, volumetric and mass flows are always the same, and an orifice will only be an additional pressure drop in your system. Nothing else.

I hope this helps in the understanding of the question! (otherwise, please ignore...)

[ajayd]

Dear Herrani,

Thanks For Your Responce But Still I Am In Confusion if downstream Volumetric flowrate across the orifice is higher than the upstream Vol Flowrate ( Means Through BDV ) Then How We are reducing the capacity of flare because the Vol Flowrate is Increased Because Of Orifice.

Please explain in details.