10 replies to this topic

[PTSHeng]

Good evening all,

I know that a RO is used to drop the pressure in a pipeline, however I would like to know how it works to reduce the flow rate as many articles online do not explain how a flow is reduced. 

For example, for a given upstream pressure and pressure drop, how would I halve the flow rate going through the pipe line? (600kg/h before the plate to 300kg/h after the plate).
 

[breizh]

PTSheng ,

 

 http://en.wikipedia....i/Orifice_plate

 

Certainly good to read .

 

Note : Mass flow rate is constant !

 

Breizh 

Edited by breizh, 09 January 2015 - 03:43 AM.

[PTSHeng]

Hi Breizh,

Thanks for your response. That was the first article I read!

The problem I keep running into is that the equations provided all account for constant flow rate. There is no way to calculate the pressure drop to reduce the flow.

[fallah]

PTSHeng,

 

Basically the flow through a RO will vary with upstream and downstream pressures hence pressure drop across it. The size of a RO is to be specified  based on the range of operating conditions can occur across the relevant orifice. Then a RO is normally sized either to limit the flow based on possible range of pressure drop or to limit the pressure drop based on possible range of flow. But in any case for a specified line size along which a RO has been installed, only there is a specified flow corresponding to a specified pressure drop and to reduce the flow downstream pressure is to be increased (or upstream pressure is to be reduced) hence the pressure drop has to be reduced.

[processengbd]

Once I thought about simulating these scenarios in Dynamic Simulator (HYSYS or OLGA). 

 

"Flow through a RO will vary with upstream and downstream pressures hence pressure drop across it. "

Would it be worth doing this simulation? As two independent variable is concerned. It is hard (It was hard while I was thinking) to grasp the interdependency of orifice dia, pipe size for such a young engineers like me. But in my view experienced engineers can design this thing with normal intuition only.

 

Best Regards,

processengbd

Edited by processengbd, 09 January 2015 - 07:08 PM.

[paulhorth]

PTSheng,

 

The problem I keep running into is that the equations provided all account for constant flow rate. There is no way to calculate the pressure drop to reduce the flow.

 

Just rearrange the equation so that the DP is on the left and the flow is on the right! Then enter the flow you want and calculate the necessary DP. Repeat for different orifice sizes.

 

I'll do it for you:

 

Orifice area       A         =              0.621  W_________     

                                                                           C.Y.sqrt((P₁ – P₂).r₁)

 

Where:             A         =          orifice area, mm2

                        W         =          required flowrate, kg/hr

                        P₁               =          initial upstream pressure, bar a

                        P₂               =          downstream pressure, bar a

            r₁         =          upstream fluid density, kg/m³

                        C          =          discharge coefficient:

                       Y = expansion factor  (1 for liquids)

 

So:    (P₁ - P₂)   = (0.621W/(A.C.Y))²/r1

 

Solve and tabulate (P₁ - P₂) vs W for different A.

 

 

Processengbd,

 

There is no need to go anywhere near a dynamic simulation to solve this simple equation for different values of A and DP.

 

Paul

[AlertO]

PTSHeng,


I think you may confuse that the RO can reduce the flow between upstream and downstream RO. Actually, it does not happen like that.


The word "reducing the flow" means the flow is reduced after installing RO comparing to before RO installed. However, the flow upstream and downstream of RO is always equal.

[PTSHeng]

Hello all,

Thank you very much for your help! I will consult with my chief with the details!

[PTSHeng]

PTSHeng,


I think you may confuse that the RO can reduce the flow between upstream and downstream RO. Actually, it does not happen like that.


The word "reducing the flow" means the flow is reduced after installing RO comparing to before RO installed. However, the flow upstream and downstream of RO is always equal.

Do you mean to say, that an RO cannot reduce a flow even when sized for a pressure drop?

Say I have a flowrate of 100kg/h with a working pressure of 2 bar. I would like to put an RO such that the flow rate downstream will be 50kg/h. Is this not possible?

[paulhorth]

PTSHeng,

 

Say I have a flowrate of 100kg/h with a working pressure of 2 bar. I would like to put an RO such that the flow rate downstream will be 50kg/h. Is this not possible?

 

Not in this universe, it isn't. You would need to inhabit a world of magic to do that.

 

Mass flow is conserved. Always. If you put 100kg/h into a pipe then 100 kg/h must come out of it, somewhere.

 

If you want 50 kg/h then you must either reduce the feed or else take out the extra 50 kg/h from a side branch.

 

Paul

[Propacket]

PTSHeng,


I think you may confuse that the RO can reduce the flow between upstream and downstream RO. Actually, it does not happen like that.


The word "reducing the flow" means the flow is reduced after installing RO comparing to before RO installed. However, the flow upstream and downstream of RO is always equal.

Do you mean to say, that an RO cannot reduce a flow even when sized for a pressure drop?

Say I have a flowrate of 100kg/h with a working pressure of 2 bar. I would like to put an RO such that the flow rate downstream will be 50kg/h. Is this not possible?

I can understand your confusion but its not like that as you say. Yes RO can be used to reduce flow but you must understand what does it really mean.

Just take example of a BDV/RO located on a pressure veseel for its depressuring. If you try to depressurize the vessel without an RO, depressuring will take place quickly and depressuring flow will be very high. When you put an RO, it will reduce the depressuring flow and depressuring time will increase.

Coming back to your question, if you want to reduce flow from 100 to 50 by using an RO you must send the remaining 50 kg/hr to somewhere else.

Haseeb Ali