8 replies to this topic

[OlusolA]

Good day all.

Is it possible to have an undersized restriction orifice and how do i know this.

I am carrying out a check of an existing blowdown system which has restriction orifices downstream the blowdown valves. The restricition orifices are sized for a max flow of about 5% of the maximum to be expected through the blowdown valve. Now i understand that the function of the RO is to restrict the gas flow so that the flare doesnt become over loaded, but when is the restriction too much such that we are not able to safely depressurize the facility within the required time frame. Hope this is clear. 

 

Thanks in advance

 

Sola

[PaoloPemi]

a RO usually controls the flow while BDV starts blowdown sequence (no flow control, typically it's a full bore valve),

for gas providing a sufficient dP is available across RO (i.e. Pin > 2 * Pout) flow is critical and you can evaluate the maximum mass flux knowing inlet density and speed of sound,

for liquids (vaporizing) you can adopt HEM or HNE models to estimate the maximum mass flux.

I am not sure about the scope of a

*RO sized for a max flow of about 5% of the maximum to be expected through the blowdown valve*

is the flow controlled by the valve ?

[OlusolA]

PaoloPemi,

Flow is not controlled by the valve. It is an on/off valve required to open only during emergencies while other gas outlet would be closed, and the restriction orifice is downstream this valve. The intent is that, during emergencies, the valve opens and the vessels are depressurized to 7barg within 15mins. Now my question is that, when is the restriction too much, such that the vessel is not able to depressurize. I currently have that the maximum flow through the RO is about 5% of the flow through the Valve.

 

Thanks

[PaoloPemi]

"Now my question is that, when is the restriction too much, such that the vessel is not able to depressurize. I currently have that the maximum flow through the RO is about 5% of the flow through the Valve"

 

the flow through BDV and RO is not constant, for gas (usually) there is a initial peak,

you can simulate the blowdown (15 -> 7 Bar.a) or estimate a peak flow to size the RO,

if the flow allowed by RO is lower that required blowdown will require more time.

[fallah]

Now my question is that, when is the restriction too much, such that the vessel is not able to depressurize. I currently have that the maximum flow through the RO is about 5% of the flow through the Valve.

 

Sola,

 

Answer to your question: When the RO being applied would be one with lower Beta ratio than has been obtained from proper simulation e.g. by Hysys...

 

Would you please specify how did you recognize that the flow through the RO is about 5% of the flow through the valve (probably without the RO in downstream?)...

[RoyenG]

Hi Sola,

 

checking your RO is undersized or oversized can be based on the original design of your system (say, 15 minutes to reduce the normal operating pressure to half of design pressure or to final pressure of 7 barg).

 

safe or not safe depressurization in terms of relief load will be depend on your flare tip capacity and also consideration to the other BDVs that open simultaneously.

[PaoloPemi]

"safe or not safe depressurization in terms of relief load will be depend on your flare tip capacity and also consideration to the other BDVs that open simultaneously"

 

this is a important point to consider, there could be factors limiting maximum allowable flow in original design,

a common way to size a RO is to simulate blowdown,

nowadays almost all simulators do include a specific procedure for that,

I use the utility in PRODE PROPERTIES but other members here have hysys (results should be comparable),

another possibility (for vessels containg gas) is to size RO with a simplified procedure (errors are limited).

[OlusolA]

Hi All,

Thanks for your responses.

 

Fallah,

I wasnt able to obtain a beta ratio from HYsys - i am asuming this means the ratio of the diameters. From the hysys simulation, i had a peak flow through the valve and it was this value i compared with the maximum capacity of the restriction orifice based on its diameter. I think i shouldnt have gone that route though.

 

I have now been able to reach a reasonable conclusion. I determined the inventory in the vessel and pipework(from HYsys) from initiation of depressuring to the endpoint of 7barg or 0.5DP and calculated the time it would take to go through the installed RO. If it was greater than 15mins, i took it that the RO was undersized. I also checked the flare capacity to be sure that wasnt what was dictating the size of the RO.

 

Thanks for your help everyone

 

Regards

[PaoloPemi]

that is the usual way to simulate blowdown cycle with a simulator,

you define as feed vessel's capacity + (if any) heat exchanged, piping + RO and backpressure (if variable) vs. time

(with a flare backpressure in your system may show some variation),

the software should report pressure (inside vessel) vs. time plus flows and temperatures (gas,walls etc.)

(I use Excel and create a graph with PRODE),

from that you should be able to evaluate if RO size is acceptable (or not).

Edited by PaoloPemi, 13 January 2014 - 08:39 AM.