9 replies to this topic

[vaibhav]

Dear Respected Members

 

I would like to seek  expert advise / suggestions on the following issue.

 

we had nitrogen gas depressurization system where  nitrogen gas is depressurizing via scrubber vessel to ATM. in the system PCV ( self actuated pressure control valve ) is installed to limit the downstream pressure @ 0.5 bar g.  total time for depressurization operation is considered as 30 min which is confirmed by client. Upstream pressure at PCV is 5 bar g which is varying as the operation progress. ( process Conditions that is pressure and flow rate of Nitrogen gas is varying ) .

 

with the current design of PCV , IT Takes almost 3.5 to 4 hours to get operation completed. what is the reason to cause this problem?

I am again doing resizing of the PCV but not able to reach best possible solution .

 

whether we have to go for control valve ( Modulating type ) to overcome this problem or with already installed valve whether by doing internal modification we can achieve our target .

 

 Awaiting for your valuable suggestion.

 

regards

VP

 

[fallah]

 

 

we had nitrogen gas depressurization system where  nitrogen gas is depressurizing via scrubber vessel to ATM. in the system PCV ( self actuated pressure control valve ) is installed to limit the downstream pressure @ 0.5 bar g.  total time for depressurization operation is considered as 30 min which is confirmed by client. Upstream pressure at PCV is 5 bar g which is varying as the operation progress. ( process Conditions that is pressure and flow rate of Nitrogen gas is varying ) .

 

with the current design of PCV , IT Takes almost 3.5 to 4 hours to get operation completed. what is the reason to cause this problem?

I am again doing resizing of the PCV but not able to reach best possible solution .

 

whether we have to go for control valve ( Modulating type ) to overcome this problem or with already installed valve whether by doing internal modification we can achieve our target .

 

 

VP,

 

With your limited info and lack of a clarified sketch just can be say the PCV is undersized. Please upload a simple sketch and additional info such as process type (batch or continuous) final pressure,...to get a better and proper response

[vaibhav]

 

 

 

we had nitrogen gas depressurization system where  nitrogen gas is depressurizing via scrubber vessel to ATM. in the system PCV ( self actuated pressure control valve ) is installed to limit the downstream pressure @ 0.5 bar g.  total time for depressurization operation is considered as 30 min which is confirmed by client. Upstream pressure at PCV is 5 bar g which is varying as the operation progress. ( process Conditions that is pressure and flow rate of Nitrogen gas is varying ) .

 

with the current design of PCV , IT Takes almost 3.5 to 4 hours to get operation completed. what is the reason to cause this problem?

I am again doing resizing of the PCV but not able to reach best possible solution .

 

whether we have to go for control valve ( Modulating type ) to overcome this problem or with already installed valve whether by doing internal modification we can achieve our target .

 

 

VP,

 

With your limited info and lack of a clarified sketch just can be say the PCV is undersized. Please upload a simple sketch and additional info such as process type (batch or continuous) final pressure,...to get a better and proper response

 

 

Dear Mr Fallah

 please find attached valve sizing calculation report for your reference. this nitrogen depressurization operation is batch process . over a period of depressurization operation flow rate and up stream pressure of nitrogen gas is changing. currently PCV installed in the system is self actuated pressure control valve ( Globe type ) . At present time required for depressurizing the nitrogen gas via scrubber vessel after the pigging operation gets over is 3.5 to 4 hours instead of 30 min which is the anticipated time for the operation. client now ask what is the root cause for this ?  is any additional details of input data is required pl. let me know.

awaiting for your valuable suggestion .

 

I am attaching sketch for the system also.

 

thanks

 

VP
 

Attached Files

•  calculation.pdf   169.83KB   54 downloads
•  ABE-XXX-PRO-PID-01 Model (1).pdf   45.79KB   38 downloads

[Bobby Strain]

What makes you believe that setting the downstream pressure at 0.5 bar is the correct value? We can't help you if we don't know the volume of the system that you wish to depressure. How did you arrive at the design flow you show on the datasheet? Lots of missing information to offer any guidance.

 

Bobby

[vaibhav]

What makes you believe that setting the downstream pressure at 0.5 bar is the correct value? We can't help you if we don't know the volume of the system that you wish to depressure. How did you arrive at the design flow you show on the datasheet? Lots of missing information to offer any guidance.

 

Bobby

 

Mr Bobby

pl. find attached calculations for more information & basis for the flow rate of nitrogen gas considered in PCV sizing calculations.

regarding down stream pressure of 0.5 bar g , if it increase to let us say 2 bar g does it help us to resolve the issue ?  flow rate is  based on 30 min time for depressurization operation which is confirmed by client.

 

Noise limit set during depressurization operation is 85 dB ( A ) at 1 meter distance.

 

hope to get best possible solution for PCV .

VP
 

Attached Files

•  Vent line hydraulics for scrubber system.docx   118.71KB   17 downloads

[Bobby Strain]

I presume that you are reporting actual observed values. Then you will be able to increase the flow by setting the downstream pressure higher. And 2 bar should reduce the time in half. If you set it at 4 bar, the valve should go wide open. Your calculations did not include hydraulics for the 6" pipe. Looks like you didn't do the right calculations.

 

Bobby

Edited by Bobby Strain, 03 December 2017 - 11:49 PM.

[vaibhav]

Dear Members

 

any valuable advice for PCV issue ?

[serra]

when sizing the valve consider that inlet pressure is not constant and flow depends from density and valve dP (limited to critical  flux),

you can estimate how much fluid to discharge (from mass of fluid at operating pressure - mass of fluid at final conditions)

and adopt a procedure as direct integration to simulate the cycle,

selecting a suitable number of steps (say 5 or more) and an accurate correlation for valve flow (ISA) results are accurate,

 

as alternative there are several correlations for estimating  pressure / time , see for example

 

https://www.ijee.ie/...3-2/ijee924.pdf

[vaibhav]

when sizing the valve consider that inlet pressure is not constant and flow depends from density and valve dP (limited to critical  flux),

you can estimate how much fluid to discharge (from mass of fluid at operating pressure - mass of fluid at final conditions)

and adopt a procedure as direct integration to simulate the cycle,

selecting a suitable number of steps (say 5 or more) and an accurate correlation for valve flow (ISA) results are accurate,

 

as alternative there are several correlations for estimating  pressure / time , see for example

 

https://www.ijee.ie/...3-2/ijee924.pdf

 

how to simulate the cycle ? can someone will guide me in simple way ?  how to reduce the depressurization time from 3.5 hours to half an hour using the same installed PCV ? This is the main concern. I had already provided sizing calculations for PCV.

 

Bobby

I  had check the hydraulics for 6" line & found correct. the flow rate for nitrogen gas is based on the hydraulics only & depressurization time of 30 min.

 

hope to hear from members ASAP.

 

[serra]

of course to reduce the time you need to increase port diameter (valve CV) which could be (or not) possible (you may ask manufacturer),

you can start calculating mass to discharge = Volume *(Dinitial-Dfinal) where Dinitial and final are densities at initial and final conditions,

since inlet pressure is not constant you should subdivide the whole process in several steps,

for example start from 5 Bar, you know CV, calculate the time required to discharge 1/5 of total (Volume *(Dinitial-Dfinal)),

recalculate pressure in vessel (with the ideal gas law or a EOS),

go on with another step (now inlet pressure is that you calculated) till the final pressure is that specified,

adjust CV (with the values provided by manufacturer) to reduce the total time below your spec.