Urgent: basic understanding

[TheDruid]

Hi folks,

i have an urgent question which involves basic understanding (which i seem not to have)

Oxygen, 23 bar, 20 °C
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pipe DN20 (inner diameter 22,9 mm), 250 m long
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pressure reducer from 23 bar to 2 bar
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pipe DN50
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PSV setpoint 3 bar

What is the mass/volume flow, for which the PSV has to be designed ? The worst case would be a defective pressure reducer (full open).

According to a calculation of a colleaque, he calculated a flow speed of 135 m/s. With diameter this sums up to 200 ANm3/h. Which density do i use for mass calculation ?

thank you very much for your answers.

Alex.

 

[CMA010]

Depends on your approach.

You can calculate the maximum flow through your reducer or more rigorously calculate the maximum flow trough the reducer and tubing. In this case you can't ignore the length of your tubing so i would use the second approach. Considering the length i doubt you flow will be sonic.

Use Crane Technical Paper 410M (e.g. formula 3-20) or another compressible flow formula you fancy to calculate the maximum flow through your 250m tubing and reducer. Use a pressure drop from 23 bar(g?) to 3 bar(g?) + 10%.

The maximum flow you calculated will have to be relieved by your PSV.

The density to use for your mass flow calculation depends on which velocity your colleague calculated. making the most sense this would be the velocity at the end of your tubing (where P = 3 bar(g?) + 10%).

 

[EGT01]

TheDruid,

It looks like you have taken your colleague's velocity and multiplied by the cross-sectional area of your DN20 pipe to get your flow of 200 ANm3/h.

Your units of ANm3/h are confusing to me. Tyically the "A" would indicate "actual" meaning volumetric flow based actual temperature and pressure of flow. If you are referencing the DN20 pipe, that is 23 bar, 20C.

However, "N" typically indicates "normal" meaning volumetric flow based on some defined temperature and pressure conditions such as 0C, 1 atm or whatever you have defined as "normal".

Having the "A" and "N" together doesn't make sense. I would suggest you discuss this with your colleague to make sure you understand the information they have provided and make sure you have used the proper designation for the units. Then you can decide which density to use.

 

[TheDruid]

Hi folks,

@EGT01: sorry for the mixup, should be Am3/h of course.

@CMA010: i calculated speed for inlet (23 bara) and outlet (3 bara) without friction and speed in pipe with friction. Last one was lowest, so i chose it as limiting factor.

I'll redo the calculation with +10% flow and use the results for the PSV.

thank you very much

TheDruid

PS: The velocity my "colleague" calculated was wrong to start with

 

[CMA010]

To clarify, the pressure drop is 23 - (3 + 10%) = 23 - 3.3.

10% is the amount of accumulation.

 

[MortenA]

TheDruid

You have to ask yourself a basic question: What will you do with any information you get from this tread? You dont understand the basics and you will thus have no way of telling if its right?

You have to aquire the knowledge - either by yourself or buy the knowledge e.g. from a a consultant. This forum is not really a "free consultancy" because we cannot be held responsible for any advice - or your interpetation!

Best regards

Morten

 

[TheDruid]

Good morning MortonA,

thank you for advice, and of course is this forum no free consultancy, it wasn't even intended for consultancy, just for clarification. As you don't know the background, i will explain in a few words: a chemical engineer of different company did the calculations of flow, which i thought of erroneous - and you proved me right. As for forums i find it more useful to ask questions in a "stupid" way, to get the best answers. Please be assured, that i know what i'm talking about, but as english isn't my native language, sometimes misunderstandings may occur.

best wishes

 

[JoeWong88]

TheDruid,

The flow passing "pressure reducer" would be critical. You may consider dP = 23 - critical pressure.