8 replies to this topic

[PTSHeng]

Hi all, 

I am using the InstruCalc orifice sizing program right now and I have a question I wonder if anyone can help.

Right now we are sizing an orifice plate for a customer and this line is used by intermittent users for multiple users. We are hitting a snag in the sizing part right now.

The first question is :

What is the difference between the differential range and overall pressure loss? Does the differential range refer to the transmitter range and overall pressure loss, the actual pressure loss after the fluid has gone through the orifice? The calculation file is uploaded in this thread.

If the above is true, how would one be able to adjust to calculate the actual pressure loss without changing the meter differential range? Even with other calculation software? 

Thank you all for your help!

Attached Files

•  ISOPROG.pdf   67.55KB   221 downloads

Edited by NicholasY, 06 March 2014 - 04:15 AM.

[chemks2012]

NicholasY

 

Looking to the attached file and calculation I ran through it appears that

 

1. Your instrument has differential range of maximum of 5000mm H2O through that orifice. Meaning if you have pressure difference more than this, instrument will not read it. And your understanding is correct that the differential range refer to the transmitter range. Please see below as well.
2.  Normal flow differential (1701 mm H2O): It appears that if you keep pressure difference of 1701mm H2O across the orifice [inlet to orifice 16.9barg and outlet to orifice 16.734barg], you would get normal flowrate of 1.75T/hr
3. Also, when you allow maximum pressure difference of about 5000mmH2O across orifice [inlet to orifice 16.9barg and outlet to orifice 16.403barg], you will get maximum flowrate of 3T/hr
4. It is not very clear what is Max Pressure Loss 3841mmH2O – Other member would help you?
5. Sorry, I didn’t understand your question, please elaborate: If the above is true, how would one be able to adjust to calculate the actual pressure loss without changing the meter differential range? Even with other calculation software? 

 

Hope this helps

KS

Edited by chemks2012, 07 March 2014 - 05:01 AM.

[PTSHeng]

Hi ChemKS,
 

Thank you very much for your help.

I realised that the 2nd question was already answered in your response.

Once again, thanks for clarifying these doubts for me.

As for max pressure loss, I will check around and let you know what it means if I do find out what it is!

[katmar]

I agree with what chemks2012 has written in points 1 to 3.  Just a note - the pressure drop through an orifice is proportional to the square of the flow rate so you will see that (3 / 1.75)² * 1701 = 4999 = approx 5000

 

Regarding Point 4 - the pressure differentials mentioned above all refer to the differences in pressure between the two tappings on the orifice assembly.  The downstream tapping is put close to the vena contracta to ensure the maximum differential is measured and the best accuracy is obtained.  But in flowing from the vena contracta downstream the fluid undergoes some pressure recovery (Bernoulli effect of velocity being converted back to pressure).  The overall (or permanent) pressure drop is usually measured between two tappings with the first at 2-5 pipe diameters upstream of the orifice and the second tapping about 10 diameters downstream.  This permanent pressure loss will be less than the differential across the tappings at the orifice because of the pressure recovery. The given value of 3841 mmH2O corresponds with the permanent pressure drop for a flow rate of 3 tonne/h.

[chemks2012]

Hi katmar,

 

Thanks for your input.

 

When you say : The overall (or permanent) pressure drop is usually measured between two tappings with the first at 2-5 pipe diameters upstream of the orifice and the second tapping about 10 diameters downstream. 

 

I would like to know the significance of measuring pressure drop at these points please.

 

Thanks in advance.

 

KS

[katmar]

The significance of the permanent pressure drop is for pipe sizing or pump sizing exercises.  Here you are not interested in the differential across the tappings at the orifice itself because it is the overall or permanent pressure drop that the pump or pipeline has to deal with.  The overall pressure drop is seldom measured with tappings, but we need to know what it is so that we can size the pipe and pump.  In references like the Crane 410 manual and in line sizing software the pressure drop calculated for an orifice will be the permanent pressure drop.  On the other hand, the instrument engineers will use different formulas to calculate the differential at the orifice itself for their flow instruments.

[chemks2012]

Hi katmar,

Thanks for that.

 

So, I would think the maximum pressure loss of 3841mm H2O stated is during normal flowrate of 1.75T/hr when you have pressure drop of 1701mmH2O across orifice plate?

 

Your input would be highly appreciated.

Regards,

KS

[katmar]

No, by my calculation the 3841 mmH2O corresponds with the 3 tonne/h (See post 4 above).  My calculation at 3 t/h is actually 3898 mmH2O but the coefficients for permanent pressure drop have not been as thoroughly investigated as those for close tappings so the difference between AioFlo and Instrucalc is not surprising. For pipe sizing this accuracy is acceptable, but not for flow indication.  At 1.75 t/h I get a permanent pressure drop of 1326 mmH2O.

[chemks2012]

Thanks katmar.