Dear Friends;

I want equation/Formula to calculate delta P when there is a pressure/temp. change of process fluid for orifice plate flow measurement.

e.g., if for steam design press. 9 Kg/Cm2(g) Flow =5000 Kg/Hr Delta P:- 1000 mmWC.

Now if the Steam press. changes to 5 Kg/cm2(g) what will be the new delta P? (flow range remain the same.) Obviously there will be temp. change corresponding to press. (for steam.)

 

Here is what I think.

If P = 9 kg/cm2 is designed pressure, then flow is calculated according to the following formula:
Q = k *sqrt(Deltap).

In your particular case: DeltaP = 1000 mmH2O = 9806.65 Pa and Q = 5 t/h. This will lead to k = 0.0505.

Now, if absoulte pressure is lower, you need to make correction according to the following (approximate) formula:
Q = k*sqrt(Deltap) * sqrt(Pactual/Pdes) = 3.73 t/h

I think that would be it, but it would be good if someone else verify this.

Cheers

 

Dear Chrees;

I have cross checked your calculation using another formula; it is correct. but what about delta P? As per my supplier--for these changed conditions the delta P is 1623 mmWC. but i'm not getting how he calculated it.

 

I don't know either about deltaP.

However, his question was (if I understood correctly) to determine new flow rate when pressure is changed and transmitter is measuring same DeltaP of 1000 mm H2O.

Regards,
Mik

 

The question is if design pressure changes; what will be the new delta P for the same design flow 5000 kg/hr.

 

how you have decided delta P as 1000 mmWC? is there any standard or general practice?

 

> how you have decided delta P as 1000 mmWC? is there any standard or general practice? <

whatever data i have given is for existing orifice which is designed by supplier.

so 1000 mmwc is the designed deltap for 5000 Kg/Hr flow & Given designed flow 5 Kg/Cm2 (g)

now due to some circumstances the pressure is changing to 10 Kg/Cm2 (g). I asked my supplier to design a new Dp for changed conditions. He calculated it 1024 mmwc for same 5000 Kg/hr flow rate. I need those calculations if any body have. i.e. if pressure or temp of the process liquid changes what will be the new deltaP if the flow remains constant?

 

The differential pressure developed by an orifice plate is affected by the fluid density as well as the flow. If your fluid is a gas, then temperature and pressure will both alter the performance. Temperature will also change the dimensions of the orifice through expansion.

The best thing for you to do is to obtain a copy of an orifice sizing program, that lets you see what happens under different circumstances. One I have used for this is FESizer which has a demo available for download that can be used.

Otherwise you can obtain a copy of the relevant standards used t size orifices in your plant and work trough the calcs - but I wouldn't wish that on you as it can be a nightmare process.

Bruce.