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# Flow calculation through orifices in series

## Flow calculation through orifices in series

(OP)
I have two orifices ( Capillary type ) in series in H2 service at 30 kg/cm2_g and 40 Deg C. Line size is 0.75 inch. As per their datasheet
1st Orifice: Do = 0.51 mm & L = 50 mm ; Pressure drop at Max flow : 6.89 kg/cm2_g
2nd orifice: Do = 0.51 mm & L = 50.83 mm ; ; Pressure drop at Max flow : 3.44 kg/cm2_g

Data sheet mentions that both are restriction (choke) type venturi.
However, when I calculated critical pressure under the given condition to check the whether the flow is choked or not, Critical pressure comes out to be 15.2 kg/cm2_g, which is less that the combined pressure drop the two orifices can provide.

In plant, pressure drop measurement is only available across the last orifice.I need to calculate flow rate under operating conditions.
Need expert guidance.

### RE: Flow calculation through orifices in series

Did you consider the separation, pressure recovery, and that Pcrit1 ≠ Pcrit2?

Good luck,
Latexman

Engineers helping Engineers

### RE: Flow calculation through orifices in series

(OP)
My understanding is that Pcrit is property of service gas at given temperature and pressure. Pcrit is not a function of orifice dimensions.

Also, since it is restriction orifice, I am assuming no pressure recovery.

### RE: Flow calculation through orifices in series

It does look a little strange. Are you sure of all the pressures used?

Also what is the definition of restriction(choke)? It doesn't say choked flow.

A drawing of the orifice and the actual data sheet would help.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Flow calculation through orifices in series

(OP)
I am also not sure what 'restriction(choke)' mean. I am only assuming it means that choked flow.
Assuming it not choked flow, D/L ration is 0.009. Hence usual orifice formula wouldn't apply.

I do not have drawing of the orifice. Data sheets mentions ' It is a restriction(choke) type venturi and is to be omni-directional'.
Max design flow is 0.226 Kg/hr.
I can not share actual data sheet as this being a propriety information. I can help you with more detail if needed.

### RE: Flow calculation through orifices in series

#### Quote (R4j4n)

My understanding is that Pcrit is property of service gas at given temperature and pressure.

Yes, but it applies dynamically and will be affected by the separation between the two orifices and depending on how that separation affects pressure recovery. Basically, Pcrit2 depends on Pcrit1 and the loss in orifice 1. T2 ≠ T1 and P2 ≠ P1, so Pcrit2 ≠ Pcrit1.

On the multiple orifices I have seen and designed, enough separation is given (5 pipe diameters) so pressures recover between orifices.

Good luck,
Latexman

Engineers helping Engineers

### RE: Flow calculation through orifices in series

What is it though that you're actually trying to do?

You have an inlet pressure of 30kg/cm2;. At max flow you have a certain pressure drop in two stages.

What is changing? you mention "operating conditions". Are these different to the figures you've supplied?
Orifices are great simple devices, but they are not good when process conditions change.
You have max flow I assume from the data sheet.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Flow calculation through orifices in series

Long or short cone venturi devices are not called orifices. Venturi flow meters are employed in services where high pressure recovery is required. If I remember correctly, permanent dp is less than 20% of pressure drop at vena contracta for venturi devices. If both devices are operating in choked flow, would agree there is something odd about the 6.9kg/cm2 dp for the first venturi; the second one with dp of 3.4kg/cm2 dp seems more reasonable at first glance.

A 7kg/cm2 dp on the first device seems more like the performance of a regular thick plate restriction orifice, provided you have the appropriate t/d on the plate thickness.

Use of terminology here is confusing: capillary(?) orifice ; orifice ; venturi

### RE: Flow calculation through orifices in series

(OP)

#### Quote (LittleInch (Petroleum))

What is it though that you're actually trying to do?

At current conditions:
DP across the second orifice is 2.8 Kg/cm2_g. I am trying to calculate flow in this case.

### RE: Flow calculation through orifices in series

Critical flow venturis are used where precise restrictions are required.

https://www.ceesi.com/services/critical-flow-ventu...

As long as the dimensions are unchanged, and the device has been recently inspected, the flow rate is only determined by the upstream pressure.

Most plants do not have certified measurment lab facilities...

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