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Flow Orifice For Gas Service

Flow Orifice For Gas Service

Flow Orifice For Gas Service

(OP)
Good day, forum

Sorry for english - I have rare practice.

A flow orifice is the cheapest and simplest (and therefore reliable) means to control gas flowrate. I have some particular questions concerning flow orifice calculation. There is a lot of information about critical flow and flow orifice calculation. So, if you have a worth link, source or file - you are welcomed. If you have some general information - please do not flood. And I know that wikipedia is not a worth engineering source but it is the most convenient when you work on internet.

All sources I have found talk about critical (sonic, choked) flow of ideal gas through ideal orifice. E.g. Perry's Chemical Engineers' Handbook 8th ed. provides with calculation for ideal gas and ideal orifice. But one deals with a real gase, a real piece of metal and a real turners. How a real gas affects gas flowrate through a real orifice? Some sources talk about more problems with a real gas. E.g. Wikipedia talks:

Quote (Wikipedia)

The technical literature can be very confusing because many authors fail to explain whether they are using the universal gas law constant R which applies to any ideal gas or whether they are using the gas law constant Rs which only applies to a specific individual gas.
So my questions are:
Q1a: How compessibility shall be taken into account?
Q1b: Does isentropic index shall be used instead of adiabatic index? How I can easy and fast evaluate isentropic index for real gas mixture?
Q1c: Does temperature and density drop in orifice bore shall be taken into account? When?

Some sources talk about thick and thin plates. E.g. Wikipedia talks:

Quote (Wikipedia)

The flow of real gases through thin-plate orifices never becomes fully choked. The mass flow rate through the orifice continues to increase as the downstream pressure is lowered to a perfect vacuum, though the mass flow rate increases slowly as the downstream pressure is reduced below the critical pressure.
When sources talk about a real orifice some correction factor (discharge coefficient) appears. E.g Wikipedia talks:

Quote (Wikipedia)

The value of Cd can be calculated using the below expression:

So my questions are:
Q2a: When a plate is thin or thick?
Q2b: What is the min thickness of plate to guarantee critical flow?
Q2c: How to calculate the correction factor? How it depends on shape and dimensions of bore, e.g cant.
Q2d: How flow fluctuations affect calculation accuracy? Does upstream and downstream pipe require straight run as those for flow restriction devices? How straight run requirements can be estimated?

One deals with not only gases. A wet gas contains a small portion of liquid upstream of orifice which flashes when pressure drops. Some gases do not contain liquid but produces liquid when pressure drops. Liquid can condense into orifice bore and flashes downstream. Liquid can condense into orifice bore and partially flashes downstream. Liquid can condense downstream of orifice. So my questions are:
Q3a: How liquid flash/condensation affects critical flow?
Q3b: How liquid affects metal? When erosion shall be considered? How metal can be protected from erosion (process means, manufacturing means)?
Q3c: A small drain bore can be provided to prevent liquid accumulation ustream of flow orifice. How this drain bore affects total gas flow through flow orifice? Can e.g. cemicircle bore be provided in case wet gas? How to calculate e.g. cemicircle bore?

Flow orifice can be purchased in professional company or manufactured in situ by a local turner or even by personnel using hand drill. So my questions are:
Q4a: Please share your positive/negative experience with flow orifice procurement? When it can/should/shall be purchased/manufactured and how?

Update: I mentioned flow orifice not for flowrate measurement, but for choked flow conditions to control gas flowrate.

RE: Flow Orifice For Gas Service

evaluation of Choked Two Phase Flow is not easy...
thermodynamic libraries can include several methods to predict speed of sound for single and two phase flow, for example my copy of Prode Properties has HEM (vapor and liquid traveling at same speed), HNE-DS (delayed vaporization), HNE (homogeneous non equilibrium) etc..
However even with accurate thermodynamic models as GERG 2008 you can observe large differences comparing HEM vs. HNE vs ... (there is a Excel page for that)
Anyway, assuming you are able to obtain a decent estimate of speed of sound then there is the problem to evaluate discharge flow coefficient, there are several papers discussing this topic, you may expect 30% (or higher) differences comparing different methods, which could be acceptable depending from application...

RE: Flow Orifice For Gas Service

Quote (shvet)

A flow orifice is the cheapest and simplest (and therefore reliable) means to control gas flowrate.

Well yes, but only if you have fixed parameters. Change gas composition, pressure, temperature, wear on the orifice, dirt, and flow changes.

A lot of this depends on how accurate you need to be.

You seem to be concentrating on critical flow which is then really a subset called restriction orifices. Velocities can be very high and erosion becomes a real issue. Anything which results in two phase flow or starts with two phase flow will complicate life a lot and accuracy of any calculation will be subject to larger variations.

There are vendors and companies which supply these who have more experience of real life flows than anyone else. Find one.

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

RE: Flow Orifice For Gas Service

(OP)

Quote (LittleInch)

There are vendors and companies which supply these who have more experience of real life flows than anyone else. Find one.
We contact with professional companies for flow orifice procurement. Sometimes I have to deal with some kind of clients when.... hmmm what is the reason to purchase when a device can made of scrap? To provide a good engineering practice I have to have tight arguments and an international standard is the best for this purpose.

Anyway I would like to understand this issue better.

RE: Flow Orifice For Gas Service

according our experience, with two phase flow, I must agree with apetri and LittleInch, better to ask manufacturers...

RE: Flow Orifice For Gas Service

Q1a: How compessibility shall be taken into account?

Ans It is taken into account by the compressibility factor Z, as discussed in Perry's and in Flow Handbooks.

Q1b: Does isentropic index shall be used instead of adiabatic index? How I can easy and fast evaluate isentropic index for real gas mixture?

Ans. As the temperature changes when subject to pressure drops it is necessary to use the adiabatic rule.

Q1c: Does temperature and density drop in orifice bore shall be taken into account? When?

That is taken into account in the meter factor and in the pressure correction.

Thin plate orifices refers to a class of orifice plates used in flow measurement, thick plate orifices are commonly encounted in flow restriction such a feed water recycle restrictions for small BFW pumps.

There are several flow measurment texts, and several flow meter standards that are relevant, get your credit card handy.



RE: Flow Orifice For Gas Service

From your last sentence I guess you are talking about restriction orífices, no orífices for flow rate measurements.
About the restriction orífices, the choked flow conditions and thin/thick plates, I answer some of your questions.
The difference between thin and thick plates is related to its value of t/d (t is the plate thickness and d the hole diameter). Some authors state that the thick orifice plates are those with t/d equal or greater than 1, but this is for values of d very small (2 mm or less). In the industrial plants as oil refineries and nuclear or thermal power plants, my personal experience is that the thin plates corresponds to those with t/d equal or less than 2.
The thin orifice plates don´t have choked flow if they have the hole with a sharp-edge entrance, but if the hole have a rounded entrance , it is as a nozzle and has choked flow.

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