Pressure drop through an orifice

[DMay121]

Howdy!

I am trying to understand the fluid mechanics theory to understand a problem I am trying to solve.

Say I have a system that has 2640 psig inlet pressure. This flow goes through a regulator and then through an orifice which leads to a pressure gauge that has a range of -30-0-30 psig.

If the regulator fails, will the downstream pressure (relative to the regulator) equalize to the upstream pressure i.e. Bernoulli equation?

If this is the case, then I need to calculate the total pressure drop across the orifice assuming an upstream pressure of 2654.7 psia.

How can I determine the pressure on the downstream side of the orifice to make sure the gauge will not blow if the regulator were to fail full-open?

Also, the orifice manufacturer lists the orifice being rated for 8.1 scfm. What does that number mean exactly?

EDIT: I want to acknowledge that in an infinite system, the pressure would drop through an orifice, but what about if the orifice leads into a dead-end such as with the gauge? There will be a pressure drop initially, but wont the upstream and downstream pressures relative to the orifice equalize over time since there is no outlet on the downstream side?

 

[bimr]

If the regulator fails open, then yes, the downstream pressure will be the same as the upstream. That is not an application of the Bernoulli equation, that is just steam passing a broken valve.

What does "8.1 scfm" mean? Standard Cubic Feet per Minute at the design pressure.

http://www.tlv.com/global/TI/calculator/air-flow-rate-through-orifice.html

Depends what type of regulator that you have. One type of regulator will sense downstream pressure and control to that pressure. Look at the valve in the attachment.

 

[DMay121]

But wont the orifice within the regulator result in a drop in pressure across the regulator even when it fails?

And I know what scfm means, I just do not understand what an orifice rated for 8.1 scfm entails. Flow changes with u/s pressure and incoming flow, so how is it rated for a certain flow rate?

 

[bimr]

Depends on the failure mode. You are asking for a prediction on how a valve fails without knowing what type of valve.

Q = Cd A\sqrt(2gh)

where Q = flow (cubic metres per second)
C(d) = coefficient of discharge
A = area of orifice (square metres)
g = acceleration from gravity (9.81 m/s^2)
h = head acting on the centreline (m)

With the orifice equation, the outlet flow Q depends on the area of the orifice, the coefficient, and the head (pressure).

 

[DMay121]

Which equation should I use to determine the resulting pressure downstream of an orifice given an upstream pressure, orifice size, etc.?

 

[bimr]

Look at this website, it should help:

http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm

 

[Latexman]

Since you are asking about the intermediate pressures, generalizations are meaningless without the details on the regulator and orifice and piping!!! Please characterize the regulator and orifice and interconnecting piping. A sketch with details, lengths, diameters, Cv's, fittings, etc. would be wonderful.

For example, what if the regulator had a 1/16" orifice and the orifice was 1" diameter? Then, what if the regulator had a 1" orifice and the orifice was 1/16" diameter? The intermediate results would be quite different while the total flow may be about the same. The devil is in the details.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[hydtools]

The flow rating of scfm implies you are dealing with air or other gas. The flow rating is probably choked flow rate for that orifice.

Ted

 

[LittleInch]

DMay121,

Your post seems to be missing some vital data and possibly understanding,

In short you need to consider the whole system and where and how much flow is going.

Taking your OP in turn

You a rather precise and really quite high inlet pressure. However you don't say anything about flow or pressure the regulator regulates to. To have a pressure gauge which measures vacuum conditions implies that the pressure d/s the orifice is being brought below atmospheric by some sort of device. BTW you can't get lower than -15 psig....

If the regulator fails (presumably full open), there will normally be some sort of pressure drop as even fully open there is restriction across the valve. This depends on flow and size of the valve internals. If there is no flow then yes the pressure will equalise.

Only if your regulator is huge will it have no appreciable pressure drop when fully open assuming that there is flow.

The pressure downstream will depend on what the flow is, what are the conditions downstream, how are they affected by an increase in flow. You can't look at this in isolation without getting incorrect results.

The orifice manufacturer will have assumed or used certain pressures, both upstream and downstream, fluid densities, to provide this flow figure. It is not possible to provide a volume flow figure without that data.

Your edit - Yes, of course it will. How long it takes will depend on flow (that word again), d/s volume and pressures, compressibility factors etc. The time period is infinite to exactly equalise, but within normal margins you will get equalisation if the d/s volume is fixed with no outflow and fluid is continuing to enter.

does this help your understanding?


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

 

[DMay121]

@Latexman

The orifice in question has an opening of 0.025" and is definitely smaller than the regulator.

@LittleInch

I am looking at a bunch of nitrogen panels and there are orifices right before a lot of the pressure gauges (which I do not understand) of different pressure ranges. These pressure gauges are dead end lines and do not require certain flow, so I don't understand the purpose behind those specific orifices. Some of the gauges are do experience a vacuum due to components in the panel, but a large majority of the gauges with orifices have a pressure range of 0-300 psig and are located on the d/s side of a regulator. This regulator has u/s pressure of 2640 psig and a d/s pressure of 0-250 psig, but if it fails open it will be much larger than that. This lead me to determine if the orifice will protect that gauge from blowing if the regulator were to fail.

On the flip side, there are orifices before some flow meters, but that makes sense to me. What I am trying to determine is the pressure drop across the orifice to see what pressures the components d/s of the orifice will experience if a regulator fails. The flow meter has an MAWP of around 100 psig. If the regulator fails, then the u/s pressure of 2640 will in large part be passed down to the orifice, so if the orifice can't offset that pressure, the flow meters will blow.

All of these systems are already installed, just trying to verify which components are at risk to be damaged if the regulators fail.

Thanks again for all of the input!

 

[bimr]

DMay121,

What is the system that you are working on?

 

[LittleInch]

Aaaah, extra information always works.

It would be great to draw this out so we all can see what you see in front of you, but I think I get the issue now.

The key issue is that you have a large pressure source (your N2 bottles) and by the sound of it lower pressure rated equipment d/s of a regulator. normally such equipment is protected against over pressure by a relief valve and often some sort of mechanical or instrumented shutoff system in the event of failure of the regulator. to not have one or both is a failure of design and potentially a significant safety issue, but please check and see if your system has one installed.

I suspect the rather small orifices are simply a version of pressure guage "snubbers", which limit the rise of pressure if the gas header is pressurised too fast. Many instruments don't respond well to being shocked and can either break or start to read incorrectly. for the avoidance of doubt, in a non flowing situation as you describe orifice plate WILL NOT limit pressure downstream and will equalise pressure.

The orifice plates at the meters might be there to limit pressure on the basis of max flow through the regulator meaning you have choked flow, but without details it is not at all clear.

Basically - fit a full flow pressure relief valve set at the lowest MAWP of your system d/s the regulators and then all your problems just go away. regulators are normally quite reliable and won't normally fail fully open.

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

 

[bimr]

Since you appear to be inspecting a new system, you should verify there is a nitrogen hazard warning sign. Nitrogen cannot be detected by the sense of smell. A nitrogen-enriched environment, which depletes oxygen, can be detected only with special instruments. If the concentration of nitrogen is too high (and oxygen too low), the body becomes oxygen deprived and asphyxiation occurs.

http://www.csb.gov/assets/1/19/SB-Nitrogen-6-11-031.pdf

 

[DMay121]

@bimr

This system has been in use for 10+ years and is a gaseous nitrogen.

@LittleInch

Okay this is all making sense now, but I have one question:

This specific situation has 2 panels. The second panel has no relief valve but is entirely fed by the first panel that has a relief valve d/s of the regulator that has a set pressure of 165 psig and is capable of maintaining system pressure at 104 psig if it were fully open (API 520 calcs). The cracking pressure of the relief valve is 157 psig, so in actuality the d/s portion of the system will experience a maximum pressure of 157 psig. If the regulator fails, it allows 576 scfm of flow, calculated using Cv calcs. The relief valve can offload enough scfm to maintain that pressure no problem. However, in the second panel there is a flow meter that has an orifice proceeding it. This flow meter has an MAWP of 100 psig. The entire line containing the flow meter has an isolation valve that will only be opened when the user port at the end of the line is active. So for the line to be at risk, it will be a full flow line and not dead ended at any time.

So, I have to calculate if the orifice will reduce the 157 psig to below 100 psig to protect the flow meter.

 

[bimr]

"a gaseous nitrogen" Are there other available forms of nitrogen for process plants?

Don't know your background or experience level, but to be clear, plenty of people have been inadvertently killed in control rooms by nitrogen leakage and it is important to promote safe work practices.

If this system has been working for 10 years, what is the concern at the present time?

 

[LittleInch]

If the set pressure is 165, that's what it protects at. The 104 figure is meaningless as the spring will shut the valve closed well before that.

So now you know your upstream pressure. To work anything out you need to figure out flow and pressure down the system. You might need to start with a number for either and see if it all works then start to iterate. TBH, you might find excess flow is your issue rather than pressure. What type of meter is it?

What do you know about this system ds the meter (flow, pressure, end user)?

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

 

[DMay121]

@bimr

The concern at the present time is that the designers of the system might have incorrectly designed it so that components are in danger of failure if a regulator fails.

@LittleInch

I know that the 104 is meaningless, but in API 520 terms, its what the relief valve could be set at to relieve all of the flow coming through the regulator if it were to fail. If the calculated pressure was higher than the set pressure, then the RV is not adequately sized.

The flow meter is rated at 60 SCFH, so 3600 scfm, which is way more than what the regulator can let through even when failed. The low MAWP of 100 psig for the flow meter, in my mind, is indicative that the orifice was intended for some sort of pressure protection. Heres a link for the flow meter:

http://www.omega.com/pptst/FL4200_4600.html

its the FL4216 model.