cm^3/s to ppmV 1

[ajdowdell]

Ok, so I work at a facility building valves. I'm being asked to meet differing fugitie emission standards. there are 3 that I know of, and I'm trying to get a handle units of measurement. I think this has been sorta hashed out before on here, but the thread was closed.

The 3 standards I'm looking at are:

1. ISO 15848-1 (Type testing) very in depth requires heat cycling, 6 bar, casing around the packing, data acquisition, 500 strokes or even 1000 for CO2 service, requires measurement of ALL helium that leaks past the entire stem diameter (ppmV) or a sniffer test that reads in ppmV. there also some other options

2. ISO 15848-2 (production testing) also requires that the device read in ppmV

3. Shell Spec MESC SPE 77/312 requires high pressures, qualified technicians and units to be atm*cm^3/s

 

[ajdowdell]

OK, now I have a varian 959 manual mass-Spectrometer. it measures in atm*cc/s.

I know the discussion that cam about before you could not convert between atm*cc/sec and ppm, but where doesn ppmV fall into this? It means part per million of volume.

Basically the end goal is this: I want to have one device that can perform all ISO 15848-2 AND the Shell spec. I'm having a tough time finding one that reads in ppmV, is there something small I'm mising here?

In the ISO spec is says in multiple spots that 1ppmV= 1 mL/m^3 = 1 cm^3/m^3. So 1 ppmV = volume/volume

if the reading out of the varian is atm*cm^3/s is there a conversion that will get me to ppmV?

 

[btrueblood]

Does the ISO test specify a casing volume over the stem, and a time limit? You could then convert by multiplying

leak rate X time = volume of leakage

and

volume of leakage / volume of casing = ppmV

 

[ajdowdell]

ISO 15848-2 the production test does not specify a casing at all.

The -1 ISO spec has several different methods.

1. Vacuum method-- sealed chamber is evacuated, then the valve is allowed to leak, the device pulls the helium through it. atm*cc/s or Pa*m^3/s

2. flushing method-- the sealed chamber is "flushed" with a known rate of a known gas, the valve is allowed to leak, and the difference of the 2 nets the ppmV of the Helium

3 sniffer method--NO chamber, simply sniff around the packing and read. And this is where it's very clear to decipher between ppm "global" and ppmV "local". ppm "global" is the total leakage and ppmV is the ppm of the volume of helium going through the detector.

ISO 15848-2 is the sniffer method, with the same ppmV requirement

Shell MESC SPE 77/312 is also a sniffer test but requires the atm*cc/s unit.

 

[ajdowdell]

I would prefer to do the sniffer method for everything as it's difficult to encase the valves we build because they cover just about every size/pressure class/actuator known to man.

 

[btrueblood]

Ugh. I understand the first two, but not the third. I mean, I understand the leak test, but not their assumption of units. The global value (ppmGlobal) I would assume to be the background He concentration, and the ppmV is the measured He concentration pulled thru the sniffer...but one has to make some assumptions about mixing and diffusion to come up with a leak rate from that. Or I'm missing something.

 

[ajdowdell]

Here's their wording in the description of the sniffing method:

A portable instrument is used to detect leaks from valves. The instrument detector type is not specified. But
the selected detector and its sensitivity shall be able to meet the tightness class limits. This procedure is
intended to locate and classify leaks only, and is not used as a direct measure of mass emission rates from
individual sources.
The detector probe (sniffing) method, see Figure B.1 and Figure B.2, allows the measurement of the local
emission of the stem sealing system (production test) and body seals.
The measured concentration is expressed in parts per million volume (1 ppmv = 1 ml / m3 = 1 cm3 / m3).
Some helium mass spectrometers are capable of measuring a local volumetric leak rate. It is expressed in
millibars per litre per second or equivalent atmospheres per cubic centimetre per second.
To avoid any correlation between local and global measurement, measurement with the sniffing method is
expressed in parts per million volume (1 ppmv = 1 ml / m3 = 1 cm3 / m3).

 

[ajdowdell]

So it says that it is to "classify" leaks only not quantify...

But how do I use a device that measures in atm*cc/s to classify?

 

[ajdowdell]

I'm thinking you would have to quantify the flow rate that is being pulled through the sniffer. Once I knew that rate, I could use the cc/min rate given and basically divide the two and get a percentage (probably in the ppm range) and the per seconds would cancel out leaving volume/volume correct? Well I guess atm*volume/volume...

Is that correct?

 

[btrueblood]

Well, not sure what classify means, other than to locate and gauge the severity of.

Yes, you have to know the flow rate thru the sniffer probe to know, or be able to convert to, a leak rate of helium into the local atmoshphere near the probe tip. Our sniffer here has a calibrated orifice in the line, so you know the rate of flow into the vacuum leak detector, and can thus back out, from the concentration in the vacuum/ion chamber, what the leak rate must be (near the probe tip). The leak detector manufacturer should be able to tell you what the probe orifice flow rate is, or perhaps you could calibrate it using an air flow meter or air-under-water beaker flow test at 1 atm differential.

On our detector, the conversion is done automatically in the software, and you "must" use their calibrated probe tip. Gee, wonder if they want to make a few extra bucks on that?

Sorry not to get to this point earlier, I had to go read the manual on the leak detector (it belongs to our sister company) to verify the above. Looks like you got there too. Good luck.

 

[ajdowdell]

I think classify basically means pass/fail.

I wonder if ours has a calibrated oriface, I think that would require a pretty constanct vaccuum to be applied, so the operator would have to be careful to maintain the required distance. Last time I checked our probe had a short section of polypro tubing on the end of it, I assume to give a little more reach, that will probably have to go...

the Varian 959 is a "manual" helium tester, so there is no software for it, and that calculation would have to be manual. Basically I would do the calc ahead of time, and (pulling numbers from thin air) would say if the helium cc/s goes over 10^6 that is over 50 ppmV that it is allowed, so it fails.

Whats an air under water beaker flow test? IS that where you invert a water column and catch the air bubbles and read the volume on the graduations?

 

[NGiLuzzu]

Maximum emission limits expressed in terms of gas flow rates (atm*cc/s, mbar*l/s or equivalent units) should be applicable to GLOBAL detection techniques only, i.e. when all the possible emissions from a sealing system are captured and sent to the detector, like in the VACUUM method described in the Annex (see VDI 2440, 2000 edition, pages 95 & 96, and/or ISO 15848-1, Annex A.1) or in the FLUSHING method with a tight enclosure around the sealing under test (see ISO 15848-1, Annex A.2).

Those methods are not always easy and quick to set up, especially for valve packings, so they are usually adopted for TYPE testing qualifications only (e.g.: according to TA-Luft and/or ISO 15848-1), while a more practical technique is needed for PRODUCTION testing.

So SNIFFER method is used (see ISO 15848-1, Annex B) even if it is a LOCAL and semi-quantitative technique only (see ASME B&PV Code, Section V, Article X): for this reason, it seems more appropriate to express the results of a sniffer detection in terms of He concentration (ppm), as this depends on the measureament point, its distance from the leak sources, etc...

If the mass spectrometer leak detector gives a reading in terms of flow rate (atm*cc/s, mbar*l/s or equivalent units) also in SNIFFER mode, of course that should be considered as just the product of the He concentration in the measurement point times the total sucked-in air flow rate (hence the reading is proportional to concentration, as long as the sucked air flow is constant), but must NOT be mixed up with the result of a GLOBAL detection technique.

Unluckily, VARIAN does not publish useful data about intaken air flow rate (probably because the exact value may depend on some variables and it is hardly repeatible...?).
However, it is possible for example to establish an empirical correlation between instrument readings and helium concentrations (ppm) by recording the results obtained with the sniffer probe in front of two or more sources of helium at known and certified concentrations (e.g.: 10 ppm, 50 ppm, 100 ppm, etc...): this "system calibration" should be repeated at the beginning of each test session.



Hope this helps, 'NGL

 

[ajdowdell]

The problem with all of that though is that ISO call for ppmV (volume) NOT concentration. They are VERY specific that it is volume, not actual ppm.

I am primarily interested in doing ISO 15848-2 and the SHell spec, the -1 ISO is beyond the scope of prodution testing.

the -2 ISO only references the -1 for methods of testing, and the units required for the -2 are different than those required by the -1.

My understanding of the -2 ISO is that the only way to do a quick and easy test like this is to consider local emmissions only. So if you move the sniffer around the packing, if at any point the concentration exceeds a certain level their must be a leak in that area that would exceed a global measurement standard. Someone has done the empirical comparison between the local and global and has decided what levels are acceptable.

And yes the ONLY unit of measurement is atm*cc/s, in any setting. i have searched the market and that is the ONLY unit of measure I can find on a helium sniffer.

 

[NGiLuzzu]

ajdowdell,
please refer to ISO 15848-1 (2006), § 3.3 and to the relevant NOTE: ppm_V (parts per million volume, or equivalent ml/m³ or cm³/m³) are a unit of measure of CONCENTRATION indeed!!


If you perform a deeper search, you'll find leak detectors that also give readings in ppm (especially portable instruments); besides, it's only matter of measuring one more flow rate and calculating a ratio...!

See, for example:

-

http://lacotech.com/product_pages/GasCheck_Handheld_Sniffer_Leak_Detectors_2385714426.html

-

http://www.adixen.com/helium_leak_d..._leak_detectors_asm_102s_asm_142s-43-2-p.html

Best Regards, 'NGL

 

[ajdowdell]

I would prefer to use our current varian 959 if at all possible... yes it is concentration but by volume. Not a mass or molar concentration which is the normal ppm designation and why they are so specific as to require units in ppmv not ppm. most testers read in cc/s, and that is a volume per second of helium. So why doesn't taking measured helium volumetric flow rate divided by the total volumetric flow rate, not give ppmv? All units cancel appropriately. Never once do i see ppm (without the v) in the -2 ISO or the Shell spec.



How do you get from ppm to ppmv? Something like this?

ppmv = (mg/m3)(273.15 + °C) / (12.187)(MW)

where: ppmv = ppm by volume

mg/m3 = milligrams of gaseous pollutant per cubic meter of ambient air

MW = molecular weight of the gaseous pollutant
°C =ambient air temperature in degrees Centigrade

 

[btrueblood]

"Whats an air under water beaker flow test? IS that where you invert a water column and catch the air bubbles and read the volume on the graduations? "

Yes. Measure with a 1-atm differential if possible. Alternatively, and more accurately, let the sniffer tube pull (suck) air from inside a full (of air), inverted beaker, and measure the volume change.

"So why doesn't taking measured helium volumetric flow rate divided by the total volumetric flow rate, not give ppmv? "

It does.

But, if you had a mass flow rate (which some detectors may also report), it wouldn't.

Regarding calibration, does the Varian unit not have a built-in calibrated leak (Helium source)? If not, you may need to have the unit calibrated with a portable He "leak".

 

[ajdowdell]

The Varian 959 does not have a built in Helium leak, but we have it calibrated very frequently, and it's expensive!

Since this is true: -->So why doesn't taking measured helium volumetric flow rate divided by the total volumetric flow rate, not give ppmv?

Am I confused about anegri's first post or was he confused on what I was asking?

Because it doesn't really matter how far you are away for the packing for the 15848-2 test, if you measure a ppmv (converted from atm*cc/s as mentioned above) concentration higher than the posted limit, anywhere, it fails, right? That's the whole point of using qualifying pass/fail instead of quantifying right? That's why it appears that the shut off numbers appear to be so so so tight. Because you are measuring local, ppmv (or atm*cc/s for shell) helium concentration, NOT global, quantitative, I know all of the helium that is leaking out of this packing.

 

[btrueblood]

Um, I struggled with his first post too (I think perhaps English is not his native language), but don't think he said that the ratio is incorrect. Just that the specific usage of units (ppmV for sniff tests, and atm-cc/sec for sealed or "global" leak tests) is maintained in the ISO testing spec's. to try to ensure nobody is confused about what test was conducted (global vs. sniffer). In his second post he mentions that calculating ppm is simply computing a ratio of flow rates.

 

[ajdowdell]

"In his second post he mentions that calculating ppm is simply computing a ratio of flow rates."

He must have left off the "V" then because-->

"If you perform a deeper search, you'll find leak detectors that also give readings in ppm (especially portable instruments); besides, it's only matter of measuring one more flow rate and calculating a ratio...!"

I wasn't sure what to make of that statement because I didn't think ppm (no V) was as easy to measure (well or the conversion was much more difficult, and complicated with more equipment needed for our 959).

It's tough to make sure that the correct notation is ALWAYS used...

 

[NGiLuzzu]

ajdowdell,
yes, you're right: confusion arose just because many times I just implied the "V" for volume after ppm, even if I NEVER meant molar or mass concentration: sorry for that!

Probably you are absolutely right about notation, but I'm afraid I'm not the only one to leave that "V" off... including the He leak detectors' Manufacturers (I mean: may be the "ppm" readings are in volume already... I should check).

In any case, this is not so important, because - as you correctly guessed - once the solute and the solvent (and the relevant molar masses) are known, it is possible to convert between two different ways of measuring concentration: see, for example

http://www.lenntech.com/calculators/ppm/converter-parts-per-million.htm

.


In the case of Helium in air, for example:

1 ppm_V = 1 ml/m³ = 0.176 mg/m³ = 0.136 mg/kg = 0.136 ppm_M

1 ppm_M = 1 mg/kg = 1.29 mg/m³ = 7.32 ml/m³ = 7.32 ppm_V

(please double-check against other sources, if you like...)



Back to previous posts, I said that "... a reading in terms of flow rate (...) in SNIFFER mode(...) should be considered as just the product of the He concentration in the measurement point times the total sucked-in air flow rate...", in formulas:

q_He = C_He * q_Tot

where q represents flow rates and C represents concentrations, both volumetric (but they could be both in mass, as well).


You asked "... doesn't taking measured helium volumetric flow rate divided by the total volumetric flow rate, not give ppmv?" My answer, as well as btrueblood's one, is affirmative. In formulas:

q_He / q_Tot = C_He

which is the same as above!


After all, the empirical correlation (system calibration against known He concentrations) I proposed and the air under water beaker test proposed by btrueblood are different ways to experimentally determine q_Tot.


About distance from packing, please note that ISO 15848-1, para. B.2.7.2-c) states that the sniffer probe shall be "as close as possible to the potential leak source"; moreover, if you search in ASME B&PV Code, Section V, Article X, you will find a maximum scanning distance (and a maximum scanning rate, too).
Of course, if you detect a high helium concentration at a certain distance from the packing already, this concentration will increase as you approach it (otherwise... helium is coming from another source!).


I hope I clarified my positions enough... but don't hesitate to ask me other questions, if you need!



Thanks and Regards, 'NGL