Converting PPM to a leak rate.

[StephenDiM]

Hi all,

I am curious, how does one convert between a contamination level of process flow measured in PPM to its corresponding leak rate.

As an example given a process flow with a known mass flow rate, gas species, pressure, temperature and allowable contamination level (say 0.2 PPM) how do we determine the maximum allowable leak rate.

I would appreciate any feedback.

 

[StephenDiM]

I think I have come across a method in High-Vacuum Technology - A Practical Handbook (Marsbed H. Hablanian):

It states that 'if the temperature and species of gas are known, the pressure term in this case can be directly converted into density and hence mass flow.' So then, my example:

For air leaking into a helium line resulting in a maximum 0.2 PPM (by mass) for 10 g/s flow rate of helium. This would result in 2x10^-7 g/s of containment flow.

m' = 2x10^-7 g/s = V'ρ.

At STP air has a ρ ~ 1.204x10^3 g/m^3

So V' = 1.667X10^-10 m^3/s

Therefore Leak Rate = V'P = (1.667X10^-10 m^3/s)(101325Pa)
Leak Rate = 1.68875x10-5 Pa m^3/s

Does anyone see any glaring flaws?

 

[StephenDiM]

EDIT* Final answer is off by a factor of 10 so its 1.68875z10^-4

 

[bimr]

You can use the method in Appendix B of The EPA Protocol for Equipment Leak Emission Estimates.

http://www.epa.gov/ttnchie1/efdocs/equiplks.pdf

 

[StephenDiM]

I believe you are missing the point entirely. This section of the EPA protocol requires experimental data, where as I want to derive an estimate of the maximum acceptable leak rate into the process line to maintain a specific purity.

The equipment cannot be tested as we are not in possession of it yet, instead this leak rate will be set as a guideline to be placed in a contract for it.

 

[TD2K]

You're talking about a leak (say from a heat exchanger) into a process stream that you describe as "known mass flow rate, gas species, pressure, temperature and allowable contamination level"? Assuming that's true:

If the 0.2 ppm is on a mass basis, take your process flow in mass terms and multiply by 2e-7, that will give you your allowable leak rate.

If the 0.2 ppm is on a mole or volume basis, take your process flow in mass terms and convert to moles per time unit. Multiply by 2e-7, that will give you your allowable leak rate in moles per time unit and you convert back to mass or whatever units you want to put in the contract.

 

[SeanB]

x / y * (1,000,000) = 0.2 ppmw

If y is your known flow rate you can calculate x - the leakage rate.

 

[StephenDiM]

Again leak rates have units of Pressure*Volume/s not Mass/s. When detecting the leak, the leak rate is measured in that fashion giving units of Pa L /s, as an example.

So its not as simple as finding what fraction of the flow the contamination is.

 

[chicopee]

Do a search in the Local Exhaust Ventilation forum.I am confident you'll find your answer there.

 

[SeanB]

Why isn't it? If you have mass units and you know pressure and temperature can't you do the conversion?

 

[StephenDiM]

Well SeanB, that is what I am asking. I am unsure only because I have seen only one vague reference to the procedure.

And thank you chicopee I will try looking into it.

 

[SeanB]

If you have your mass flow rate you can convert that into a volume flow rate using the Ideal Gas Law : PV = nRT.

 

[StephenDiM]

Right, but a volume flow rate such as m^3/s != Pa*m^3/s, which is a leak rate.

 

[SeanB]

So if you took the volume flow rate and mulitplied it by pressure, what units would you have?

 

[bimr]

I see. Most people are usually concerned by leakage outward. You should have said inleakage.

ppm is parts per million, percent is parts per hundred. Just divide by ten thousand. Ten ppm is 0.001%

Note that ppm can be figured based on weight ratio, volume ratio, or molar ratio. The percent figure above will be on the same basis.

 

[ports394]

In my area, we use PPM to be mg/L.

For water treatment.