Trace components and autoignition temperature
Trace components and autoignition temperature
(OP)
Hi All,
Can anyone point me in the right direction for dealing with trace components in a gas mixture, and whether these need to be taken into account when determining temperature code for equipment?
E.g. I have a gas where the bulk components would warrant T2 (<300 °C). However, there's <100 ppm of a component which would warrant T6 (<85 °C) if it were a pure component. There is no way for the trace component concentration to increase beyond a few hundred ppm.
Somebody is asserting that because this component is in the gas, we must specify T6 for all equipment in the area. This seems a little over the top to me. If this was a flammability range determination, I'd do a calculation of the mixture flammability range, rather than using pure components, and use the mixture limits for design (with a healthy safety factor). Without any discussion of how to deal with mixtures in the standard, it seems that this person favours following the standard to the point of absurdity, and asserts that even 1 ppb of this substance would warrant a T6 rating, even if the remaining components were completely inert.
Any help appreciated!
Can anyone point me in the right direction for dealing with trace components in a gas mixture, and whether these need to be taken into account when determining temperature code for equipment?
E.g. I have a gas where the bulk components would warrant T2 (<300 °C). However, there's <100 ppm of a component which would warrant T6 (<85 °C) if it were a pure component. There is no way for the trace component concentration to increase beyond a few hundred ppm.
Somebody is asserting that because this component is in the gas, we must specify T6 for all equipment in the area. This seems a little over the top to me. If this was a flammability range determination, I'd do a calculation of the mixture flammability range, rather than using pure components, and use the mixture limits for design (with a healthy safety factor). Without any discussion of how to deal with mixtures in the standard, it seems that this person favours following the standard to the point of absurdity, and asserts that even 1 ppb of this substance would warrant a T6 rating, even if the remaining components were completely inert.
Any help appreciated!





RE: Trace components and autoignition temperature
(0.999995 times MW1 times Auto1 + 0.000005 time MW2 times Auto2)/MWmixture always equals Auto2
The only exception that I've seen to this is with liquids that can stratify and the trace can accumulate. With gases this doesn't happen.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Trace components and autoignition temperature
RE: Trace components and autoignition temperature
You're right though, in that most of the gases in that table are in a similar range of temperatures, and if I had a mixture of a few of them, I'd pick the lowest and call it a day. In my situation though, there is an outlier in terms of ignition temperature, that also only happens to be present at ppm levels. I'm arguing that we should neglect this component, or do a weighted average as per David's example (effectively neglecting it), and thereby not have to specify every piece of equipment in the plant at T6.
The situation reminds me of a similar one trying to explain distillation to some people:
RE: Trace components and autoignition temperature
With a lot less effort, by examination of this equation, and knowing how mixing rules for these physical props are configured, we can see that thermal conductivity, density and specific heat for this NG mix would hardly register a change with 100ppmv CS2 in it. Hence we could safely say that AIT for this gas is >400degC, by looking at table 26-10.