SLPM vs. LPM

[EDSantos]

So there seems to be some confusion at my plant over what the definition of a SLPM is. Some say it is a conversion to account for the fact that we are not measuring at STP, whereas others see it as converting the compressed liters into standard liters i.e. 5LPM @ 100psi = 38LPM @ 14.6psi. It would be interesting to see what you guys think.

 

[LittleInch]

Please define your terms and also is this liquid or Gas?

Generally anything with "S" in front of it is a conversion back to "Standard" conditions - usually 60F/15C at atmospheric. The marginal difference between these two standards is relevant only for fiscal metering

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EDSantos]

Thanks for your input. This is in a gas application. Lets say STP is 25C and 1atm

 

[LittleInch]

This question comes up all the time in compressed air systems where there are many different way of quoting the same air flow. In your case there will be a big difference between the two figures, but it is all in the definition. Standard anything relates to conversion to standard conditions. I confess I don't really understand your first point about not measuring at STP.

I'll confess I didn't know there were so many definitions of STP - I've always used the metric and oil and gas versions, but the wiki page attached shows that there are many versions. It is important everyone uses the same one(!)

http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EDSantos]

I agree. What I meant by not measuring at STP is that you would get two different results if you measured a flow through an orifice at sea level, and then again at elevation.

 

[zdas04]

LittleInch,
You can clearly see from your link that "everyone uses the same one" simply is not going to happen. What is really important (and achievable) is for everyone in a particular transaction to clearly state their "standard" temperature and pressure. I review a lot of gas sales contracts and that is the first thing I look for, if the contract is silent on the definition of "standard" then I throw a huge monkey wrench in the works until everyone agrees on the values (I couldn't care less what value they pick, just that they have both signed off on some value). Not specifying it is good for my full employment (as an expert witness), but I don't love that kind of work.

EDSantos,
I certainly hope that you are not using "1 atm" as your standard. The generally agreed value for that is "local atmospheric pressure" which is very much elevation specific and a bottle filled to 200 "std" Liters in Denver would be significantly underfilled in New Orleans. A fairly large minority wants it to be equal to 1 bar (14.5 psia or 100 kPaa), but everybody thinks they understand it and never bother to convert it to a value you can read on a gauge.

Also, it is very rare to talk about industrial gases in LPM (that I probably why I've never seen SLPM used in a contract). We tend to talk about gases either in mass units or in multiples of a standard cubic meter.

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

 

[LittleInch]

zdas04,

what I meant was what you said - I don't expect everyone in the world to use the same system , though it be nice if they did, only that - as you said more eloquently than me - everyone in a particular location / plant / company, uses the same definition. I agree that "ATM" can be misinterpreted unless specified as a particular pressure.

If you measure flow through an orifice, unless that orifice is open to atmosphere at one side, you should get the same differential pressure. The guage reading might need to be adjusted, but differential should be the same.

I've not come across LPM for gas unless it is a very small flow - normally it is volume in m^3 or cc or ft^3. Gas sales are often energy, but use standard volume as a key factor in that calculation.

Also puzzled why this is in the valve forum but hey - you got an answer....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way