STD & NTP conditions

[Pons19]

Hello,

I have a query on STP and NTP conditions.In process calculation, we use Standard operating conditions (STP) and sometimes normal operating conditions, why?. In real life the actual condition exists in the plant, still why we use STP and NTP?

I Really would appreciate your reply.

Thanks

 

[Homayun]

Sometimes we would want to compare physical properties of some material against some other material at a predetermined condition. for example some times you want to know the relative density of a liquid compared to water at a given condition (specidic gravity).

Some times the aparatus that measures physical properties, like density, or viscosity, or analyzers operate at a much more milder conditions than the actual plant conditions. That's when we use "normal" or " standard" conditions" so that every one knows at what conditions those parameters and properties were measured.

I hope this helps..

 

[zdas04]

"STP" does not mean "Standard Operating Conditions" it means "Standard temperature and pressure". Converting a volume flow rate to a predetermined Temperature and Pressure allows comparison of the volume flow rate from one process (at operating pressure and temperature) to another process (at a different operating pressure and/or temperature). You could do the same thing with mass flow rate and save some of the confusion, but industries decided to use STP (or some contractual variation of STP) and we're stuck with it.

The biggest limitation of either volume flow rate at standard conditions or mass flow rate is that these parameters don't allow direct calculation of velocity. With volume flow rate at "actual" conditions, the bulk velocity is directly calculated from the pipe area. That is often a useful thing to know. If you have a volume flow rate at STP and divide by the pipe area you'll get a number that means absolutely nothing. I see people do that all the time and they're always wrong.

I don't know what your NTP is. Often metric flows are stated in nm3/hr meaning "cubic meters per hour at normal standard conditions". (why they chose and "n" instead of an "s" seems to be plain mean spiritedness to make sure that they're not confused with U.S. measurements). Is that what you mean?

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[mbeychok]

Pons19:

There is no universally accepted "Standard temperature and pressure" or "Normal temperature and pressure" for gases. There are at least a dozen or more different sets of reference temperature and pressure that are referred to as "standard" or "normal". Read this article in the online Citizendium article:

[tab]Click here ==> Reference conditions of gas temperature and pressure

Instead of simply referring to set of reference temperature and pressure as "standard" or "normal", it is much better to always clearly state the reference temperature and pressure.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[MortenA]

As it is stated above ist a reference temperature and pressure used together with a reference volume and its just a different measure for a fixed number of moles. Some mechanical engineer must have been involved ;-). For a cubic meter of gas, if your reference is 0 barg and 0 deg C then is 44.62 moles, if its 0 barg and 15 deg C then its 42.29 moles.

Best regards

Morten

 

[HEC]

There are standard references for the "STP" and "NTP" conditions. STP is generally the US and imperial version 101.325kPa 0C while NTP is the preferred metric 101.325kPa 20C. Why it is diferent I do not know!

Mark Hutton

 

[katmar]

It seems the OP's question was not what standard and normal conditions are, but why they are used in flow calculations.

The only reason is to make flowrates under widely differing conditions comparable. What we pay for is the amount of mass or matter that is transferred, rather than the volume. Converting to standard or normal conditions is just a convoluted way of converting volumetric quantities into mass (or molar) based quantities. I have been kicking against this convention and trying to get people to just work in mass flows directly for the last 30 years. And I have been spectacularly unsuccessful at it. Its a convention that is not going to go away, so you and I will just have to learn to live with it.

Just think of STP or NTP as being another way of expressing mass. And, as David has pointed out, don't forget to convert to actual conditions when you want to calculate velocities or pressure drops.



Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Pons19]

Many Thanks for your replies.

Am mentioning about Std Temp & Pre (STP) and Normal Temp & Pre (NTP).

In plant while measuring the units in DCS, we look in which conditions, STP,NTP or actual conditions.

We measure and supply some chemicals to customer that will be in actual conditions, isn't it?

Thanks
Pons

 

[zdas04]

No, custody transfers are always done at the conditions specified in the contract. I've never seen a contract that didn't specify the pressure and temperature that gas measurement was to be adjusted to. It would be good if industries would use mass, but they don't so we're stuck with imaginary conditions. As is said above, "STP" is no more standard than women's clothes sizes. The governing document is the sales contract (or the regulatory requirements for reporting volumes to the state).

David

 

[JLSeagull]

Where the signals exist, it is common to apply the gas laws for temperature and pressure compensation for flow rates etc. I find this helpful when I get to play on the console such as routing a gas stream through a chiller and watching the flow rate. Consider regulations and contract requirements for custody transfer meters. (I am with a contractor these days and rarely get to play at the console.) Also, operators probably won't like my description of play. ;-)

 

[Pons19]

I got your points.

Have few queries...The value displayed in DCS will be in actual condition or in STP/NTP conditions?

Monitoring the flow based from DCS is based on actual conditions?

Thanks
Pons

 

[zdas04]

The data displayed in the DCS will be based on the conditions that it was programed to display. It may show flows at contractual STP, it may show mass flow rate, it may show flows at actual conditions (rare but sometimes done).

David

 

[JLSeagull]

Lacking other instructions the display will be whatever units are available from the transmitter. Normally this is not adjusted for the operating conditions.

You may have pressure and temperature available. You may also have multivariable transmitters. In such cases one could apply the ideal gas laws or even use the process data to provide something like "mass flow" information.

Pressure and temperature compensated flow data does not happen automatically. Someone must decide that they want corrected flow rates and pay for the effort to make it happen. I did this when I worked for an operating company building and operating cryogenic natural gas plants. Not everyone wants the corrected flows. Few would pay for the additional calculations. The company paid me whether I was working in my office or playing in the plants. I had fun with such requests.

 

[MortenA]

Mark (HEC) asks why different reference conditions are used in the various unit systems - my guess: Love of "round numbers". Its especially "funny" that in the imperial unit system the reference temperature is 60ºF=15.56ºC. The engineers working in metric unit will however be familiar with a SM3 where the reference temperature is 15ºC...

Best regards

Morten

 

[Pons19]

Hi

I got your points. Thanks a lot for your valuable replies.

Regards
Pons