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Standard Conditions Of Temperature And Pressure




A lot has been said about standard conditions of temperature and pressure in chemical engineering texts, international standards (API, ISO, DIN, JIS etc.). Probably the best reference to this very debatable topic is the “Wikipedia” article given at the following link:

http://en.wikipedia....re_and_pressure


In this blog entry I am providing my opinion on the various standard conditions mentioned in the above article.

Let me begin with standard pressure. I have no issues with the standard being defined as 101.325 kPa (abs) in SI units and translating as 14.696 psia in USC units. Note that I have used the term “USC” units which is an abbreviation for “United States Customary” since SI units are now being used even in United Kingdom for all chemical engineering work. An important point to note here is the “Wikipedia” article has mentioned that the OPEC (Organization of Petroleum Exporting Countries) nations which presently are listed as 12 nations and a majority of the natural gas industry in North America follow a standard pressure condition of 14.73 psia (USC units) which translates into 101.56 kPa (abs) in SI units. The current API standards (e.g. API STD 2000) define standard conditions of pressure in terms of standard pressure as 14.7 psia (USC units) and in terms of normal pressure as 101.3 kPa (abs). Note that API restricts itself to single decimal places for both the standard and normal pressure conditions instead of defining it to the second or third decimal place. This is very confusing since both are the same if you ignore the second decimal place.

Standard Temperature provided an altogether different picture. When talking in terms of USC units and using the terminology “Standard” the standard temperature used is 60°F. Whereas in SI units and using the terminology “Normal” the normal temperature is 0°C. In the Wikipedia article, several other international and national organizations have used the standard temperature as 15°C and 20°C. This is where in my opinion lies the biggest ambiguity because of the major difference between 15 or 20°C and 0°C.

In my opinion there is no rationale for using 0°C as a reference for defining a standard condition of temperature. What is my logic for arriving at this conclusion? It is quite simple. How many times and places across the planet and at mean sea level do you actually observe an ambient temperature of 0°C. The premise that 0°C could be the normal temperature around the planet is in itself faulty. The origins of 0°C as a normal temperature emanate from the “International Union of Pure and Applied Chemistry” (IUPAC) which first proposed this a reference condition. For laboratory purposes, this value is fine since all laboratory experiments are conducted under a controlled environment. For the chemical process industry the reference condition of 0°C does not hold ground.

The reference condition (call it “standard” or “normal”) for temperature should be 15 or 20°C in my opinion. This is much more logical than 0°C because an ambient temperature of 15 or 20°C is much more likely to be seen at mean sea level across the entire planet than a temperature of 0°C.

Many of the top operating companies in the chemical process industry (oil and gas, refining) have realized the futility of using 0°C as a reference and now define the reference or standard condition for temperature as 15°C. Shell now defines standard conditions of temperature and pressure as 15°C and 101.325 kPa (abs) for all their oil and gas operations and installations.

In one of my forum posts on “Cheresources” I had called the term normal “archaic” not because of the English language word “normal” but for the reason that it implies 0°C and is defined such in many international standards and engineering texts. Even today I stand by my calling.

I would love to hear comments from the members and readers of the “Cheresources”.


Regards,
Ankur.




I certainly share your frustration with there being so many "standards". You have made a good job of summarising the situation here. The only way around all this is for us to maintain the discipline of referencing our standard conditions in every document and specification we issue.

One other mismatch which you didn't mention is also due to IUPAC. It is customary for chemical engineers to do some courses in chemistry and physics as part of their degree curriculum. These courses are usually given by scientists rather than by engineers and so they take the IUPAC rulings quite seriously. About 25 years ago IUPAC changed their standard pressure for gases from 101.325 kPa to 100.000 kPa. It seems very rare for engineers to use this standard, but it is what is taught in the "scientific" courses to engineers at undergrad level - unfortunately driving in the wedge further between the scientists and the engineers. See http://goldbook.iupac.org/S05910.html
I share your frustration with the lack of standardisation among the 'standards'.

Personally I think this has gone beyond anyones ability to resolve and perhaps reflects of the impact of globalisation (where every individual country's 'standards' are now impacting on their trading partners).
The growth of internet-based systems is also allowing internal (corporate, national, etc.) systems to be imposed on external communications. For example, the EPA have creating their own 'standard' conditions (based on 25°C) which differ from those generally used elsewhere.
The potential for confusion is not helped when companies fail to even indicate that a quoted gas volume flow is based on 'standard' conditions or start quoting gas flows and reservour/storage capacity in energy units such as GJ/hr (GigaJoules per hour) and PetaJoules.

My recommendation is that you should always quoted the reference conditions to support any reference to a 'standard' gas volume flow. Where the flow/volume is in energy terms the associated heating value basis should also be included.
If someone fails to provide this information to me then I either ask for it to be provided or use an appropriate set of 'standard' conditions (and heating value) and include them in my response.

Note that dictionary definitions for "normal" include meanings relating to 'standard' other than "typical" so I think that this is just another aspect of the confusion surrounding the use of these standard conditions. With regard to the use of 0°C as a reference temperature this is as relevent as any other definition when applied to the range of temperatures experienced in Alaska or Saudi Arabia.
to summarize my understanding STP = 15.6 deg C and Pressure of 1 atm (14.7 psi / 101.3 Kpa) What about NTP ? It is usually referred for fan capacities right ?

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