REGARDING MASS MEASUREMENT…
“Maybe that’s because when it gets hot (as in USA) they need to supply less mass, but when it gets cold (Canada) they need to supply more mass. My own opinion is that the only real measurement of flow is mass flow. Anything else is a holdover from the days when liquids were sold in buckets.”
- “because of the time lag due to thermal masses and the thermal conductivity of “earth”, I suspect the wee hours of the am would not be the lowest temperature an underground gas tank could reach due to overnight lower surface temperatures., but your answer begs the question about mass.
- do you mean to say that ordinary gasoline pumps are smart enough to compensate for temperature-induced density changes in gasoline? That could lead to a further question about their ability to compensate for a density change in gasoline due to the addition of 10% oxygenated additives like ethanol. So, are ordinary pumps really smart?”
…BUT NOT EVERYONE SAW IT THAN WAY…
“Anyway, look, you and I pay for gasoline, at the pump, by the gallon; in most other countries one pays by the liter; in all countries one pays on the basis of volumetric flow, – so why bring in mass flow measurement at all ? And when your tank is full, that means the volume of your gas tank is now filled with gasoline, — not that your car is carrying a given maximum amount of kilograms or pounds of gasoline, right ? Further, as far as I know, gas pumps use turbine flometers now, – probably used displacement types at one time. Both measure volumetric flow, right ? So why bring “mass of gas” into it ?”
“I hate to differ, but most gasoline pump calibration is done using a Seraphin Can, which is a purely volumetric measurement. (See www.seraphinusa.com) The old adage is to purchase gasoline early in the morning when the weather is hot. At this time, the temperature is at a low, and you end up with more mass per gallon.”
… AND A FEW PRACTICAL MATTERS…
“I am enjoying your series. We have an interesting variant in Australia – the refineries charge the service station on a volume ‘as-dipped’ (ie, no T-comp). But the dispensing equipment IS temperature compensated. The independent stations are claiming the refiners are short changing them by 2-3% by delivering product as hot as possible as fast as possible. To which the reply is????”
BUT IN THE US… “Gasoline does not come directly from the refinery. It goes first to a bulk depot, where tank trucks are loaded from above ground storage tanks. The temperature of the product in large ASTs fluctuates a great deal (shadow vs sun side of tank, etc.). It might also heat up during transport (in a tanker travelling 1-2 hours in 100F).”
“A colleague pointed out to me that the mass of a rail tanker changes as it crosses the US/Canada border, as US standard volume is at 60F and Canada at 15C (not 15.5C). As the volume in the tanker isn’t allowed to change (it has been sold at $/gal), the railroad changes the mass of the car to calculate their charges. Einstein missed his vocation – I dont want to calculate the energy involved in such a calculation.”
“I see that you learned a great deal about the retail sale of gasoline. I was recently told by a woman that her father told her, when she first started driving, not to fuel at a station if she saw that a delivery was being made.I believe that this was becasue all the muck in the tank was churned up during the delivery. Much of the uncertainty that you refer to is accommodated for in the tolerance. In general, the allowable error is as much as 0.5%.”
“Thanks for sending the Ezine to me. One of my favorite topics. Thought you were going to present the maximum variation analysis in hopes of converting the US to either temperature compensation or mass. While the US W&M have argued your point – that the variation is small, Brazil – for example – is in process of moving to temperature compensation. Apparently, much of their position is based on the opportunity for fraudulent practices.”
“A lot of fuel is stored above ground, such as aviation fuels, so as a pilot, I am going to check and see if the pumps are temperature corrected, I had always assumed they were. The price of Avgas has gone over $3.00 per gallon so it is not a mote point anymore.”
“One of the biggest points in our business is that the LPG tanks are not below ground, and there is a lot more opportunity for temperature variation to present error in measurement – 1% for each 6 degrees variation.”
…AND SOME PHILOSOPHICAL THOUGHTS…
“That’s why careful consideration of given conditions based on knowledge, understanding, and experience, in conjunction with appropriate simplifying assumptions work towards technical problem solving, as opposed to the time and effort expended wandering through theoretical minutiae on the way to the solution.”
My previous e-mail containing readers’ responses to the e-zine generated additional responses that might be of interest. The writers quoted below (and in the previous e-mail) agreed to allow their comments to be shared in this manner. I will summarize my thoughts in the next e-mail (after ISA/2001). My commentary is IN CAPS between quotes.
“Working for a company that sells both volumetric and mass flowmeters, I find it interesting when a specification calls for a mass meter and the operating conditions specify the flow rate in volumetric units and the desired output is volumetric. For many years users thought in terms of volumetric measurements, but now that very highly accurate meters are available for mass flow. Users want to try the newer technologies, but they just do not want to step off into a new measurement that they have not had a lot of experience with at this point. Not unlike the belt and suspenders level of comfort.”
“Here’s an example of what happens when a measurement standards committee doesn’t include a Measurement Engineer:
- In the US, the dispensing of CNG (Compressed Natural Gas) for vehicles is typically measured with coriolis (mass) meters that are calibrated by weight, i.e. filling and weighing the change of a test cylinder. However, the pump’s display, and pricing, is in units of “Equivalent Gallons” of gasoline!! To make this conversion, requires two factors; 1) the (assumed constant) Btu/gallon of gasoline, and 2) the Btu/lbm of the CNG being dispensed (frequently assumed to also be constant). The idea of selling CNG by the lbm or kg was not considered to be ‘marketable’ to the US public.”
…BEATING THE MEASUREMENT…
“You sure struck a very touchy subject. It is interesting to see that much of the comments relates to beating the measurement and tipping the scale in some ones favor. Thanks for keeping me in the loop.”
“I will stir the pot a bit by suggesting that, as a user, and if a patient one, the best tactic is to “pour slow”, as the error SUBSTANTIALLY increasingly favors the customer at the low end of flow, due to meter “slip” – true for most metering systems including PD. Our times have not made us advocates of “slow” unfortunately!”
…ON DIFFERENT TYPES OF FLOWMETERS…
“At the time, the only meters in dispensers were PD. I had developed a configuration I wanted to foster which used a 1/2″ turbine, putting the meter and display at the nozzle – eliminating the dispenser altogether. W/M accuracy at the time made that uncertain, and we abandoned it.”
…A SAFETY ISSUE…
“I always enjoy reading your application experience type letters. I found the gasoline flow article interesting. Here is some “FUEL” for thought. After reading the one about the gasoline, I thought you may not want warn all these people about EXPANSION of gasoline. When topping off their vehicle gas tank when it was extremely cool, then letting it set in another area where the temperature is allot higher can be dangerous. The expansion of gasoline in when people northern Minnesota have filled their cars in the dead of winter, then put them in a heated garage has caused many explosions and fires.”
…BUT WHAT IS REALLY BEING SOLD?
“And speaking of the ‘real accuracy’ of measuring gasoline; cars need energy (Joules, Calories, or Btu) to go putt-putt up the hill, not volume or mass. So why (rhetorical question) don’t the gasoline stations post the specific energy content or the price per unit of energy of the gasoline they sell? Does anyone have any data showing the variations in gasoline heating values across the US? So David, maybe next time your flow meter examples might use something simpler, like a kwh meter. ”
I received two more readers’ responses to the e-zine regarding fuel measurement that might be of interest. The writers quoted below (and in the previous e-mails) agreed to allow their comments to be shared in this manner.
“I had the pleasure of re-reading a number of your Ezine issues this morning. While I don’t want to get into all the permutations of why someone “should” measure by mass instead of volume, especially in retail exchanges, I did want to comment on one point. That being, the best way to infer energy content of a mixed hydrocarbon steam… for instance, gasoline, CNG or just boring old natural gas supplied to your home.
When measured by volume, the BTU per Scf of different hydrocarbons changes dramatically. Methane at approx. 1000, Propane at Approx. 2500 for instance. If the max variation is examined, variations from 1000 to 4000+ are possible (for individual components, not the resulting mixture). In comparison, the variation in BTU per LB of any hydrocarbon is much less. Methane comes is at 23,890, Propane at 21,670 and Hexanes at 20,800. The typical variations of a hydrocarbon mixture are less than +/- 4%… This is an argument for making a direct mass measurement using a technology that is insensitive to composition change… of course, a Coriolis meter is the one single device that can accomplish this. A neat plug for using Coriolis in CNG dispensers… and an easy way to infer energy content from hydrocarbon streams like gasoline flows. Anyone interested in using a Coriolis for a Gas Pump now?”
“Have read your tutorials on flow measurement with interest. I have one for you that might be of interest.
In measuring natural gas flow in a power plant, the measurement may be used both for accounting and for matching the Btu flow to the requirement for combustion air. The process is further complicated by the meters owned by the gas companies and their contract for the gas sold to the power company. In addition heat loss analysis, power generated, etc. can determine thermal efficienies and back into additional values of the gas flow.
Invaribly meters will not check and only after very careful attention to all the detail can they be made to even come close. This is further complicated by the fact that the control manufacturer’s metering is designed by different people using different assumed and measured values such as internal pipe diameter, flowing pressure and temperature, % saturation, and standard conditions.
Standard conditions are a particular problem. Invaribly the plant’s gas contract will have different standard conditions than those given the control equipment manufacturer. There are a number of different standard conditions in play. Some pf these are 14.4 psia, 14.4 psia plus 8 ounces, 14.7 psia, 30 in. Hg., 14.696 psia, and 15.025 psia. In addition the Btu value determined by the gas company and given to the plant is often on a basis of 30 in. Hg. saturated, since the process of determining Btu value saturates the sample. As I recall the value between saturation and dry is 1.732%, and the actual flowing saturation is not given. Because of this 50% saturation is often assumed, even though the gas may be perfectly dry.
For this type of meter for natural gas, that is only part of the story. What is really being measured is Btu’s. Typical natural gas may vary in Btu content from about 950 to over 1100 Btu/scf. The variation is normally due to different percentages of CH4, C2H6, C3H8, and possibly very small percentages of other gases. Typically the composition depends on the field the gas comes from. he gas, however, comes through a pipe line andd it is not at all uncommon for the Btu content to fluctuate over a plus or minus 5% band. If there is a very small amount of inerts such as Nitrogen, the sp.gr. changes in the same direction as the Btu content. Thus should the Btu change so does the sp.gr. in a partially (about 2/3) compensating manner. This means that such a meter is closer to a Btu flow meter than a meter measuring scf of natural gas. This is fine as far as fuel-air ratio is concerned since combustion air flow is needed based on Btu flow.
The problem comes when inert gases such a N2 are present in a large enough percentage to cause Btu content to be lowered as sp.gr. increases. This really throws a kink in the fuel-air ratio. by causing more scf for the same Btu flow but a significant sp.gr. error. High N2 amounts are found in some Oklahoma gas fields.
The answer to this problem is not a mass flow meter since such a meter would show a higher mass flow with not only the combustible gas a part of the mass flow but also the inert N2. In addition there is a variation in the Btu content per lb. of various combustible components.
If you know all this ahead of time, no problem, since you can cope with it. If you don’t understand it, an otherwise good engineer may beat his brains out trying to fix an unfisable.”
THE FOLLOWING IS FROM AN ONGOING CONVERSATION WITH A COLLEAGUE:
In the first comment, the implied conclusion does not logically follow from the facts presented. The natural gas supplied to your home is principally methane, so only a small percentage of its content would be questionably measured by non-mass techniques. In addition, gas companies attempt to control the energy content per unit volume of the gas delivered. These issues would be further complicated by whether the measurement is in the gas production, transmission, or distribution systems.
In the second comment, variations of the internal pipe diameter (flow area) is perhaps the largest source of error when one or both of the measurements are orifice plates. It is suggested that such natural gas measurement systems utilize a metering assembly consisting of the upstream/downstream piping, pressure tap, temperature sensor/well, and orifice. It should be supplied, assembled, and calibrated by the vendor, and installed as one piece. In addition, the second comment mentioned customer complaints when the difference between their measurements and the utility company’s measurements was excessive. To achieve a higher level of confidence in these installations, the flowmeters should use different measurement principles.
In addition, information about standard conditions and inferential mass flow measurement that address other issues raised are discussed on page 64 or 67 (depending on edition) of “Flow Measurement” (David W Spitzer, editor — available at isa.org bookstore).
“The author is quite right about the mass flow measurement of Nat. gas. For pure hydrocarbons (no inerts) mass flow measurement and btu measurement track each other somewhat more closely than other methods..but as the author noted they vary according to the molecular contruction of the hydrocarbons of the mixture, this is exacerbated in distribution systems up north where propane is used in peak shaving periods to maintain the btu level of the transmitted gas…nitrogen is used by man to control btu content in some instances. Natural gas distribution heating values determined by gas calorimetry techniques together with restricted volume flow techniques(i.e. all measurements are reduced to a set of reference temperature and pressures, ..one needs consistent reference standards) is still probably the best and most practical method to measure how many btus one is recieving…but to insure accuracy of measurement…both calorimetric and volume flow measurement especially for power plants and other large custody transfer should be done at both the suppliers and vendors sites. Mass flow measurements can be made but requires a gas chromatography analysis(to determine composition) used to calculate btu content…gas chomotography can also be used in conjunction with volume flow techniques. In LNG measurement gas chromotography techniques are essential to good flow measurement(btu content implied)….as you can see the implications further exacerbates the problems that thermal mass flow meters present, which at best, only implies mass flow measurement under very restricted conditions. I hope that some of this clears up the problems somewhat..but could confuse the novice. I have discussed this in some of my published papers on “Mass Flow Measurement’ (ASME/ISA?)and others on LNG measurements in AGA proceedings..if you like I would dig up these references. As one can see …good flow measurement is not always easy to come by and is seldom accomplished accurately by the use of a single measurement technique\”
“David: Re measurement of nat gas into power plants, the method usually used, especially where large or medium volumes are involved, is an ultrasonic meter (corrected for press & temp) and a gas chromatograph for the BTU determination. Output signals from both are fed into a flow computer which then reads out in total energy (therms). This combo is becoming commonplace now and is generally very accurate. The ultrasonic meters can measure flow to about +/-0.5% over a 30:1 turndown, and the GC’s are accurate to +/-.5 BTU even with large percentages of N2 and/or CO2. Re standard conditions, the prevailing values used by most of the gas transmission companies I have run across are 14.73 psia & 60 deg F.”
I have just been through some discussions on fuel supply to a gas turbine cogeneration system. The turbine manufacturer specifies a maximum Wobbe Index value ie. Lower Heating Value/sqrt(Specific Gravity) which makes sense – no one can condense the water of combustion to extract the latent heat. But the pipeline operator specifies a Wobbe Index based on Higher Heating Value, and a maximum percentage inerts. In truth, they vary from about 3 to 7% depending on operating conditions in the gas wells and plants feeding the pipeline. Neither specified a calorific value specification, and the Wobbe tolerance is +/- 5%. So determining the thermal efficiency of the system requires laboratory analysis of the fuel gas through the test runs (as the turbine manufacturer won’t accept the data from the on-line GC which he supplies as part of the contract)! Yet the gas is sold by energy content, even though there isnt a spec for it. It is a ‘declared value’. Even more interesting, some of the gas coming into the plant is on-sold (as energy at the same ‘declared value’) so we must ignore the ‘truth’ of a on-line measured value and use a value determined on the pipeline operators history (or the accountants will be disturbed).”
“You are obviously getting much interest in your e-zines. The guy or girl responding to you with the report you were kind enough to send me is clearly knowledgeable and understands the significance of knowing what is being measured and what it really means. There are metering systems that actually do this well by combining full data of the gas mass and composition but as you well know such measurements are expensive.
For proper gas flow measurement one must use flow rate, temperature, pressure and gas composition. If these parameters are accurately measured then mass and energy content can be accurately provide. At that time all guessing is off. Right?”
FROM FLOWMETER MANUFACTURERS…
“There is no doubt that mass measurement is the best solution when it comes to gas measurement, and mass flow meters are used in CNG dispensers. My background is primarily with liquids, where volumetric measurement is traditional and usually the most cost effective solution (in terms of system cost, operating cost & accuracy achieved) – even with temperature compensation added.
The really difficult application in this field is LPG. In the US & Canada, LPG = 100% Propane, which has a distinct table of coefficients of expansion. In many warm climate markets (souteast Asia), LPG is a mix of Propane & Butane, where the latter has coefficients of expansion about 2/3 of that of Propane. Butane makes up from 20% to 40% of the total mix. Since the ratio of the mixture varies from delivery to delivery, even mass flow meters cannot keep up (without knowledge of the specific ratio being metered).”
“Your e-mail … was passed on to me to read. I found your points very well taken and I certainly agree that most all applications for fuel flow measurement are intended to ultimately determine BTU content and delivery.
We … manufacture thermal mass flow meters and are periodically asked to measure natural gas flow rates. In order to make such measurements with any degree of accuracy, the ideal measurement method involves blending in the precise components of the gas mixture as specified by the customer and then calibrating the flow meter accordingly. We, and usually our customer realize that the blend components can change with time and thus significant calibration errors are quite likely. As you point out, when nitrogen gas is injected into the gas line, then we see that BTU content will go down and the thermal mass flow meter will not be able to make compensation for this inert gas entry into the flow stream.
I certainly agree that the coriolis meter will be superior in the measurement of various blend gases when compared to a thermal mass flow meter. This is particularly true when very large gas flow rates are to be measured since most calibration facilities that calibrate thermal mass flow meters will use air as cal gas and rely on “K” factors to adjust their calibration using cal lab “Black Book” know how to make such calibration adjustments more closely fit the true calibration. In these large flow rate usages of thermal mass flow meters, significantly larger mass flow rate errors will occur. The Coriolis flow meter, however, does not have to resort to “K” factors or to make any adjustments when calibrating using air and then being put into natural gas service. Yet, even the Coriolis flow meter will prove inaccurate if nitrogen gas is inserted into the natural gas flow stream since it will not be able to correctly determine BTU content.
In your final two paragraphs … you said in part the following: “The answer to this problem (i.e. nitrogen gas injection into the natural gas flow stream) is not a mass flow meter……..” “If you know all this ahead of time, no problem, since you can cope with it.”
I would appreciate it if you would expand on your thoughts regarding the above two sentences since I do not exactly understand what you are inferring.”
“…I had a couple of comments to pass along. Looking first at the comments regarding the use of mass flow meters to infer energy content in a hydrocarbon stream, the writer brushes over a couple of points quite lightly. The statement that the energy content in mixed streams of a CNG flow will vary only +/-4.0% is about right with the assumption being that is the type result one could expect with a mass coriolis meter. The problem is an accuracy of +/-4.0% in measurement for a point-of-sale system is hardly a result worth bragging about.
Secondly, though coriolis meters are used in CNG dispensers, the leap to the assumption they are a viable candidate for use in gasoline pumps or home/business natural gas metering fails to take into account the relative costs between coriolis meters and the positive displacement meters used in most gasoline pumps or the bellows style meters used in most residential and commercial natural gas meter runs. Having investigated both these markets at some length, I can attest that the cost of the meters used in gasoline dispensers range from $50.00 to $150.00 per unit. Residential natural gas meters cost even less. I personally have visited two major natural gas utility companies that run extensive repair facilities for their residential and commercial bellows style natural gas meters. Both of these utilities have set a value of about $16.00 as the point above which a meter repair is less financially viable than a complete replacement of the meter for a new unit. I have heard of no coriolis mass meter that is capable of reaching anywhere near these cost levels and to do so with the long-term reliability and accuracy required for such applications. So.to answer the reader’s question.no I do not think we’ll be seeing mass coriolis meters showing up at your neighborhood filling station or in front of our homes any time in the near future.
With respect to the second email concerning the problem of energy content in power plant applications, there are solutions available for the scenario described. We supply large turbine meters for power plant projects around the world and, yes, the quality of the gas can vary significantly and even from one day to the next. These variations are far more than the special case on CNG dispenser fuel discussed in the first email and have much higher variations in energy content. In these applications, NO flowmeter is capable of making the measurement and inferring the energy content by itself. That is why we see most of these plants incorporating both turbine meters and gas chromatographs to determine energy input. We are seeing more and more specifications for these projects being written with ever-tighter accuracy and overall thermal efficiency specifications. In some cases, the specifications are so tight (+/-0.25%) that only by utilizing the excellent repeatability of the turbine meter, enhanced signal conditioning and very sophisticated calibration techniques is it possible to meet the requirements but it can be done. Could a coriolis meter be used in place of the turbine meter in these applications??….Probably, at least for the smaller installations where they have sufficient capacity but most likely at a higher cost. Could the coriolis meter eliminate the need for the gas chromatograph in these applications??…No, not for the accuracy specifications being demanded in these projects.
My point in all this would be that to tout any one technology as the “do-all, end-all, one-size-fits-all” of flow measurement is a fool-hardy argument that cannot stand-up to the light of day upon detailed examination.
Thanks for sending us the emails and for this opportunity to respond.”