The AGA 3 and 8 Calculations reference atmospheric pressure. So does Directive 017 and API 21.1. But how exactly does it effect to gas measurement? If you refer back to your high school chemistry you will recall that base pressure is defined as 101.325 kPa. This is also the atmospheric pressure at sea level. When we refer to a volume of gas, it is always assumed to be at base conditions. (101.325 kPa and 15° C). As you move above or below sea level atmospheric pressure decreases or increases accordingly. Because most pressure sensors are not installed at sea level, we need to correct accordingly to ensure that a given volume is gas is always that volume, regardless of what the atmospheric conditions are.
So what effect does an incorrect atmospheric pressure correction have on gas volume? This post will outline two examples to illustrate the ramifications of an incorrect meter configuration.
The first illustration is taken from a booster station sales meter in South Eastern Alberta.
The flowing parameters and meter information are listed below:
For the purpose of this example we will assume that temperature correction factors are applied correctly at 15° C and that orifice and pipe reference temperatures are configured correctly at 20° C (room temperature). The atmospheric pressure at this given location has been determined to be 91.350 kPa. Using a combination of the AGA 3 and 8 calculations we can determine the gas volume at this metering point to be 169.133 e3m3/day.
This is the correct volume for this meter, but let’s explore the effects of the two most commonly made mistakes when configuring a flow computer.
Mistake #1: Entering all atmospheric pressures as 93.08 kPa
In Directive 017 there are 7 example scenarios used to determine the accuracy of a flow computer’s calculation. In these examples the atmospheric pressure is always references as 93.08 kPa. Often times when reviewing a flow computer’s configuration this is the value found entered. While this isn’t a valid excuse for a technician entering this value, you can understand where they came up with it. On the plus side, at least they are looking at the regulations.
Mistake #2: Leaving atmospheric pressure at default of 101.325 kPa
Flow computers are manufactured under laboratory conditions where the atmosphere and temperature are regulated to base conditions. Many flow computers come from the factory still configured to base conditions. If a technician is unaware that the atmospheric pressure needs to be changed in the configuration it may remain at this value for quite some time.
So what is the effect of these two mistakes?
The table below shows the effects on volume between the original example and the two incorrect atmospheric pressure configurations.
At first glance it doesn’t seem like a lot. 0.093% difference per day seems pretty marginal, and is well within the acceptable accuracy range of the performance evaluations mentioned above in the ‘Mistake #1’ scenario. From a meter technician’s point of view this really seems like a dead issue. But what if we were to start associating dollar amounts to this value?
Let’s assume a spot price of $3.62 / mmBtu. The table below outlines the financial difference on a daily basis and also per year.
Assigning a dollar amount to this makes it seem slightly more significant, but as some of you may have already noticed; 1) this is an error in the meter operator’s favor, 2) it is still only 0.158% of the total ~$21,750.00 per day going through this meter. For the unethical producer out there it might be a consideration to purposefully not fix this error. In the event that this were to come to light in a measurement audit, the payout would be fairly painless for the operator.
The second illustration is taken from a coal bed methane well, feeding into the meter from example one. The flowing parameters and meter information are listed below:
Again, for the purpose of this example we will assume that temperature correction factors are applied correctly at 15° C and that orifice and pipe reference temperatures are configured correctly at 20° C (room temperature). To help illustrate the effects of incorrect atmospheric pressure correction on a lower pressure well we will again assume the atmospheric pressure at this location to be 91.350 kPa. Using the AGA 3 and 8 calculations we can determine the gas volume at this metering point to be 2.183 e3m3/day.
Again, this is the correct volume for this meter, but let’s look at what effect the same two effects have on this well head meter.
This is quite a difference from our first example. Almost 5% in the worst case scenario and even in the lesser of the two mistakes we find the volumes are still outside of the 0.25% recommended by the performance examples in Directive 017. Even though the volumes are over 80 times less, the volumetric error is approximately 2/3 of that in the original example. Let’s look and see what the financial impact is.
Again, assigning a dollar amount to this volume makes it seem more meaningful. It still is only approximately 5% of what is going through the meter, but that is a lot to overstate and possibly have to repay with interest down the road. Consider now that this error could feasibly have been made at all of the other well head meters in this field, compounding this issue into something catastrophic. Significant indeed.
So why the difference?
It all comes back to absolute pressure. To refer back to high school chemistry again, you may remember that:
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
Let’s express this in terms of our two examples:
In the first example we see that the addition of atmospheric pressure to our gauge pressure yields a relatively small increase, and as such makes for a small percent difference. In our second example we see that this becomes almost an order of magnitude more significant and is indeed a real consideration.
So what can we take away from this? Obviously in the measurement world we seek to ensure everything is as accurate as possible, correct atmospheric pressure determination is certainly one important, and often overlooked, piece of this puzzle. Contrary to initial instincts if this is an area that needs attention in your field, perhaps it makes the most sense to tackle the lowest pressure wells first?
Stay tuned for the next post where I will show you an easy way to determine atmospheric pressures and elevations automatically…