METHOD MEASURES BTU REDUCTION IN GAS PLANT

Jim Bieda
Union Pacific Resources Co.
Fort Worth

Accurate calculation of the heating-value (BTU) reduction as the result of natural gas being processed is essential for determining revenues for producers and feedstock costs for processors.

Here is a method for calculating that change.

BTU reduction in gas plants refers to the change in quality of a gas stream that occurs as a result of the stream's being processed in a plant. By processing, the plant purposely extracts or removes certain components from the gas and thereby changes its physical makeup.

It is very important that this change be measured and accounted for accurately because it is the basis for determining the amount of revenue the producer ultimately receives for the gas he produces and the amount the processing plant pays for its feedstock.

ROLE OF THE PLANT

The terms "plant" and "processing" are somewhat generic and are often used indiscriminantly to describe various kinds of equipment and services such as separation, filtration, dehydration, and compression.

Here, however, "plant" will specifically refer to a facility whose primary purpose is to process gas where "processing" specifically refers to the steps required to extract and separate natural gas liquid (NGL) products.

Gas processing is a manufacturing operation in which NGLs are made. As such, the profitability of the operation depends greatly on the cost of the raw materials and how efficiently they are used.

In this case, the raw material is the gas itself, and the amount of it used is carefully measured and paid for by the plant. It is the plant's most significant operating expense and is call "PBR" for "plant BTU reduction."

The size of the PBR is directly related to the price of the gas (i.e., what it sells for at the time it is used) and the amount of product made. As the gas price goes up, PBR goes up; as the gas price goes down, PBR goes down.

In general, the plant's profitability is favorably affected by low gas prices because they reduce PBR. Also, it is important to understand that, although processing "shrinks" or reduces the volume of the gas stream produced at the wellhead, processing actually increases the stream's overall value.

The value of each cubic foot is enhanced when the correct balance of gas and NGL is achieved because the combined value will exceed the value of only the gas.

This is more than a happy coincidence. The correct balance is carefully determined by comparing the value of all components individually to their gas and liquid-phase values to determine how much of each should be extracted as NGL. This can be a complicated decision depending on the pricing makeup of the inlet gas, but it is one that must be made accurately if the overall value is to be enhanced.

COMPOSITION

Two changes occur to the gas stream as a result of processing:

The volume is reduced by a measurable amount (15- 20%,commonly).
The composition changes because certain components are removed. Table 1 illustrates the change in composition for a typical gas stream before and after processing.

It is very important that these two changes be accounted for in a way that represents accurately the use that has taken place. The method for this accounting must address the fact that a thousand standard cubic feet (Mscf) of propane is more valuable than an Mscf of methane.

The most widely accepted method is to quantify the BTUs removed (or the reduction of the BTU content, hence the term "plant BTU reduction") as a result of processing.

The PBR may be calculated in several ways, two of the most common are:

PBR = Inlet BTUs - residue BTUs (1)

PBR = Product BTUs + fuel BTUs + flare/loss BTUs (2)

The correct way depends on a variety of circumstances unique to each location but it will always be defined in a .,processing agreement," which is a contract that describes every business detail of processing. It is an important document and should always be consulted if a conflict or question arises.

Not all processing agreements are alike, and one must therefore be careful to refer to the correct contract for the situation.

To demonstrate the PBR calculation, Equation 2 will be used because it requires a liquid BTU calculation (for the NGL) and a gas BTU calculation (for fuel, flare, and losses).

GAS BTU CALCULATION

The calculation of the BTU content of a certain volume of gas is performed by separating the total mixture into individual component volumes and then multiplying by the corresponding BTU factor; these factors are well documented and usually come from GPA Standard 2145 pursuant to the processing agreement.'

Therefore, in order for the calculation to be performed, the following data are required:

Water content (dry or saturated)
A component analysis
Total measured volume
The pressure and temperature base of the volume.

The water content is important because it affects the volume of gas measured,

However, plants always dehydrate the gas as an early step in processing. Therefore it is always dry by the time it is measured.

This fact will allow us to skip calculating a water-volume adjustment, but if the gas is not designated as "dry" or has not been dehydrated, a separate calculation is required.

It is not a difficult one and it can be found in many references such as Spink .2

The pressure and temperature bases are important for the same reason: They directly affect the volume measured.

Volumes are almost always reported with a pressure/temperature base, but in the rare cases when they are not, the person using the data may assume the gas is at the correct conditions according to the contract and then clearly state this assumption in the presentation of subsequent calculations.

As a hypothetical example, we will calculate the BTU content of the fuel used by "Plant X" for a given day. The plant reported fuel use to be 4.87 MMcfd at 14.696 psia and 60 F.

But the contract states that all volumes must be reported at 14.65 psia and 60 F. Therefore, before the BTUs can be computed, the volume must be corrected, which is easily done with the following calculation: (reported pressure base/desired pressure base) x reported volume = volume at desired pressure base; in this case: (14.696/14.65) x 4.87 MMcfd (@14.696 psia) = 4.89 MMcfd (@14.65 psia).

The basis for this calculation is Boyle's Law which can be written a number of ways, but here it is:

(P1 . V1)/P2 = V2

In our example it should be clear that the initial pressure base of 14.696 psi and volume of 4.87 MMcfd correspond to Pi and Y, and that the desired base of 14.65 psi and corrected volume correspond to P2 and V2- The remainder of the calculation is best handled in a tabular format (Table 2) and is quite compatible with the spreadsheet programs available for all personal computers.

In Table 2, Column A values are provided by laboratory analysis; Column B values = Column A values/100, for each component; Column C values = total measured volume x Column 6 values, for each component; Column D contains individual component factors from GPA Standard 2145. Note that the factors in Standard 2145 are at 14.696 psia. This means that every factor with "cubic feet" in the units must be corrected to 14.65 psia.

Because the Column D factors have "cubic feet" in their denominators, the net effect is to multiply Column D values by 14.65/14.696 0.997, hence Column E: Column E = Column D x 0.997; and finally, Column F = (Column C x Column E)/1,000, for each component.

The sum of all components in Column F yields the total MMBTU content of the 4,890 Mcf of fuel consumed.

Referring to Equation 2 for calculating PBR, one can see that the majority of terms are gases, and each one will have its own calculation. Although they may vary in volume or composition, the procedure will be the same as shown so long as they are in the gas phase.

In order to complete the calculation, the BTUs of the liquid products (NGLS) must now be computed.

LIQUID BTU CALCULATION

The liquid calculation is very similar in concept to the gas calculation but requires three additional steps:

One step converts the liquid volume (gallons) to an equivalent gas volume (Mcf). Two more steps convert the volume and heating-value constants to the proper pressure base (in this case, to 14.65 from 14.696).

The pertinent steps are shown in Table 3 and are based on the following hypothetical case:

On a given day, "Plant X" produced 5,600 bbl of a single mixed product. The composition, determined by lab analysis, is a mandatory piece of information and is shown in Column A.

In Table 3, Column G (Column B x Column D x Column F)/l x 106, for each component. Column B is the conversion of the total product volume to the individual component volumes by the use of component volume percents (Column A).

Columns C and E are factors from the previously referenced GPA Standard 2145, and Columns D and F are the same factors corrected to a 14.65 pressure base. This procedure is valid for all liquid-phase streams regardless of composition and volume.

Now that the procedures for calculating the gas and liquid phase BTUs have been demonstrated, the PBR becomes only a problem of identifying and measuring the pertinent streams and adding them, as shown in Equation 2. PBR, then, is an accounting method that measures the amount of raw material used.

The method involves calculating the heating value (BTU content) of all the streams removed from the inlet-gas volume and summing them to arrive at one number that represents the use of several different components with a wide variation in physical properties. The calculation itself is not difficult but requires knowledge and consistency of two key conditions:

The water content (dry vs. saturated) of all gas phase streams
The pressure base of all measurements vs. the pressure base required by contract (the processing agreement). With these conditions under control, the computation requires several multiplication steps using physical constants that are readily available.

REFERENCES

"GPA Standard 2145," Gas Processors Association, Tulsa, 1989.
Spink, L. K., Principles and Practice of Flow Meter Engineering, Foxboro Co., Foxboro, Mass., 1972.