Application:

US EPA requirements for flares used to comply with US EPA air emission standards are specified in 40 CFR Section 60.18. The requirements of 40 CFR Section 60.18 are for air-assisted, steam-assisted and non-assisted flares. US EPA 40 CFR Section 60.18 provides detailed specifications of: flare tip exit velocity, visible emissions, pilot flames and net heating value of gas being combusted. The US EPA requirement for net heating value of gas being combusted requires that the net heating value of the gas being combusted be 300 BTU/SCF or greater. Typically, the flare gas is controlled at the required minimum of 300 BTU/SCF by the addition of natural gas, commonly known as "auxiliary fuel". It is generally desirable to automatically control the addition of the auxiliary fuel. This is particularly true for flare operations that must accommodate gases with large fluctuations in volatile organic carbon (VOC) compositions. The Flo-Cal Calorimeter is used to provide a high- speed calorific value measurement of flare gas, thereby, providing a key component of the information necessary for automatic control of auxiliary fuel addition.

Flaring involves piping VOCs to a remote and generally elevated location for combustion. The combustion takes place in an open flame such that > 98% of the VOCs are destroyed. Flaring operations must be designed to accommodate fluctuations in VOC concentration, flow rate, heating value and concentration of non-combustible components (inerts). If the flaring operation is designed to relieve emergency process upsets in addition to the combustion of VOC streams, the aforementioned fluctuations can be severe. In order to provide accurate, reliable, and high-speed calorific value measurements under these conditions Flo Cal Calorimeter is configured for either 0-2500 BTU/SCF or 0-1000 BTU/SCF. Each measurement range is tuned to provide optimal measurement precision at calorific values less than 1000 BTU/SCF. Selection of full-scale value is governed by the expected maximum calorific value of the gas being flared.

Consider, for example, the 0-2500 BTU/SCF measurement range. Measurements between approximately 1000 BTU/SCF and 2500 BTU/SCF are used primarily to trend the heating value of the flare gas at the beginning or end of an upset condition. In this range, the Flo Cal reliably reports a decline or increase of the flare gas heating value. Once the flare gas heating value falls below approximately 1000 BTU/SCF, precise measurements of calorific value are necessary for the auxiliary fuel control system to reliably maintain the minimum heating value of 300 BTU/SCF.

The Flo-Cal measurement precision between 1000 BTU/SCF and 2500 BTU/SCF is less than +/- 7% of full scale (2500). This level of measurement precision at values between 1000 BTU/SCF and 2500 BTU/SCF is generally considered acceptable for trend information. The Flo-Cal measurement precision at values below 1000 BTU/SCF is less than +/- 1% of full scale (2500), thereby, providing a precision level suitable for control of the flare gas heating value at 300 BTU/SCF.

The Flo-Cal also accommodates the possibility of the flare gas becoming inert (i.e., non-combustible). For the purposes of this discussion, heating values < 200 BTU/SCF are considered inert. Consider, for example, that the flare gas heating value may possibly approach inert conditions prior to control being established at 300 BTU/SCF. The Flo-Cal configured for the 0-2500 BTU/SCF measurement is fitted with the Flo-Cal auxiliary gas option. The Flo-Cal auxiliary gas option delivers approximately 200-300 cc/min methane to the Flo-Cal burner at a flow held constant over the entire measurement range of 0-1000 BTU/SCF or 0-2500 BTU/SCF. The calorific value of the methane auxiliary gas is set as instrument zero. This addition of approximately 200-300 cc/min methane to the Flo-Cal burner enables the Flo-Cal to reliably report flare gas heating values below about 200 BTU/SCF, thus, preventing Flo-Cal flame out conditions should the flare gas become inert.