Global Processing

Flare Gas Monitoring: An oil and gas company leaves nothing to chance when it comes to monitoring combustible toxic gases

July 11, 2003

A major oil and gas company in Mississippi needed to ensure the flow rate of combustible toxic gases in a vapor header line connected to its incinerator were measured accurately with a high level of repeatability. During the refining process, combustible toxic vapors result and must be captured for safe disposal. Incorrectly measuring the rate of vapor flow in the header line during the refining process could result in an improper mix of toxic waste gases that would fail to combust in the facility''s incinerator. If these gases are not properly incinerated, there is a risk of their accidental release into the environment resulting in costly plant closures.
The problem

The accidental release of combustible vapors and dangerous toxins into the atmosphere can result in emergency plant shutdowns. These plant closures, which are mandated by various government agencies, can be expensive and time consuming to remediate. The lost profit, production time, and the cost of restoration make this process critical to the plant''s safe and effective continuous operation.
At a refining facility in Mississippi, the company''s process engineers determined they were experiencing a highly variable vapor density. The vapors in the header line included a mix of paraxylene, benzene and other combustible gases. This condition led them to conclude that mass (rather than volumetric) flow rate was the most important parameter to consider in the incineration process and to assist in the plant''s mass balance calculation.
The variable flow rate in the header lines ranged from just 0.5 to 150 scfm. At the low end of the range (0.5 scm), the process required a highly sensitive mass flow sensor to recognize the presence of gas. The pressure range in the process varied from 0 to 50 psig at temperatures from 68 to 149 degrees F.
It is generally a challenge for process engineers to measure multiple variable gas mass composition accurately, which can typically require the use of multiple flowmeters. The cost of installing and maintaining multiple flowmeters, however, along with the cost of keeping different spare instrument items, led the plant''s instrumentation team to consider another option.
The refinery''s process engineers contacted a manufacturer of mass flowmeters to discuss the development of a highly intelligent flow transmitter to handle the task. There are two major types of mass flowmeters — Coriolis and thermal.
For this particular application, the instrument team chose thermal mass flowmeters, which apply the thermal dispersion principle and measure mass flow indirectly, which can require separate devices to measure process temperature and pressure along with a flow computer to calculate the actual mass flow rate.
These flowmeters also can be designed with no moving parts, which means they require little or no maintenance. Moreover, they have a broad flow range, detecting a very low flow rate to a much higher flow rate up to a turndown ratio of 1,000:1. For this reason, a single meter can be applied in an application to measure wide flow ranges reliably instead of applying multiple flowmeters in series to support an application over the same wide flow range.

The solution
In this application, the refinery process engineers chose a highly intelligent microprocessor-based flowmeter. When networked with a densitometer, this flowmeter accepts a process input signal and selects a flow calibration curve that precisely matches the waste gas vapor composition in the line connected to the facility''s incinerator.
For example, the instrumentation team at this facility set the densitometer cutovers at 35, 50 and 75 percent. The flowmeter automatically changes to a flow calibration curve stored in its intelligent transmitter that is appropriate to the gas composition at each of these different cutover settings. A signal is then sent via the plant''s distributed control system that adds the necessary amount of natural gas to the toxic gas vapors for optimal incineration.
In this application, the accuracy of the flowmeter was enhanced with the use of a tab-type flow conditioner. The flow conditioner, which is then inserted in the process piping, features a series of tabs that create a highly predictable flow profile with little or no pressure loss. The tabs create vortices that eliminate swirl and other process distortions that are often caused by inadequate straight pipe runs and other flow disturbances such as elbows, tees, valves, etc.
Flowmeters, such as the one chosen for the vapor header line application, support a broad variety of gas flow measurement tasks in the oil/gas, chemical, wastewater, general process and pollution monitoring industries. The flowmeter chosen by the refinery''s engineers is compatible with four popular communication buses: HART, Ethernet, Profibus and MODBUS.
The standard flow accuracy of the flowmeter selected is ±1 percent of reading ±0.5 percent full scale over a flow range of 0.25 to 1,600 sfps. It operates at pressures of up to 1,000 psig at temperatures ranging from -100 to 850 degrees F. Repeatability is ±0.5 percent of reading or better. A number of manufacturers offer instruments with similar performance characteristics.
The flowmeter chosen for the refinery application includes electronics that are addressable via a built-in LCD display and keypad or through its RS-232C serial port. The LCD and keypad allow the refinery''s process engineers to perform in-field programming to change zero, span, switch points and units to measurement or to perform instrumentation, verification, troubleshooting and other critical functions. The serial ports also support access to computers or ASCII terminals.
The flowmeter''s smart transmitter features two independent analog signal outputs that are field programmable. Analog outputs may be 4-20mA, 600 ohms maximum load; 0-10 Vdc, 5,000 ohms minimum load; 0-5 Vdc, 2,500 ohms minimum load; and 1-5 Vdc, 2,500 ohms minimum load.
Since the installation of the first mass flowmeter supporting the vapor header line application at this facility, the company has experienced no further problems with this process over the past year. In addition, the process team has installed seven more flowmeters on other vapor lines at the facility. The company plans to utilize this same solution at several other locations for similar applications.

Shawkat Alam is Product Development Manager at Fluid Component International. For more information, contact FCI at 800-854-1993 or email info@fluidcomponents.com.