Due to more stringent methane emission regulations, natural gas-powered level control systems in oilfield separators are no longer practical. Thankfully, many of these separators can be retrofitted easily with new electric level control technologies, as discussed in my article, titled “Improved oil separator control strategies,” in the October 2023 edition of Process Instrumentation. Please find a summary of the article below, and the entire article can be viewed by clicking on the link above.
Oilfield separators are passive devices that separate mixed-phase feed streams into their oil, gas, and water components for further processing and transport.
A typical oil separator uses a level transmitter and an oil outlet valve (lower right) to control oil level, and a level interface transmitter and a water outlet valve (lower left) to control the oil/water interface level.
Maintaining appropriate oil and oil/water interface levels is necessary to ensure that oil, water, and gas are completely separated, and then exit the separator only via their designated outlets.
Historically, oil separator systems often used readily available natural gas pressure to power level transmitters, controllers, and valves, but recent emission regulations have eliminated this option.
One common low-cost solution is replacing existing equipment with point level switches and electric valves, which alternately and frequently open and close to keep level within an acceptable range—but this continuous cycling wears out valves quickly, resulting in high maintenance and replacement costs, lost production, and potential downstream impacts.
Compared to level switches, level transmitters are much better suited for this application because their output varies with level, and it can therefore be used to drive a control valve. This methodology provides steady-state control with limited required valve movement, extending valve life and using far less energy.
Of the level measurement technologies available, guided wave radar transmitters and displacer transmitters are both effective, but typical designs require substantial modifications to the existing separation vessel. Meanwhile, differential pressure transmitters tend to plug over time.
Oil separator level technologies can employ guided wave radar (left) or DP transmitters (right). Guided wave radar usually requires vessel modifications, and DP transmitters are prone to plugging problems.
The Fisher L2t Liquid Level Controller is a new, cost-effective solution designed for easy installation in existing vessels, and it solves the problems facing retrofit design teams. Using proven displacer technology, the device is equally suited for measuring oil or interface level.
The recently introduced Fisher FIELDVUE L2t Liquid Level Controller is a cost-effective option that can be installed through a 2-inch hole in the side of a horizontal oil separator vessel, or through the top of vertical vessel. The transmitter uses displacer technology, which performs well in these types of applications, where plugging and coating often affects other technologies.
In typical level measurement applications, a separate controller is used to maintain a precise setpoint, but an oilfield separator does not require such precision. Calibrating the L2t Liquid Level Controller across a relatively short range and wiring it directly to the valve is sufficient to keep level within a narrow range, and to establish a steady-state equilibrium of inflow and outflow.
The ability to easily replace the level transmitter and eliminate the controller can make oil separator retrofits much less costly. The new arrangement also saves significant energy and control valve maintenance cost by minimizing valve movement during operation.
An operator in the Marcellus Shale area of the U.S. using differential pressure transmitters was having problems with plugging. After upgrading to the L2t Liquid Level Controller solution described above, the oil separator controls now operate reliably, with minimal required maintenance expected.
In western Canada, two operators decided to implement this same solution after experiencing the ongoing maintenance pains of valve cycling in systems based on level switches. The retrofit dramatically reduced service efforts and costs going forward.
The L2t Liquid Level Controller solution is straightforward and inexpensive to install in existing oil separator vessels, and it significantly reduces maintenance and lost production costs, especially compared to level switch solutions. It is also emission free, a significant improvement over systems using natural gas as a motive force.
Visit the Fisher FIELDVUE L2t Liquid Level Controller section on Emerson.com to know more about this environmentally conscious solution with zero emission ideal for level on gas separators and scrubber applications.
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