Improved Valve Control Can Significantly Reduce Methane Emissions
Submitted by Rotork Instruments
on Wed, 08/14/2019
on Wed, 08/14/2019
IntroductionThe natural gas industry commonly uses pneumatic devices powered by pressurized natural gas as valve controllers and pressure regulators. However, these devices are one of the main sources of vented methane emissions in the industry, with 51 billion¹ cubic feet released per year in the production sector alone.
These levels can be reduced or even completely eliminated by replacing high bleed and low bleed devices by retrofitting existing natural gas pressure regulators. Reducing the number of components in a system and improving maintenance practices can also be profitable.
Data from the United States Environmental Protection Agency shows approximately 400,000 pneumatic devices are used in the production sector to control and monitor gas and liquid flows in dehydrators and separators, temperature in dehydrator regenerators and pressure in flash tanks. Approximately 13,000 gas pneumatic devices are used in the processing sector for compressor and glycol dehydration control in gas gathering/booster stations and isolation valves in processing plants. In the transmission sector an estimated 85,000 devices actuate isolation valves and regulate gas flow and pressure at compressor stations, pipelines and other storage facilities.
Electric actuators and motorized regulators offer a number of benefits to reduce methane emissions, even enabling the design of zero emissions systems.
It is normal for pneumatic devices to bleed some natural gas into the atmosphere and so they are a major source of methane emissions from the natural gas industry. The actual bleed rate or emissions level mostly depends on the device’s design. According to the Natural Gas STAR Program (an emissions reduction framework for partner companies with oil and gas operations in the US), a high bleed pneumatic device is any which bleeds more than 6 standard cubic feet per hour which equates to over 50 thousand cubic feet each year.
The above picture shows a schematic of a gas pneumatic control system on a city gate station. These stations, located at points where gas is delivered from transmission pipelines to a distributor’s high- pressure lines, are where gas metering and pressure regulation is carried out. Clean, dry, pressurized natural gas is regulated to a constant pressure, usually around 20 psig. This gas supply is used both as a signal and a power supply. The control system processes the measurement of the process conditions and sends an electrical 4 to 20 mA signal to an electro-pneumatic control device (e.g. I/P Converter, Pressure Controller), where its variable pressure is used to control a valve actuator or a main pressure regulator. Pneumatic devices are available in three basic designs to perform specific roles:
- Continuous bleed devices – used to modulate flow, liquid level or pressure and generally vent gas at a steady rate.
- Actuating or intermittent bleed devices – used for fast-acting control and only release gas when either stroking a valve open or closed, or while throttling gas flows.
- Self-contained devices – release gas into the downstream pipeline instead of the atmosphere.
Alternative solutions can be offered to reduce or eliminate emissions either by using a standalone actuator to remove the need to pilot gas or an actuator combined with a precision pressure regulator. This can be fulfilled using either a Rotork PAX1 or PAXL electric actuator. Although both are linear, the PAX1 is uni-directional and designed purely for push action, meaning a spring or other device is required to provide the pull action when used on a regulator or small valve, while the PAXL is bi-directional and features an attachment which allows for both push and pull action with a non-rotating rod.
1. Motorized RegulatorsThese can reduce emissions by providing a zero gas bleed solution as an alternative to a standard electropneumatic controller.
This diagram shows a schematic of a gas pneumatic control system using a PAX1 motorized regulator. The regulator can be chosen as a non-bleed version, in order to avoid the continuous gas bleed as depicted in the first picture on this page. Then, the regulator would only require to vent gas when decreasing the set pressure.
Technical Advantages of Motorized Regulators
- Flexible operating modes – set point adjustment through the existing Analog Mode (4 to 20 mA loop), Pulse Mode to increase or decrease set points or Modbus® communication.
- No need for constant electric supply – power is only needed when a change to the set point is required, otherwise solar solutions are suitable.
- Inherent Lock in Last Place setting – as the PAX1 actuates a pressure regulator there is no need to maintain an electrical signal. Instead, control systems can be put on ‘sleep mode’ until a new change in set point is required.
- Choice of output pressure ranges – pressures range from 0 to 0.5 psig (0 to 35 mbar) to 0 to 3,000 psi (0 to 207 bar).
- No pressure reducing regulators needed – PAX1 actuators paired with Rotork’s Fairchild brand of regulators can handle higher input pressures compared to standard electro-pneumatic controllers.
- Explosionproof enclosure – FM, CSA and ATEX certified with IP68, 7 meters for 72 hours (PAX1) or IP66 (PAXL) ingress protection rating.
These can eliminate emissions by providing an electric, low power, direct operating device adapted to natural gas regulators, eliminating the need to pilot gas.
2. Standalone Linear Electric Actuators
The above image shows a schematic of a natural gas control valve being directly actuated by a PAX1 or PAXL electric actuator with the use of a PAX1 motorized regulator. Existing installations of natural gas regulators can be retrofitted and automated through adapting kits available for the most popular natural gas regulators.
Technical Advantages of Using a Standalone Electric Actuator
- Reduced components – PAX1 and PAXL solutions both directly drive the main regulator or valve without the need for pilot control.
- Safer operation – no venting of hazardous gases.
- Flexible operating modes – set point can be adjusted through the existing Analog Mode (4 to 20 mA loop), Pulse Mode to increase or decrease set points or Modbus communication.
- No need for a constant electric supply – the PAX1 solution only needs power when a change in set point is required, while solar solutions can be used.
- Inherent Lock in Last Place setting – there is no need to maintain the electrical signal as the PAX actuates either a pressure regulator or valve.
Using a standalone PAX actuator offers significant economic and environmental benefits with the potential for methane emissions to be reduced to between 45 and 260 thousand cubic feet per device per year. These reduced emissions can also lead to financial gains which can range from $225 to $1,300 per year per device¹, and in many cases the cost of implementation is recovered in less than 18 months. These two benefits combined with the retrofit or complete replacement of older units can provide better plant-wide performance and reliability to improve the monitoring of parameters including gas flow and pressure.