Preventing or Reducing Pipeline Freezing with Chemical Injection
Wintry conditions present a unique set of challenges for oil and natural gas pipelines, with freezing posing a particularly serious problem. When pipeline freezing occurs, ice crystals can cause flow blockages and damage system components. Sluggish flow can reduce volumes traveling through pipelines.
To ward off freezing problems, many pipeline owners and operators use chemical injection methodologies, where a precisely controlled amount of methanol or similar chemical is injected into the pipeline or well site. The chemical works as an anti-freeze by reducing the dew point of the gas, preventing the forming of line blocking gas hydrates (ice formations that encapsulate gas molecules).
Petroleum pipeline freezing can occur in two primary ways. If water particles are present, ice can form in gas streams at temperatures below 32°F, the freezing point of water. Freezing can also occur with the formation of hydrates, which are a combinations of water and hydrocarbons that can freeze at temperatures above 32°F, depending on operating conditions and gas properties.
Pressure drops and flow restrictions can contribute to freezing. As temperatures drop, so do pressures. While a gas flow stream may operate normally with an internal temperature above freezing and an external temperature below freezing, the internal temperature could drop below freezing with a reduction in pressure. If the gas has any water vapor or condensate present, freezing can occur. Hydrates can form balls of ice that can damage or block pipelines.
Inhibiting Freezing with Technology
A variety of techniques have been used to prevent freezing and hydrate formation. One approach is to remove the free and dissolved water from the system with separators, glycol dehydrators, molecular sieves, or other methods. This technique may not be viable in remote locations, or where access is limited. Temperatures and pressures can be adjusted to prevent hydrates from forming and keeping all phases fluid. This often requires significant operational adjustments and ongoing monitoring.
Chemical injection can offer a dependable, measurable way to ward off freezing. Inhibitors can be injected into the pipeline or well site in specific concentrations to reduce the likelihood of freezing.
Chemical Injection Pumps
Using modern technology, chemicals can be injected and monitored precisely, often with remote equipment that does not require staff to be on-site. Chemicals are dispensed with positive displacement pumps designed to fit various pressure and flow requirements. Injections can be controlled by temperature, with pumps switched on at certain temperature and off at another temperature. Electric pumps are commonly used, operating off either AC or DC power. Solar DC power can be used in remote areas.
Pneumatic pumps can run off natural gas or compressed air. They can be used in applications that require hazardous location equipment, do not have accessibility to AC grid power or where solar power is not feasible.
Electric pumps can be powered by solar or conventional power sources.
Pneumatic pumps run off regulated natural gas or compressed air.
Accurate chemical dosing is critical to successful freezing inhibition. Under-dosing can limit effectiveness of the inhibitor, while over-dosing can lead to waste and excess costs. When sizing pumps, it is important to know what the volume and pressure output of the pumps should be as well as the chemical compatibility requirements of the seals.
Controllers Provide Brains
Controllers serve as the “brains” of chemical injection systems, and can be selected based on power requirements, levels of automation desired, and other factors. They can provide a simple time-based control for chemical injection, cycle control or an adaptive flow control process which improves accuracy and provides flow assurance. Remote system monitoring and control is also available, with automation features including: tank level monitoring, pressure, flow & temperature measurement and leak detection.
Controllers are available for both AC (left) and DC (right) power sources.
Complete Chemical Injection System
To complete the chemical injection system, other components could include a solar panel, stand, batteries, and charge controller if solar power is used. Other components could also include chemical storage tanks, piping, fittings, calibration column and various other accessories to complete the system.
A chemical injection system in use at an oil-field site
“How To Prevent Freezing In Gas Pressure Reducing Systems: Part One,” https://www.lowflowvalve.com/prevent-freezing-gas-pressure-reducing-systems-part-one/
“Freeze Protection for Natural Gas Pipeline Systems and Measurement Instrumentation,” Tom Fay, Welker, Inc. https://asgmt.com/wp-content/uploads/pdf-docs/2011/1/T06.pdf
“Preventing Formation of Hydrate Plugs,” PetroWiki, https://petrowiki.org/Preventing_formation_of_hydrate_plugs