Things to Keep in Mind When Choosing Valves
(P&GJ) — Oils processing occurs in multiple stages and requires specialized piping systems to handle different fluids. There are dedicated piping systems that transport crude oils, natural gases, liquids and semisolid materials. Service fluids will exhibit diverse physical and chemical characteristics at every stage. The design and size of pipeline components change as the fluid moves from one process to the other.
Oils and their products are explosive, with traces of hazardous gases. As such, valves and other components of the piping systems must provide leak and emission-free transportation of fluids. Valves undergo stringent testing that adheres to American Petroleum Institute (API), American National Standards Institute (ANSI), International Organization for Standardization (ISO) and Manufacturer Standardization Society (MSS) standards.
These tests are vital for defining the temperature and pressure limits and the leakage classes of valves. There are traces of hydrogen sulfide, carbon dioxide, chlorides and water present in crude oil. Gas traces convert into weak acids that corrode internal pipe components. Other oil applications operate at low temperatures, requiring the use of cryogenic valves.
The complex nature of oil applications demands that valves meet the highest industry standards. What factors should engineers consider when choosing valves for oil applications?
Valve Actuation
Oil pipelines have different flow control requirements. Piping systems rely on valves to start or stop the flow and vary flow rates and control direction. Some of these processes occur frequently and demand high levels of precision, while others operate for long without alteration.
Frequently cycled applications require valves that respond fast to flow control signals. The flow control mechanism must move from close to open, or vice versa, within the shortest time. These applications use actuated valves that receive power from electrical, hydraulic or pneumatic sources.
Actuators increase the response time of the valves and enhance the accuracy of critical oil processes. Always consider the torque requirements for actuating valves. Large valves require more force for actuation, increasing the energy consumption and operating costs. Pneumatic actuators are preferable for most oil applications because of their quick response. They enhance the precision of multiple oil processes.
Applications where flow conditions remain constant for an extended period feature manually operated valves like gate and globe valves. If the flow rates vary from time to time and the application requires a quick operation, use actuated butterfly, ball or solenoid valves.
Companies in the oil and gas sector prefer to automate plants and actuate process valves, even if they do not require frequent operation. It enhances the accuracy and safety of oil applications.
Valve Material
Oil pipelines transport mixtures of gases, liquids and semisolids with complex chemical properties. Changes in the temperature and pressure conditions of the pipeline initiate chains of chemical reactions to form corrosive substances. As the fluid flows past valves, they corrode wetted surfaces. Semisolid particles present in crude oil accumulate around valves, increasing the probability of cavitation and choked flow.
The manufacturing of valves for oil applications requires corrosion-resistant materials. They should be chemically inert and provide defense against the abrasive action of service fluids. Preferable materials for valves in oil applications include stainless steel with 2% to 3% molybdenum content, Inconel, Monel and nickel-molybdenum-chromium alloys with high pitting resistance values.
Using molybdenum, Monel, and nickel in the manufacture of oil application valves increases the corrosion resistance and strength of steel alloys. Inconel enhances the performance of valves under extreme temperatures and mechanical stress, while high percentages of chromium improve the corrosion resistance of stainless steel alloys.
Most valve applications are long-term and should resist corrosion by the service media, extending their longevity, reliability and safety. The presence of hydrogen sulfide in oil applications increases the corrosive effect of service fluids. Valves for these applications must comply with National Association of Corrosion Engineers (NACE) standards. NACE-compliant valve materials have high resistance to corrosion and sulfide stress cracking (SSC) and are suitable for both upstream and downstream applications.
Pipeline Maintenance
Oil pipelines require continuous inspections to evaluate their structural strengths and identify weaknesses. The same applies to valves, actuators and pumps. Pipeline maintenance involves the replacement of damaged pipe sections, pipe hangers and supports.
Performing maintenance requires the isolation of pipeline sections. The valve must be strong enough to stop the flow of fluids for a predetermined amount of time. The design of the valve end connections determines how fast maintenance teams can disassemble pipes and fittings. Disassembling valves with flange end and socket connections requires little effort, compared with butt- and socket-weld counterparts.
Internal pipe walls of some oil applications require cleaning using pipeline intervention gadgets (PIGS) occasionally. Ensure that valves for such applications provide full paths for the movement of PIGS. Where internal pipe cleaning is mandatory, choose a piggable valve, like full-port ball valves.
Butterfly valves will complicate internal pipe maintenance interventions. The valves and their actuators should be easily accessible and have minimal requirements for maintenance or replacements. Therefore, the valves should contain only a few internal mechanical components.
Leakage Classes
Oil pipelines traverse expansive areas, significantly impacting ecosystems. Their designs must conform to multiple statutory regulations. Pipes, valves, fittings and other components should not emit the service fluid, process gases or wastes to the environment without proper treatment or control.
Valves for oil applications must provide sufficient sealing to prevent service fluid leakages and fugitive emissions that can explode under elevated temperatures or cause harm to the ecosystem. Valves for oil applications need to conform to API 598 standards.
The predominant leakage classes for oil application valves are Class IV and Class VI. The valves use materials of suitable hardness, corrosion resistance and grain composition to facilitate fluid flow under high pressure and temperature.
Temperature, Pressure
Oil application processes operate at extreme temperatures and pressure. Process pressure can exceed 1,400 psi (97 bar) while the temperature can go beyond 7,232 degrees F (4,000 degrees C). Pipelines for processing natural gas can operate at subzero temperatures.
Since oil pipelines are severe service utilities, the valves need to sustain the extreme operating ranges without failure. Wall thicknesses of the valves must contain the stress exerted by the fast-flowing service media. The end connections, bolts and sealing materials must sustain massive temperature variations.
The stems for these valves should be internally loaded, with adequate blowout-proof mechanisms to prevent explosion-related valve accidents. For cryogenic oil applications, the valve material must permit the flow of fluids without undergoing structural damage or contraction under the subzero media temperatures.
Pressure Drop
Flow rates of a piping system determine the limits of pressure losses across valves. Since the operating conditions of the pipeline vary over time, valves must provide predictable performances irrespective of fluid flow changes. The valve should have a high-pressure recovery factor to prevent turbulent flow in the subsequent pipe sections.
The desired pressure drop requirements across the piping system determine the valve trim style and the type of manufacturing materials to use. The intensity of pressure drop affects the selection and sizing of pumps. As the pressure drop across valves increases, energy requirements for pumping the service media increase. The demand for more energy pushes the pumps to optimal operating levels. It may cause overpressurization of the pipeline, leading to the weakening of seals.
Viscosity of Fluid
Naturally, fluids resist movement. Oil applications deal with viscous fluids that present several challenges when sizing valves. When fluid passes through valves, it exerts frictional resistance against internal components, causing infinitesimal surface corrosion. After several cycles, the valves lose the strength to control the flow of fluids.
Valves in oil applications are prone to clogging as the valve acts as a constriction, limiting the flow rates of the pipe system. The size of the valve’s bore plays a crucial role in permitting the flow of fluids. Less energy is required to pump viscous fluids through a full port valve than one providing a partial port.
Most oil applications operate under extreme temperatures and pressure. These applications require reliable valves that can support the desired system flow rates, prevent leakages and guarantee the safety of processes. Globe, gate, plug, needle, butterfly and ball valves are high-pressure valves suitable for the different categories of pipelines in oil and gas applications.
Ball valves are a popular choice for oil applications because of their internal construction and performance characteristics. A ball valve consists of a valve body with a rotatable ball.
The ball has a bore through its center, which is either a standard port or a full port. The bore of a full port valve has almost the same diameter as the pipes connecting to it, while the standard port is closer to a pipe one size smaller.
Full port ball valves induce only tiny pressure drops, making them reputable flow control devices. Ball valves use a simple and effective design that requires minimal maintenance interventions, improves durability and enhances flow control characteristics.
