Some explanation is in order when speaking of pipeline gate valves. Gate valves used in this service are different from the wedge-type gate valves common in the downstream petrochemical and refining industries. The pipeline gates come in two basic types: slab and expanding wedge. The slab type utilizes a large slab that floats slightly in the valve body and seals downstream with the aid of upstream pressure. Spring-loaded seats are often employed to increase the sealing efficiency. The expanding gate, on the other hand, uses a split-disc design and separator mechanism that tightly expands the gate both upstream and downstream as the valve is closed. This type then reverses the process upon opening. The tighter closing design enables the valve to seat more effectively at lower pressures.
A QUESTION OF INTEGRITY
Valve integrity along with pipeline integrity is of prime importance to the pipeline owner as well as those who live and work close to the line. A complex formula for risk assessment is used to guide pipeline operators with inspection programs. The assessment criteria include the product, age of the pipeline, and proximity to population centers, local housing and occupied structures. The pipeline itself must be inspected at specified intervals. This line inspection is usually performed by “smart pigs,” complex devices that roll through the line to perform radiography, remote visual, ultrasonic evaluation and other inspections.
Valves, on the other hand, need their own inspection programs. The U.S. Department of Transportation has developed natural gas pipeline valve inspection criteria detailed in CFR Title 49, part 192, “The Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards.” Paragraph 192.745 of that title states, “Each transmission line valve that might be required during any emergency must be inspected and partially operated at intervals not exceeding 15 months, but at least once each calendar year.” Similar requirements are published for crude oil and hazardous liquid pipelines in CFR Title 49, part 195, “Transportation of Hazardous Liquids by Pipeline,” paragraph 195.420.
Proper valve maintenance is always vital, and pipeline valves are no exception. Since most pipeline valves have a seat sealant injection feature to facilitate tight closure, the sealant must be properly introduced into the seat seal area. New valves typically require more sealant top-off than those that have been in operation for a year or two.
All pipeline operators have preventive maintenance (PM) and repair programs to ensure the life and functionality of their valves. Most companies will use a combination of in-situ repair along with shop refurbishments for tough repair cases. “We spend over 25% of our time in valve shops to get the valves just like we want them,” says MPL’s Daigle.
Because of the importance of proper pipeline valve repair, a specification that describes the repair procedure is in place: API 6DR, “Repair and Remanufacture of Pipeline Valves.”
HOW PIPELINES WORK
Understanding how pipelines operate provides a better understanding of how valves are used in pipeline service. Major pipelines receive input from either smaller gathering lines, tank farms or, in the case of finished products, refineries and petrochemical plants. Because of friction losses, the arriving pressure of the fluid is much too low to provide enough energy to send the product very far through the line. Most transmission pipelines in the United States operate at maximum pressures of less than 1440 psi. Common maximum target pressures range from 700-725 psi and 1300-1400 psi, which equates to ANSI classes 300 and 600 respectively. These maximum pressures would only be found immediately downstream of pumps or compressors.
Because of the pressure drop in the line, booster pumping stations at intervals along the line are needed. In the case of a liquid such as crude oil, a minimum pressure of about 25-50 psi is needed for the suction side of the booster pumps to operate. Each booster pumping station is equipped with manifolds containing many valve types, including gate, ball, check, and in areas where pigging is not required, reduced port, lubricated plug valves. Additionally, control valves often are used to regulate flow from the stations.