Progressing Cavity Pumps Simplify Crude Transportation

For more than six decades, a Canadian energy company has been delivering energy, including oil and gas, across North America. A large part of the company’s business is the operation of the world’s longest crude oil and liquids transportation system, conveying crude oil and other liquid hydrocarbons from the point of supply to refining markets in the midwestern U.S. and eastern Canada. This company had two goals for its pumping system. First, it wanted to streamline the system for its waste oil storage tanks. Second, the company needed a system to improve its diluent blend into heavy crude before the product joins the main pipeline for distribution. The energy pipeline company asked a pump manufacturer to design and build customized pumping solutions for greater operational efficiencies and lower maintenance.

Waste Oil Storage Tanks

This energy company relies on pumping stations to power the liquid fuels through the pipeline. At each pump station, a 5,000-gallon (19,000-liter) underground storage tank is buried below the frost line to prevent the liquid from freezing. Each tank is used as a collection point for waste oil from service work performed on the main pipeline pumps. After completion of the service work, the liquid must be pumped from the storage tank back into the pressurized pipeline. This process had previously consisted of a two-pump system with a cantilever pump used to lift the waste oil out of the tank and a high-pressure piston plunger injection pump used to move the oil back into the pipeline. “The main issue for the end user is that two pumps for each storage tank means twice the maintenance,” the consulted pump manufacturer’s engineer said. “Additionally, with the high pressure required to operate the piston pump, pulsation was causing pipe stress. The energy company wanted to eliminate the pipe stress and simplify the system with one pump that could provide smooth, almost pulsation-free conveyance.”

A Customized Solution

After several meetings with the energy company to understand the process and the challenges of their two-pump system, the pump manufacturer’s engineers designed a custom 10-stage vertical, semi-submersed, progressive cavity sump pump that can lift the heavy oil out of the tank and that has the capacity to achieve 700 pounds per square inch (psi) (48 bar) of differential pressure if the system required it in an upset condition. The pump manufacturer addressed many technical issues, including the use of a mechanical seal, to the energy company’s specifications. A common pump length with a drop tube and strainer was specified to accommodate any changes in sump depth. The pump manufacturer also performed chemical compatibility tests on several different elastomers. The unique pump selection allowed the manufacturer to eliminate universal joints by incorporating a flexible connecting rod. Design engineers also considered the electrical service at the site by selecting a 20-horsepower driver that corresponded with the existing two-pump electrical arrangement. The housing was designed to accept the user selected mechanical seal with the capacity to operate at 700 psi (48 bar). The selected pump was ideal for this application, with dimensions (length, height, mounting flange) to fit the energy company’s existing underground tank. This pump also operates in reverse, requiring the seal to be on the discharge side and the entire housing to be pressurized. The rotor and stator system is located at the lowest point, which helps prevent dry running. The space-saving design of the entire pump (except for the drive and discharge flange) disappears into the tank, providing a clean installation. Additional heating and isolation of external equipment is unnecessary—an important attribute in ambient operating temperatures as low as -40 F. With the critical operating nature of the oil and gas transport industry, the energy company expects high-quality control requirements. Before placing the order, the energy company sent a team to the pump manufacturing plant to verify that the manufacturing processes would meet its high-quality standards. The selected pumps are produced in accordance with American Petroleum Institute (API) 676 and shaft seals in accordance with API 682. To date, more than 20 vertical pumps are in operation with no field issues.

Diluent Injection Control

This same energy company finished an infrastructure project that included the construction of 15 new storage tanks ranging in size from 250,000 to 530,000 barrels. This project also encompassed all associated piping, manifolds and booster pumps to facilitate the crude oil transfer to and from the storage facility, the mainline piping system and other connecting carriers and terminals. With a goal of reaching the highest operational efficiencies at the storage facility, the energy company wanted a pumping system to blend the heavy crude oil from the tanks with diluents to lower the viscosity of the crude to be pumped to the main pipeline for distribution. The energy company conducted an in-depth evaluation of pumping systems. It originally settled on a multi-stage canned pump for this application but still had reservations about viscosity and injection control. For this pump type to work, it would need to operate at full speed and capacity and modulate the injection flow through a control valve. The irregular viscosity of the product through the drawdown of the storage tank could cause problems. As the crude oil sits in storage tanks, lighter product moves to the top, and the heavier product moves to the bottom. “Crude oil is drawn off the bottom of the tanks first, which is where most of diluent injection is required,” said Luke Bauer, a senior application engineer with the pump manufacturer. “As the product’s viscosity changes, the amount of diluent being used must be adjusted. We approached the design of this system with progressing cavity pumps to take care of operating issues associated with constantly changing viscosity.” The extremely cold operating temperatures during the winter months was a critical design issue. With ambient conditions of -22 to -40 F (-30 to -40 C) and diluent temperatures as cold as -8 to -13 F (-22 to -25 C), the pump manufacturer’s engineers carefully considered the elastomer selection for this application. Because the progressive cavity pumps would operate outdoors, a patented reduced wall stator was selected with heat tracing to ensure that a 23 to 32 F (-5 to 0 C) operating temperature was maintained within the elastomer. Additionally, three resistance temperature detectors (RTD) were mounted in each pump to provide feedback data to the control system. The chemical compatibility of the diluent also presented a design challenge. “Two different diluent samples were given to us, and we did a lengthy immersion test with several elastomers at different temperatures to provide a better picture of how our elastomers would react,” Bauer said. “After extensive evaluation and swell testing, a special blend of Viton was designed and produced for the stator and seals that would be used.” To compensate for the cold temperatures, the pump manufacturer designed a slightly oversized rotor to maintain the interference fit at sub-zero temperatures. This presented an additional hurdle at assembly time. The stators were packed with ice to fit the oversize rotors during fabrication. Other custom design features were required as well. The energy company requested that the pumps be mounted on a metal grid platform because the standard baseplate for these pumps would not provide enough support. The pump manufacturer designed a new baseplate incorporating a reinforced 8-inch I-beam with a drip pan and lifting supports. The energy company also wanted a pump housing with added connections for a relief valve return and other monitoring equipment to be installed on-site. The pump manufacturer designed a spool piece to attach to the pump housing inlet, providing the extra connection. By using four custom-designed progressing cavity pumps piped to a common header, the pump manufacturer provided the energy company with the required level of diluent injection control. The energy company can run any combination of the pumps and, with the use of variable frequency drives, they have complete control over diluent injection. The large pump size also allows the energy company to run the pumps slower for a longer life cycle.

Long Term Solutions

Both pumping issues for this energy pipeline company were solved. With many of these pump stations in remote locations, the major benefits include lower operational cost and less field maintenance, especially in the extremely cold operating environment. All of the pumps have been in operation since 2010 without the need for maintenance or service work.