Striking the balance between efficiency and performance in changing operating conditions.
by Paul Fulghum
August 29, 2017

Pipeline operators are responsible for transporting billions of gallons of oil every year, and they are constantly looking at ways to improve efficiency and reduce costs. Depending on fluctuating network demands, this might require managing an increase or decrease in flow rate in the pipeline, a change in pressure or simply a pump upgrade to more efficient technology. So how do you change pipeline flow and pressure performance while maintaining efficiency and reliability?

From an engineering perspective, the process performance required from a pipeline can seem to change all too frequently. Veering away from the original pipeline pump design criteria makes keeping the “right fitted” pump for the current set of conditions a challenge even for the most dedicated engineer.

There can be many reasons for needing to rerate pipeline pumps: increasing throughput of deliveries is the most common requirement, but often the pipeline was overdesigned in the first place and now flow rates are less than the original design specification. Pipeline pressure derating can also be the catalyst behind a rerate or simply the need to improve or restore pump efficiencies and reliability in order to reduce power costs and improve run time.

Predicted performance curveFigure 1. Predicted performance curve (Images and graphics courtesy of Sulzer Pump Services)

The task of redesigning an entire set of pipeline pumps can be daunting. It may span several budget years, or for the more demanding rerates have a completion deadline only months away. The key to managing a successful project is first to determine if and how the user’s hydraulic requirements can be achieved. Typically a large team of qualified and experienced field engineers is then required, supported by a substantial service and engineering capacity dedicated to the specific requirements of the pipeline pump.

Project Request

A recent commission is a good example to illustrate how each project brings its own challenges and how good organization and resources are essential to stay on track and on budget.

This project began with a phone call to a field engineer. The end user was contemplating a 20 percent increase in flow rates in his pipeline and wanted to make the change with the existing pumps and motors without exceeding or moving the existing station horsepower. The end user also wanted to deliver four different grades of crude oil.

Field performance test curveFigure 2. Field performance test curve

Clearly, this project would be a challenge to not only accomplish the end user’s request but also to accomplish it in a manner that produced pumps that would operate reliably, efficiently and give the end user a normal service life.

Pump rotor before modifications.Image 1. Pump rotor before modifications.

Normally, pump hydraulic design occurs so that impeller and volute casing design are optimized, allowing casting, machining and assembly to be accomplished in a cost effective and efficient manner.

These optimized designs are then rigorously factory tested to prove the designs. Once proven, these pumps become part of a pump company’s catalog of pumps that usually encompass an assortment of pump types covering a variety of flow ranges and differential pressures, creating a range of off-the-shelf pumps.

When a rerate of one of the pumps occurs, it is typically out of the range of operation originally intended for the pump by the manufacturer. What has taken years to engineer and prove by testing now has to be repeated and condensed into a very short period of time. This is where a manufacturer’s experience, knowledge and network is a significant advantage for a pipeline pump rerate project.

Design Strategies

Within a few weeks, the pump manufacturer’s advanced engineering group had developed options for the end user. After careful consideration and communication, a strategy was developed and the project was started.

The key was the hydraulics, and designs were developed that modified the impeller and volute of the pump so that the specified flow rate and differential pressure could be achieved. Impeller and volute modifications also addressed concerns about vibration and cavitation at the increased flow rates.

Pump rotor after modifications.Image 2. Pump rotor after modifications.

The pump manufacturer developed a plan whereby one of the user’s spare pumps was reworked at the service center, thoroughly tested and shipped to site for installation in a live position. The pump that was removed was then shipped back to the manufacturer for the next hydraulic retrofit. This continued until all the rerates were installed.

As with any project, there are always challenges. During phase two of the project, a complete installation service was provided. Normal commissioning services were expanded to include foundation prep, grouting, leveling, lube oil system flush, motor testing, pipe strain and final alignment. A project management team was formed, drawing on experienced project managers, engineers and technicians to complete the pump installations.

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