Rerating Pipeline Pumps

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.

Figure 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.

Figure 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.

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.

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.

Rerating

Pipeline hydraulic pump rerates are a theoretical construct in most cases. Site variables make it very difficult to be precisely sure how the hydraulic design will perform once in service. A pump performance test at the factory to prove the modifications can be helpful. However, with rerates, it can add significant costs and time to a project schedule. A field performance test is often better and adds the benefit of testing the pump as installed and with the pipeline product.

In the pump rerate example, field acceptance testing was provided in the form of a field performance evaluation. On three of the rerated pumps, actual field flow and pressure values along with vibration data and thermal images were all employed to confirm the pump rerate was performing as predicted and also provide baseline data for future trending.

Image 3. Completed pump installation

Around the world there are over one million kilometers of crude oil pipelines and at regular intervals there are pumping stations that are responsible for keeping the oil on the move. The pumps assigned with this task are designed and built specifically for this purpose, and in many cases each pump on a pipeline has a unique design that has been tailored to deliver the most efficient performance.

It is estimated that more than 60,000 pumps are employed on the world’s oil pipelines and they consume around 150 gigawatts every year, which is similar to the annual consumption of 20 cities the size of New York. It is therefore essential to fine-tune the performance of both the pumps and the prime movers, whether they be electric motors, gas turbines or diesel engines, to minimize running costs.

It is common practice within the oil pipeline business to request an original equipment manufacturer (OEM) alter the performance of a set of pumps, usually to meet increased demand. Many pump designs can achieve higher performance levels without changing the pump configuration, but this can often be at the expense of efficiency.

Image 4. Field performance test – thermal pictures

Efficiency figures from a recent pipeline project show that an OEM was asked to develop a flow rate of 291,000 barrels per day (bpd). This could have been achieved without changing any pump components, but efficiency would have dropped from 86.1 percent to 84.9 percent.

In many industries a 1.2 percent loss in efficiency may be acceptable, but when the pumps are powered by prime movers rated in megawatts, this small change in efficiency can equate to more than 220 kilowatts per pump. The number of pumps can vary enormously, but in this case the increased energy costs were multiplied across 24 pumps and would have amounted to several hundred thousand dollars every year. In this industry even small improvements in efficiency and performance can deliver significant improvements in operating costs and productivity. Examples also highlight the importance of accurate data and the need to monitor pumping equipment regularly to ensure that optimum performance is maintained.

Improving Flexibility

Oil pipelines may be required to change the commodity that they carry, for example from heavy to light crude oil, or to vary the flow rate during a typical day. Essentially, changes in density, pressure or flow rate may be required, and if the OEM is made aware of these requirements, allowances can be made in the design that enable these changes to be implemented more cost effectively.

Image 5. Field engineers are supported by substantial service and engineering capacity dedicated to the specific requirements of the pipeline pumps.

Pump designers can take into account the required changes in density, pressure and flow. This can include the use of low-capacity impellers and changes to the throat area of the pump, which has a major influence on pump performance.

Furthermore, a properly planned rerate program will also take into account the parts inventory that will need to be held by the pump operator. It is usually possible to allow mechanical seals and bearings to be interchangeable, which greatly reduces the cost of the parts inventory and helps to minimize the operational costs of the pump.

Overseeing the health of a large population of mainline pumps takes organization and a defined plan of action for each budget year. The pump repairs can run the gamut from correcting the hydraulics for the current flow rates to material upgrades, foundation repairs, seal and seal flush improvements to name a few. A pump census is strongly recommended with as much information as possible recorded about each individual pump.