Throughout the life of a pipeline, the process conditions can change annually from the original pipeline pump design conditions. This makes keeping the pump with the “right fit” for the conditions a challenge, even for the most dedicated of engineers.
Reasons for rerating pumps
There can be many reasons to rerate pipeline pumps. Increasing the throughput of deliveries is a common requirement. But often the pipeline was overdesigned and now the pipeline flow rates are lower than in the original design. Pipeline pressure deration can also be the catalyst behind a rerate. Another reason is the need to improve or restore pump efficiencies and reliability in order to reduce power costs and improve run time.
The task of redesigning an entire set of pipeline pumps can be daunting and may span several budget years. The more demanding rerates have only months to complete the process. The key to the success of the process typically begins when the Sulzer Pumps Equipment hydraulic engineering department determines whether the customer’s hydraulic requirements can be achieved. The Sulzer network of service centers, field engineers, field service technicians, and vendors then execute the project.
More throughput needed
One project began with a phone conversation with the Sulzer Denver service center field engineer. The customer was contemplating approximately a 20% increase in flow rates in their pipeline: “By the way, we want to do this with our existing pumps and motors without exceeding our existing station horsepower. Additionally, we want to deliver four different grades of crude oil.”
This project would be a challenge not only to accomplish the customer’s request but also to realize it in a manner that produced pumps that would operate reliably, efficiently, and give the customer a normal service life. The Denver field service engineer contacted Sulzer’s advance engineering department in Portland, OR, USA, to determine whether there was a hydraulic solution available for the customer’s rerating request.
Usually, pump hydraulic design is done that the impeller and volute design are “optimized” and casting, machining, and assembly can be accomplished in an efficient and cost-effective manner. The optimized designs are then rigorously factory tested to prove the designs. Once proven, these pumps become part of the company’s catalog of pumps. This catalog encompasses an assortment of pump types that cover a variety of flow ranges and differential pressures.
When a rerate of one of the pumps occurs, it is 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 is condensed into a very short period of time. This is where Sulzer’s experience, knowledge, and network are a significant advantage for a pipeline company’s pump rerate projects.
Hydraulic development for rerates
Within weeks, the advance engineering group at Sulzer had options for the customer to consider. After careful consideration and a good deal of communication with the customer, a strategy was developed, and the project was started. The key for the optimization was a new hydraulic design. The new design has a modified impeller and volute of the pump. The Sulzer engineers calculated a performance curve (see Fig. 1) for the pumps to be sure that — with the modifications — the specified flow rate and differential pressure could be achieved. Impeller and volute modifications (see details in Figs. 2 and 3) were also made to address concerns regarding vibration and cavitation at the increased flow rates.
Smart planning of rerates
The Denver service center developed a plan whereby one of the customer’s spare pumps was reworked at the service center and then shipped to the site for installation. The pump that had been removed was then shipped back to the Denver service center for the next hydraulic retrofit. This continued until all the rerated pumps had been rebuilt and installed.
Field performance evaluations
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, such a test can add significant costs and time to a project schedule. Particularly, if the pipeline is in service, customers do not want to lose time for testing. A field performance test is often a better solution and adds the benefit of testing the pump as installed, using exactly the material pumped by the customer.
In the abovementioned pump rerate example, field acceptance testing was provided by Sulzer Field Technical Services 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 (Fig. 4) were employed. These measurements confirmed that the pump rerate was performing as predicted and provided baseline data for future trending.
Organization of pump repairs
Overseeing the health of a large population of mainline pumps takes organization. A defined plan of repair action for each budget year ensures that the neediest pumps are addressed in order of importance or criticality. 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 about each individual pump.
Rerating pipeline pumps adds a level of complexity that will challenge a company’s management, engineers, and resources. Sulzer Pumps Equipment has many years of experience in this area and can draw on many resources within the company to make this effort a successful endeavor.
Sulzer Management AG