Productivity targets are a key performance measure in any process industry and the refining business is no exception. Increasing output from a large refinery to meet market demand presents challenges. However, dealing with the pumps does not need to be one of them.
Due to changes in government regulations over the past several years, the demand for ultra-low sulfur diesel (ULSD) has increased. As a result, refineries across the country have been forced to adapt and find innovative solutions to meet increasing capacity. A refinery in Texas found itself in this situation and began investigating possible solutions with the help of a service center.
The two pumps of interest were 6 x 8 x 10.5 BB5 10 stage units in diesel charge service. The American Petroleum Institute (API) BB5 is a barrel pump that encloses a multistage axial split inner bundle with an opposed impeller configuration (Image 1). Being that the pressure boundary is radially split, these pumps are typically designed for high-temperature and/or high-pressure applications. With the capability of handling pressures and temperatures up to 6,250 pounds per square inch (psi) and 800 F respectively, the API BB5 can be suitable for applications such as water injection, oil export, boiler feed and charge service.
The subject pumps were originally sold in 2006 at a rated point of 1,110 gallons per minute (gpm) at 4,014 feet. With the change in ULSD demand, the reliability engineers at the refinery wanted to increase the capacity to 1,628 gpm at 3,450 feet (Image 2). The goal was to find the most economical and timely solution to meet the needs of the refinery.
The Design Review
When in this situation, there are three possible options:
- Purchase a new pump that is designed to deliver the desired capacity.
- Hydraulically modify or “re-rate” the existing pumps.
- Operate the two existing 100 percent pumps in parallel (depending on system curve).
Each of these options have advantages and disadvantages. For example, parallel operation would be the least costly to achieve the increase in flow.
However, the lack of system redundancy in the event of a failure is risky and can often be costly in terms of lost production.
A new pump selection to fit the application could be advantageous with regards to efficiency. But long lead times, the cost to reroute pipework, and baseplate or foundation modifications may make this option less attractive.
If re-rating pumps, the desired performance may not be achievable in the given frame size. But if feasible, re-rating can often be quicker and more economical.
Based on the criticality of this service for the overall success of the refinery operations, the possibility of re-rating was of great interest.
Engineers conducted a thorough feasibility study both hydraulically and mechanically to determine whether the goal was possible. A specific speed (Ns) based search revealed an existing proven design that would meet the refinery’s needs—provided the impellers were able to physically fit, and the inner-case volute nozzle areas could be increased enough to allow the impeller to meet as-designed performance. A review of the volute development drawings provided confidence that both items could be fulfilled.
The approach was to keep it simple:
- Select an existing standard impeller hydraulic design that was suitable for the desired performance and had been proven by at least two factory tests.
- Ensure that case modifications could be made to fit the new impeller.
- Confirm the nozzle area could be increased enough to mimic the reference pump performance.
Ideally, a factory test would have been performed after completing this work, but unfortunately, the project’s timeline did not make this possible. It was important that the proposed upgrade was supported by internal empirical data and experience to provide confidence in the accuracy of the proposed design changes. It was necessary that the predicted impeller trim was calculated correctly on the first attempt.
The Optimized Solution
With an array of impeller hydraulic designs available, engineers were able to select an existing design that would meet the new desired head and capacity of the application.