Utility Fuel Switch Case Study

An east coast public utility decided to do a fuel switch recently by going from a heavier No. 4 fuel oil to a lighter and more environmentally friendly No. 2 fuel oil. The legacy power stations subject to this fuel switch were in tight urban surroundings, which limited the options to do new pump installations. The existing pumps, of which some were centrifugal pumps and other were twin-screw pumps, were to be replaced and the choice was to use three-screw pumps customized to fit existing facilities and foundations with a minimum of plant rework. Using the flow span of the three-screw pump using various speeds, the pump selection came down to one single pump size, which greatly simplified parts, operator training and service. To do the direct replacement of the old pumps, the new reengineered pumps were designed to be assembled and configured so they could be dropped in place with limited disturbance of the daily operations and limiting any outage to an absolute minimum.

Two installation options are addressed with the new pumps. In one case, the pumps are in a deep well configuration, mounted on top of a fuel tank including sole plate, riser piping, shafting and motor mount. The design (Image 1) is configured so that it can be installed (and pulled) in sections in order not to interfere with surrounding structures and equipment. Lifting and other servicing arrangements are designed to facilitate access and future service of the pumps. Not only are older twin-screw pumps, but also deep-well centrifugal pumps, being replaced by three-screw pumps which will simplify future operations and service to benefit of the operators.

Some older pumps in two other plants or power stations operated by the same public utility will be changed out as well to accommodate the switch to No. 2 fuel oil. They will service as fuel forwarding and fuel burner pumps, replacing older screw pumps. The installation of the pumps to be replaced are in very confined spaces where access is restricted or difficult. It was decided to go with three-screw pumps in vertical execution on a common base, including piping, to achieve a limited footprint and floor space. The limited access made it necessary to split up the pump skid into three sections, which will be assembled on site (Image 2). The pumps are configured in a backup mode, allowing service on one pump while the other is in operation. The pumps can operate in parallel to increase the flow when required.

A big consideration in older plants with old piping systems is the risk of releasing sediments and dirt that has settled in old pipes and valves. The No. 2 fuel oil could act as a solvent and release some of the sediments, which could potentially damage the pump. With this in mind, a smaller skid with large duplex basket strainers was included with the pump and pipe skid. A 40-mesh size was selected to protect the pumps.

Another interesting note is the increased pumping efficiency of the three-screw pump has allowed for a smaller motor, contributing to more than 20% savings in electric power.