A gas fired, cogeneration plant located within a refinery in Germany used a boiler feedwater pump to provide 1,000 m3/h (4,400 USGPM) of water, with a head of 1,355 m (4,450 ft). The pump was set up at a fixed speed operating at 2,980 rpm and required a 4.1 MW (5,500 hp) motor to power it. Since the original installation of the pump, the customer’s production cycle had changed significantly and the pump needed to run on partial load due to changes in power demand.
In order to meet the required variable flow of between 500 m3/h (2,200 USGPM) and 1,000 m3/h (4,400 USGPM), the power plant was using a valve at the discharge to throttle the flow. This meant that the generated head was being throttled and the energy and cost for creating it was wasted. This incurred inefficiency could be avoided. In order to improve the efficiency of the feed pump, it was necessary to modify its operating range by configuring a speed control mechanism.
Initially, the customer considered two more conventional options: a variable frequency drive and a hydro dynamic speed coupling. However, both of these alternatives had a number of disadvantages: chiefly the size, inconvenience and cost of installation for the medium voltage variable speed drive (VSD) and the inherent efficiency losses of the coupling. These two options did have advantages though, the VSD offered good energy efficiency and the coupling was compact and relatively easy to fit, sitting between the main motor and pump.
Sulzer proposed the use of an innovative third option, one that was developed for the renewable power industry and delivered the benefits of both alternatives and none of the negatives. The variable speed electro-mechanical drive was offered a compact, convenient solution that could be installed between the motor and pump and was extremely energy efficient, even more so than the large VSD.
For this particular application the combination of a variable speed drive and a mechanical geared assembly would prove to be the ideal solution. The CONTRON electro-mechanical drive train allows the main motor to remain mounted in line with the pump, but uses a planetary gear arrangement driven by a high power servo motor and variable speed drive system as an override that takes over progr speed drops.
