May 15, 2007

Focus on O&M (May 2007)

Not a grate design

Montenay's existing hydraulic system was designed in Germany and uses a main power unit with 24 submerged gear pumps (Figure 5). The gear pumps set the flow rate for multiple sets of identical cylinders that feed the trash into the boiler by controlling the movement of grates within the boiler. The grates are moving plates inside the boiler that shuffle the trash along as it burns.

5. From the Old Country. Montenay's old hydraulic system was designed in Germany and uses a main power unit with gear pumps stacked together and submersed in tanks. The design makes the pumps difficult to access and repair. Courtesy: Bosch Rexroth Corp.

One problem with the old design, as Polidore knew and Metz quickly discovered, was that its fixed-displacement pumps move the grates and then stop them for variable periods of time, depending on process conditions. When the grates are stopped, weldments often occur that cause them to seize. After enough seizures, a grate must be taken out of service and fixed. Another problem with the design is that the gear pumps are stacked together and submerged in a tank, which makes them difficult to access and repair (Figure 6).

6. Six of twenty-four. The old system had 24 submerged gear pumps that set the movement rate for multiple sets of identical cylinders. Courtesy: Bosch Rexroth Corp.

The solution that Metz proposed to Polidore called for the replacement of Montenay's 24 gear pumps with four Bosch Rexroth A10VS0-DR size 18 pumps, which are pressure-compensated. A controller maintains a constant pressure within the range of the pumps, so the pump supplies only the amount of hydraulic fluid required.

The pumps then pass the flow through Bosch Rexroth Model 2 FRE 6 proportional-flow control valves. Proportional flow control valves are two-way valves that allow an electrical signal to control the fluid flow independent of pressure and temperature variations. Each valve assembly comprises a proportional solenoid with an inductive positional transducer, a metering orifice, and a pressure compensator. The valves are controlled by Bosch Rexroth VT 5010 electrical amplifier cards with electrical position feedback. In Conshohocken, the cards are managed by the plant's existing programmable logic controller.

Polidore accepted Metz's proposal and Bosch Rexroth assembled a temporary unit at its facility and delivered it to Montenay as a complete power unit for testing. Airline Hydraulics Corp. (www.airlinehyd.com), a Bosch Rexroth distributor in Bensalem, Pa., helped test the power unit as it operated the entire feed system for one boiler, to prove that it could control the cylinders using the Model 2 FRE 6 valves and VT 5010 driver (Figure 7).

7. Test before ship. Airline Hydraulics Corp. helped Bosch Rexroth test the new power unit by having it operate the entire feed system for one boiler, to prove that it could control the cylinders. Courtesy: Bosch Rexroth Corp.

"Working together, all the components in the Bosch Rexroth hydraulics package have ensured cylinder synchronization," says Bill Hludzinski, an Airline Hydraulics sales engineer.

A final tweak

Once the testing was complete, Montenay asked Bosch Rexroth and Airline Hydraulics to redesign the entire power unit for each boiler. The unit now has four top-plate pump motor groups, each powered by Bosch Rexroth A10VSO pumps and new electric motors. Previously, it took six gear pumps to power one electric motor (Figure 8); now it takes just one. Along with supplying and installing manifolds, Airline Hydraulics and Bosch Rexroth were responsible for integrating the new unit into Montenay's existing system and field piping. That completed the retrofit.

8. New pumps for old. The new gear pump design reduces from six to one the number of gear pumps per electric motor. In an operating plant, the unit is positioned upside-down, with the motor on top and the pump hanging down submerged in the fluid tank. Courtesy: Bosch Rexroth Corp.

Polidore was responsible for writing the front-end software program that converts the internal boiler conditions to signals that enable the hydraulics to react to changes in them. "The hydraulic system controls the motion of the boiler's in-feed as a function of parameters such as temperature and pressure, explains Polidore. Metz adds that the pressure-compensated pump and the pressure-compensated proportional valve control the hydraulic flow throughout the cylinder cycle to achieve optimum feed rates and combustion quality.

Montenay operated the temporary test unit during the entire changeover process because, as Polidore says, "The plant cannot afford downtime, and the trash never stops coming." He says Montenay manages a facility in California where engineers are planning to upgrade their in-feed systems with the new Bosch Rexroth hydraulics design.