Coordinated Motion, Concentrated Power

Drives Choreograph Motors for the Greatest Effect and Energy Savings. Here's How Experienced Users Gain Their Advantages

By Jim Montague

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It's all about being in the right place at the right time—even if it looks like you're just spinning wheels. 

In the case of motors and rotating equipment in process control applications, drives are what help them meet those appointments and get to the exact right positions. Lately, variable-frequency drives (VFDs) and variable-speed drives (VSDs) enable more precise motion and efficiency, but the evolution of drives isn't stopping there.

For instance, Daqing Petrochemical Corp's refinery in Daqing, Heilongjiang, China, produces 600,000 tons per year of gasoline, diesel oil, kerosene, paraffin and other petroleum products as a subsidiary of PetroChina Co. Ltd.. As a result, the crude oil pump for Daqing's 3500-ton, pressure-reduction process needs to maintain a steady flow at 9000 to 10,600 tons per day, but it consumes huge amounts of electricity, and required two full days of downtime and maintenance per year, costing more than $16,000 in lost production per day. This was mainly because the pump and its motor were always running non-stop and at full speed, and used a valve-throttle control method that was only adjusted periodically, causing increased pipe pressure, leaks and seal replacements.

To adopt more efficient pump controls, Daqing's engineers sought a medium-voltage (MV) drive solution, and recently implemented a PowerFlex7000 MV drive from Rockwell Automation. This drive has a 6-KV rectifier and a 6-KV inverter that can be built with fewer 6.5-KV components without an output transformer. It also uses current source inverter-pulse width modulation (CSI-PWM) to reduce its power devices, and it achieves this efficiency with over-current protection capabilities that don't require fuses and electronic fusing circuits to protect the power devices, which simplifies the drive's complexity.

By adopting PowerFlex7000 for pressure reduction, Daqing cut its power consumption for 9000 tons of output by about 41% from 426 KWh to 250 KWh, which saves 1.5 KWh per year. Likewise, for 10,500 tons of output, the new drive cut consumption by 32% from 585 KWh to 400 KWh, which saves 1.6 KWh per year. Also, the new drive safeguards the refinery's pump motors against over-currents, and saves about $15,000 per year in maintenance and downtime.

Grace Under Compression

Similarly, the Dez Gas natural gas collection and distribution facility, located in the Deir Es Zor desert region in northeastern Syria, takes previously flared natural gas from nearby oilfields, uses reciprocating compressors to boost pressure, and pushes it through a 250-km pipeline to the national grid. The facility includes five plants, each of which can process 20 million standard cubic feet (scf) per day.

However, because Dez Gas distributes directly without silo storage, its volume must be regulated based on actual consumer demand, which can vary considerably. And, since reciprocating compressors displace a constant volume of gas regardless of operating conditions, they must be configured to displace slightly more gas than required, so some excess gas still has to be flared. In gas plants, fuel-driven engines usually adjust compressor speed, but their disadvantages include limited process control and vibrations, which are a major problem on skids (Figure 1). Also, it's very difficult to cool this equipment in the desert's 52 °C ambient temperatures.

Consequently, Daz Gas evaluated and implemented ACS 1000 medium-voltage VSDs from ABB with help from Singapore-based integrator Gas Services International. These drives include three-level, voltage source inverters (VSIs), water and air cooling, and induction motors. By using VSDs to soft start the compressors, high starting currents, voltage dips and trips on other electrical devices on the same bus are eliminated. This smooth ramp-up protects the skids' mechanics, which reduce maintenance and prolong their lifecycles.

In addition, the VSDs eliminate vibration on the skids, while configuring them with 12- and 24-pulse rectifiers minimizes harmonics and allows gas pressure to be regulated according to demand without having to flare excess gas. Because there's no local water for cooling in the Deir Es Zor area, ABB also supplied chillers, which consist of a closed-loop, water-cooling system, enabling the VSDs to run in the desert heat.

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