Improving dual-drive fan configuration in a process plant

Plant availability and efficiency have become increasingly important as utility costs rise and profit margins dwindle. That’s why dual-drive equipment applications are rising in popularity in power generation, refineries, petrochemical, and upstream and mid-stream oil and gas facilities.

A dual-driven train improves overall plant availability by deploying an overrunning clutch which enables a driver to be disconnected from the train and stopped while the driven machine continues to be powered by a second driver.

There are economic, efficiency and flexibility benefits by doing so. With utility costs rising, the introduction of dual-driven capabilities enables a facility to switch between utility sources based on which is cheaper at a given time. Alternatively, the facility can have the option of using different drivers in order to gain higher levels of efficiency, lower maintenance costs or an increase in overall process output. By installing overrunning clutches between the drivers and driven equipment, greater operational flexibility can be achieved increasing overall system reliability and time between shutdowns for critical rotating equipment.

There are many examples of such arrangements around the world. This technology has been successfully implemented for decades. Those deploying such clutches in power and oil and gas applications have observed higher levels of reliability and efficiency.

Case in point

A process plant in the Middle East had been experiencing critical rotating equipment failures leading to plant shutdowns and major production losses. The source of the problem was the failure of sprag clutches, installed as part of a dual-drive operation established in the facility.

A sprag clutch is a one-way freewheel clutch that looks similar to a roller bearing. Instead of cylindrical rollers, non-revolving asymmetric figure-eight shaped components known as sprags are used. A sprag is an object that prevents something from moving that wants to move in a circular direction.

This sprag clutch arrangement works well in lower-power overrunning clutches but not in higher-power configurations such as those used in this facility. That plant has an induced draft fan installed with an electric motor and steam turbine, both of which can drive the fan.

The steam turbine often utilizes plant process steam as a means of increasing plant overall efficiency. The electric motor, on the other hand, is typically used as a backup driver should steam be shut down and during start up before the steam becomes available.

Keeping the markets and customers in mind, the plant could ill-afford unscheduled downtime. To increase reliability and prevent further clutch-related shutdowns, therefore, it retrofitted two overrunning clutches to operate each of its two 1.75 MW induced draft fans. It also ordered a spare for standby duty (though this has never been needed in a decade of usage). These clutches had to be rugged enough to withstand severe ambient conditions where metal surface temperatures could reach as high as 85ºC due to the intensity of solar radiation.

Known as a Synchro-Self-Shifting (SSS) Overrunning Clutch and supplied by SSS Clutch, it transmits torque through multiple gear teeth. Small pawls are used to mate with ratchet teeth to align and then shift the clutch teeth into engagement along helical splines. The teeth are engaged automatically at synchronism at any speed from rest to full operating speed. The pawls and ratchets are inactive except during the short engagement/disengagement process. Once engaged, torque is transmitted through the surface contact of involute shaped teeth. An internal oil dashpot, in-between the input and output components, cushions the engagement of the clutch.

The large induced draft fans in use at the facility mean that significant shaft movements require that the clutches are foot-mounted. These encased clutches have anti-friction bearings which enable the units to be self-lubricated.The clutch with a shaft lockallows the turbine to be maintained while the fan continues to operate when it is being driven by the motor. After many years of almost continuous operation, no further failures were experienced and the SSS clutches resulted in greatly improved plant availability and reliability.

Engage and disengage drivers

A dual-drive configuration that takes advantage of SSS clutches gives plants the ability to automatically engage and disengage two drivers. Far from being a new innovation, dual-driven configurations have been employed by a variety of power, refinery and petrochemical plant applications for around three decades.

The economics will, of course, vary with each location. The price and availability of electricity; seasonal and daily price fluctuations; and the needs of ongoing plant processes are just a few of the factors to take into account.

(Rajakumar Sreenivasan, B.E. (Hons), M.E., is currently working as Reliability Section Head at the Advanced Petrochemical Company, Saudi Arabia. For more information, email: Rajakumar.s@advancedpetrochem.com)