Q & A > Question Details
Are variable speed drivers ever used in pumps?
If not, why not?
 
Answers
27/07/2010 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
I concur with Ralph Ragsdale that there are some good opportunities for using inverter-rated induction electric motors powered by Variable Hertz Inverters on pumps.
Several decades ago, the physical size and reliability of Variable Hertz Inverters were not favorable to process services. But dramatic improvements have occurred, although negative perceptions have remained.
There also can be some confusion about the role of the VSD motor and the role of the trim discharge control valve with respect to controlling flow.
Inverter-rated induction electric motors powered by Variable Hertz Inverters have made substantial in-roads as motive drivers for barrel centrifugal compressors and axial compressors in the process industries, and increasing interest within the context of high performance air coolers with advanced winterization. Latter high performance air coolers require mandatory 100% VSD fans, and this again is often at odds with traditional perspectives.
27/07/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
YES: VFDs are increasing in numbers as operating energy cost go up, In new designs, 'right sizing' or 'design to fit' practices strive to reduce 'over-size procurement and installation' that has been so common. VFDs can be used as the primary flow control device, completely eliminating the costly control valve installation (and associated leak points.) All too often, 'economic analysis' overlooks the cost reduction from eliminating the control valve.
Existing installations often have numerous pumps with 'trimmed impellers' that reduce the pressure output , thus saving energy. Economic justification of a VFD on such circuits may not be attractive.
The savings from 'clean-sheeting' all the flow loops and sizing every component for the primary operating condition can save lots of money and avoid later equipment damage.
As an example (a very real instance), a positive displacement pump (plunger pump) with some 1500 Hp hydraulic duty was driven by a synchronous motor. The design discharge pressure was some 3400 psig. The fluid was a light lubricating oil fraction that had some 15% wax with a melting point of about 130F. The storage tank was heated and maintained at 150F.
Starting from the plunger pump crankshaft with a 'demand ' of some 1800Hp including friction losses, the coupling was sized for 100% more torque to allow for acceleration by the synchronous motor.
Then the motor was over-sized by 20% of the maximum requirement as specified by client design guides. Then the electrical supply system was over-sized by 25% of the usual size to satisfy the over-sized motor to ensure good life at high ambient temperatures (per client specs).
After over a year of trouble-free operation, 'energy conservation' efforts identified the feedstock tank heating as 'low hanging fruit' because the waxy oil did not solidify even at the lowest expected temperature--why heat the tank? Turn off that heat and save $50K of 15# steam--(the plant was having to use air coolers to condense excess 15# steam to recover the BFW, so the steam really had a NEGATIVE value--it cost more to not use it.). The Result--
During the summer--fine--but when ambient temperature dropped in the fall and the oil viscosity increased significantly due to wax particles, frictional losses in the plunger pump increased from the allowed 300 Hp to over 800Hp. Not to worry--plenty of capability in the coupling, plenty of capability in the motor, plenty of capability in the electrical supply -BUT--the PUMP CRANKSHAFT failed from excessive torque, wrecking the $1.2 million pump and causing several weeks of unit downtime.
Proper engineering analysis of the SYSTEM would have prevented the costly failure. Is it really wise to size reflux pump systems for 25% more than the maximum process duty and maximum possible charge rates?
27/07/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Each time energy efficiency surveys and studies are conducted, the lower operating costs of variable speed pumps and compressors are evaluated. Due to the higher capital costs and complexity that could affect reliability and maintenance, their use is not yet widespread.
27/07/2010 A: Raghavendra Sangam, Libra Techcon Ltd., rsangam0504@yahoo.co.in
Yes, of course! In dosing chemicals, metering of fluids or in mixing of fluids in certain ratios. Examples: (1) The VFD is the slave on a ratio controller which may control the catalyst slurry feed to a oligomer flow for further polymerisation. The oligomer flow controller itself may be a master controller for this ratio controller. The VFD modulates the rpm of catalyst pump which may be a reciprocating pump. (2) A simple reactor level control may be controlling the reactor fefd pump rpm and therefore, the flow to reactor. And, in many such cases.