An audit by Kadant identified poor condensate removal, runnability problems and energy losses. A steam system upgrade provided a five-month ROI.
An audit by Kadant identified poor condensate removal, runnability problems and energy losses. A steam system upgrade provided a five-month ROI.
A Yankee drying system involves a complex thermo-dynamic process that requires the right balance of steam supply and condensate removal to ensure stable drying and trouble-free tissue machine operation. A commonly overlooked detail is the link between thermocompressor sizing, syphon design, and piping. The thermocompressor and the Yankee dryer syphons must, of course, be correctly sized to deliver the right steam pressure and effectively remove the condensate under different condensing rates needed for the machine’s grade structure. But one should not forget or underestimate the importance of the steam and condensate piping. Correct sizing is vitally important if the drying system is to work as a stable, responsive process. If the upstream or downstream piping is a choke-point because of excessive pressure drops, the operators may find that controlling the machine’s drying system is somewhat like controlling the speed of your car with one foot on the accelerator and one on the brake. It’s ineffective, inefficient, and frustrating.
Cascades in Kingsey Falls, QC, was experiencing unstable drying controls, Yankee dryer flooding, machine runnability problems, and energy inefficiency on its PM3 tissue machine which makes tissue, toweling, and some facial tissue grades. The furnish is 100% chlorine-free recycled furnish from an on-site recycling plant.
The 124 inch trim 150 tpd machine was running a maximum speed of 1800 m/min on tissue grades and about 1560 m/min on toweling grades. The toweling grades, which make up 40% of total production, were limited by drying capacity. The maximum Yankee pressure that could be achieved was 120 psig, even thought it was designed for 150 psig. The running speed of the toweling grades could be increased if the drying capacity limitations could be solved.
To help solve these problems, Cascades enlisted the assistance of Kadant Johnson Canada to conduct a survey of the steam and condensate system and make recommendations to solve Cascades’ dilemma. After the study, Cascades’ engineering group followed through with Kadant’s recommendations and worked closely with Kadant on the implementation. The results have been dramatic; they include reduced energy consumption, increased speed on toweling grades and fewer breaks.
Condensate removal problems
Yves Martin, technical director at Cascades, explains the situation before the steam and condensate system re-design. “When the header pressure and the differential pressure dropped, condensate built up in the Yankee. The operators reacted by opening the condensate drain valve. It was money going to the atmosphere. Sometimes it took an hour to drain the condensate from the Yankee,” he says. During this period the Yankee could also lose its coating, and sheet breaks would occur. He reported that because of these difficulties the operators had lost confidence in the automatic control strategy used by the mill.
The ribbed 15 ft. diameter Sandusky Walmsley Yankee cylinder with soda straw syphons was installed in 2003. To handle extra drying capacity and differential pressure requirements, a new Kadant thermocompressor was installed in 2005. The thermocompressor was designed for the extra condensing load, but something else was limiting the differential pressure and causing dryer flooding. Hence the study was warranted.
Model calculations, actual conditions
Kadant measured the actual condensing rates on the machine using the rate of rise in the separator tank. By using actual machine conditions the models relating differential pressures and blowthrough rates would be true to life. The calculations made by Kadant evaluated the differential pressure requirements for existing condensing rates of 9200 lbs/hr on tissue grades and 10000 lbs/ hr on toweling grades. The model calculations were extended to 10800 lbs/hr to see if the speed potential on the toweling grades could be realized.
Model calculations related differential pressure to steam blowthrough rates. The calculations modelled the syphon tip velocity needed for proper condensate removal. The minimum recommended blowthrough rate corresponded to a syphon tip velocity of 75 fps. The study confirmed that the actual differential pressures were too close to the minimum for all of the production grades. Line pressure fluctuations could cause the differential pressures to be reduced below the minimum recommended value, hence flooding and extended machine instability would occur. A higher differential pressure operating range was required. But, if the new thermocompressor was correctly sized, where were the pressure losses coming from?
Undersized piping corrected
Kadant next conducted a thorough audit of the steam and condensate system components, instruments and controls. The mill worked through a checklist to ensure components were working properly and instruments calibrated. Valve sizing was checked as well.
The major causes of limitations in steam supply and condensate removal became apparent from calculations of the pressure drops across the piping runs. The calculations included pressure drops for valves, elbows and other flow restrictions. Several lines were found to be undersized and pressure drops were excessive. Figure 1 shows the pressure drop calculation for the high pressure steam line to the thermocompressor. Note that the pressure drop across the line is exponential with respect to pipe diameter. A 2-inch line creates 27 times the pressure drop of a 4-inch line.
The existing 2-inch motive steam line to the thermocompressor was one of the bottlenecks. Not enough motive steam pressure was available at the thermocompressor until the mill installed the recommended 4-inch steam pipe. To alleviate the differential pressure problems, other pipes were replaced including:
* The steam line at the exit of the thermocompressor
* The condensate line at the exit of the Yankee to the condensate separator
* The steam line between the condensate separator and the thermocompressor
* The vent line to atmosphere
In some cases, line size changes were recommended to allow automatic valves to function properly.
The piping design process involved the cooperation of supplier and mill engineers. In addition, the mill wanted to relocate much of the piping to the back of the machine to improve operator access on the tending side. They also wanted to improve operator safety through lockouts and make it easy to maintain the equipment. Operator inputs were incorporated in the design criteria.
Impressive results
The entire project was completed within three months. Daniel Guay, paper machine supervisor at Cascades, reports that the results on PM3 machine have been impressive.
These include:
* A saving in steam costs due to a reduction in condensate sewer losses.
* Lost production due to breaks has been reduced from 1.5 hours per week to zero.
* The Yankee steam pressure has the potential to be raised from 120 psig to 150 psig.
* The machine speed on the dryer limited toweling grade was increased by 100 m/min.
All told, Guay reports a return on investment for the project of five months. Martin credits the good teamwork for a well executed project and excellent results. The machine is easy to control now without the previous condensate removal problems. It’s full speed ahead!
Mark Williamson is a journalist engineer from Thornhill, ON.
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