Process Optimization Tips
How often should paper machine dryers be inspected?
Insurance carriers typically expect at least 20% of the dryer cylinders to have an internal inspection every year. On this basis, all dryers would be inspected every five years. Dryer inspections include magnetic particle inspection of dryer heads and bolts, a visual inspection of the inside dryer surface for damage, inspection of balance weights, syphon shoes, condensate piping, surface wear, and often measurement of the dryer shell thickness. Guidelines for these inspections can be found in Tappi (Technical Association of the Pulp and Paper Industry) Technical Information Paper 0404-16.
What is the difference between a “rotary” syphon and a “stationary” syphon?
Both types of syphons are attached to rotary joints. A rotary syphon, however, rotates with the dryer shell and is held in contact with the dryer shell, either by a spring-loaded support rod or by being bolted directly to the dryer shell. A stationary syphon, on the other hand, does not rotate with the dryer. Rather, it is suspended slightly above the inside surface of the dryer and it remains stationary inside the dryer as the dryer shell rotates past the syphon shoe. Internal syphon piping is normally made from stainless steel, to prevent corrosion and erosion in service. The tip of a rotating syphon (the “shoe”) is normally stainless steel as well, to prevent erosion / corrosion. The tip of a stationary syphon can also be stainless steel, but there is an increased risk of serious damage to the dryer shell if the dryer shell contacts a stationary stainless steel shoe. For this reason, stationary syphon shoes are often made from a softer material, such as gray cast iron, ductile iron, or a high-temperature material like Teflon.
How do dryer bearings handle thermal expansion?
A paper dryer shell will expand when it is heated to operating temperature. For example, a dryer with a 250” face will grow in length by up to 0.4” if it is operating at 75 psig. The amount of expansion depends on the steam temperature and the steam condensing rate and correspondingly will change as these parameters change. The back (drive side) bearing is normally fixed in the bearing housing and the thermal expansion takes place on the front (tending) side. The front bearing is normally fixed in its bearing housing as well, but the bearing housing is mounted on “rockers” that allow the entire housing to move axially to accommodate the thermal expansion. Over time, the rockers can drift until they reach their end-of-travel. When this happens, the bearing housing “falls off its rockers”, taking the dryer out of alignment, increasing the axial stress on the bearings, and causing misalignment of the rotary joints. Rockers should be inspected periodically to ensure that they are operating in range.
Why are there “weep holes” in dryers with journal insulating sleeves?
Dryer journal insulating sleeves greatly reduce the transfer of heat from the steam flowing through the dryer journal to the bearings that support the dryer journal. To be effective, the insulating sleeves must be perfectly sealed so steam does not get into the gap between the insulating sleeve and the dryer journal bore. If steam leaks into this area, the effectiveness of the insulating sleeve is lost and the risk of dryer bearing failures increases dramatically. A “weep hole” provides a passage from this gap to the outside, so the steam leak can be detected and repaired before dryer bearings are damaged.
Why are steam joints being repaired at every maintenance shutdown?
Repairing steam joints at each shutdown is often the result of reactive maintenance. Over time, the distribution of remaining steam joint life becomes very broad. For example, if the average life of a particular type of rotary joint is 4 years, eventually 25% of the rotary joints will have to be rebuilt or replaced each year. A papermaking machine with 100 dryers (200 steam joints), will have 50 joints that have to be repaired or replaced each year. This would be 5-6 steam joints every 6 weeks.
A preventative maintenance program reduces the total cost of ownership by preventing unscheduled shutdowns, reducing random failures, and providing more predictable costs and scheduling. And it avoids the need to repair steam joints at every maintenance shutdown.
What is the best configuration for flexible hoses?
The ideal configuration for most flexible metal braided hoses is the "compound hose" configuration. A compound hose has two flexible lengths connected with a 90-degree elbow. The combination of two hoses allows the rotary joint to move in response to journal run-out, seal wear, thermal expansion, and minor misalignment, without placing undue stress on the rotary joint support bearings or brackets.
Is it necessary to change more than just the seal ring when rebuilding a rotary joint?
Yes, it is important to change more than just the seal ring when the seal ring has reached its end of life. It is always a good practice to replace the "soft parts" in a rotary joint at the same time. Elastomeric seals (o-rings, glide rings, energized seals) can become inflexible and cracked after several years in service. Gasket surfaces should be cleaned and new gaskets installed. Internal seal pre-load springs can take a set, particularly in high-temperature applications. Replacing the springs, although not always required, is always a good idea. The associated metal parts can be re-used, often several times, but should be inspected for signs of wear or damage.
Can steam joint flex hoses be too short?
Yes, they can. Flexible braided hoses used on steam lines are flexible in bending, but are very stiff in compression or expansion. If a flex hose is too short (or too long), it will put very high stress on the rotary joints. This can significantly reduce the life of the rotary joint, particularly those joints that are “self-supported”. The internal bearing supports of the rotary joint will wear very quickly and greatly reduce the service life of the rotary joint.
Is there really any difference between OEM parts and "duplicate" parts?
Yes, there is a difference and the difference can be quite significant. OEM (original equipment manufacturer) parts are designed specifically for that equipment.
The materials are selected to meet code requirements, seals are tested and qualified based on the expected operating conditions, and dimensions, surface finishes, and tolerances are based on a detailed analysis and test results of the entire design assembly.
"Duplicate" parts that appear to be the same can in fact provide unreliable, unsafe, and unacceptable performance.
Always use genuine OEM parts, particularly in applications in which steam pressure must be contained.