Dec 8, 2020
This article covers recommended types of dryer inspection and their frequency. Regular paper dryer inspections should be a part of a mill's maintenance program. An inspection program is a necessary means to detect early signs of dryer component failure in an effort to maintain a safe working environment and avoid costly extended downtime.
CAUTION: Read all pertinent safety instructions before performing any of the operating or maintenance procedures in this guide. |
The following is a list of the types of inspections to be performed on a paper dryer:
The inspection frequencies that Valmet recommends are:
Immediate
One year intervals
Two year intervals
Five year intervals
A possible strategy for the five year interval inspection is to perform inspection on 20% of the dryers each year, thereby reducing the length of each outage and the labor required.
It is essential that a written procedure, including maintaining detailed records, be established to ensure that record retention of the operating history of each dryer be maintained so that one can establish the history of the dryer, and justify its integrity to the insurance underwriter and/or governing jurisdictional inspectors.
This record should contain the following items:
There are several things that should be looked for when dryers are in operation. Paper machine operating personnel should routinely visually examine dryers for:
If any of the above are found, these items should be noted for additional inspection when the dryers are stopped.
Idling dryers are stationary but have not been opened. They may be pressurized. As with operating mode inspections, thorough visual inspections should be made on idle mode dryers by the operators for:
The most detailed inspections are performed when the dryers are "shutdown" (stationary and cool). An external visual inspection of the dryer face should be made for:
If a crack should be discovered in the dryer shell, replacement of the dryer shell should be considered immediately, as additional cracks may develop, and some may progress and link up until the shell fails. Steam should be shut off to a dryer immediately upon discovery of a crack.
Appropriate safety procedures should be followed before any internal inspections are performed. The following procedure is recommended for entering a paper dryer to ensure a safe working environment:
The internal visual inspection shall consist of the following items:
When the internal visual inspection is complete, and the dryer manhole is to be closed up, it is important that the manhole cover is correctly installed to prevent it from becoming loose, with potential of injuring personnel or damaging equipment. It is important that properly tightened locknuts be used. Many Beloit dryers were originally furnished with special split locknuts which may need adjusting after removal. This split nut is adjusted by closing or opening the split with a hammer and/or hammer and screwdriver to produce sufficient drag or friction between threads of nut lock and stud.
CAUTION: Over-adjustment will damage threads! |
Manhole and handhold studs or "T" bolts are grade 2 steel. To prevent loosening or over-torquing, it is necessary to lubricate threads with thread lubricant and tighten nut according to the following chart.
Thread diameter | Torque |
---|---|
5/8" | 40 ft-lbs |
3/4" | 69 ft-lbs |
1" | 160 ft-lbs |
Warm-up dryers slowly following a proper procedure, and retorque manhole nuts after one to three hours.
It is recommended that a controlled warm up procedure be followed to avoid excessive stress concentrations in the dryers due to large variations in temperature. The controlled warm up should be tailored to your operation. Valmet recommends the following steps in warming up the dryers from a room temperature condition:
If machine has only been down for four to eight hours the length of time for Step 1 may be reduced.
If machine has been down for four hours or less omit Step A and reduce the time of Steps 2 & 3 to 30 minutes each for a total warm up time of 1 hour.
Four longitudinal scans, 90° apart, should be performed along the entire external length of the dryer shell. These scans should be continuous in nature, noting the readings every 12 inches. See the graphic following this section for a suggested form for recording the measurements taken. When using this form, a standardized reference point on the dryer should be established for continuity. It is suggested that the centerline of the manhole on the tending side of each dryer be designated as 0°. Measurement quadrants are than designated clockwise as 0°, 90°, 180° and 270° while facing the tending side of the dryer.
Capscrews should be ultrasonically inspected to see if their shanks are cracked. The ideal calibration standard is a sample bolt which is identical in geometry and material to those which are being inspected. The graphic shows how these spare bolts can be notched for use as calibrations standards.
The capscrew heads must be prepared by removing any substances or features which will interfere with ultrasound transmissions.
Light grinding may be required to do this, but do not remove ASME bolt identification markings, as they must remain visible.
At least 20% of the bolts should be ultrasonically inspected on a given dryer head. Should a cracked capscrew be found, then all capscrews on that dryer are to be inspected.
The transducer head should not travel beyond the diameter of the shank or false indications may be noted from the underside of the capscrew head. A socket-like template may be used to restrict transducer movement to the shank diameter. See the graphic for this template design.
The next graphic is a form that may be used for documenting the findings of ultrasonic testing of dryer head capscrews. When using this form, a standardized reference point on the dryer should be established for continuity. It is suggested that the centerline of the manhole on the tending side of each dryer be designated as 0°, and the capscrews be referred to in clockwise rotation from this point while facing the tending side of the dryer.
If a leak is found in the dryer-to-shell joint, the capscrews in this area should not be tightened or retorqued but replaced immediately upon leak detection.
When installing a new capscrew, use ASME Code designated material with specification SA 354 Grade BC. Follow the procedure for installation according to OEM drawing tightening specs. The threads and washer face of the capscrew must be lightly lubricated with thread lubricant. Install the new capscrew and torque to the following amount of torque.
Capscrew size | Torque |
---|---|
3/4" | 185 ft-lbs |
7/8" | 300 ft-lbs |
1" | 450 ft-lbs |
Under no circumstances should paper dryer head capscrews be retorqued.
Valmet has been advised that paper mills will, at times, retorque dryer head capscrews in an effort to stop steam leaks at the dryer head to shell joint. This practice often leads to cracked or broken dryer head capscrews which can result in accidents that damage machinery and are a threat to personal safety.
Contact your dryer OEM representative for proper bolt tightening specifications.
A method of sealing leaks in dryers is covered in Valmet's Paper Machine Maintenance course. Contact your Valmet representative for more information.
The preferred method of magnet particle testing is wet fluorescent medium, due to humidity or the dryer heads being damp from the cleaning process.
The dryer heads must be cleaned to remove dirt, grease, scale, and other surface contamination. MT can provide successful inspection over clean, tightly adhering paint coatings of paint thickness no more than 10 mils (.25 mm); one coat of high temperature aluminum paint = .4 mils when dry. However, when MT inspection is to be performed over painted dryer heads, appropriate calibration tests to insure satisfactory inspection sensitivity are recommended.
MT inspection is to cover 100% of the dryer head surface, with special attention paid to the areas around the bolt holes, the manhole, and the radius juncture between the journal and head surface.
When any irregularities are found, they should be ground out to reveal good base material by MT inspection again. Irregularities should not be ground out any deeper than .25 inch, and the opening size on the surface should be a ratio of 3:1 to the depth, radiusing the surface edges. Once a crack is discovered, replacement of the head should be considered immediately, as additional cracks may develop, and some may progress and link up until the head fails. Steam should be shut off to a dryer immediately upon discovery of a crack.
Content for this article comes from the following sources:
For more information on dryer inspection, contact your Valmet representative about Valmet's Paper Machine Maintenance course, or see the Related Links of this article.