|MECHANICAL AND ELECTRICAL INSPECTIONS IN PAPER MILLS|
Prepared by: L.E. Beck October 24,1946
It may be best to discuss the subject of Mechanical and Electrical Inspections in Paper Mills progressively, in other words, in much the same way that the raw material flows through the process, to the finished product.
With this thought in mind we will first consider the Barking Drums or Tumbler drives.
You will notice, as we proceed, that some of the functions of the process of paper manufacture are being omitted. This is not an oversight as it is the intent of this paper to deal primarily with those objects which are usually insured and which, in the event of failure, would curtail production, and those objects, which are peculiar to paper mills.
The Barking Drums which are used to remove the bark from the logs are usually driven with a wound rotor induction motor connected either by a belt and reduction gear or a reduction gear alone, to the drum.
The drums, in many plants, are located out of doors and the motors are always exposed to an unusual amount of dirt and fine bark.
These motors are forced to start an extremely heavy load due to the inertia of the logs in the drum and the friction of the supporting trunions. This causes very heavy currents in both the stator and rotor windings, during the starting cycle.
We have, on not a few occasions, found rings of solder on the stator windings which has been melted from the rotor coil tip connections, and was thrown out during the start up.
When a condition such as this is found in these or any other wound rotor motor, a request should be made for the motor to be dismantled and all of the rotor coil connections should be resoldered and reinsulated.
It is possible for some solder to be thrown out of the connections and still do no harm, however it is impossible to tell their condition without dismantling. A high resistance connection between rotor coils would mean ultimate failure in operation.
The Chippers are driven by various methods and each, of course, has it's advantages and disadvantages. The discs are heavy and quite large in diameter which means that, when starting, they come to speed slowly. If they are driven with a flat belt from a motor, the belt unless fitted with an idler, must be very tight to keep it from slipping and running off. This heavy belt tension will cause more than normal wear of sleeve bearings. therefore, the air gap clearances of the motor should be checked when possible.
A great many of the newer chippers are being driven by synchronous motors, direct connected to the chipper shaft. At this point, please bear in mind that we are not discussing design, only the inspection of equipment already installed and in operation.
The wood that is fed to the chippers is usually four or five feet long and some of the pieces will weigh several hundred pounds. In some installations there is a drop, through a shoot, from the conveyor to the disc of as much as fifteen or twenty feet at about a forty five degree angle. A heavy piece of wood, therefore, strikes the disc with a terrific force, in the axial plane.
Although the disc is very heavy, a great amount of the impact is transmitted through the shaft, to the motor spider and to the thrust bearing which is usually located at the outboard motor bearing. There are cases on record where this continual beating has caused fatigue and failure of cast iron motor spiders.
It is, therefore, important to check the spider of these motors for cracks. In many cases the manufacturers have recognized this weakness and are making the spiders of steel. This, however, does not eliminate the necessity for a thorough inspection.
It is also important to check the motor and chipper for loose or broken base bolts.
The discs should also be carefully inspected for cracks, especially at the corners of the knife openings and at the keyways in the hub.
DIGESTERS AND WASHERS
In general, the conditions in these two departments are the most adverse as respects electrical equipment of any place in a paper mill. The black liquor which is the fluid part of the pulp as it is blown from the digesters and which is removed in the washers, is an excellent conductor, much better than water. All of the motors in these two locations should be totally enclosed and particular attention should be given to the condition of the motor conduit to see that it is sealed so that no water or liquor can enter the motor through it or the motor terminal box.
All starters and safety switches in this area should be of the water-proof type.
The use of totally enclosed motors brings up the problem of bearing lubrication because most motors of this type also have totally enclosed ball bearings. These bearings are grease lubricated and there is no way of knowing whether there is too little, too much, or just the right amount of grease in them. Usually there is, or has been too much.
When too much grease is forced into the bearing housing the static pressure along with the pressure generated by the revolving balls will rupture the inner grease seal. When this happens the grease will be forced into the inside of the motor. As the motor is all enclosed this condition can not be observed, so the oiler continues to put grease into the bearing and the bearing continues to put it into the motor. Eventually the coil insulation becomes so soft that an electrical short circuit or ground occurs.
I am of the opinion that a totally enclosed motor should be dismantled once each year to inspect the bearings, seals and windings. A definite greasing program should be established based on the recommendations of the manufacturer and on information obtained from the dismantle inspections.
In the case of totally enclosed motors that are used intermittently it has been found that condensation has formed on the inside and in some instances as much as two quarts of water was removed when the motor was opened. This condition can be relieved by drilling a small hole at each end of the frame, at the bottom.
This is undoubtedly one of the most severe types of service that a motor can be subjected to. It seems to be the general opinion of paper makers that good stock cannot be furnished unless the jordan plug is screwed up to the full load of the motor and the best stock can be made if the motor is overloaded. It is true that, in most cases, finer jordaning produces better stock, but a 250 HP motor should not be expected to do the work of a 300 HP. It should be recommended that an ammeter, clearly marked with a red line at the full load of the motor, be installed where it will be visible to the man operating the jordans. The relays should be checked for proper setting at each inspection, if possible.
These Jordans are driven either with squirrel cage or synchronous motors, usually of the open type. They are located at the wet end of the paper machine and are, therefore, exposed to a very humid condition. They are also quite frequently soaked by some uninformed person who may be washing up the floor with a water hose.
Motor protection should be recommended that will not restrict the free circulation of air, but will still keep the water out.
MOTOR DRIVEN CENTRIFUGAL PUMPS
A great many pumps are used in the paper industry and the greater part of these are of the centrifugal type.
It has been our experience that a large percentage of the trouble in motor-pump sets, of this type, can be traced to misalignment. It is true that we cannot check the alignment of every unit as we inspect it but vibration, hot bearings or a noisy coupling should be sufficient reason for asking that the alignment be checked at the next shutdown.
Another frequent cause of failure in these sets is a worn out or improperly adjusted pump thrust bearing. The fillets of the motor bearings should not be expected to carry the thrust of the pump, in fact, they cannot carry it for very long. They will either burn out or wear back until the rotor fans of the motor strike the end guards and cause a shutdown.
Sometimes a pump thrust with excessive axial clearance will allow the pump and motor shaft to move back and forth, endwise. This osculating motion will draw the oil out of an otherwise well sealed motor bearing and deposit it on the windings. So oil in the motor windings is not always an indication of a faulty motor bearing. The trouble may be at the outboard end of the pump.
PAPER MACHINE DRIVES
Most of the paper machines are driven by variable speed, direct current, motors which receive their current from a motor-generator set located in a substation, or a room apart from the machine room. These motors and M-G sets are no different either in construction or operating characteristics than sets found in other plants. So no further time will be taken in discussing them.
A great many of the machines are driven with small steam turbines through reduction gears. Some of the gears are installed in the line shaft and others are belted to it with a flat belt and an idler pulley. The greatest difficulty in inspecting these machines is checking the overspeed stop.
If the reduction gear is belted to the shaft, the belt should be removed during the test. If the gear is mounted in the shaft, the couplings at each side of the gear should be disconnected. If this is not possible, run off all of the belts to the machine and then watch the line shaft closely for unbalance while the machine is being overspeeded.
It is, quite often, possible to set the tripping speed of these turbines lower than the 10% above nameplate speed. The turbines are frequently designed for a speed much higher than that required to operate the paper machine at its top speed. So the top speed at which the paper machine is to be operated should be determined and the tripping speed of the turbine set 10% above that, if that figure does not exceed 10% above the name plate speed.
We will not attempt to discuss the complete inspection of the power plant because that could easily constitute a complete article in itself. We will, instead, just consider those things peculiar to paper mill installations.
Note: When I presented this paper I used a one-line diagram on a chalk board to animate the steam flow.
In the pulp and paper mill power house (of any size) we find many different types of turbines. A typical installation is shown here.
The machine on the left is a straight non-condensing turbine taking steam at 600 pounds pressure and exhausting into an intermediate pressure header at 150 pounds.
The machine in the center is a single extraction, non-condensing type. This machine takes steam at 600 pounds, extracts at 150 pounds and exhausts at 30 pounds to a low pressure header.
The machine on the right is an extraction, condensing type which takes steam at 150 pounds, extracts at 30 pounds and exhausts to a condenser.
The 150 pound header as well as being fed by the exhaust of the non-condensing turbine and the extraction of the extracting machine, can be fed through a pressure reducing valve from the high pressure header. Likewise, the low pressure header can be fed through a reducing valve from the 150 pound header.
There will, of course, be desuperheaters in these reducing valve lines to bring the temperature of the steam down to the proper level.
You will note relief valves on both the 150 pound and the 30 pound headers and on the exhaust and extraction lines from the turbines, as well as an atmospheric relief on the condenser of the extracting-condensing machine.
You will also note non-return valves in the extraction lines. Some of them are weight loaded and others are actuated by the overspeed stop of the turbine. Either style is acceptable. The condition of these non-return valves is very important, because if the overspeed stop should close the throttle and the non-return should remain open, steam at low pressure would flow ungoverned through the low pressure element of the turbine and run it away, regardless of the throttle or the main governor.
In general, I would say that moisture, overload and lack of a specific maintenance program can account for the greater part of the failures in paper mills. Moisture is a condition that cannot be eliminated, and each installation must be delt with in the manner that the particular conditions dictate.
Overload can only be controlled by proper relay protection and constant vigilance on the part of the inspector.
A proper maintenance program cannot be carried out when the mill operates twenty four hours a day, seven days a week, with only a shut-down of three or four hours once a week to change felts and wash up.
So anything that can be done to arrange a definite operating schedule with sufficient down time to complete the repair and maintenance of the equipment is a long step toward the prevention of accidents in this industry.