The challenges of paper machine pulp dryer sprinklering.

Pulp dryer is a significant part of any paper machine and its reliable operation is a key factor for the production rate and production cost. One of the most popular type of dryer is the roll dryer where the paper pass through series of heated rollers.

Even the fact that the humidity is relatively high the risk of fire in those machine is still high and the fire damage may cause a significant losses related to down time.

Few areas are in high risk, those are the broke collection area, the air plenum and the lub oil system.

Combination of favorable conditions and the many potential source of ignition increasing the risk level and required a risk treatment to reduce the potential damages in case of fire.

A good guidelines for protection of different type of pulp dryers can be found in FM Global data sheet 7-4 covering a pulp dryers and a paper machines protection.

A sprinklering of those machines may become a big challenge to the designers and installers taking in consideration the size, construction and specific futures of their operation and maintenance. 

One of the problem is the fact that in most cases those are machines in 24/7 operation where down time tolerance is zero. The design of the whole sprinkler system and the way it’s installed must be carefully planned allowing installers to work without need machine to be stopped.

Another very critical specialty is the regular machine maintenance and repairs. Depend of how old the machine is, it is normal different drums to be taken out for maintenance and repair up to few times per month. Those are enclosed machine and the only way for those maintenance and repairs is through the hood. This automatically led to requirements sprinkler system to be designed in a way where branches can be easily disassembled and assembled again. Adding the high building roofs that cannot be used to support the piping and the limited ability dryer hood to be used for attachment of pipe supports the task become even harder.

Next issue may come from the area bellow the air plenum. As is mentioned above the dryer hood as well the false ceiling cannot be used for pipe support attachment, but in the same time the air plenum may take almost half of the machine width and depend of the machine size this may be equal to 7-8 meters. The pipe branch can’t be supported along their length and obviously protection of such an area can’t be done with regular sprinklers because the insufficient branch support. In small machines, where this distance is up to 4 m a standard coverage side wall sprinklers can be used. The situation with larger machines is different. Standard coverage side wall sprinklers can’t be used but in the same time the temperature of extended coverage side wall sprinklers is too low for the application and they also can’t be used.  One of the possible approaches is use of extended coverage pendent sprinklers installed as far from the wall is possible. This may provide adequate protection when air plenum is 6-7 m wide.

Both sprinklers under the hood and bellow false ceiling should penetrate the machine enclosure trough suitable openings that need to be reliably covered, otherwise the increased heat losses will led to add additional cost to production.

Another tricky part is the broke collecting area. The challenges here are different. Those areas usually are much colder and extended coverage side wall sprinklers can be used successfully. The issue may become the machinery used to clean the area, usually those are small front loader or fork lifts. The sprinkler piping should be design on a way that is naturally protected by structural elements, otherwise pipe damages are very likely situation.

Serious issue my become protection of those sprinklers during regular maintenance. In many cases the regular maintenance include heavy welding works, and situation where melted metal activate the broke collection area sprinklers is quite often. To prevent this from happening a suitable shielding screen should be installed above the sprinklers. This will also prevent their activation during the regular machine operation when a hot water jets may reach a sprinkler.

The designers must take in consideration a different type of sprinklers used and applicable standard and regulation requirements. If not negotiated with the AHJ those sprinklers should not be mixed in one system, so different alarm valves must be installed.

Frequently an additional support structures are required that may change significantly the cost of the sprinkler system.  The maintenance-ability of the system itself also should be taken in consideration during the design.

Finally the high humidity and temperatures / part of the system will be expose to temperature close or even above 100 C/ also should be taken in consideration. High temperature may affect some fittings when high humidity combined with high temperature made galvanized pipes very bad choice. In such a systems properly protected black steel pipe should be used or even better choice are stainless steel pipes.

The wide variety of pulp dryers require detailed study of machine structure, operating and maintenance procedures before design activities even be started.