Steam harness components must all be sized and installed properly in order to have consistent pellet mill operation. Pellet mill downtime is expensive, as lost tonnage is only made up with increased operational hours.
Typically, the pellet mill is the heart of the feed mill, and steam supplied to the conditioner ahead of the pellet mill must be both adequate (from the boiler) and consistent (pressure, quality and flow rate). In order to have consistent pressure and volume of steam to each conditioner ahead of the pellet mill, a steam header must be used.
A steam header is simply a containment tank. The diameter should be four times the diameter of the high-pressure supply line from the boiler, and the length is determined by the number of pelleting lines—13 feet long for two lines and 21 feet long for three lines.
The header is installed horizontally and located relatively close to the pellet mills. High-pressure steam from the boiler goes in the top of the header (center) and steam lines then come out the top of the header—equally spaced from the steam inlet pipe—and go to each pelleting line.
The purpose of the header is to act as a shock absorber so that, when steam is turned on or off on one pelleting line, it is not in short supply or oversupply to the other pellet mill(s). Since pellet mills are often operated near pellet mill full load amperage, any sudden change in the moisture (steam) can cause the pellet mill to choke. A sudden reduction of moisture will cause the pellet mill to over amperage, and a sudden increase of moisture will cause a roll slip, commonly referred to as a wet choke. The steam header greatly reduces this fluctuation in steam pressure and/or volume, allowing the pellet mill to operate more consistently.
Should a steam header be used if there is only one pelleting line? The benefit is less, but in many cases the answer is yes. When a boiler is operating and more steam is required, the boiler will go from low fire to high fire and, depending on the boiler, this change of the boiler will result in more volume and/or pressure for a short time. The opposite is true also when the boiler returns to low fire, decreasing volume and pressure.
When these changes pass through to the conditioner and pellet mill, they can cause the pellet mill to choke, resulting in downtime. This is especially true when the pellet mill is operating near its die choke point (the maximum moisture and feed rate before the rolls will slip or the pellet mill motor will over amperage).
From the steam header, high-pressure steam goes to a pressure reducing valve, an on/off valve and then to a flow control valve, which then controls the amount of steam that flows into the conditioner.
The steam header is just one component of a proper steam harness, but a correctly sized and installed steam header will help keep the pellet mill running smoothly.
Other components of the steam harness (pressure reducing valve, shut-off valve(s), flow control valve and pipe sizes) will be addressed in another segment.
To learn more about the importance of steam in pellet mills, or to contact a salesperson, visit: http://www.cpm.net/equipment/pellet-mills.