See part one of this series, "How to Prevent Dryer System Fires and Explosions. Knowing what causes fires and explosions is the first step in prevention.
Unplanned and uncontrolled fires and explosions are never welcome in an industrial plant. Certain conditions create and contribute to an environment that allows and supports such events. Part 1 of this two-part article series, “How to Prevent Dryer System Fires and Explosions” (February 2015, page 16), outlined the central causes that allow fires and explosions to occur in dryer systems. In this article, I’ll explain three simple things you can do to reduce the risk of fires and explosions in a dryer system. They are:
Reduce the amount of oxygen in the system.
Keep ductwork clean.
Adjust operations and controls in specific ways.
If you operate a dryer in your facility, read on for ways to mitigate your risks.
No Oxygen = No Combustion
If ogres are like onions, fires are like people. They do not last long without oxygen. Therefore, reduce the risk of fires and explosions by minimizing oxygen in the dryer system. This is the most important precaution to take in order to minimize fires and explosions. Because the dryer system will inherently have an ignition source (burner and heat) and fuel (product, natural gas, propane, etc.), the only element that can be easily minimized is oxygen.
The first step is to use an oxygen sensor to monitor oxygen within the system. With most drying systems, the lower the oxygen percentage is in the system, the better it is. Ideally, the maximum oxygen level in any dryer system should be 12 percent. If your oxygen is above 12 percent while operating, try these tips:
Employ exhaust-gas recycling if you are not already doing so. This increases the percentage of water vapor and inert combustion gases, thereby driving down the percentage of oxygen.
Find and fix air leaks into the dryer system. Train operators and maintenance staff to seal manway covers and other openings into the system correctly.
Make drum seals work the way they are intended. Smooth and round the seals and sealing surfaces. Bear in mind, it is impossible to round the sealing surface well enough without the drum tires being round to less than ±0.030" and the trunnions also being round to less than ±0.003".
If it does not have structural integrity and flexes while it rotates, you may need to add stiffening to your drum to attain these specifications for your drum seals and trunnions. Drums without structural integrity rotate like a water balloon rolls down the sidewalk — always wider than tall though perhaps not as visibly. As you can imagine, the seals and sealing surfaces will never match up. Round drum and seals will give you “happy yellow minions” while flexing, non-mating seals will give you “unhappy purple minions,” whose jaws do not quite match up. Rounding your drum and making it rigid will go a long way toward better sealing surfaces.
It is important to note that if the drum has a floating- or walking-style tire — common in industrial dryers — it is very difficult to make a drum round and rigid. In this case, shim the walking tire to make it rotate at the same rate as the drum before attempting to round the tires and seal rings.
A final tip for keeping your oxygen level below 12 percent is to install an airlock for each cyclone in the system (assuming it utilizes cyclones as a particulate-separation device). Pressure differences between cyclones not separated by airlocks can cause undesirable flow characteristics such as particulate from one cyclone getting sucked up through another cyclone and placed back into the gas stream. As a general rule, airlocks' blades should be refurbished or replaced every other year (or more often if necessary). Proper rotor-to-housing clearance is crucial for minimizing airflow and particulate back into the gas stream.
Just Say No to VOCs
Another factor that must be considered to reduce the risk of fires and explosions is to keep ductwork clean. An initial cleaning of system ducts may be required, but if you are able to implement the following tips, you should not see as much buildup as before.
First, to keep ductwork clean, keep volatile organic compounds (VOCs) from condensing. For distillers’ dried grains with solubles (DDGS) systems, one of the predominant oils released when the product is thermally damaged condenses around 215°F (101°C). Keeping the temperature in the whole drying system — from start to finish — above 220°F (104°C) will help keep the buildup to a minimum. Of course, keeping the dryer above the condensation temperature is a practice for all dryer systems, not just DDGS. Process operating parameters may need to be modified by adjusting the control system and increasing fan flows. If you have tried everything else without success, adjusting drum rotation speeds may help.
Second, stop producing VOCs. This will certainly help keep your ductwork clean. Measures should be taken to minimize the production of VOCs in order to stop thermally damaging the product. Most thermal damage comes from radiant heat when the product is able to “see” the flame. Just as it is difficult to get sunburned when you are in the shade, if the flame is hidden from the product — around a corner or behind a shield — thermal damage will decrease. This win-win bonus for stopping the thermal damage to the product can yield many dollars’ worth of profit each year, all while minimizing the risk of fires and explosions.
Operations and Control Systems
In my first article in this series, I shared that the majority of fires and explosions in dryer systems occur during startup, shutdown or upset conditions. The refractory, metal and high volume of gases hold a great amount of inertia — a thermal flywheel — that must be overcome any time product flow is changed or when system temperature setpoints are adjusted. The control system can react much faster than the physical dryer system, so caution must be used when making adjustments. Using the following tips will help keep your dryer safe and in tip-top shape.
First, raise the temperature slowly during startup. When infeed is started, it also must be ramped up in small increments — for example at 20, 25 or 30 percent — until you are putting in 100 percent. Dryer systems perform best and safest when changes are made slowly.
Second, for the same reasons, it is best to ramp down the system in a similar fashion. Ease off on product infeed and allow the control system to slowly take energy out of the system. You do not want to spontaneously stop product flow, cut the burner or use the “E-Stop” button unless it is truly an emergency. Use the moisture of the incoming product to absorb energy as you scale down the system. If it helps, think of yoga music in the background while starting up and shutting down. Relax. Breathe in. Breathe out. No rush.
Third, maintain equilibrium in the dryer system during normal operation. The safest state for a drying system is a steady state within proper specifications. In this manner, the burner, fans, feed rate and feed moisture remain consistent. The drum only holds so much volume. Thermal energy changes cannot come into effect immediately.
Fourth, train operators to spot conditions that may develop into adverse situations. Also, train them on how to alleviate an adverse condition before it develops into an incident. This may mean the operator lets the control system do its thing, or he may switch to manual control and handle the situation without control system help. The best choice will depend upon the conditions, but a well-trained operator will be prepared to take the best course of action. Operators must know how to deal with startup, shutdown and adverse situations.
Smoldering fires are a bit of an anomaly because they can start in low oxygen environments and remain undetected. Think of the embers you remove from your fireplace. They were not glowing until they had access to oxygen, but they are hot enough to start a fire in the presence of oxygen.
In process applications, smoldering fires start when the product being dried remains on a sufficiently hot surface for a long enough time to exceed the autoignition temperature of the product. Such fires can develop undetected in low oxygen environments, and they are most common during shutdown or upset conditions.
The last thing an operator wants to do is introduce oxygen onto the smoldering fire. A properly designed fire-suppression system that includes water sprinklers or steam purging can be activated to put out the smoldering fire before oxygen is introduced, preventing a larger fire or explosion. Never open any manway or access port until it is certain that a fire or potential fire is out.
See the related web exclusive, "Industrial Dryer System Maintenance Tips," to learn about how maintenance affects dryer safety, emissions, efficiency, reliability and durability.
Sometimes, there is no manner of preparation a plant can do to fully eliminate the risk of fires and explosions. Drying systems should be equipped with a pressure-relief system that is placed in such a way that personnel will not be in the direct line of release should an incident occur. A pressure-relief system will go a long way toward protecting the equipment and personnel in an adverse situation.
In summary, every direct-fired dryer system has fuel, ignition and oxygen, and, therefore, fires and explosions are a real risk. In the first part of the series, I discussed why unplanned fires and explosions are undesirable and should be prevented in an effort to protect personnel, equipment and your business. This article supplied some important tips that will help reduce the risk of unwanted fires and explosions in a dryer system:
Stop producing VOCs.
Train your operators for proper startup, shutdown and adverse situations.
Adjust the control system to properly control startup, shutdown and adverse situations.
As much as possible, operate the dryer in a steady-state manner and make adjustments slowly.
Be aware of the risk of smoldering fires during upset conditions, and never introduce oxygen until certain that the fire-suppression system has put out any embers.
To paraphrase that famous bear we all grew up with, only you can prevent dryer fires and explosions.