SILO GAS FOREWORD RECOMMENDED PROCEDURES - A SUMMARY

Transcription

1 SILO GAS The Canada Plan Service prepares detailed plans showing how to construct modern farm buildings, livestock housing systems, storages and equipment for Canadian Agriculture. This leaflet gives the details for a farm building component or piece of farmstead xquipment. To obtain another copy of this leaflet, contact your local provincial agricultural engineer or extension advisor.

2 SILO GAS PLAN M REVISED 88:06 FOREWORD In 1977, near Gananoque, Ontario, the Conners were harvesting their corn crop somewhat early because of planned highway construction nearby. On Monday, September 12, they continued filling the silo but stopped on Tuesday because of rain. On Wednesday, September 14, they decided to resume harvesting. The silage distributor had not been adjusted and silage had piled about 5 m higher against the silo wall at the chute side. From there it sloped steeply downwards towards the opposite side. Edward Conner (a son) apparently decided to level the silage and adjust the distributor before restarting filling operations. Because all the chute doors were closed except the top one, he decided instead to go in through the roof opening. Using a 7 m rope ladder, he let himself down the 9 m drop. Jack Conner (the father) returned from town at about 11:00 a.m. and noticed that Edward was missing. A hired man, William Carr, climbed the outside silo ladder, looked in, shouted that something was wrong, then jumped in. He landed part way up the silage slope. Hearing Carr's shout, Jack Conner climbed the outside ladder; he saw Edward lying at the bottom of the slope and William lying part way up. Mr. Conner yelled out that the boys were in trouble. Another brother, Eldon Conner, ran from the house and up the silo chute, followed by another hired man, Don Littlejones. Eldon kicked in two silo doors and entered. He tried without success to move Carr, then went farther down the slope to help his brother, Edward, and collapsed beside him. The father watched all of this from the top of the outside ladder! Don Littlejones tried twice to pull Carr up to the silo chute. Jack went back down the outside ladder, started up the tractor and forage blower to ventilate the silo, then went up the chute. Firemen and ambulance (called by Mrs. Conner) arrived on the scene. Then the tractor running the blower ran out of fuel. Littlejones changed tractors, but broke a shear-pin when restarting the blower. This had to be replaced before silo ventilation could be continued. The am bulance attendant tried to climb the silo chute with an air-pack, but it was too big for the chute and had to be dropped. Without the air-pack, his rescue attempts were futile. The bodies of the three men were finally removed with an aerial platform, from which were suspended two ladders lashed together and lowered through the roof door to reach the silage surface. At the corner's inquest, the forensic scientist reported that the victims' lungs showed massive bleeding and contained high levels of methemoglobin, evidence of exposure to a strong oxidizing agent. This was attributed to nitrogen dioxide (N02), or silo gas. Another more recent case emphasizes that N02 is not the only gas hazard in farm silos. In 1987, Wayne Smith, a farm employee of Dwight Gilmer and Sons, South Mountain, Ontario, entered a tower silo. It had been lined with a suspended plastic bag, converting the silo to an oxygen-limiting storage for high moisture shelled corn. Smith was working between the concrete wall and the slack plastic liner above the corn, tramping down some corn that remained around the inside perimeter of the bag after the last of the freeflowing corn had been augered out. Apparently the liner ripped open at one of the seams, perhaps spilling C02 gas into the space where Smith was working. His body was found some time later, where he had tumbled through the ripped liner and into the gas -filled space inside. These tragic events remind farmers and silo service people of the deadly nature of the gases sometimes found in and around farm silos. This publication explains the silo gas problem and gives some safety rules for preventing further accidents of this kind. RECOMMENDED PROCEDURES - A SUMMARY 1. POST A WARNING SIGN in an obvious place such as on the feed room door or beside the silo chute. Many silo builders provide such signs, some with operating instructions warning of gas and injury hazards peculiar to their own brand of silo. The message can be quite simple, such as DANGER, SILO GAS. 2. Check to see if your LOCAL FIRE DEPARTMENT (or similar emergency service) has the pressure demand breathing apparatus with a long air-hose for such emergencies. Diving gear (SCUBA) is not suitable because the back-pack air tank is too big to be worn while climbing the silo chute or the outside ladder-cage. 3. VENTILATE THE FEED ROOM in case silo gas spills out of the silo doors or is blown out by the silo unloader. 4. During silo filling operations DON'T GO INTO THE SILO just to level the silage; instead, make adjustments to the silage distributor at the top of the silo to keep the silage leveled during filling. 5. It is sometimes necessary to go into a silo after filling (such as to level and tramp the silage, install a plastic cover sheet, or to setup the silo unloader). DO THIS AS SOON AS THE LAST LOAD IS PUT IN, AND LEAVE THE BLOWER RUNNING while inside. Don't wait until the next day. When opening a chute door into the silo, climb farther up the chute (not down), in case silo gas spills out of the doorway.

3 6. Previous bulletins have recommended a safety harness (or at least a loop of rope under the armpits) with a rope leading to a helper at the top (the 'buddy system'). This was not a bad idea, but if you fall, ONE PERSON ALONE COULD NEVER PULL YOU UP THE SILO WALL and out through the roof hatch or silo chute doors to safety. Two helpers is the minimum! 7. Before entering a previously-filled silo, check the depth of the settled silage. To make sure that the air-blast is reaching all the way down into the headspace, attach a tube adaptor to the blower pipe (see Figures 4 and 5). Leave the chute doors closed and VENTILATE THE SILO by running the forage blower. Larger silos and deeper headspaces require more ventilation time (see Table 1). KEEP THE BLOWER RUNNING while you are in the silo. 8. Once operating, the silo unloader can ventilate the silo quite effectively. However, if the silo unloader fails and requires service in a recently filled silo, one must assume that some gas may have accumulated. In this case, reventilate with the forage blower- and drop-pipe as in recommendation 7, above. 9. If you discover someone collapsed in the silo, the first and most important step is to start the forage blower and VENTILATE THE SILO. Providing a fresh air supply is the most positive and immediate help to both victim and rescuers. 10. As soon as the victim is outside in the fresh air, APPLY ARTIFICIAL RESUSCITATION, if necessary, to restore breathing. Obtain medical assistance. 11. Chemical gas detectors presently available are accurate enough but they are not suitable for operators who do not also have the remote air supply equipment for silo entry. Tests for C02 and NOx, should be carried out using two tubes for each gas. Take samples just above the lowest point of the silage surface. 12. Oxygen-limiting silos are a special case. If you have to enter one of these silos for any reason, you must wear an external air supply. This consists of a pressure tank, a supply hose long enough to reach into the silo, a 'pressure-demand' type of supply regulator and a full-face mask. The acid content of the silage continues to increase over a period of several weeks (depending on the chemistry of the silage material and the air tightness of the silo). Eventually, the increasing acidity kills off the acid-forming bacteria that produced it and inhibits the growth of molds and fungus that could further damage the silage as feed. This is the normal silage making process which takes place in a silo with reasonably airtight walls. ONE SILAGE GAS (C02) IS ALWAYS PRODUCED in this process. The period of most rapid gas production is during the first 6-7 days after filling. During this period the total volume of the various gases represents several times the volume of the silo! For the next 3-4 weeks the gas production tapers off. Some gases stay trapped within the silage during the storage period and silage settlement continues to squeeze the gases into the headspace above (see Figure 1). Another family of gases, the OXIDES OF NITROGEN (NOX), can also be produced. Any weather condition, cultural practice or silage additive that increases the dissolved nitrate (N03) content of the plant material also raises the risk of producing this group of deadly gases. The period of greatest danger is up to 3 weeks after filling. For example, a drought period during the corn growing season can cause a buildup of nitrate in the soil and the corn plants. The corn is then harvested before the thirsty plants can convert the nitrate into useful food proteins, and the unwanted nitrogen is released as nitrous oxide (N20) and nitric oxide (NO). The N20 is produced in only minor quantities and is relatively harmless (laughing gas, an anaesthetic), but the NO is another matter. This unstable NO gas can be produced in considerable quantities and it quickly combines with any oxygen remaining in the silo headspace, forming deadling N02 gas. Other factors that can increase the nitrate content of silage are nitrogen fertilizer overdose (including manure), black soils that are high in organic matter, and silage additives containing nitrate nitrogen (for increasing the protein equaivalent of cattle feed). For example, the Conners, prior to their accident, had been pumping ammonium nitrate solution through the forage blower while filling the silo. Urea is another nitrogen chemical sometimes used this way. MAKING SILAGE Fresh silage material (corn, grains, grasses or legumes), when chopped and blown into silos, is living plant material. During the ensiling process this living material quickly uses up the limited supply of atmospheric: oxygen entrapped within the compressed silage mass, and dies. In this respiration process the oxygen is converted into some water and carbon dioxide (C02). At the same time, acid-forming anaerobic microbes multiply rapidly in the warm moist silage. They feed on part of the sugars and starches in the silage material and convert them to organic acids (lactic, acetic, etc.). These acid-forming bacteria also produce more C02. SILO GAS PROPERTIES CARBON DIOXIDE (C02) is not itself damaging to health. Its danger lies in the factthat in silos and similar tight spaces it replaces the oxygen of the air (normal air is about 21% oxygen, 79% nitrogen and 0.03% carbon dioxide). In oxygen-limiting silos, the contained atmosphere will be almost entirely C02 and nitrogen (N2); human survival in this atmosphere depends entirely on a safe, external air supply. Carbon dioxide is an odorless, tasteless, colorless gas that is 53% heavier than air. Humans and animals fortunately have a built-in warning system for C02; when it reaches a certain concentration in the bloodstream, it triggers a nervous impulse

4 and the victim gasps for air. At higher concentrations this reflex action is inhibited and the victim is asphyxiated. The C02 'threshold limit value' (TLV-STEL, explained later) is ppm (parts per million), or 1.5% by volume. In conventional top-unloading silos with roofs, C02 can accumulate to dangerous concentrations when trapped at low points near the silage surface, and this risk should not be disregarded (see Figures 1 and 2). NITROGEN DIOXIDE (N02) is a deadly gas, appearing as a reddish to yellowish-brown haze, with an acrid or bleach-like odor. Like C02, it is heavier than air (its relative density is 1.58, or 58% heavier), so it too accumulates at low points in the silo headspace, feed room, feed alleys, etc. The threshold limit value (TLV-STEL) is set at only 5 ppm. At this level it is barely detectable by smell. When produced in sufficient quantities it is so dangerous that it can be a threat to man and beast by spilling over into the silo chute and draining down into the feed room and connecting stable. Nitrogen dioxide is deadly because when inhaled it dissolves with moisture on the wetted internal surfaces of the lungs, instantly forming potent nitric acid (4N02 + 2H > 4HN03). Here it 'burns' the sensitive oxygen-transfer surfaces of the lungs, effectively stopping any further oxygen supply to the 1 silo chute doors, closed for filling 2 top of silage just after filling 3 settled silage 4 dense silo gas, squeezed out of the silage, remains in the headspace 1 wind blows through open roof hatch and top of chute, ventilates silo dome and top of headspace 2 highest concentration of silo gas is at the lowest part of the headspace 3 workman opens chute door, silo gas pours down into chute Figure 1 Typical unsealed tower silo just after filling Figure 2 Behavior of silo gas on opening of a silo chute door

5 body. Tiny blood vessels in the lungs break down, causing massive bleeding, and death results (When Eldon Conner went down the sloping silage to try to rescue his brother, his father watched him drop unconscious almost immediately. Even if rescue could have been completed within minutes, it is very unlikely that either Eldon or Edward could have been revived). Even small periodic dosages of N02 (such as from working each day in a poorly-ventilated feed room at the bottom of the silo chute) has been blamed for a host of chronic respiratory problems, including shortness of breath, coughing and fluid in the lungs. These symptoms can also result from a single non-lethal exposure and may be first noticed several weeks afterwards. This delayed effect has sometimes made an accurate diagnosis difficult. Another of these pumps, made by Drager in Germany (Figure 3), consists of a small hand-squeezed rubber bellows fitted with check valves. One or more specified strokes of the bellows are required, depending on the concentration of the gas (the weaker the mix the more strokes to be counted). Research showed that the Drager bellows type was easier to use than the piston type and more suitable for the wide ranges of temperature and humidity encountered in and around farm silos. Consult the Drager chemical tube specifications and take special care when interpreting the readings at temperatures below freezing. DETECTING SILO GASES During the danger period after filling the silo, use the following silo gas indicators: Acrid bleach-like odor or a brownish to yellowish haze at the silage surface or on the feed room floor can indicate nitrogen dioxide. Sometimes an unnatural bright-yellow to orange coloration in the silage is another indicator. Unnatural breathing, or coughing of livestock and people, may indicate either N02 or C02, or both. Dead flies, cats or mice on the feed room floor, or dead birds in the silo, may indicate silo gases. Animals on the floor will be exposed to heavier gas concentrations than a standing person. In addition, chemical detectors are available for many hazardous gases, including C02, combined NO and N02 (NOX), and N02 alone. The simplest of these is a paper tape that changes color when exposed to N02 in silos and feed rooms, but it doesn't indicate how much gas is present. The tape is therefore not recommended. CHEMICAL TUBE DETECTORS, on the other hand, can be calibrated and should be considered for use by trained technicians and rescue teams (firemen, etc.). This type of detector consists of a sealed glass tube containing a specific chemical reagent that changes color progressively along its length when the gas in question is drawn through the tube. Each tube is activated by snapping off both ends of the glass and plugging it into a small hand-operated air pump. Operating instructions specify the number of pump strokes required to draw a specified air volume through the sampling tube. The gas concentration is read either directly or by comparing the length of tube that has changed color to tables or charts reading in parts per million (ppm), by volume. Several approved brands of tube detectors (MSA, Kitagawa, Drager, and Gastec) are available at safety supply stores. One type of air pump consists of a piston and cylinder somewhat like an oversized hypodermic syringe. One pull of the handle draws a precise quantity of the air in question (typically 100 ml) through the chemical tube. Figure 3. Drager gas tester consisting of bellows pump, attached chemical tube and extension hose for remote sampling. A practical difficulty is that all of these chemical tube detectors are calibrated with the glass tubes plugged directly into their air pumps. This means that the operator himself must enter the silo or other danger area, which is hazardous unless he is wearing a remote air supply system. Use of the tube detector will give the best indication that it is safe for others to follow without the protection of a remote air supply system. For practical purposes, an extension tube up to 5.5 m (18 ft) long (provided by the manufacturer) is probably accurate enough when reading near the threshold limit values. At least two chemical tubes for each gas (C02 and NOX) should be used to check a silo. In many cases the extension tube is not nearly long enough to reach from the safety of the outside silo ladder down to the sampling point inside the silo. THRESHOLD LIMIT VALUES (TLV) are the upper safety limits of hazardous gas concentrations. The American Conference of Government and Industrial Hygienists (1980) has published two values: the TLV-TWA (time weighted average) value is the upper safety limit for workers repeatedly exposed for the typical 8-hour workday; the TLV STEL (short-term exposure limit) is the upper safety limit for four daily exposures of up to

6 15 minutes, separated by 1-hour periods of no exposure. The latest published values for silo gases are: TLV-TWA TLV-STEL (ppm) (%) (ppm) (%) Carbon dioxide (C02) Nitric oxide (NO) Nitrogen dioxide (N02) 3 5 Oxides of nitrogen (NOX) 3* 5* * Based on the most dangerous possibility that all the NOX can convert to N02 VENTILATING THE SILO AND FEED ROOM PORTABLE EXHAUSTER FANS were tried in controlled experiments with C02 in full-size farm silos. Two sizes of electric blowers were tested, each being connected to 15 m (50 ft) of clothes-drier hose. This simple adaptation improved remarkably the penetration of the air-blast deep into the silo headspace. The makeshift arrangement shown in Figure 2 was experimentally effective, but there is too much chance that in a real gas -hazard situation the tube could be dislodged by air and vibration. This could leave someone in the silo without a continuing air supply. Therefore the tube should be tied securely in place. 'Spring Flex' tubing, by Flexhaust Co., 11 Chestnut St., Amesbury, Mass., U.S.A. Available from Indus trial Rubber products, 90 Commander Blvd., Agincourt, Ont. M1S 3H7, and ForestTube Inc., 3555 boul. Cremazie E., Montreal, Que., H1Z 2S3. The larger blower was rated at 215 L/s when connected to the collapsible tubing. Once in place and operating, it reduced the C02 concentration in the silo headspace to that of normal air (around 3000 ppm) in 15 to 20 minutes. However, it was quite difficult to place the tubing at the far side of the silo while standing safely in the ladder cage outside the roof. Probably few farmers would buy and routinely use such special-purpose equipment. THE FORAGE BLOWER can move more air than any other easily-portable system, and it has the important advantage of already being in place at the end of silo filling. Air-moving capacities of various forage blowers running 'empty' at rated tractor speed were from 800 to 1000 L/s ( cfm); this is about five times the capacity of the larger portable exhauster that was tested! After the initial 5-8 days of most rapid gas production, the forage blower is the most practical way to remove silo gases. A forage blower, equipped at the top of the silo with the side-fill deflector-type of silage spreader (Badger, Lancaster Multi-flo, etc.), is effective as long as the silage level is not more than 5 m (16 ft) below the distributor hood. At greater headspace depths, however, the splitter vanes appear to break up the airflow and prevent the air stream from penetrating deep enough to properly mix and displace the bad air deep inside the silo. With the various types of rotating center-fill silage dis tributors (Even-flo, Butler, etc.) some of the air blast is lost from the open underside of the long arched gooseneck, and the rotating distributor baffle-plate further deflects the remaining air-blast toward the silo wall. With a center-fill spreader attached, the forage blower gives rather poor ventilation, especially where the headspace from distributor to silage is greater than 5 m. To better direct the air-blast downward and to prevent interference by the silage distributor (either type), a 5 m (16 ft) length of 200 mm (8 in.) wire-wound flexible exhaust ducting * was stuffed into the throat of the blower pipe (Figure 4). Figure 4. 1 forage blower 2 fixed filler pipe, 230 mm (9 in.) mm (8 in.) Spring Flex wire-wound tubing 4 air blast penetrates to bottom of headspace 5 mixed and diluted silo gas escapes through open roof hatch Silo ventilation by forage blower and flexible drop-tube stuffed into throat of silo filler pipe

7 Really, a better method of attachment is required. Figure 5 shows a suggested adaptor for the rotary center fill type distributors. At this time, we are recommending to silo equipment manufacturers that they develop and market these important safety devices. Another important fact was discovered. Contrary to popular opinion, we also found that the forage blower was more effective as a silo ventilator with all the CHUTE DOORS CLOSED and the ROOF PANEL OPEN. This procedure also avoids the dangerous possibility of the silo gas simply moving from the silo down into the feed room and connecting stable. 1 attachment fitting and rectangular-to-round transition piece; bolts to throat of gooseneck, bolt heads smooth and flush inside throat 2 diverter flap-valve spring-loaded to 'open' position 3 external crank and cable pulls valve to 'bypass' position for ventilating the silo mm (8 in.) 'Spring Flex' wire-wound ventilaon tubing, gear clamp to transition piece 1 Figure 5. Proposed gooseneck attachment for diverting the ventilation down into the silo headspace (gooseneck design by Lancaster Level-Flo, Inc., Lancaster, Pennsylvania 17602, USA).

8 In an emergency, black plastic drainage tubing 200 mm (8 in.) diameter, without perforations, could be substituted for the more durable and lightweight wire-wound ventilation ducting. Table 1 gives calculated ventilation times for various sizes of silo headspace, assuming ventilation with a forage blower at 800 L/s (full rated tractor pto speed of 540 rpm) and an initial gas concentration of 10% C02. In headspaces of 6 m (20 ft) and deeper, this table is valid only with the drop pipe attachment in place. Also, this table is not applicable to the period of most rapid gas production, for 5-8 days after filling - during this extreme danger period, even the forage blower is too small to assure enough gas dilution for safety. TABLE 1 MINIMUM TIME TO VENTILATE TOWER SILO HEADSPACES Silo Ventilation time* (minutes) for headspace diameter depths of 3 m 4.5 m 6 m 7.5 m 9 m m (ft) (10 ft) (15 ft) (20 ft) (25 ft) (30 ft) 3.6 (12) (16) (20) (24) * based on C-Ci = e KQt/V Co-Ci where C = C02 gas concentration at time t (assumed 0.015, the TLV-STEL) Ci = inlet C02 concentration (assumed , as in fresh air) Co = original C02 concentration (assumed 0.10 in the headspace) K = mixing factor (assumed 1.0) Q = ventilation rate (assumed 48 m³/min) t = ventilation time, min V = headpsace volume, m³ e = THE SILO UNLOADER (once installed and operating) is an effective silo ventilator, but it provides no ventilation (and hence no protection) before it is lowered to the leveled silage surface, adjusted and started up. This start-up time is, unfortunately, the time of greatest risk from silo gas. FEED ROOM VENTILATION is important as well. There is a real danger that silo gas can fill up to the level of the open chute doors and spill down the silo chute. This is of little consequence if the silo stands alone outside and is connected to the feed room with only a feed conveyer. In most cases, however, the silo chute connects directly through the roof of an attached feed room, and the feed room connects in turn with the barn. Exhaust ventilation fans in the barn can easily draw silo gases through the crack under the feed room door, endangering livestock as well as people. The problem is potentially worse where the feed room floor is below outside ground level (a common situation with bank barns). This prevents effective natural ventilation that could be accomplished by opening an outside feed room door. A simple and inexpensive solution is to move one of the barn exhaust fans (the one that runs most) to the feed room. This will exhaust the barn air through the feed room, instead of the reverse. To make sure that the heavier-than-air silo gases are effectively removed, build a box duct over the inside of the fan and extending down to about 150 mm (6 in.) from the feed room floor. The opening areas of the duct cross-section and the inlet at the floor should each be at least twice that of the fan opening. The feed room should be insulated to prevent condensation on the walls and ceiling. A further advantage of this arrangement is that in winter the feed room is heated at no cost by warm air drawn from the livestock barn. It will be necessary to use some closure such as a silage funnel attachment to close off the bottom of the silo chute. This is to ensure that the fan will draw air from the stable, not from the silo. The silage funnel should be tied off with a plastic bag (or other suitable closure) except when the silo unloader is operated. REFERENCES 1. Airborne Contaminants Committee TLV's, threshhold limit values for chemical substances and physical agents in the workroom environment, with intended changes for Amer. Conf. of Govt. Industrial Hygienists, PO. Box 1937, Cincinnati, OH 45201, U.S.A. 2. Wells, D Respiratory protection for agricultural tower silos and manure pits. Farm Safety Assoc. Inc., 340 Woodlawn Rd. W., Guelph, Ont. N1H 7K6. 3. Sabourin, H.M., W.S. Reid, J.E. Turnbull and M. Ihnat Silo gas production, detection, ventilation and related safety procedurs. Can. Soc. of Agric. Eng. paper no , presented at ann. conf., Vancouver, B.C. Contribution no. I-421, from Eng. and Stat. Res. Center, Research Br., Agriculture Canada, Ottawa K1A OC6. 4. Turnbull, J.E Silo gas - what can be done? Proc., Int. Silo Safety Conf., Kitchener, Ont. Nov. 28. Contribution no. I-660, from Eng. and Stat. Res. Center, Research Br., Agriculture Canada, Ottawa K1A OC6. 5. Meiering, A.G., M.G. Courtin, S.F Spoelstra, G. Pahlow, H. Honig, R.E. Subden and E. Zimmer Fermentation kinetics and toxic gas production of silages. Trans. of ASAE. Vol. 31.