7
The Risks of Enclosed Spaces
7.1 Introduction
The reduction of oxygen (O2) concentration in air by inert gases is a risk of enclosed spaces. A further risk is intoxication by absorption of hazardous toxic materials through the lungs. Inhalation of dust and particulate matter can also be a serious respiratory problem. The focus in this chapter is on the first two risks. Toxic materials can be classified in terms of their physiological action:
- Irritants, which includes corrosive gases/vapors that attack the mucous membrane surfaces of the body. Sulfur dioxide and chlorine are examples.
- Asphyxiants, which are substances that interfere with the oxidation processes in the body. The simple asphyxiants are physiologically inert gases, which dilute or replace the oxygen required for breathing. Dilution of air by the simple asphyxiant nitrogen (N2) was probably the cause of the accident described in Section 7.3, the section concerning industry. Consumption of oxygen in air by fire was probably the cause of many casualties at a fire described in Section 7.4.1, part of the section on society. Chemical asphyxiants react with an essential body function involved with the transportation of oxygen (O2) from the lungs via red blood cells to body tissues. In such cases, asphyxiation results even though the air contains an adequate concentration of O2. Chemical asphyxiation by carbon monoxide (CO) was the cause of the accident described in Section 7.2, the section regarding transport. A further case of chemical asphyxiation by probably both hydrogen sulfide and hydrogen cyanide is described in Section 7.4.2, part of the section on society.
- Anaesthetics and narcotics, which depress the central nervous system and lead to unconsciousness.
- Systemic poisons, which injure or destroy internal organs of the body.
7.2 Transport
Lethal accident aboard the Dutch ship Lady Irina in 2013 [1].
Event
The chief engineer of the Dutch ship Lady Irina died aboard the ship after having entered the bow thruster space in the front part of the ship on July 13, 2014. A Danish coroner established that the chief engineer had died from poisoning by carbon monoxide (CO). The ship was on its way from Archangelsk in Russia to Kolding in Denmark. It carried wood pellets.
Ship, Cargo, and Trip
See Figure 7.1. The ship has an overall length of 88 m and a maximum width of 14.4 m. Its gross tonnage is 3323. Its maximum velocity is 13.5 knots (25.0 km h−1). Eight men were aboard the ship at the time of the accident. The Lady Irina regularly loaded wood pellets at Archangelsk and carried them to various European harbors, one of which is Kolding.
Figure 7.1 Ship Lady Irina.
Source: Courtesy of Dutch Safety Board, The Hague, The Netherlands.
Detailed Description of the Accident
The chief engineer met the first chief on the bridge of the ship at approximately 19.00 h on July 13, 2014. They discussed the removal of splash water aboard the ship by a pump. He left the bridge between 19.30 and 19.45 h for the engine room. At that time, it would have taken approximately 4 h to Kolding. The captain took over from the first chief on the bridge at 20.00 h. The first chief then went to the chief engineer's cabin to discuss the work of the next day. They used to have such a meeting at that time of the day. However, the chief engineer was not present. That did not surprise the first chief; he assumed that the chief engineer was busy because of the planned arrival at Kolding. The first chief returned to the chief engineer's cabin at 21.45 h. The chief engineer still was not present. The first chief then went to the engine room; however, he did not see the chief engineer. He noticed in the engine room that the pump for the removal of splash water was running. That surprised him because the removal of splash water usually did not take longer than 20 min. The first chief thereupon went to the bow thruster space in the front part of the ship where, he assumed, the chief engineer could be present. He found that the door of that space was open. The compartment has two floors. On entering the space, it is possible to descend a stairs leading to the lower floor. The first chief found the chief engineer lying on the lower floor. He noticed that the chief engineer did not breathe anymore and his heart had stopped beating. The first chief then went at about 22.00 h to the bridge and informed the captain and the second chief. The captain then decided to sail to the nearest harbor, i.e. Frederica in Denmark at a distance of approximately four sea‐miles. The first chief ordered the second chief to get a stretcher and go to the space where the chief engineer laid. The first chief then went to the day‐room to alert the crew and asked them to go to the compartment where the chief engineer was lying. He put on his working‐clothes and arrived in the specific space a few minutes later to supervise the activities to remove the chief engineer from the bow thruster space. The alerted crew members had started with the reanimation of the chief engineer; however, they did not use a breathing apparatus because they thought that the chief engineer had fallen from the stairs. It did not occur to them that the air quality was the problem.
At that time, all crew members, except the captain, were involved in the rescuing operation. The group consisted of six men, i.e. the first chief, the second chief, the apprentice‐engineer, the cook, and two sailors. The ship arrived at about 22.45 h at Frederica. The apprentice‐engineer and a sailor thereupon left the compartment where the chief engineer was lying for the engine‐room to start the rotation of the bowscrew.
The second chief left the space when it became clear that it was not possible to get the strongly built chief engineer out of the compartment with the stretcher. He went to look for a neck support and an oxygen‐breathing trunk. He went for the neck support because the crew thought that the chief engineer had fallen from the stairs and he went for the oxygen‐breathing apparatus because the chief engineer did not breathe anymore.
When the second chief got back, he saw that the first chief had become unconscious and was lying on the floor. One crew member tried to move him. A further crew member walked around as if he was drunk. The second chief entered and put the oxygen‐breathing apparatus over the mouth of the first chief and heard that it functioned. He also requested the crew members still standing on their feet to leave the compartment. He then also left the compartment because he became unwell.
The ship moored at 22.50 h. The Danish fire‐brigade got the chief engineer and the first chief out of the bow thruster space. A coroner established that the chief engineer had died because of poisoning by CO. The first chief regained consciousness after oxygen had been administered to him. Three crew members, i.e. the first chief, a sailor, and a cook, were hospitalized but could return to the ship after a couple of days.
The Cause of the Accident
Toxicological Properties of CO
CO is a particularly dangerous chemical because it cannot be detected by the natural senses of the body [2]. It is toxic because it competes successfully with oxygen (O2) for the binding sites of hemoglobin (Hb), the O2‐carrying hemoprotein in the blood of mammals. The affinity of hemoglobin for CO, calculated from the pressure of CO required for half‐saturation, i.e. [Hb] = [HbCO], is 200–300 times that for O2.
The TLV‐TWA (ACGIH) is 25 ppm by volume. TLV stands for threshold limit value, TWA stands for time‐weighted average, and ACGIH stands for American Conference of Governmental Industrial Hygienists. The inhalation of air containing 400 ppm by volume of CO will result in headache and discomfort within 2–3 h. Inhalation of air containing 4000 ppm by volume proves fatal in less than 1 h. Inhalations of air in which the concentration of CO is high can cause sudden collapse with little or no warning.
Measurements
The Danish fire‐brigade carried out measurements in the aforementioned space after the compartment had been ventilated for approximately 1.5 h. 80 ppm was measured in the upper part, whereas 20 ppm was measured in the lower part. The door giving access to the compartment was subsequently closed and the measurements were repeated after 36 h. 690 ppm was measured in the upper part, whereas 555 ppm was measured in the lower part. Also after 36 h, the fire‐brigade measured the CO‐concentration in a compartment adjacent to the aforementioned space. In that compartment, the CO concentration appeared to be higher than 2000 ppm, the maximum that could be measured with the equipment at hand. Further measurements were carried out after the ship had been unloaded. CO could be detected in neither the aforementioned compartment nor in the space adjacent to that compartment.
CO Formation
Wood is a combustible material. The burning of wood is a chemical reaction between wood and oxygen (O2) at elevated temperature. The reaction between wood and O2 proceeds, albeit at a low rate, also at ambient temperature. If, at the reaction between wood and O2, there is an abundant supply of O2, the reaction product is carbon dioxide (CO2). Otherwise, CO is also formed. Wood pellets are a bulk material and it means that about 55% by volume of the bulk material is wood and about 45% by volume of the bulk material is, at unconditioned storage, air. That means that the wood pellets expose, in m2 kg−1, a large area to air. As a result of chemical reactions between wood and oxygen, CO is formed. The separations between, on the one hand, the bow thruster space and an adjacent compartment and, on the other hand, the cargo‐space are not airtight and thus CO could enter, by both diffusion and natural convection, the mentioned compartments. The measurements carried out by the Danish fire‐brigade prove that this actually happened. The main part of the CO in the bow thruster space and an adjacent compartment probably entered them as follows. A cargo‐space ventilation duct passes through the space in which the accident happened. There is a ventilation duct inspection hatch in that compartment. An inspection after the accident revealed that the hatch was not airtight.
A CO concentration of 500 ppm by volume in air does not imply a significant lowering of the O2 concentration in air. Air normally contains approximately 21% by volume of O2 and a CO concentration of 500 ppm by volume is 0.05% by volume. So, a considerable CO level (in view of its toxicity) hardly affects the O2 level.
Safety Procedures
Before the Accident
A procedure for entering an enclosed space was applicable for the Lady Irina. So, the hazard was known. However, that procedure was not adhered to for the compartment in which the accident happened. A shortened procedure was applicable. That procedure implied that, some time before entering the space, the door had to be opened. Personnel was allowed to enter the compartment after the door had been open for 15–20 min. During the day, the door was left open. The space was entered frequently during the day. Applying the official procedure would have been a roundabout way. Measurements of gas concentrations were not carried out.
After the Accident
A new procedure for enclosed spaces in all ships of the fleet to which the Lady Irina belongs is now applicable. The new procedure defines a “First Entry.” A “First Entry” is any entry when the enclosed space is closed. The first person entering the space must carry an instrument with which it is possible to measure the concentration of various gases in the compartment. The instrument gives an acoustic signal if the concentration of a dangerous gas is too high or the O2 concentration is too low. The MSA Altair 4X Multigas Detector is a typical instrument. It can test for, e.g. LEL (lower explosion limit), O2, CO, hydrogen sulfide (H2S), sulfur dioxide (SO2), and nitrogen dioxide (NO2). Four of these measurements can be carried out simultaneously. At least three of these instruments have to be available aboard a ship. The instruments must be maintained and calibrated with calibration gas regularly. A board on the door giving access to the enclosed space outlines the new procedure.
Remarks
The detailed description of the accident indicates that CO is a dangerous gas indeed because it cannot be detected by the natural senses of the body. The crew of the Lady Irina was aware of the risks associated with the cargo of the ship. However, it did initially not occur to the crew that the CO concentration in the mentioned compartment was too high.
The Danish fire‐brigade could get both the chief engineer and the first chief out of the bow thruster space. The ship's crew could not do this. These two facts give rise to the question whether it would be necessary to have, on board of the ship, means available to, if need be, hoist a human from, e.g. the bow thruster space. However, such provisions have not been included in the new approach.
7.3 Industry
Lethal accident during maintenance of a phosphorus furnace at Flushing in The Netherlands in 2009 [3].
Event
Two employees of Thermphos at Flushing in The Netherlands entered a phosphorus furnace during maintenance activities on May 15, 2009 without a compressed air breathing apparatus. The air in the furnace probably contained less than 10% by volume of oxygen and that probably caused their death.
Thermphos
Thermphos at Flushing was part of Thermphos International B.V.. The company went bankrupt in 2012. Competition by a company in Kazakhstan was an important aspect in the considerations concerning the bankruptcy. The company had activities in Europe, North‐ and South America, and Asia. The location at Flushing was the main seat of the company. The production of white phosphorus, ortho‐phosphoric acid, and sodium tripolyphosphate (NTPP) were the main activities at Flushing. Thermphos International B.V.. employed approximately 1200 people. The workforce at Flushing was 450.
Phosphorus
Phosphorus is an element indicated by P. As an element, phosphorus exists in two major forms, i.e. white phosphorus and red phosphorus. Phosphorus is never found as a free element on the earth due to its high reactivity. Phosphorus‐containing minerals are usually present in their fully oxidized state, that is, as inorganic phosphate rocks.
White phosphorus emits a faint glow upon exposure to air due to oxidation (reaction with oxygen) of the material. The term “phosphorescence,” meaning glow after illumination, derives from this property of white phosphorus. It is, therefore, usually stored under water in which it is insoluble. Red phosphorus does not emit a faint glow upon exposure to air.
White phosphorus is metastable. It is yellow when it is slightly impure. Its melting point is 44.1 °C and its boiling point at atmospheric pressure is 280.5 °C. White phosphorus can be converted into red phosphorus.
White phosphorus burns to phosphorus pentoxide (P2O5) and diphosphorus tetroxide P2O4, depending on the amount of oxygen present. Finely divided white phosphorus particles are extremely reactive and ignite spontaneously in air.
White phosphorus is used for the manufacture of ortho‐phosphoric acid (H3PO4) mainly. 50% of the H3PO4 from white phosphorus is used for the manufacture of phosphates for detergents, whereas also 50% is used for the production of phosphates for foodstuffs, pharmaceuticals, and animal feeds. It is also possible to manufacture H3PO4 directly by the reaction between sulfuric acid and phosphate rock. However, H3PO4 thus obtained is less pure than H3PO4 obtained from white phosphorus. The reason is that, during its production, white phosphorus leaves the phosphorus furnace as a vapor that is subsequently condensed in water. The white phosphorus thus produced is pure.
Phosphorus Production
Thermal phosphorus manufacture (in an electric furnace) starts nearly always from fluoroapatite (Ca5(PO4)3F). The reaction equation is
The second reactant is silica in the form of flint pebbles or gravel, whereas the third reagent is coke. The three raw materials are, in a particulate form, continuously added to the reactor. The chemical reaction is endothermic, which means that heat must be supplied to the reactor. The reaction temperature is in the range of 1400–1800 °C and the pressure is slightly higher than atmospheric pressure to avoid the ingress of air as it contains oxygen. Figures 7.2 and 7.3 illustrate the construction of an electric furnace in which white phosphorus was produced at Flushing and in which the accident occurred. The top view shows that the top of the reactor has the form of an equilateral triangle with rounded corners. There are three electrodes situated at the corners of a smaller equilateral triangle. Electric currents pass from the electrodes to the floor of the furnace and the reacting mass is heated by the heat development due to the electric resistance offered by the reacting mass. The triangular arrangement of the electrodes avoids the existence of live and dead phases. The feed lines are near the electrodes. It is important that the grain sizes of the three raw materials are the same. That results in virtually constant gas permeability of the reacting mass. The lines through which the gaseous reaction products white phosphorus and carbon monoxide leave the furnace are visible in the upper part of Figure 7.2. Figure 7.3 shows the large dimensions of the furnace used by Thermphos; its diameter is approximately 9 m. Large furnaces, like the furnaces used by Thermphos, have electrodes with a diameter of 1.3–1.5 m.
Figure 7.2 Top view of the phosphorus furnace. Ovenuitgang, Furnace exit.
Source: Courtesy of Dutch Safety Board, The Hague, The Netherlands.
Figure 7.3 Cross section of the phosphorus furnace. Werkvloer, shop‐floor; Electrode‐opening, opening for electrode; vulpijp, filling pipe; Betondeksel, concrete lid; Chamollestenen, Chamolle stones; Steunring, support ring; Wandkoeling, wall cooling; Koolstofstenen, carbon stones; Stampmassa, pound mass; IJzersteenkoeling, cooling for iron stone; Ringgoot, annular drain; Bodemkoeling, bottom cooling.
Source: Courtesy of Dutch Safety Board, The Hague, The Netherlands.
The other two reaction products leave the reactor as slags. First is the calcium silicate slag. Approximately eight metric tons of calcium silicate per metric ton of white phosphorus leave the reactor continuously as a viscous liquid. Its temperature is approximately 1350 °C. A second, much smaller, slag flow leaving the reactor intermittently is ferrophosphorus. Oxides of iron are impurities in fluoroapatite. They give rise to the formation of ferrophosphorus.
The gases leaving the reactor enter an electrostatic precipitator for the removal of dust. Next, the virtually dust‐free gases, having a temperature of approximately 400 °C, enter a condensation system. Gaseous phosphorus is condensed in wash towers where it is brought into contact with water. Carbon monoxide passes the condensation step and is processed elsewhere.
Description of the Accident
The accident occurred in the early hours of May 15, 2009. Two phosphorus furnaces were being maintained at that time. Thermphos had three phosphorus furnaces in operation at Flushing. The maintenance activities at the furnace in which the accident occurred were almost ready. An operator got the request to hoist a faulty old valve into the furnace through an electrode opening. The probable reason for this request is that Thermphos wanted to get rid of the valve. The molten iron of the valve would add to the ferrophosphorus slag flow. The electrode opening mentioned was the only opening of the furnace as the two other electrodes had already been installed. He asked a colleague to assist him. The two employees noticed dust in the air in the empty phosphorus furnace. After the accident, it has been concluded on the basis of an investigation that the dust particles must have consisted of P2O5 or part of the dust particles must have consisted of that material. They decided to enter the furnace without breath protection because they could not find breath protection. The operator who had received the request to hoist the valve into the furnace climbed down a ladder and reached the floor of the furnace. The operator wanted to remove the valve from the tackle. He asked his colleague to assist him. The colleague then started to climb down the ladder. At that moment the operator in the furnace told him to leave the furnace because “something is wrong.” The colleague thereupon left the furnace. The operator in the furnace also wanted to leave the furnace and started climbing the ladder. However, he fell 3 m down from the ladder on the bottom of the furnace. His colleague then informed the shift leader who worked in the vicinity. The shift leader asked for a compressed air breathing apparatus but did not wait for this provision and entered the furnace to provide first aid to the operator lying on the bottom of the furnace. He too fell 3 m down from the ladder while climbing down. Two Thermphos employees subsequently entered the furnace to assist their colleagues. These employees both wore a compressed air breathing apparatus. They reanimated their colleagues in the furnace and removed them from the furnace. Reanimation in the furnace and removal from the furnace lasted approximately 1 h. Reanimation was continued outside the furnace. The employees that had been lying in the furnace without breath protection died in the morning of May 15, 2009. The employee who had assisted his colleague for hoisting the valve and had been in the furnace for a short time was admitted to the hospital for observation.
The Cause of the Accident
Table 7.1 appears in [3]. All times in the table are times after the accident happened. The ppm values (parts per million) stated in the table are probably by volume. The height of the furnace is approximately 5 m. It is probable that a low oxygen concentration has caused the unconsciousness and the subsequent death of the two Thermphos employees. Humans faint almost immediately in air containing less than 10% by volume of oxygen. Prolonged exposure to such an atmosphere results in death.
Table 7.1 Concentrations of oxygen, phosphine, and carbon monoxide in the Thermphos Furnace after the accident.
| Time on May 15, 2009 (h) | Oxygen (% by volume) | Phosphine (ppm) | Carbon monoxide (ppm) | Height a (m) |
| About 03.15 | 13.3 | 17.7 | 21 | 10 |
| About 04.24 b | 5.5 | 33 | 62 | 7 |
| About 04.24 c | 7.1 | 29 | 59 | 7 |
| Note d | 20.1 | 1 | ||
| About 11.30 | 20.9 | 0 | 0 |
a Meters above ground level; see Figure 7.3.
b Instrument No. 1.
c Instrument No. 2.
d After activation of the main ventilation.
The furnace was not ventilated at the time of the accident. It appeared that inert gas had leaked into the furnace via a raw material feeding line. During production, inert gas enters this raw material feeding line to prevent ingress of carbon monoxide. A valve in the inert gas line had not been fully closed.
Carbon Monoxide, Phosphine, and Phosphorus Pentoxide
The question may be raised whether the two Thermphos employees could have been harmed as a result of the measured carbon monoxide and phosphine levels and the observed dust in the furnace.
First, the toxicity of carbon monoxide will be reviewed shortly [2]. See also Section 7.2. Carbon monoxide was in focus at Thermphos because it is a by‐product of the reaction in which white phosphorus is produced. It follows from the data in Section 7.2 that carbon monoxide cannot have been the cause of the death of those two employees.
Next, the toxicity of phosphine will be mentioned [4]. Phosphine was in focus at Thermphos because it is known that electrodes resorb phosphine. Phosphine is a highly poisonous gas. Its formula is PH3. For humans, concentrations of 400–840 mg m−3 over a period of 30–60 min are extremely dangerous or even lethal, whereas concentrations of 140–260 mg m−3 are tolerated under these conditions. However, one death has been observed following exposure to 10 mg m−3 for 6 h. To convert mg m−3 into ppm by volume at atmospheric pressure and 20 °C, multiply by 0.70. Also in view of the short time of exposure, it is improbable that phosphine has been the cause of the death of those two Thermphos employees.
Finally, the effect of the dusty atmosphere is discussed. As stated, it has been concluded on the basis of an investigation that the dust particles in the furnace at the time of the accident must have consisted of P2O5 or part of the particles must have consisted of that material. The Dutch Safety Board does not exclude the harmful effect of dust on the two affected Thermphos employees. I think that it is improbable that the dust has caused their death.
Remarks
Thermphos erred in not considering the furnace to be an enclosed space during the final phase of the maintenance activities. The furnace should, according to Dutch regulations, have been considered an enclosed space during that phase of the maintenance activities [5]. The definition of an enclosed space is, according to [5]: “A closed or partially open environment having or not having a narrowed access and insufficient or bad natural ventilation, which is not designed for the stay of humans, and where activities occur that entail risks concerning safety, health and well‐being.” The Dutch regulations require a detailed analysis of the risks of an enclosed space. Such an analysis could have brought to light that it would have been necessary to block the inert gas line to eliminate the possibility of inert gas flowing into the furnace. Probably Thermphos did not consider the possibility of inert gas flowing into the furnace. Such an analysis could also have led to the conclusion that it would have been necessary to have means available to, if need be, evacuate persons from the furnace.
7.4 Society
7.4.1 Fire in a Nightclub at West Warwick, Rhode Island in the United States in 2013
Event
A fire occurred in The Station Nightclub at West Warwick, Rhode Island in the United States in the evening of February 20, 2003 [6]. A band performing that evening used pyrotechnics that ignited polyurethane foam lining the walls and the ceiling of the part of the building where they performed. The foam provided acoustic insulation. The fire spread quickly along the walls and the ceiling. Once most of the foam was consumed, the fire transitioned to a wood frame building fire. Hundred people lost their lives in the fire. The main cause of death was probably simple asphyxiation.
Additional Remarks
The Station Nightclub was a single‐story building having an area of approximately 412 m2. The polyurethane foam present in The Station did most probably not contain fire retardants. It has been estimated that the conditions in the greater part of the nightclub would have led to severe incapacitation or death within approximately 1.5 min after ignition of the foam for anyone remaining standing, and not much longer for those occupants close to the floor.
The capability to suppress the fire in its early stage of growth was insufficient primarily because automatic fire sprinklers were not installed. The building was equipped with hand‐held fire extinguishers. The situation in 2003 was that sprinklers would have to be installed in new constructions; however, such provisions would not have been required for an existing structure like The Station Nightclub. The investigators of the fire recommend the installation of automatic sprinkler systems according to NFPA 13 for places such as The Station Nightclub. NFPA stands for National Fire Protection Agency (U.S.A.).
A heat detection/fire alarm system had been installed in The Station Nightclub, which activated (sound and light strobe) 41 s after ignition of the polyurethane foam, by which time the crowd had already begun to move toward the exits.
Three exits were available: the double doors of the front main entrance on the north (limited by the single door into the vestibule), the single door on the west near the performance platform, and the single door on the east near the main bar. There were no emergency exits.
Egress from the nightclub was hampered by crowding at the main entrance to the building. The probable number of occupants was in the range 440–458, whereas the occupant limit amounted to 420 people. The building had several windows. These windows became the secondary route of escape.
7.4.2 Slurry Silo at Makkinga in The Netherlands in 2013
Event
A lethal accident occurred at a livestock farm for dairy cattle at Makkinga in The Netherlands on June 19, 2013 [7]. An employee of a company specialized in slurry silo cleaning was cleaning a silo while wearing breath protection. The cleaning was necessary because the silo mixer had to be repaired. A second man working for the same company watched his colleague outside the silo standing on a ladder while the first man was working. The man inside the silo became unwell. The man outside the silo thereupon called for assistance and subsequently descended into the silo by climbing down a second ladder. Three further men working at the farm subsequently also descended into the silo. The stock farmer was one of these three men. One of these three men was, while he climbed down the ladder, instructed to go back because it was not safe in the silo. The three men having entered the silo to assist the man cleaning the silo also became unwell. The unwellness of all four men present in the silo has most probably been caused by the inhalation of air containing toxic gaseous components resorbed by the slurry in the silo. Those toxic components are hydrogen sulfide and hydrogen cyanide. Another person present at the farm called the emergency number. The fire‐brigade succeeded in making a hole in the silo wall to evacuate the four men. Three men had by that time died in the accident and one was hospitalized heavily injured. The heavily injured man has partially recovered but will probably not recover completely.
The Farm and Procedures
The farm is in the northern part of The Netherlands. At the time of the accident, there were approximately 110 dairy cattle at the farm. Both the faeces and the urine of the cattle in the stables were collected together in a cellar under the cattle. The cellar is called the slurry cellar. Typically, a slurry cellar has a depth of 2 m. The slurry is transferred to farmland again. In The Netherlands, it is not allowed to transfer slurry to farmland from about September 1 till about February 1. The reason for this standstill order is that nutrients can be taken up by vegetation only if the vegetation is present and if it is growing weather. The aim is to prevent pollution of ground‐water, surface water, and air. Furthermore, it must be possible to store the slurry production of minimum 7 months in a silo. The rationale of the 7‐month storage time instead of 5 months is weather and the availability of personnel to transfer the slurry to farmland. The livestock farm at Makkinga has a silo having a total volume of 924 m3. Its diameter is 14 m and height is 6 m. Slurry is, typically once a month, transferred by means of a pump from the slurry cellar to the slurry silo. The slurry in the cellar is not homogeneous. It is necessary, to enable pumping, to regularly homogenize the contents of the slurry cellar. That process step is called mixing, and it is indeed performed by means of a mixer. The slurry, on arrival in the silo, segregates again. Before the transfer from the silo to a tankcar, it is often necessary to homogenize the slurry again by means of a mixer.
Gaseous Toxic Components
Enclosed Space and Mixing
Two different situations can be considered. The first one is the situation when slurry is mixed in the cellar under the animals. This first situation considers what happens to people and animals present at ground level. Ground level refers to both inside and outside the stable. The second one is the situation where slurry is present in an enclosed space accessible to humans or animals. An enclosed space can be a cellar, a silo, a tank, or a container. The accident at Makkinga occurred in a silo.
Measurements
The ppm values (parts per million) stated in this section are probably by volume. Measurements have been carried out at 22 dairy cattle livestock farms in the northern part of The Netherlands before and during mixing in (probably) 1986 [8]. The measuring period lasted approximately 1 year. Thirteen farms of the 22 had already experienced problems concerning toxic gases emerging from slurry, whereas 9 farms had not yet experienced such problems. The problems were that mainly cattle became unwell and that in some instances, died. Mixing occurred, on average, once per 3–4 weeks and lasted 2–4 h.
First, measurements were taken before mixing. On average, 0.055 vol.% of carbon dioxide (CO2) and 3 ppm of ammonia (NH3) were found in the stables of the 13 problem farms. In the stables of the 9 reference farms, 0.039 vol.% of CO2 and 4.9 ppm of NH3 were found. Measurements concerning other compounds were not carried out before mixing. The measurements were carried out in the stables at the height of an animal.
Second, measurements concerning various compounds were carried out during mixing. They were started when the mixing was stationary. The measurements were carried out in critical areas. Area near the mixer and “stagnant” zones in a stable were defined as critical areas. The first series of measurements is summarized in Table 7.2. The figures in the table are averages of the highest values measured at the farms. The second series of measurements in both 13 problem farms and 9 reference farms concerns hydrogen cyanide only. The concentration of hydrogen cyanide was measured during mixing as a time weighted average (TWA) over a period of 15 min. The majority of the data is in the range 10–100 ppm. A significant difference between the two farm categories cannot be noticed. One measurement is as high as 144 ppm.
Table 7.2 Analytical data of air in dairy cattle livestock farms during mixing – averages of the highest values measured at the farms.
| Compound | 13 problem farms | 9 reference farms |
| Hydrogen cyanide (HCN) | 165 ppm | 145 ppm |
| Hydrogen sulfide (H2S) | 376 ppm | 339 ppm |
| Carbon dioxide (CO2) | 0.52% by volume | 0.5% by volume |
| Ammonia (NH3) | 5.7 ppm | 11.9 ppm |
| Methane (CH4) | 15.2% of the LEL a | 10.2% of the LEL a |
| Oxygen (O2) | 20.63% by volume | 20.70% by volume |
a LEL stands for lower explosion limit.
Toxicological Data
TLV stands for threshold limit value and TWA means time weighted average.
Hydrogen Cyanide
TLV‐TWA 10 mg m−3 (8.3 ppm) [9]. It is the highest concentration allowable for human exposure in air, 8 h a day, and 40 h per week. Exposure of humans to concentrations exceeding approximately 100 ppm can result in sudden collapse and stopping of breathing [10].
H2S
TLV‐TWA 5–10 ppm by volume [11]. Exposure of humans to concentrations in air between 300 and 400 ppm for periods between 15 min and 1 h causes severe respiratory distress and acute asthenia. Exposure of humans to concentrations in air higher than 1000 ppm causes immediate loss of consciousness and respiratory distress.
CO2
TLV‐TWA 0.5% by volume [12]. Exposure of humans to air containing 5% by volume of CO2 causes an increase in the breathing rate by a factor of approximately 3. Concentrations higher than 5% by volume rapidly cause unconsciousness and death.
NH3
TLV‐TWA 25 ppm [13].
Evaluation of the Measured Data
The concentrations of HCN and H2S are high. The concentrations of CO2 and NH3 are harmless. A vapor/air explosion cannot occur. The air in the stables in question contains the right amount of oxygen. The accident at Makkinga in 2013 occurred in an enclosed space. The measurements in the northern part of The Netherlands in (probably) 1986 make it plausible that the Makkinga accident has been caused by HCN and/or H2S.
Prologue of the Accident
Approximately 36 m3 of purge water had been added to the silo on February 16, 2013. The purge water came from a pig livestock farm. It had been used to remove NH3, dust, and smelling gaseous components from stable air in a scrubber. Purge water is added to the slurry because it contains nitrogen in the form of NH3. Moreover, it decreases the viscosity of the slurry and thus facilitates the distribution on the farmland. The farmer transferred approximately 800 m3 slurry from the silo on February 18, 2013. However, it was noticed during the distribution of the slurry on the farmland that the slurry had not been mixed properly. It was assessed that the silo mixer had not functioned adequately. Slurry remained in the silo. The slurry height in the silo was about 90 cm. The farmer then asked a specialized company to repair the mixer. It is necessary to empty the silo before the repair can be attempted.
Detailed Description of the Event
See Figure 7.4. Two employees of the company specialized in silo cleaning and repair arrived at the farm in the morning of June 19, 2013. They started the work at approximately 07.30 h while wearing watertight suits. A ladder was put up against the silo wall, a manhole in the roof of the silo was opened, and a second ladder was installed at the other side of the silo wall to be able to descend into the silo. One of the two employees entered the silo. He had equipped himself with breath protection. The breath protection consisted of a cap through which air was passed into the silo. The air came from a compressor located outside the silo. The compressor was driven by an electromotor. It was assessed after the accident that the compressor still could function. The air passed through a hose to the cap. The employee in the silo also wore detectors to detect dangerous concentrations of H2S and low O2‐concentration in the air. The second employee stood, while the first employee worked, on the ladder outside the silo and watched the first employee. The work in the silo proceeded as follows. The employee in the silo forced the slurry toward a central outlet in the silo bottom by means of a water jet. The water came from a water container outside the silo. Next, the slurry was pumped into a tankcar having a volume of 17 m3. A filled tankcar was emptied on the farmland. A second tankcar having a volume of about 36 m3 arrived at the farm at approximately 10.30 h to assist at the slurry removal. At that time, the driver of the 17‐m3 tankcar had left the farm to empty the tank on the farmland. The driver of the 36‐m3 tankcar then connected his tankcar to the silo and stayed at the farm.
Figure 7.4 Approximate reproduction of the situation during the accident at Makkinga in 2013. (1) Slurry silo; (2) container filled with water; (3) tractor and car with pump; (4) breathing air compressor; (5) tankcar for slurry transfer; (6) opened manhole in silo roof.
Source: Courtesy of Dutch Safety Board, The Hague, The Netherlands.
The employee in the silo became unwell at about 11.00 h. The two employees of the specialized company had, by that time, worked for about 3 h. The height of the slurry layer had been decreased from about 90 cm to about 10 cm. The second employee watching the man in the silo called for help and then descended into the silo without breath protection. The driver of the 36‐m3 tankcar also went into the silo without breath protection. The farmer also descended into the silo without breath protection. He had been working on the farm with a gardener. The gardener also descended into the silo. However, when he had almost reached the bottom, the farmer told him to go back because it was not safe in the silo. The gardener succeeded in climbing the ladder and leaving the silo. He saw the farmer turning the victims so that they could breathe. The farmer then also tried to get out of the silo. He came as far as halfway the ladder and then fell back into the silo.
The farmer's father thereupon called the emergency number. The fire‐brigade succeeded in making a hole in the silo wall and recovered the victims. However, three men had already died by that time and the driver of the 36‐m3 tankcar was heavily injured. The heavily injured man has recovered partially but will probably not recover completely.
The Compass of the Accident
The data and figures in this section apply for The Netherlands. There are about 20 000 dairy cattle livestock farms. There are more than 8000 pig livestock farms. The number of dairy cattle is approximately 1 500 000 and the number of pigs is about 7 000 000. Thus, the average number of dairy cattle per farm is about 75 and the average number of pigs per farm is about 875. The farm at Makkinga where the accident happened is slightly larger than the average farm.
The annual slurry production from cattle amounts to more than 65 000 000 metric t. About 50 000 000 metric t are annually transferred to the farmer's own farmland. The remaining 15 000 000 metric t per annum are used elsewhere, the greater part is transferred to a different farmland. Safety issues concerning slurry exist for slurry originating mainly from dairy cattle and pigs. More problems are experienced at dairy cattle livestock farms than at pig livestock farms.
Before 1987, silos did not have a roof. As from 1987, new silos should have a roof. As from January 1, 2013, all silos have to have a roof. The rationale of the gradual closing of silos is the reduction of emissions. Still, silos have an open connection with the atmosphere.
Low‐emission grate floors have been installed in stables in the last couple of years. Low‐emission grate floors allow the passage of manure from the stables through slits into manure cellars. However, the escape of gases from the manure cellars through the same slits into the stables has been made difficult by, e.g. rubber strips. At farms where the dairy cattle stays indoors, new stables and extensions have to be low‐emission stables and low‐emission extensions. As from 2015, that statement is valid for all new buildings and extensions. However, these measures are not sufficient to prevent HCN and H2S entering the stables when mixing is performed.
Slurry in a dairy cattle livestock farm cellar tends to develop a hard and dry crust at the surface. The slurry must therefore be mixed regularly. The slurry is often also mixed in a silo before transfer to a tankcar. As a rule, slurry from a pig livestock is directly, i.e. without mixing, pumped into a silo. Such slurry is often mixed in a silo before the transfer to a tankcar.
In recent years, on average, three serious accidents occurred annually in The Netherlands. Almost 90% of the accidents occurred in a silo, cellar, or tank till approximately 5 years ago. More than half of the accidents in the last 5 years occurred in the stables or in the environment of the stables during mixing the slurry in the slurry cellars.
Addition of Materials to the Slurry
It has been stated previously that 36 m3 of purge water had been added to the silo at Makkinga on February 11, 2013. The background has also been indicated. Transferring purge water to farmland is allowed in The Netherlands; however, mixing purge water with slurry is not allowed. The reason is that additions to slurry could mix up the bookkeeping of materials transferred to farmland. Additions of materials to slurry are not actively checked by the authorities in The Netherlands. The addition of further materials also occurs, e.g. the addition of fertilizers. It is possible that addition of materials to slurry has an effect on the formation of gaseous toxic gases.
Protections Used and Remarks
Breath Protection
The breath protection used by the employee working in the silo consisted of a cap on the head of the employee through which air was passed on to the silo. Thus, there was an open connection between the atmosphere in the cap and the atmosphere in the silo. Such protection is inadequate when there are high concentrations of dangerous substances in the air of the silo. It is neither suitable for spaces in which the air contains less than 17% by volume of oxygen. A compressed air breathing apparatus would have been required for the man working in the silo at Makkinga in June 2013. Moreover, a second compressed air breathing apparatus should have been present at the farm. The employee working in the silo wore detectors to detect a dangerous concentration of H2S and too low a concentration of O2. These detectors are useful but cannot replace breath protection. In addition, the accident has possibly rather been caused by HCN than by H2S. As to the former compound, the employee working in the silo did not wear a detector to detect dangerous concentrations of HCN.
Construction
The rule to have all slurry silos equipped with a roof as from January 1, 2013 to protect the environment turns the silos into enclosed spaces. Thus, the risks for personnel working in the silos increase.
Tackle
Means to evacuate persons from the silo if need be were absent. Such means should have been present.
Hatches
There was no hatch in the silo wall at ground level that could have been opened to assist and evacuate persons from the silo if need be. I think that such a hatch should have been present.
Stairs
Putting up ladders at both sides of the silo wall to get into the silo is improvising. It would have been better to have a permanent stairs at the outside of the silo with a platform at the top of the silo. Both stairs and platform would have to be provided with hand‐rails. A permanent ladder at the inside of the silo could be used for descending into the silo.
Ventilation
The presence of a hatch in the silo wall at ground level would enable forced ventilation through the silo by means of a fan. The air could leave at the top of the silo through the manhole.
Concluding Remarks
Four concluding remarks are made. The first one is that the safety measures during the cleaning of the slurry silo at Makkinga were inadequate. The second one is that the activities at both dairy cattle livestock farms and pig livestock farms have been scaled up in the last decades. At the same time, the rules have been tightened. The silos must have a roof and the period in which it is not allowed to transfer slurry to farmland has increased. The safety aspects have insufficiently been taken into account at the scaling up of the activities at dairy cattle livestock farms and pig livestock farms and the tightening of the rules. The third remark is that it is not good practice to add materials to slurry without knowing the effect it can have on the formation of toxic gases. The fourth and final remark concerns the standard of safety measures for the two types of livestock farms mentioned. The latter aspect receives the attention of the Dutch Safety Board. In the (petro)chemical industry, storage tanks that have contained chemical compounds are cleaned. This activity can be compared to the cleaning of a slurry silo for the two types of livestock farms mentioned. The Dutch Safety Board has asked a company specialized in cleaning industrial tanks to design a procedure for cleaning a slurry silo still containing a layer of viscous slurry. It strikes that this procedure takes better account of the risks of cleaning slurry tanks than the procedure in actual fact adhered to at Makkinga on June 19, 2013 [14].
References
- [1] Dutch Safety Board (2015). Carbon Monoxide in Bow Thruster Space, 1–22. The Hague, The Netherlands: Dutch Safety Board (in Dutch).
- [2] (2001). Carbon Monoxide. Ullmann's Encyclopedia of Industrial Chemistry, Wiley Online Library.
- [3] Dutch Safety Board (2013). Lethal Accident during Maintenance of a Phosphorus Furnace, 1–18. The Hague, The Netherlands: Dutch Safety Board (in Dutch).
- [4] (2001). Inorganic Phosphorus Compounds. Ullmann's Encyclopedia of Industrial Chemistry, Wiley Online Library.
- [5] SGS Nederland B.V. (2011). Working Safely in Enclosed Spaces, 1–42. The Hague, The Netherlands: Sdu Uitgevers bv (in Dutch).
- [6] Grosshandler, W., Bryner, N., Madrzykowski, D., and Kuntz, K. (2005). Report of the Technical Investigation of the Station Nightclub Fire, vol. 1, xvii–xxvi. Washington, DC: United States Government Printing Office.
- [7] Dutch Safety Board (2014). Lethal Accident in a Slurry Silo at Makkinga, 1–70. The Hague, The Netherlands: Dutch Safety Board (in Dutch).
- [8] Factory Inspectorate and Animal Health Service, both in the Northern Part of The Netherlands (1988). Dangers at the mixing of slurry in dairy cattle stables: the resorption of hydrogen sulphide and hydrogen cyanide (in Dutch).
- [9] (2001). Hydrogen Cyanide. Ullmann's Encyclopedia of Industrial Chemistry, Wiley Online Library.
- [10] Braker, W., Mossman, L., Effects of Exposure to Toxic Gases – First Aid and Medical Treatment, Matheson Gas Products, East Rutherford, NJ, 1970, pp. 16, 17.
- [11] (2001). Hydrogen Sulphide. Ullmann's Encyclopedia of Industrial Chemistry, Wiley Online Library.
- [12] (2001). Carbon Dioxide. Ullmann's Encyclopedia of Industrial Chemistry, Wiley Online Library.
- [13] (2001). Ammonia. Ullmann's Encyclopedia of Industrial Chemistry, Wiley Online Library.
- [14] Dutch Safety Board (2014). Lethal Accident in a Slurry Silo at Makkinga, 30. The Hague, The Netherlands: Dutch Safety Board (in Dutch).