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Confined Space Manure Storage Ventilation Systems

Discussion on how to determine the required amount and type of ventilation to reduce risk when entering a confined space manure pit.

Introduction

Many on-farm manure storages are considered a confined space. A confined-space is defined as one that “(1) is large enough and so configured that an employee can bodily enter and perform assigned work; (2) has limited or restricted means for entry or exit; and (3) is not designed for continuous employee occupancy.” (OSHA, 2011). The hazards contained within a confined space manure storage may include a lack of oxygen, toxic and flammable gases, and exposure to drowning. Many of these hazards may be mitigated through the use of the engineering safety standard ANSI/ASABE S607, Ventilating Manure Storages to Reduce Entry Risk (ASABE, 2010).

The research basis for the ANSI/ASABE S607 safety standard are summarized in a series of refereed engineering and agricultural safety journals (Zhao, et al., 2007a; Zhao, et al., 2007b; Pesce, et al., 2008; Zhao, et al., 2008a; Zhao, et al., 2008b). These publications describe the development and validation of the computational methodologies and data used to generate the ventilation capacities and ventilation times published in the standard.

The purposes of this publication are to: (1) Summarize the most important provisions of ANSI/ASABE S607, Ventilating Manure Storages to Reduce Entry Risk; and (2) Demonstrate how to use the S607 standard to determine the required amount and type of ventilation to reduce risk when entering a confined space manure pit. The discussion in this fact sheet is limited to covered, confined-space manure storage, transfer, or reception pits with covers that may be solid, partially slotted or totally slotted. Other publications in this series discuss the hazards associated with manure pit entry (E 51, Confined Space Manure Storage Hazards), selection and use of monitoring devices for determining manure gas and oxygen concentrations in manure pits (E 52, Confined Space Manure Gas Monitoring), and appropriate safety protocols for confined space manure pit emergencies (E 54, Confined Space Manure Storage Emergencies).

ANSI/ASABE S607 Basic Provisions and Organization

ANSI/ASABE S 607. Ventilating Manure Storages to Reduce Entry Risk, was first approved and published as a national engineering safety standard in October, 2010 by the American Society of Agricultural and Biological Engineers. The purpose of the standard is “…to reduce risk from asphyxiation, poisoning and explosions when entering confined space manure storages by specifying the positive pressure, forced ventilation requirements,… for “…empty or nearly empty covered or partially covered confined-space, on-farm, manure storages, reception tanks, agitation tanks … prior to entry.” The standard considers evacuation of contaminant manure gases such as hydrogen sulfide, methane and carbon dioxide from the manure pit using ventilation. It also considers replenishment of oxygen into the manure pit using ventilation.

This standard recommends layouts for positive pressure, mechanical ventilation systems for a wide range of confined-space manure storage facilities commonly found on livestock farms in the United States. The standard includes separate sections for manure pits with solid, totally slotted, and partially slotted covers. Solid covered tanks may be located beneath or outside the animal living quarters with square, rectangular or circular footprints. Because of the solid cover, they are considered independent and separated from the animal living quarters. Partially and totally slotted cases are limited to rectangular and square footprints and are assumed to be directly below animal living quarters.

The ANSI/ASABE S607 standard tabulates the minimum ventilation times to reduce contaminant gas concentrations to below acceptable levels for long-term human occupancy and to replenish oxygen to 20 % by volume for:

  1. Each solid or slotted cover category;
  2. Several geometries and sizes;
  3. AC (Air Exchange) rate = 1.5 air changes per minute;
  4. The maximum observed contaminant gas concentrations as reported in the literature;
  5. The maximum observed gas emission rates in manure storage facilities as reported in the literature;
  6. Fresh air intake located directly on top of manure pit cover for solid covered cases; and
  7. Fresh air intake located in contaminant free area for slotted covered cases.

The manure tank sizes and geometries in the standard are those identified by a nationwide survey of Natural Resource and Conservation Service (NRCS) waste management specialists. The maximum observed contaminant gas concentrations and maximum gas emission rates for each gas presented in Table 1 were identified from a comprehensive survey of the international technical literature. Acceptable concentrations for human occupancy for each contaminant gas were defined by the American Conference of Governmental Industrial Hygienists (ACGIH) recommended 8-hour Threshold Limit Values (TLV’s) presented in Table 1. The contaminant manure gases considered in the standard are hydrogen sulfide (H2S), carbon dioxide (CO2), and methane (CH4).

The standard also includes tabulated times to replenish oxygen levels inside each manure storage tank category from 0 percent (by volume) to 20 percent (by volume) when the manure storage tank is ventilated at 1.5 AC/m.

For each type of manure storage section, the standard presents transformation equations or adjustment factors for:

  1. Alternative ventilation strategies;
  2. AC rates different from 1.5/min;
  3. Initial contaminant gas concentrations less than maximum documented levels (for example, less than 10,000 ppm for H2S); and
  4. Fresh air intake location different from tabulated case
Table 1. Initial and Final Gas Concentrations and Gas Emission Rates Used to Determine Tabulated Minimum Ventilation Times Prior to Entry
Gas Maximum Concentrations¹
(ppm)
TLV²
(ppm)
Maximum Gas Emission Rate¹
mg m-2 s-1
Hydrogen Sulfide (H2S) 10,000 1 0.48
Methane (CH4) 700,000 1,000 2.35
Carbon Dioxide (CO2) 450,000 5,000 7.25

¹From Zhao et al. (2008b)  ²From ACGIH, 2010 

Each adjustment case is next discussed in detail.

(1 and 2) Adjustments to Tabulated Ventilation Times for Fan Location and Capacity

Tabulated ventilation times are increased by 10 to 20 percent if ventilation fans are located differently from those recommended in the standard. Also, ventilation times for rectangular and square shaped tanks are modified for air exchange rates different from 1.5 AC/min in accordance with equation 1 (Equation 4 in ANSI/ASABE S607).

Tx = Tmax(ACx/1.5)y, (Equation 1)

where Tx = ventilation time at the desired air exchange rate (seconds)
Tmax = the tabulated ventilation rate at 1.5 AC/min (seconds) from ANSI/ASABE S607
ACx = the desired air exchange rate (AC/m)
y = dimensionless exponent dependent up the tank cover type (See Table 2)

For tanks with solid covers and circular footprints, equation 2 (Equation 5 in ANSI/ASABE S607) modifies ventilation times for air exchange rates different from 1.5 AC/min.

Tx = -14.2(ACx) + Tmax + 21 (Equation 2)

Table 2. Exponent Values for Ventilation Air Exchange Rates(AC) for Square and Rectangular Manure Storages1
Cover Type Gas
H2S
Gas
CO2
Gas
CH4
Gas
O2
Solid -0.90 -0.80 -0.90 -0.80
Totally Slotted -0.90 -0.85 -0.95 -0.85
Partially Slotted      -0.90 -0.85 -0.90 -0.80

1Table 2 above is equivalent to Table 2 in ANSI/ASABE S607

(3)Adjustments to Tabulated Ventilations Times for Initial Contaminant Gas Concentration

If measured initial contaminant gas concentrations in the manure tank are less than the respective maximum concentrations given in Table 1, the tabulated ventilation times can be reduced by Equation 3 (Equation 2 in ANSI/ASABE S607) for hydrogen sulfide and carbon dioxide and by Equation 4 (Equation 3 in ANSI/ASABE S607) for methane.

T1 = 0.11(Tmax + 1)[Ln(C01)] -1.0 (Equation 3)

T1 = 0.07(Tmax + 59)[Ln(C01)] – 59.3 (Equation 4)

where T1 = ventilation time for the measured initial gas concentration (seconds)
Tmax = ventilation time for the maximum initial concentration in Table 1 (seconds)
C01 = measured initial gas concentration (ppm)

(4)Adjustments to Tabulated Ventilations Times for Pit Ventilation Air Intake Location

If the ventilation air intake source for a solid covered manure tank has zero contaminant gas concentration and oxygen levels of approximately 20.4 percent (by volume), tabulated ventilation times before entry can be reduced by 20 percent. The tabulated ventilation times for partially or totally slotted covered manure tanks are for ventilation intake sources with zero contaminant gas concentration and oxygen levels of approximately 20.4 percent (by volume). If the ventilation intake source for a manure tank with a partially or totally slotted cover is from directly above the slotted cover, ventilation times prior to entry must be increased by the gas dependent factors in Table 3. It is recommended, but not required, that ventilation intake air be from a contaminant free source with oxygen levels of approximately 20.4 percent by volume.

Table 3. Factors for Increasing Tabulated Storage Ventilation Times for Manure Storages with Partially or Totally Slotted Covers when the Storage Fan Fresh Air Intake is Located on Top of the Storage Cover Inside an Animal Living Space1
Cover Type Gas
H2S
Gas
CO2
Gas
CH4
Gas
O2
Totally Slotted 1.2 1.2 1.2 1.2
Partially Slotted 1.5 1.5 1.5 1.5

1Table 3 above is equivalent to Table 6 in ANSI/ASABE S607

Evacuating Animals from Above Ventilated Slotted-Covered Manure Pits

The animal living space above slotted covered manure tanks must be vented at the hot weather ventilation rate, as defined in ASABE EP 270.5 for a fully stocked animal facility for at least 5 minutes before, and during, ventilating the pit. In some instances all animals and personnel must be evacuated from the living space prior to ventilating the pit. For example, if the initial hydrogen sulfide concentration within the manure tank is greater than 80 ppm, all personnel and animals must be removed from the building prior to ventilating the manure tank. Similarly, all personnel and animals must be removed from the animal living quarters above the manure tank if the initial carbon dioxide concentration is above 32,000 ppm or if initial methane concentration is above 25,000 ppm (2.5 percent by volume).

The next sections of this fact sheet illustrate how to use the provisions of the standard to determine the ventilation system requirements and ventilation time requirements for ventilating manure tanks prior to entry. Examples include cases for manure storages with solid and slotted covers, as well as when the ventilation intake air is from directly above the storage or when the intake air is contaminant free.

Not all information necessary to calculate the ventilation time requirements for the given scenarios in Examples 1 to 4 are presented in this fact sheet. Some of the necessary information to complete the following examples are directly from S607 and stated in the examples. If one wants to determine the ventilation time requirements for situations or scenarios different than the following examples, one will need to obtain a copy of S607 to determine the ventilation time requirements for different situations.

Example 1: Solid Covered Tank

Example 1 illustrates using the standard to plan a ventilation system for a 20 ft square by 15 ft deep manure tank with a solid cover and with the fan and ventilation air outlet located as shown in Figure 1. The measured initial concentration of hydrogen sulfide gas in the tank is 500 ppm. The ventilation fan is located directly above the manure tank cover at the location shown in Figure 1. The air outlet is located as shown in Figure 1. The fan capacity is 5,000 cubic feet per minute (cfm)

Figure 1. Sketch of the solid-covered manure tank for Example 1
Figure 1. Sketch of solid-covered manure tank

Step 1: Determine the tabulated ventilation time from S607.

The tabulated ventilation time (Ttab), from Table 1 in S607, for this tank is 8 minutes and 56 seconds, or 536 seconds. This ventilation time is based on an initial hydrogen sulfide concentration of 10,000 ppm and an air exchange rate of 1.5 per minute.

Step 2: Adjust the tabulated ventilation time for the measured hydrogen sulfide initial concentration of 500 ppm.

The adjusted ventilation time for an initial hydrogen sulfide concentration of 500 ppm (T500), calculated using Equation 3 is 366 seconds.

T500 = 0.11(Tmax + 1)[Ln(C01)] -1
Tma = Ttab = 536 s and C01 = 500 ppm
       = 0.11(536 + 1)[Ln(500)] – 1
       = 59(6.21) – 1 = 366s

Step 3: Adjust the ventilation time for an air exchange rate of 5,000 cfm.

The fan capacity must first be converted to air changes per minute. The empty tank volume is 20 ft. x 20 ft. x 15 ft., or 6000 cubic feet. From this the air exchange rate is

AC per minute = Fan Cap/Volume
                    = 5000/6000 = 0.833 AC/minute

The required ventilation time prior to entry using the 5,000 cfm fan (T1) calculated using Equation 1 and an exponent of -0.9 from Table 2 is 622 seconds, or 10 minutes 22 seconds.
T1 = T500(AC/1.5)y
    = 366(0.83/1.5)-0.9 = 622s or 10m 22s
Rounding up to the nearest minute, the required ventilation time is 11minutes.

Example 1 Commentary: Had the fan intake air source been ducted to the fan from a location with approximately zero contaminant gas concentration and 20.4 % oxygen (O2), the ventilation time could be reduced, per Section 6.1.7 in S607. The minimum ventilation time prior to pit entry reduces to 0.8(622) or 498 seconds, or 8 minutes and 29 seconds, which when rounded up to the nearest minute is 9 minutes.

Example 2: Slotted Covered Tank - Fan directly above slotted cover- Initial H2S Concentration = 80 ppm.

The second example uses the standard to plan a ventilation system for an 8 ft wide by 150 ft long by 12 ft deep manure tank with a totally slotted cover and with the fan and ventilation air outlet located as shown in Figure 2. The measured initial concentration of hydrogen sulfide gas in the tank is 80 ppm. The ventilation fan is located directly above the manure tank cover at the location shown in Figure 2. The slotted floor openings serve as the air outlet. The fan capacity is 8,000 cubic feet per minute (cfm).

Figure 2. Sketch of the totally slotted covered manure tank

Figure 2. Sketch of the totally slotted covered manure tank

Step 1: Decide if animals and personnel need to be evacuated from quarters above the manure pit prior to ventilating the manure tank.

From section 6.4.9 and Table 7 in S607, animals and personnel need to be removed from the quarters above the slotted floor manure tank if initial hydrogen sulfide concentration inside the tank exceeds 80 ppm. Thus the animals and personnel do not have to be evacuated prior to ventilating the manure tank. However, Section 6.4.9 of S607 stipulates that the quarters above the slotted cover should be ventilated by fan or natural ventilation at the maximum summer ventilation for a fully stocked facility for at least five minutes prior to and while ventilating the manure tank.

Step 2: Determine the tabulated ventilation time from S607.

The tabulated ventilation time (Ttab), from Section 6.4.2 and Table 4 in S607, for this tank is 554 seconds or 9 minutes and 14 seconds. This ventilation time is based on an initial hydrogen sulfide concentration of 10,000 ppm, an air exchange rate of 1.5 per minute, and for contaminant free intake air at the manure tank fan. None of these conditions are met in the Example 2 manure tank.

Step 3: Adjust the tabulated ventilation time for the fan intake located directly above the tank.

Section 6.4.7 in S607 and Table 3 specify a 20 percent increase in tabulated manure tank ventilation times prior to entry if the fan intake air is taken from directly above the slotted cover. The adjusted ventilation time (T'tab) for fan intake air source above the slotted cover is 1.2 x 554, or 665 seconds or 11 minutes and 8 seconds.

Step 4: Adjust the tabulated ventilation time for the measured hydrogen sulfide initial concentration of 80 ppm.

The adjusted ventilation time for an initial hydrogen sulfide concentration of 80 ppm (T80), calculated using Equation 3 is 320 seconds or 5 minutes and 33 seconds.
T80 = 0.11(Tmax + 1)[Ln(C01)] -1
Tmax = T'tab = 665 s from Step 3 and C01 = 80 ppm
       = 0.11(665 + 1)[Ln(80)] – 1
       = 73(4.38) – 1 = 320s

Step 5: Adjust the ventilation time for an air exchange rate of 8,000 cfm.

The fan capacity must be converted to air changes per minute. The empty tank volume is 8 ft. x 150 ft. x 12 ft., or 14,400 cubic feet. The air exchange rate equals the fan capacity, in cubic feet per minute, divided by the empty tank volume in cubic feet, or 8,000/14,400 = 0.56 AC/minute. The required ventilation time prior to entry using the 8,000 cfm fan (T1) calculated using Equation 1 and an exponent of -0.95 from Table 2 is 815 seconds, or 13 minutes 35 seconds.

T1 = T80(AC/1.5)y
= 320(0.56/1.5)-0.95 = 815s or 13m 35s
Rounding up to the nearest minute, the required ventilation time is 14 minutes.

Example 3: Slotted Covered Tank – Intake air ducted from contaminant free source.

Example 3 illustrates using the standard to plan a ventilation system for the same fully slotted covered manure tank and the same conditions in Example 2 except the fan intake air is ducted from a source with contaminant free air and an oxygen content of approximately 20.4 percent by volume as illustrated in Figure 3.

Figure 3. Sketch of the totally slotted covered manure tank

Figure 3. Sketch of the totally slotted covered manure tank

Steps 1 and 2: These planning steps are identical to those for Example 2 except the tabulated ventilation time doesn’t have to be reduced by 20 percent. The tabulated ventilation time prior to entering the pit is 554 seconds.

Step 3: Adjust the tabulated ventilation time for the measured hydrogen sulfide initial concentration of 80 ppm.

The adjusted ventilation time for an initial hydrogen sulfide concentration of 80 ppm (T80), calculated using Equation 3, is 266 seconds or 4 minutes and 26 seconds.
T8 = 0.11(Tmax + 1)[Ln(C01)] -1
Tmax = T'tab = 554 s from Step 3 of example 2 and C01 = 80 ppm
        = 0.11(554 + 1)[Ln(80)] – 1
        = 61(4.38) – 1 = 266s

Step 5: Adjust the ventilation time for an air exchange rate of 8,000 cfm.
The air exchange rate is the same as in Example 2, or 0.56 AC/minute.

The required ventilation time prior to entry using the 8,000 cfm fan (T1) calculated using Equation 1 and an exponent of -0.95 from Table 2 is 678 seconds, or 11 minutes 18 seconds
T1 = T80(AC/1.5)y
    = 266(0.56/1.5)-0.95 = 678 s, or 11 m 18 s

Rounding up to the nearest minute, the required ventilation time is 12 minutes.

Example 4: Slotted Covered Tank – Fan directly above slotted cover- Initial H2S Concentration = 500 ppm

This tank and ventilation system details are exactly the same as those in Example 2 and Figure 2 except the initial hydrogen sulfide concentration inside the tank is 500 ppm.

Step 1: Decide if animals and personnel need to be evacuated from living quarters above the manure pit prior to ventilating the manure tank.

From section 6.4.9 and Table 7 in S607, animals and personnel need to be removed from the living quarters above the slotted floor manure tank if initial hydrogen sulfide concentration inside the tank exceeds 80 ppm. Thus the animals and personnel must be evacuated prior to ventilating the manure tank. Section 6.4.9 of S607 also stipulates that the living quarters above the slotted cover should be ventilated by fan or natural ventilation at the maximum summer ventilation for a fully stocked facility for at least five minutes prior to and while ventilating the manure tank.

Step 2: Determine the tabulated ventilation time from S607.

The tabulated ventilation time (Ttab), from Section 6.4.2 and Table 4 in S607, for this tank is 9 minutes and 14 seconds, or 554 seconds. This ventilation time is based on an initial hydrogen sulfide concentration of 10,000 ppm, an air exchange rate of 1.5 per minute, and for contaminant free intake air at the manure tank fan. None of these conditions are met in the Example 4 manure tank.

Step 3: Adjust the tabulated ventilation time for the fan intake located directly above the tank.

Section 6.4.7 in S607 and Table 3 specify a 20 percent increase in tabulated manure tank ventilation times prior to entry if the fan intake air is taken from directly above the slotted cover. The adjusted ventilation time (T'tab) for fan intake air source above the slotted cover is 1.2 x 554, or 665 seconds or 11 minutes and 8 seconds.

Step 4: Adjust the tabulated ventilation time for the measured hydrogen sulfide initial concentration of 80 ppm.

The adjusted ventilation time for an initial hydrogen sulfide concentration of 80 ppm (T500), calculated using Equation 3 is 453 seconds or 7 minutes and 55 seconds.

T500 = 0.11(Tmax + 1)[Ln(C01)] -1
Tmax = T'tab = 665 s from Step 3 and C01 = 80 ppm
        = 0.11(665 + 1)[Ln(500)] – 1
        = 73(6.21) – 1 = 453 s

Step 5: Adjust the ventilation time for an air exchange rate of 8,000 cfm.

The air exchange rate is the same as in Example 2, or 8,000/14,400 = 0.56 AC/minute.

The required ventilation time prior to entry using the 8,000 cfm fan (T1) calculated using Equation 1 and an exponent of -0.95 from Table 2 is 1155 seconds, or 19 minutes 15 seconds.

T1 = T80(AC/1.5)y
    = 453(0.56/1.5)-0.95 = 1155 s or 19m 15 s

Rounding up to the nearest minute, the required ventilation time is 20 minutes.

Summary

The basis and general provisions of the new standard, ANSI/ASABE S607, Ventilating Manure Storages to Reduce Entry Risk, have been presented. This was followed by four design examples of solid covered and slotted covered manure tanks to illustrate how to use correctly the provisions of S607. Further details related to confined space manure tank hazards, testing for contaminant gases and oxygen concentrations in manure tanks, and emergency rescue procedures were identified (fact sheets E 51, E 52, and E 54, respectively). Users of the ANSI/ASABE S607 standard are reminded that ventilating the storage tank prior to entry does not preclude the need for best safety practices for entering confined space manure storages. Even when a manure storage is ventilated in accordance with the provisions of S607, anyone entering the storage must abide by the following entry procedures:

  • Post appropriate warning signs near entrances of confined manure storage facilities.
  • Keep ventilation instructions including required ventilation time near the confined manure storage facility for easy access in an emergency.
  • Never work alone when entering a tank
  • Monitor gas levels in the tank prior to and during the entire entry event
  • Ventilate the tank for the rate and time specified in ANSI/ASABE S607 prior to and during the entry event
  • Use a safety harness and emergency retrieval system whenever entering

References

  • American Society of Agricultural and Biological Engineers, 2003. ASAE EP270.5. Design of Ventilation Systems for Poultry and Livestock Shelters. St. Joseph, MI. 19pp.
  • American Conference of Governmental Industrial Hygienists. 2010. Hydrogen Sulfide: TLV® Chemical Substances 7th Edition Documentation. Publication #7DOC-316. ACGIH, Cincinnati, Ohio
  • American Society of Agricultural and Biological Engineers (ASABE). 2010. ANSI/ASABE S607. Ventilating Manure Storages to Reduce Entry Risk. St. Joseph, MI. 11pp.
  • Hill DE, Murphy DJ, Steel JS, Manbeck HB. 2011. Confined space manure storage emergencies. E 54. The Pennsylvania State University, College of Agricultural Sciences, Department of Agricultural and Biological Engineering, University Park, PA. 4 pp.
  • Occupational Safety and Health Act. 2011. 1919.146 Permit-required confined spaces. Date accessed: June 2011
  • Pesce EP, Zhao J, Manbeck HB, and Murphy DJ. 2008. Screening ventilation strategies for a confined-space manure storage. Journal of Agricultural Safety and Health 14(3):283-308.
  • Steel JS, Murphy DJ, Manbeck HB. 2011. Confined space manure storage hazards. E 51. The Pennsylvania State University, College of Agricultural Sciences, Department of Agricultural and Biological Engineering, University Park, PA. 4 pp.
  • Steel JS, Murphy DJ, Manbeck HB. 2011. Confined space manure gas monitoring. E 52. The Pennsylvania State University, College of Agricultural Sciences, Department of Agricultural and Biological Engineering, University Park, PA. 5 pp.
  • Zhao J, Manbeck HB and Murphy DJ. 2007a. Computational fluid dynamics simulation and validation of H2S removal from fan ventilated confined-space manure storages. Transactions of ASABE 50(6):2231-2246.
  • Zhao J, Manbeck HB and Murphy DJ. 2007b. Hydrogen sulfide emission rates and inter-contamination measurements in fan ventilated confined-space manure storages. Transactions of ASABE 50(6):2217-2229.
  • Zhao J, Manbeck HB and Murphy DJ. 2008a. Computational fluid dynamics modeling of ventilation of confined-space manure storage facilities: applications. Journal of Agricultural Safety and Health 14(4):405-429.
  • Zhao J, Manbeck HB and. Murphy DJ. 2008b. Computational fluid dynamics simulations of gas evacuation and O2 recovery times for ventilated confined-space manure pits. Transactions of ASABE 51(6):2135-2149.

Prepared by Harvey B. Manbeck, Professor Emeritus, Dennis J. Murphy, Professor, and Joel S. Steel, Senior Research Associate

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Article Details

Title

Confined Space Manure Storage Ventilation Systems

Code

E-53

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Contact Information

Dennis Murphy
  • Nationwide Insurance Professor and Extension Safety Specialist
Email:
Phone: 814-865-7157