MANURE PIT-SAFETY VENTILATION DESIGN INFLUENCE ON HYDROGEN SULFIDE GAS CONTAMINATION IN ATTACHED BARNS

Open Access
Author:
Hofstetter, Daniel William
Graduate Program:
Agricultural and Biological Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 28, 2017
Committee Members:
  • Virendra M. Puri, Dissertation Advisor
  • Dennis J. Murphy, Committee Chair
  • Eileen E. Fabian, Committee Member
  • Harvey B. Manbeck, Committee Member
  • Justin K. Watson, Outside Member
Keywords:
  • manure pit-safety ventilation
  • hydrogen sulfide
  • barn air contamination
  • CFD
  • computational fluid dynamics
  • concentration scaling
  • transient gas measurement
  • ventilation simulation
Abstract:
Current manure pit-safety ventilation research has been limited to determining the minimum amount of time required to evacuate hazardous gases to allow human entry into confined-space manure storages. Commonly installed negative pressure pit ventilation systems reduce the level of contaminant gases in the manure pit air space, but the negative pressure pit fans are usually too small to effectively ventilate a manure pit for safety purposes prior to and during human entry. The preferred method for ventilating a manure pit for human entry is to use a positive pressure pit-safety fan that delivers fresh outside air into the manure pit air space at higher air exchange rates. However, there may be cases where ventilating for a longer duration at lower air exchange rates may be desirable, such as when a pit has a slotted cover and there is an occupied animal living space located above. In these cases, too high an air exchange rate in the pit can result in dangerous contaminant gas levels in a portion of the animal living space. This has been one barrier to adoption of pit-safety ventilation practices. It is not often convenient, or even possible, to relocate all of the animals because of the size of the herd and layout of the facility. For these cases, it is desirable to determine the best combination of pit-safety fan locations and pit air exchange rates that will result in satisfactory air conditions for welfare of the animals above yet ventilate the pit in a reasonable length of time for human entry. This research develops methodologies and protocols for evaluating barn air contamination hazards during positive pressure pit-safety ventilation and to demonstrate that manure pit ventilation configuration and fan capacity do influence the level of air contamination hazard in the barn. Hydrogen sulfide (H2S) gas decay was measured at several locations inside a swine nursery room during manure pit and room ventilation. A computational fluid dynamics (CFD) model of the nursery room was developed, and transient simulations of pit-safety and room ventilation were performed. Simulation results were compared to measured gas concentrations at 15 locations. Results indicated that the simulated temporal gas concentrations at 8 of 15 locations agreed favorably (within validation criteria) after adjusting measured values for a first-order instrument response. A trend observed during analysis of CFD simulation results for a given pit and barn shape and ventilation configuration (the same pit-safety fan location and flow rate with the same barn ventilation rate) with different uniform initial manure pit H2S gas concentrations (C0) suggested that the ratio of concentration (C) at each time step during ventilation was equal to the ratio between the initial concentrations inside the manure pit. It was determined that C/C0 scaling could be used to expand the results from one CFD simulation at one C0 value to a wide range of C0 values. The maximum error when comparing simulated to estimated C/C0 values was ± 2.5% for the global maximum H2S gas concentration over time. Simulations were performed for a 12.20 m wide × 30.49 m long (40 ft wide × 100 ft long) barn located above a full-sized manure pit with a fully-slotted cover. Tunnel ventilated and mechanically cross-ventilated barn configurations were studied to determine how manure pit-safety ventilation fan configuration (location and flow rate) affects the distribution of H2S gas in the barn airspace during a barn and manure pit-safety ventilation event. Simulation results were analyzed to determine the affected area in the barn and the duration of time when the concentration of H2S gas was 50 ppm or greater, the maximum H2S concentration in the barn airspace, and how much time was required to reach safe H2S gas entry levels in the manure pit. During pit-safety ventilation, the maximum concentration in portions of the airspace within both tunnel ventilated and mechanically cross-ventilated barns was equal to the initial manure pit H2S concentration, requiring animals and personnel to be evacuated from those zones when C0 ≥ 50 ppm. The tunnel ventilated barn was divided lengthwise into five 6.10 m long × 12.20 m wide (20 ft long × 40 ft wide) quintiles for analysis. For the tunnel ventilated barn simulated in this study, animals should always be evacuated from the quintile nearest the tunnel ventilation barn exhaust fans when C0 ≥ 50 ppm during pit-safety ventilation. When C0 ≥ 200 ppm, the pit-safety fan location at the longitudinal and transverse centerline of the barn resulted in more contaminated area in the barn overall and in the three center barn quintiles than all other cases, making this the worst choice for pit-safety ventilation fan location for tunnel ventilated barns. However, there were large contiguous clear areas in the three center barn quintiles for all other pit-safety fan locations and flow rates, including the case with no pit-safety fan, when the initial manure pit H2S concentration was 300 ppm or lower. In general, pit-safety fan locations near the barn exhaust fans (counterflow locations) resulted in longer times to reach safe H2S gas entry levels inside the manure pit compared to the case with no pit-safety fan, and pit-safety fan locations near the barn air inlets (parallel flow locations) resulted in shorter times. Parallel flow with the pit-safety fan located along the longitudinal centerline of the barn resulted in less overall contaminated area in the barn than all other cases as well as the case with no pit-safety fan. The mechanically cross-ventilated barn was divided into five 2.44 m long × 30.49 m wide (8 ft long × 100 ft wide) quintiles for analysis. For the mechanically cross-ventilated barn simulated in this study, animals should be evacuated from at least portions of the quintile nearest the barn exhaust fans and the two quintiles farthest from the barn exhaust fans when C0 ≥ 50 ppm during pit-safety ventilation. However, there were large contiguous clear areas in the remaining two barn quintiles for all simulated cases when the initial manure pit H2S concentration was 200 ppm or lower. All pit-safety fan locations and flow rates resulted in shorter times to reach safe H2S gas entry levels in the manure pit compared to the case with no pit-safety fan. This work demonstrates the potential for evaluating alternative pit-safety ventilation configurations to reduce the need for animal evacuation from portions of barns located above positive pressure safety ventilated manure pits. Parallel flow pit-safety fan locations were more effective for the tunnel ventilated barn, with more contamination near the barn exhaust fans and along the barn sidewalls. In general, parallel flow pit-safety fan locations with higher flow rates resulted in shorter times to reach safe H2S gas entry levels in the manure pit, but increased contaminated area in the barn airspace. Animals do not need to be removed from barns during pit-safety ventilation when the maximum initial H2S concentration inside the manure pit is less than 50 ppm. The protocols developed for this study can be used by engineers when designing and evaluating manure pit-safety ventilation systems to reduce the risk of creating hazardous conditions inside the barn during pit-safety ventilation.