Design Criteria for Ventilation of the Enclosure of the NPDGamma Liquid

Hydrogen Target Gas Handling System on Flight Path 12 at ER2 Location: TA-53, building MPF-30, flight path 12 Organization: P-23 Date: December 01, 2004 By seppo penttila Purpose of document A purpose of this document is to identify design criteria for the hydrogen ventilation line from the enclosure of the gas handling system of the NPDGamma liquid hydrogen target so that the design will be in accordance with requirements. System Description The NPDGamma Collaboration has built a 21-liter Liquid Hydrogen (LH2) Target System for the n+p->d+γ experiment that is planned to run in flight path 12 at the Lujan Center in 2005. The LH2 target system was built and preliminary tested in the Indiana University and then moved to Los Alamos where it was reassembled for testing in the bldg MPF-35 at TA-53. To date, we have tested without cryogenics the cryostat, gas handling system, and instrumentation. Before moving the target to the experiment in flight path 12, it has to go a thorough testing in MPF-35 including hydrogen operation. The description of the LH2 target system and its safety can be found in the NPDGamma Liquid Hydrogen Target Engineering Document [1] that is available at web site www.iucf.indiana.edu/U/lh2target/expoert-files/. The LH2 target system consists of a gas handling system (GHS) that is shown in Attachment A. The GHS is enclosed to an aluminum box with a ventilation line attached. In a case of a leak in GHS the hydrogen gas is contained by the enclosure and by buoyancy the hydrogen gas flow safely in the ventilation pipe outside of ER2. The main purpose of the GHS ventilation pipe is to conduct safely any hydrogen released from GHS components away from ignition sources outside of ER2. This document is dealing with design criteria of the piping of the ventilation system. The design criteria of the enclosure are given in another document. During a normal operation the hydrogen supply is valved off by V131 and V132 and GHS contains only a small quantity of hydrogen gas in its piping, see Attachment A and B. During the typical hydrogen target filling, hydrogen gas flow rate to GHS system is limited to less than 10 SLM by V130, a restrictive orifice in the H2 supply manifold. The GHS enclosure will be monitored for hydrogen levels. Set points are so that there will be a warning at 20% and 30% LEL (LEL being defined as 4% hydrogen in air by volume). At 40% LEL the supply line valves V100 and V114 (see Attachment A) will be closed. There will also be an emergency shut down of the target and shut down of electrical power to the cave and to GHS.

The design of the vent stack should meet requirements given by CGA G-5.5-2004 “Hydrogen Vent Systems” [2]. The design criteria document addresses also requirements of NFPA 50A, Standard for Gaseous Hydrogen Systems at Consumer Sites [3] although strict compliance is not achieved due to operational requirements and space constraints. The enclosure should be in compliance with - NFPA-70, The National Electrical Code - NFPA-497 Classification of Flammable Liquids, Gases or Vapors and of Hazardous Locations. The particular requirement of concern is that GHS does not in its location have the required 15’ setback from conventional electrical equipment. By enclosing the system and venting the enclosure, a boundary is established isolating it from the ignition hazards associated with the electrical equipment. This also addresses the requirements of NFPA 50A, Standard for Gaseous Hydrogen Systems at Consumer Sites although strict compliance is not achieved due to operational requirements and space constraints. This system falls within the category of 400 to 3500 SCF of hydrogen within the code The enclosure installation should meet the following of those requirements; - Electrical equipment within the cabinet will meet the requirements for Class 1,

Division 2, Hazard Group B - The cabinet is fabricated from aluminum, non-combustible material - Ventilation is provided as required.

Since we cannot be in compliance with all the code requirements we have instituted a number of administrative and engineering controls that should bring us to achieve equivalent compliance. - GHS can only be operated by trained personnel - formal training and qualification

of operations staff are authorized by line management - extensive operating procedures - extensive procedures for leak testing of the system - continuous monitoring of hydrogen levels - welded joints or VCR joints - controls that automatically stop the filling and dump the contents of the system

upon detection of hydrogen leaks in the enclosure.

Path of the relief line piping The GHS ventilation pipe assembly is shown in Attachment C. Hazard Analysis The primary hazard of concern in design of GHS and ventilation of the enclosure is the flammability and a chance of fire in the system. The following are the failure scenarios that have been considered. Those of very low probability are not addressed by this design:

1. Rupture of low pressure pipe or fittings is considered to be very low probability because the enclosure provides protection from the physical damage, because pressure relief valves are provided to protect the system, because pipes and

fittings are rated for higher pressures than used in the system, and because the system is carefully pressure tested.

2. Minor leaks at valves and fittings are probable events and can have three categories;

a. small leaks that yield a concentration less than the hydrogen sensor alarm level.

b. leaks higher than in a) but below the automatic GHS stop filling/dump level.

c. above the stop filling/dump level. When these leaks occur the average density of hydrogen/air mixture within the enclosure and ventilation line is less than the surrounding air. This will cause a "stack-effect". The rate of the flow is determined by the difference in densities and the height of the stack. Regardless of the leak rate, the buoyancy of hydrogen causes it flow up the ventilation line as is assumed in the code. The molecular weight of hydrogen is 2 compared to the average molecular weight of air of 29. If the leak results in a concentration less than the alarm level, it is discharged up the ventilation line until the source is depleted. If it is between the alarm and the stop filling/ dump levels then operating procedures mandate that the operators would allow the leak to dissipate before opening the enclosure. Above the stop filling/dump level, the controls discharge the system through the vent line thus diverting the leak. Estimate for the flow capacity of the gas handling system ventilation line is given in Appendix D. Design Requirements

1. The ventilation piping shall be constructed of type 304L stainless steel pipe with CF-flanged joints as recommended in the Compressed Gas Association guideline for Hydrogen Vent Systems, section 5.5 to prevent corrosion and to ensure structural integrity in a fire

2. The design shall not include geometries that can result in deflagration or detonation.

3. The vent shall be sloped upward and shall not contain traps so that hydrogen will not concentrate within it.

4. Grounding of the vent shall be ensured by routing a lighting conductor the by bonding each component of the system to the building’s existing lighting conductors.

5. The ventilation line discharge shall be located using the setback required in NFPA-51A for locating outdoor, unenclosed gaseous hydrogen systems with storage capacity less than 3500 SCF.

6. The design of all other aspects of this system such as structural support and lighting protection shall comply with the LANL Facility Engineering Design Manual

ATTCHMENT A SCHEMATIC OF THE GAS HANDLING SYSTEM

ATTCHMENT B SCHEMATIC OF THE HYDROGEN SUPPLY MANIFOLD

ATTCHMENT C ASSEMBLY OF THE GAS HANLING SYSTEM VENTILATION LINE

ATTCHMENT D SIZING OF THE GAS HANDLING SYSTEM VENTILATION LINE – CALCULATION OF STACK-EFFECT The worst-case hydrogen leak will occur if the line from the H2 supply bottles is severed upstream of the H2 purifier (PRFR, see Attachment A) and inside the enclosure. Therfore, it makes sense to limit the mass flow with a restrictive orifice (V130, see Attachment B) to the maximum possible flow value at 150 psig which is the 10-SLM maximum delivery rate of the H2 purifier. H2 concentration less than 4% will not combust, 2% concentration gives a safety factor of 2. So, if a 10 SLM hydrogen mass flow rate is chosen, then airflow of (10 SLM H2)/0.02 = 500 SLM (18 CFM) is required through the enclosure for safety. The ventilation pipe of GHS has to be designed for the air flows at least 18 CFM. Assumptions are that:

1. Stack-effect is caused by the difference in densities of gases involved and the height of the stack

2. Complete mixing of gases 3. Steady-state condition 4. No wind effect

For estimate of stack-effect we use the following parameter values:

1. Inner diameter of the pipe 8” 2. Stack height = Z = 45 ft 3. Length of the pipe is = 65 ft 4. Losses caused by joints and other frictions – add about 50% to the length. The

total equivalent length =100 ft is used 5. Air density ρ = 0.057 lb/cft at elevation of 7500 ft and at sea level ρ=0.075 lb/cft 6. Hydrogen density at 7500 ft ρH2 = 0.00397 lb/cft

Pressure that creates the stack effect (SE) is

PSE = Z(ρair-ρH2) ,

where P = pressure (lb/sq ft), Z = stack height (ft), and ρ = density (lb/cft ρH2(98% air and 2% hydrogen at 7500 ft) = 0.98(0.057lb)+0.02(0.00397 lb) = 0.056 lb.

Only 2% H2 concentrations are considered since the available engineering procedures do no permit modeling of concentrations less than 2% because of the flow resistance in the stack becomes too small. For the stack effect we get

PSE = 45 ft (0.057-0.056)lb/cft = 0.045 lb/sqft. We need to convert this pressure to one that can be used in a sea level nomograph.

R = ((0.056 lb/cft)/(0.075 lb/cft))0.81 = 0.789

From an engineering formula for correction factor of density, we have

PSE(at sea level) = (1/R) x PSE (at 7500 ft)=1/0.789 x 0.045 lb/sqft = 0.057 lb/sqft If the length of pipe is 100 ft then we have 0.057 lb/sqft /100 ft. The stack -effect should be same as friction loss in the exhaust line. The friction loss of 0.057 lb/sqft/100 ft can be read from Ashrae nomograph for 8” diameter pipe. We get an air flow rate of about 60 CFM which is about three times larger than air flow rate for the worst-case scenario. This gives for 2% H2-air mixture we can have maximum of 1.2 CFM (34 SLM) hydrogen leak rate. The H2 flow was limit to 10 SLM with the orifice. A gas cylinder has 220 cf in STP, time to empty the bottle through a leak of 1.2 cfm is about 3h.