Use of Dispersion Modeling Software In Ammonia

Use of Dispersion Modeling Software In Ammonia Refrigeration Facility Design By: Martin L. Timm, PE Corporate Process Safety Manager For the UW-Madison IRC R&T Forum, May 8-9, 2013

Introduction My IIAR paper (March 2013) documents a case study of a hypothetical machinery room along with indoor and atmospheric dispersion model results for various design options. This presentation will address:

Building Use Emergency Ventilation Considerations Modeling Considerations Indoor Results Outdoor Results Conclusions

This is an overview. Please see the paper for additional details. Available at IIAR.org May 1, 2013

Building Use: CO2 Production Facility

Carbon Dioxide raw gas is purified and liquefied to make beverage-grade CO2

Machinery room is much like any ammonia refrigeration machinery room

The 2-stage ammonia refrigeration process uses oil-flooded screw compressors, shell and tube heat exchangers, evaporative condensers, etc.

The refrigerant charge (inventory) 15,000 lbs. (6,804 kg) ammonia

Subsequent work has identified opportunities to reduce the ammonia charge to below 10,000 lbs

Results of the original work are still valid and instructive

Ventilation decisions include:

the design volume flow rate of air

the number and type of exhaust fans

their location

the orientation and velocity of the air discharge

provisions for makeup air to replace the exhausted air

whether machinery room pressure must be maintained negative relative to other adjacent spaces

location, function, and type of refrigerant leak detectors,

location and function of ventilation controls,

design basis for heating equipment to maintain a minimum room temperature in cold climates

This paper concentrated on the items in yellow

Sidewall

mounted fan

Roof-mounted

upblast fan

Roof-mounted

upblast fan with

entrainment

(multiple flow impeller to

induce (mix) additional

outdoor air with the fan

discharge

Fan Types Considered in This Case Study

Case Study Machinery Room Dimensions and Volume

Room contains: •Ammonia compression equipment •CO2 compression equipment •Purification vessels, filters, etc. •Heat exchangers and low-side vessels

Outdoors: •Evaporative condensers •High pressure receiver

Flow Rate Comparison With and Without 1:1 Entrainment

Cases Considered for leak modeling: Corresponds to approximate hole size at 180 PSIG: 0.09” to 1.9” (2 mm to 48 mm) for saturated ammonia vapor 0.02” to 0.55” (0.6 mm to 14 mm) for saturated ammonia liquid

lb/min kg/min

1 0.45

10 4.5

100 45

500 227

The 500 lb/min case was chosen as an upper limit

because 1,000 lb/min would have resulted in a leak rate

that exceeded the ventilation fan capacity for those fans

designed for 2.8 ACH.

It should not be assumed that this leak rate is credible

for all or most facilities during their lifetimes. The

consideration of probability of leaks of various sizes is beyond the scope of this

paper.

Discharge Conditions Two ammonia state conditions were chosen for the modeling:

+77°F (25°C) superheated vapor at atmospheric pressure

(indoor releases)

-28°F (-33.3°C) two-phase 0.12 liquid fraction by weight

(indoor and outdoor releases)

Leak Duration was specified at 100 minutes

Note: Case Study Inventory of 15,000 lbs. would be exhausted in 30 minutes at the flow rate of 500 lb/min

UDM Plume Geometry for Outdoor Continuous Release

Warm gas will continuously rise Cold aerosol may be dense gas and slump to ground Lift-off can occur once gas warms

IRC R&T Forum 10 May 8-9

Discharge of Warm Gaseous Ammonia At Four Different Locations, 30 ACH @ 3000 fpm

Downwind Extent of 200 PPM

at Leak Rate of 500 lb/min

Discharge of Cold Two-Phase Ammonia At Four Different Locations, 30 ACH @ 3000 fpm


at Leak Rate of 500 lb/min

Discharge of Cold Two-Phase Ammonia Roof fan w/o entrainment 30 ACH


at Leak Rate of

500 lb/min

Discharge of Cold Two-Phase Ammonia Roof fan with entrainment 30 ACH


Sample Dispersion Visualization Import of Concentration Contours into 3D CAD Program

Dispersion of Stack Exhaust Reaching an Air Inlet


Conclusions (1 of 3): Dispersion modeling is useful for understanding

tradeoffs in air volume, discharge velocity, fan type, orientation, and location

The existing IIAR recommendation for upblast fans with a discharge velocity of at least 2,500 ft/min (762 meters/min) is justified

Leaks that result in the creation of a cold aerosol outdoors near ground level are expected to have the potential for reaching the longest downwind distance to concentrations of concern

Outdoor downwind concentration can be potentially reduced by placing the potential leak source inside a well-ventilated IIAR-compliant machinery room

Conclusions (2 of 3):

For “large” leaks in “small” rooms, a ventilation rate of 30 ACH in the machinery room provides a better assurance than lower ventilation rates of staying below the LFL Note: “Large” and “small” are relative terms!

Lower ventilation flow may be effective at staying below flammable limits if:

• Total ammonia inventory in the system is low enough, or

• potential leak size is minimized through an effective mechanical integrity program

• the inventory can be reliably isolated into smaller volumes when an alarm is triggered, to prevent all of the inventory from being released by a single leak

Conclusions (3 of 3):

Design factors that can potentially improve outdoor dispersion effectiveness include increasing discharge velocity, increasing discharge height, or use of fan types that entrain additional outdoor air for pre-dilution air at the fan outlet

There may be benefit to IIAR and other industry participants considering explicitly allowing the use of an engineering analysis to justify lower ventilation rates when room volume is relatively large and ammonia inventory is relatively small

If relying on isolation systems to justify lower ventilation rates in new construction, consideration should be given to evaluating the reliability of the detection and isolation systems with a quantitative or semi-quantitative tool such as layer-of-protection analysis (LOPA)

Closing Remarks:

The information presented for this case study is essentially a consequence analysis for various design options

Owners and operators may find this type of information useful to support decisions, but other information will likely be required such as local regulatory requirements, proximity of sensitive public receptors, etc.

A consequence analysis does not consider the credibility or probability of the events

When an even more refined understanding of the resulting risks is desired, probability may be evaluated as well, using a tools such as LOPA or Quantitative Risk Analysis (QRA), which build on the consequence analysis

Additional Material


A Key First Decision

Equipment location

Indoors?

Outdoors?

Combination?

Impacts:

First cost

Operating cost

Equipment life expectancy

Maintenance

Reliability

Consequences of leaks and how to respond

In this case study, the climate dictated putting compression equipment and heat exchangers indoors

Many end users choose to put at least some of the ammonia equipment indoors.

So ventilation design becomes relevant to achieve a safe, cost effective installation

Moisture in CO2 raw gas must be removed; must keep some vessels, heat exchangers, and condensate drains above freezing

Required Ventilation Calculated from ASHRAE Standard 15

Required Ventilation ASHRAE 15 Vs. 12 ACH and 30 ACH

This table shows air flow from room Table on next slide shows air discharged when dilution air at 1:1 ratio is entrained and added to the room air

Indoor Dispersion Modeling 1. UDM not suitable for estimating the change in

ammonia concentration in the room over time

2. For long duration steady leaks, could calculate leaving concentration after a long time from air flow and leak rate, independent of room size

3. For short duration and/or to account for room volume, use equation like:

4. PHAST implementation of this algorithm is “INBU”

Note on Indoor Modeling

• Method assumes instantaneous uniform mixing within the room

• In reality, mixing will be non-uniform

• Any leak will have a “zone” close to the release point that goes through the UFL and LFL

• Indoor modeling is still useful for understanding trends


Indoor Discharge of Warm Gaseous Ammonia Indoor Conc. @ Leak rate of 10 lb/min (4.5 kg/min)

Indoor Discharge of Warm Gaseous Ammonia Indoor Conc. @ Leak rate of 500 lb/min (225 kg/min)

Indoor Discharge of Warm Gaseous Ammonia Indoor Conc. @ Leak rate of 100 lb/min (45 kg/min)

Outdoor Dispersion Modeling Many programs available from various software

vendors

One class of programs uses Uniform Dispersion Model (UDM) for outdoor dispersion, solving a set of differential equations

In this study PHAST 6.7 was used

Simplifying assumptions: one exhaust fan flat terrain no influence from surrounding structures

CFD programs have more extensive computational capabilities, but are labor and cost intensive to model even simple arrangements; beyond the scope of this case study

Discharge of Cold Two-Phase Ammonia Roof fan w/o entrainment 12 ACH


at Leak Rate of

500 lb/min

Discharge of Cold Two-Phase Ammonia Roof fan with entrainment 12 ACH


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Use of Dispersion Modeling Software In Ammonia