SCBA Charging Determining an appropriate cascade volume

Executive Summary:

Given an owner specified SCBA compressor capacity of 5 SCFM, there is no technical rationale to support more than one or two cascade flasks (5000 psi ASME flask = 1.47 ft3). The size of a vessel/crew complement is not a factor in determining an appropriate cascade volume. Due to a wide variety of compressor capacities/cascade volumes (with no discernable design criteria) the results from a sampling of previous ship designs are inconclusive. In the absence of an owners requirement for a specific cascade charging capability, the cascade volume is determined solely as a function of compressor capacity and is provided only to provide for proper compressor operation. Operationally, if the Coast Guard were to decide a larger immediate SCBA charging capability were worthwhile, recommend reconsideration of the SCBA compressor size and/or the SCBA system design pressure.

Discussion:

There are a number of possible approaches to answering the question of how much cascade volume is appropriate for an SCBA charging system. Engineering design documents and regulatory body requirements (ABS, CFR, and NAVSEA Design Criteria Manuals & Gen. Specs) were consulted, but provide no guidance specific to SCBA charging. Recent ship designs were reviewed as well; however, no consistency is evident in the as-built results or specification requirements regarding SCBA cascade volumes. Similarly, while larger than the 5 SCFM compressor on OPC, the capacities on T-AKE & T-AGM 25 (13 SCFM), T-AO(X) (16 SCFM) and LCS (20 SCFM) reflect no consistent sizing criterion. The size of an SCBA compressor and cascade volume appears to be more an owners requirement, reflecting the individuals or organizations involved, than a result of hard requirements. In the absence of sizing criteria to shed light on the basis for other system designs, such a sampling is simply inconclusive.

Based on firefighting scenarios provided by the Coast Guard, the number of SCBA bottles required by ABS Naval Vessel Rules will allow the crew to engage a fire with continual 3-men teams for 4 hours assuming no SCBA bottles are charged during firefighting operations. If SCBAs are recharged during firefighting operations, the 4 hours can be extended to almost 7 hours. Either way, this has no affect the size of an SCBA cascade volume. For a long term charging operation, only the capacity of the compressor will significantly affect the speed at which bottles can be recharged; cascade volume will have little impact on this operation other than to allow the compressor to run without an SCBA bottle attached.

In the operation of any air compressor, a receiver volume is always recommended by compressor manufacturers. The reason for this is to insure that when a compressor starts, that it runs long enough to come up to temperature (for reduced friction) and that the oil comes up to temperature as well (to evaporate accumulated water from the crankcase). Short cycling is damaging to the equipment and will result in increased wear and associated problems. While SCBA bottles can provide a sufficient volume when connected, this volume will always be dependent upon the number of bottles being charged and the residual bottle pressure at the beginning of charging. These factors cannot be assured under all conditions. For an SCBA system, a receiver (cascade) volume also provides the ability to quickly recharge an empty SCBA bottle without the compressor in operation, but unless specific criteria regarding such a charging ability is required by the owner, cascade volume is not determined by this ability to recharge. Proper operation of the compressor is the only consideration taken into account.

In light of these factors, the question of cascade charging volume comes down to a matter of compressor operation and SCBA charging without use of the compressor. Bauer Compressors (a leading manufacturer of SCBA charging systems) recommends no more than 4 starts per hour (to avoid short-cycling) and provides a formula for calculating starts/hour as well as a method of easily calculating the available system capacity through cascade charging.

Calculations:

1) What volume is required to provide Bauer recommended 4 starts/hour? Per Bauer, 4 starts per hour can be calculated by the following formula:

VR = 58 x (QC/P)

VR = volume of storageQC = compressor capacityP = dead band of compressor (difference between starting/stopping pressure)

58 x 5/450 = .64 ft3

2) How many empty 4500 psi, 45 minute SCBA bottles can be charged from two fully charged ASME 5000 psi flasks?

a) An ASME 5000 psi cylinder has a volume of 1.47 ft3 and a capacity of 436 scf. b) Per the table below, at 4500 psi, 1 ft3 of air = 274.85 scf. 1.47 ft3 x 274.85 = 404.03 scf.

c) 436 scf 404 scf = 32 scf available capacity in a single 5000 psi ASME flask operating between 5000 and 4500 psi.d) Per Scott Safety, a 4500 psi, 45 minute SCBA bottle has a capacity of 65 scf.e) Available volume from one cascade flask is 32 scf. Each SCBA bottle is 65 scf. 2 ASME 5000 psi flasks are required to recharge one SCBA bottle to 4500 psi.

The results indicate two things: 1) The volume required to avoid short cycling is easily achieved by a single ASME 5000 psi flask and 2) ASME 5000 psi flasks do not provide a significant recharging capability for 4500 psi SCBA bottles.

Given the cascade volume required for charging a single SCBA bottle, there is not a significant additional charging capability to be gained by adding cascade flasks without reconsidering compressor capacity and/or system design pressure. Bauer manufactures a Mini Unicus recharging system (such as might be provided for OPC) that is typical of charging system designs and includes an integral rack with space for two cascade flasks. In the absence of direction from the Coast Guard (other than compressor capacity), 2 ASME flasks (2.94 ft3) will suffice. Bauer generally recommends a cascade volume of 50 100% of compressor capacity; two ASME flasks would provide approximately 60%.

Based on the above discussion, there is no technical rationale to support more than one or two cascade flasks. Operationally, if the Coast Guard were to decide a larger immediate SCBA charging capability was worthwhile, it is recommended that the size of the compressor and/or the specified system design pressure be reconsidered.

Conclusion:

With a 5 SCFM compressor, two ASME 5000 psi flasks (approximately 3 ft3) provide sufficient volume for compressor operation while allowing for the charging of a single SCBA bottle without use of the compressor.