|
Storage systems increase the overall efficiency of a compressed breathing air
system. A properly sized storage system allows peak demands to be met with a
smaller compressor than would be otherwise be required. Stored air can also be
used to carry on operations in the event of compressor failure, loss of power to
run a compressor, or in areas where compressors are not available. When
selecting a storage system to address your specific needs, there are several
factors to consider.
ASME vs. DOT
There are two primary types of storage vessels for compressed breathing air
commonly referred to as ASME and DOT. ASME vessels are designed and constructed
in accordance with Section Eight of the American Society of Mechanical Engineers
(ASME) code for Unified Pressure Vessels. DOT cylinders are designed and
constructed to Department of Transportation (formerly the ICC or Interstate
Commerce Commission) standards. Older cylinders bear ICC, rather than DOT
markings.
OSHA Regulations
OSHA requires all air receivers and safety valves to be constructed in
accordance with Section VIII of the 1968 edition of the ASME Boiler and Pressure
Vessel Code. According to this section, it is recommended that:
- Air receivers should be installed to ensure easy accessibility for
service and maintenance.
- Air receivers should be equipped with a drain valve, pressure relieve
valve, isolation valve and pressure gauge.
- No valve of any kind should be placed between the air receiver and the
safety valve
- Safety valves should be tested frequently to ensure they are in good
operating condition.
The chart summarizes the differences between ASME vessels and DOT cylinders. As
indicated, OSHA regulations require the use of ASME vessels for applications
where the high pressure storage system is permanently affixed to the compressor.
Peak Refill Requirements
A storage system allows you to pump a large quantity of air in anticipation of
your future refilling needs. If the storage system is properly sized, you will
be able to refill a large number of BA cylinders quickly without having to wait
for the compressor to fill each breathing air cylinder individually.
A properly sized storage system should handle your peak refilling requirements.
A 454 cubic foot storage receiver pressurized to 5000 psig will fill seven 2216
psig empty 45 cubic foot SCBA cylinders without starting the compressor. The
eighth SCBA cylinder filled from the storage receiver will have 2300 psig when
pressure equalizes between the receiver and BA cylinder. The compressor must be
started to complete the filling of the SCBA cylinder. A fifteen horsepower
compressor will complete the filling in approximately one minute. Additionally,
SCUBA cylinders can be filled by adding storage receivers to the storage system
and/or by starting the compressor at the beginning of the cylinder refill
process.
Storage System Sizing Guide (Chart B) reflects
the refill capability of various storage vessels operating at the specified
pressure and volume. Refill calculations are based upon the storage vessel(s)
being piped in a bulk or cascade configuration.
Bulk Storage vs. Cascade Storage
Storage receivers can be piped separately or together as a single storage bank.
The cascade method of refilling BA cylinders provides a high degree of refilling
efficiency. Using this method the operator must allow the BA cylinder to be
refilled or equalized, with the storage band operating at the lowest air
pressure within the storage system. Following the equalization of pressure, the
operator must sequentially select the storage bank with the next highest
pressure to complete the refilling process.
When compared to the bulk storage method of filling, cascade operation has two
major drawbacks:
- The sequential method of operation is complex and labour intensive.
- Both the initial purchase price and long-term maintenance costs are
higher, due to the number of valves, gauges and other components involved.
The major advantage of the cascade operation is its efficient use of storage
system pressure. With a stand-alone storage system, more cylinders can be
refilled using the cascade method. However, when a compressor is added to the
storage system, the efficient use of storage pressure is less critical since the
compressor is able to "top off" BA cylinders, making up any pressure deficit
that may exist. When a properly sized compressor is used, cascading is
unnecessary.
Actually, when both a storage system and a compressor are used, there is no
difference in the amount of compressed air required to fill BA cylinders using
either the cascade or bulk storage fill method.
System Refill Capability
The following formula will help you determine the number of cylinder refills
that can be obtained from a complete cylinder refill system. Since the formula
does not consider all the potential variables involved, it should be used as a
guideline only. When compared to actual practice, the calculations tend to be
very conservative since the chart values are based on refilling totally empty BA
cylinders.
Cylinder Refill System Fills per Hour =
Compressor Fills per Hour (Chart A) + Fills from Storage
(Chart B) + Residual Factor (Chart B)
Example:
- Compressor: 15 horsepower
- Storage: 3-DOT Receivers (509 scf at 6000 psig)
- Piping: Bulk
- BA cylinder: 45 cf, 2216 psig
Compressor fills/hr.=
11.3 Storage fills=15.0 Residual factor=+7.0 Cylinders
filled per hr=33.3 (SCBA, 45 cf, 2216 psig) |
