ASHRAE 15 - 1994 SAFETY CODE FOR MECHANICAL REFRIGERATION
IMPLICATIONS FOR MECHANICAL ROOM OWNERS AND OPERATORS
INTRODUCTION
First issued as a safety code (Standard B9) in 1930,
the same year that CFC (chlorofluorocarbon) refrigerants were
introduced in the United States, ASHRAE Standard 15 has undergone
unusually rapid change over the last half-decade. During this
same period, the air conditioning industry and its customers have
been faced with The Clean Air Act Amendments of 1990, which legislate
the phaseout of CFC production by the end of 1995. Although the
regulations permit the responsible use of these refrigerants,
intentional venting is strictly prohibited.
At the time of this legislation, neither ASHRAE nor
ARI safety standards had been updated to address the new alternative
refrigerants that were already appearing in the marketplace. Thus,
ASHRAE 15 was revised in 1992 and again in 1994. The current standard
is now being used to formulate codes for the safe installation
and operation of mechanical refrigeration systems throughout the
United States and beyond.
CODE COMPLIANCE AND SAFETY
ASHRAE Standard 15-1994 was intentionally written
in code language so it could be adopted nearly verbatim, if desired,
by model code associations. The primary goal of this standard
is to mitigate safety risks to the environment, to mechanical
room operators and, ultimately, to the general public.
With the introduction of alternative refrigerants
such as HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbons),
and mixtures of refrigerant compounds known as blends, new refrigerant
safety classifications have been added under the ASHRAE 34-1992
Standard, Number Designation and Safety Classification of Refrigerants.
Allowable exposure limits for the various refrigerants range from
a low of 10 ppm (parts per mil-lion) to a cap of 1,000 ppm. Some
blends are comprised of one or more flammable compounds, introducing
yet another safety concern. Using ASHRAE 34-1992 as a starting
point, ASHRAE 15-1994 and its subsequent addenda have been written
to help professionals minimize risk and promote safety stewardship.
Because ASHRAE 15-1994 is written in code language,
it can be difficult to read and understand. The purpose of this
article is to provide a basic overview of mechanical room safety
issues in relation to the ASHRAE Standard. It is not intended
to be a comprehensive review of all requirements. While this is
by no means a substitute for reading the document itself, it will
help you to know what to look for when reading the standard.
MECHANICAL ROOM SAFETY CHECK
For many building owners, mechanical rooms are virtually
unknown portions of the property. "Out of sight, out of mind"
rings true in far too many cases. Most owners understand only
that the equipment in the mechanical room supplies chilled water
for air conditioning. Equipment may be ten, 20, 30 or even 50
years old - each unit installed to comply with the codes of the
day.
Given the new ASHRAE Standard and the approaching
CFC and HCFC production phaseout, now is the right time for owners,
operators and engineers to conduct a complete mechanical room
safety check and review potential liability problems. The following
checklist highlights the many important items to look for (Fig.
1):
Are exhaust outlets located near inlet vents?
Safety relief devices on chillers are typically vented to the
out-side, most often through the roof. It is important to know
how close this vent is to the air intakes on the roof and to insure
that there is at least 20 feet between them.
Are roof drains vulnerable to collecting refrigerant?
If you lose refrigerant from an exhaust outlet and it vents a
few feet above a flat, curbed roof, the refrigerant could very
well lay across the roof and find the first exit - probably down
the roof drains. Most roof drains lead directly to a sewer system.
Where does the purge vent?
A conventional purge (not a new, high-efficiency model) loses
three to 20 pounds of refrigerant for every pound of noncondensible
air it removes from the system. Most vent directly into the mechanical
room, which may invite unnecessary problems. All purges should
vent outside.
fig. 1
Where are safety rupture disk outlets located?
On a CFC-11 or HCFC-123 chiller, the safety device is a thin carbon
rupture disk that's meant to shatter at 15 psig, allowing the
charge to safely exit the system. These disks are often vented
through the roof or side of the building. Yet in climates with
warm days and cool nights, moisture often condenses in the pipes
before it can evaporate off. Then, along with the rust and other
accumulations in the pipe, it ends up "sitting" on the
rupture disk. Over time, it may corrode the joint and result in
failure of the seal.
Are rupture lines the right size and length?
For convenience, safety devices sometimes run into a common header.
However, an improperly sized header can exacerbate problems in
an emergency, when the discharge capacity of safety lines is put
to the test.
Are chiller drain valves secured?
More often than not, chiller drain valves are an easy way for
an inexperienced technician to get into trouble. Make sure the
valves are locked off.
Is access to the mechanical room restricted?
Most mechanical rooms have neither a door nor anything to restrict
people from entering unsupervised. (There may be a fire door,
but it is typically left open.) Not only are mechanical rooms
potentially hazardous places, they contain systems critical to
a building's operation. Restricting access is only common sense.
The mechanical room door should have a tight seal to isolate the
room in case of an emergency.
Is a self-contained breathing apparatus (SCBA)
located outside of the room? In an emergency,
a properly- used SCBA can mean the difference between life and
death - but not if it's stored inside the room where the contamination
is likely to occur.
Are there any pits (low areas) in the room?
Check the condition of any pits or other areas below floor level.
They may house a host of unwanted chemical residues such as acids,
spilled refrigerant, cleaning solvents, etc.
Where do the floor drains empty?
Most floor drains were constructed to handle wash-water and empty
into the sewer system. But if your chiller and occupied space
share the same floor, they may also share drainage systems. Check
to see if drains in the occupied space (i.e., in restrooms) are
connected to those in the mechanical room. Imagine losing a full
charge and "finding" it in a restroom!
These are some of the many factors to consider when
evaluating the safety of your mechanical room. For potential building
owners, the condition of the mechanical room and its equipment
are significant considerations in the decision to purchase a particular
property.
APPLYING ASHRAE 15-1994
What is your compliance obligation to ASHRAE 15-
1994? If you're a professional engineer, a manufacturer or an
owner, you are liable for ASHRAE 15-1994 simply by being aware
of the standard. Compliance to the standard is also mandated in
the following situations:
Contracts. The specification
that equipment and service "must be in compliance with the
latest ASHRAE 15" has become standard contract language for
installation submittals and service agreements.
Equipment Design. Manufacturers' labels reference compliance with the standard.
Safety Stewardship. Professionals who can prove that
they have gone to the furthest extent possible to promote safety
will minimize their legal liability.
Mechanical Room Changes.
The CFC production phaseout has raised the critical issue of what
to do with an existing CFC chiller. Each solution - containment,
conversion or replacement - brings with it new con-cerns. Yet
it is important to understand that if the type of refrigerant
in a chiller is changed or if the chiller itself is replaced,
ASHRAE 15-1994 applies. Section 5.3 of the standard reads:
A change in the type of refrigerant in a system
shall not be made without the notification of the authority having
jurisdiction, the user and due observance of safety requirements.
The refrigerant being considered shall be evaluated for suitability.
The cost of mechanical room upgrades - typically
11- 13% of the total chiller conversion/ replacement project cost,
depending on the room's age and location - should be factored
into the refrigerant planning cost analysis. On the other hand,
the addition of containment devices to your existing CFC equipment
does not trigger application of the standard.
Building Code Compliance.
The major U.S. building code jurisdictions are currently in various
incorporating ASHRAE 15-1994 into their model codes. As this progresses,
the updated codes will ultimately be passed down to state and
municipal levels. For this reason, it is always a good idea to
meet with your building inspector up-front, before mechanical
room changes have been made. Find out what's happening to codes
in your area, then decide how to proceed.
ASHRAE 15-1994 OVERVIEW
A walk through ASHRAE 15-1994 begins with a general
look at the property in question and its mechanical refrigeration
systems. The type of occupancy - institutional, public assembly,
residential, commercial, large retail, industrial or mixed occupancy
- determines which system application rules apply.
The next step is to identify the type of refrigerant
system involved: Direct or Indirect. Direct Systems are those
in which the chiller sends refrigerant out into the cooling coils
near the occupied spaces. In Indirect Systems, the refrigerant
runs only through the chiller and chilled water in a separate
circuit produces the cooling in the airside system. Indirect systems
include Double Indirect Open Spray Systems, Indirect Closed Systems
and Indirect Vented Systems (all described in ASHRAE 15-1994).
The probability of refrigerant reaching the occupied
spaces depends in large part upon the type of system involved.
To characterize the degree of risk, ASHRAE 15-1994 introduces
the broad categories of Low-Probability and High-Probability Systems.
Low-Probability Systems are those in which the leakage of refrigerant
from a failed component cannot enter the occupied spaces. Examples
of Low-Probability Systems include Indirect Closed Systems, Double
Indirect Systems and Indirect Open Spray Systems where the secondary
coolant pressure exceeds refrigerant pressure.
Conversely, High-Probability Systems are described
in the standard as those in which leakage of refrigerant from
a failed component will enter the occupied space. High-Probability
Systems include all Direct Systems as well as Indirect Open Spray
Systems where the refrigerant pressure always exceeds the secondary
coolant pressure.
REFRIGERANT CLASSIFICATIONS
In its 1992 update, ASHRAE 15 introduced a refinement
in the way refrigerants are classified for toxicity. Refrigerant
compounds were divided into two groups, low-toxicity and high-toxicity,
designated by the letters A and B.
High
Flammability |
| |
Low
Flammability |
|
|
No Flame
Flammability |
|
|
fig. 2
Refrigerants with allowable exposure limits (AELs)
of more than 400 ppm are classed as type A (lower toxicity) refrigerants,
and those with AELs of less than 400 ppm are type B (higher toxicity).
ASHRAE 15-1994 further classifies refrigerants according
to their degree of flammability: (1) no flame propagation, (2)
low-flammability, and (3) high-flammability (Fig. 2). Ammonia,
for example, is considered a low-flammability refrigerant, while
refrigerants like HCFC-22 and HFC-134a have no flame propagating
properties.
It should be noted that the ingredients R-600a and
R-1270a, found in some blends, are flammable and would fall into
Class 2 or 3 if they were used as single compounds. Although their
flammability is mitigated when they are mixed into a blend, flammability
may still be an issue in leak situations. When blends leak, the
highest-pressure compound exits first, mixed with the next highest-pressure
compound. The lowest-pressure compound, then, is likely to be
left in the vessel, and in this state the mixture may be flammable.
A recent addendum to ASHRAE 34-1992 reflects the fact that the
flammability classification of a blend may change as it leaks.
The addendum states that a blend will be classified to reflect
the highest potential flammability of its components.
COMPLYING WITH ASHRAE 15-1994
When examining a mechanical equipment room for compliance
with ASHRAE 15-1994, here are some of the primary issues to consider:
Maximum Refrigerant Without a Mechanical Room.
In Table 1 of the ASHRAE Standard, allowances for all refrigerants
are listed in pounds of refrigerant per thousand cubic feet of
occupied space. Often misinterpreted, the figures in Table 1 (of
ASHRAE 15) represent the maximum refrigerant levels allowable
without a separate mechanical equipment room. If you have more
than the allowable amount of refrigerant, the system must be housed
in a separate mechanical room.
Purge System and Relief Devices.
The standard specifies that all purge systems, including high-efficiency
systems, and other relief devices must vent outside. Note: It
may be economical to tap into existing rupture disk lines, but
do consider putting a drain valve in the line or wrapping the
line in heater tape. As discussed above, even the highest-efficiency
purges lose some refrigerant. When warm gas hits a cold pipe it
will con-dense down into the pipe, depositing refrigerant against
the safety rupture disk. These disks are usually a 0.03"
thick layer of carbon and are not tolerant of corrosive conditions.
Sizing of Relief and Rupture Devices.
ASHRAE 15- 1994 specifies the appropriate sizing of pressure relief
devices based on a minimum required discharge capacity in pounds
of air per minute. The equation,
C = minimum required discharge capacity of relief device in pounds of air per minute (kg/s)
D = outside diameter of vessel in feet (m)
L = length of vessel in feet (m)
f = factor dependent upon type of refrigerant
fig. 3
shown in Fig. 3, includes an "f factor,"
which is identified in the standard for most refrigerants (if
not, consult your equipment or refrigerant manufacturer). When
converting a chiller to a new refrigerant, it is critical to check
the size of existing safety devices to be sure they are suitable.
The new standard also includes rating formulas for Discharge Capacity
of Rupture Members and Maximum Length of Discharge Piping (Figs.
4, 5).
Refrigerant Sensors. According
to ASHRAE 15-1994, all mechanical rooms must have a sensor capable
of detecting refrigerant loss. The sensor should be positioned
in areas where vapor from a refrigerant leak would most likely
concentrate, thus providing personnel with a means to avoid catastrophic
refrigerant loss (Fig. 6). Sensors should be calibrated so that
the allowable exposure limit for the refrigerant will not be exceeded.
C = rated discharge capacity in pounds of air per minute(kg/s) and
d = smallest of the internal diameter of the inlet pipe, retaining flanges, fusible plug, and rupture member in inches (mm)
p = (rated pressure psig [kPa gage] x 1.10) + 14.7 (101.33)
fig. 4
Rather than a refrigerant detector, ASHRAE 15-1992
required an oxygen deprivation sensor for Class A1 refrigerants.
The standard stated that the sensor should be capable of detecting
a .5% decrease in room oxygen levels (air is typically 20% oxygen,
so the sensor would detect a drop to 19.5%). In this old scenario,
67,000 ppm refrigerant concentrations could accumulate before
the sensor issued a warning! Safety stewardship and refrigerant
containment are better served by using a quality refrigerant sensor
with all refrigerants.
Mechanical Ventilation to the Outdoors.
Mechanical rooms must be vented to the outdoors using mechanical
ventilation. The formula ASHRAE provides for calculating ventilation
capacity requirements is shown in Fig. 7.
Access Restrictions. The
revised standard specifies that access to mechanical rooms should
be restricted to authorized personnel. In addition, the room must
have tight-fitting doors that open and close freely (i.e., no
fire doors), and any other opening that would permit the passage
of refrigerant must be sealed.
Refrigerant Storage. Section
11.5 of ASHRAE 15- 1994 reads: "The total amount of refrigerant
stored in a machinery room in all containers not provided with
relief valves and piped in accordance with the standard should
not exceed 330 lbs."
Cr = rated discharge capacity as stamped on the device by the manufacturer in pounds of air per minute (kg/s)
d = internal diameter of pipe in inches (mm)
L = length of discharge pipe in feet (m)
fig. 5
fig. 6
This is designed to allow owners to take separate,
approved storage tanks and store adequate quantities of refrigerant
for chiller servicing. This is completely allowable under EPA,
and many owners are opting to contain their CFC chillers and safely
store service refrigerant on-site. It should be noted, however,
that the ASHRAE recommendation is for a maximum of 330 lbs. per
system. Local building and fire codes should be referenced for
possible exceptions.
Self-Contained Breathing Apparatus.
The updated standard requires that a self-contained breathing
apparatus (SCBA) be located outside of the mechanical room, along
with a second SCBA for back-up. It's a good idea to check OSHA
regulations regarding SCBA use (i.e., SCBAs cannot be worn by
personnel with beards, and potential users must pass a pulmonary
test). Another issue is the optimum location of the SCBA gear
in applications where the mechanical room opens to the outdoors:
Should the gear be housed in an enclosure? Should the enclosure
be locked? How does that affect access?
Be sure to purchase a SCBA with an adequate air supply.
Large mechanical rooms may require an extended air supply in an
emergency situation.
Relief Discharge Location.
The discharge location of relief devices must be positioned at
least 20 feet away from any ventilation openings, and not less
than 15 feet above ground level (to avoid spraying someone with
refrigerant).
Combustion Device (i.e., Boiler) Limitation.
ASHRAE 15-1994 prohibits the location in a mechanical room of
any open-flame device using combustion air from inside the room.
On the other hand, an open-flame device such as a boiler may be
located in a mechanical room if combustion air is drawn from out-side.
Two acceptable alternatives are to duct outside, sealed ventilated
air in or to install a sensor that shuts the flame off in the
presence of refrigerant gas. Most refrigerant sensors are multi-port,
so they can run to several spots in the mechanical room and be
programmed to shut the boiler down if a chiller leak is detected.
Yet another option (though not specified in ASHRAE 15) is to isolate
the boiler or open-flame device with a wall or separate enclosure.
Room Dimensions. The new
standard defines an appropriately sized mechanical room as one
that allows access to all equipment, including adequate space
for service and maintenance as well as operation.
Periodic Testing. Ventilation
systems and sensors must be periodically tested in accordance
with the manufacturer's recommendation and/or local jurisdiction.
This is particularly important for refrigerant sensors that are
detecting compounds with low allowable exposure limits. Many of
these sensors require frequent recalibration.
READ ASHRAE 15-1994
Clearly, there are many issues for professionals
to consider in order to achieve compliance with ASHRAE 15- 1994.
Your best strategy is to obtain a copy of the standard and read
it thoroughly, perhaps using this article as a point of reference.
ASHRAE 15-1994 offers many good, safe solutions.
Not only is it a good idea to follow the new standard, it may
very soon be the law in your jurisdiction. To find out how the
standard is being adopted in your area, invite your local code
inspector to discuss ASHRAE 15-1994...before you make any changes
to your mechanical room.
Q = the air flow in ft. per min. (liters per second)
G = the mass of refrigerant in lbs. (kgs) in the
largest system, any part of which is located in the machinery
room 3
fig. 7