Hydrostatic Relief Protection in Ammonia Refrigeration Systems
In July 2006 OSHA issued a written interpretation related to the potential hazards
associated with trapping liquid ammonia
(http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25576). The opening sections of this interpretation are duplicated below:
Question 10: Has OSHA taken a position with regard to the elimination of safety
devices based on “trained operators”?
Response 10: Your scenario addresses a requirement in ANSI/IIAR 2 related to
the control of an over-pressure hazard created when equipment containing liquid
ammonia (NH3) is isolated. Isolated equipment has potential to be over-
pressured due to thermal expansion effects of the liquid which can result in
rupture and loss of containment from the equipment (liquid expansion hazard).
As you mentioned, ANSI/IIAR 2 contains requirements to address this hazard.
Given the serious nature of the liquid expansion hazard, you question whether
the administrative control alternative listed in ANSI/IIAR 2 — 1999, Section 7.3.4
is adequate to control this hazard. You also question whether OSHA accepts
administrative controls exclusively when engineering controls, i.e., safety
devices, are also identified as a control measure for the same hazard/condition.
It is important to understand that the ammonia refrigeration industry, through
ASHRAE 15, ASME 31.5 and ANSI/IIAR-2, has identified this liquid expansion
hazard as a hazard with potentially serious/catastrophic consequences. The use
of trained operators/technicians can reduce the probability that a part of a liquid-
filled NH3 system might be incorrectly isolated. However, if the system relies
exclusively on a trained operator to ensure safe operations, operator or
procedural deficiencies (e.g., improper isolation of the equipment due to an
operator error, inadequate procedure, inadequate supervision, communication
error, etc.), may result in a catastrophic release because there is no other means
to control/relieve the overpressure. Therefore, OSHA does not accept the use of
trained technicians/operators as the sole means to control this hazard...
Thus OSHA appears to reject the notion that there are times when administrative
controls such as training, written operating procedures, and lockout/tagout procedures
would not provide sufficient hydrostatic relief protection. The written interpretation
seems to indicate that engineering controls, such as hydrostatic relief valves, need to be
installed wherever ammonia could be trapped between two valves. Field response to
the this interpretation has included some inspectors insisting on the installation of
hydrostatic relief devices on all refrigerant lines with isolation valves, even if the lines
contain only ammonia vapor.
The IIAR agrees with OSHA that the potential hazards associated with trapped liquid ammonia need to be identified (for example during Process Hazard Analysis studies)
and adequately controlled though the use of engineering and/or administrative controls.
We dispute the notion, however, that it was ever the intent of ASHRAE 15, ASME B31.5
or ANSI/IIAR 2 to require engineering controls in all cases where liquid ammonia could
be trapped between two valves. The interpretation appears to negate good engineering
practices available within the refrigeration industry while assigning greater reliance on
OSHA field inspectors for refrigeration system expertise which they may not possess.
There are situations where engineering controls (such as hydrostatic relief protection)
are the primary control provided to protect against trapped liquid. For example, Section
220.127.116.11 of ANSI/IIAR 2-2008 states that “A hydrostatic or differential pressure relief
device (or non closeable vent pipe) shall be used for pressure protection of a liquid
pump and its associated piping.” This is because liquid can be automatically trapped
between the pump discharge and the automatic solenoid valves downstream.
But there are many other situations where designers and system operators must rely on
administrative controls to provide adequate control. Attachment 1 contains an example
where manual valves are installed to provide isolation for a solenoid valve in a liquid
supply line to a vessel. The attachment also shows a hydrostatic relief installed in the
discharge line of a liquid pump. Both examples are circled in this attachment. If
maintenance is required on the solenoid valve, liquid must be removed from the line and
the manual valves must be closed using administrative controls before maintenance can
be safely performed on the solenoid valve. We believe that providing an additional
engineering control, such as a hydrostatic relief device, in this line would actually
increase rather than decrease potential hazards in this situation. The designer would
be faced with one of two unpleasant options when installing this hydrostatic relief valve:
The hydrostatic relief valve could be piped so that it is vented to atmosphere.
The problem with this approach is the risk that large quantities of liquid ammonia
would be vented directly to atmosphere if the hydrostatic relief valve were to
open. This risk should be avoided at all costs and is discouraged in the
The hydrostatic relief valve could be piped so that it relieves to another part of
the system. The problem with this approach is that hydrostatic relief devices
“shall not be used as shut off valves” per section 11.4.3 of ANSI/IIAR 2-2008. At
least one manual valve would need to be installed downstream of the hydrostatic
relief device as a service valve. When doing maintenance on the solenoid valve,
the service valve would have to be closed to fully isolate the solenoid valve from
the rest of the system. Thus the facility would ultimately need to rely on
administrative controls since the hydrostatic relieve device would be isolated and
out of service during this maintenance operation once the service valve is closed.
During normal operation, the piping would be unrestricted and not subject to over
The installation of hydrostatic relief devices requires service and maintenance of these
devices which increases the number of isolation valves and, indeed, the need to either provide “administrative” control procedures or additional hydrostatic protection devices
to enable such service. At some point, administrative controls are ultimately required.
We believe that one of the reasons the OSHA clarification was written is the
appearance of a “conflict” between ASHRAE 15 and IIAR-2. ASHRAE 15 appeared to
allow no provisions for the use of administrative controls (such as “trained technicians”)
in lieu of hydrostatic relief protection. ANSI/IIAR 2 and ANSI/ASHRAE 15 have since
been amended to clarify administrative controls which should be provided to address
the thermal expansion effects of liquid ammonia.
The following language is contained in ANSI/IIAR 2-2008 (Addendum A):
11.4.1 The manual isolation for any purpose of equipment and piping sub-
section(s) shall be undertaken by trained technician(s) taking all necessary
precautions to protect against overpressure due to hydrostatic expansion of
trapped liquid refrigerant.
NOTE: This typically involves the implementation of energy control procedures
and training in compliance with OSHA 29CFR1910.147 [ref.18.104.22.168].
11.4.2 Equipment and piping sub-section(s) that can be isolated automatically in
accordance with 11.4(b) shall be protected against overpressure due to
hydrostatic expansion of trapped liquid refrigerant by either:
a. A hydrostatic relief device relieving to another part of the system or to an
appropriately engineered location, or
b. An expansion compensation device.
The following language is contained in Addendum g to ANSI/ASHRAE 15-2007:
22.214.171.124 If trapping of liquid with subsequent hydrostatic expansion can occur
automatically during normal operation or during standby, shipping, or power
failure, engineering control(s) shall be used that are capable of preventing the
pressure from exceeding the design pressure. Acceptable engineering controls
include but are not limited to the following:
a. pressure-relief device to relieve hydrostatic pressure to another part of the
b. reseating pressure-relief valve to relieve the hydrostatic pressure to an
approved treatment system.
126.96.36.199 If trapping of liquid with subsequent hydrostatic expansion can occur only
during maintenance—i.e., when personnel are performing maintenance tasks—
either engineering or administrative controls shall be used to relieve or prevent
the hydrostatic overpressure.
Thus both standards clearly state that administrative controls can be used to relieve or
prevent hydrostatic overpressure and that there are times when engineering controls
are not required.
The IIAR supports the Process Safety Management (PSM) program to enhance training
for industrial refrigeration operators. Such training is appropriate and should be
supported by OSHA. In fact, the PSM Standard and Risk Management Program rule
are, themselves, administrative controls that OSHA and EPA stress as required
practices. The written interpretation related to hydrostatic relief protection is not
consistent with the philosophy provided in these regulations.
In conclusion, we propose that the written clarification be amended to indicate that:
The need for hydrostatic relief protection, either provided by engineering controls
or administrative controls, should be evaluated in ammonia refrigeration systems
following the guidelines provided in ANSI/IIAR 2 and ASHRAE 15.
The most common way to conduct this evaluation is during a hazard analysis of
the refrigeration system. If the system changes, Management of Change (MOC)
procedures should be followed if the facility is subject to OSHA’s PSM standard
and the EPA’s Risk Management Program requirements.
There are situations, for example during maintenance operations, where
administrative controls are the primary controls used to prevent hydrostatic
It is important to have strong administrative controls in place. These controls
may include training, written operating procedures, energy control
(lockout/tagout) procedures, signs, tags and locks.
EXAMPLE: LIQUID SUPPLY LINE and
LIQUID PUMP HYDROSTATIC RELIEF