Understanding Liquid
and Hydrostatic Relief
INDUSTRIAL REFRIGERATION CONSORTIUM
RESEARCH & TECHNOLOGY FORUM
MAY 2-3, 2012
Douglas Reindl
Industrial Refrigeration Consortium
University of Wisconsin-Madison
University of Wisconsin-Madison
Principal Codes & Standards for
Ammonia Refrigeration
• ASME
• B&PV Code (Pressure vessels)
• B31.5 (Refrigeration Piping)
• ASHRAE
• Std. 15
• IIAR
• IIAR 1, IIAR 2
• Codes
• International Mechanical Code or
• Uniform Mechanical Code
• Other State & Local Codes
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1
What is required?
• All ASME stamped vessels & components require
overpressure protection [UG-125]
• Vessels & other stamped components that are to
operate completely filled with liquid shall be
equipped with pressure relief devices designed for
liquid service, unless otherwise protected against
overpressure. [UG-125(f)]
• Relief valves must be certified [UG-129]
What’s a liquid service relief?
• Liquid service relief valve: a pressure-actuated
valve closed by a spring or other means,
designed to automatically relieve liquid pressure
in excess of its setting.
• Intended for use with ASME [rated] pressure
vessels or other stamped equipment.
• Certified by the National Board and rated in U.S.
gallons per minute of 60°F [16°C] water flow at
110% of its setting
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What is required – nonstamped equipment?
• ASME B31.5-2010
• 501.4.2 Fluid Expansion Effects (Increased Pressure)
• Consideration must be given to expansion of liquid
refrigerant trapped in or between closed valves and a
means provided to prevent overpressure.
Hydrostatic relief device
• Hydrostatic relief device: pressure-actuated valve
designed to automatically relieve hydrostatic
pressure in excess of its setting caused thermal
expansion of liquid in a volume-constrained part
of a system.
These relief valves can be used to protect piping
and other NON ASME rated [i.e. “stamped”]
vessels/equipment from overpressure
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ASHRAE 15-2010
9.4.3 Hydrostatic expansion. Pressure rise resulting from hydrostatic
expansion due to temperature rise of liquid refrigerant trapped in or
between closed valves shall be addressed by the following.
9.4.3.1 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 system
b.) reseating pressure-relief valve to relieve the hydrostatic pressure to an
approved treatment system.
ASHRAE 15-2010
9.4.3.2 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.
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IIAR 2-2008 (Addendum A)
• §11.4 states:
This section applies where specifically required by other
sections in this standard and either of the following is true:
a. Equipment or piping sub-section(s) are isolated manually for any
purpose, or
b. Equipment or piping sub-section(s) can be isolated automatically as a
function of normal operation, shutdown [by any means, including alarm
or power failure], standby, or equipment or component fault and can
trap liquid refrigerant in the isolated section.
IIAR 2-2008 (Addendum A)
• 11.4.1:
• The manual isolation for any purpose of equipment and piping subsection(s) shall be undertaken by trained technician(s) taking all
necessary precautions to protect against overpressure due to
hydrostatic expansion of trapped liquid refrigerant.
• 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.
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IIAR 2-2008 (Addendum A)
• 11.4.3:
• Hydrostatic relief valves shall not be used as shut off valves (See 10.3.4.)
What references 11.4?
• Air-cooled condensers and desuperheaters (7.1.1.3)
• Evaporative condensers (7.2.1.4)
• Non-coded S&T, P&F chillers, double-pipe
condensers, (7.3.1.4, 7.4.1.4, 7.5.1.4, 8.3.1.2, 8.4.1.4)
• Secondary coolant lines (7.3.1.5, 7.4.1.5, 7.5.1.5, 8.4.1.5)
• Forced air evaporators (8.1.1.3)
• Check valves upstream of solenoids (10.3.2)
• Piping (11.3.5)
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Relief design basis
The situation and criteria that can create overpressure in a
component or subsystem
• Examples
• Vessels: overpressure caused by the change in state from
liquid to vapor due to an external heat addition
(Vapor relief)
• Based on the radiative heat gain from a fire to a vessel or
protected piece of equipment (we called this an “external load”
situation)
• Could include other sources of vapor addition: internal reliefs,
booster compressors, etc.
Relief design basis
Oil cooling S&T heat exchangers for screw compressors:
•
Oil-side: designed to address the overpressure created by
external heat gain to the oil-cooler (Liquid service relief)
• Historical practice – relieved to the floor of the machinery room
(AN UNACCEPTABLY BAD PRACTICE)
• Current practice – relieve internal to the system back to the oil separator
•
Refrigerant-side: designed to address overpressure created by
heat gain to the refrigerant-side of the oil cooler if compressor
is started and OC is valved-out (Vapor relief)
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Relief design basis
• Equipment: heat gain internal to a component (like
a heat exchanger) from a product or a secondary
fluid (Vapor or liquid)
• When heat gain occurs at a temperature greater than the
saturation temperature for the relief device set pressure
• Causes overpressure due to state change of the refrigerant
(liquid to vapor)
• Piping: heat gain due to external heat addition
(generally Hydrostatic relief)
Set Pressure Determination
Marked set pressure on PRV shall not be greater than
the MAWP [UG-125(c)(3)(b)]
• ASHRAE 15-2010
9.5.1 Pressure-Relief Valve Setting. Pressure-relief valves shall
start to function at a pressure not to exceed the design pressure
of the parts of the system protected.
Exception: See Section 9.7.8.1 for relief valves that discharge into
other parts of the system.
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Oil-Side Internal Relief
Oil Separator
Oil Cooling HX
MAWP
Max PRV Set P
Oil Cooler
Oil Separator
250
250
300
250
50
400
300
100
400
250
150
Not compliant
If this is your plant, retrofit it!
There are systems like
this that have 300
psig rated oil coolers
and 300 psig rated oil
separators!
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What to do?
• Oil cooler and oil separator have same design pressure
1. Replace oil cooler with higher MAWP
2. Install a 50 psig lower set pressure relief on the oil
separator to accommodate a 50 psig set pressure PRV
3. Relieve oil cooler to an atmospheric vented tank
4. Relieve oil cooler to another suitable treatment
system
Cardinal Rule – no liquid directly to atmosphere!
Non-stamped Equipment:
Internal Relief
• Let’s assume we are going to use a small relief
regulator like an HA2BK
• Hydrostatic relief service
Design Pressure
Component
Downstream
300
250
400
300
400
250
Max HA2BK Set P
Be sure to determine regulator’s capacity at the
design differential pressure!
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Sizing Analysis
• The driver: Heat Addition (Q)
GPM 
1 b Q
500.5 S  c p
where
GPM required liquid volume flow rate [gpm]
Q heat addition [Btu/hr]
b volumetric coefficient of expansion [1/oF]
cp specific heat [Btu/lbm-oF]
S
specific gravity of fluid (density relative to water at 62.4 lbm/ft3)
Sizing Scenario – External Heat
Load
• Analogous to vapor reliefs
• Q = 9,000 Btu/hr per ft2 of projected area
• fHS
• = 0.035 for ammonia
• = 0.0178 for oil
• Multiply fHS by 2.5 if combustible materials present
GPM  f HS  D  L
where
GPM
D
L
volume flow rate [gpm]
pipe outer diameter [in]
pipe length [ft]
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Example: Oil-side of oil cooler
• Oil cooler (ammonia system):
• Overall dimensions: 8” x 54”
• No combustible materials within 20 ft
• What flow is required for hydrostatic
overpressure protection of the oil side of the HX?
GPM min  0.0178  8"4.5 ft  0.64 gpm
Example: Oil-side of oil cooler
• The pressure drop in the outlet piping line can be
calculated using traditional relations for single phase
flow pressure drop due to frictional and fitting effects
𝐿 𝑣2
∆𝑃 = 𝑓
𝑑 2𝑔
• Need to consider potential effects of static head in
outlet piping if outlet piping rises
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In summary,
• When an engineering control for hydrostatic
overpressure protection for piping and other
equipment is required, following can be used:
• Certified liquid service relief valve (coded equip)
• Relief regulator (non-coded equipment)
• Other appropriate device
• All options require consideration of valve set or
opening pressure
Questions?
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