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Understanding Liquid and Hydrostatic Relief

University of Wisconsin-Madison

Douglas Reindl Industrial Refrigeration Consortium University of Wisconsin-Madison

INDUSTRIAL REFRIGERATION CONSORTIUM

RESEARCH & TECHNOLOGY FORUM

MAY 2-3, 2012

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 2

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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 – non-stamped 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 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.

• 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 Cooling HX

Oil Separator

MAWP Max PRV Set P

Oil Cooler Oil Separator

250 250 Not compliant

300 250 50

400 300 100

400 250 150

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 Max HA2BK Set P

Component Downstream

300 250

400 300

400 250

Be sure to determine regulator’s capacity at the design differential pressure!

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Sizing Analysis

• The driver: Heat Addition (Q)

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)

pcS

QGPM

b

5.500

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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

where

GPM volume flow rate [gpm]

D pipe outer diameter [in]

L pipe length [ft]

LDfGPMHS

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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?

gpmftGPM 64.05.4"80178.0min

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

• Need to consider potential effects of static head in outlet piping if outlet piping rises

∆𝑃 = 𝑓𝐿

𝑑

𝑣2

2𝑔

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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?