pressure relief system design practices
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Pressure Relief SystemDesign Practices
May 1, 2008
By Richard Akinretoye
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Slide 3
Copyright © Siemens AG 2006. All rights reserved.
Historical Timeline From 1870 to 1910 there were about 10,000 steam boiler
explosions (including 27 boilers at one time in a PA mine)
In 1911 ASME set up the Boiler and Pressure Vessel Committee(eventually led to ASME, Boiler and Pressure Vessel Code)
In 1955, API published first document on Pressure Relief Systems.
In 1969, API published 1st Edition of API RP 521 separate from
API RP 520. In 1976 AIChE formed DIERS (Design Institute for Emergency
Relief Systems)
In 1993, OSHA 1910.119, “Process Safety Management of Highly
Hazardous Chemicals” went into effect
More than 34 citations for violation of OSHA 1910.119 d(3)(i)(D)
since 1998.
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Slide 4
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Equipment Requiring Protection
Pressured Vessels
ASME Section VIII coded vessels
Design pressure greater than 15 psig
ASME Section I Vessels
Fired vessels and steam boilers
Low Pressure Tanks
API 620 and 650 tanks
Design pressure less than 15 psig to atmospheric
Machinery and other equipment
Steam turbines, pumps, filters etc
Refrigeration equipment
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Slide 5
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Standards & Guidelines
API RP 520 and API Standard 521
Applies to ASME Section VIII coded vessels
Applies to non-refrigeration equipment
ASME Section I Vessels
Applies to fired heaters and steam boilers
NFPA 30 and API Standard 2000
Applies to low pressure tanks
Applies to LPG tanks
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Slide 6
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Pressure Relief Devices
Provide last line of defense against equipment
malfunctions and human errors
Are relatively simple
Are relatively reliable
Provide perfect separation between basic process control
systems and Safety Instrumented Systems (SIS’s)
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Slide 7
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API Standard 521
Overpressure Scenarios List
1. Closed outlets on vessel
2. Cooling failure to condenser
3. Top tower reflux failure
4. Sidestream reflux failure
5. Lean oil failure to absorber
6. Overfilling vessel
7. Accumulation of non-condensables
8. Entrance of highly volatile
material
9. Abnormal heat input
10. Failure of control valves
11. Split exchanger tube
12. Internal explosions
13. Chemical reaction
14. Hydraulic expansion
15. Exterior fire
16. Power failure
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Slide 8
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Example Distillation System
FV-1
PV-1
PSV-1
LV-1
To Flare
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Slide 9
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Example Distillation System
FV-1
PV-1
PSV-1
LV-1
Inlet Control ValveFails Open
To Flare
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Slide 10
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Example Distillation System
FV-1
PV-1
PSV-1
LV-1
Inlet Control ValveFails Open
Blocked Outlet
To Flare
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Slide 11
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Example Distillation System
FV-1
PV-1
PSV-1
LV-1
Inlet Control ValveFails Open
Blocked Outlet
Loss of Overhead
Coolant
To Flare
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Slide 12
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Example Distillation System
Tube Rupture
in Reboiler
FV-1
PV-1
PSV-1
LV-1
Inlet Control ValveFails Open
Blocked Outlet
Loss of Overhead
Coolant
To Flare
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Slide 13
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Example Distillation System
Tube Rupture
in Reboiler
FV-1
PV-1
PSV-1
LV-1
Inlet Control ValveFails Open
Blocked Outlet
Loss of Overhead
Coolant
To Flare
External Fire
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Slide 14
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Next Step
Identified applicable overpressure scenarios
for equipment Quantify the required relief rate
Utilize available Industry guidelines or company
guidelines
Determine the relief device capacity
Utilize Industry guidelines and standards
Utilize engineering software
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Slide 15
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Types of Pressure Relief Devices
Re-closing relief device
Spring Operated relief device
Conventional
Balanced Bellows
Pilot Operated
Weighted Pallets
Conservation vent
Emergency manways
Non re-closing relief device
Rupture disk
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Slide 16
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Spring Operated Relief Valve
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Slide 17
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Key Definitions
MAWP – Maximum Allowable Working Pressure
Accumulation – The pressure increase over the MAWP of a vessel
during a relieving event. ASME Code regulates the accumulation thathas nothing to do with the set pressure.
Built-up back pressure – Back pressure developed as a result of flow
through the valve
Overpressure – The pressure increase over the set pressure of a relief
valve. It is the same as the accumulation when the valve is set at the
MAWP
Set pressure – Inlet gauge pressure at which a pressure relief device isset to open under service conditions
Relief pressure – The pressure at which the relief valve capacity is
evaluated. By definition this is the valve set pressure plus overpressure
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Slide 18
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Conventional Relief ValveCharacteristics
Opens on differential pressure
Generally not suitable with variable superimposed backpressure
Not generally suitable for tie-in to a flare header
May simmer above 90% of set pressure
Inlet pressure drop limited to 3% of set pressure
Suitable for most services
Lowest cost and maintenance
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Slide 19
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Bellows Relief ValveCharacteristics
Opens on inlet pressure regardless of back pressure
Back pressure reduces capacity above 30% of set pressure
Suitable for variable superimposed back pressure
Additional cost and maintenance associated with bellows
May simmer above 90% of set pressure
Inlet pressure drop limited to 3% of set pressure
Suitable for most services
Some conventional valves can be modified with bellows
conversion kits
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Slide 20
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Pilot-Operated Relief ValveCharacteristics
Most designs open on inlet pressure regardless of back
pressure
Back pressure reduces capacity above 50% of set pressure
Suitable for variable superimposed back pressure
Instability due to high inlet pressure drop can be a remediedwith remote sensing
Leak tight very close to set pressure
Not suitable for dirty service
Additional installation costs
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Slide 21
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Low Pressure Vents
Vents include conservation vents and emergency relief
manways
Typically set pressure of inches of H2O
Vent must be set to provide adequate capacity at tank design
pressure/vacuum plus any allowable accumulation.
Check tank design code for these limits
Direct acting vents require around 100% overpressure to
achieve full opening (so typically set at 50% of tank’s designpressure.
Pilot-operated vents can be set closer to design pressure
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Slide 22
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Set Pressure & Accumulation
Limits as Percentage of MAWP
Multiple ValveSingle Valve
Operating Only
1st Valve
Additional Valve(s)
Fire Only1st Valve
Additional Valve(s)
Supplemental Valve
100
---
100
---
---
110
---
121
---
---
100
105
100
105
110
116
116
121
121
121
ContingencySet
Pressure
Accum.
Pressure
Set
Pressure Accum.
Pressure
(per ASME B&PVC, Section VIII)
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Slide 23
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Single Valve Installation
For Operating Contingencies
MAWP = 100 psig
Allowable Overpressure = 10 psig
Maximum Accumulated Pressure = 110 psig
Relief Header
P(relieving) = 110 psig
P2 = Back Pressure
P(set) = 100 psig
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Slide 24
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Single Valve Installation
For Fire Contingencies
MAWP = 100 psig
Allowable Overpressure = 21 psig
Maximum Accumulated Pressure = 121 psig
Relief Header
P(relieving) = 121 psig
P2 = Back Pressure
P(set) = 100 psig
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Slide 25
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Multiple Valve Installation
For Fire Contingencies
Allowable OverpressureValve 1 = 21 psig
Valve 2 = 16 psig
Maximum Accumulated Pressure = 121 psig
Relief Header
P(relieving) = 121 psig, both valves
P2 = Back Pressure
P(set) = 105 psigP(set) = 100 psig
Valve 2Valve 1
Combined capacity of two or more
valves used to alleviate overpressureMAWP = 100 psig
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API limits non-recoverable losses to 3% of set pressure based on the
rated capacity of the relief valve
If directly connected to a vessel, 3% applies to the total non-recoverable (frictional) loss from the vessel to valve inlet
If connected to process line, 3% applies to the total non-
recoverable loss in non-flowing inlet line plus incremental pressure
drop in process line
A rupture disk used in combination with a relief valve must be included
in the inlet pressure loss calculation
High inlet pressure drop may result in chattering and reduced capacity
Pressure Relief Device Inlet PipingPressure Drop
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Slide 27
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Pressure Relief Device OutletPiping Consideration
For discharges to flare, outlet line should be free-draining to main
header (no pockets) or have a low point drain
Atmospheric discharges should have weep hole
All block valves should be full port and locked or carsealed open
Check discharge piping for low temperature concerns
Properly support outlet piping (stress analysis may be necessary)
Impact of back pressure dependent on valve type
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Slide 28
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Pressure Relief SystemsDeficiencies (Industry)
No Relief Device
8% Undersized
Relief Device
6%
Instal lat ion
Issues14%
Flare System s4%
Data
Discrepancy
8%Meets Standards
60%
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Slide 29
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Additional Information
Mary Kay O’Connor Process Safety Center
2-day detailed training seminar by Siemens
Includes capabilities of proprietary software
Industry Standards and Guidelines
API 520 Recommended Practices
API Standard 521
API Standard 2000
NFPA 30
ASME Section I
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Slide 30
Questions?