Pressure Relief System Design Practices

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Pressure Relief SystemDesign Practices

May 1, 2008

By Richard Akinretoye



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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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



Slide 5


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



Slide 6


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)



Slide 7


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



Slide 8


Example Distillation System

FV-1

PV-1

PSV-1

LV-1

To Flare



Slide 9



FV-1

PV-1

PSV-1

LV-1

Inlet Control ValveFails Open

To Flare



Slide 10



FV-1

PV-1

PSV-1

LV-1


Blocked Outlet

To Flare



Slide 11



FV-1

PV-1

PSV-1

LV-1


Blocked Outlet

Loss of Overhead

Coolant

To Flare



Slide 12



Tube Rupture

in Reboiler

FV-1

PV-1

PSV-1

LV-1


Blocked Outlet

Loss of Overhead

Coolant

To Flare



Slide 13



Tube Rupture

in Reboiler

FV-1

PV-1

PSV-1

LV-1


Blocked Outlet

Loss of Overhead

Coolant

To Flare

External Fire



Slide 14


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



Slide 15


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



Slide 16


Spring Operated Relief Valve



Slide 17


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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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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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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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



Slide 21


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



Slide 22


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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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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Single Valve Installation

For Fire Contingencies

MAWP = 100 psig

Allowable Overpressure = 21 psig


Relief Header

P(relieving) = 121 psig

P2 = Back Pressure

P(set) = 100 psig



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Multiple Valve Installation

For Fire Contingencies

Allowable OverpressureValve 1 = 21 psig

Valve 2 = 16 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



Slide 26


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



Slide 27


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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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%



Slide 29


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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Questions?

Pressure Relief System Design Practices

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