1. “Grace under pressure” – Ernest Hemingway
Pressure Relief
“Grace under pressure”
– Ernest Hemingway
Harry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil)
and Professor Emeritus Art Sterling

2. What is the Hazard? Despite safety precautions …
Equipment failures
Human error, and
External events, can sometimes lead to …
Increases in process pressures beyond safe levels, potentially resulting in …
OVERPRESSURE due to a RELIEF EVENT

3. What are Relief Events? External fire Flow from high pressure source
Heat input from associated equipment
Pumps and compressors
Ambient heat transfer
Liquid expansion in pipes and surge

4. Potential Lines of Defense
Inherently Safe Design
Passive Control
Active Control
Low pressure processes
Overdesign of process equipment
Install Relief Systems

5. What is a Relief System? A relief device, and
Associated lines and process equipment to safely handle the material ejected

6. Why Use a Relief System?
Inherently Safe Design simply can’t eliminate every pressure hazard
Passive designs can be exceedingly expensive and cumbersome
Relief systems work!

7. Pressure Terminology MAWP Design pressure Operating pressure
Set pressure
Overpressure
Accumulation
Blowdown
Page 1
Operating Pressure:
Pressure usually subjected to during normal operation
Design pressure:
Set by process conditions
Pressure which will provide a suitable margin above the operating pressure in order to prevent nuisance trips
Normally set at greater of 25psi or 10% of set pressure above the operating pressure
MAWP
Maximum Allowable Working Pressure
Defined in the construction codes for unfired vessels (ASME)
Set by metallurgical conditions based on type of material and its thickness
Must be equal to or greater than the design pressure
Set pressure
Inlet pressure at which the valve is adjusted to open
Liquid service valve starts to open
Vapor service valve pops
Accumulation
Pressure above MAWP that results during a release. Code allows 10%.
Overpressure
Any pressure above the SP.

8. Code Requirements General Code requirements include:
ASME Boiler & Pressure Vessel Codes
ASME B31.3 / Petroleum Refinery Piping
ASME B16.5 / Flanges & Flanged Fittings
ASME Section 8 for unfired vessels
ASME B31.3, “Process Piping”, This Code prescribes requirements for materials and components, design, fabrication, assembly, erection, examination, inspection, and testing of piping.
ASME B16.5, “Pipe Flanges and Flanged Fittings”, This Standard covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings.

9. Code Requirements
Relieving pressure shall not exceed MAWP (accumulation) by more than:
3% for fired and unfired steam boilers
10% for vessels equipped with a single pressure relief device
16% for vessels equipped with multiple pressure relief devices
21% for fire contingency
Go over bullets.

10. Relief Design Methodology
LOCATE RELIEFS
CHOOSE
TYPE
DEVELOP SCENARIOS
SIZE RELIEFS
(1 or 2 Phase)
CHOOSE WORST CASE
DESIGN RELIEF SYSTEM

11. Locating Reliefs – Where?
All vessels
Blocked in sections of cool liquid lines that are exposed to heat
Discharge sides of positive displacement pumps, compressors, and turbines
Vessel steam jackets
Where PHA indicates the need
LOCATE RELIEFS

12. Choosing Relief Types Spring-Operated Valves Rupture Devices CHOOSE

13. Spring-Operated Valves
Conventional Type
CHOOSE
TYPE

14. Picture: Conventional Relief Valve
CHOOSE
TYPE

15. Superimposed Back Pressure
Pressure in discharge header before valve opens
Can be constant or variable
Superimposed back pressure is always present
In the example the suction pressure of the pump is always on the outlet of the pressure relief valve. It can be constant or variable depending on the nature of the system.
Superimposed back pressure can give us problems if variable when trying to specify a PRV.
What will the back pressure be when the valve is called upon, i.e. will it be lifting alone or as part of a plant wide release?
Could affect the type of pressure relief valve chosen.
More of a problem for valves with lower set points as pressure variation will be a greater percentage of set pressure.
CHOOSE
TYPE

16. Built-up Back Pressure
Pressure in discharge header due to frictional losses after valve opens
Total = Superimposed + Built-up
Pressure which develops as a result of flow in the discharge header after the pressure relief valve opens
Is not present when valve is closed.
In the example no built-up back pressure exists until the valve opens. Upon opening the built-up back pressure develops and increases as the flow through the valve increases until full lift.
The sum of the superimposed and built-up back pressure will give the total back pressure on the pressure relief valve outlet. The PRV does not care where the back pressure comes from.
CHOOSE
TYPE

17. Spring-Operated Valves
Balanced Bellows Type
CHOOSE
TYPE

18. Picture: Bellows Relief Valve
Venting arrangements must be carefully selected and designed to meet the following requirements:
1.Manufacturer’s shipping plugs must be removed from the bonnet vent holes before a new valve is commissioned.
2.Each PR valve must be installed so that the bonnet vent does not allow released vapors to impinge on lines or equipment,or towards personnel walkways. Where necessary, a short nipple and elbow should be added to direct flow away from such areas. In these cases, the vent piping should discharge horizontally to avoid entry of rainwater and debris, and should terminate in a position which is accessible for leak testing.
3.In cases where bellows failure would release flammable, toxic or corrosive liquids through the vent, a short nipple and elbow should be used to direct leakage to an open funnel which is piped to grade and ties into a catch basin or manhole through a sealed inlet connection.
4.Although venting to the atmosphere is preferred, an alternative is to tie into a closed low pressure system, if available.
CHOOSE
TYPE

19. Pros & Cons: Conventional Valve
Advantages
Most reliable type if properly sized and operated
Versatile -- can be used in many services
Disadvantages
Relieving pressure affected by back pressure
Susceptible to chatter if built-up back pressure is too high
Now that we’ve described these valves, let’s try to summarize the advantages and disadvantages of the various types.
Go over bullets.
CHOOSE
TYPE

20. Pros & Cons: Balanced Bellows Valve
Advantages
Relieving pressure not affected by back pressure
Can handle higher built-up back pressure
Protects spring from corrosion
Disadvantages
Bellows susceptible to fatigue/rupture
May release flammables/toxics to atmosphere
Requires separate venting system
Continue going over bullets.
CHOOSE
TYPE

21. Rupture Devices
Rupture Disc
Rupture Pin
CHOOSE
TYPE

22. Conventional Metal Rupture Disc
CHOOSE
TYPE

23. Conventional Rupture Pin Device
The buckling pin concept is derived from Euler’s formula. Euler’s formula relates the force or pressure needed to buckle a long, thin column to the length 2 , diameter 4 , and modulus of elasticity of the column.
The key to the accuracy of the buckling pin valve is the repeatability of the buckling pin.
CHOOSE
TYPE

24. When to Use a Spring-Operated Valve
Losing entire contents is unacceptable
Fluids above normal boiling point
Toxic fluids
Need to avoid failing low
Return to normal operations quickly
Withstand process pressure changes, including vacuum
CHOOSE
TYPE

25. When to Use a Rupture Disc/Pin
Capital and maintenance savings
Losing the contents is not an issue
Benign service (nontoxic, non-hazardous)
Need for fast-acting device
Potential for relief valve plugging
High viscosity liquids
CHOOSE
TYPE

26. When to Use Both Types
Need a positive seal (toxic material, material balance requirements)
Protect safety valve from corrosion
System contains solids
CHOOSE
TYPE

27. Relief Event Scenarios
A description of one specific relief event
Usually each relief has more than one relief event, more than one scenario
Examples include:
Overfilling/overpressuring
Fire
Runaway reaction
Blocked lines with subsequent expansion
Developed through Process Hazard Analysis (PHA)
DEVELOP SCENARIOS

28. An Example: Batch Reactor
Control valve on nitric acid feed line stuck open, vessel overfills
Steam regulator to jacket fails, vessel overpressures
Coolant system fails, runaway reaction
DEVELOP SCENARIOS

29. Sizing Reliefs Determining relief rates Determine relief vent area
SIZE RELIEFS
(Single Phase)

30. Scenarios Drive Relief Rates
Overfill (e.g., control valve failure)
Fire
Blocked discharge
Maximum flow rate thru valve into vessel
Vaporization rate due to heat-up
Design pump flow rate
SIZE RELIEFS
(Single Phase)

31. Overfill Scenario Calcs
Determined maximum flow thru valve (i.e., blowthrough)
Liquids:
Gases:
SIZE RELIEFS
(Single Phase)

32. Fire Scenario Calcs
API 520 gives all equations for calculating fire relief rate, step-by-step
Determine the total wetted surface area
Determine the total heat absorption
Determine the rate of vapor or gas vaporized from the liquid
SIZE RELIEFS
(Single Phase)

33. Determine Wetted Area
SIZE RELIEFS
(Single Phase)

34. Determine Heat Absorption
Prompt fire-fighting & adequate drainage:
Otherwise:
where
Q is the heat absorption (Btu/hr)
F is the environmental factor
1.0 for a bare vessel
Smaller values for insulated vessels
Awet is the wetted surface area (ft2)
SIZE RELIEFS
(Single Phase)

35. Determine Vaporization Rate
where
W = Mass flow, lbs/hr
Q = Total heat absorption to the wetted surface, Btu/hr
Hvap = Latent heat of vaporization, Btu/lb
SIZE RELIEFS
(Single Phase)

36. Determine Relief Vent Area
Liquid Service
where
A is the computed relief area (in2)
Qv is the volumetric flow thru the relief (gpm)
Co is the discharge coefficient
Kv is the viscosity correction
Kp is the overpressure correction
Kb is the backpressure correction
(r/rref) is the specific gravity of liquid
Ps is the gauge set pressure (lbf/in2)
Pb is the gauge backpressure (lbf/in2)
SIZE RELIEFS
(Single Phase)

37. Determine Relief Vent Area
Gas Service
where
A is the computed relief area (in2)
Qm is the discharge flow thru the relief (lbm/hr)
Co is the discharge coefficient
Kb is the backpressure correction
T is the absolute temperature of the discharge (°R)
z is the compressibility factor
M is average molecular weight of gas (lbm/lb-mol)
P is maximum absolute discharge pressure (lbf/in2)
c is an isentropic expansion function
SIZE RELIEFS
(Single Phase)

38. Determine Relief Vent Area
Gas Service
where
c is an isentropic expansion function
g is heat capacity ratio for the gas
Units are as described in previous slide
SIZE RELIEFS
(Single Phase)

39. A Special Issue: Chatter
Spring relief devices require 25-30% of maximum flow capacity to maintain the valve seat in the open position
Lower flows result in chattering, caused by rapid opening and closing of the valve disc
This can lead to destruction of the device and a dangerous situation
SIZE RELIEFS
(Single Phase)

40. Chatter - Principal Causes
Valve Issues
Oversized valve
Valve handling widely differing rates
Relief System Issues
Excessive inlet pressure drop
Excessive built-up back pressure
Go over bullets.
We will examine each of these causes and propose solutions.
SIZE RELIEFS
(Single Phase)

41. Worst Case Event Scenario
Worst case for each relief is the event requiring the largest relief vent area
Worst cases are a subset of the overall set of scenarios for each relief
The identification of the worst-case scenario frequently affects relief size more than the accuracy of sizing calcs
CHOOSE WORST CASE

42. Design Relief System
Relief System is more than a safety relief valve or rupture disc, it includes:
Backup relief device(s)
Line leading to relief device(s)
Environmental conditioning of relief device
Discharge piping/headers
Blowdown drum
Condenser, flare stack, or scrubber
DESIGN RELIEF SYSTEM

43. Installation, Inspection, and Maintenance
To undermine all the good efforts of a design crew, simply …
Improperly install relief devices
Fail to regularly inspect relief devices, or
Fail to perform needed/required maintenance on relief devices

44. ?? Reduced Inlet Piping
Reduced
Inlet Piping
Anything wrong
here?

45. ?? Plugged Bellows, Failed Inspection, Maintenance
Anything wrong
here?
Signs of
Maintenance
Issues
Bellows plugged
in spite of sign
Failed
Inspection
Program

46. ?? Discharges Pointing Down
Anything wrong
here?
Discharges
Pointing Down
Anything wrong
here?

47. ?? Long Moment Arm
Long
Moment Arm
Anything wrong
here?

48. ?? Will these bolts hold in a relief event
Anything wrong
here?

49. Mexico City Disaster
Major Contributing Cause:
Missing Safety Valve

50. Summary Pressure Relief Look forward to …
Very Important ACTIVE safety element
Connected intimately with Process Hazard Analysis
Requires diligence in design, equipment selection, installation, inspection and maintenance
Look forward to …
Two-phase flow methodology/exercise

51. References
Crowl and Louvar – Chemical Process Safety, Chapters 8 and 9
Ostrowski – Fundamentals of Pressure Relief Devices
Sterling – Safety Valves: Practical Design, Practices for Relief, and Valve Sizing

52. END OF PRESENTATION .