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“Grace under pressure” – Ernest Hemingway

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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

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

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

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


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