ERT 422/4 Piping and instrumentation diagram (P&id) MISS. RAHIMAH BINTI OTHMAN (

Fundamentals of Pressure Relief Devices

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

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

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

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

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

BIOREACTOR CONTROL SYSTEM

Pressure Terminology u MAWP u Design pressure u Operating pressure u Set pressure u Overpressure u Accumulation u Blowdown

Pressure Relief Devices What’s coming u Basic terminology u Code requirements u Safety relief valves u Rupture discs

Pressure Terminology u Operating pressure u MAWP u Design pressure u Set pressure u Accumulation u Overpressure u Blowdown

Code Requirements General Code requirements include: –ASME Boiler & Pressure Vessel Codes –ASME B31.3 / Petroleum Refinery Piping –ASME B16.5 / Flanges & Flanged Fittings

Code Requirements u All pressure vessels subject to overpressure shall be protected by a pressure relieving device u Liquid filled vessels or piping subject to thermal expansion must be protected by a thermal relief device u Multiple vessels may be protected by a single relief device provided there is a clear, unobstructed path to the device u At least one pressure relief device must be set at or below the MAWP

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

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

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

Choosing Relief Types u Spring-Operated Valves u Rupture Devices CHOOSE TYPE

Spring-Operated Valves u Conventional Type CHOOSE TYPE

Picture: Conventional Relief Valve Conventional Relief Valve CHOOSE TYPE

Spring-Operated Valves u Balanced Bellows Type CHOOSE TYPE

Picture: Bellows Relief Valve Bellows Relief Valve CHOOSE TYPE

Pros & Cons: Conventional Valve u Advantages +Most reliable type if properly sized and operated +Versatile -- can be used in many services u Disadvantages –Relieving pressure affected by back pressure –Susceptible to chatter if built-up back pressure is too high CHOOSE TYPE

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

Rupture Devices u Rupture Disc u Rupture Pin CHOOSE TYPE

Conventional Metal Rupture Disc CHOOSE TYPE

Conventional Rupture Pin Device CHOOSE TYPE

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

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

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

Rupture Discs u A rupture disc is a thin diaphragm (generally a solid metal disc) designed to rupture (or burst) at a designated pressure. It is used as a weak element to protect vessels and piping against excessive pressure (positive or negative). u There are five major types available –Conventional tension-loaded rupture disc –Pre-scored tension-loaded rupture disc –Composite rupture disc –Reverse buckling rupture disc with knife blades –Pre-scored reverse buckling rupture disc

Rupture Discs u They are often used as the primary pressure relief device. –Very rapid pressure rise situations like runaway reactions. –When pressure relief valve cannot respond quick enough. u They can also be used in conjunction with a pressure relief valve to: –Provide corrosion protection for the PRV. –Prevent loss of toxic or expensive process materials. –Reduce fugitive emissions to meet environmental requirements.

Rupture Discs Are Well Suited For Some Applications When compared with PR valves, rupture discs have: Advantages +Reduced fugitive emissions - no simmering or leakage prior to bursting. +Protect against rapid pressure rise cased by heat exchanger tube ruptures or internal deflagrations. +Less expensive to provide corrosion resistance. +Less tendency to foul or plug. +Provide both over pressure protection and depressuring. +Provide secondary protective device for lower probability contingencies requiring large relief areas.

Rupture Discs Are Less Well Suited For Other Applications When compared with PR valves, rupture discs have: Disadvantages –Don’t reclose after relief. –Burst pressure cannot be tested. –Require periodic replacement. –Greater sensitivity to mechanical damage. –Greater sensitivity to temperature

Conventional Tension-Loaded Metal Rupture Disc

Pre-Scored Tension - Loaded Rupture Disc

Disc Corroded Through

Composite Rupture Disc

Reverse Buckling Rupture Disc With Knife Blades

Typical RD/PRV Installation

Anything wrong here?

Pressure above RD Reduced inlet piping

Damaged during Installation

Classic Alligatoring

Conventional Rupture Pin Device

VALVES & PIPE SIZING

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

Determine Relief Vent Area u Liquid Service where  A is the computed relief area (in 2 )  Q v is the volumetric flow thru the relief (gpm)  C o is the discharge coefficient  K v is the viscosity correction  K p is the overpressure correction  K b is the backpressure correction  (  /  ref ) is the specific gravity of liquid  P s is the gauge set pressure (lb f /in 2 )  P b is the gauge backpressure (lb f /in 2 ) SIZE RELIEFS (Single Phase)

Determine Relief Vent Area u Gas Service where  A is the computed relief area (in 2 )  Q m is the discharge flow thru the relief (lb m /hr)  C o is the discharge coefficient  K b 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 (lb m /lb-mol)  P is maximum absolute discharge pressure (lb f /in 2 )   is an isentropic expansion function SIZE RELIEFS (Single Phase)

Determine Relief Vent Area u Gas Service where   is an isentropic expansion function   is heat capacity ratio for the gas  Units are as described in previous slide SIZE RELIEFS (Single Phase)

JADUAL 5.2 Pelega Untuk Fasa Cecair Menggunakan Persamaan 9-4 (Crowl & Louvar) CoCo =0.61 Anggap Terlebih Tekanan = 25% Anggap tekanan balik sentiasa maksimum, iaitu 50 % Pelegaq (kg/j)r (kg/m 3 )Q v (gpm)r/r ref KvKv KpKp KbKb P s (kPa)P s (lb f /in 2 )P b (lb m /in 2 )A (in 2 )A (m 2 ) PSV-23.49E PSV-38.13E PSV-44.51E PSV PSV PSV-75.26E PSV-81.82E PSV E PSV E PSV

FERMENTER CONTROL SYSTEM

a)LIQUID STREAM Equation; 5-14 (R.K Sinnot): For carbon steel pipeline; d i, optimum = 282G 0.52  Where; G = Flow rate, kg/s  =, kg/s For stainless steel pipeline; d i, optimum = 282G 0.50  Where; G= Kadar aliran, kg/s  = Ketumpatan aliran, kg/s

a)LIQUID STREAM Equation; 5-14 (R.K Sinnot): For carbon steel pipeline; d i, optimum = 282G 0.52  Where; G = Flow rate, kg/s  = Density of fluid, kg/s For stainless steel pipeline; d i, optimum = 282G 0.50  Where; G= Flow rate, kg/s  = Density of fluid, kg/s DIAMETER SIZING CALCULATION

b)VAPOR STREAM For long pipeline, the velocity of vapor = sonic velocity (Crowl& Louvar 1990). Sonic velocity; where,  = 1.32 untuk gas terion g c = ft.ib m /s 2.ib f R g = 1545 ft.ib f /ib - mol. o R T o R= 1.8T O C DIAMETER SIZING CALCULATION

Flow rate, Q m =  UA (kg/s) Where; U = a = halaju sonic (m/s) A= Luas keratan rentas paip (m 2 ) DIAMETER SIZING CALCULATION b)VAPOR STREAM-CONT’

EXAMPLE OF CALCULATION (DIAMETER OF PIPELINE FOR VAPOR STREAM) Valve T (°C)T (°R)Mg = Cp/Cva (m/s)ρ(kg/m3)Qm (kg/j)Qm (kg/s)A (m2)D (m)D (in) PSV PSV PSV gc32.17ft.lbm/lbf.s2 Rg1545lbf/lb-mol°R

SPRAY TOWER CONTROL SYSTEM

PIPE THICKNESS SIZING CALCULATION From British BS 3351 standard, the thickness of pipepine is given by equation (R.K. Sinnot) ; Where; P = internal design pressure, bar. d = outside radius of pipe, mm. σ d = allowable stress of pipe material at design temperature, N/mm 2.

?? Reduced Inlet Piping Anything wrong here? Reduced Inlet Piping

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

?? Discharges Pointing Down Anything wrong here? Anything wrong here? Discharges Pointing Down

?? Long Moment Arm Anything wrong here? Long Moment Arm

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

Mexico City Disaster Major Contributing Cause: Missing Safety Valve

Summary u Pressure Relief –Very Important ACTIVE safety element –Connected intimately with Process Hazard Analysis –Requires diligence in design, equipment selection, installation, inspection and maintenance

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