Chemical & Process Technology

Pressure Relief load estimation is commonly estimated by taking system is approximately at "steady state condition". The estimated relief load could be excessive due to conservative assumption, unrealistic external energy inputs, etc. Current trend is to utilize Dynamic simulation in order to derive realistic but still preserve conservatism, integrity and safety of plant pressure relief and overpressure protection systems. This approach inline with API std 521 recommendation and probably the way to go in near future.

This study presented the dynamic simulation of a gas compression system, proving the viability of operational philosophy and emergency shutdown logic with quantitative process responses in various situations. To avoid unnecessarily high peak in an initial stage of blowdown, this study employed the controlled blowdown and investigated its safety level.

This study concluded that dynamic simulation of start-up and emergency operation improved the operability of the whole process. The revealed transient behavior demonstrated that PSVs sized to API standard led to chattering because the standard gives excessive size. Choice of properly sized PSVs eliminated the chattering with a decrease in relief loads by 40%. The blowdown valves and PSVs are likely to be oversized if the API RP 521 is observed. The dynamic simulation gave precise estimates, consequently decreased the flare loads, and better safety. The controlled blowdown system mitigated the flare load to about 60% of the conventional blowdown system. Its safety was more reliable than that of the conventional, satisfying SIL 2 of IEC 61508.

Pressurized liquid or vapor-liquid in equilibrium or vapor only system during normal operation, when it is expose to external fire attack, heat inputs into vessel (or system) may possibly increase internal pressure and temperature. For system with high design pressure (or Maximum allowable working pressure, MAWP), the pressure relief valve (PRV) protecting the vessel (or system) may have same set pressure as the design pressure (or MAWP). Subject to design code of the vessel, overpressure allowed by code is different from code to code. Typically for ASME unfired vessel section VIII, the maximum allowable overpressure is 10% of set pressure. This will results maximum allowable accumulation pressure (or relieving pressure) reach at 110% of set pressure.


In some event, the relieving pressure is higher than the fluid critical pressure. For example, a CO2 injection compressor, the injection pressure at is 65 barg. The design pressure may be 72 barg and relieving pressure is approximately 79.2 barg, is higher than CO2 critical pressure of 72.9 barg. The PRV is relieving at supercritical condition.

Conventional method in determining PRV orifice size is presented in API RP-521. Part 1. It considered all “un-wetted” vessels are same regardless the fluid is supercritical, a vapor or a gas. Nevertheless, one shall take note that  this method based on the physical properties of air and the perfect gas laws with no change in fluid temperature. 

API method may be conservative, there are chattering and oversized PRV and discharge problem. A  rigorous method has been discussed by R.C. DOANE in "Designing for pressure safety valves in supercritical service" published in Hydrocarbon Processing Jan 2010. This method assuming all thermodynamic paths are well defined by a Process Simulator. Thermodynamic path of fluid from operating condition to relieving and drop to back pressure to PRV may be defined by four typical steps.

For Step 1, earlier post "Constant Density To Obtain Relieving Condition" has discussed similar subject previously. 

For the step 3, another definition of PRV relieving path can be Isentalpic from relieving to throat follow by isentropic from throat to PRV backpressure as discussed in "Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve".

In the "Designing for pressure safety valves in supercritical service" article, table 1 "Supercritical relief valve sizing example problem—normal butane" has tabulated step-by-step calculation. This table has incorporated equation 1 to equation 10 in this article. The calculation consist of segment 1-to-2, 2-to-3, 3-to-4 and 4-to-5. In recent work in establishing similar task carried out by this example, one suspicious discrepancy is identified. Details as follow.

From segment 1-to-2 to segment 4-to-5, volumetric flow is increased from 1304 ft3/hr to 1315 ft3/hr (segment 2-to-3) and decreased in subsequent segments to 1250 ft3/hr. The volumetric flow is in the range of 1250 to 1315 ft3/hr. HOWEVER, mass flow is increased almost double from 6,374 lb/hr to 13,405 lb/hr (segment 2-to-3) and decreased in similar range (11620 to 12,371 lb/hr). Why there is significant different in segment 1-to-2 compare to other segment ?

Detail checking found that Mass Flux (G in lb/ft2s) is calculated by dividing Orifice Velocity (v) by Specific Volume (V) at orifice condition for segment 1-to-2. On the other hand, Mass Flux (G in lb/ft2s) is calculated by dividing Orifice Velocity (V) by Specific Volume (V) at outlet condition for other segments. Different method in calculating Mass Flux has created the discrepancy.

Opinion

As the flow is choked (or highest velocity for subsonic) at the PRV nozzle,  Mass Flux (G in lb/ft2s) should be calculated by dividing Orifice Velocity (v) by Specific Volume (V) at orifice condition.

Well... Above query will be raised and response will be posted once we have received it.

Related Topics

• Tedious & Simple Method In Determining Specific volume For Isentropic Nozzle Flow Mass Flux
• Constant Density To Obtain Relieving Condition
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve
• API Std 520 Part 1 Dec 2008 is Released
• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Requirement of overpressure protection devices on system design to PIPING code

Earlier post "API Std 520 Part 1 Dec 2008 is Released" highlighted one of the main  changes was to use of isentropic nozzle flow method and/or ideal gas specific heat ratio for vapor/gas relief flow calculation.

Isentropic nozzle flow method is considering one dimensional homogeneous flow is limiting in Pressure Relief Valve (PRV) nozzle with isentropic (adiabatic and reversible) expansion across the PRV nozzle. Maximum mass flux across the PRV nozzle will be determined follow by mass flow passing the PRV. In determining the maximum mass flux, an integration of specific volume (v) time differential pressure (dP) is required. This integration involve substantial derivation of specific volume (v) which change with pressure (P). If the system can be modelled in HYSYS, you may consider the following tedious and simplified method in determining the specific volume (v) with varies pressure (P).

The detailed method involve the fluid expanded isentropically across multiple expander with fix differential pressure (dP). Entire model is simulated using HYSYS with Isentropic expansion using Expanders. Stream 1 is the feed to PRV nozzle. Stream 1 is expanded isentropically (polytropic efficiency =100%) to stream 2 with fix differential pressure. Stream 2 is further expanded isentropically to stream 3, Stream 4 is further expanded isentropically to stream 4,.............

Mass density is extracted from each stream in order to use for integration of specific volume (v) time differential pressure (dP). Following is the example of specific volume (Inverse of Mass density).

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Pressure safety valve (PSV) or pressure relief valve (PRV) is commonly used to protect a pressure containment part i.e. vessel, column, etc from overpressure. It is one of the code approved type of overpressure protection devices. This type of device is reclosing type where the mechanism of devices is designed such that it will stop relief when the pressure is reduced to it reseat pressure. Besides PSV, rupture disc (RD) and rupture pin are also code approved type of overpressure devices. This type of device is non-reclosing type where it will continue to relief until all inventory is completely evacuated from the system or with operator intervention.

Earlier post "Visualise Pressure Safety Valve (PSV) Assemblies & Operation" presented some features about the PSV type and also a video clip for typical construction and operation of a conventional PSV. This post will present PSV valve disc movement and flow distribution inside a PSV.

PSV valve disc movement and flow distribution inside a PSV is simulated using CFD modeller, CFX 11.0 . Following video clip shows how a spring loaded pressure relief valve works under over-pressure condition.



You may notice that high mass flux is on the disc facing PSV outlet nozzle.

Following is another video clip shows oscillating movement of PSV disc during relieving condition.

Above oscillating movement generate a oscillating forward flow which is believe one of the cause for valve chattering. Read more about PSV chattering in "PSV Chaterring is Destructive...The ways to Prevent...".

Thanks to Songxguan

Related Post

• Rupture Disc Installation Guide Videos Reduce Premature Openning Risk
  
• Several Impact of Backpressure on Conventional PRV
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 3
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 2
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 1
• Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?
• Should we install Butterfly valve for Pressure Relief Valve (PSV) isolation ?

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Rupture disc (RD) is one of the non-reclosable type pressure relief device for vessel overpressure protection. It has been used in many application due to it uniqueness and special characteristic as covered in "Why Rupture (RD) Upstream of Pressure Relief Valve (PRV) ?". In some application, two RDs are placed in series to minimize inventory lost cause by premature opening of RD. Premature opening of RD may be cause by RD quality during manufacturing. Besides, premature opening of RD may be caused by wrong handling and installation of RDs.


Elfab, a RD manufacturer offer a few RD installation videos which could very useful for most operator. you may download from the following links or visit Elfab.

• Conventional Domed (CD)
• Domed Composite (DC)
• Flat Composite (FC)
• Opti-Gard (OP)
• Opti-Gard SoLo (OPS)
• Safe-Gard (SG)
• Universal Arma-Gard (AGS)

Related Post

• Why Two Rupture Discs in Series ?
• Why Rupture (RD) Upstream of Pressure Relief Valve (PRV) ?
• Tube Rupture : Pressure Relief Valve (PSV) or Rupture Disk (RD) ?
• Criteria for Requirement of Pressure Relief Device for Tube Rupture
• Some Comments on Providing External Insulation as Protective Measure against FIRE
• Protective Measures against FIRE other than Pressure Relief Device (PRD)
• PSV for Shell-and-Tube HEX Tube Side Overpressure Protection against External Fire Attack ?

A pressure vessel or system expose to external fire, entire system may be isolated automatically by a plant wide emergency shutdown system (ESD) and emergency depressuring system will be initiated to depressure system to safe level and evacuate the inventory from high risk area (expose to external fire) to disposal system. Depressuring system is known as one of the most effective measures against external fire risk. Besides, there are others measures as discussed in "Protective Measures against FIRE...".

Pressure relief device (PRD) may not protect pressure vessel or system from external fire. Having said that PRD may serve in some circumtances to "buy time" for operator action. Besides, design code and/or local regulation demand a PRD in pressure vessel or system as ultimate protection.

One may have process simulation for normal production system. The stream for vessel or system expose to fire is at normal operating pressure and temperature. How shall one can adjust the process simulation to bring it up the relieving pressure ? One may consider a constant density method.

Constant density method
Pressure vessel expose to fire will be isolated and settled-out. Read more in "Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS". Assuming no credit for automatic depressuring system and operator intervention, the inventory trapped in the pressure vessel will remain as trapped vapor mass (M_V0) with vapor volume (V_V0) and liquid mass (M_L0) with liquid volume (V_L0) at normal pressure (P₀) and temperature (T₀) when ESD is just initiated. Total trapped mass (M_T0) will be M_V0 + M_L0 and total trapped volume (V_T0) will be V_V0 + V_L0. Mix denstiy will be M_T0/V_T0.

Trapped inventory will be heated up with external fire. Heat added into the trapped system will cause temperature rise, more liquid vaporise and pressure rise upto relieving pressure, before pressure relief device is popped open. At relieving condition (relieving pressure and temperature), trapped vapor mass (M_Vr) with vapor volume (V_Vr) and liquid mass (M_Lr) with liquid volume (V_Lr). Total trapped mass (M_Tr) will be M_Vr + M_Lr and total trapped volume (V_Tr) will be V_Vr + V_Lr. Mix denstiy will be M_Tr/V_Tr. As there is no inventory evacuated from the system, total trapped mass (M_T0) and volume (V_T0) at normal condition will be same as trapped mass (M_Tr) and volume (V_Tr) at relieving condition.


Similar mix density (M_T0/V_T0) will remain same.


This is commonly known as constant density method.

Related Topic

• Don't misunderstood depressuring
• Depressuring Flow - Quick Manual Method
• Protective Measures against FIRE other than Pressure Relief Device (PRD)
• Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS
• Simple Manual Method for Settle Out Condition Estimation
• Simple Method For Compressor Settle Out Using HYSYS
• Depressuring - Save Some Time in HYSYS - FLARENET Iteration

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This post is intended to provide supplementary information for earlier post "API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section".

Addendum May 2008
API has released the Addendum details to API Std 521. It describes in detail the modified sections compare to earlier release in 2007. You may download Addendum May 2008 from here.

Errata June 2007
For those who don't not aware the Errata in June 2007 for API Std 521 released in 2007, you may read "ERRATA - API Std 521, Pressure Relieving and Depressuring Systems" and download Errata June 2007 from here.

Related Topic

• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• ERRATA - API Std 521, Pressure Relieving and Depressuring Systems
• Workbook for Chemical Reactor Relief System Sizing
• A must have book...Emergency Relief System Design Using DIERS Technology
• Requirement of overpressure protection devices on system design to PIPING code
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

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Recently there have been many discussion and argument among engineers from operating company, design and engineering, manufacturer and consulting on one simple but important term relates to Pressure Relief Device (PRD), "backpressure". A lot of terms have been used and mixed and lead to confusion. Beside, there are also some new terms created unintentionally which further confuse the discussion. This post is intended to clarify common definition used in Chemical Process Technology

Several definition of "backpressure" have been pointed out in the discussion :

Every companies and engineers may have different definition and understanding about "backpressure". The combination of terms used within a code or company may be correct. However, one specific term used in one code or company shall not mix with another term used in another code or company.

For example, instrument datasheet is used to transfer and communicate information between an instrument engineer in design company and engineer in PRD manufacturer. The terms used possibly are Constant backpressure, Variable backpressure and Total backpressure. However, a process engineer in design company may use Superimposed backpressure, Built-up backpressure and Backpressure to transfer and communicate similar informations to instrument engineer in design company. Whenever the discussion among three parties (process engineer, instrument engineer & manufacturer engineer), correct combination of terms shall be clarified and used. Confusion will occur when different combination are used e.g. built-up backpressure mix with constant and variable backpressure.

Common Definition of "Backpressure" in Chemical Process Technology
A pressure relief valve (PRV) in "Ready-to-operate" mode and "relieving" mode during plant operation will expose to different type of pressure. "Ready-to-operate" mode is the PRV's disc keeping PRV in closed position when inlet pressure (Pi) is lower than or equal to PRV set pressure (Ps). "Relieving" mode is PRV disc away from seat allowing fluid passing the PRV valve nozzle when the inlet pressure (Pi) is higher than PRV set pressure (Ps).

As PRV may be in Ready-to-operate and Relieving mode, the "backpressure" exist at different modes will vary. In many event, understanding of this "backpressure" creates a lot of confusion among engineers in operation, design, vendor, manufacturer, etc. It is important to make the defintion clear prior to any discussion.

Superimposed backpressure (Pbs) is the static pressure that exists at the outlet of a pressure relief device (PRD) at the time the device is required to operate ("Ready-to-operate" mode). Superimposed backpressure is the result of pressure in the discharge system coming from other sources i.e. Pressure control valve (PCV), pressure relief valve (PRV), etc. Superimposed backpressure may be constant or variable.

Built-up backpressure (Pbb) is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device (PRD) opens ("relieving" mode).

Backpressure (Pb) is the pressure that exists at the outlet of a pressure relief device (PRD) as a result of the pressure in the PRD discharge system. Backpressure is pressure result of both superimposed backpressure caused by other sources and built-up backpressure due to the relief flow during "relieving" mode. Backpressure is the sum of the superimposed and built-up backpressures.


Above defintion are inline with API STD 520 Part 1, Dec 2008 - Sizing, Selection & Installation of Pressure-Relieving Devices in Refinery.

"Backpressure" will affect the performance of Pressure Relief Valve (PRV). "How "backpressure" affect PRV performance ? Read more in "Several Impact of Backpressure on Conventional PRV".

Related Post

• Several Impact of Backpressure on Conventional PRV
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 3
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 2
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 1
• Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?
• Should we install Butterfly valve for Pressure Relief Valve (PSV) isolation ?
• ERRATA - API Std 521, Pressure Relieving and Depressuring Systems
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

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American Petroleum Petroleum (API) released the API Std 520 Part 1, Sizing, Selection and Installation of Pressure-relieving Devices in Refineries Part 1 - Sizing and Selection Eighth Edition, Dec. 2008 in December 2008. First far most important change is it became a STANDARD, instead RECOMMENDED PRACTICES. User shall remember although it became a standard, it is solely users responsibility to make sound, scientific, engineering, safe, environment friendly judgment. Neither API nor its employee, etc make warranty for the use of this standard. Detail refer to "Special Note" in relevant Standard.

Applicability
User of this standard shall understand the applicability of this standard.
This standard

• applies to the sizing and selection of pressure relief devices (PRD) used in refineries and related industries for equipment that has maximum allowable working pressure (MAWP) of 15 psig (103 kPag) or greater
• applicable to PRD protecting unfired pressure vessels and related equipment
• applicable to steady state flow of Newtonian fluids
• supplement to ASME Section VIII Pressure Vessel Code
• is not applicable Atmospheric or Low pressure storage tank (cover under API Std 2000)
• is not applicable fired vessels (cover under ASME section 1 and ASME B31.1)

Revised/Added Section
Quick glance through this document, several section are revised/added. The updated / added sections are (not limted to) :

• Additional attention shall be taken when appreciable of non-condensable gas present in the flashing liquid. Leung Omega Two Point method for flashing liquid and non-condensable gas may be referred.
  

• Used of HEM method is easy but rigorous. Understanding of the thermal equilibrium for fluid properties is important and required.

• Two phase Vapor Liquid Relief method presented in Annex B have not been validated by test (accoring to API) and there is no any recognized procedure for certifying the capacity of PRVs in two phase flow service.

Related Topics

• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Workbook for Chemical Reactor Relief System Sizing
• A must have book...Emergency Relief System Design Using DIERS Technology
• Requirement of overpressure protection devices on system design to PIPING code
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

Conducting overpressure scenario analysis, derivation of relief load, follow by pressure safety valve (PSV) sizing and finally selection of PSV are common activities in oil and gas project. Nowadays, API Std 521 is commonly used for sizing and hence the required API effective discharge area is the result of the calculation. PSV with effective discharge area larger than required API effective discharge area shall be selected. However, many PSVs' certified area are presented in ASME area.

How to relate API effective discharge area and ASME area ?

This is a very common question and problem face by many young engineers. Sometime engineers may wonder why a PSV with ASME "D orifice" is sufficient for a relief scenario required API "E orifice". Reason for the difference between API and ASME is dated back to 1962 when ASME Section VIII Code was changed to derate all certified relieving capacities by 10%. PSV manufacturers have decided to increase PSV flow area by 10%, instead of derating their capacity by 10%. Nevertheless the API "orifice" still remain unchanged.

A PSV with ASME flow area (A_ASME), ASME discharge coefficient (K_ASME) and API discharge coefficient (K_API),

A calculated effective area based on K_API = 0.973, A_API = 0.12 inch2. A API "D orifice" with 0.11 inch2 is insufficient. A "E orifice" with 0.196 inch2 is required.

Let check the National Board published data, a ASME "D orifice" will have K_ASME = 0.859 and flow area of A_ASME = 0.15 inch2, the equivalent API effective discharge area would be

A_API = K_ASME x A_ASME / K_API

In recent Hydrocarbon Processing November 2008 release, an article "What You should know about liquid thermal expansion" has discussed about issue related relief due to liquid thermal expansion. There was similar discussions in previous posts such as :

• Thermal Relief of Non-Flashing Liquid in Pipe
• Do i Normally Calculate Thermal Relief Load ?
• Simple Flow Chart to Determine Requirement of Thermal Relief

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Similar to what has been presented in "Simple Flow Chart to Determine Requirement of Thermal Relief", this article presented a Decision Flowchart assist in determining requirement of Thermal Expansion Relief Valve (TERV or TRV) in detail and brief in TRV relief rate, location of TRV and discharge point. Certain points discussed in this article is a bit brief and quickly jumped into the conclusion. Thus, the intention of this post is to further expand it, provide some highlights and pre-caution so that reader and user can take serious considerations on decision of provision of TRV.

Definition of Liquid Filled System & Cushion effect

R1 : No TRV is required if it is not completely liquid-filled system. A system is considered completely liquid-filled at 95% or greater volume. For a two-phase system with lower liquid fraction and gas system, a TRV is not ussually required because existing gas can absorb the increase in liquid volume due to thermal expansion.

Consideration if gas "cushion" effect for liquid system is acceptable and TRV may not be required, one shall take note that the "cushion" effect may not be applicable to condensing vapor and/or vapor is highly soluble in liquid. As heat input (external or internal) to the system, liquid expands and pressure increase. Vapor may condense to form liquid or vapor may dissolve in liquid rapidly. This quickly removed the "cushion" effect readily available in vapor and results pressure rise rapidly.

The recemmendation of not having TRV in 95% liquid system may be only applicable to some conditions but not all conditions. It is very much subject to liquid expansion factor. Liquid with high expansion factor i.e. light HC would tends to remove the vapor "cushion" effect quickly. It is always recommended to study the possibility of overpressure due to thermal expansion using constant density method. Besides, for close system, it has some level of difficulties to ensure or guarantee of less than 95% liquid volume requirement. Thus, this rule shall be applied carefully.

Non-Blocked in Section Shall be Drained... Losses

R2 : Process plant piping (on-plot piping) is not normally blocked in, but storage or transport piping sections are regularly shut-in during normal operation. This is why few process piping lines have TRVs.

This recommendation is valid for process piping is not normally blocked in during normal operation. When it is blocked, part of liquid is possible to be drained (sufficient vapor space) to avoid overpressure due thermal expansion. Quantity of liquid to be drain to provide "sufficient" vapor is arbitrary and shall be stuided when this philosophy is adopted. Provision of 10% vapor space could be a good start.

Generally process piping within a plant is not equipped with TRV based on above consideration. However, some operating owner may not agree to lose any inventory i.e. long distance pipeline, due to draining activities. Thus, TRV may still be required. If above philosophy is adopted, the draining requirement after blocked in shall be agreed and captured in the operating procedure. Used of Locked-Open (LO) manual isolation valve couple to strict permitting procedure is one of the good practice to minimize potential of complete blocked-in.

TRV Not Required for "Piping" ?

R3 : The heat source can be internal or external. An internal heat source is.... External fire is not considered a heat source if thermal expansion is studied for a system consisting of only pipes...

Above could be a consequence of code mis-interpretation where pressure containing part designed to PIPING code i.e. ASME B31.3, there is waiver from overpressure protection. For example, a DN1500 pipe and a DN 150 filter containing same fluid, same design pressure and temperature, the volume contains by the DN1500 pipe significantly higher than DN150 filter. As per code interpretation, PSV is provided for DN150 filter but not required for DN1500 pipe. This consideration is unreasonable as the risk for DN1500 pipe is much higher than DN150 filter.

Requirement of PSV shall NOT be 100% judged from the design code itself. The RISK and CONSEQUENCE e.g. INVENTORY associates risk & consequences shall come into consideration to define if a overpressure protection i.e PSV is required. This has been discussed in "Requirement of overpressure protection devices on system design to PIPING code".

Concluding remark : TRV is Just a Small & Simple Overpressure Protecting Device
TRV is a small, simple and non-costly overpressure device. Compare to other major equipment such as compressor, turbine, column, etc, it is just almost nothing to compare from cost aspect A TRV can be a DN20 x DN25 PSV. it is nothing to compare from the associated consequences for not providing it. Seriously analyse it before decision on not providing it for thermal expansion relief.

Related Post

• Thermal Relief of Non-Flashing Liquid in Pipe
• Do i Normally Calculate Thermal Relief Load ?
• Simple Flow Chart to Determine Requirement of Thermal Relief
• Why Rupture (RD) Upstream of Pressure Relief Valve (PRV) ?
• Why Two Rupture Discs in Series ?
• Tube Rupture : Pressure Relief Valve (PSV) or Rupture Disk (RD) ?
• Criteria for Requirement of Pressure Relief Device for Tube Rupture

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Earlier post in "Few Concerns & Recommendations of PSV Discharge Tail pipe", there are many points and requirements related to PSV discharge tail pipe have been discussed. This mainly focus on PSV discharge in proper designed close collection system. In the other post "Concerns & Measures on PSV Discharge to ATM", the focus is on those PSV discharge to ATM directly. Concern on creature and rain water entering the PSV tailpipe have been discussed. Nevertheless, detail analysis on the recommendations, they are mainly tackle the issues of creatures and rain water enter. However, there are other issues such as noise, hazards, etc which subject to fluid and flow. For example, PSV discharging steam may normally high noise level. It is hot and possess hazard to operator if drain hole is provided. This post will present more recommendations for this situation.

Many of you as process engineer, plant engineer or operation engineer due with steam system. One of the safety device in this steam system is Pressure safety valve. "Steam Team Know-How ", an article discuss step-by-step guide for proper sizing and installation of safety valves which will equips you with the tools you need to specify these safety devices with confidence.

We have been discussing many stuffs related to safety relief device in Chemical & Process Technology. We believe many points presented in this article have already been discussed and highlighted in depth in many previous post. However, in this article one of the interesting point is the "drip pan elbow" of safety relief valve protecting steam system.

The drip pan elbow is an elbow changes the outlet of the safety device from horizontal to vertical. at the corner of elbow, there is a connection which allow rain water and condensed steam drain to safe location. This is another alternative for Pressure Safety Valve relieve directly to ATM as discussed in "Concerns & Measures on PSV Discharge to ATM". See following image.



Second advantage for this "drip pan elbow" is it avoid PSV attach directly to the discharge vent piping as connecting vent pipe directly to PSV will place undue stress and weight on the valve body.

Another interesting point related to this "drip pan elbow" is relieved steam will not escape from the drip pan elbow if the vent line is sized correctly. This reduce the safety concern of steam leak through the drain line and poses hazard to personnel around the drain point.

Following is one of the recommended installation of PSV protecting steam system.