chemical engineering: Noise

Showing posts with label Noise. Show all posts

Sunday, May 2, 2010

More Low Noise Trim to Minimize AIV Problem

High frequency acoustic excitation downstream of pressure reducing device potentially results downstream piping failure due to Acoustic Induced Vibration (AIV). Earlier post "Principle in Eliminating & Minimizing AIV Impact" discussed about common principles in minimizing Sound Power level (PWL). Several type of low noise trim in control valve is typical device used to reduce Sound Power Level. There are multi-stage type, multiple path type and Labyrinth-disk type. Details can be read found in "Measures & Technique In Eliminating / Minimizing PWL" and typical low trim trim e.g. WHISPER trim from EMERSON FISHER and V-LOG from DRESSER MASONEILAN are included. This post is to include another two more low noise trim from FLOWSERVE VALTEK.

Two type of low noise trims are provided by FLOWSERVE VALTEK. They are MEGASTREAM and TIGERTOOTH.

MEGASTREAM

MEGASTREAM trim eliminates the problem of control valve noise by dealing effectively with gaseous pressure reduction, and by controlling turbulence carried into the downstream piping using multi-stage expansion and distribution. Each stage is designed to take a small pressure drop and prevent high velocities present in single-throttling-point trims.

Below is the typical noise reduction / Sound power attenuation curve for MEGASTREAM low noise trim. Maximum attenuation can be as high as 28 dB.

Minimize High Stress Concentraction to Reduce Likelihood of AIV Failure

High frequency acoustic excitation downstream of pressure reducing device potentially results downstream piping failure due to Acoustic Induced Vibration (AIV). Earlier post "Principle in Eliminating & Minimizing AIV Impact" discussed about common principles in minimizing Sound Power level (PWL). Several proposed measures to reduce Sound Power Level at source discussed in "Measures & Technique In Eliminating / Minimizing PWL".

High Risk Area

Piping downstream of AIV source expose to high Sound Power Level, past experiences shown that high risk location is at circumferential piping with high stress concentration and/or asymmetric piping. Typical example are

Welded tee or branch in particular large main pipe with small branch,
Main pipe supported vent / drain
Main pipe supported instrument connection
Welded support
Connection (Tee or elbow) subject to thermal cyclic
Line / connection subject to sonic flow

Recommendation

The principle is tackling high risk area are minimizing high stress concentration area, reduce asymmetric connection and provide reinforced connection. A few good engineering practices may reduce (but not 100%) risk of AIV problem :

Avoid using threadolet fittings

AVOID

Use Sweepolet instead of Weldolet

MAXIMIZE

MINIMIZE

Use forged type Tee and fittings instead of welded type.

Welded type connection increases tendencies of localised high stress concentrated area compare to forged type.

USE

Use Full Wrap Around reinforcement for welded type tee

USE

Reinforce welded pipe support
Use thicker pipe wall (strengthen) of main pipe

Related Topic

Sunday, April 18, 2010

Some Recommendation for Process Critical Line

Earlier post “Process Critical Line” has presented a checklist of process critical line. During design phase, these process shall be checked in detail to minimize or avoid problem such as vibration, hammering, capacity reduction, cavitation, etc to occur. This post will further present good engineering practice for process critical line.

Gravity Flow
Any line subject to gravity flow e.g. drain, flare, vent, etc, low pocket shall be avoided. Liquid or solid accumulate in low pocket potentially result corrosion and blockage. Line should be sloped (and/or free draining) from sources to receiver.

Pump Suction
Line to pump suction should be designed to allow self floating as far as possible where lowest liquid level is above the pump highest point.

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Ensure minimum submergence of tank to avoid potential vapor being sucked in to pump suction line due to vortex. If positive submergence is not achievable, installation of vortex breaker is another option. Read more in “Vortex Breaker to Avoid Vapor Entrainment”.

Any high pocket shall be avoided and provision of eccentric reducer at the pump suction to avoid potentially vapor lock prior to pump start-up.

Ensure NPSHa is always higher than NPSHr with a positive margin e.g. 1m for entire operation range (turndown to design capacity) and operation conditions (highest operating temperature). There are 17 Ways to Reduce Likelihood of Pump Cavitation.

Minimize suction length and fitting as much as possible to minimize potential of pump cavitation.

Compressor Suction
Compressor suction knock out drum (KOD) may be equipped with mist eliminator e.g. wiremesh to promote droplet coalescing and separation.

KOD vapor exit nozzle should be designed large enough to minimize exit momentum (rho V2 less than 6000 Pa) in order to minimize reentrainment of coalesced liquid droplet into vapor.

Compressor in general can tolerate small amount of liquid. If absolute no liquid is allowed enter compressor as imposed by compressor manufacturer, one may consider provision of insulation to minimize ambient and JT cooling and heat tracing to compensate heat loss due to above mentioned cooling.

Absolute no low pocket shall present in the compressor suction line as low pocket can accumulate liquid and slug of liquid can cause severe damage to compressor.

May consider a compressor suction strainer for start-up and commissioning. As compressor is sensitive to suction line pressure drop, any additional fitting and device at compressor suction can lead to capacity reduction, installation of suction strainer shall be analyzed in detail during design phase.

Flashing / Two phase Gas-Liquid Flow
Slugging and plugging flow in vertical and horizontal potentially results significant vibration to piping. During process design phase, slugging and plugging flow shall be avoided for entire operating range (turndown to design capacity) and operating conditions.

May consider provision of vapor liquid separation and run separator separate header for vapor and liquid line if slugging / plugging flow is unavoidable. For steam header, provide sufficient steam traps to drain-off condensate and minimize potential of slugging flow.

Extra and strengthen support may be provided to avoid severe vibration and failure on pipe crack.

Liquid-Liquid Coalescer
Vapor generation in liquid-liquid coalescer may accumulate and result under-performed liquid-liquid separation. May consider to provide sufficient static head to suppress vapor generation in liquid-liquid coalescer. It is always recommended to provide a vapor equalization line back to separator to release any vapor form in liquid-liquid coalescer.

Low Pressure Line
Minimizing pressure drop in low pressure line is the key factor to ensure proper performance of system. Minimize line length, fittings, elbow, etc and use of smooth surface pipe e.g. stainless steel may be considered.

Potential Surge Line
Steam supply line experience heat loss and condensation due to partially damaged insulation and extreme low ambient temperature. Flashing condensate with steam return to collection header mix with cold condensate. Both condition would results sudden steam collapse and lead to implosion. Steam implosion would generate severe movement of condensate in the collection header and severe vibration of header. Therefore proper maintenance of insulation is extremely important in keep steam line from transient surge. Besides, provide sufficient steam trap to eliminate condensate from steam line.

Long pipeline transferring incompressible fluid e.g. LNG rundown line, produce water injection line, etc potentially experience transient surge (water hammer) in the event of closure of shutdown valve. Transient surge analysis shall be conducted during design phase to ensure surge is avoided. Slower closure of shutdown valve is one of the key component in minimizing surge in long pipe line. Non-slam check valve on the pump discharge may also assist in minimizing surge in long pipeline with pump. Surge suppression system may be considered in the event surge is unavoidable. One shall take note that provision of pressure relief valve may not help to eliminating surge due to slow response time of PRV.

Pressure Relief Valve Inlet & Outlet
May consider discussion and recommendation in :

Control Valve & Restriction Orifice
Flow Induced Vibration (FIV) and Acoustic Induced Vibration (AIV) may be studied to identify location of piping which potentially experience high risk of low frequency and high frequency vibration. Minimizing small bore connection may be considered e.g. provision connection with more than 2 inches, avoid using connection smaller than 2 inches. For small bore connection, may consider brazing and extra support to strengthen the connection and avoid pipe cracking.

Anti-cavitation trim could be considered for control valve potentially experience cavitation. Similarly provision of multiple restriction orifice (RO) in series or multi-ported RO may be considered if cavitation occurs in RO.

Related Topic

Tuesday, April 13, 2010

Process Critical Line

Process design involve line sizing and pressure profile definition. All line size will be presented in Piping & Instrumentation Diagram (P&ID). Nevertheless, there is no line length, elbow and elevation define in P&ID. Upon receipt of P&ID, Piping engineer will begin the piping routing activities and assign necessary length, elbow and elevation to the line. This piping routing may not consistent with assumption taken by process engineer during earlier process design. Significant increase in pressure drop, wrong routing of pipe , incorrect sloping, etc could lead to severe vibration, valve chartering, reduced capacity, under-perform equipment, etc. Therefore it is important for a process engineer to identify Process Critical Line for detail isometric checking. Following will tabulate typical line may experience potential problem and required detail process checking.

Gravity flow

In general most fluid is transferred by pressure from source to destination during normal operation. Pressure head available at source will overcome frictional loss, velocity head and static head. This allow fluid transfer from low point to elevated point. Typical example is transfer liquid from closed drain drum to production separator with the pressure head develop by a reciprocating pump. This kind of pipe is typically know as pressurized pipe. Nevertheless, there are some fluid is transferred by gravity force (or static head). Typical system is closed drain network, process line designed for gravity transfer, etc. Improper design of gravity flow would lead to reduce or no flow.

Pump Suction

Cavitation is phenomenon cause by bubble generation follow by bubble collapsed. More thorough discussion on cavitation phenomenon, cavitation damages and the way to minimize / avoid cavitation can be found in following post :

Typically to minimize / avoid cavitation damage is to ensure Net Positive Suction Head required (NPSHr) by the pump is lower than the NPSH available (NPSHa) by the system itself. Pump suction line size and routing is a dominant factor affects NPSHa. Improper design of pump suction line would lead to severe cavitation, vibration and pump damage.

Centrifugal Compressor Suction

Centrifugal Compressor capacity is subject to designated flow and compressor inlet condition. Any changes in suction condition (e.g. decrease in density) would seriously affect compressor capacity (e.g. decrease in capacity). Improper design of line between Compressor suction Knock-out drum (KOD) and compressor inlet nozzle would lead to high pressure drop, subsequently lower density and capacity decrease.

Long compressor line (from KOD to compressor) would increase potential of heat loss to ambient (severe during winter time) and results condensation. Present of condensate in vapor to compressor and impinge on compressor impeller when vapor is accelerated potentially damage compressor impeller and severe vibration in compressor.

Flare/Vent Collection Header

Flare / vent collection header has significant impact on built-up backpressure to pressure relief valve. (PRV) Severe pressure drop can lead built-up back pressure exceed it allowable limit e.g. 10% for conventional type PRV. Warm fluid mix with cold fluid in flare header may results two phase gas liquid flow in flare header. Similarly, severe flare header vibration can occur with the present of slugging / plugging flow. Low point in flare line potentially results liquid accumulation in flare line and corrosion may occur. In the major relief event, high velocity vapor pushing accumulated liquid would results slugging flow in the flare line. Liquid column flowing at vapor velocity knocking of elbow/bend may generate severe vibration.

Flashing / Two phase Gas-Liquid Flow

Liquid at saturation point coming from separator potentially flash and two phase gas liquid flow. Typical flow regime is Bubbly flow. Similarly saturated vapor experience ambient cooling and line frictional loss results condensation and two phase gas liquid flow. Typical flow regime is Mist flow. Both Bubbly and Mist flow are not destructive in nature and properly a normal support would be sufficient. Nevertheless, slugging and plugging flow in vertical and horizontal potential results significant vibration to piping. Extra and strengthen support is required to avoid severe vibration and failure on pipe crack. More discussion on Problems Caused by Two Phase Gas-Liquid Flow.

Liquid-Liquid Coalescer

Saturated liquid from separator feeding liquid-liquid separator, any pressure drop increase potentially lead to vapor accumulation and under-performed liquid-liquid separation.

Low Pressure Line

Low pressure stream e.g. overhead from amine regeneration column, end flash gas from end flash column, etc is very sensitive to frictional loss.Low pressure here is pressure very close to atmospheric pressure. Any increase in frictional loss will seriously reduce flow through the pipe.

Potential Surge line

Steam supply line experience heat loss and condensation due to partially damaged insulation and extreme low ambient temperature. Flashing condensate with steam return to collection header mix with cold condensate. Both condition would results sudden steam collapse and lead to implosion. Steam implosion would generate severe movement of condensate in the collection header and severe vibration of header. Long pipeline transferring incompressible fluid e.g. LNG rundown line, produce water injection line, etc potentially experience transient surge (water hammer) in the event of closure of shutdown valve. Piping surge is severe in nature and potentially lead to pipe crack and support failure.

Wet Corrosive Service

Some line is normally flow with vapor contains CO2 & H2S and sulfide stress corrosion cracking (SSCC) and general CO2 corrosion is not expected as only vapor flow. During winter low ambient temperature and under turndown operation, ambient cooling potentially lead to vapor condensation and induced SSCC and general corrosion on under-designed piping. Typical example is Condensate stabilizer overhead. Similarly warm wet flare header is normally dry due to continuous dry gas purging. In the event, PRV passing leaks wet vapor into warm wet header or any PRV open follow by closure of PRVs, wet vapor potentially condensed and accumulate in low point and results general corrosion.

Critical Pressure drop line

Line normally design for low pressure drop, any increase in pressure drop could to capacity reduction and/or under-perform downstream unit. Typical example is high pressure gas feeding liquefaction Main Cryogenic Heat Exchanger. Any reduction in Feed pressure to MCHE would lead to higher heat of vaporization and reduce LNG production.

Pressure Relief Valve Inlet

Under normal design condition, PRV inlet line non-recoverable pressure loss shall be limited to 3% of PRV set pressure Any significant increase in line length and elbow (due to piping routing) will results non-recoverable pressure loss increase and lead to PRV chattering.

Pressure Relief Valve Outlet

Upon opening of PRV, instantaneous large gas or vapor passing PRV. High frequency noise is generated results acoustic induced vibration (AIV) which potentially cause discharge pipe cracking. Instantaneous large gas/vapor flow accelerated from zero velocity to maximum velocity will induced high reaction force to downstream piping. Under-designed pipe may crack on high reaction force.

Control valve and Restriction Orifice

Fluid passing control valve and restriction orifice continuously will generate low frequency noise. This noise wave will be transmitted to downstream piping and result Flow Induced Vibration (FIV) which potentially
leads to pipe cracking in particular at small bore connection to large line

Saturated liquid passing a control valve or restriction orifice, pressure will began to decrease and lowest pressure closed to vena contracta, follow by pressure recovery once is passed the vena contracta. Lowest pressure point could be lower than vapor pressure of fluid. Vapor bubble will begin to form and once fluid passed through the vena contracta, vapor will start to collapse and results jet wave impacting control valve or restriction or piping. Above phenomenon generally known as cavitation which generate severe vibration to the piping.

Any scenario is normally ignore or miss by engineer where control valve downstream piping may not design for occurrence sonic flow downstream piping. This typically occur in line with control valve discharge to flare/vent header. Sonic flow potentially reduce flowing capacity and how reaction force to piping.

Related Topic

Saturday, January 23, 2010

Control Valve Cavitation Damage and Solutions

Cavitation damage and fatigue due to acoustically induced vibration have discussed several times in previous posts such "cavitation" and "AIV". Control valve is known as one the common element / component in a plant potential source of cavitation and AIV related problems. Many efforts in combating both issues were proposed.

Typical solution is anti-cavitation trim e.g. staged trim, multi-flow path trim, labyrinth-disk type trim, etc.

i) Staged trim low noise
Staged trim low noise trim is adopted multi-stage pressure letdown similar to multi-stage RO.

ii) Multiple flow path type low noise trim

Multiple flow path low noise trim is one of the very effective noise (Sound Pwer Level) attenuator. It possibly reduce the noise level up to 40 dB. Image below shows a Multiple flow path low noise trim installed in a control valve.

Detail construction of multiple path low noise trim (red circle) shown in below image. Typically it adopt the simple principle as use in RO. Multiple flow path follow by multiple expansion stage.

Read more in "Fisher® WhisperFlo® Aerodynamic Noise Attenuation Trim".

iii) Labyrinth-disk type low noise trim
Another type of low noise trim is the Labyrinth-disk type low noise trim. It works approximate the same way as Multiple flow path type low noise trim.

Above trims design is commonly based on general principle in cavitation prevention which is ensure the operating pressure along the flow path in valve trim above fluid vapor pressure. See below image.

Below are some old useful articles related cavitation and multi-stage disc trim available for download. The post part of the continuation post from "Useful Documents Related to Control Valve"

Fluid kinetic energy as a selection criteria for control valve
A selection criteria is provided that assures a control valve will perform its control function without the attendant problems of erosion, vibration, noise and short life. The criteria involves limits on the fluid kinetic energy exiting through the valve throttling area. Use of this criteria has resolved existing valve problems as demonstrated by retrofitting of the internals of many valves and vibration measurements before and after the retrofit. The selection criteria is to limit the valve throttling exit fluid kinetic energy to 70 psi (480 KPa) or less.

Multi-stage valve trim retrofits vibration eliminate damaging
Through the RHR valve trim retrofit at Quad Cities with multi-stage, tortuous-path, pressure reducing disks and an emergency capacity cage, the damaging vibration previously experience during system test operation has been eliminated. Further, an unlikely repetition of the previously experienced valve blockage by a Rad bag or any other medium has been precluded by the 50% over-capacity cage in the last 20% of valve stroke. Also, previous concerns regarding possible piping fatigue failures within the RHR system as a result of past severe vibration problems have been eliminated.

Specifying control valves for severe-service applications
Large number of the process control valves used in fossil-fired power plants must operate under severe-services conditions—that is, in high-pressure and / or high-temperature applications. When specifying valves for such applications, extreme care must be taken to avoid costly premature failures. This article discusses the stringent requirements that valves must meet to safety operate and deliver long-term performance under severe service conditions. Requirements are examined for both generic and specific applications.

Solving cavitation and Sand Erosion problems
In combating cavitation and erosion, principle in eliminating proposed are multistage velocity control and proper material of construction. Few valve applications in oil and gas production are more destructive and require more continuous maintenance than separator level-control valves. Over the years, the industry has frequently come to accept poor service life in this application. Service lives of a few weeks between complete rebuilds are common. But acceptance of poor service life is no longer necessary.

Control Valve Cavitation
Trim exit velocity is one of the parameter to be considered in control valve selection. Nevertheless, it may not completely explains entire physical phenomena occur in a control valve.The trim velocity approach may not reliable enough in solving problem related to control valve cavitation. The critical pressure drop method and the sigma method which will be introduced proves that the single stage valve may not experience cavitation, despite a trim exit velocity much higher than 100 ft/sec.

Impact of control valve design piping vibration
Vibration of the recycle piping system on the main oil export pumps from a platform in the North Sea raised concern about pipe breakage due to fatigue. Failures had already occurred in associated small bore piping and the instrument air supply lines. and control accessories on the recycle flow control valves. Concern also existed due to the vibration of non-flowing pipe work and systems such as the deck structure, cable trays and other instrumentation, which included fire and gas detection systems. The vibration was finally solved by changing the control valve to a trim that added enough pressure stages to assure the trim exit velocities and energy levels were reduced to levels demonstrated historically as needed in severe service applications. This vibration energy reduction was more than 16 times. This was achieved by reducing the trim exit velocity from peaks of 74 m/s to 12 m/s.

Special thanks to Control Component Inc.

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Monday, December 21, 2009

Calculate Combined Sound Power Level (PWL) Using Analytical Method

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In process plant, there will be scenario for two and/or more pressure reduction devices (PRD) downstream piping discharge to a common header. Typical example is blowdown / restriction orifice to flare header. During plant wide total plant blowdown, all blowdown valves may be opened simultaneously or opened in group. Different PRD will results different level of PWL.When two Sound power sources are combined, it is understood that the total combined Sound Power Level will increase due to two energy stream are combined. However these energy streams are transmitted in wave form, the resultant Sound Power level will not be added arithmetically i.e. 1+1=2. In earlier post "Calculate Combined Sound Power Level (PWL) Using Graphical Method", an graphical method using PWL adder is presented.

The total combined Sound Power Level is equal to "PWL adder" (which estimated base on PWL difference between both stream and from several experience equations ) plus maximum PWL out of both streams.

Combined PWL = Maximum PWL + PWL Adder

Analytical Method
This post will present another analytical method to calculate combined PWL for multiple streams (PWL1, PWL2...)

Total combined Sound Power Level

PWL_C = 10 Log₁₀ [+ 10^(PWL₁ / 10) + 10^(PWL₂ / 10)+...]

Example
Two Pressure control valves with PWL of 160 dB and 166 dB discharging to a flare header. Calculate combined PWL.

Graphical method
Assumed PWL attenuation due to piping is ignored.
PWL,diff = 166 - 160 = 6 dB

PWL adder = 10 ^ (0.4771 - 0.0795 x 6) = 1 dB (refer earlier post).

Combined PWL = Maximum PWL + PWL Adder
Combined PWL = 166 + 1 = 167 dB.

Analytical method

From above analytical equation,
Combined PWL = 10 Log10 [10^(166/10)+10^(160/10)]
Combined PWL = 167 dB

Obviously analytical method is simpler and faster.

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Wednesday, December 2, 2009

Sound Power Level Attenuation due to Pipe Length

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Rules of thumbs said that every 50D pipe length results approximately 3 dB Sound Power Level (PWL) attenuation where D is in meter (m). This is presented in "Measures & Technique In Eliminating / Minimizing PWL". This post will base on this simple rule-of-thumbs to generate a simple equation for Sound Power Level attenuation due to pipe length.

Derivation
For every pipe length of 50D, the PWL loss will be 3 dB,

PWL loss per meter of pipe length = 3 / 50D = 0.06/D

Sound Power Level attenuation for any pipe length of L,

PWL_Loss,L = 0.06 L/D

where
PWL_Loss,L = PWL loss per meter pipe length (dB/m)
L = Pipe length (m)
D = Pipe internal diameter (m)

Following is typical graph for Unit PWL versus pipe ID.

Related Topic

Monday, November 23, 2009

PWL Reduction By Splitting Flow

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As discussed in "Measures & Technique In Eliminating / Minimizing PWL", one of the measures to reduce Sound Power Level (PWL) at sources PWL is to split the flow. Half the original flow would result approximately 3 dB reduction. This post will discuss how this 3 dB reduction is derived.

A common acceptable method to predict Sound Power Level (PWL in dB) of AIV source is as follow :

Refer to "Sound Power Level (PWL) Prediction from AIV Aspect" for more discussion on above equatio.

Above equation will provide PWL_W for a stream with W mass flow. If the stream is split into two stream, the mass flow will be W/2.

Sound Power level for single half-stream (W/2),

PWL_W/2 = PWL_W + 10Log [(1/2)^2]
PWL_W/2 = PWL_W - 6

When two half-streams with PWL_W/2 are mixed, the resultant PWL of two half-streams may be estimated base on method as discussed in "Calculate Combined Sound Power Level (PWL)".
From this post,

Combined PWL = Maximum PWL + PWL Adder

Both half-streams have same PWL. The PWL difference (PWL,diff) is zero (0).
PWL adder = 10 ^ (0.4771 - 0.0795 x PWL,diff)
PWL adder = 10 ^ (0.4771 - 0.0795 x 0)
PWL adder = 3 dB

Both half-streams have same PWL.
Maximum PWL = PWL_W/2 = PWL_W - 6

Therefore, Combined PWL is :
Combined PWL = PWL_W/2 - PWL adder
Combined PWL = PWL_W - 6 - 3
Combined PWL = PWL_W - 3

So, half the flow will results approximately 3 dB reduction from total flow.

Ref.
i) "Designing Piping Systems Against Acoustically Induced Structural Fatigue", E.L. Eisinger, Journal of Pressure Vessel Technology, Aug 1997.

Related Topic

Wednesday, November 18, 2009

Calculate Combined Sound Power Level (PWL) Using Graphical Method

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Earlier post "Measures & Technique In Eliminating / Minimizing PWL" discussed about several measures can be considered in eliminating and minimizing PWL for pressure reduction devices. In similar post, Sound Power Level attenuation due to extended "strengthened" pipe has also been presented. This post will focus on approach to estimate combined sound power level (PWL mix) for two or more streams.

In process plant, there will be scenario for two and/or more pressure reduction devices (PRD) downstream piping discharge to a common header. Typical example is blowdown / restriction orifice to flare header. During plant wide total plant blowdown, all blowdown valves may be opened simultaneously or opened in group. Different PRD will results different level of PWL. Now the queries will be :

How these PWL are interact between each and other ?
How to estimate resultant PWL ?
Are different PWL addition arithmetically ?

When two Sound power sources are combined, it is understood that the total combined Sound Power Level will increase due to two energy stream are combined. However these energy streams are transmitted in wave form, the resultant Sound Power level will not be added arithmetically i.e. 1+1=2. The total combined Sound Power Level is equal to "PWL adder" (which estimated base on PWL difference between both stream and from several experience equations ) plus maximum PWL out of both streams.

Mathematically,

Combined PWL = Maximum PWL + PWL Adder

PWL Adder
PWL adder is estimated from PWL difference between both stream and the following equations.

If PWL Difference (PWL, diff)

between 0 to 6 dB, PWL adder = 10 ^ (0.4771 - 0.0795 x PWL,diff)
between 6 to 10 dB, PWL adder = 10 ^ (0.5651 - 0.0942 x PWL,diff)
between 10 to 14 dB, PWL adder = 10 ^ (0.5432 - 0.092 x PWL,diff)
more than 14 dB, PWL adder = 10 ^ (0.7794 - 0.1088 x PWL,diff)

Example
Two Pressure control valves with PWL of 160 dB and 166 dB discharging to a flare header. Calculate combined PWL.

Assumed PWL attenuation due to piping is ignored.
PWL,diff = 166 - 160 = 6 dB

PWL adder = 10 ^ (0.4771 - 0.0795 x 6) = 1 dB

Combined PWL = Maximum PWL + PWL Adder
Combined PWL = 166 + 1 = 167 dB.

Related Topic

Saturday, October 24, 2009

Principle in Eliminating & Minimizing AIV Impact

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High frequency acoustic excitation downstream of pressure reducing device potentially results downstream piping failure due to Acoustic Induced Vibration (AIV). Whenever there is pressure drop with mass passing through the valve, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping, in particular at discontinuity section i.e fabricated Tee, small bore connection, welded pipe and pipe support, etc. This acoustic excitation phenomena is generally involve high frequency (more than 1000Hz) acoustic energy. When high frequency acoustic energy is matches with mechanical natural frequency of piping and its component, excitation amplitude is at maximum and lead to increased stress level. AIV phenomenon and assessment methodology have been discussed in previous post. Sound Power Level (PWL) is commonly used in quantifying acoustic energy in AIV. Higher PWL, higher the potential of vibration level and higher the risk of AIV.

Principle in Eliminating & Minimizing AIV Impact
This post will focus in some common measures and techniques to eliminate and/or minimize PWL generation and transmission. The main principles in avoiding and minimizing the impact of AIV are :

Eliminate or reduce vibration level at pressure reduction device
Damper vibration level
Increase resistance to vibration
Minimizing vibration transmission

All shall measures are not exclusive between each and other but all measures shall be taken and apply all (if possible). However, It shall follow the sequence from principle no. 1 to principle no.4.

Application Example
A control valve discharge gas from high pressure vessel to flare header results high vibration level (or sound power level, PWL) and lead to AIV problem. The consideration shall be focused on reducing the PWL by using special trim control valve (principle no. 1). Splitting flow into several control valves in parallel may reduce the PWL (principle no.1). However one shall remember this sometime is not a good idea as common mode failure may worsen the situation. Next may consider a silencer insert downstream of control valve to damper PWL (principle no.2). One may also consider to increase piping resistance to vibration i.e. increase wall thickness (principle no.3). This high resistance piping may be extended to decrease the PWL to certain acceptable limit before it is tie in to downstream piping which is less resistance (principle no. 4).

Concluding Remark
In eliminating and minimizing AIV impact, all principles as mentioned above are not exclusive and shall be applied to maximum (if possible).

In coming post "Measures & Technique In Eliminating / Minimizing PWL", the discussion will focus on
- type of AIV source
- measures in tackling each AIV problem

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Sunday, July 26, 2009

Heat Radiation For Pain & Blistering Threshold

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Flare is commonly installed in oil and gas process plant to burn hydrocarbon and/or toxic gas to avoid formation of combustible mixture, to minimize green house effect (GHE), to minimize health hazards to personnel on site, etc. Flaring hydrocarbon gas may generate carbon dioxide & water for complete combustion and soot (contribute to smokeless level) & unburnt components (contributes to toxic environment) for incomplete combustion. Besides, heat and noise are generated and radiated and transmitted around the flare tip.

Heat radiated from flare may transmitted in sphere form around the flare tip. Along the transmission, the energy is distributed in sphere form and this lead to reduction in heat radiation level (heat flux, kW/m2). Personnel or equipment along the transmission path will expose to this heat radiation. Personnel or equipment closer to flare tip will experience higher heat radiation level.

With studies conducted by Stoll and Greene (1958), following graph relate heat radiation versus time for Pain threshold and Blister threshold. With heat radiation of 6.3 kW/m2 (2 000 Btu/h·ft2), the pain threshold is reached in 8 s and blistering occurs in 20 s.

The following equation derived from above graph and can be used to relates heat radiation with time for pain and blister thresholds.

Pain threshold :

q = 25.544 x t ^-0.6742

Blister threshold :

q = 75.691 x t ^-0.8399

where :
q = heat radiation (kW/m2)
t = time (seconds)

Ref :
(i) A. M. STOLL and L. C. GREEN, The Production of Burns by Thermal Radiation of Medium Intensity, Paper Number 58-A-219, American Society of Mechanical Engineers, New York, 1958

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Saturday, July 18, 2009

Energy Input or E-method In Assessing AIV

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High frequency acoustic Excitation downstream of pressure reducing device and potential of downstream piping failure on Acoustic Induced Vibration (AIV) has raised concern in many process plant

Earlier post "Extra Attention to Common Point and Similarity on AIV Failure" has discussed the common points and similarity of AIV such as typical failure location, system experienced failure in the past, failure time & period and Mach no. An engineer assessing AIV shall pay extra attention in these factors.

There are several methods have been discussed earlier in assessing AIV problem. There are "D-method" and "D/t-method". These two methods are widely used by many engineering contractor and consultants in many previous projects. There is another method, the Energy Input method or "E-method" which has been introduced in mid 1990.

Energy Input or E-method In Assessing AIV
The "E-method" is focus on the acoustic energy transmitted along the pipe, downstream of pressure reducing devices. The acoustic energy can be measured by Ma2.dP where Ma2 is the Mach number at downstream pipe and dP is the pressure drop across pressure reducing devices. The higher the acoustic energy or Ma2.dP, higher the risk for AIV failure.

Similar to the other two methods, based on data available in Carucci & Mueller (1982) studies, "No failure" and "Failure" points have been plotted in following chart with Ma2.dP versus Pipe Diameter / Wall thickness (D/t).

Note :
1) A "failure" point (54.8, 0.35) is below curve. It is failed on bad welding. No further failure after good welding.

2) Red point are "failure" point and Blue point are "No failure" point

From above chart a clear distinctive limit line can be established. This line may be used to assess potential failure of piping downstream of pressure reducing device. Any point above the line potentially fail on AIV, piping treatment or redesign required to minimise the risk of AIV failure.

*As limit line is straight or following any pattern, a representative equation is yet to be developed.

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Monday, June 8, 2009

Assess AIV with "D/t-method" with Polynomial PWL Limit Line

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High frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure on Acoustic Induced Vibration (AIV) has raised concern in many plant design. Earlier post "Extra Attention to Common Point and Similarity on AIV Failure" has discussed the common points and similarity of AIV such as typical failure location, system experienced failure in the past, failure time & period and Mach no. An engineer assessing AIV shall pay extra attention in these factors.

Earlier post "Assess AIV with "D/t-method "has presented a Sound Power level (PWL) limit line (logarithm line) to assess the possibility of Acoustically Induced Fatigue failure based on D/t method. Nevertheless, there is still some concern on the area where there is no published data shows the possibility of AIV failure (shown as pink triangle area in below image). Eisinger PWL limit line and Logarithm PWL limit line consider this area is non-failure.

Taking conservative approach, this area may be defined as area potentially failure until more real field or experiment data available. Therefore, a new Polynomial PWL limit line is proposed.

PWL_limit =C₄.r⁴ + C₃.r³ + C₂.r² + C₁.r + C₀

Where
PWL_limit = Maximum allowable PWL at Pipe diameter D meter, (dB)
r = 1000D/t
D = Pipe Outside diameter (m)
T = Pipe wall thickness (mm)
C4 = 0.000000032
C3 = 0.000023
C2 = 0.0057
C1 = 0.691
C0 = 192

Refer following graph. New polynomial presented as violet line.

Note :
1) A "failure" point (0.3m, 165dB) is below curve. It is failed on bad welding. No further failure after good welding.

2) Red point are "failure" point and Blue point are "No failure" point

3) May be used for 0.2m < D < 0.9m. Use at risk for D < 0.2m and D > 0.9m

This line may be used to assess potential failure of piping downstream of pressure reducing device. Any point above this line potentially fail on AIV, piping treatment or redesign required to minimise the risk of AIV failure. This will be discussed in future...

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