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The Emerson severe service team provides global customers with Fishersevere service control valve solutions. Whether it issevere service applications for the power, hydrocarbon, chemical or pulp and paper industry, these technical experts deliver

sound solutions to address critical applications for aerodynamic noise, cavitation and out-gassing issues, as well as particulateerosion. Please visit our website or contact your local Emerson Process Management sales ofce for more information on howthe severe service team can help you.

Volume 11 Issue 2

Outgassing Versus Flashing - What are the Differences?

While outgassing and ashing may seem similar to manyprocess engineers, the driving forces and the effects ona control valve are quite different. Flashing occurs whenthe pressure of a uid falls below its vapor pressure. At

this point, the uid begins to change from a liquid to avapor, both of which have the same chemical makeup.The vapor pressure is a function of the uid temperatureand, therefore, ashing is a function of both the pressureand temperature of the uid. For the uid to ash, heatmust transfer from the liquid during the vaporizationprocess and this requires time.

Outgassing results in a liquid and gas mixture as well,but the driving force is strictly due to the uid pressurebeing below the saturation pressure of a gas dissolved ina liquid. Once the uid pressure is below that saturationpoint the gas comes out of solution and the liquid

and gas have a different chemical makeup. The bestillustration of outgassing is what occurs when a bottleof champagne is opened. Immediately, the pressureinside the bottle drops to atmospheric, which is belowthe saturation pressure of the dissolved carbon dioxide.The same holds true in a control valve. A uid with therequired heat transfer takes a certain amount of timeto ash whereas outgassing requires no heat transferand occurs much faster. Downstream of the valve thetwo phenomena can appear to be similar, but at thethrottling point the difference can be dramatic.

Flashing is well understood behavior and valve sizing forashing is covered by standard liquid sizing calculations.Outgassing, which can occur in many combinationsof liquids and gases, does not follow a speciedthermodynamic path and special valve sizing routines

are required to handle it. Likewise, valve and trim stylesas well as material selected for ashing applicationsmay not be suitable for outgassing. So how can you tellif your application may have an outgassing or ashingproblem? Two indicators are the liquid and gas exitingthe valve have different molecular weights and the valveis providing liquid-level control for a phase separationprocess.

Your local Emerson Sales or Local Business Partner ofcecan help obtain the best solution to your outgassing orashing problems.

http://www.fishersevereservice.com/

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FisherControl Valves Combat Outgassing

Outgassing can cause problems in a control valvebecause of the sudden increase in ow volume oncethe gas comes out of solution. Because the gas has amuch larger volume than the liquid, the ow area needsto be larger or else the velocity of ow will increase. Arestricted ow area can create a stream of high velocityliquid droplets with the potential to be erosive to anysurface they impinge upon.

Another problem is the release of energy that the valveand piping are subjected to and the potential for highvibration levels. A valve designer must reduce thesedamaging effects. To design or select the proper trim,the engineer must know the pressure and volume ofthe gas coming out of solution. The ow passagescan then be correctly designed to control the velocityof the uid through the trim. Since outgassing is

not a thermodynamic process that can be predictedaccurately, the designer must account for this inaccuracyin sizing and selecting a suitable trim.

The simplest and oldest solution to outgassing is to usea single-stage pressure drop. A ow-down angle bodydesign, such as the Fisher 461, has been successfullyused for many years. Its expanded outlet reduces thevelocity effects and a swage in the downstream pipingcan further reduce the velocity of ow. The anglebody design helps the uid to change direction gentlyand avoid an increase in turbulence and the directimpingement of the uid on the plug and seat ring.

An alternative is to split the pressure drop into two ormore stages. However, as the number of stages increasesit becomes increasingly difcult to correctly account for

the change in volume. Some manufacturers try to stagethe pressure drop using a series of ow restrictors in thetrim, but the results are questionable and difcult to

justify given the increase in complexity and cost.

The Fisher DST-G valve reduces the pressure dropthrough dened reduction stages. Between thesestages are open volumes that allow the uid to recoverbefore entering the next stage to ensure that thevelocity between stages is low. The last stage is a seriesof vertical slots around the circumference of the trim,owing from the inside to the outside. These slots splitthe total ow into smaller pockets thereby reducing theenergy released by the uid in each ow passage andreducing vibration and velocity effects on the outlet.The small passages also increase the frequency of noisegenerated so that it does not transmit as well through

the downstream piping.

For low volume outgassing, it is acceptable to droppressure through more stages and the Fisher NotchFloDST valve uses a series of notches cut into the valveplug to create these. In this design, the ow movesupward through the trim and plug notches each being adenitive pressure drop.

Good shutoff is another important feature for valvesused in outgassing applications. Any leakage betweenthe plug and seat will generate outgassing and quicklydamage the seating surfaces. In the pressure-staged

designs described above, Class V shutoff is standard andany leakage that does occur is staged through the trim atlow velocity.

Fisher 461 Valve Fisher DST-G Valve Fisher NotchFlo Valve

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Fisher and NotchFlo are marks owned by one of the companies in the EmersonProcess Management business division of Emerson Electric Co. Emerson ProcessManagement, Emerson, and the Emerson logo are trademarks and s ervice markof Emerson Electric Co. All other marks are the propert y of their respectiveowners.

2011 Fisher Controls International LLC. All Rights Reserved.

The Severe Service Journal is published quarterly by the Fisher severe service team and is distributed by email.

To subscribe, go to www.FisherSevereService.com .

FisherDST-G Valves Selected for ChallengingHydrocracker Application

Reneries in Europe are building new hydrocracker unitsto meet the increasing demand for diesel fuel and toprocess heavier sour crude. Hydrocracking is one of the

most versatile of all petroleum rening processes. Ashydrocracking takes place, sulphur, nitrogen, and oxygenare almost completely removed and olens are saturatedso that products are a mixture of essentially pureparafns, naphthalenes, and aromatics. This exibilitygives the hydrocracking process a particularly importantrole as reneries attempt to meet the challenges oftodays economic climate.

In a typical hydrocracking process the feedstock isheated and fed to a reactor containing a catalyst wherethe reaction takes place. The reactor efuent is sent

to a hot separator where conversion products areashed overhead and heavy unconverted products aretaken as hot liquid bottoms. The vapors from the hotseparator are fed to a cold separator where recycle gasis separated from the product. The product is sent tothe fractionation and the recycle gas is returned to thereactor via the recycle gas compressor.1

The separator letdown valves are uniquely challengeddue to outgassing, which can result in noise, erosion,vibration, and plugging due to the presence of thecatalyst. In a control valve operating in a pressure rangeof 2,000 to 3,000 psid (138 to 207 bar), the exiting

mixture of vapor and liquid can result in severe damageto the valve trim and also result in noise and vibration.

The Emerson sales ofce in Spain recently was awardeda project consisting of 20 valves for a hydrocrackerproject in Europe. As a part of the project, Emerson is

supplying Fisher DST-G valves with alloy steel bodiesand hard-faced trim to withstand erosion and a pressuredrop of 1800 psid (124 bar) at temperature of 608 F(320 C). The DST-G valve, with its unique passage anda slotted cage on the downstream, can withstand thecatalyst particles and also attenuate noise. The valves

were selected based on Fishers proprietary bracketingmethod of sizing which takes into account the amountof vapor content at the outlet of the valve and estimatesthe downstream area to accommodate the volumetricexpansion at the outlet of the valve. The plant isscheduled for startup in mid 2011.

1Robert A. Meyers, Handbook of Petroleum Rening Processes,

Third Edition.

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