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CONTROL VALVES

general market demands to the highestflow capacity (cv x 100)/DN2 at lowestinitial cost.Today, the inherent character-istics of globe valves are somewhat modi-fied equal percentage in the competitiverange of published highest cv values.To becompetitive, valves need to be offeredmostly with the largest seat diameter(smallest nominal size DN), if not speci-fied otherwise.Cam and positioner signal technologyused to linearize any mismatching is notthe “door to heaven”; rather, advantagesand disadvantages need to be discussedseparately.[4]Figures 7d and 7e show that there is an in-teraction between the valve characteristicform, the flow capacity and the powerconsumption.This is shown here with liq-uid measurements of xFz characteristicsversus load cv/cv100.Valves with higherxFz values convert the power [dp x q xconst.][1] better to heat by flow frictionthan high flow capacity valves, which con-vert more to flow velocity and thereforecavitation occurs at smaller pressure drops.

5 Trends and definitions of inherentvalve characteristics for globe androtary valves

Gain requirements of valve inherent char-acteristics are defined in IEC 543 2-4 witha tolerance band of +-10 % within thelimits: 0.5 < ∆cv / ∆s < 2.This was acompromise to the former stricter nation-al regulations of VDI 2173 with 0.7 < ∆cv / ∆s < 1.3 and +-10% of thecv100 value. See Figures 7a to 7c.Because today there are many types of in-herent valve characteristics, from theglobe control valves to quarter-turn con-trol valves, IEC 534 2-4 has defined basicrequirements for the characteristic quality(see Figures 7a to 7e).In general, all kinds of characteristics aresupported, but they are to be published ifthey are not of the ideal linear or equalpercentage type, i.e. outside the toleranceband defined in IEC 534 2-4.The idealequal percentage characteristic of globevalves from former times cannot beachieved with modern economic standardglobe valves, which also have to follow

Plant design and controlvalve selection under

increasing cost and timepressure - Part II

This is the second and final section of Mr Siemers´ technical paper addressing plant design andcontrol valve selection when working under increased time and cost pressure. The first section (Aprilissue) focused on control valve operating points and provided a case history involving a mismatchbefore introducing better valve sizing practices. Part two starts by explaining the trends anddefinitions of inherent valve characteristics before focusing on “quick and dirty” sizing. The paperthen addresses cavitation before concluding with the expert software available to help select theoptimum valve characteristic form.

By Dipl. Ing. Holger Siemers, SAMSON AG

Part I

1. Plant design under cost and time

pressure

2. Control valves today are converting

links between budgets!

3. From traditional to modern

Development and Engineering

Practice (DEP) for plant designers

4. The new DEP for trouble-shooting

the mismatched case study in

section 2

Part II

5. Trends and definitions of inherent

valve characteristics for globe and

rotary valves.

6. Detail engineering-sources for plant

and valve designers have dried out!

7. Noise reduction and getting the

plant power under control.

8. Selecting the optimum valve

characteristic form

9. Using software to increase control

quality, reduce cost and save time

for creativity

Note: given the amount of detail in some of the graphics in this article, readers might like to note that a high resolutionPDF version can be viewed at: www.valve-world.net/magazine/controlvalves.asp.

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CONTROL VALVES

Fig. 7a Fig. 7b

Figs 7a to 7e: Inherent valve characteristic versus flow capacity versus power converting capability -xFz f(cv/cv100)-.

Fig. 7c

Fig. 7d Fig. 7e

▲

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CONTROL VALVES▲

Modern control valves are designed forspecific marked segments like chemicals,food & beverage and pharmaceuticals. Ona higher price level they are used for thedownstream hydrocarbon processing in-dustries and oil and gas market.And atthe top price level they are mainly appliedin the onshore and offshore upstreammarket, in oil and gas exploration, separa-tion, storing and transport, or they aretailored for special demands. In the HPImarket, control valves are more often“power converting machines” up to 300MW used as flare valves, blow down oranti-surge control valves and for otherspecial demands.Those valves in the se-vere service area in all nominal sizes andpressure ratings are on average approxi-mately four times more expensive -in-cluding more detail engineering- thanvalves of non critical standard applicationsin chemical plants in the range of DN <= 6 inch and PN <= Class 300.

6 Detail engineering-sources forplant-and valve designers dried out!

Plant and valve designers in the field ofsevere applications need time to optimizecontrol quality, the sound level and powerconsumption as well as to handle increas-ing regulation paperwork and economicaspects. Sizing control valves from a totalpoint of view is a challenge for the projectengineer as well as for the valve manufac-turer's specialists, even if they use mod-ern powerful in-house sizing programsand tools.Those “valve guys” need long-term experience and high skills in meas-urement and control, mechanical engi-neering and thermodynamics. Incomparison to the past, the time availablefor major projects has been more thanhalved, the specification volume -includ-ing the increasing paperwork associatedwith standards, special regulations andtailored customer requirements havemore than doubled.Negative effects of today are: valve speci-fication sheets are often of low quality,operating points are missing or not logi-cally sorted to qmax, qnorm, qmin, Im-portant property data (like the vapourpressure) may be missing, no informationabout the worst-case conditions like dur-ing start-up, no control loop informationetc. No wonder that sources of compe-

tence for high-level engineering for valveswith higher demands have dried out andthe risk of “quick and dirty” sizing is in-creasing.Given today's quality standards ISO9001:2000 and the upcoming SIL (IEC61508) requirements even for non emer-gency shut down ESD serial valve prod-ucts, do we really need SIL for special se-vere service valves? Also, as products aremore often being offered under e-pur-chasing conditions characterized by an in-creasing cost and time pressure, the au-thor would like to point out that careneeds to be taken in this area, particularlyfor severe service valves. Proper controlvalve selection requires detail engineeringwith competence.We should not save ini-tial costs by cutting back on detailled en-gineering and shifting higher cost to otherafter-sales budgets, e.g. troubleshooting,maintenance and, at worst, plant shut-down or accidents.The CONVAL software looks at plant pa-rameters (pipework, pipe devices, flowmeters and valves) from an overall pointof view with expert system features tocompensate for the negative trends de-scribed above.The integrated mightyvalve database can store every brand valvefunction like inherent valve characteristicas well as all defined valve recovery factorfunctions.

7 Noise reduction and getting theplant power under control is not thateasy under time pressure.

Figures 8a to 8d show the interaction ofdifferent valve authority parameters withthe liquid sound emission and the chal-lenge to select the most economic valvefor each unique plant design. Figure 8ashows examples of a pump-generatedplant system with total valve authorityV=0.3 and a tank level control processwith total valve authority V = 0.7.All possible valve authorities versus valveload cv/cv100 are shown in Figure 8b.The hatched area may include > 80% ofall applications. Cost pressure drives thetrend towards the bottom line V < 0.1,unfortunately with a negative impact ongood control parameters.This sometimescalls for undersizing cv 100 < cv max toavoid excessive gain fluctuations.Figure 8c shows different standard globe

valve qualities with comparison measure-ments of seven globe valves.Adjusted tothe same cv100 value the valves createSPL differences between 65 and 95 dB(A)depending on valve design parameterslike top-guided parabolic, top seat-guidedcontour plug or top seat-guided parabolicAC trim. If no measurements from thevalve database are available, valve recov-ery factors versus valve load cv/cv100 areused in the software for flow and SPL cal-culation as a first approximation for para-bolic plugs (flow to open).Figure 8e shows a real-time test rig forcomparison measurements as demon-strated in Figure 8f with xFz measure-ments -xFz versus valve load cv/cv100-for different plug guidance principles. Incritical cases detailled engineering is nec-essary to find valves with xFz characteris-tics Figure 8f overlaying the curves in Fig-ure 8b to avoid cavitation over the entirecontrol range.In applications with steam or gases, highlysophisticated noise reduction measuresalso need to be taken, giving priority tothe valve outlet velocity, especially in caseof flashing, which is not noise sensitive.[1,part 2]

8 Selecting the optimum valve charac-teristic form

This is the last point to improve controlquality but also the most difficult. In caseof pressure control from a static point ofview, an equal percentage characteristicstands for a more constant gain ∆p/ds ;p=p1, p2, or ∆p independent of thevalve authority. But anti-surge pressurecontrol mostly calls for a linear charac-teristic because of dynamic reasons toprotect the flow machine as quickly aspossible.The software looks at all loop parametersusing questions and answers as illustratedin the table in Figure 10.The recommen-dation then is the characteristic formwith advantages in comparison to others.It does not mean that other characteris-tics are not practicable. It only meansthat they will produce higher gain fluctu-ations, which can be checked separatelyby the graphical support of the software.Figure 9 shows the borders of valve au-thorities where the program shifts to thenext better valve characteristic form.

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CONTROL VALVES

Fig. 8a: The interaction of valve authority parameters todifferent installed flow characteristic.

Fig. 8b: Characteristic of the plant parameterxF as a function of the load cv/cv100 andthe valve authority. Fig. 8e: Real-time test rig at SAMSON AG.

The valve authority characteristic isindependent from the valve characteristic.The hatched area includes > 80% of allapplications.

Valve authority versus load cv/cv100

Up

and

dow

nstr

eam

pres

sure

[bar

_abs

]So

und

pres

sure

leve

ldB

(A)

Plant characteristic, pump power 50 [kWatt] Globe valve comparison: IfxF > 0.5, risk of high SPLdB(A) and resonating stemfor top guided parabolictrims DN > 1.5 inch.Difference > 30 dB(A)between valve Type d) andAC Anti-Cavitation TrimSystem-Top and seat guidesspecial parabolic trim.

Type of valves: a to e, Cv100 from 91.7 to 131Plant adjustment: dp_100/dp0 = 0.1 at V713 and direct replacement with a to e.

Flow [m3/h]Fig. 8c: The interaction of globe valve design parameters to the liquid sound emission.

Fig. 8d: Approximation of valve recoveryfactors for standard valves (flow to open -FTO), if only one value at load cv/cv100 =0,75 is given.

Fig. 8f: Typically measurements ofxFz-characteristic for parabolic plugtrims (FTO) and circumferenceguided trims

Typical globe valves xFz-characteristic

curves for sound SPL dB(A) prediction

regarding IEC 534-8-4

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CONTROL VALVES

Nevertheless, attention should still bepaid to some more specific loop parame-ters for the final software decision of thebest characteristic form. (See Figure10b.)After calculating the specific valve au-thority the software expert system goesthrough the tables by Questions and An-swers and recommends the optimumvalve inherent characteristic form (Fig-ure 10a).

tool for pipelines and pipe devices. Includ-ing material and property database formore than 3,000 substances like hydrocar-bons and chlorine.About 60 industrial flu-ids are calculated very accurately usingequations of state developed by Ruhr Uni-versity in Bochum, Germany. (Seewww.conval.de for more details.)If operating conditions are given with one,two or three operating points the plantsystem is defined in the standardized dif-ferential pressure versus flow diagram.The inherent cv characteristic of any valveas well as all other valve characteristicsxFz, Fl, xT, Fd, etc. are stored in a mightyvalve database in form of equations orpolynomial coefficients. Every valve in-stalled characteristic like flow, gain, andvalve authority, sound, inlet and outlet ve-locity as well as cavitation, flashing andchoked flow areas are presented in graphicform.A dynamic ruler indicates all resultsincluding alarms at any valve travel posi-tion.The program combines expert valvesizing with powerful plant optimizationand troubleshooting.

▲

Fig. 10a: Questions and answers in CONVAL to select the best characteristic form for the control variable flow, level, temperature and pressure.See also Fig. 10b.

9 Using software to increase controlquality, reducing cost and savingtime for creativity

The CONVAL® 6 software treats theplant and valve sizing parameters from anoverall point of view, issuing dynamicgraphics with installed characteristics con-cerning flow, power, gain, and outlet ve-locity as a function of the valve coefficientcv and the valve travel.The software is amanufacturer-independent optimization

Recommendations to improve the optimum inherent valve characteristic form. The valvemanufacturer cannot influence the valve upstream and downstream pressure characteristic - plantdesign responsibility. The valve manufacturer can influence the valve authority V if the optimumcv100 valve is chosen. CONVAL recommend five typically characteristics forms of valve familiesfound today.

Example for flow control with measurements dp proportional to flow.If other characteristics are chosen from other aspects of view, eg, dynamic aspects, the control loopmay have more gain variations. (If not out of limits, who cares?)

Fig. 9: Borders of the inherent valve characteristics forms as a function of the valve authority.

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Recommendations forimproving the basiccharacteristic formIt is assumed that only one of the

disturbance quantities acts as the main

disturbance quantity.

The symbol “=” in front of the

characteristic means that the control

loop parameters remain constant;

“�” before the characteristic means

that the parameters change in the

event of a disturbance or setpoint

change.

Fig. 10b: CONVAL question and answers Table to select flow control, level control, temperature control and pressure control.

CONTROL VALVES

▲

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The software successfully detected valvesizing mistakes in several of the 1,200control valves installed in a German refin-ery before the regular plant shutdownafter five years' operation.With the soft-ware “troubleshooting features” the refin-ery therefore ordered spare parts in timeand started up without any delays.The software provides a bi-directionalCOM link to spreadsheets and CAE sys-tems as well as in-house valve sizing pro-grams which companies can use to storevalve data e.g. sound measurements, ad-ministration of inquiry and quotation sys-tems as well as pricing and drawings. (SeeFigure 11.)

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CONTROL VALVES

About the authorMr Holger Siemers, who

gained a Dipl. Ing.

Degree from the

University of

Furtwangen, started his

career with ECKARDT

AG company. He was

former head of the

Control Valve Sales

Department in ECKARDT; ECKARDT-

FOXBORO; SIEBE_INVENSYS. He was one of

the founder members of VDMA 24422 79 and

89 which becomes worldwide Standard IEC

534 8-4 - Sound Prediction Methods for

liquids.

He was also chairman in the VDI Working

Group: Valve Characteristics, gave support and

copyright of CONVAL (the plant- and valve

optimization tool) and was involved in the

R&D of SILENCER Technology - Multi-hole

orifices with deliveries of 2500 silencers in the

past twenty years.

He has trouble-shooting experience with

sound, loop stability, life cycle and plant

performance problems and been involved in

valve optimization for demanding applications

in petrochemicals and refineries (hydro-

carbon-processing, HPI, LNG, PTA, ...).

Publications, Training and Support: Application

and plant design, control loop optimization,

selecting and sizing control valves.

After 30 years' plant design with control valves

he joined SAMSON AG in 1999. Now he is

SAMSON's Severe Service Control Valves

Marketing Manager in the International Sales

& Marketing Department.

■

Fig. 11: Valve database for sizing, calculation and optimization of all kinds of valves andcommon plant components and the bi-directional interface structure.

References:[1] Siemers: Control Valve Design Aspects for critical applications in petrochemical plants. Part I.

Valve World Magazine June 2004.

[2] Siemers: Selecting Valves by choosing the optimum flow characteristic, chapter 9, Fig. 9. 3rd

International Conference 27-29 March 1990 Developments in valves and actuators for fluid

control Bournemouth UK BHR Group.

[3] Dr. Kiesbauer: Control Valves for Critical Applications, Hydrocarbon Processing, June 2001.

[4] H.D. Baumann : Valve Primer, A User`s Guide Chapter 7 ISA Edition.

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