Rotary Globe Valve A novel innovation in flow control

Transcription

1 Rotary Globe Valve A novel innovation in flow control By: Jari Kirmanen and Vesa Lempinen Neles Product Line, Metso Automation, Finland Abstract Valves are commonly categorized by stem motion. The linear globe valve is a well-known traditional design, but nowadays, rotary valves have encroached on the domain of linear motion valves in control applications. This has naturally resulted in various technical and commercial battles between linear globe valve and rotary control valve vendors. In general, each valve type has its own pros and cons. Rotary valves are less prone to clog in dirty service, and emissions through the stem packing can be controlled more easily without expensive bellows seals. By contrast, linear globe valves typically have better cavitation resistance and more trim variation than rotary valves. The ever growing trend in the process industries is to improve efficiency and minimize costs, which in turn provides challenges to valve vendors. They are forced to develop their products further, increase their flow control and application know-how and reduce costs. In recent decades, intelligent positioners have been a major innovation in the control valve industry, but there have not been many major valve-related innovations. In this paper, a novel rotary valve innovation for flow control is presented. The valve design, based on well-known proven technologies, combines linear and rotary valve features in an innovative way. A globe-valve body provides enhanced cavitation performance, and a rotary motion stem provides improved emission control. Flow characterization can be easily achieved by replaceable trims in this rotary valve. Nowadays, modern computer technology can be utilized in valve development. Flow through the valve trim can be detected even before production of the first valve prototype, but real tests in laboratories and adequate field testing are still needed to verify valve performance under various conditions. Computational flow studies are presented and compared to real flow measurements. A long-lasting control valve requires an inherently reliable design, which is selected and sized according to the application. In addition, intelligent positioners can provide the means of detecting and maintaining performance throughout the valve life cycle. Minimizing the life cycle cost of the control valve can have a significant impact on profit. Factors affecting valve life cycle cost are also discussed.

2 1. Introduction Valves are commonly categorized by stem motion. The linear globe valve is a well-known traditional design but, nowadays, rotary valves have encroached on the domain of linear motion valves in control applications. In general, each valve type has its own pros and cons. Rotary valves are commonly less prone to clog in dirty service, and emissions through the stem packing can be controlled more easily without expensive bellows seals. By contrast, linear globe valves typically have better cavitation resistance and more trim variation than rotary valves. The foundation of reliable control valve operation is the valve itself, but this is not enough if the valve is chosen for an application for which it is not suitable. Hence, in addition to the inherent reliability of the valve design, application knowledge is important when long-lasting valve performance is targeted. Sufficiently accurate knowledge of both process conditions and valve sizing is crucial in ensuring reliable control valve operation. It would help valve engineers to optimise valve selection commercially and technically. It is well known, but too often forgotten, that most of the total life cycle cost of a control valve occurs after the purchasing phase. Intelligent reliability combines intelligent field devices with applicationbased control valve selection and inherently reliable products. The ever growing trend in the process industries is to improve efficiency and minimize costs, which in turn provides challenges to valve vendors. They are forced to develop their products further, increase their flow control and application know-how and reduce costs. In recent decades, intelligent positioners have been a major innovation in the control valve industry, but there have not been many major valve-related innovations. In this paper, a novel rotary control valve is presented. The valve design is based on wellknown proven technologies and combines linear and rotary valve features in an innovative way. Enhanced cavitation perfomance is provided by a globe valve body, and improved emission control by a rotary motion stem. Flow characterization can be easily achieved by replaceable trims in this rotary valve. Nowadays, modern computer technology can be utilized in valve development. Flow through the valve trim can be detected even before production of the first valve prototype, but real tests in laboratories and adequate field testing are still needed to verify valve performance under various conditions. Computational flow studies are also presented and compared to real flow measurements.

3 2. Intelligent, reliable rotary control 2.1 Inherent reliability The foundation of reliable, long-lasting control valve application is the valve itself. In addition to methods such as fault-tree analysis, modern computer-aided technology can be used to simulate and study a valve s inherent reliability and performance. For instance, flow inside the valve trim can be studied at the development phase, even before manufacturing the first prototype. Advanced computer technology can improve valve reliability, but valve parameters - such as the differential pressure ratio of incipient cavitation or flow capacity - must be verified by measurements. Laboratory testing and verification of valve performance in the field, is important for reliable valve design. Rotary control valves typically use metal or soft polymer seats. Metal-seated technology is proven to provide a reasonably tight shut-off, long-lasting performance and a relatively low torque requirement. Metal-seated technology is capable of handling temperatures from cryogenic conditions up to +600 ºC and even beyond, in a manner no soft polymer seat technology can match. Excessive friction may cause poor performance, i.e. stick-slip in the control valve. However, friction in rotary valves can be minimized with well-engineered designs, even though the seat and trim are in continuous contact. In eccentric plug and high-performance butterfly valves, seat friction is eliminated because the plug (or the disc) detaches from the seat as soon as the valve is opened. Anti-cavitation and noise-reduction trims are also available for rotary valves, which provide high capacity combined with good cavitation resistance and noise reduction in a manner that no sliding stem valve can match. These types of rotary trim utilize similar technology to multistage globe valves, i.e. pressure staging and dividing the fluid into multiple jets. In contrast to multi-stage globe valves, these valves can handle very dirty fluids because the trim rotates with the closure member. The same trim can be used for liquid, gas or steam applications. 2.2 Application-based valve selection An inherently reliable valve may not perform well if it is chosen for an application for which it is not suitable. Every valve type has its own pros and cons, which must be considered when a valve is selected. Modern valve-sizing software is capable of modelling installed behaviour, which helps the user to detect 'hidden' errors related to valve selection. Unfortunately, the engineer making the valve selection all too often sees only the datasheet and not the full picture of the application, which would help him sometimes to optimise valve selection. In addition to careful design selection, valve size and flow characteristic should be selected for the application. Controllability The basis of control loop performance is correct control valve selection and sizing. Even an advanced control method with the most accurate transmitter cannot compensate for problems caused by poor valve selection. To optimise valve size, valve opening should, under normal process conditions, be within the range from 60% to 80%. To optimise controllability, it is important to aim for an installed flow characteristic that is as linear as possible within the control range. Linearity of the installed flow curve can be characterized by the installed gain i.e. the slope of the installed curve. Even though both valve sizing and selection software is widely used, in many cases the installed curves have not been analysed. Sometimes the installed curve is difficult to model, because process conditions are

4 undefined or vary from time to time. Trim flow characteristic can be selected to match the application and nowadays intelligent valve controllers (or positioners) have travel characterisation features to enhance the linearity of the installed curve. However, in many instances the actual control range needed in a particular application is so small that the correct valve size with a reasonably low installed gain is more important than the valve characteristic. Installed gain can be used to estimate relative flow error with following equation [3] Q= G h (1) where Q= relative flow error, G = installed gain and h=relative valve travel error (e.g. valve dead band). A common problem is an oversized valve. This means that the valve operates with openings that are too low, within a very narrow opening range and with high installed gain. As can be seen from equation 1, high installed gain means that even small changes in the control signal, and respectively in valve travel, cause relatively large changes in flow. To control such a loop accurately could be very difficult. Demanding fluids Difficult fluids such as high-viscosity slurries, fluids that have a tendency to stick, polymerise or crystallize or fluids containing impurities are usually very demanding applications for a control valve. Such valves tend to stick easily, become clogged or eroded. Conventional cage-type linear globe valves may not be suitable for these applications. Linear valves in general tend to draw dirt into the gland area, which typically increases leakage and friction. By contrast, rotary valves do not draw dirt into the packing or bearing area (see Fig 1). Eccentric plug valves in particular have proved their excellence in dirty fluid services, because there are no cavities inside the valve where fluid can stick. Therefore they can handle fluids prone to crystallize or polymerise. Fig. 1: Rotary stem vs. rising stem. The rising stem (on the right) tends to draw process media and dirt into the gland area. Fugitive emissions Fugitive emissions are a very hot topic in the hydrocarbon industry nowadays due to everincreasing environmental pressures. Environmental issues are, of course, very important and should be considered carefully. Typically, rotary control valves offer better emission control than standard linear control valves, simply because a rising stem tends to move process media into the gland packing. Standard linear control valves are therefore typically more prone to leakage problems than rotary control valves.

5 Valve outlet velocity Valve outlet velocity should not be too high to avoid the risk of damage. The outlet velocity limit for globe valves with standard trim can be lower due to high velocity that may arise and hit the globe valve body downstream of the valve orifice. Because rotary valves have a typically higher inherent capacity than standard globe valves, there is a possibility that rotary control valves may be smaller than globe valves in certain applications. Cavitation Conventional rotary control valves typically have lower cavitation resistance than linear globe control valves. However, rotary valves with anti-cavitation and noise-reduction trim improve the cavitation resistance up to globe valve level. Predicting cavitation damage is not necessarily simple, because it depends on several factors, such as fluid type, temperature or pressure drop across the valve. On the other hand, if the pressure drop across the valve is low (approx bar), the risk of cavitation damage is not necessarily high. However, in order to keep cavitation-damage prediction simple, a method based on noise prediction has been developed. This method can be applied to most applications. There are two factors involved in estimating cavitation damage: first, the pressure drop across the valve should not exceed the terminal pressure and secondly, the predicted noise level should not exceed the limits given per valve size. This cavitation prediction method has been proven to be conservative and safe. Noise A choked gas flow typically causes a rapid increase in aerodynamic noise in the control valve. In general, the higher noise level correlates to the severity of the application. Choked flow does not necessarily cause damage, if the valve outlet velocity and noise level are not too high. Especially high velocity with a condition where liquid droplets may form should be considered carefully. However, simply avoiding choked flow, without further analysing the valve noise level or outlet velocity, may not lead to optimal valve selection. Life Cycle Cost The above factors help the valve engineer to find the optimal commercial and technical solution for the application. But the purchase price is commonly only a fraction of the cost of the whole valve life cycle, a fact that is unfortunately forgotten way too often. In general, valve life cycle costs could typically contains factors such as purchase costs, installation, operating costs and disposal costs. Purchase costs include engineering costs and purchase price. Operating cost might include maintenance, energy costs, loss of production due to unexpected shutdowns or high variability due to poor performance, etc. Disposal cost is simply the cost of waste during the whole valve life cycle. About 70-80% of valve life cycle cost is estimated to come after the purchasing phase. The most efficient valve life cycle costs can be reduced to optimise the operating costs. This can be achieved by utilizing predictive maintenance, which is discussed in the next chapter. 2.3 Intelligent reliability Intelligent reliability combines inherently reliable products and application-based control valve selection to intelligent field devices, which can be utilized in predictive maintenance and the diagnostics of a control valve. The aim of predictive maintenance and continuous performance monitoring is to indicate decreasing valve performance and to warn the user before failure is so serious that it causes excessive process variability or even an unexpected shutdown. The most efficient way to carry out predictive maintenance and on-line diagnostics is to utilize valve controllers, which can store results in their memory and send warnings and alarms based on performance limits stored in their memory. In this way, no additional manpower is needed to continuously analyse and study the results, because the intelligent

6 valve controller, with the help of advanced asset management software, can measure valve performance automatically (Fig 2). Valve-performance measurement results saved in the positioner's memory can be presented in various ways in asset management software. Results are normally shown as counters, trends or histograms. In order to visualize valve problem areas more clearly and to show all the important parameters in one display, an innovative valve diamond has been introduced. The valve diamond is a visual summary of statistical diagnostic measurements, which shows at a glance the problem areas in valve performance (Fig 2). The corner points of the diamond represent the warning limit of each measured trend value. The asymmetric diamond, which is mostly inside a larger diamond, shows the actual measured values of trends. Data analysis StartUp Plant is running Shut Down Fig. 2: Diagnostic data is gathered during plant is running in 2 nd diagnostic. Data analysis before a planned shutdown. generation on-line 3. Innovative rotary control valve Standard linear globe valves are capable of handling high pressures and usually have better cavitation resistance than standard rotary valves, such as butterfly or ball valves. Better cavitation resistance is mainly due to the curvature of the flow channel and drag in the trim. Rotary valves with anti-cavitation and noise-reduction trim improve the cavitation resistance up to globe valve level, which although it may be technically suitable is not always a very attractive solution commercially for small size valves. By contrast, as discussed in the previous chapter concerning rotary valves, they have inherently low emission through the gland packing. Accurate valve sizing is important especially at low flow rates and with small valve sizes. This would commonly require several different trim options per valve nominal size. Linear globe valves usually have more trim variations available than standard rotary valves, which make accurate valve sizing easier. The rotary control valve design shown in Figure 3 combines a globe valve body with a rotary stem in a simple way. The curved "S-shaped" flow channel, similar to that of conventional globe valves, provides improved cavitation resistance also for rotary valves without anticavitation trims. The rotary valve's inherently low emission control through the gland packing can be improved by utilising spring-loaded packings, which eliminate packing re-tightening in emission-critical applications. Trim modifications and characteristics are easy to carry out in a cylindrical plug and seat assembly, which also provides sealing and radial bearing functions. Typically eccentric rotary plug valves are used in dirty applications where linear

7 cage-type globe valves may not be suitable. By contrast, resistance to dirt can be improved by a design capable of cleaning the sealing surfaces by scraping. Such cleaning techniques have been successfully used in ball valves, for example. The thrust (axial) bearing is located outside the flow channel. This minimizes corrosion and the load carried by the axial bearings. In addition, the bearing construction incorporates a stem anti-blow out feature. Thrust bearing Bonnet and actuator bracket Gland packing Plug and seat Valve body Fig. 3: Rotary control valve combining a linear globe valve body with a rotary stem. Fig. 4: Standard plug and seat assembly (on the left), and pressure balanced, anti-cavitation trim for high-pressure differential applications (on the right).

8 Computational fluid dynamics (CFD) were used to study flow through the valve. The results were always verified and compared to results measured in the flow laboratory. Figure 5 shows the simulated and measured flow capacity coefficient (Cv) as a function of valve opening. The simulated results correspond to measured results within a reasonable accuracy level, except at very low openings Flow capacity, CV Measured Simulated Opening, deg Fig. 5: Measured and simulated flow capacity coefficient as a function of valve opening ( 1" valve). Simulated velocity contour curves of fluid (water) flowing through the valve are shown in Figure 6. As can be seen in Figure 6, the maximum velocity is achieved in the throttling point at the outlet of the valve orifice. In contrast to the flow channels seen commonly in conventional linear globe valves, the outlet flow port was straightened to minimize the impact of fluid on the rotary globe valve body.

9 Fig. 6: Velocity contour curves inside the valve. The pressure drop for incipient cavitation (xfz) was also defined by measurement. CFDsimulation was also utilized to detect areas of cavitation by using pressure contour plots. Based on CFD-study, possible cavitation areas or locations can be studied qualitatively, but defining xfz values within the required accuracy can be very difficult. It may even require acoustical modelling in combination with fluid simulation, which makes modelling very complicated. Figure 7 shows an example of hydrodynamic noise testing including predicted noise level based on VDMA (1979) and IEC (2005). As Figure 7 shows, both noise prediction methods produce corresponding measurement results. The pressure recovery factor, FL, as well as the incipient cavitation factor have to be defined by measurements. CFD programs can be utilized to study the phenomena, but it is very difficult to define the pressure recovery factor by using CFD-programs, especially for valves requiring 3D flow modelling. Modelling of the choking phenomenon would need 2-phase flow simulation with a cavitation model, which makes simulation very challenging and timeconsuming.

10 Sound pressure level, dba IEC Measured VDMA v xf Fig. 7: Measured and predicted noise levels as a function of pressure drop ratio xf at constant valve opening. Pressure-balanced two-stage trim shown in Figure 4 enhances the valve's resistance to cavitation even further than standard trim. In addition to multiple stages, this trim is less vulnerable to cavitation-erosion damage caused by flow jets coming out from the 1st stage which are guided to an area not close to the surface of the plug (see Fig 8). The second stage of the trim is very similar to standard trim. Because flow is distributed evenly around the trim, it does not create an excessive increase in dynamic force and therefore the trim can handle high pressures without a large increase in torque. A A Fig. 8: Fluid jets flowing through 1 st stage of balanced trim at cross sectional cut A A.

11 The pressure differential ratio factor, xt, is used to define choked conditions in compressible fluid flow through control valves. In choked flow conditions, sonic speed in the valve flow channel is reached. The mass flow rate of air through the valve was also simulated by CFD software. The simulated mass flow rate corresponds well to the measured rate; above choked conditions, the simulated flow rate tends to deviate from measurements (see Fig 9). Simulation of compressible flow through choked conditions is rather demanding, hence the xt factor must be defined by measurements. Definition of the xt factor by gas (air) flow measurement using variable upstream pressure is shown in Figure Mass flow rate, kg/h Measured Simulated Pressure differential rati, DP/P1 Fig. 9: Measured and simulated mass flow rate of air as a function of pressure drop ratio, DP/P1, at constant valve opening.

12 Flow functions W1, W xt W1(measured) 1000 W2(DP/P1) DP/P1 Fig. 10: Measured and calculated flow functions as functions of pressure differential ratio to define xt. Flow functions W1 and W2 in figure are defined from the standardized gas flow w T Z sizing equation: W1= and W 2 = N8 Cv Y x M. P1

13 4. Conclusions The foundation of reliable, long-lasting control valve application is the control valve itself. Modern computer technology can be used to simulate valve performance, for instance flow inside the valve trim. However, based on studies made with the CFD simulation program, critical valve sizing parameters, such as differential pressure ratio or incipient cavitation, must still be verified by measurements. Intelligent reliability combines intelligent field devices, such as smart positioners capable of detecting valve performance, and application-based valve selection to produce inherently reliable products. The aim of predictive maintenance is to warn the user before failure is so serious that it causes excessive process variability or even an unexpected shutdown. Consideration of these factors brings the availability of a control valve to a significantly higher level and has a high potential to reduce valve life cycle costs. A new rotary control valve is presented which combines a conventional linear globe body with a rotary stem in a simple and innovative way. The flow channel is similar to that of conventional linear globe valves, which improves cavitation resistance compared to standard rotary valves. Hence this valve can handle higher pressure-differentials than conventional rotary valves without cavitation. The rotating stem provides the inherently low fugitive emission control commonly seen in rotary control valves. Accurate control valve sizing is important especially at low flow rates and with small valve sizes. This would typically require several different trims. The plug and seat design presented makes trim modification easy to assist accurate control valve sizing. 5. References [1] Kirmanen, J. " Intelligent reliability in control valves", Valve World Conference, 2006 [2] Kirmanen, J., " Reliable, intelligent rotary control valves for hydrocarbon industry myth or reality", Valve World, April 2006, p [3] Metso Automation, Flow Control Manual [4] IEC , Industrial-process control valves Part 8-4: Noise considerations Prediction of noise generated by hydrodynamic flow [5] IEC , Industrial-process control valves Part 8-3: Noise considerations Control valve aerodynamic noise prediction method [6] IEC , Industrial-process control valves Part 2-1: Flow-capacity- Sizing equations for fluid flow under installed conditions [7] VDMA , 1979