An advantage of concentric butterfly valve is that the media going through the valve will not contact the body, since the seat is constructed as a sleeve or cartridge inside of the body. This zero-offset valve design is also called Resilient Seated or Rubber Seated because it relies on the flexibility of the seat rubber to efficiently seal the flow when closed. This helps to reduce the size and cost of the control valve, gives a more linear installation curve, and offers an opportunity to select a linear valve. Figure 1: Val-Matic graph demonstrating Inherent characteristic curves for different valve types Sourced from Surge Control in Pumping Systems 2018. The installation curve can also be defined by considering the Kvr at all loads against the perfectlysizedKvsof69.2. Wafer type butterfly valve do not transfer the weight of the piping system directly through the valve body. This section will consider the various options of valve characteristics for controlling water and steam systems. The valve can only be sized on the application conditions. For example: It can be seen from Table 6.5.4, that at the maximum flowrate of 10 m/h, the Kvr is 8.06. The physical shape of the plug and seat arrangement, sometimes referred to as the valve trim, causes the difference in valve opening between these valves. The selection of the appropriate control valve characteristic is dependent on the needs of the system. A linear valve achieves this by having a linear relationship between the valve lift and the orifice passarea(seeFigure6.5.3). This could result in a smaller valve but also a larger heat exchanger, because the heat exchanger operating temperature is now lower. FromEquation6.5.2:bar. Valves are often oversimplified during modeling. Consider if the pass area of a fully open valve with a Kvs of 10 is 100%. The relationship between valve lift and orifice size (and therefore flowrate) is not linear but logarithmic, and is expressed mathematicallyinEquation6.5.1: The maximum flowrate through a control valve with an equal percentage characteristic is 10 m/h. What is it? As already calculated, the Kvr at the maximum flowrate of 10 m/h is 8.06, and the Kvs of the DN25 valve is 10. An oversized equal percentage valve will still give good control over its full range, whereas an oversized linear valve might perform less effectively by causing fast changes in flowrate for small changes in lift. This compressive stress is eased by the tensile stresses imposed by the internal fluid pressure. While valves come in all shapes and sizes, rarely will a valve have a completely unique geometry. Triple offset butterfly valves are often made of metal seats to create a bubble-tight shut-off. There is less backpressure in the condensate system. The following are some characteristics for butterfly valves when used for modulating service. There is a sudden change in the shape of the graphs at roughly 90% of the load; this is due to the effect of critical pressure drop across the control valve which occurs at this point. It is interesting to compare linear and equal percentage valves having a Kvs of 10 against the installation curve for this example. High performance butterfly valve designs use the pressure in the pipeline to increase the interference between the seat and the disc edge. Find the steam temperature at this reduced load. (For more information on critical pressure, refer to Module 6.4, Control valve sizing for steam). If no characteristic curve was defined, it can be fully defined here as before and saved to the Optional tab's Open Percentage Data table automatically. Presently, butterfly valves can be found in almost every chemical plant handling a variety of diverse fluids. By observing the curve for the equal percentage valve, it can be seen that, although a linear relationship is not achieved throughout its whole travel, it is above 50% of the flowrate. These butterfly valves have higher pressure ratings and are prone to less wear. The frictional resistance of the pipework changes with flowrate. A valve with a Kvs of 69.2 is not commercially available, and the next larger standard valve has a Kvs of 100withnominalDN80connections. The results aregraphedinFigure6.5.10. Specifying a transient valve closure using a pre-defined characteristic curve is almost as easy as assigning the curve itself. The linear characteristic valve plug is shaped so that the flowrate is directly proportional to the valve lift (H), at a constant differential pressure. If the pump pressure had remained constant across the whole range of flowrates, the installation curve and the curve for the linear valve would both have been straight lines. Updated April 29, 2020. However, it can be observed from Figure 6.5.8, that if the linear valve is properly sized, it will perform perfectly well in this type of water application. Instead, it is easy to categorize different valves into the usual suspects: ball valves, globe valves, butterfly valves, plug valves, quick opening valves, etc. The disc is positioned in the center of the pipe. It is worth noting how these valves would perform should they have been selected, and as shown in Table6.5.10andFigure6.5.11. In many cases, a valve closure is simplified with a linear decrease in Cv. 1. Using the data in Table 6.5.2, the required valve capacity, Kvr, can be calculated for each incremental flowrate and valve differential pressure, by using Equation 6.5.2, which is derived from Equation 6.3.2.The Kvr can be thought of as being the actual valve capacity required by the installation and, if plotted against the required flowrate, the resulting graph can be referred toastheinstallationcurve. The self-acting type of control valve tends to have a plug shape similar to the fast opening plug in Figure 6.5.1. Nick holds a Bachelor of Science in Chemical Engineering with a Minor in Economics from the Colorado School of Mines. If a Cv vs. Open Percent was already defined in the Optional tab, it will automatically be transferred for use here. The plug position responds to changes in liquid or vapour pressure in the control system. Below is a table of the flow rates for Butterfly Valves at different opening stages between zero degrees (fully closed) and ninety degrees (fully open). Conclusion The user must also define the Time Increment for the Cv vs. Time table. Because the disc of butterfly valves moves into the seat with a wiping motion, most butterfly valves are capable of handling fluids with solids in suspension and, depending on the robustness of the seatings, also powders and granules. Lug type design is preferred if a valve may be required to remain in place in closed position on removal of the pipe on one side for a temporary operation, it must be flanged for bolting to a pipeflangeon the other side. Concentric butterfly valves are commonly used for low pressure ranges. Taking the valve flowrate and valve P from Table 6.5.2, a Kvr for each increment can be determined from Equation 6.5.2; and these aretabulatedinTable6.5.3. Butterfly valves have a relatively simple construction. At 3.32 bar a, hfg = 2 153.5 kJ/kg, consequentlyfromEquation2.8.1: Using this routine, a set of values may be determined over the operating range of the heat exchanger, asshowninTable6.5.7. From Equation 6.5.2: The water flowrate varies between 0 and 10 L/s (kg/s). Various types of flow characteristics are available. By using Equation 6.5.4 the required valve lift at full-load istherefore: Therefore, both the pass area and valve lift at any flow condition is simply the Kvr expressed as a proportion ofthevalveKvs. In the Optional tab of the Valve Properties window, there is a button to "Create Cv/Open%", highlighted in Figure 2. An equal percentage valve will require exactly the same pass area to satisfy the same maximum flowrate, but its lift will be different to that of the linear valve. These valves have a valve plug shaped so that each increment in valve lift increases the flowrate by a certain percentage of the previous flow. Therefore, two valves, one giving a 80% flow for 50% lift, the other 90% flow for 60% lift, may both be regarded as having a fast opening characteristic. Required flow through the valve = 10 m/ h It will offer a more constant gain as the load changes, helping to provide a more stable control loop at all times. Lug-style butterfly valves used in dead end service generally have a lower pressure rating. For example, if a valve travel change from 10% open to 20% open produced a 60% change in flow-rate, then a valve travel change from 20% open to 30% open would produce another 60% change in flow-rate. As the volumetric flowrate through any valve is proportional to the orifice pass area, Equation 6.5.3 can be modified to give the equal percentage valve lift in terms of pass area and therefore Kv. Open Percentage can also be added to output via Output Control for additional analysis, as demonstrated in Figure 3. A rod passes through the disc to the outside of the valve. Butterfly valves are generally associated with reduced maintenance because there are a minimal number of moving parts and there are no pockets to trap fluids. Do you have questions about our website, our products or any of our services? The application data is approximate, consequently an additional safety factor is included. A valve using this type of plug is sometimes referred to as having an on/off characteristic. The lower part of its curve is relatively shallow, offering slower opening during its initial travel, and is better for controlling steam flow than the linear valve in this case. Consider a valve with an equal percentage inherent characteristic This opens a window where you can specify the Cv vs. Open Percent Type, as well as specifying the full open Cv. The Kvs of the selected valve is 100,consequentlytheliftis: Using the same procedure, the linear valve lifts can be determined for a range of flows, and aretabulatedinTable6.5.9. Cavitation and choked flow are two potential concerns. The next larger standard valves have a Kvs of 160. A boiler water level control system a water system with atwo-portvalve It can be seen from Figure 6.5.11 that both valve curves have moved to the left when compared to the smaller (properly sized) valves in Figure 6.5.10, whilst the installation curve remains static. In practice, it would always be the case that the selected valve would be at least one size larger than that required, and therefore have a Kvs larger than the application Kvr. Finally, the valve can either be closing from fully open or opening from fully closed. The orifice pass areas will be the same. This double eccentricity of the stem enables the rotating disc to rub over the seat for only about one to three degrees. This arrangement did not provide a leak-tight closure, but did provide sufficient closure in some applications (i.e., water distribution lines). Efforts to adapt butterfly valves to wider temperature and pressure ranges have led to the development of a family of butterfly valves that may be fitted with seatings of a variety of construction materials to meet the operational requirements. Therefore, both the pass area and valve lift at any flow condition is simply the Kvr expressed as a proportion of the valve Kvs. therefore: Using the same routine, the valve lift required at various flowrates can be determined from Equation 6.5.4 andisshowninTable6.5.6. valvetwo-portvalve. By using our website, you consent to our use of cookies. This fact, together with the ability of most metals to handle compressive loads of up to twice their limit for tensile loads, strongly recommends the use of the wafer body. Flanged bodies, on the other hand, have to carry all the tensile stresses imposed by the line strains, and the tensile stresses from the line pressure are cumulative. This effectively means that the valve is operating as a fast acting valve for up to 90% of its range. The preferred body configuration for butterfly valves is the wafer, which is clamped between two pipeline flanges. For example, at 50% open, the flow-rate is 50% of maximum flow. When the valve is 50% open, it will pass 1.414 m/h, an increase of 48% over the flow of 0.956 m/h when the valve is 40% open. If the linear valve has a Kvs of 10, for the valve to satisfy the required maximum flowrate,thevalvewilllift: Using the same routine, the orifice size and valve lift required at various flowrates may be determined for the linear valve, asshowninTable6.5.5. Forexample. Another set of advantages accrues from larger heat exchangers operatingatlowersteampressures: A balance has to be made between the cost of the control valve and heat exchanger, the ability of the valve to control properly, and the effects on the rest of the system as seen above. Typical trim shapes for spindle operated globe valves are compared inFigure6.5.1. The resulting application curve and valve curves for the application in Example 6.5.2 for both the linear and equal percentage inherent valve characteristics are shown in Figure 6.5.8. This is not the best type of inherent characteristic for this type of steam installation, as it is usually better for changes in steam flow to occur fairly slowly. Rotary valves (for example, ball and butterfly) each have a basic characteristic curve, but altering the details of the ball or butterfly plug may modify this. This tutorial discusses the three main types used in water and steam flow applications: fast opening, linear, and equal percentage flow; how they compare, and how (and why) they should be matched to the application in which theyareused. Butterfly Valves are quarter-turnrotary valves in which a disc-shaped closure member is rotated through 90 degree or less to open or close or regulate the flow passage. As discussed in Modules 6.2 and 6.3, valve capacities are generally measured in terms of Kv. Rotating therod, through hand-wheel or actuator, turns the disc either parallel or perpendicular to the flow. This applies regardless of the inherent valve characteristic. Figure 3: Using the Valves button in Output Control to add Open Percentage to output. Lug type butterfly valve allows dead-end service or removal of downstream piping. The user can also use the "Show Graph" button to see the spread of Cv and the shape of the valve's curve. There is less flash steam produced in the condensate system. Sizing routines that include operational factors such as an over-zealous allowance for fouling. Although the orifice pass area of both valves will be exactly the same, the shape of the equal percentage valve plug means that it operates further away from its seat, reducing the risk of impact damage between the valve plug and seat due to quick reductions in load at low flowrates. It can be seen that, as the valve with a Kvs of 69.2 is perfectly sized for this application, the maximum flowrate is satisfied when the valve is fully open. Monarch Gaskets & Seals Ltds impressive start to 2022 includes the Privacy Policy | Cookie Policy | Accessibility | Site Map. This installation curve can be thought of as the valve capacity of a perfectly sized valve for this example. There are different kinds of butterfly valves, each adapted for different pressures and different usage. As valve position becomes more open, flow-rate changes diminish with incremental changes in valve travel approaching zero change as the valve position nears full open. When the stem is located right behind the center-line of the disc, the valve is called Single-Offset Butterfly Valve. If the valve has a turndown of 50:1, and is subjected to a constant differential pressure, by using Equation 6.5.1 what quantity will pass through the valve with lifts of 40%,50%,and60%respectively? Such a valve would be perfectly sized for the example, and would describe the installation curve, as tabulated in Table 6.5.8, and drawn in Figure 6.5.9. Butterfly valves are easy and fast to open and close. While there are some benefits to considering open percentage in steady-state, it is essential to account for valve characteristics during transient analysis as it can lead to drastically different results. It can be seen that, as the valve is perfectly sized for this installation, the maximum flowrate is satisfied when the valve is fully open. If it were impossible to increase the steam supply pressure, a solution would be to install a heat exchanger that operates at a lower operating pressure. The disc of butterfly valve is always present within the flow, therefore a pressure drop is always induced in the flow, regardless of valve position. These characteristic curves are important in almost all cases, especially when evaluating a valve for a control circumstance. A few other inherent valve characteristics are sometimes used, such as parabolic, modified linear or hyperbolic, but the most common types in manufacture are fast opening, linear,andequalpercentage. Quick opening Quick opening flow characteristic means that flow-rate through the valve increases very rapidly for incremental changes in valve travel when valve position is near closed. The available pressure drop over the control valve at full-load is low. The pressure within the boiler will vary as a function of the steam load, the type of burner control system and its mode of control. When the valve is 60% open, it will pass 2.091 m/h, an increase of 48% over the flow of 1.414 m/h when the valve is 50% open. (This phenomenon is discussed in further detail in Module 6.3). A wafer-style butterfly valve is the most economical version and it is sandwiched between two pipe flanges and the flange bolts surround the valve body. A triple offset butterfly valve is more efficient and allows for less wear. The initial U value of 1 500 W/m2 C is reduced by 4%, so the temperature required in the steam space may becalculatedfromEquation2.5.3: If TLM = 100C, and T1, T2 are already known, then Ts may bedeterminedfromEquation2.5.5: The saturated steam pressure for 137C is 3.32 bar a (from theSpiraxSarcosteamtables). Figure 2: Specifying the relevant valve type and Full Open Cv to define the valves characteristic curve. The seat surface takes a conical shape which coupled with the same shape at the ridge of the disc, results in minimal contact before full closure of the valve allowing the disc to seal against the seat with no friction. The boiler feedpump performance characteristic is tabulated in Table 6.5.2, along with the resulting differential pressure (P) across the feedwater valve at various flowrates at, and below, the maximum flow requirement of 10 m/h of feedwater. By using our website, you consent to our use of cookies. At the maximum water flowrate of 10 kg/s, the steam valve Kvr is 69.2. The fast opening characteristic valve plug will give a large change in flowrate for a small valve lift from the closed position. The sealing between the valve and pipe flanges is accomplished by using a tightly fitted seal, such as an O-ring, gasket, precision machined, along with a flat valve face on the downstream and upstream sections of the valve. As in Example 6.5.2, the equal percentage valve requires a much higher lift than the linear valve to achieve the same flowrate. For more information,viewtheAFT Privacy Policy. 2022 Spirax Sarco Limited. For more information,viewthe, Secret Variable-g Feature in AFT Software? In steam temperature control systems, the pressure drop over the control valve is deliberately varied to satisfy the required heat load. Conclusion - In most applications, an equal percentage valve will provide good results, and is very tolerant of over-sizing. The effect of a control valve which islargerthannecessary. When the disc plane is at right angle or perpendicularto the center line of pipe, butterfly valve is closed. The unique feature of this valve is its adaptability for lined pipes due to negligible disc protrusion beyond the body laying length. Consider a valve with a linearinherentcharacteristic. Please note: Given the opportunity, it is better to size steam valves with as high a pressure drop as possible at maximum load; even with critical pressure drop occurring across the control valve if the conditions allow. In this way, the pressure drop would increase across the control valve. The steam pressure at various heat loads. The Kvr of 69.2 satisfies the maximum secondary flow of 10 kg / s. Early use of butterfly valves focused on water applications but new designs and component materials have allowed them to be utilized in growing industrial fluid applications. Luckily, many characteristic curves for different valve types are built into AFT software for the user's convenience. Note: The valve P is the difference between the pump discharge pressure and a constant boiler pressure of 10 bar g. Note that the pump discharge pressure will fall as the feedwater flow increases. It can be determined from Table 6.5.2 that the fall in the pump discharge pressure is about 26% from no-load to full-load, but the fall in differential pressure across the feedwater valve is a lot greater at 72%. Figure 4: The Cv vs. Time From Open Percent window is where the transient event itself is defined. A design's safety margin may be severely impacted by considering the valve's characteristics. Disc movement is unguided and affected by flow turbulence. If the steam pressure supplying the control valve is given as 5.0 bar a, and using the steam pressure and steam flowrate information from Table 6.5.7; the Kvr can be calculated from Equation 6.5.6, which is derived from the steam flowformula,Equation3.21.2. It is generally agreed that control valves find it difficult to control below 10% of their range, and in practice, it is usual for them to operate between 20% and 80% of their range. Find the heat transfer area required to satisfy the maximum load. Such seatings may be metal-to-polymer or metal-to-metal, and may be designed to satisfy the requirements of fire-tested valves. A valve with a Kvs of 8.06 would be perfectly sized, and would describe the installation curve, as tabulated in Table 6.5.4, and drawn in Figure 6.5.7. Butterfly valves should normally be mounted with the stem horizontal since this allows debris in the pipe to be swept clear as the valve is closed. These provide a good approximation while manufacturer data has yet to be determined. Nick Vastine is a Business Applications Engineer at Applied Flow Technology (AFT). This relationship between a valve's Open Percent and Cv is called the valve's inherent characteristic curve. Luckily there are only a few extra steps to make your model a bit more precise. Comparing the linear and equal percentage valvesforthisapplication. Figure 5: Comparison of a 10 second closure with an increment of 5 seconds and 0.1 seconds. For this example: The installation curve is constructed, by taking the ratio of Kvr at any load relative to the Kvs of 8.06. The calculated Kvr is only slightly higher than the Kvs of a standard valve, and the next larger size has to be selected. 2. It may also be observed that the change in flowrate is large for a relatively small change in the lift. Figure 1 demonstrates a few characteristic curves for different valve types. The pump characteristic. Using this routine, the Kvr for each increment of flow can be determined, as shown in Table 6.5.8. The UK Government have announced changes attempting to make it Is it the glorious sunshine? Because there are several factors to be considered, a complete system analysis is required to determine precisely which is the optimum characteristic. This varying steam flow varies the pressure (and hence temperature) of the steam in the heat exchanger and thus the rate of heat transfer. When fully open, the disc of butterfly valve creates little turbulence or resistance to flow. This also ensures installation close to other pipe fittings. The installation curve can be thought of as the valve capacity of a valve perfectly sized to match the application requirement. An equal percentage valve will require exactly the same pass area to satisfy the same maximum flowrate, but its lift will be different to that of the linear valve. - The best choice in these applications is usually a valve with a linear characteristic. Above 86% load in this example, it can be shown that the steam pressure in the heat exchanger is above 2.9 bar a which, with 5 bar a feeding the control valve, is the critical pressure value. 3. Furthermore, when opening the valve, the bottom of the disc should lift away from solids that may have accumulated on the upstream side of the disc. Single-offset butterfly valves have given way to double offset and triple offset butterfly valves. Find the log mean temperature difference. This will always be the case for an equal percentage valve with rangeability of 50. At the full-load condition, from Table 6.5.2: The equal percentage valve offers an advantage over the linear valve at low flowrates. If the valve closes so the pass area is 60% of the full-open pass area, the Kvr is also 60% of 10 = 6. In this Module, the term valve lift is used to define valve opening, whether the valve is a globe valve (up and down movement of the plug relative to the seat) or a rotary valve (lateral movement of the plug relative to the seat). Example6.5.3. The graphs in Figure 6.5.10 refer to linear and equal percentage valves having a Kvs of 100, which are the next larger standard valves with suitable capacity above the application curve (the required Kvr of 69.2), and would normally be chosen for this particular example. Figure 6: Valves in same system, same closure time, and same initial Cv resulting in different surge pressures. If the falling differential pressure across the valve is not taken into consideration when sizing the valve, thevalvecouldbeundersized. As a valve with a Kvs of 8.06 is not commercially available, the next larger standard valve would have a Kvs of 10 with nominal DN25 connections. For example, should the heat exchanger working pressure be 4.5 bar a, and the maximum available steam pressure is only 5 bar a, the valve can only be sized on a 10% pressure drop ([5 4.5]/5). For interest, if a valve has a rangeability of 100, the incremental increase in flowrate for a 10% change in valve lift is 58%.