butterfly valve characteristic curve

2021/05/21

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. 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. 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. Sizing routines that include operational factors such as an over-zealous allowance for fouling.

Because of this, the installed and inherent characteristics are always similar and linear, and there will be limited gain in the control loop. Unlike a ball valve, the disc is always present within the flow, therefore a pressure drop is always induced in the flow, regardless of valve position. This simply means the percentage of Kvr to Kvs, or in other words, the percentage of actual pass area relative to the full open pass area. The overall heat transfer coefficient (U) is 1 500 W/m2C at full-load, and reduces by 4% for every 10% drop in secondary water flowrate. 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. 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. 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. (For more information on critical pressure, refer to Module 6.4, Control valve sizing for steam). Note that the equal percentage valve has a significantly higher lift than the linear valve to achieve the same flowrate. However, conditions may not allow this. From this window, the user has several options: The user must also specify the overall Transient Duration time that open percent is linearly varied. A design's safety margin may be severely impacted by considering the valve's characteristics. Some of the ones more frequently used are: The majority of butterfly valves are of the interference-seated type in which the seat has a rubber liner.

Butterfly valves are generally favored because they cost less than other valve designs, and are lighter weight so they need less support. As discussed in Modules 6.2 and 6.3, valve capacities are generally measured in terms of Kv. All Rights Reserved. a technical knowledge base for all the process piping professionals around the world, Tags: butterfly valvelug typevalvewafer type, Download Free E-Books on Process Piping Engineering. Thisvariationiscausedby: Example 6.5.2 Select and size the feedwater valveinFigure6.5.6. It will offer a more constant gain as the load changes, helping to provide a more stable control loop at all times. For interest, if a valve has a rangeability of 100, the incremental increase in flowrate for a 10% change in valve lift is 58%. 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. 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. Not until this is established can the following be found: The steam temperature at various heat loads. This varying steam flow varies the pressure (and hence temperature) of the steam in the heat exchanger and thus the rate of heat transfer. 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. - The best choice in these applications is usually a valve with a linear characteristic. Consider, at a 10% flowrate of 1 m/h, the linear valve only lifts roughly 4%, whereas the equal percentage valve lifts roughly 20%. 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 corresponding lifts for linear and equal percentage valves are shown in Table 6.5.9 along with the installation curve.

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. Butterfly valves are relatively inexpensive and cost effective. Consider a valve with a linearinherentcharacteristic. If a Cv vs. Open Percent was already defined in the Optional tab, it will automatically be transferred for use here. 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. This double eccentricity of the stem enables the rotating disc to rub over the seat for only about one to three degrees. In a particular steam-to-water heat exchange process,itisproposedthat: Using this data, and by applying the correct equations, the following propertiescanbedetermined: Heat load is determined from Equation 2.6.5: The heat transfer area (A) can bedeterminedfromEquation2.5.3: At this stage, TLM is unknown, but can be calculated from the primary steam and secondary water temperatures,usingEquation2.5.5. The application data is approximate, consequently an additional safety factor is included. The pump characteristic. Example6.5.3. The stem passes through the center-line of the disc which is further placed in the center of the pipe bore and the seat is the inside diameter periphery of the valve body. A linear valve achieves this by having a linear relationship between the valve lift and the orifice passarea(seeFigure6.5.3).

Circumstances that canleadtoover-sizinginclude: For example, if the steam supply pressure is 4.5 bar a and the steam pressure required in the heat exchanger at full-load is 4 bar a, this only gives an 11%pressuredropatfull-load. The valves in these different categories will often share common traits regarding how a valve's open percent relates to its ability to restrict flow. The following are some characteristics for butterfly valves when used for modulating service. Table 6.5.1 shows how the change in flowrate alters across the range of valve lift for the equal percentage valve in Example 6.5.1 with a rangeability of 50 and with aconstantdifferentialpressure. The UK Government have announced changes attempting to make it Is it the glorious sunshine? The preferred body configuration for butterfly valves is the wafer, which is clamped between two pipeline flanges. As flowrate is decreased, the differential pressure between the pump and boiler is increased (this phenomenon is discussed in further detail in Module 6.3). Using this routine, the Kvr for each increment of flow can be determined, as shown in Table 6.5.8. However, as in the water valve sizing Example 6.5.2, it is undesirable to select a perfectly sized valve. However, just like handbook data, heuristics, and correlations, valves too can be reasonably approximated using their common characteristics. This type of connection is designed for sealing against bi-directional differential pressures and to prevent backward flow in systems designed for universal flow. A Triple-Offset Butterfly Valve (TOV or TOBV) is often used in critical applications and is designed similar to a double offset butterfly valve with one more offset. Comparing linear and equal percentage valves, a linear valve might have a 25% valve opening for a certain pressure drop and flowrate, whilst an equal percentage valve might have a 65% valve opening for exactly the same conditions. This applies regardless of the inherent valve characteristic. 1. The results aregraphedinFigure6.5.10. Butterfly valves have a relatively simple construction. The equal percentage valve offers an advantage over the linear valve at low flowrates. 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. Nick Vastine is a Business Applications Engineer at Applied Flow Technology (AFT). These characteristic curves are important in almost all cases, especially when evaluating a valve for a control circumstance. The metal seats allow butterfly valves to be used in higher temperature ranges.

There is less flash steam produced in the condensate system. This will always be the case for an equal percentage valve with rangeability of 50. Two sets of bolts connect pipe flanges to each side of the bolt inserts without nuts. 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. Figure 3: Using the Valves button in Output Control to add Open Percentage to output. Click on the logos above to visit their website. The advent of Elastomers, PTFE, Metal Seatings etc has initiated the rapid development of tight shut-off butterfly valves in which these elements serves as the sealing element between the rim of the disc and the valve body. The Kvr of 8.06 satisfies the maximum flow condition of 10 m3/h for this example.

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. Nick holds a Bachelor of Science in Chemical Engineering with a Minor in Economics from the Colorado School of Mines.

Triple offset butterfly valves are often made of metal seats to create a bubble-tight shut-off. Valves of any size or inherent flow characteristic which are subjected to the same volumetric flowrate and differential pressure will have exactly the same orifice pass area.

It is worth while considering what effect the next larger of the linear or equal percentage valves would have had if selected.

The effect of a control valve which islargerthannecessary. 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. 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. Find the heat transfer area required to satisfy the maximum load. The lug-style butterfly valves, unlike the wafer-style, carry the weight of the piping through the valve body. Do you have questions about our website, our products or any of our services? 2022 Spirax Sarco Limited. In AFT Impulse and AFT Fathom XTS, this begins in the valve's Transient tab. It may also be observed that the change in flowrate is large for a relatively small change in the lift. A Butterfly Valve is from a family of valves calledQuarter-Turn Valves. Water is heated from 10C to a constant 60C. At the maximum water flowrate of 10 kg/s, the steam valve Kvr is 69.2. Valves are often oversimplified during modeling. In a simplified example (which assumes a constant boiler pressure and constant friction loss in the pipework), a boiler is rated to produce 10 tonnes of steam per hour. For more information,viewtheAFT Privacy Policy. Lug type butterfly valve allows dead-end service or removal of downstream piping.