In the convergent section, the steam velocity increases as the pressure falls, though the specific volume of the steam also increases with the lowered pressures. Kvr 3.8 The initial part of the control valve travel, during which this lowered control effect is seen, is greater with the selection of the larger control valves and the accompanying small pressure drop at full load. This is due to the high pressure and high temperature of steam and because the measurement parameters vary with the type of steam. There is less tendency for the heating surfaces to scale and foul as the required steam temperature is lower. A few examples of how these may be used are shown below: Connection required. The same procedure can be used to determine the conditions of the downstream steam for other upstream conditions. In this example, a lack of calibration during a long period of time caused the steam provider to claim that the steam flowmeters were not accurate. A limitation of spoolpiece ultrasonic flowmeters is that the transducers can become overheated due to the temperature of the steam. Screwed or flanged connections, and which type of flange, for example, ASME, EN 1092 or DIN? When the flow increases, more pressure drop is created. In vapor service, vapor may be allowed to condense in some of the impulse piping to form a liquid seal between the hot vapor and transmitter in order to protect the transmitter from heat. Read the Kv at this crossing point, i.e. Downstream pressure for maximum, normal and minimum loads. The ability of vortex meters to measure steam flow will encourage suppliers of alternative technologies to further develop and promote their alternative technologies for steam flow measurement, knowing that they can always fall back on their vortex meters for difficult applications. Drop a vertical line from the resulting intersection point, to meet the 200 kg/h horizontal and read the Kv at this crossing point i.e. ... turbulence is always likely to be present in the valve outlet. Example 6.4.1 The valve outlet velocity and drying/superheating effect. If we happened to measure 6.1 kPa of differential pressure across this same venturi tube as it flowed sea water (density = 1.03 kilograms per liter), we could calculate the mass flow rate quite easily using the same equation (with the k factor of 404.3): Credits : Tony R. Kuphaldt – Creative Commons Attribution 4.0 License. Briefly, an application needing on/off control (either fully-open or fully-closed) may require a valve characteristic suited to that purpose, whereas an application calling for continuous control (any degree of opening or closing), might perform better with a different type of valve characteristic. Manufacturers give the maximum lift Kvs values for their range of valves. Maximum differential pressure for shut-off. Proper expertise is available from bona fide manufacturers, and this should always be sought when steam is the prime energy source. The allowable outlet velocity will depend on the downstream steam condition as previously outlined in this section, and observed in Example 6.4.2. The system used to generate the steam is called a boiler. However, there are some technical limitations that make steam flow a difficult application for ultrasonic flowmeters. Dealing with challenging flow measurements. When the control valve chosen is small enough to require a ‘critical pressure drop’ at full load the effect disappears. Control valves can be compared to convergent-divergent nozzles, in that each has a high-pressure region (the valve inlet), a convergent area (the inlet between the valve plug and its seat), a throat (the narrowest gap between the valve plug and its seat), a divergent area (the outlet from the valve plug and its seat, and a low-pressure region (the downstream valve body). The effectiveness of a DP flow transmitter for steam flow depends on the primary element used. The design thickness and body jointing methods also have an effect. For example, IEC 60534 is designed to calculate other symptoms such as the noise levels generated by control valves, which are subjected to high pressure drops. All differential pressure flowmeters rely on the velocity-pressure relationship of flowing fluids for operation. It seems that control valves of differing types may appear to reach critical flow conditions at pressure drops other than those quoted above for nozzles. It is good practice to keep the downstream steam velocity in the valve body typically below 150 m/s for saturated steam and 250 m/s for superheated steam. First, it is necessary to determine the steam state for the downstream condition of 5 bar a. One solution to this problem is to fit a larger bodied valve with the same Kvs of 10 to reduce the wet steam outlet velocity. It is not obvious why more suppliers have not chosen to offer this meter, because, in many ways, the target meter is most like a vortex meter. Steam pressure upstream of valve = 9 bar a There are a variety of very complex sizing formulae available, but a pragmatic approach, based on the ‘best fit’ of a mathematical curve to empirical results, is shown in Equation 3.21.2 for globe valves throttling saturated steam. Includes 53 different calculations. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. Compared to shell-and-tube exchangers, they can operate at lower pressures for the same duty, but because of their high heat transfer characteristics, and a lower requirement for oversizing, they are still smaller and less expensive than a comparable shell-and-tube exchanger. If the nozzle outlet is too small, the steam has not expanded enough, and has to continue expanding outside the nozzle until it reaches the required downstream pressure in the low pressure region. This means that when steam condenses, the pressure in the steam space is reduced. While as many as seven different types of flowmeters can be used to measure steam flow, the vast majority of steam flow measurements are made with differential-pressure (DP) and vortex flowmeters. The valve is held fully open (see Figure 6.4.1). This is then taken as zero and the function within the square root sign becomes unity, and the equation is simplified as shown in Equation 6.4.3. A larger pressure drop that allows the valve to apply effective and accurate control over the pressure and flowrate for most of its travel. If the supply steam is dry and/or the valve encounters quite a large pressure drop, (as in Example 6.4.1), the steam will be more superheated. The principle can be explained by looking at how nozzles work and how they compare to control valves. Dr. Yoder can be reached at jesse@flowresearch.com or 781-245-3200. The shell and tube heat exchanger, Traditionally, the shell-and-tube heat exchanger has been used for many steam heating and process applications across a broad spectrum of industries. Steam demand of heat exchanger = 3 000 kg/h Complicated equations are required to calculate noise emitted from control valves and these are difficult to use manually. A less accurate but useful method to estimate whether noise will be a problem is by calculating the velocity in the valve outlet port. The condensate can harm the blades, and interfere with measurement accuracy. Correct selection for steam is not just a matter of pressure/temperature compatibility. Critical pressure. As a result, the differential pressure produced by the flowmeter can also vary significantly during operation. The reduced pressure in the steam space means that a pressure difference exists across the control valve, and steam will flow from the high-pressure zone (upstream of the control valve) to the lower pressure zone (the steam space in the equipment) in some proportion to the pressure difference and, ideally, balancing the rate at which steam is condensing. The flow and expansion of steam through a control valve is a complex process. Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate. In other words, a larger heating coil or heat exchanger will be required. Control valves will be manufactured to a standard relating to shut-off tightness. What is the approximate flow rate into a circular 1,000-gallon vertical tank that is filled in 10 minutes? For example, if the heat exchanger working pressure is 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) = 0.1. However, a simple steam valve sizing equation, such as that shown in Equation 3.21.2 for saturated steam, is perfectly adequate for the vast majority of steam applications with globe valves. Providing problem solving and educational information for topics related to industrial steam, hot water systems, industrial valves, valve automation, HVAC, and process automation. He has 20 years of experience as an analyst and writer in process control. This technology accounts for about 21% of the world market for flowmeters. It is very easy to buy a heat exchanger designed for liquid use and wrongly assume that it will perform perfectly when heated with steam. This will allow the smaller DN25 control valve to be used, and is preferred because: If the apparatus working pressure is not known, it is sometimes possible to compromise. Another reason for the limited use of variable-area flowmeters is that they have low accuracy. The steam pressure at this minimum flow area or ‘throat’ is described as the ‘critical pressure’, and the ratio of this pressure to the initial (absolute) pressure is found to be close to 0.58 when saturated steam is passing. It is usually considered that the control valve will produce unacceptable noise if the velocity of dry saturated steam in the control valve outlet is greater than 0.3 Mach. Thus, once the critical pressure drop is reached at the throat of the nozzle, or at the ‘vena contracta’ when an orifice is used, further lowering of the downstream pressure cannot increase the mass flow through the device.