For example, an SG iron valve could have a PN16 rating and may also be Appendix 1 Saturated steam valve sizing chart, Appendix 1 Saturated steam valve sizing chart, Appendix 2 Superheated steam valve sizing chart. Variable-area meters do have the advantage of being low cost though, so they are a good fit for noncritical measurements where high accuracy is not a requirement. Because of this, the heat transfer area required to achieve the same heat load must be increased. 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. Consider the next largest valve, a DN32 (but with a 25 mm trim). The design thickness and body jointing methods also have an effect. Multivariable differential-pressure flowmeters can be used to measure liquid, gas, and steam. Impulse piping routes the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow. Mead O'Brien supports our customers with Application & Engineered design consulting, Surveys & Assessments, Field Service, and In-house Assembly & Repair. Nevertheless, when sizing a control valve for a critical process application, such software is indispensable. The dryness fraction of the lower pressure steam is the quotient of the two total heat figures. Another reason for the limited use of variable-area flowmeters is that they have low accuracy. Diaphragm seals can sometimes be applied in these applications. At 10 percent of full scale flow, the differential pressure flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. The heat is transferred to water, which generates steam. When the flow increases, more pressure drop is created. Coriolis flowmeters can be classified as an emerging technology for steam flow measurement. Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate. The heat exchanger design pressure is 5 bar a, and the total heat in dry saturated steam at this pressure is 2 748.65 kJ/kg (from the steam table). Bringing together the information for steam valve sizing, Certain minimum information is required to determine the correct valve size: When the pressure drop across a valve is greater than critical, noise can be generated by the large instantaneous exchange from kinetic energy to heat energy in the low pressure region, sometimes exacerbated by the presence of supersonic steam. The design and material of the control valve must be suitable for the pressure of the system in which it will be fitted. For example, if the pressure upstream of a control valve is 10 bar a, the maximum flowrate through the valve occurs when the downstream pressure is: DP flow transmitters together with primary elements account for well over half of the flowmeters sold for steam flow applications. The speed of sound (Mach 1) can be calculated from Equation 6.4.2. Gas flow measurement is still a relatively new application for Coriolis meters, and the use of Coriolis meters to measure steam flow is just beginning to occur. Control valves will be manufactured to a standard relating to shut-off tightness. With saturated steam, temperature and pressure are strictly related; therefore controlling the steam pressure easily regulates the temperature. This technology accounts for about 21% of the world market for flowmeters. Saturated steam flow through a control valve. The higher the temperature, the lower the allowable pressure, resulting in a typical pressure/temperature graph as shown in Figure 6.4.10. 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. This can give the advantage of smaller and less expensive control valves, whilst minimising the cost of the heat exchanger itself. With superheated steam the ratio is about 0.55, and for air about 0.53. These include fossil fuels, such as coal, natural gas, or oil, as well as other sources of heat, including nuclear fuel, waste fuels, solar energy, and geothermal energy. The valve is held fully open (see Figure 6.4.1). 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. The steam will expand after passing the throat such that, if the outlet area has been correctly sized, the required downstream pressure is achieved at the nozzle outlet, and little turbulence is produced as the steam exits the nozzle at high velocity. The Industrial Steam, Valve, and Process Control Blog, Limitorque QX Electronic Actuator User Instructions. In this situation, sizing on critical pressure drop would have unduly reduced the size of the control valve, and the heat exchanger would be starved of steam. It will give better regulation as it is more appropriately sized to handle changes in the steam load. From steam tables, total heat (hg) in the upsteam dry saturated steam at 12 bar g = 2 787 kJ/kg. 16. First, it is necessary to determine the steam state for the downstream condition of 5 bar a. Therefore the speed of sound in the valve outlet: As the steam is superheated in the valve outlet, the criterion of 0.5 Mach is used to determine whether the valve will be noisy. Alternatively, valve-sizing or Kv charts can be used. What is the steam velocity in the valve outlet? Previous - Control Valve Sizing for Water Systems. It assumes that critical pressure drop occurs at 58% of the upstream pressure. For steam control valves, a leakage rate of 0.01% is perfectly adequate for most applications. A value of 1.3 is chosen for the isentropic exponent ‘γ’ due to the steam in the valve outlet being superheated. Generally, it is better to design the system so that the plate exchanger operates with critical pressure drop (or the highest possible pressure drop) across the control valve at full load. = 0.98 x 2108.23 kJ/kg By entering wet steam at 10 bar a, and 0.96 dryness into the Spirax Sarco website wet steam table, it can be seen that the total heat (hg) held in the 10 bar wet steam is 2 697.15 kJ/kg. ∗Superheated steam: Temperature and pressure are independent variables. Valve ‘A’ will give a higher flowrate for the same pressure drop. This family-owned business started out making variable flow switches for the automotive industry and today services concrete, defense, chemical processing, mining, wastewater, energy, electronics, laboratories, medical, and many other industries. Complicated valve sizing equations can be used to take these and other criteria into consideration, and more than one standard exists incorporating such equations. 2. As a result, more attention is being paid to steam flow measurement. • The system will not control well at low heat loads, effectively reducing the ‘turndown’ capability of the valve. This can be achieved by increasing the valve body size, which will also reduce the velocity in the valve outlet and the likelihood of excess noise. Kvr 3.8 DP flow is also the most studied and best understood method of flow measurement, and multivariable DP flow transmitters are capable of measuring more than one process variable, such as differential pressure, process pressure, and temperature, which in turn enables the measurement of mass flow. Even though target meters were developed after 1950, they are classified as a traditional-technology flowmeter, due to their primarily mechanical means of construction. The advantage of this relatively simple formula is that it can be used with the aid of a simple calculator. Steam pressure upstream of valve = 9 bar a The speed with which an ultrasonic wave travels through metal may be different than the speed of the wave through steam. The speed of sound in steam will depend upon the steam temperature and the quality of the steam, but can be calculated from Equation 6.4.2 if the conditions are known (Mach 1 = speed of sound). When the control valve chosen is small enough to require a ‘critical pressure drop’ at full load the effect disappears. The pressure drop ratio at full load is larger than 0.42, so critical conditions apply and Equation 6.4.3 may be used to find the required Kvr. Control valves are not as efficient as nozzles in changing heat into kinetic energy. 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. The steam demand of the plant can be determined using a number of different methods: Calculation By analysing the heat output on an item of plant using heat transfer equations, it may be possible to obtain an estimate for the steam consumption. The system used to generate the steam is called a boiler. At first, the velocity increases more quickly than the specific volume, and the required flow area through this part of the nozzle becomes less. 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. Dr. Yoder specializes in flowmeters and other field devices, including pressure and temperature products. (This is in contrast to a sharp-edged orifice, where a vena contracta occurs downstream of the orifice. Steam is most stable at the boiler, but cold spots can occur in the line, bringing about condensation. You will now continue to the existing site for Spirax Sarco. As the costs of energy rise, many companies are looking to increase efficiencies in their energy and power generation. This can often make the control unstable, increasing the possibility of ‘hunting’, especially on reduced loads. It is recommended that the maximum exit velocity for wet steam is 40 m/s in the outlet port. Potential steam flow measurement errors can add up to millions of dollars over years of operation. Tim Old, VP of Aftermarket at Celeros Flow Technology, talks about the company’s ambition to be a full life-cycle partner for its customers and the challenges of launching during the COVID-19 pandemic. Calibration should generally be performed in-situ when possible and provisions to do so should be addressed during the design phase. Downstream pressure equals atmospheric pressure. Comparing two DN15 valves from different sources shows that valve ‘A’ has a Kvs of 10 and valve ‘B’ a Kvs of 8. The condensate can harm the blades, and interfere with measurement accuracy. An alternative method of simple valve sizing is to use a Kv chart, Figure 6.4.8. In other words, a larger heating coil or heat exchanger will be required. 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.
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