280 SERIES STEAM CONVERTING VALVE SIZING AND SELECTION. 1. Introduction Design Properties Sizing and Selection Criteria...

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1 Contents 1. Introduction Design Properties Sizing and Selection Criteria Definitions Operating Conditions Valve/Pipe Dimensioning Valve Design Alternative Steam Converting Systems Benefits General Benefits of the Series 280 Valve Advantages compared to a system using a pressure reducing valve and spray nozzle Application Examples Steam Converting Control Station Appendix I: Sizing Criteria Tables

2 1. Introduction "Steam converting," also known as "desuperheating," refers to the process in which superheated steam is cooled to a lower desired temperature. There are several different methods for achieving this goal, most commonly heat exchangers, water bath coolers, or direct water injection desuperheaters. The simplest and most common method of steam conditioning is by direct water injection. This is a method in which water is directly added to the steam flow, evaporated into the steam by the excess heat of the superheated steam, and in the process drawing heat from the superheated steam to reduce the temperature. This method is most commonly carried out using either a type of injection nozzle or a desuperheating valve. Steam Converting Valves (desuperheating valves) are control valves that are used to simultaneously reduce the temperature and pressure of superheated steam. These special control valves generally inject water directly after the throttling process, utilizing the very high velocities created, which allows for an excellent mixing of the steam and water. SAMSON offers the 280 series for steam converting valve applications. The purpose of this guideline is to provide important sizing and selection criteria for SAMSON 280 series steam converting valve. The selection criteria specified below is automatically monitored in the steam converter sizing program. Supplemental Documentation T8251, T8252, T8265, T8266 Data sheets for types 3281 and 3286 steam converting valves EB 8251 Mounting & operating instructions for types 3281 and 3286 steam converting valves TV-SK 9778 Installation Conditions for Steam Converters in Pipelines 2

3 Design Properties Like most desuperheating valves, the 280 series steam converting valve reduces steam temperature by utilizing the throttling process inside a control valve to assist in mixing steam and water. Water is lead through the bonnet of the valve and into the flow divider, where high velocity steam leaving the throttling area comes in contact. The steam breaks the water into very fine droplets (like fog), where the excess energy of the steam is absorbed by the water during the evaporation process, and the steam reaches the final desired outlet temperature. 3

4 Sizing and Selection Criteria The following criteria details the sizing and dimensioning requirements for the 280 series steam converting valves, however this information is also integrated into the steam converter sizing program and is automatically monitored when sizing a steam converting valve using this program. The steam converter sizing program can be obtained at: The associated cooling water valve can be sized using ecat or SVSS. The required process data for the water valve can be found on the sizing sheet of the series 280 valve (water mass flow W 3, water upst. pressure p 4, water downst. pressure p 3, water temp. T 3, required water valve size) 3.1 Definitions T 1 Inlet Temperature T 2 Outlet Temperature T 3 Cooling Water Temperature p 1 Inlet Pressure p 2 Outlet Pressure p 4 Cooling Water Supply Pressure p Steam Differential Pressure (p 1 -p 2 ) x differential pressure ration ( p/p 1 ) subscript ast above saturation temperature Ex. T 2,ast = Difference between T 2 and T p 2 WS% - Water/Steam Ratio 3.2 Operating Conditions Variable Symbol Moderating Variable(s) 1.1 Water/Steam Ratio W/S% T Min. Pressure Drop p p Min. Outlet Temp Above Saturation Min. Cooling Water Temp Max. Cooling Water Temp Min. Cooling Water Pressure T 2,ast WS% x T 3 T 1 Limit max. 20%, depending on process conditions See Appendix I, Figure 2 depending on process conditions See Appendix I, Figure C, depending on process conditions See Appendix I, Figure C, depending on process cond See Appendix I, Figure 3 T 3 p 3 Flashing Not Allowed! p 4 p 1 p 2 p 4 =p (p 1 )+2bar 4

5 3.3 Valve/Pipe Dimensioning 2.1 Max Valve Velocity 100 m/s up to maximum 120 m/s 2.2 Min & Max Pipe Velocities Min: 10 m/s, Max: 80 m/s, Optimal: 50 m/s 2.3 Max Water Valve Velocity 3 m/s 2.4 Temp Sensor Distance (AT) 10 m (required distance calculated by sizing program) 2.5 Straight Pipe Length 5 m (required distance calculated by sizing program) 3.4 Valve Design 3.1 Bonnet and Packing Combination T C: PTFE Packing + Standard Bonnet 220 C < T C: Graphite Packing + Standard Bonnet 350 C< T C: PTFE Packing + Insulating Section 3.2 Flanges 3.3 Cooling Water Connection 3.4 Steam Trap 3.5 Characteristic All three flanges (inlet/outlet/cooling water) have the same pressure to match the nominal pressure. Cooling water connection on the steam converter is always connected over a flange and not a welding end. (Even when main connections are welding due to repair capability) If condensate is expected to collect in the body (e.g. turbine bypass or standby station), a connection for an automatic steam trap must be fitted in the bottom of the valve. Standard: Equal percentage. For split range: linear in small valve and equal percentage in large valve. 3.6 Trim Material Standard: Stellited Seat and Plug 5

6 4. Alternative Steam Converting Systems While the 280 series steam converting valve covers a large range of applications, there are some cases in which this valve is not suitable. The following additional steam conditioning products can be offered for the below specified applications. The Type Selector in the steam converter sizing program helps you to select the correct steam converting system. Spray Water Desuperheater (Samsomatic Type DK) Temperature must be reduced with no (or very low) reduction in pressure Water Bath Desuperheater (Samsomatic Type ) Saturated steam in the outlet is required Motive Steam Desuperheating Valve (Schroedahl Type DUP) Required water/steam ratio exceeds the limits of the 280 series Valve (predominantly angle) with different inlet and outlet sizes Pressures above PN 160 / Class 900 or sizes above DN 500 / NPS 20 6

7 5. Benefits 5.1 General Benefits of the Series 280 Valve 7

8 5.2 Advantages compared to a system using a pressure reducing valve and spray nozzle It is very common for a desuperheating system to consist of a standard control valve to reduce the steam pressure, followed by a spray water nozzle to reduce the steam temperature. In many cases, a 280 series steam converting valve can be used instead to provide better temperature control, lower installation costs, and longer service life: Lower installation costs Both pressure and temperature reductions take place inside the 280 series steam converting valve. In contrast, a pressure reducing valve with downstream spray nozzle are two separate devices which may require much more time and money to install. Better temperature control The spray water is injected at the point where the steam velocity is at its highest in the 280 series steam converting valve, which results in a uniform mixing of the steam and cooling water. In contrast, when using a spray nozzle, the steam and cooling water often do not mix along the entire crosssectional area of the pipe. Consequently, the steam at the edge of the pipe is hotter than in the middle of the pipe. As a result, the wrong steam temperature might be measured, leading to inaccurate temperature control. Longer service life The water droplets injected by a spray water nozzle may directly hit the pipe wall, accelerating wear in the pipe. However, in the 280 series, the water is injected into the steam converting valve, providing better protection against water droplets. 8

9 6 Application Examples Heating up substances in the chemical industry High heat exchanger efficiency by using steam near the steam saturation curve Use of high-pressure or low-pressure steam Safe and efficient heating of products Steam box control in pulp & paper mill Provide constant pressure and temperature to shower Helps paper dry more efficiently, resulting in higher paper speeds Can use low pressure steam and water which is normally wasted Protection of paper from over pressure, over temperature and water droplet damage Sterilization and cooking in food industry Provides steam at near saturation for constant temperature heat transfer Accurate temperature control ensures proper sterilization Precise steam flow control results in faster cooking times 9

10 7 Steam Converting Control Station 10

11 Appendix I: Sizing Criteria Tables Minimum Required p Minimum Required p Figure 1: Minimum Required p in Steam Converting Valve 11

12 Min. Required Outlet Temperature Above Saturation (depending on the differential pressure ratio x) Figure 2: Minimum Required Outlet Temperature above Saturation Examples: Water/Steam Ratio = 4%, x (( p/p 1 ) = 0,1 Min. Required Temperature Above Saturation = 9 C Water/Steam Ratio = 7%, x = 0,3 Min. Required Temperature Above Saturation = 7 C Water/Steam Ratio = 10%, x = 0,5 Min. Required Temperature Above Saturation = 5 C Temp. Above Saturation 10 C Max. Water/Steam Ratio = 20% 12

13 Minimum Required Cooling Water Temperature (as long as there is no flashing in the water valve) Figure 3: Minimum Required Cooling Water Temperature 13