Steam Energy Saving------Selection of Steam Traps, Part 3

Thread Content

Steam energy savings——Part 3 of selecting steam traps. Technical Service Center of Hangzhou Watt Energy Saving Engineering Co., Ltd.; Li Shaopeng, Steam Technology Engineer. When selecting steam traps, the safety factor is an important consideration, as the pressure before the steam trap is not always stable at the designed value, especially during startup conditions. For the same discharge hole, factors such as the temperature of the condensate water, the static pressure head before the valve, the smoothness of discharge, the structural design of the heat exchanger, the layout of the flow channels, and the difficulty in collecting the condensate water within the heat exchanger all affect the discharge of the condensate water. To ensure safe and timely discharge, a certain degree of redundancy must be considered. To accommodate changes in the condensate load, a certain safety factor should be applied to the calculated condensate load. The table below shows the safety factors adopted in most applications based on experience. Applications: General systems with regulatory control; primary heat exchangers: x3, n/a; shell-and-tube heat exchangers: x2, x3; storage heaters: x2, x3; heat exchangers: x2, x3; air heaters: x2, x3; ironing machines: X2; small high-pressure sterilizers: X2; large high-pressure sterilizers: X3; evaporators: X2; rotary drums (continuous type): X3, x6; submersed coils (low-level drainage): X2; submersed coils (siphon drainage): x3; air heating coils: x2, x3; bypass heat pipes: x2, x3. The table above indicates that, under normal conditions, the safety factor should be at least 2; for some temperature control systems, the safety factor needs to be 3. It is actually difficult to determine an exact safety factor. In most cases, the steam trap will be able to drain more condensate than calculated based on the load. If the safety factor is too high, it may lead to the selection of a model that is too large, resulting in higher costs compared to products from competitors. Moreover, a steam trap that is too large has larger discharge holes, which means more steam will leak out when the trap fails for some reason. If the safety factor is too low, the drain valve selected will be undersized, failing to remove condensate and air quickly as required, which reduces the efficiency of the equipment. Selection of steam traps: All Watt (Hangzhou Watt and Taiwan Watt) steam traps come with selection charts based on pressure difference and condensate load; these charts allow us to determine the heat condensate load at a given pressure difference. On some selection diagrams, the volume of cold condensate (during startup) is also indicated with dashed lines. On the Spiraq sales brochure, red lines and blue lines are used respectively to represent the hot condensate flow rate and the cold condensate flow rate. For single-port steam traps such as pressure-balanced steam traps, metal steam traps, or thermodynamic steam traps, the condensate discharge volume can be determined once the minimum pressure difference and maximum condensate load (with a safety factor taken into account) have been calculated. For mechanical steam traps (float-type steam traps and inverted bucket steam traps), the situation is different due to their special internal components. The pressure difference provides the corresponding internal components. The lower the pressure difference, the larger the effective diameter of the discharge hole. The basic models of inverted barrel steam traps come with a suffix number such as (/12, /48). This number is approximately what fraction of 64 of the hole’s diameter (in inches). Therefore, the clearance bore diameter of a 212/12 steam trap is 12/64\" (3/16\"). When selecting a float-type steam trap or a inverted bucket steam trap, the most important factor to consider is the maximum steam pressure, as it may be higher than the maximum operating pressure difference. For example, in a system where the upstream pressure is 10 bar and the backpressure is 6 bar, the pressure difference is 4 bar. It might seem appropriate to choose a float-type steam trap with a pressure difference of 4.5 bar, as this value is closest to the required pressure difference and will allow for the maximum amount of condensate to be discharged. But once the back pressure is removed, the rising pressure difference will act on the float, preventing it from rising to open the valve. The condensate will not be able to be discharged. Therefore, what is needed at this time is a float-type steam trap with a pressure difference of 10 bar. The following is an example of selection for a heating process: Application – Heat exchanger with temperature control; Type of drain valve – Float-type drain valve equipped with a thermodynamic venting device; Material requirement – Ductile iron; Maximum system pressure – 10 barg (saturated steam); Normal system pressure – 5 barg (saturated steam); Pressure in the condensate pipeline – 1 barg; Maximum pressure difference ΔP – 8 bar; Minimum pressure difference ΔP – 3 bar; Lift height after the drain valve – 10 m; Calculated maximum condensate load – 150 kg/h; Safety factor – 3. The float-type drain valve is the best choice for draining water from heat exchangers with temperature control; therefore, in this case we can select drain valves made of malleable cast iron from the FQ series. The calculated condensate flow rate of 150 kg/h represents the operating load; multiplying this by a safety factor of 3 yields a maximum starting load of 450 kg/h. The back pressure is the pressure in the condensate pipe, which is 1 bar, plus an increase of 10 meters due to the lift provided by the steam trap; thus, the total back pressure is 1 bar + 1 bar = 2 bars. A pressure relief valve with a differential of 5 bar can meet the requirement of a working pressure differential of 5–2 bar = 3 bar, but it is not suitable when the system pressure rises to 10 bar, as the differential at that point would be 8 bar (10–2). Therefore, the correct choice should be a steam trap with a pressure difference of 10 bar, so that it can ensure good performance under all conditions and thus facilitate the effective drainage of condensate at the steam saturation temperature. As we have mentioned, for general applications, some engineers can choose a steam trap based on experience alone, without needing detailed data. For inverted bucket steam traps, using internal components designed for high pressure differences in applications with low pressure differences can affect the proper operation of the trap; therefore, the selection should be based on the minimum pressure difference.