Thread Content
Steam energy savings——Selection of steam drain valves, Part 4. Technical Service Center of Hangzhou Watt Energy Saving Engineering Co., Ltd.; Steam technology engineer: Li Shaopeng. After correctly selecting the type and diameter of the drain valve, it is important to install it properly to ensure its most effective operation. Different methods can be used to install steam traps on pipes. Each installation method has its own advantages and disadvantages, which are briefly described as follows: Additionally, a swivel joint can be used to assist with the installation of steam traps; this swivel joint facilitates quick and easy removal or replacement of the trap without the need for special tools or interrupting the operation of the system. The swivel joint is mounted on the pipe and is connected to the body of the drain valve by only two screws. Its connection surface uses stainless steel spiral gaskets to eliminate leaks. Generally, in the case of isolation at both ends, replacing a drain valve only takes a few minutes. The steam drain valve is just one of the important components of the entire steam drainage station. Whether the various components in the steam dehydration station are installed correctly or not has an impact on its overall operation. Basic Installation Guidelines for Steam Trap Systems • All steam traps come with detailed installation and maintenance manuals. It includes all the information on how to properly install the drainage valve. The following points are things to keep in mind in the application of a good drain valve. • It must not exceed the maximum allowable pressure and temperature (PMA and TMA) specified on its valve body. • It is not possible to exceed the maximum working pressure (PMO), otherwise it is difficult to ensure safe and effective operation. • To ensure safe and reliable isolation, a stop valve should be used during maintenance. Depending on the application equipment, options include corrugated tube stop valves, plug valves, or ball valves. • All steam drain valves have small discharge holes and moving parts; if impurities, weld slag, rust, etc. enter the drain valve, it is very easy to cause blockages, which in turn can lead to leakage or damage of the valve. Therefore, it is necessary to install a Y-shaped filter upstream of the drain valve. Some models of water-repellent valves come with an internal Y-shaped filter, so there is no need to install a separate filter. However, for some drain valves equipped with built-in baffle filters, it is still recommended to install a separate Y-shaped filter to facilitate cleaning. • A drain valve detection valve should be installed upstream of the steam drain valve, to facilitate the detection of any malfunction in the drain valve or of water accumulation in the system during inspections and tests. Some applications install cameras downstream of the water drainage valve, but this is becoming less and less common. For thermodynamic steam traps, the sight glass should be installed at least one meter downstream of the spray-type steam trap. • When there is backpressure in the drain pipe, a check valve should be installed downstream of the steam trap to prevent a drop in inlet pressure or water accumulation in the steam space during shutdown. The same check valve should also be installed 1 meter downstream of the spray-type drain valve. • Failing to turn off the bypass valve can lead to steam leakage or problems with the drain valve, and it may also increase the pressure in the condensate return pipes. Therefore, we do not recommend installing a drainage bypass. • The steam drain valve should be installed below the steam system, and there should be a reduction pipe with a small diameter of about 150 mm upstream of the drain valve. Otherwise, at low loads, the sealing bags/components of the pressure-balancing drain valve and the dual-metal drain valve cannot be controlled by the temperature of the chilled water, which can lead to incorrect operation of the drain valves. • For steam drain valves connected by welding, arc welding should be used, as it eliminates the need to remove internal components when making the weld. Using other welding methods may cause the valve body to deform or damage its internal components. • Careful attention must be paid to ensuring the correct installation direction of the steam trap, especially for mechanical steam traps with vertical lifting mechanisms (such as float ball steam traps and inverted bucket steam traps). If the installation direction is incorrect, it will cause the drain valve to wear out, become ineffective, and result in steam loss. • It should be installed in the correct direction as indicated on the body of the drain valve. • Although the Watt thermal power drain valve can operate in any installation orientation, it achieves the longest service life when installed horizontally. • Thermostatic steam traps cannot be installed with the inlet facing downward. At low loads, such an inverted installation causes the sealing capsule or components to become submerged in cold, stagnant condensate water, which results in an excessive opening of the trap, an increased flow rate, and subsequently attempts to close the trap again. These two opposing effects can lead to incorrect operation of the water drainage valve. Furthermore, impurities such as weld slag in the pipeline can easily enter the valve cover, affecting the proper functioning of the sealing gasket/components. • Pressure-balanced drain valves and dual-metal drain valves cannot be installed on the side, as this will cause the sealing capsule/component to be partially submerged in condensate and partially exposed to steam, resulting in incorrect operation of the drain valve. • Some customers, in order to save costs, use a single drain valve to discharge the condensate from several devices; this is known as group drainage. But in reality, due to the different working loads of various devices, water accumulation in the system occurs. Therefore, we recommend that a separate drainage system be provided for each piece of equipment. Improper group hydrophobicity • It is necessary to ensure that the diameter of the hydrophobic piping is correct, especially that of the piping on the discharge side of the hydrophobic valve; otherwise, an excessively high flow rate of flash vapor can affect the proper operation of the hydrophobic valve. When the water discharge valve is operating at near its maximum capacity, the downstream pipeline may need to have an increased diameter to prevent excessive flow rates and frictional pressure losses. • If the installation position of the drain valve has to be above the drainage point, a rising pipe with a small diameter should be used and inserted into the U-shaped water seal. This type of installation is typical in small processing tanks, such as plating tanks. It is also advantageous to have a small downpipe in front of the drain valve. If a swing-bucket type drain valve is installed, a check valve must be installed in front of it to prevent the drain valve from losing its water seal. A small-diameter rising pipe in a small water tank is inserted into a U-shaped water seal. • In situations where air accumulation may occur, a thermodynamic type air release valve should be used; if there is no air barrier disc, installing the thermodynamic air release valve with its head facing downward can solve the problem. • When thermodynamic steam drain valves are installed in cold and humid environments, a shielding cover must be fitted over the valve cap to prevent excessive heat loss, which could lead to frequent operation and subsequent premature wear. • Inverted buckets and ball-type drain valves should be installed as close as possible to the outlet of the equipment being drained, in order to prevent steam lockage. When the pipe between the condensate outlet and the drain valve is filled with steam, the condensate will not be able to reach the drain valve, resulting in a steam lock. This will cause water to accumulate in the system, affecting the efficiency of the equipment. This is very similar to air locking in water systems. The most common device in a steam lock is a rotating drum, through which the condensed water is discharged via siphon tubes. After the floating ball water-repellent valve drains all the condensed water, the siphon tube is filled with steam. Since the drum is also filled with steam, there is no temperature difference between the wick tube and the drum; as a result, the steam in the wick tube does not condense, preventing the condensed water from reaching the drain valve and causing water to accumulate in the drum. The method to resolve steam lock is to use a thermal static evacuation device along with a steam lock prevention device’s drain valve. The condensed water is discharged through the drain valve. After the water is drained, steam enters the wick tube. The steam in the wick tube does not condense, which prevents steam lock and allows water to accumulate in the drum. Steam lock in rotary drum equipment • If a ball-type drain valve is installed in an exposed environment, a temperature-insulating layer must be added. • If pressure-balanced drain valves or dual-metal drain valves are used in steam spaces where water accumulation is not allowed, a section of cooling pipe must be installed upstream of the drain valve; this pipe does not require temperature control. Generally, the length of the cooling pipe upstream of a pressure-balancing drain valve is 2 meters, while that upstream of a dual-metal drain valve is 3 meters. • If a inverted bucket type drain valve is used in superheated steam, a check valve should be installed at the inlet to prevent the drain valve from losing its water seal. Because when the temperature of superheated steam drops under system pressure, it often causes water to evaporate or flash vaporize. The Watt inverted bucket type drain valve is standardly equipped with an internal check valve. When a superheated condition exists, due to the low amount of condensed water, it is sometimes necessary to add water before starting up to ensure water sealing. • In some special applications, the inability of the lift-type drain valve to remove air quickly can result in an extended preheating time and water accumulation in the steam space; in such cases, installing a separate air drain valve can ensure effective operation. • For inverted bucket type drain valves, if a bypass is required, it must be located above the drain valve. If installed below the drain valve, a leak in the bypass or failure to close it properly can cause the drain valve to lose its water seal, resulting in steam waste. • If a dual-metallic type drain valve is installed in a system with non-zero back pressure, changes in back pressure will affect the condensate discharge temperature. As the back pressure increases, the discharge temperature of the condensed water drops, thereby increasing the likelihood of water accumulation. Installation rules for steam drain valves ‘ABC’: • A = Install in a location that is easy to inspect and maintain as much as possible. • B = as low as possible below the hydrophobic point. • C = As close as possible to the hydrophobic point to prevent steam lock.