Steam traps Selection

Partial waterlogging of heater batteries can lead to early failure due to differential thermal expansion. Steam trap selection should take account of this. 

Steam Trap Selection and Application Guide", Sarco Co., P. O. Box 119, Ailentown,... 

For a steam coil to operate efficiently, it must have all the latent heat in the steam. This is achieved by the use of a steam trap. The correct trap type must be selected for the particular application in order to prevent waterlogging. All condensate, air, or other noncondensable must be removed from the system without delay otherwise,... 

Steam Utilization Table 22.8 Selecting steam traps... 

Select steam traps for the following five types of equipment (1) where the steam directly heats solid materials, as in autoclaves, retorts, and sterilizers (2) where the steam indirectly heats a liquid through a metallic surface, as in heat exchangers and kettles where the quantity of liquid heated is known and unknown (3) where the steam indirectly heats a solid through a metallic surface, as in dryers using cylinders or chambers and platen presses and (4) where the steam indirectly heats air through metallic surfaces, as in unit heaters, pipe coils, and radiators. 

Determine the condensate load. The first step in selecting a steam trap for any type of equipment is determination of the condensate load. Use the following general procedure. a. Solid materials in autoclaves, retorts, and sterilizers. How much condensate is formed when 2000 lb of solid material with a specific heat of 1.0 is processed in 15 min at 240°F by 25-psig steam from an initial temperature of 60°F in an insulated steel retort ... 

A safety factor must be applied to compensate for radiation and other losses. Typical safety factors used in selecting steam traps are... 

Has the steam trap been properly selected and sized ... 

A typical steam tracing system is shown in Figure 7-84. The keystone of this system is the steam trap. Correct selection and application of other components is to no avail if the steam trap is improperly chosen. Selection must be based on two choices type and size of the trap. 

On this basis, the selection of a group of steam traps to handle all steam tracing situations is possible. Actually this consists of a 3/8-inch nominal size trap but many steam trap manufacturers rate their traps by orifice size rather than by connection size. Suitable traps should have 150 psig steam condensate capacities from 350 to 600 pounds per hour at saturation temperature, and from 550 to 850 pounds per hour at 30°F below saturation temperature. Using 15 psig steam, the condensate capacity should range between 135 and 250 pounds per hour at saturation temperature and from 225 to 400 pounds per hour at 30°F below saturation temperature. 

Pressure tests of pipelines, jackets, steam coils, and arrangement to isolate and take out coils for maintenance are necessary. Steam traps shall be selected for the removal of condensate as soon as formed. There shall be no cooling of condensate in the jacket or coils (unless system is specially designed to recover heat from hot condensate also). 

Plant engineers should consider the following for selecting a suitable steam trap ... 

Maximum superficial condensate velocities are about 25 m s . In a given line, the velocity increases as the pressure drops and more liquid vaporizes. Vendors of steam traps and condensate systems can provide guidance and line-sizing charts. Water lines usually have velocities up to 3ms . The designer must consider water hammer as well as velocity and pressure drop. Selection of valves and their closure time becomes important. Compressed air usually is transported at 5-8 m s. ... 

Thus, selection and sizing of steam traps are critical for the management program. The steam trap should be sized for either start-up or operating conditions, depending on how often the unit will start up. If it is sized for operating conditions, then additional manual drainage should be considered for start-up. Six factors are listed in detail for selection of steam traps (Kenney, 1984) ... 

It is important to know that steam traps should not be sized based on pipe size. A common problem is poorly sized traps that cause premature failure through excessive cycles and wear on internal parts as well as excessive steam leaks. Therefore, steam traps are selected primarily on duty second, on duty variation for transient cases and third, on equipment requirement (Kenny, 1989). Proper installation and maintenance together with regular vendor service can maintain good steam trap performance and long life. 

Large condensate loads are frequently present in evaporator systems. Large quantities of condensate are best handled in an instrumented condensate pot or tank in which liquid level is controlled. Liquid-level control will reduce steam consumption by 2-17%, depending on the steam pressure and trap selected for... 

In order to compensate for these variations in operating conditions, the designer uses a capacity safety factor to increase the calculated condensate load. This safety factor should be selected with care. One short-cut method that should be avoided sizing steam traps to equal line size. This practice is never a substitute for analyzing process conditions it invariably leads to specification of the wrong size steam trap. 

Considerations for selecting the right type of steam trap are as follows ... 

Each specific steam trap has a finite, relatively narrow range that it can handle effectively. For example, an inverted-bucket trap designed for up to 15-psi service will fail to operate at pressures above that value. An inverted-bucket trap designed for 125-psi service will operate at lower pressures, but its capacity is so diminished that it may back up the system with unvented condensate. Therefore, it is critical to select a steam trap designed to handle the application s pressure, capacity, and size requirements. 

When properly selected, installed, and maintained, steam traps are relatively trouble-free and highly efficient. The critical factors that affect efficiency include capacity and pressure ratings, steam quality, mechanical damage, and calibration. 

Each type and size of steam trap has a specified capacity for the amount of condensate and noncompressible gas that it can handle. Care must be taken to ensure that the proper steam trap is selected to meet the application s... 

As discussed previously, each type of steam trap has a range of steam pressures that it can effectively handle. Therefore, each application must be cateliilly evaluated to determine the normal and maximum pressures that will be generated by the steam system. Traps must be selected for the worst-case scenario. 

Steam traps arc designed fc r a relatively constant volume, pressure, and condensate load. Operating practices should attempt to maintain these parameters as much as possible. Actual operating practices are determined by the process system, rather than the trap selected for a specific system. 

The required orifice continuous flow capacity is determined at steam chest pressure to condensate system pressure at a flow 6 to 8 times design. If designed for normal flow the trap would have to be open 100% of the time. Then, as stated above, a body size is selected that can contain the required orifice (not be above the stated... 

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