Steam Trap Selection: Understanding Specifications

Following the first article on how the steam trap application affects steam trap selection, this second article will offer an overview of how operating conditions influence the steam trap model and its specifications.

Operating Conditions Effect on Trap Specifications

System conditions determine the minimum trap specifications for pressure, temperature, discharge capacity, material, and connection type.

Body Material

Trap body material is one of the first items to look at when selecting a trap. The material is selected based on the maximum operating temperature and pressure at the condensate discharge location (CDL), the surrounding environment, and requirements for longevity/ minimal maintenance. The material must also meet the pressure test and maximum pressure and temperature piping design specifications.

The materials used for the steam trap body, cover, and other pressure-resistant parts are no different from those used in other types of valves. Some examples are:

• Gray Cast Iron/ Ductile Cast Iron
• Carbon Steel
• Stainless Steel

The maximum applicable pressure and temperature of the body material are not necessarily equivalent to the maximum operating pressure and temperature of the trap. This is because the maximum operating pressure and temperature can be limited by the pressure/temperature resistance of other parts such as gaskets and other internal components.

In addition, different standards such as ASME or DIN can affect the maximum operating pressure / temperature of the trap material. For example, A126 cast iron has a maximum allowable pressure of 13 barg (190 psig) according to DIN standards, but 16 barg (250 psig) according to ASME standards. Also, stainless steel traps have recently become more and more popular because they are typically easier to maintain and offer a longer service life.

Sizing

A large number of steam users improperly select trap size based on the size of existing piping. However, trap size should closely match the size of the piping on the outlet side of the equipment that supplies condensate to the trap.

It is generally recommended to size condensate piping on the discharge side of equipment that supplies condensate to the steam trap according to the following table:

Maximum Condensate Load	Equipment Outlet Piping Size
Less than 200 kg/h [440 lb/h]	15 mm [1/2 in.]
200 - 500 kg/h [440 - 1100 lb/h]	20 mm [3/4 in.]
0.5 - 1 t/h	25 mm [1 in.]
1 - 2 t/h	32 mm [1 1/4 in.]
2 - 3 t/h	40 mm [1 1/2 in.]
3 - 5 t/h	50 mm [2 in.]
Over 5 t/h	65 - 100 mm [2 1/2 - 4 in.]

^* Provided as a general reference. Please consult a steam specialist such as TLV if you are unsure about trap selection or piping design.

Generally, the trap should never be sized smaller than the equipment outlet piping because this can lead to waterlogging and ensuing damage and / or heating problems.

In addition, pipe sizing at the trap outlet should not be based on trap size, but instead should be designed to deliver the required flow rate and limit pressure loss for two-phase flow. For more information on this topic, please read: Condensate Recovery Piping

Connection Type

Most steam users typically require threaded (screwed), socket-welded, or flanged steam trap connections depending on the standard national, industry, or company codes and specifications.

Threaded connections cost much less than flanged connections to install, but need to be screwed-in during installation, meaning that either the trap outlet piping needs to remain disconnected or a union needs to be used to allow for easy trap replacement. On threaded connection steam traps, it is important that the trap threads follow official standards to help minimize poor connection sealing to the connected piping.

Traps with socket weld connections are generally preferred in some plants to limit the amount of steam leaks, but socket weld connections can be more difficult to remove during replacement, and may also have higher installation or maintenance costs. Additionally, some areas may have shortages of qualified welders, which can reduce the overall installation or repair efficiency.

Traps with flanged connections can be easily removed and replaced only if the new trap has the exact same size and face-to-face dimension. It is best to require a strict face-to-face dimension according to a trap manufacturer’s standard production item when specifying flanged traps on new construction projects.

Example of Trap with Flanged Connections

After selecting the trap specifications according to operating conditions and environment, the next step is evaluating the necessary discharge capacity that includes the safety factor, and selecting the most economical trap. For more info on these topics, please read part 3.

Steam Trap Selection: How Application Affects Selection

Steam Trap Selection: Safety Factor and Life Cycle Cost