Understanding Steam Trap Selection

Understanding the Types of Steam Traps

As essential components in steam systems, steam traps are primarily responsible for discharging condensate, air, and other non-condensable gases while simultaneously retaining live steam. Selecting the correct trap is crucial for system efficiency and heat transfer. Steam traps can be categorised into three main operating principles: thermodynamic, thermostatic, and mechanical.

• Thermodynamic Traps

Thermodynamic traps operate based on the difference in dynamic response between hot condensate/flash steam and cooler condensate. Their operation is governed by Bernoulli’s principle.

• Principle of Operation: When the system starts up, the pressure of the incoming fluid (cool condensate, air, and non-condensable gases) lifts a disc (or valve), allowing a free and rapid discharge. As the system heats up, the hot condensate begins to flow through the inlet passage at high velocity. When this high-velocity, hot condensate enters the lower pressure chamber, it undergoes flash steam creation. This flash steam moves rapidly above the disc and into a control chamber.
• Sealing Action: The high-pressure flash steam in the control chamber exerts a strong downward force on the top of the disc, seating it firmly against the trap’s seating rings and sealing the outlet.
• Discharge Cycle: The flash steam in the control chamber loses heat by radiation to the atmosphere, eventually condensing back into condensate. As this happens, the pressure above the disc drops. The force exerted by the incoming condensate from below then lifts the disc, and the condensate is discharged, restarting the cycle. This results in an intermittent or cycling discharge action.
• Applications: They are highly robust and widely used in main steam distribution lines, drip legs, tracers, and high-pressure applications due to their tolerance for water hammer, superheated steam, and high operating pressures.

• Thermostatic Traps

Thermostatic traps operate solely on the temperature difference between steam and sub-cooled condensate, making them excellent for removing condensate that is below the saturation temperature of steam.

• Principle of Operation: These traps utilise an internal temperature-sensitive element (such as a bellows filled with a volatile liquid or a bi-metal strip) that expands and contracts in response to temperature changes.
• Start-up Phase (Open): During system start-up, the lines are full of cool air and condensate. The cool temperature causes the thermostatic element to remain contracted, keeping the valve fully open. This allows a massive volume of cool condensate and, critically, large amounts of air and non-condensable gases (which would otherwise hinder heat transfer) to escape quickly.
• Operating Phase (Modulating): As hot condensate or steam reaches the trap, the high temperature causes the volatile liquid in the bellows to vaporise or the bi-metal element to expand. This expansion drives the valve close to its seat, sealing the trap against live steam.
• Condensate Discharge: The trap remains sealed until the condensate cools down (sub-cools) to a pre-set temperature below the saturation temperature of steam. This temperature drop causes the element to contract, opening the valve and discharging the cooled condensate.
• Designations: They are also commonly known as balanced pressure traps when using a bellows element, as the pressure inside the element is balanced against the steam pressure outside.
• Applications: They are ideally suited for applications requiring excellent air venting capability and the ability to discharge sub-cooled condensate, such as air venting, jacket tracing applications, and small process equipment.

• Mechanical Traps

Mechanical traps are distinct in that they operate based on the difference in density between steam (low density) and condensate (high density). They provide a modulating, continuous removal of condensate immediately upon formation.

• Principle of Operation: These traps utilise an internal float or bucket that rises and falls with the level of condensate inside the trap body. This movement directly controls the opening and closing of a valve mechanism.
• Sub-Types: The two primary types of mechanical traps are:
  Float Traps: These use a sealed, spherical float that rises and falls with the condensate level. The float is connected via a leverage system to a main valve. As the condensate level rises, the float lifts the valve off its seat, allowing a continuous, smooth discharge. Float traps often include a separate thermostatic element for automatic air venting during start-up.
  Inverted Bucket Traps: These use an open-bottomed bucket inverted over the trap’s outlet. When condensate flows into the trap, it sinks, causing the bucket to sink (pulling the valve open) and discharge the condensate. When steam enters the bucket, it imparts buoyancy, causing the bucket to rise and pull the valve shut. Air and non-condensable gases bleed slowly through a small vent hole at the top of the bucket.
• Applications: Mechanical traps are highly valued for their ability to discharge condensate at steam saturation temperature and their reliable, robust operation. They are primarily used in process lines and heat exchangers where maximum heat transfer and immediate, continuous condensate removal are essential.

Selecting the Right Steam Trap: A Comprehensive Approach

Selecting the appropriate steam trap is a critical decision in maintaining an efficient and safe steam system. While numerous types of traps exist, a thorough assessment of the process conditions and requirements is paramount before making a final selection.

Key Factors in Steam Trap Selection

General considerations that significantly influence the choice of a steam trap include:

• Discharge Requirements: The trap must be capable of discharging condensate effectively under all operating conditions, including handling air and non-condensable gases.
• Operating Pressure and Temperature: The trap’s design must be compatible with the maximum expected operating pressure and temperature of the system.
• Load Requirements: The trap must handle the maximum, minimum, and start-up condensate loads efficiently.
• Safety: The selection must adhere to all safety regulations and ensure reliable, safe operation, preventing issues like water hammer or steam locking.
• Installation Orientation: Some traps, such as certain float traps, require a specific orientation (e.g., horizontal or vertical), which must align with the installation space.

The proper sizing of a steam trap is just as vital as selecting the correct operating principle. An undersized trap will lead to condensate backup, reduced heat transfer efficiency, and potential water hammer, while an oversized trap can cause rapid cycling and premature failure.

The selection and sizing process should follow a systematic procedure:

1. Understand the Application

The function and location of the trap dictate the primary requirements. Trapping is typically required for:

• Distribution Lines (Mains): Used to drain condensate formed from radiation losses in the steam piping. These require traps that can handle relatively steady but sometimes low loads and discharge condensate close to steam temperature to prevent water hammer.
• Process Equipment (Heat Exchangers, Reboilers, Coils): Used to remove condensate quickly from heat transfer surfaces to maximise efficiency. These applications often involve modulating steam pressures and varying condensate loads.
• Jacket Tracing or Steam Tracing: Used to maintain the temperature of product lines. These involve small, continuous condensate loads, often with superheat.

2. Calculate the Steam Load Required for the Application

Accurate calculation of the condensate load is the cornerstone of proper sizing. Three key load scenarios must be quantified:

• a. Maximum Condensate Load: The highest amount of condensate the trap will ever need to handle under normal running conditions (e.g., maximum product flow, full steam pressure). This is the base for selecting the nominal size.
• b. Minimum Condensate Load: The lowest amount of condensate the trap will encounter. This is important to ensure the trap operates efficiently without losing steam (steam loss) or rapid cycling (short service life).
• c. Start-up Condensate Load: The significantly higher volume of cold condensate and air that must be discharged when the system is brought online from a cold state. While some traps are designed to handle this, specialised air venting is often required. The trap must at least be able to handle the start-up load without flooding the equipment.

3. Apply the Minimum Recommended Safety Factor

To account for potential variations in operating pressure, temporary overload conditions, and manufacturing tolerances, a safety factor (SF) is always applied to the maximum running condensate load. This safety factor is only considered if the sizing is based on the maximum running load.

The required trap capacity must be equal to or greater than (Maximum Condensate Load x Safety Factor).

4. Understand the Operating Conditions and Trap Specification

The selected trap’s physical and performance specifications must align perfectly with the system environment:

• a. Pressure:
  Maximum Operating Pressure (MOP): The highest pressure the trap will encounter.
  Maximum Allowable Pressure (PMA): The maximum pressure the body can safely withstand.
  Differential Pressure (ΔP): The difference between the steam pressure upstream of the trap and the back pressure in the condensate return line. Trap capacity is heavily dependent on the ΔP.
• b. Temperature: The trap must be rated for the maximum saturation temperature of the steam and ambient conditions.
• c. Material of Construction (MOC): The body and internals (e.g., stainless steel, cast iron, cast steel) must be compatible with the fluid (steam/condensate) and rated for the required pressure/temperature envelope.
• d. Orientation: The physical installation requirement (horizontal or vertical pipe run) must match the trap’s design, particularly for float traps.

By systematically following these steps, engineers can ensure the required trap capacity meets or exceeds the factored load, resulting in a safely and efficiently managed steam system.