Introduction - Why Steam Traps?

Considerations for steam trap selection

Air venting

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At &#;start-up&#;, i.e. the beginning of the process, the heater space is filled with air, which unless displaced, will reduce heat transfer and increase the warm-up time. Start-up times increase and plant efficiency falls. It is preferable to purge air as quickly as possible before it has a chance to mix with the incoming steam. Should the air and steam be mixed together they can only be separated by condensing the steam to leave the air, which must then be vented to a safe place. Separate air vents may be required on larger or more awkward steam spaces, but in most cases air in the system is discharged through the steam traps. Here thermostatic traps have a clear advantage over some types of trap since they are fully open at start-up. Float traps with inbuilt thermostatic air vents are especially useful, while many thermodynamic traps are also quite capable of handling moderate amounts of air. However, the small hole in fixed orifice condensate outlets and the bleed hole in inverted bucket traps both vent air slowly. This could increase production times, warm-up times, and corrosion.

Condensate removal

Having vented the air, the trap must then pass the condensate but not the steam. Leakage of steam at this point is inefficient and uneconomical. The steam trap has to allow condensate to pass whilst trapping the steam in the process. If good heat transfer is critical to the process, then condensate must be discharged immediately and at steam temperature. Waterlogging is one of the main causes of inefficient steam plant as a result of incorrect steam trap selection.

Plant performance

When the basic requirements of removing air and condensate have been considered, attention may be turned to &#;plant performance&#;. Simply put, unless specifically designed to waterlog, for a heat exchanger to operate at its best performance, the steam space must be filled with clean dry steam. The type of steam trap will influence this. For instance, thermostatic traps retain condensate until cooled to below saturation temperature. Should this condensate remain in the steam space, it would reduce the heat transfer area and the heater performance. The discharge of condensate at the lowest possible temperature may seem very attractive, but generally most applications require condensate to be removed from the steam space at steam temperature. This needs a steam trap with different operating properties to the thermostatic type, and this usually means either a mechanical or thermodynamic type trap.

Before choosing a particular steam trap it is necessary to consider the needs of the process. This will usually decide the type of trap required. The way in which the process is connected to the steam and condensate system may then decide the type of trap preferred to do the best job under the circumstances. Once chosen, it is necessary to size the steam trap. This will be determined by the system conditions and such process parameters as:

• Maximum steam and condensate pressures.
• Operating steam and condensate pressures.
• Temperatures and flowrates.
• Whether the process is temperature controlled.

These parameters will be discussed further in subsequent Modules within this Block.

Reliability

Experience has shown that &#;good steam trapping&#; is synonymous with reliability, i.e. optimum performance with the minimum of attention.

Causes of unreliability are often associated with the following:

• Corrosion, due to the condition of the condensate. This can be countered by using particular materials of construction, and good feedwater conditioning.
• Waterhammer, often due to a lift after the steam trap, sometimes overlooked at the design stage and often the cause of unnecessary damage to otherwise reliable steam traps.
• Dirt, accumulating from a system where water treatment compound is carried over from the boiler, or where pipe debris is allowed to interfere with trap operation.

The primary task of a steam trap is the proper removal of condensate and air and this requires a clear understanding of how steam traps operate.

Flash steam

An effect caused by passing hot condensate from a high pressure system to a low pressure system is the naturally occurring phenomenon of flash steam. This can confuse the observer regarding the condition of the steam trap.

Consider the enthalpy of freshly formed condensate at steam pressure and temperature (obtainable from steam tables). For example, at a pressure of 7 bar g, condensate will contain 721 kJ/kg at a temperature of 170.5°C. If this condensate is discharged to atmosphere, it can only exist as water at 100°C, containing 419 kJ/kg of enthalpy of saturated water. The surplus enthalpy content of 721 - 419 i.e. 302 kJ/kg, will boil off a proportion of the water, producing a quantity of steam at atmospheric pressure.

The low pressure steam produced is usually referred to as &#;flash steam&#;. The amount of &#;flash&#; steam released can be calculated as follows:

If the trap were discharging 500 kg/h of condensate at 7 bar g to atmosphere, the amount of flash steam generated would be 500 x 0.134 = 67 kg/h, equivalent to approximately 38 kW of energy loss!

This represents quite a substantial quantity of useful energy, which is all too often lost from the heat balance of the steam and condensate loop, and offers a simple opportunity to increase system efficiency if it can be captured and used.

A check valve is a type of valve that allows fluids to flow in one direction but closes automatically to prevent flow in the opposite direction (backflow). Check valves are used in a wide variety of locations, but the focus of the discussion in this tutorial will be the installation of check valves at the steam trap outlet side.

Questions are often asked about the need and purpose of check valves, such as:

• Is it always necessary to install a check valve downstream of a steam trap?
• Does installation of a check valve prevent water hammer?

Let&#;s discuss these two points.

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Is it always necessary to install a check valve downstream of a steam trap?

Check valves are necessary if there is a risk of condensate backflow. For example, when a trap discharges into a common condensate collection line, there is the potential risk of backflow from condensate discharged from other traps, so as a rule a check valve should be installed. Preventing this backflow is important because it can not only diminish process heating efficiency, but can also damage steam traps. In contrast, when there is a single downward sloping pipe that is not submerged at any point, there is almost no possibility of backflow, so a check valve is not necessary.

Single Dedicated Line

If the trap is gravity drained through a downward sloping dedicated line that releases to atmosphere without being submerged at any point, there is usually no risk of backflow, so there is no need to install a check valve.

Note: Depending on piping configuration after the steam trap, back flow can also occur in open lines if equipment goes to vacuum or if there is positive pressure from a too small vent or pipe rise which can cause unwanted return line pressure. In such cases, installing a check valve may help prevent back flow.

Common Collection Line

If the trap outlet piping is connected to a common condensate collection line, condensate discharged from equipment in operation may backflow into equipment that is out of service unless there is a check valve installed at the trap's outlet.

Illustration C: No check valves at trap outlets

If a check valve is installed, even if the trap outlet piping is connected to a common collection line, the condensate discharged from equipment in operation will not backflow into equipment that is out of service.

Illustration D: Traps with check valves

Does installation of a check valve prevent water hammer?

There are various mechanisms that generate water hammer. A principal cause of water hammer in condensate recovery lines is condensate flowing back down in vertical rises. The installation of a check valve at each of these locations is very effective in preventing water hammer due to backflow.

Water hammer caused by condensate backflow in vertical rises of condensate recovery lines

Illustration E: No check valve on vertical risers

If the condensate discharge piping on a pump with intermittent operation (e.g. TLV PowerTrap® series or motorized pump with ON-OFF control) has a long horizontal run followed by a vertical rise, any condensate that falls back down the vertical rise becomes backflow that may collide with newly discharged condensate, resulting in water hammer. Similarly, in situations where a PowerTrap® discharges high temperature condensate, the combination of flash steam and backflow could be another possible cause for water hammer.

In such cases, water hammer may be prevented by installing a check valve at critical locations within the system (e.g. the beginning of a vertical rise).

Illustration F: Check valve on vertical riser

Additional Info

Check valves can also help prevent water hammer caused by a pulsating flow of low temperature condensate in condensate transport piping.

For more information on water hammer, please visit the water hammer tutorial.

Check Valve Restrictions

When high pressure hot condensate is discharged through a steam trap to lower pressure, flash steam is generated. If this flash steam then flows into a return line that contains sub-cooled condensate at a much lower temperature, an instant collapse (condensing) of the flash steam will occur as it gives off its latent heat to the condensate, and water hammer may result. Installation of a check valve is generally not effective in this situation.

Check valves prevent backflow, but not back pressure. It is not possible to discharge low pressure condensate into a higher pressure line. Even if a check valve is installed after a steam trap, condensate will not flow if the pressure upstream of the trap is lower than the downstream (return) side.

Additionally, it should be noted that if a check valve is installed at the outlet of a trap operating under an extremely large operating differential pressure, the check valve itself becomes a point of resistance (i.e., the check valve has a pressure drop as well), which means that it is necessary to calculate pressure drop very carefully.

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