Steam Trap Explained Totally

There are many types of steam traps each having its unique characteristics and system benefits. Deciding which type of trap to use is sometimes confusing and, in many cases, more than one type can be used. The following is intended to point out system conditions that may be encountered and the characteristics of each type of trap.

Steam Trap Functional Requirements

Within steam systems, important considerations must be taken into account. These considerations include venting of air during start-up; variations of system pressures and condensing loads; operating pressure and system load; continuous or intermittent operation of system; usage of dry or wet return lines; and overall probability of water hammer.

Air Venting

At start-up all steam piping, coils, drums, tracer lines, or steam spaces contain air. This air must be vented before steam can enter. Usually the steam trap must be capable of venting the air during this start-up period. A steam heating system will cycle many times during a day. Fast venting of air is necessary to obtain fast distribution of steam for good heat balance. A steam line used in process may only be shut down once a year for repair and venting of air may not be a major concern.

Modulating Loads

When a modulating steam regulator is used, such as on a heat exchanger, to maintain a constant temperature over a wide range of flow rates and varying inlet temperatures, the condensate load and differential pressure across the trap will change. When the condensate load varies, the steam trap must be capable of handling a wide range of conditions at constantly changing differential pressures across the trap.

Differential Pressure Across Trap

When a trap drains into a dry gravity return line, the pressure at the trap discharge is normally at O psig. When a trap drains into a wet return line or if the trap must lift condensate to an overhead return line, there will normally be a positive pressure at the trap discharge. To assure condensate drainage, there must be a positive differential pressure across the trap under all load conditions.

Water Hammer

When a trap drains high temperature condensate into a wet return, flashing may occur. When the high temperature condensate at saturation temperature discharges into a lower pressure area, this flashing causes steam pockets to occur in the piping, and when the latent heat in the steam pocket is released, the pocket implodes causing water hammer. Floats and bellows can be damaged by water hammer conditions.

When traps drain into wet return lines, a check valve should be installed after the trap to prevent backflow. The check valve also reduces shock forces transmitted to the trap due to water hammer. Where possible, wet returns should be avoided.

Application

The design of the equipment being drained is an important element in the selection of the trap. Some equipment will permit the condensate to back up. When this occurs the steam and condensate will mix and create water hammer ahead of the trap. A shell and tube heat exchanger has tube supports in the shell. If condensate backs up in the heat exchanger shell, steam flowing around the tube supports mixes into the condensate and causes steam pockets to occur in the condensate. When these steam pockets give up their latent heat, they implode and water hammer occurs, the water hammer often damages the heat exchanger tube bundle. The trap selection for these types of conditions must completely drain condensate at saturation temperature under all load conditions.

Steam mains should be trapped to remove all condensate at saturation temperature. When condensate backs up in a steam main, steam flow through the condensate can cause water hammer. This is most likely to occur at expansion loops and near elbows in the steam main.

Applications such as tracer lines or vertical unit heaters do not mix steam and condensate. In a tracer line, as the steam condenses, it flows to the end of the tracer line. Back up of condensate ahead of the trap does not cause water hammer. Steam does not pass through condensate.

Vertical unit heaters normally have a steam manifold across the top. As the steam condenses in the vertical tubes, it drains into a bottom condensate manifold. Because steam does not pass through the condensate, water hammer should not occur.

TRAP OPERATION

A review of the trap operating principle will show how various types of traps meet the different system characteristics.

Float & Thermostatic Traps

During start-up the thermostatic vent is open to allow free passage of air.

The thermostatic vent will close at near saturation temperature. The balanced design will allow venting of noncondensables that collect in the float chamber, when operating at design pressure.

The condensate port is normally closed during
no load. As condensate enters the float chamber, the seat opens to provide drainage equal
to the condensing rate.

Primary Applications

Heating main drip traps.
Shell & tube heat exchangers.
Tank heaters with modulating temperature
regulators.
Unit heaters requiring fast venting.
Steam humidifiers.
Air blast heating coils.
Air pre-heat coils.
Modulating loads.
Fast heating start-up applications.

Advantages

Completely drains condensate at saturation temperature.
Modulates to handle light or heavy loads, continuous discharge equal to condensing load.
Large ports handle high capacities.
Separate thermostatic vent allows fast venting of air during start-up.
Modulating ports provide long life.
Cast iron bodies.

Disadvantages

Float or bellows may be damaged by water hammer.
Primary failure mode is closed.
Does not withstand freezing temperatures.
Pressure limit of 175 psig.

Bucket Traps

The trap body must be manually primed at initial start-up. Under operation the body will remain full of condensate.

During start-up, air is vented through the bleed hole in the top of the bucket into the return line.

Condensate entering the trap will flow around the bucket and drain through the open seat.

As steam flows into the trap it collects in the
top of the bucket. The buoyancy of the steam
raises the bucket and closes the seat.

An optional thermal vent installed in the bucket allows faster air venting during start-up.

Primary Applications

Process main drip traps.
Where condensate is lifted or drains into wet return line.
Drum type roller dryers.
Steam separators.
Siphon type or tilting kettles

Advantages

Completely drains condensate at saturation temperature.
Open bucket will tolerate moderate water hammer.
Available in pressures up to 250 psig.
Normal failure mode is open.
Cast iron bodies.

Disadvantages

Marginal air handling during start-up.
Cycles fully open or closed.
May lose prime during light loads and blow live steam.
Requires manual priming to provide water seal.
Does not withstand freezing temperatures.

Thermostatic Bellows Type Trap

Thermostatic traps are normally open. This
allows fast venting of air during start-up.

Cold condensate during start-up drains through the trap. As temperatures reach 10° to 30° F of saturation, the trap closes.

During operation, thermostatic traps find an equilibrium point to drain condensate approximately 10° to 30°F below saturation at a continuous flow.

Primary Applications

Radiators, convectors, unit heaters.
Cooking kettles.
Sterilizers.
Heating coils.
Tracer lines.
Evaporators

Advantages

Sub-cools condensate usually 10° to 30°F.
Normally open at start-up to provide fast air venting.
Follows steam saturation curve to operate over wide range of conditions.
Brass bodies.
Self draining.
Energy efficient.
Compact size and inexpensive.
Fast response to changing conditions.
Fail open models.

Disadvantages

Water hammer can damage bellows.
Superheat can damage bellows if it exceeds trap temperature rating.
Pressure limit of 125 psig.
Cooling leg required in some applications.

Disc Traps

Thermodisc steam traps provide dependable performance for applications with light to moderate condensate loads. Thermodisc traps are excellent for high pressure drip and steam tracing applications.

Because the disc is the only moving part, the traps are rugged and resistant to damage. However, if the seat and disc require servicing they may be easily replaced without removing the trap body from the piping.

Disc Trap Operation

The disc is pushed off The disc is pushed off the seat by the inlet pressure and is held open by the impact force of the condensate hitting the disc.

As the condensate nears saturation temperature, greater amounts of flash steam will appear. Some of the flash steam escapes to the area above the disc, causing the pressure above the disc to increase, pushing the disc closer to the seat.

When all the condensate is discharged, flash steam enters the seat-disc chamber at high velocity. This high velocity causes a sudden pressure drop at the lower side of the disc and it snaps closed against the seat.

At the instant the disc snaps closed on the seat, the pressure above the disc is approximately equal to the upstream line pressure. The disc is held closed because the pressurized area above the disc is much larger than the inlet area. The pressure above the disc decreases either by steam condensation or by non-condensables being removed via the micro-bleed on the disc. When the pressure is low enough, the disc is pushed off the seat and the process is repeated.

Primary Applications

Steam tracer lines where maximum temperature is required.
Outdoor applications including drips on steam mains.
Drying tables.
Tire mold press and vulcanizing equipment Dry kilns.
Pressing machines.
Rugged applications (superheat & water hammer).

Advantages

Completely drains condensate at saturation temperature.
May be installed vertically, to drain trap body when steam is off, to prevent freezing.
Compact size.
Easily serviced in line, replaceable seat and disc (some models).
All stainless steel.
Will tolerate water hammer and superheat.

Disadvantages

Noise.
Sensitive to dirt, prevents tight closing of disc.
Available in sizes up to 1” only.

Orifice Traps

Primary Applications

Should be limited to constant load continuous operation.

Advantages

No moving parts to wear.

Disadvantages

Does not close against steam.
Small hole easily plugs due to dirt.
Backs up condensate on heavy loads and during start-up.
Does not respond to modulating loads.
Does not vent air when handling condensate—causes slow system start-up and may cause water hammer.
Not easily recognized as trap during energy survey.
Built-in small screen plugs easily.
Discharges condensate at saturation temperature with some live steam, often causes excessive condensate temperatures and cavitation at condensate pumps.
Wastes energy.
Sizing is critical.