Functioning of Different Types of Steam Traps

Steam trap selection is very important in any steam/condensate plants. This blog provides an introduction to various types and functioning of different types of steam traps.

What are the different types of steam traps?

Steam traps are divided into three main categories. They are;

• Mechanical Steam Trap
• Thermostatic Steam Trap
• Thermodynamic (Kinetic) Steam Trap

Some steam trap professionals tend describe steam trap types in terms of their properties such as “mechanical density,” “thermostatic/temperature” or “kinetic energy.” To the uninitiated, it is best understood to describe traps in terms of their generic operation modes, such as “continuous flow” and “intermittent flow”.

Continuous flow traps will, to one degree or another, continuously discharge condensate. These are float, thermostatic and fixed orifice traps. The thermostatic trap is a hybrid. It can be considered either a continuous flow or an intermittent flow, depending on the condensate load. Under heavy condensate load or at start-up, it will tend to have a continuous discharge.

Intermittent traps will cycle open and closed. They have a pattern of hold-discharge-hold. These traps are the thermodynamic, inverted bucket and bimetallic.

Explain the functioning of a Mechanical Steam Trap.

Mechanical steam traps work on the principle of differentiating between the density of steam and condensate. For example, in one type of mechanical trap, a float (the valve) rises with the level of the condensate to open; yet in the presence of steam only, does not become buoyant, but seats securely over the orifice to close.

In other words Mechanical trap is designed to mechanically sense the phase difference between condensate and steam.

Mechanical traps are buoyancy operated based on the difference in density between steam and condensate. These traps discharge condensate close to the saturated steam temperature.

There are three types of mechanical traps: inverted bucket, open bucket and float traps.

Explain the functioning of an Inverted Bucket Steam Trap.

The most common type of trap is an inverted bucket trap shown on the left. Instead of a float rising, we have an inverted bucket. Inverted bucket traps have a “bucket” that rises or falls as steam and/or condensate enters the trap body. Opposite to the float, when the bucket is in the raised position, the trap is closed. When the bucket is down, the trap is open to drain the condensate. When steam is in the body, the bucket rises closing a valve. As condensate enters, the bucket sinks down, opening a valve and allowing the condensate to drain.

Steam and water enter from the bottom of the trap at “E” into the inverted bucket. Being less dense than condensate, initially the steam will push the inverted bucket “A” up and it causes the bucket to rise, closing the trap.

Condensate entering the trap will flow around the bucket and when the trap fills with water, the inverted bucket “A” will drop and the water will be blown out into a condensate line at “B”. 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 i.e., the hole at “B” closes. The cycle repeats itself.

We still have to remove air. For that, the bucket has a small vent “C” at the top. Air (and very small amounts of steam) escape through this vent and leave the trap. It doesn’t remove air rapidly, which isn’t a problem during normal operation. But on start-up, we have large amounts of air we would like to vent quickly. For this reason, some manufacturers have added another vent hole for start-up.

This means that we will trap less steam. An optional thermal vent installed in the bucket allows faster air venting during start-up. This feature should be specified where frequent start-up occurs. To prevent too much steam from passing through the holes, the manufacturers have placed a bimetallic disc near the additional vent hole. During start-up, the air is vented rapidly through the two vent holes, but when steam hits the bimetallic disc, the heat of the steam warps the disc until it closes off the additional vent hole. This is called a Bucket Trap with Optional Thermal Vent.

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.

As the steam condenses, the bucket falls, opening the trap to drain the condensate.

There are two ways to prime this type of trap. One is to insert a removable plug in the trap. The other, and most likely, is to let the warm-up load do it naturally.

If the trap fails, it probably fails with the bucket down and in the open position. Once again, imagine you have your straw. You can get enough pressure differential to lock the trap shut, even though gravity is trying to make the bucket fall. Since the trap rises and falls, its operation is cyclical and it is suitable for steady loads.

Usually, when this trap fails, it fails open. Either the bucket loses its prime and sinks or impurities in the system may prevent the valve from closing.

Advantages:

• Since this is not a hollow float but an open one, the possibility of water hammer damage is slight. For this trap to properly work, we need a small amount of condensate in the bottom, otherwise the steam may escape from under the bucket without lifting it. Inverted bucket traps are ideally suited for water hammer conditions completely drains condensate at saturation temperature.
• Open bucket will tolerate moderate water hammer
• Available in pressures up to 900 psig
• Normal failure mode is open
• Cast iron, ductile iron or cast steel

Disadvantages:

• Marginal air handling during start-up
• Cycles full open or closed
• May lose prime during light loads and blow live steam
• Requires manual priming to provide water seal
• Since this is a trap that needs priming, not all of the condensate will drain. Therefore, make sure this trap is not exposed to freezing conditions, or it may be damaged. It does not withstand freezing. Proper insulation may prevent freezing in low temperature climates to some extent.

Primary Applications

1. Process main drip traps
2. Where condensate is lifted or drains into wet return line
3. Drum type roller dryers
4. Steam separators
5. Syphon type or tilting kettles

Explain the functioning of an Open Bucket Steam Trap.

This inverted bucket trap differs from a bucket trap. The open bucket trap is not as popular as the inverted bucket trap, so we will not discuss it. Just realize they are two different traps, and they operate differently. To make matters more confusing, many people refer to an inverted bucket trap as a bucket trap. Nonetheless, if you can imagine it and go through the same analysis as we have done so far, you will be able to come up with its characteristics.

Explain the functioning of a float and thermostatic Steam Trap.

Float and thermostatic traps consist of a ball float and a thermostatic bellows element. As condensate flows through the body, the float rises or falls, opening the valve according to the flow rate. The thermostatic element discharges air from the steam lines. They are good in heavy, and light loads and on high and low pressure, but are not recommended where water hammer is a possibility.

When these traps fail, they usually fail closed. However, the ball float may become damaged and sink down, failing in the open position. The thermostatic element may also fail and cause a “fail open” condition.

Explain the functioning of a float style Steam Trap.

The float style steam trap has a float connected to a lever arm. On the lever arm is a pin. As condensate enters the trap, the float rises, pulling the pin from the seat and thus allowing the condensate to drain. When there is very little or no condensate present, gravity pulls the float down, keeping the trap closed.

In this position, there is little chance for air to escape, so to remove air, a thermostatic element is usually added to this particular style of traps. However, in this case, its only function is to remove air, not the condensate. The element is located above the waterline in the trap so that the water does not block its removal path. Now the trap is known as a float and thermostatic (F&T) trap.

These traps come with a seat pressure rating. Don’t confuse seat pressure rating with the pressure rating of the trap body. They are two completely different things.

The easiest way to explain this is to imagine the float is down and the pin is in the seat. Now imagine yourself on the other side of the seat. You have a straw connected to the seat. You create a differential pressure on the straw (you suck on the straw.). Can you suck hard enough on the straw to hold the pin in place?

The seat pressure rating is a measurement of, in this example, how hard you need to suck on the straw to keep the pin in the seat, even though the water is rising and trying to bring the float up with it. Now the water level is above the float and the pin is still in the seat.

Obviously, there is not going to be someone sucking on a straw to keep the pin in place. However, there may be a large enough difference in pressure from the inside of the trap to the outside to lock it shut in much the same way as in our straw example.

We need to ensure that the buoyant force of the ball is greater than the force locking the trap shut. Since force is pressure times area (F = p x a), if we decrease pressure of the steam or the area of the seat, we decrease the force necessary to lock the trap shut.

We’re not going to change the pressure of the steam, but we can change the seat area (size). That will change our seat pressure rating. Remember, that will also change our trap capacity.

However there is one caution on this seat pressure-rating concept. It is not unusual to control pressure by using a temperature-sensing device. If we need more heat, a pilot increases the pressure. But by increasing the pressure, we may lock the trap shut.

This can happen with a new heat exchanger. As the heat exchanger becomes fouled, less heat is transferred across its surface. The temperature sensor detects this and increases the steam pressure. If we are not careful, the pressure differential may become enough to lock the trap shut. Next, the heat exchanger floods and there is even less heat transferred across its surface.

Since we are using a float, temperature has no effect on the float rising and falling. Therefore, we can drain condensate at saturation temperature without having to subcool it. Also, since the float operates based on condensate level, the trap must be plumb and level.

Along that same line of thinking, the rate of condensate drainage depends on the float level, so this trap modulates based on load. Therefore, a good application for this trap would be one where the load varies, such as heating water for a shower room.

Since the float is hollow, water hammer may damage the float. A damaged float probably causes the trap to fail closed.

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
• Limited to 175 psig

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
• Applications that require fast heating at start-up

Explain the functioning of an Inverted Open Float Steam Trap.

In an Inverted open float steam trap the outlet valve is operated by means of the inverted bucket. Normally the bucket or float hangs downward with the valve in the open position and condensate entering the trap flows under the rim of the bucket and then out through the discharge valve. When steam enters the trap, it blows into the bucket and causes it to float thus shutting the outlet valve. When the steam in the bucket condenses, the bucket sinks and once again opens the valve. Any air which enters the trap, escapes through a small vent hole in the top of the inverted bucket.

Explain the functioning of a Thermostatic Steam Trap.

Thermostatic steam traps are temperature-sensing traps that utilize the temperature differences between the condensate and steam to provide the closing and opening forces on the valve. The thermostatic steam traps operate by sensing the temperature of condensate. As steam condenses, the condensate so formed is at steam temperature, but as it flows to the steam trap, it loses temperature. When the temperature has dropped to a specified value below the steam temperature, the thermostatic trap will open to release the condensate.

Thermostatic traps consist of a bimetallic or bellows element that distinguishes between steam and subcooled condensate – opening a valve when condensate is present. The bimetallic thermostatic trap utilizes a metal element with the proper coefficient expansion for the application.

The operation of a thermostatic trap is simple. The key component of the thermostatic trap is the thermostatic element inside. The element is generally filled with an alcohol-water mixture that will boil at a lower temperature than the temperature of saturated steam. Therefore, as steam reaches the element, the alcohol-water mixture boils.

As it boils, the bellows element that contains the mixture quickly expands. As it expands, it drives a pin into a seat to prevent steam from leaving before it gives up its latent heat.

To open, the element must cool down until the mixture condenses. Then the bellows will contract, pulling the pin from its seat and opening the trap to drain the condensate. The condensate must cool down below saturation temperature to open. Therefore, you have to provide a way for the condensate to cool in addition to an area for it to cool. This place is known as a cooling leg. It’s usually a length of pipe long enough to meet the stated requirements. You need to ensure that the cooling leg is not accidentally insulated. If it is, subcooling cannot take effect and the trap will not open to drain condensate.

Another characteristic to note here is that this particular type of trap is pressure independent. By that, we mean it follows the saturation curve of steam. To explain it another way, the higher the pressure at which the process operates, the higher the temperature of the steam.

Along the same reasoning, the higher the pressure, the higher the temperature required to boil the alcohol-water mixture. This is not to say there is no limit to pressure. The trap body still has a pressure rating that must be followed.

Other things we can deduce about this type of trap is that the element is probably made of a thin material. Therefore, it is susceptible to water hammer. So if water hammer is a problem for your facility, you want to ensure that the thermostatic element has some type of protective case.

You may also opt for a thermostatic element that is filled with a heat-expanding solid, such as wax, instead of the liquid mixture. But if you do, remember that it will not follow the saturation curve, and it will be much slower acting.

Another thing we can deduce is that this type of trap quickly vents air.

Also, it is important to note that when this trap fails, it probably fails open from element damage or dirt. Therefore, you need to know how important it is to your system that when a trap fails, it fails a certain way. The other thing is that it will most likely fail a certain way, if it fails. But, depending on the circumstances following the failure, it may fail in a different position than what you anticipate.

A thermostatic trap could work well with tracer lines and heating equipment.

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

Cold condensate during start-up drains through the trap. As temperatures reach 100 to 300 F. of saturation the trap closes.

During operation thermostatic traps find an equilibrium point to drain condensate approximately 100 to 300 F. below saturation at a continuous flow.

There are three types of thermostatic traps: bellows, bimetallic and expansion traps.

Applications:

These traps are typically used in high pressure applications and where some condensate back-up is allowable.

• Radiators, convectors, unit heaters
• Cooking kettles
• Sterilizers
• Heating coils
• Tracer lines
• Evaporators
• NOTE: A solid fill expansion element thermostatic trap should be used where water hammer (cavitation) may occur.

Explain the functioning Thermostatic Bellows Type Steam Trap.

Thermostatic traps have, as the main operating element, a metallic corrugated bellows that is filled with an alcohol mixture that has a boiling point lower than that of water. The bellows will contract when in contact with condensate and expand when steam is present. Should a heavy condensate load occur, such as in start-up, the bellows will remain in a contracted state, allowing condensate to flow continuously? As steam builds up, the bellows will close. Therefore, there will be moments when this trap will act as a “continuous flow” type while at other times it will act intermittently as it opens and closes to condensate and steam, or it may remain totally closed.

These traps adjust automatically to variations of steam pressure but may be damaged in the presence of water hammer. They can fail open should the bellows become damaged or due to particulates in the valve hole, preventing adequate closing. There can be times when the tray becomes plugged and will fail closed.

The kinetic traps operate based on the different flow characteristics of steam and condensate. The three types of kinetic traps are thermodynamic or disk, impulse or piston and orifice. The thermodynamic or disk trap consists of one moving part, a disk that is lifted off its seat to open the discharge port. These traps are best suited for high-pressure steam systems. Impulse or piston traps open and close the valve port based on pressure. These traps are susceptible to sticking or clogging based on the small discharge orifice. Orifice traps have no moving parts and continuously pass condensate based on the density differences between steam and condensate. These traps operate best under steady pressure and load conditions such as is supplied by steam mains.

Advantages:

• Sub cools condensate usually 100 to 300 F
• Normally open at start-up to provide fast air venting
• Follows steam saturation curve to operate over wide range of conditions
• Brass or cast steel bodies
• Self draining Energy efficient
• Small and inexpensive
• Fast response to changing conditions
• Fail open or fail closed models

Disadvantages:

• Water hammer can damage bellows
• Super heat can damage bellows if it exceeds trap temperature rating
• Pressure limit is 300 psig
• Requires cooling leg to prevent backing up condensate in applications where this may cause water hammer.

Explain the functioning Bimetallic Thermostatic Bellows Type Steam Trap.

The bimetallic steam traps can conserve energy by discharging sub-cooled condensate in those applications, which can utilize sensible heat. They are the most robust of all the thermostatic steam traps, able to withstand water hammer and corrosive condensate.

Range: DN15 to DN40;

Pressure up to 45 bar;

Material : Cast steel, alloy steel and stainless steel;

Capacities : up to 9 000 kg/h.

Advantages:

• Sub cools condensate
• Energy efficient (will not pass live steam)
• Adjustable temperature range of condensate discharge (some models)
• Normally open at start-up for fast air venting
• Modulating action (provides long life)
• Cast steel or forged steel
• Rugged design withstands water hammer
• Not affected by super heat
• Available in pressures to 2850 psig
• Completely drains condensate when system is off, may be used outdoors in freezing weather conditions

Disadvantages:

• Requires cooling leg to prevent backing up condensate in applications where backing up condensate may cause water hammer.

Applications:

• Tracer lines Unit heaters
• Process
• Greenhouse coils
• Super heat steam lines
• Drip traps on steam lines

Explain the functioning Thermodynamic Steam Trap.

Thermodynamic steam traps operate on the principle of difference between the flow of steam over a surface compared to the flow of condensate. Steam flowing over a surface creates a low-pressure area, this phenomenon being used to move a valve towards the seat and eventual closure.

Thermodynamic traps are phase sensing traps that discriminate between condensate and steam. There are three types of thermodynamic traps: disc, piston and lever traps.

This trap is a good general type trap where steam pressures remain constant. It can handle superheat and water hammer but is not recommended for process, since it has a tendency to air-bind and does not handle pressure fluctuations well.

A thermodynamic trap usually fails open. There are other conditions that may indicate steam wastage, such as “motor boating,” in which the disc begins to wear and fluctuates rapidly, allowing steam to leak through.

Explain the functioning Thermodisc Steam Trap.

Another very popular type of trap is the thermodisc trap, a rugged, simple, and easy-to-maintain trap. The only moving part is a disc. These traps have a disc that rises and falls depending on the variations in pressure between steam and condensate.

When condensate is present, it nudges the disc aside and drains. As condensate builds up it reduces the pressure in the upper chamber and allows the disc to move up for condensate discharge. Because the downstream side of the trap is lower in pressure than the upstream side, some of the condensate will flash into steam as the condensate reaches saturation temperature. Flash steam will escape to the area above the disc.

After the condensate has drained, steam velocity across the trap will increase, causing a sudden drop in pressure under the disc and snapping it closed. Steam will tend to keep the disc down or closed.

The pressure on both sides of the disc will be approximately equal. However, the disc is held closed because the area above the disc being acted on by the steam is larger than the area being acted on below the disc.

As the steam on the downstream side of the trap condenses, pressure will drop. When it drops low enough, the disc can be nudged off its seat and the process repeated. Pressure can also be relieved as air is removed via a micro-bleed path etched into the disc.

Based on how this trap operates, we can see that it is well suited for outdoor, high-pressure, and water-hammer-type applications. Because of its size and the design of the disc, you can see that this type is not as efficient at removing air. Also, the path is small and can easily become clogged. Therefore, a strainer should be used upstream of this trap.

Explain the functioning Orifice Steam Trap.

An orifice trap is rather simple; there are no moving parts, just a plate with a small orifice. Fixed orifice traps contain a set orifice in the trap body and continually discharge condensate. They are said to be self-regulating. As the rate of condensation decreases, the condensate temperature will increase, causing a throttling in the orifice and reducing capacity due to steam flashing on the downstream side. An increased load will decrease flashing and the orifice capacity will become greater.

Knowing just that, we can see that it can only be used in applications that operate continuously and have a constant load. It will not be able to handle air and condensate at the same time, and will allow some steam to pass. It may also back up condensate on start-up if the warm-up load is not considered.

There is the possibility that on light loads these traps will pass live steam. There is also a tendency to waterlog under wide load variations. They can become clogged due to particulate build-up in the orifice and at times impurities can cause an erosion and damage the orifice size, causing a blow-by of steam.

If it is sized for the start-up load, it may be too small for the running load, so sizing is critical. This is a trap that can easily become plugged. It would not be the best choice for an HVAC application or an application with a varying load.