Thermodynamic Steam Trap

What is a thermodynamic steam trap?

A thermodynamic steam trap is made up of one moving part. This part is a disc made of stainless steel and it serves as a valve. This steam trap operates by use of flash steam dynamic effect. A thermodynamic steam trap is simple in design and is well suited for serving medium and high-pressure applications. This steam trap is small, free from water hammer effects, and it can be installed in any position be it vertical or horizontal. Thermodynamic steam trap manufacturers design the trap with a compact design and versatile for a broad pressure range. Such characteristics have made thermodynamic steam traps more preferred for wide use in tracing, specific light processes, and drip steam applications.

Figure: Thermodynamic steam trap.

How does a thermodynamic steam trap work?

Two main types of thermodynamic steam traps are thermodynamic disc steam traps and thermodynamic impulse steam traps. The disc type trap has a valve that opens and closes as per the force changes that take place on a flat disc valve. For the impulse trap, the piston disc movement is used to control flow. The thermodynamic impulse steam trap can be regulated to either increase or reduce flow. Both of these types of thermodynamic steam traps eject condensate intermittently. 

Working of disc and impulse types of thermodynamic steam trap

Types of Thermodynamic steam trap

There are two types of thermodynamic steam traps which are thermodynamic disc steam trap and thermodynamic impulse steam trap. Between these two traps, the most common one is the disc trap. The impulse trap is not common because it tends to leak steam and it can fail in presence of a small amount of dirt clogging the pilot channel. 

Opening and closing of disc valve in a thermodynamic disc steam trap

When steam moves at high speed under the valve disc, it causes a decrease in pressure under the valve. This makes the disc be forced onto the seat of the valve due to the great pressure in the chamber and the valve is closed. In a thermodynamic disc steam trap, the controlled steam can be live steam or flash steam. When condensate enters the steam trap and changes phase because of pressure reduction it is said to be flash steam. When the condensate load is very small or if the trap design is compromised enough not to guard against steam loss then it is referred to as live steam. Thermodynamic disc steam trap manufacturers tend to use the best design that helps to eliminate or minimize the use of live steam so that the trap can operate by use of steam where possible. Pressure chamber control steam exerts force downwards on the valve disc top surface which is equal to the product of the pressure and area (pressure*area). On the valve disc underside, the controlled steam makes the pressure reduce under the disc due to its high speed (from Bernoulli’s principle increase in speed results to reduction in pressure). Thermodynamic disc steam trap manufacturers design the trap in a way that makes it close condensate near steam temperature. This happens when the accumulated condensate is discharged. Once the closing force magnitude is high enough to overcome the opening force, then the valve closes. 

Working of a thermodynamic disc steam trap

Thermodynamic impulse steam trap 

A thermodynamic impulse steam trap is a steam trap that cannot provide a tight shutoff. As such, its use is limited relative to the thermodynamic disc steam trap which is common in the market. 

How does a thermodynamic impulse steam trap work? 

1. The thermodynamic impulse steam trap is made up of a hollow piston. The piston is associated with a piston disc which works in a tapered piston that acts as a guide. 
2. During the start-up of this trap its main valve seats on its seat. This makes it leave a flow passage via piston and cylinder clearance and hole at the piston top section. 
3. When condensate and airflow are increased they exert force on the piston disc causing the main valve to lift off from its seat giving increased flow. 
4. The other part of the condensate flows via the disc and piston gap through point E and away via the outlet. 
5. While the condensate nears the temperature of the steam part of it flashes to steam as it flows via the gap. 
6. This is bled away via hole at point F but it creates intermediate pressure above the piston. This helps to effectively position the main valve so that it can meet the load. 
7. The trap can be modified by moving point B on the piston relative to the trap seat. However, the trap can be affected by backpressure.     

Thermodynamic disc steam trap 

A thermodynamic disc steam trap has condensate flow controlled by a valve disc that opens and closes against a valve seat. In this trap, the valve disc is detached from other parts of this trap and lies on the top of the seat. The valve seat is made up of two seat rings that are inner ring and the outer ring. The use of the inner ring is to separate the inlet hole of the fluid from the outlet hole. This helps to prevent steam short-circuiting to the outlet. Steam leakage from the pressure chamber over the disc to the exit section is controlled by the outer ring. 

How does a thermodynamic disc steam trap work? 

This type of trap has irregular, cyclical operating characteristics. A valve mechanism comprising of seat rings and a disc is used to open to eject condensate for a few seconds after which it closes for a longer time till a new discharge cycle starts. The action to open and close comes from the difference in force that acts on the bottom and top of the valve disc. The acting forces depend mainly on pressure and kinetic energy variations for general fluids involved that are condensate, air, and steam. During start-up, coming fluids (air, condensate, or steam) exert lifting/opening force at the valve disc bottom side. This causes the valve to lift and open. This allows the condensate to flow. While in the open position, two primary forces acting on the disc valve that is a force due to steam pressure on the disc top section and force due to steam under the disc. The steam that acts to open and close the disc valve is called control steam.

Advantages of thermodynamic steam traps

Advantages of thermodynamics disc steam trap 

• This type of steam trap can work in their whole working range without the need to adjust or change their internal components. 
• Lightweight, compact and simple designs and the ability to accommodate a large amount of condensate enabled by large openings. 
• Operate in superheated and high-pressure steam and they are not damaged by vibration or water hammer. 
• Using ferrous materials such as stainless steel known for high strength and high corrosion resistance. 
• Strong against condensate freezing and they rarely experience freezing once installed with a disc in a vertical orientation as they discharge freely to the atmosphere. 
• The moving part is disc only which makes maintenance of this trap to be very easy as it can be done without removal of trap from its line but rather removing the top cap which is fixed by threads or bolts. 
• Click sound when they open and close. This helps in testing the trap before.

Disadvantages of thermodynamic disc steam trap 

• Closing the disc in this steam trap needs the pressure below the disc to low. This is possible only when the flow speed below the disc valve is high. At a higher speed, higher differential pressure will be required. As such, the thermodynamic disc steam trap does not work well when the differential pressure is very low. 
• Eject large quantities of air during start-up when inlet pressure accumulates slowly. However, if the pressure builds up rapidly, it will cause high-speed air to shut your trap making it air-bind. The same will happen if steam is used. 
• Tends to be noisy which restricts its use in some areas such as in operating theatre or outside the hospital ward. If a thermodynamic disc steam trap must be used, then a diffuser will be needed to help reduce noise during discharge.  
• Design of oversize thermodynamic disc steam trap tends to increase time to complete a cycle which causes increased wear on the trap. 

Advantages of thermodynamic impulse steam trap 

• High capability to handle condensate of their size. 
• These steam traps are capable of venting air without being air bound. 
• They can be used in applications of super-heated and high-pressure steam applications. 
• Can work over a broad range of pressures without the need to change valve size. 

Disadvantages of thermodynamic impulse steam trap 

• Unable to provide tight shut-off and they blow steam even at a very light load. 
• Cannot work on a back pressure exceeding 40% of the pressure at the inlet.
• Thermodynamic impulse steam traps get affected by dirt easily because the clearance between the cylinder and the piston is very small. 
• These steam traps pulsations cause noise, mechanical damage, and water hammer.

Applications of thermodynamic steam trap 

• Drip applications. Thermodynamic steam traps are widely used in removing condensate that forms in steam lines after steam losses its heat energy. These traps are widely used in drip applications due to their air vents in the pipeline which remove air from the piping system. 
• High temperatures or steam mains drainage in tracer discharge. 
• Process application. Thermodynamic steam traps are used in heat transfer processes such as in heat exchangers or radiators to remove both air and condensate. 

Troubleshooting thermodynamic steam trap 

When a trap is cold and no discharge 

• Very high pressure 

• Orifice has been enlarged due to wear. Replace the valve. 
• Reducing valve out of order. Put the valve in order. 
• Malfunctioning pressure gauge. Give the gauge pressure a lower reading. 
• Return line has a very high vacuum. Check the vacuum pressure as recommended. 

• Zero steam or condensate coming to the thermodynamic steam trap 

• Pipeline is clogged. Removed blocking materials. 
• Valve ahead of the steam trap has failed. Replace the valve. 
• Blocked strainer. Remove dirt or materials plugged in the strainer. 

• The internal mechanism of the trap has a fault

• Replace the broken internal component of the thermodynamic steam trap. 

• The trap body is filled with unnecessary materials 

• Check and remove dirt or any materials unnecessary in the trap. 
• Installation of strainer before using the trap. 
• Clean trap strainer if necessary

When a trap is hot and no discharge 

• Condensate is not coming to the trap 

• Thermodynamics steam trap by-pass valve is leaking. Check the valve and repair.
• Syphon pipe is broken. Replace the broken pipe. 
• Water heater vacuum halting drainage. A vacuum breaker needs to be installed between the trap and heat exchanger. 

The thermodynamic steam trap is hot and it is losing steam

• Valve is not seating 

• Valve parts are worn out. Replace the valve.
• Some dirt got stuck in the orifice. Remove the dirt. 

Thermodynamic steam trap experiencing continuous flow 

• The size of the trap is very small

• Use a larger trap or employ more traps in a line. 

Summary 

A thermodynamic steam trap is a team trap that is versatile and compact meant for a wide range of pressure applications. These traps employ simple design and can operate either in vertical or horizontal positions. Such features make thermodynamics steam traps more common for use in different applications such as rip, tracing, and other light process applications. Two types of thermodynamic steam traps is the thermodynamic disc steam trap and the thermodynamic impulse steam trap. Between the two types, the disc type is the one mostly sued because the impulse type tends to leak pilot steam and can fail even when working on a slight amount of dirt. 

In the thermodynamic disc steam trap, condensate flow is controlled by a valve disc that opens and closes against a valve seat. This valve is detached from other parts of the trap to rest on the valve seat. During the startup of a thermodynamic impulse steam trap, a valve resting on the seat leaves flow passage via clearance between cylinder and piston. When the flow of condensate and air is increased the valve lifts resulting in more flow. These thermodynamic steam traps are used in various applications such as tracer discharge, drip drainage, and process applications among others. Thermodynamic steam traps have excellent features which give them several advantages compared to other traps such as the ability to handle the high flow, ability to work in superheated and high pressure, ability to handle air venting, can work over a wide range of pressure among others.