What you need to know about venturi orifice steam traps

"A steam trap is a device used to discharge condensate and non-condensable gases with a negligible consumption or loss of live steam. Most steam traps are nothing more than automatic valves. They open, close or modulate automatically."

The first sentence defines the function of a steam trap. All steam traps must remove condensate and non-condensable gases without losing any live steam. The second sentence explains the function of a steam trap in the simplest way. The keyword to note from this sentence is - most. This is where orifice type steam traps differ. Only mechanical type traps contain a valve. Orifice type traps do not contain any moving parts. So how do they function? 

There are two main types of orifice traps, (1) fixed or plate orifice trap, and (2) venturi orifice traps. Unlike a conventional mechanical steam trap, an orifice steam trap continuously removes condensate from a system. They allow any condensate present in the steam line to pass into the condensate return system as it is formed.  The operation of the trap is based on the difference in density between water and steam. At low pressures, the density of condensate is about 1,000 times greater than that of steam. In the case of a small orifice, the condensate flows with much lower velocity through the opening, than steam. If both media are present, the much denser condensate will stop the steam from passing through. This means that no live steam will leak through the trap. Although fixed and venturi traps work on the same principle, there is one key difference between them. 

A fixed or plate orifice trap consists of a small orifice machined into a plate. The size of the orifice used is determined by the pressure and condensate flow through the trap. As condensate is forced through the orifice it passes from an area of high pressure into a lower pressure region. Water’s capacity to contain energy reduces as pressure reduces and so any excess energy which cannot be contained due to the pressure drop serves to instantly evaporate a proportion of the liquid water. This evaporation is known as flashing, and the instantaneous expansion of the flash steam creates a back pressure which prevents live steam from passing, and protects the orifice from erosion. However, this means that fixed orifice traps can only effectively drain condensate for a specific condensate flow condition, meaning that this type of trap cannot handle loads that are varying. If the amount of condensate which needs to be discharged reduces, the condensate plug at the orifice, created by localized back pressure from the flash expansion, will not be present. This will lead to live steam being lost. Conversely, if the condensate load exceeds the design flow for the orifice plate, then condensate will back up which may affect process temperatures or lead to waterhammer.

Figure 1: Fixed or plate orifice trap set up

If you search for the definition of a steam trap Google will give you the following result:

A venturi orifice trap works on the same principle, but with is one vital difference - It is designed to handle varying loads. Following the orifice there is a cleverly configured staged throat that is individually sized for its application and varying condensate loads. The staged throat is designed to create a variable back pressure at the orifice as the load changes. At lower flow rates the condensate flashes close to the orifice whereas at higher load conditions it flashes further down the stage throat. This restricts the condensate flow through the orifice depending on the load. So venturi traps can self-regulate their capacity across a range suitable for the vast majority of industrial applications.

Figure 2: GEM steam trap set-up

"As long as the GEM steam trap is sized correctly there can be no loss of live steam. This results in the trap being 10-20% more efficient than a mechanical trap."

The GEM steam trap is a venturi orifice type trap. The engineers at Thermal Energy International ensure that our GEM traps are sized correctly; maximizing the energy saved compared to conventional mechanical traps and fixed (or plate) orifice traps. Mechanical traps tend to have much larger orifice sizes, so when failing open, or even partially open, the steam losses can be substantial.  Mechanical traps can often fail closed which can have catastrophic consequences on a site due to the resulting waterhammer.

Key advantages of a venturi orifice trap:

1. No loss of live steam – The trap is sized so there is always a plug of condensate at the orifice. This ensures that no live steam can escape and protects from erosion. 
   
2. No moving parts to wear and replace – The trap is fundamentally a piece of high grade stainless steel with a cleverly designed orifice and staged throat configuration machined into it, so contains no moving parts that can break like mechanical steam traps.
   
3. Require minimal maintenance and eliminate downtime – As the traps contain no moving parts there is no reason for unexpected shut downs or wasted maintenance time used to fix them.
   
4. No air vents required – The GEM trap does not need any air vents as any air in the system can discharge through the orifice at start up. The trap never closes to block air from discharging.
   
5. Reduce waterhammer and increase safety – As the GEM trap does not have parts that can fail closed there can be no back up of condensate resulting in waterhammer.
   
6. 10-20% Energy savings – As long as the GEM steam trap is sized correctly there can be no loss of live steam. This results in the trap being 10-20% more efficient than a mechanical trap. Some mechanical traps use steam within their operation, which is a waste of steam. Mechanical traps can also fail open losing huge amounts of live steam.
   
7. Reduce condensate return line pressure – When mechanical traps fail open whilst connected to a condensate return system the line can become pressurized. This can be critical for some sites as this can affect other processes. As the GEM trap cannot fail open or pass live steam in to the condensate line it can reduce condensate line pressurization and keep it at a constant pressure. 
   

Thermal Energy's GEM steam traps are the most efficient and reliable steam traps on the market.  Our high quality product and service enables us to supply GEM steam traps with a 10-year performance guarantee. See why our customers prefer our GEM steam traps over conventional steam traps.