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Steam Trap

A steam trap is an automatic device that discharges condensate generated during processes and in steam transport lines while preventing any steam from escaping ('trapping' the steam). 

Need of Steam Trap 

Steam is formed when water vaporizes to form a gas. In order for the vaporization process to occur, the water molecules must be given enough energy that the bonds between the molecules (hydrogen bonds, etc.) break. This energy given to convert a liquid into a gas is called 'latent heat'.

Steam-based heating processes use latent heat and transfer it to a given product. When the work is done (i.e. steam has given up its latent heat), steam condenses and becomes condensate. In other words, condensate does not have the ability to do the work that steam does. Heating efficiency will therefore suffer if condensate is not removed as rapidly as possible, whether in steam transport piping or in a heat exchanger.

So, in order to improve the quality of steam and heating efficiency of steam, steam trap is must.

Earlier Period

During initial period condensate is drained manually instead of steam trap, the valve will be adjusted the manually to drained the condensate.

However the manual valve is not in the position to respond for the variation of parameters of Steam/Condensate to discharge the condensate accordingly.

So, hence throttling of manual valve for draining the condensate is not viable.

Later on steam trap has been introduced to discharge the condensate properly with response of steam/condensate condition (Load/Pressure/Temperature/ETC…….)

Importance of Steam Traps

Steam Traps are key to system optimization, major function of the steam trap to keep the steam dry and improve the heat transfer efficiency of steam.

Major Function of Steam Traps

• Discharge the condensate
• Prevent Steam Loss
• Discharge the incondensable gases

Discharge the Condensate

Discharging of a condensate whenever occurs in the system it will help to improve :-

To keep quality of steam dry throughout the steam transport

Improve the heat transfer efficiency of a steam heating equipment

Prevent the Steam Loss : -

Steam Trap will prevent the steam loss, it will ensure the optimization of steam utilization

Discharge the Incondensable Gases :

In condensable gases in the steam system will act as a barrierfor a heat exchanger, in turn it will reduce the heat transfer efficiency and also corrodes the pipe line which in turn start leaks.

With help of steam traps the incondensable gases will be discharged automatically.

Considering the all above features, no steam system is complete without  a crucial component “ Steam Trap”

Steam Trap is a most important equipment in the condensate loop, since it connect steam utilization & condensate return equipment.

Consideration of Steam Trap Selection:

Air venting

During ‘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.

Due to the air in the system it will reduce the heat transfer efficiency, so it has to be discharged immediately

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 in heating equipment

Condensate removal

Once air has been vented the trap has to be discharge the condensate without letting steam. Leakage of steam through trap, it seems the trap is inefficient or uneconomical

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:

Application where the traps are to be utilized (Steam Header/Heating Equipment/Tracing Lines, etc….)

Upstream & Downstream Pressure

Condensate Load across the traps

Reliability

Good Performance of a steam traps will improve the optimization of steam in the system. .

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 feed water conditioning.

Water hammer, 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 172 kcal/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 100 Kcal/kg of enthalpy of saturated water. The surplus enthalpy content of 172 - 100 i.e. 72 kcal//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:

Flash Steam Produced = Sensible Heat before the Trap (HP) – Sensible Heat after the trap (LP) / Latent heat at Lower Pressure

 = 72 kcal/kg / 540 Kcal/kg = 0.134 Kg of steam per kg of condensate or 13.4 %

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.

Types of steam traps:

Various types of Traps has been developed to discharge the condensate automatically with help of the following three basic mechanism, an all three are classified by International Standard ISO 6704:1982.

Thermodynamic (Dynamic)

Mechanical (Density)

Thermostatic (Temperature)

Thermodynamic (operated by changes in fluid dynamics) 

The    trap    operates    by    means    of    the    change    inDynamic   effect   of   condensate   &   flash   steam   as   it passes    through    the    trap.    These    types    of    trap specially    design    to    discharge    condensate    from Steam header immediateafter it forms.  Main  aim  in this  type  of  operation  is  to  drain  out  the  condensate immediately.   These   traps   are   very   much   compact, lightweight,   have   very   simple   mode   of   operation. These    have    a    blasting    operation    and    discharge condensate at almost steam temperature

Working Principle:-

Initially condensate at steam pressure raises the disc and goes out through outlet ports.   As it expose to comparatively    low    pressure,    it    flash,    High    flash steam  velocity  create  low  pressure  zone  at  bottom  of  disc,  some  part  of  flash  steam accumulate  above  disc  and  forced  to  disc  down  and  weight  of  disc  acts  together  to shut the disc  over seat,  and  trap stop discharge condensate.  As  the flash steam  above the  disc condense,  by  means  of  latent  heat  dissipate  to  ambient  through  trap  cover, disc again raises  by condensate load the  cycle repeat

Mechanical (Operated by changes in Fluid Density) :-

The   trap   operates   by   sensing   of   the   change   in   density   of   condensate   and   steam. These   types   of   traps   especially   design   to   discharge   condensate   from   Steam heating   equipment. Density   of   water   is   much   denser   than   steam.   There   is   one moving   part   that   floats   on   water   but   not   in   steam.   There   are   two   type   of   traps basically used in industries.

Ball Float Trap:

Discharge     condensate     continuously. One SS circular ball float fix inside the circular   trap   cover.   At   start-up   float rest    at    bottom    and    trap    in    close condition. When   condensate   reaching to the trap, the ball float rise (floating over condensate),lifting the valve off its seat        and        releasing        condensate. Continuous condensate flow keep trap continuously     open.     As     condensate stop,   float   will   rest   at   bottom,   trap close once again.  A water seal not allow passing    live    steam    at    outlet.    These types of traps have Steam Lock Release (SLR), Automatic   Thermostatic Air Vent (TV) facility.

Inverted Bucket Trap:

Discharge condensate intermittently.  One Metallic Bucket upside down fix inside the trap cover.  At start-up   float   rest   at   bottom   and   trap   in   open condition,   trap   discharge condensate.   When   live Steam   reaching   to   the   trap low   density   steam cause bucket float and rise, valve inside trap close the   outlet   orifice.   Once   again   when   condensate reach  at  the  trap  it  force  bucket  to  sink,  at  that time  live  steam  inside  the  bucket  release  through hole  top  of  the  bucket  and  bucket  sink  down,  trap open and      discharge  condensate. Operation repeats.

Thermostatic (operated by changes in fluid Temperature):  –

The trap   operates by sensing of   the change in temperature of   condensate.   These types  of  trap  specially  design  to  discharge  condensate  from  Steam  Tracing  Line. This  type  of  trap  discharge  condensate  at  20  °C  lower  temp  than  saturation  temp  at operating   pressure.   One   special   type   of   fluid   having   almost   parallel   pressure   Vs evaporation  temp  curve  with  water  (up  to a  certain  pressure)  use  in  this  trap.  These traps are very much compact, lightweight.

Working Principal:

Fluid use in Thermostatic  Steam  trap having 20?C less  evaporation  temp  than  water  /  condensate  at same  pressure.  This fluid keeps in a metallic bellow capsule over orifice. On   start-up   fluid   remain   in liquid state in capsule and capsule is in contracted position, trap is open and discharge condensate. When  condensate  at  steam  temperature  reach  at orifice,     fluid     inside     capsule     vaporizes,     vapor pressure  increase,  capsule  expands  and  closes  the trap.  Once  again,  when  condensate  near  capsule cool  up  to  20  °C  less  temp  than  saturation  temp, fluid inside capsule condense and capsule contract and trap discharge condensate.