Steam Trap Selection Guide
Steam trap selection guide - Float & Thermostatic, Inverted Bucket, Bimetal Thermostatic, Impulse and Thermodynamic Disc steam traps.
A steam trap is a self contained valve which automatically drains the condensate from a steam containing enclosure while remaining tight to live steam, or if necessary, allowing steam to flow at a controlled or adjusted rate. Most steam traps pass non-condensable gases like air while remaining tight to live steam.
A steam trap should
- discharge condensate immediately and completely
- not leak steam
- discharge non-condensable gases like air
There are three primary categories of steam traps:
- mechanical
- thermostatic
- thermodynamic
Popular traps in these categories includes the float steam trap, the thermostatic steam trap, the inverted bucket steam trap and the thermodynamic disc steam trap. Which one is preferred depends on the application.
A steam trap prime missions is to remove condensate and air preventing escape of live steam from the distribution system. The steam trap must adapt to the application. A disc thermodynamic steam trap should never be used together with a modulating heat exchanger - and a floating ball steam trap may be overkill for draining steam pipes.
The table below can be used as a short guide for the selection of steam traps:
Type of Steam Trap | Operation | Normal Failure Mode | |||
---|---|---|---|---|---|
No or little load | Light Load | Normal Load | Heavy Load | ||
Float & Thermostatic | No action | Usually continuous. May cycle. | Usually continuous. May cycle. | Continuous | Closed |
Inverted Bucket | Small dribble, may leak steam with very little condensate load | May dribble | Intermittent | Continuous | Variable |
Bi-metal Thermostatic | No action | Usually dribble action | May blast at high pressures | Continuous | Open |
Impulse | Small dribble | Usually continuous with blast at high loads | Usually continuous with blast at high loads | Continuous | Open |
Thermodynamic Disc | No action | Intermittent | Intermittent | Continuous | Open |
Thermostatic Steam Traps
There are two basic designs for the thermostatic steam trap, a bimetallic and a balanced pressure design. Both designs use the difference in temperature between live steam and condensate or air to control the release of condensate and air from the steam line.
In a thermostatic bimetallic trap it is common that an oil filled element expands when heated to close a valve against a seat. It may be possible to adjust the discharge temperature of the trap - often between 60oC and 100oC.
This makes the thermostatic trap suited to get rid of large quantities of air and cold condensate at the start-up condition. On the other hand the thermostatic trap will have problems to adapt to the variations common in modulating heat exchangers.
- intermittent operation
- fair energy conservation
- fair resistance to wear
- good corrosion resistance
- poor resistance to hydraulic shocks (good for bi-metal traps)
- do not vent air and CO2 at steam temperature
- good ability to vent air at very low pressure
- excellent ability to handle start up air loads
- excellent operation against back pressure
- good resistance to damage from freezing
- good ability to purge system
- excellent performance on very light loads
- delayed responsiveness to slugs of condensate
- fair ability to handle dirt
- small comparative physical size
- poor ability to handle flash steam
- open or closed at mechanical failure depending of the construction
Float Steam Traps
In the float steam trap a valve is connected to a float in such a way that a valve opens when the float rises.
The float steam trap adapts very well to varying conditions as is the best choice for modulating heat exchangers, but the float steam trap is relatively expensive and not very robust against water hammers.
- continuous operation but may cycle at high pressures
- no action at no load, continuous at full load
- good energy conservation
- good resistance to wear
- good corrosion resistance
- poor resistance to hydraulic shocks
- do not vent air and CO2 at steam temperature
- excellent ability to vent air at very low pressure
- excellent ability to handle start up air loads
- excellent operation against back pressure
- poor resistance to damage from freezing
- fair ability to purge system
- excellent performance on very light loads
- immediate responsiveness to slugs of condensate
- poor ability to handle dirt
- large comparative physical size
- poor ability to handle flash steam
- closed at mechanical failure
Inverted Bucket Steam Trap
In an inverted bucket steam trap a leverage system multiplies the force provided by the bucket to open a valve against the pressure. The bucket is open at the bottom and robust against water hammers.
- intermittent operation - condensate drainage is continuous, discharge is intermittent
- small dribble at no load - may leak steam, intermittent at light and normal load, continuous at full load
- good energy conservation
- good resistance to wear
- good corrosion resistance
- excellent resistance to hydraulic shocks
- vents air and CO2 at steam temperature
- poor ability to vent air at very low pressure
- fair ability to handle start up air loads
- excellent operation against back pressure
- good resistance to damage from freezing
- good ability to purge system
- good performance on very light loads - but may leak steam if there almost no condensate load for the bucket to float
- immediate responsiveness to slugs of condensate
- excellent ability to handle dirt
- large comparative physical size
- fair ability to handle flash steam
- open at mechanical failure
Thermodynamic Disc Steam Traps
The thermodynamic trap is an robust steam trap with simple operation. The trap operates by means of the dynamic effect of flash steam as it passes through the trap.
- intermittent operation
- poor energy conservation
- poor resistance to wear
- excellent corrosion resistance
- excellent resistance to hydraulic shocks
- do not vent air and CO2 at steam temperature
- not recommended at low pressure operations
- poor ability to handle start up air loads
- poor operation against back pressure
- good resistance to damage from freezing
- excellent ability to purge system
- poor performance on very light loads
- delayed responsiveness to slugs of condensate
- poor ability to handle dirt
- small comparative physical size
- poor ability to handle flash steam
- open at mechanical failure
Related Topics
-
Design of Control and Safety Valves
Sizing and dimensions of control valves & equipment in steam and condensate systems. -
Steam and Condensate
Design of steam & condensate systems with properties, capacities, sizing of pipe lines, system configuration and more.
Related Documents
-
Condensate Generated in Cold Steam Pipes - SI Units
Huge amounts of condensate are generated when cold steam pipes are heated up must be drained from the pipes. -
Condensate Generated in Cold Steam Pipes - Sizing of Steam Traps
When cold steam pipes are heated up they generate huge amounts of condensate that must be drained away from the pipe through steam traps - in Imperial Units. -
Condensate Pipe Lines - Sizing
Flow and pressure loss in condensate return lines - SI Units. -
Condensate Pumping
High temperatures and danger of impeller cavitation is the major challenge for condensate pumping in steam systems. -
Flash Steam Generation - Imperial Units (psig)
When condensate passes steam traps - flash steam is generated. -
Flash Steam Generation - SI-units
When condensate leaves the steam traps - flash steam is generated. Amount of flash steam generated at different pressures - kN/m2. -
Insulated Steam Pipes - Condensate Generated
Heat loss from steam pipes generates condensate which must be drained from the system - imperial units. -
Insulated Steam Pipes - Condensate Generated (kg/h per 100m)
Heat loss from steam pipes generates condensate which must be drained from the system. -
Properties of Saturated Steam - SI Units
Saturated Steam Table with steam properties as specific volume, density, specific enthalpy and specific entropy. -
Saturated Steam - Properties - Imperial Units
Steam table with sensible, latent and total heat, and specific volume at different gauge pressures and temperatures. -
Sizing Steam Pipes (lb/h)
Steam is a compressible gas where the capacity of a pipe line depends on the size of the pipe and the steam pressure. -
Steam Flash Generation (bar)
The amount of flash steam generated depends on steam pressure and pressure in the condensate lines. -
Steam Heating Processes - Load Calculating
Calculating the amount of steam in non-flow batch and continuous flow heating processes. -
Steam Heating Systems - Design
An introduction to the basic design of steam heating systems. -
Steam Pipes - Installation of Drip Legs
Properly draining steam pipes for condensate. -
Steam Pipes - Sizing
Sizing of steam pipe lines - major and minor loss in steam distribution systems. -
Steam Traps - Back Pressure and Capacity
A back pressure in a condensate systems will reduce steam trap capacity -
Steam Traps - Safety Factors
Selection of steam traps and their safety factors.