Steam Traps - Introduction

Expired

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 allowing steam to flow at a controlled or adjusted rate. Most steam traps will also pass non-condensable gases while remaining tight to live steam.

Basic Steam Trap Types

Most steam traps used in the chemical process industries fall into one of three basic categories:

Mechanical Traps

Mechanical traps are actuated by a float, which responds to changes in condensate level. Mechanical traps, use the density difference between steam and condensate to detect the presence of condensate. This category includes float-and-thermostatic traps and inverted bucket traps.

Thermostatic Traps

Thermostatic traps are actuated by temperature sensitive devices, responding to changes in condensate temperature. They operate on the principle that saturated process steam is hotter than either its condensate or steam mixed with condensable gas. A thermostatic trap opens its valve to discharge condensate when it detects this lower temperature. This category of trap includes balanced pressure and bi-metal traps as well as liquid expansion thermostatic traps.

Thermodynamic Traps

Thermodynamic traps are actuated by the principles of thermodynamics and fluid dynamics. which use velocity and pressure of flash steam to operate the condensate discharge valve.

Steam Trap Applications

Steam Trap application falls into the following categories:

Drip Traps on steam lines

Steam Trap on Drip Trap
STEAM TRAP - DRIP TRAP
Image courtesy: PROTECTOSEAL

Drip applications are by far the most common application for steam traps. This application refers to removing the condensate that forms in steam lines when steam loses its heat energy due to radiation losses. Traps used in these applications are referred to as drip traps. Generally speaking, traps used for these applications require relatively small condensate capacities and don’t normally need to discharge large amounts of air. (Air removal is the primary function of air vents and process traps located throughout the system.) The most common trap choices for drip applications are thermodynamic for line pressures over 30 PSIG, and float & thermostatic for line pressures up to 30 PSIG. Inverted bucket traps are also commonly used for drip trap applications due to their ability to handle large amounts of dirt and scale often found in this type of application.

Steam Tracing on piping or equipment

Steam tracing refers to using steam to indirectly elevate the temperature of a product using jacketed pipes or tubing filled with steam. A typical application would be wrapping a high viscosity oil pipeline with steam tubing. The steam inside the tubing heats the oil to lower its viscosity, allowing it to flow easily thru the pipeline. Similar to any steam applications, a steam trap must be used on the end of the steam tubing to discharge unwanted condensate. Steam traps used in these applications are referred to as tracer traps. The most common trap choice for tracing applications is the thermostatic type.

Process applications

Steam Trap Process Application
STEAM TRAP - PROCESS APPLICATION
Image courtesy: PROTECTOSEAL

Process trap applications refer to removing condensate and air directly from a specific heat transfer process such as a heat exchanger that could be making hot water or a radiator heating a room. Traps used in these applications are referred to as process traps. Generally speaking, traps used for process applications require larger condensate handling capability and also need to be able to discharge large amounts of air. The most common trap choices for process applications are float & thermostatic traps and thermostatic traps. Both are known for their excellent condensate and air handling capabilities. In contrast, thermodynamic traps and inverted bucket traps, which have poor air handling ability, would normally make a poor choice for process applications.

Steam Trap Sizing

A steam trap must be sized based on condensate load. Once the condensate load is calculated, the trap should be sized using a safety factor. The sizing of condensate lines, downstream sub-headers and headers shall ensure there is no excessive back pressure on the trap.

During start-up conditions, condensate forms at a rapid rate as the piping is at ambient temperature. Safety factors are therefore necessary to take into account the start-up conditions or any other abnormal operating conditions.

Applicable Industry Codes and Standards

  • FCI 69-1 - Pressure Rating Standard for Steam Traps.
  • FCI 85-1 - Standard for Production Testing for Steam Traps.
  • ASME PTC39 - Steam Traps.

Comparison of Steam Traps

TYPES OF STEAM TRAPS ADVANTAGES  DISADVANTAGES RECOMMENDED SERVICE
Float-and thermostatic
  • Continuous condensate discharge.
  • Handlesrapid pressure changes.
  • High noncondensible capacity.
  • Float can be damaged by water hammer.
  • Level or condensate in chamber can freeze, damaging float and body.
  • Some thermostatic air vent designs are susceptible to corrosion
  • Heat exchangers with high and variable heat-transfer rates.
  • When a condensate pump is required Batch processes that require frequent start-up of an air -filled system.
Inverted bucket
  • Rugged
  • Tolerates water hammer without damage
  • Discharges non-condensibles slowly (additional air vent often required).
  • Level of condensate can freeze, damaging the trap body (some models can handle some freezing).
  • Must have water seal to operate, subject to losing prime.
  • Pressure fluctuations and superheated steam can cause loss of water seal (can be prevented with a check valve)
 Continuous operation where noncondensible venting is not critical and-rugged construction is important
Thermodynamic
  • Rugged, withstands corrosion, water hammer, high pressure and superheated steam.
  • Handles wide pressure range compact and simple.
  • Audible operation warns when repair is needed
  • Poor operation with very low-pressure steam or high back-pressure.
  • Requires slow pressure build-up to remove air at start-up to prevent air binding.
  • Noisy operation.
  • Steam mains drips, tracers.
  • Constant-pressure, constant-load applications.
  • Installations subject to ambient conditions below freezing.
Balanced pressure thermostatic
  • Small and light-mass.
  • Maximum discharge of noncondensible start-up.
  • Unlikely to freeze.
  • Some types damaged by water hammer, corrosion and superheated steam.
  • Condensate backs up into the drain line and/or process.
  • Batch processes requiring rapid discharge of noncondensibles at start-up (when used for air vent).
  • Drip-legs on steam mains and tracing.
  • Installations subject to ambient conditions below freezing.
Bimetal thermostatic
  • Small and light-mass.
  • Maximum discharge of noncondensibles at start-up.
  • Unlikely to freeze, unlikely to be damaged if it does freeze.
  • Rugged, withstands corrosion, water hammer, high pressure and superheated steam.
  • Responds slowly to load and pressure changes.
  • More condensate back-up than Balanced Pressure trap.
  • Back-pressure changes operating characteristics.
  • Drip legs on constant – pressure steam mains.
  • Installations subject to ambient conditions blew freezing.
Hits: 14964