Failure Modes of Steam Traps

Understanding what will happen to a steam system when a steam trap fails is very important.  The longevity of a steam trap is impacted by factors such as system operating conditions (pressures, temperatures, and load), the presence of foreign material, improper system operation, and neglect.

 As you are aware there are many types of steam traps that use various technologies to operate.  Within a given steam trap technology, when a steam trap fails it does so in a foreseen manner (i.e. fail open or fail closed).  The position of failure is of course determined by the method in which the trap operates.  It however must be understood that anything is possible.  A given steam trap that is designed to fail closed may actually fail open.  There are always unforeseen circumstances in a system that may cause abnormal failure, such as dirt, backpressure, or operator action.

 Typical Trap Fail Positions:

Thermodynamic (Disc) – Fail Open (Based on seat and disc erosion)

Float & Thermostatic – Fail Closed (Based on float or linkage erosion)

Inverted Bucket – Fail Open (Based on linkage failure)

Thermostatic:

            Bellows Type – Fail Open or Fail Closed (Based on bellows failure)

            Bi-Metal Type – Fail Open or Fail Closed (Based on metal strip failure)

 Nicholson Thermostatic Steam Trap Failure Modes:

N125, N450, TA, N650 – Available in both fail open and fail closed designs

Liquidator 450, A, B, C, and CS – Available in fail open only

To achieve both fail open and fail closed modes; Nicholson uses different welded stainless steel bellows.  To achieve fail open the bellows material spring rates are adjusted as well as filling the bellows under atmospheric conditions.  When the bellows is punctured it will fail in its normal position, which is open.  In the fail-closed design, the bellows is filled under a vacuum causing it to tightly contract against the trap cover.  When the bellows is ruptured it will expand and drive the trap valve against the seat.

Trap Selection Based on Failure Mode:

Normally in steam trap applications failure mode may never be addressed.  This is just one topic that only the steam savvy or the owner of a critical application may ask.  Ask the following question to yourself when faced with a steam trapping application, “What will happen to my system if the steam trap fails in its normal fail open or fail closed position?  Based on the given application either open or closed may be desirable.  It must be emphasized that there may not be one type of trap, based on failure position, that best suits all trapping applications.  This of course forces us to ask a few questions if the topic is brought up.

Fail Open -    When a steam trap fails open it passes live steam directly into the condensate                                           return system.

            Advantages:

1.      Allows continuous operation of the system

2.      Permits continual system drainage that prevents degraded efficiencies and equipment damage

Disadvantages:

1.      Creates excessive return system backpressure which may lead to additional trap failures and reduced system drainage capacities

2.      System will now take increased quantities of steam to operate and system-operating costs will drastically increase due to energy losses.

Fail Closed – When a steam trap fails closed it prohibits the passage of steam and condensate completely.

            Advantages:

1.      Prohibits the passing of live steam to the condensate return system and thus eliminating difficulties associated with increased back pressures

2.      Subsequently conserves energy by conserving steam consumption

3.      Ability to determine steam trap failure when adverse parameters or system operation is noted.

Disadvantages:

1.      A failed closed steam trap causes condensate in a system to back up and become flooded, which can cause:

a.      Degraded heat transfer in the process that is being drained

b.      Excessive condensate in a steam system will cause water hammer that can lead to equipment damage and personnel hazards.

c.      Freezing of steam and condensate lines in cold climates

d.      Solidification of process fluids in steam tracing applications.

2.      Systems that become unable to drain shut down completely if not diagnosed and repaired.

It must be understood that the particular failure mode of a steam trap depends on an application and the end-user’s priorities.  Steam Trap users in a process facility usually choose to have a trap that fails open.  In this case the production and a facility’s continual operation is much more important than the blowing of live steam.  It is however just as important in such a facility that a good steam trap maintenance program is in place.

On the other side of the spectrum, a steam trap user that predominately deals with heating applications may have another approach.  If a steam trap were to fail on a unit heater he would learn about it immediately.  Once the trap fails closed the condensate would decrease the rate of heat transfer, as the coils were flooded.  As soon as the heater stops heating personnel in a given space the maintenance crew is usually called to diagnose the problem.  In this particular case fail closed would be desirable due the energy savings and the quick problem identification.