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Selection of Steam trap

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SELECTION OF STEAM TRAP

The selection of steam traps for specific applications is made in two steps :

A. Choice of type
B. Choiche of size


Before discussing these steps it is necessary to make a general comment from the economic point of view.
Giving as granted that the condensate must be discharged , it is highly important not to loose live steam in this
process. Assuming that today’s cost of steam is approximately 0.02 U.S. $ to a kilogram ( and this is very
conservative ) it follows that a trap , sized for 200 Kg / hour losing 10% of its steam , in a refinery onstream 24
hour / day , costs in one year ( 200 x 0.1 x 365 x 0.02 ) U.S. $ 146 .
If this refinery has 1000 traps incorrectly sized and therefore in such conditions , the loss will be 146000 U.S. $
per year !
One can easily calculate what happens if the trap has failed in the open position instead of just losing some
steam. The choice of the type and size fo a steam trap is a matter of great importance .


A. CHOICE OF TYPE

The main criteria for the selection of the type are ( they cannot be listed in order of importance since it varies
from application to application ) :

- Resistance to freezing
- Installation versatility
- Air venting
- Resistance to water hammer
- Cold condition ( if waterlogging is not allowed the trap must be the open type )
- Type of discharge ( with temperature regulating control valves , the modulating type is preferable )
- Heat exchange efficiency ( traps discharging subcooled condensate do not allow an efficient heat exchange )
- Sensitivy to back pressure
- Reaction to load changes
- Pressure variations ( types requiring cahnges of orifices for different pressure are unfit for wide variations )
- Dimension and weight

For specific suggestion see “ steam traps applications and selection “ table .


B. CHOICE OF SIZE

There are 3 parametres to take into account for a correct sizing :

1. Differential pressure
2. Condensate load to be discharged
3. Safety factor


1 – DIFFERENTIAL PRESSURE

The differential pressure is simply the difference between the pressure upstream and downstream of the trap.
When a trap discharges at the atmosphere the downstream pressure is zero ( we always refer to relative and not
absolute pressure ) and the differential pressure is the same of the line. When there is a condensate return
system , there is always some pressure inside it due to friction and line lifting. The best way to know the value of
downstream pressure ( also called backpressure ) is to install a pressure guage just after the trap. If this is not
practical one should calculate the amout of backpressure by formulas of pressure drop in water ducts adding
approx. 0.1 bar for each meter of rise .

2 – CONDENSATE LOAD

This is the second parameter to be introduced into the capacity tables. For draining of steam mains the quantity of
condensate is related to the size of the pipe , to the steam pressure , to the efficiency of thermal insulation , to the
outside temperate , to the wind force if any and to the temperature of the line ( cold start – up or running
conditions ). In all the other applications traps are used to drains machines utilizing steam as a heating medium.
In these cases the quantity of condensate to be removed will be equal to the amount of steam used by the
machines to give the desired performance .

3 – SAFETY FACTOR

For many reasons the steam trap will be not able to handle on field the condensate loads given in the capacity
tables. These reasons are :

- Type of discharge ( intermittent or continuons )
- How the condensate reaches the trap
- Presence of large quantities of air
- Influence of other traps discharging in the same return line

Moreover there may be incorrect assumptions in the condensate load calculation and it is necessary to take into
account that at cold start – up th equantity of condensate to be discharged is a lot more than at running
conditions. To summarize , the size of the trap is selected entering the capacity tables with the differential
pressure and with the condensate load multiplyed by the safety factor. A minimum safety factor 1.2 / 1.5 must be
always taken into consideration. Higher safety factors 2 / 4 are required for certain applications .

INSTALLATION

Specific suggestions for a correct installation depend on the application and on the type of select trap. The
following are some general comments :

- The trap should always be installed below the drain point
- Try to avoild condensate lifting. If this is necessary install a check valve just after the trap
- Always install “ Y “ type strainer upstream, unless the trap has a built in “ Y “ type strainer
- Mechanical and thermodynamic traps should be installed as close as possible to the drain point
- Thermostatic traps should be installed at 1 – 2 mt from drain point. Do not insulate this cooling leg
- It is avdvisable to install a check valve up stream of an inverted bucket trap to prevent water seal loss
- A sight glass fitted down stream the trap allows a continuous check of the trap operation
- Always install isolating valves upstream and downstream for maintenance purposes
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