The selection of steam traps for specific applications is made in two steps :
A. Choice of type
B. Choice 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 to 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):
For specific suggestion see “ steam traps applications and selection “ table.
B. CHOICE OF SIZE
There are 3 parameters 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 gouge just after the trap. If this is not practical one should calculate the amount 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 :
Moreover there may be incorrect assumptions in the condensate load calculation and it is necessary to take into account that at cold start – up the quantity 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 multiplied 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 .
Specific suggestions for a correct installation depend on the application and on the type of select trap. The
following are some general comments :
|STEAM TRAP SELECTION AND APPLICATIONS|
|D = Thermodynamic
T = Balanced pressure thermostatic
B = Bimetallic thermostatic
I = Inverted bucket
G = Ball float with thermostatic air vent
|STEAM MAINS||D – T|
|TRACING LINES||D – B|
|THANKS||Storage tanks||B - T|
|HEATHER BATTERIES||Unit heaters||G – I|
|PANS||Jacketed pans||G – I|
|HEAT EXCHANGERS||Water heaters||G – I|
|Fuel oil preheaters|
|DRYING CILINDERS||Paper dryers||G – I|
|PRESSES||Plywood presses||D – T|
|Tire mold presses|
|OVENS||Dressing sterilizers||G – T|
|IRONING MACHINES||I – D|
|TURBINES||D – I|
|MARINE APPLICATIONS||D – B|