Steam traps : calculate steam loss
What is the steam flow through a failed steam trap ?
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1. Steam traps
2. Steam traps problems
5. Steam traps : steam loss estimation Excel calculator
1. Steam traps
Steam traps are critical elements within the steam network of process industries. They allow to catch and evacuate steam condensates that should not stay within the steam network (condensate will cause issues such as poor heat transfer, damages due to hammering...) and to send it to the condensate network where it can be channeled back to the boiler for instance.
While doing so, the steam traps also have the function to prevent steam to escape at this purging point which would cause a large loss of energy (generating steam is requiring a lot of energy, if steam escapes without having done any work then all the energy, and money, spent, is lost).
2. Steam traps problems
Steam traps however face many failure modes and are not considered as particularly reliable pieces of equipment. On large process plants, such as refineries, it is estimated that 10 to 20% of the steam traps can have an issue at a given point of time. Such issues can be :
- The steam trap remains open, or partially open (then steam can escape)
- The steam trap is always closed (then condensate cannot be purged)
When the steam traps remain open, as we have seen above, it leads to a loss of energy and thus a cost. The calculation below allow to estimate what is the flow of steam lost through the steam trap and then determine what would be the savings that could be done by repairing the traps and improving the maintenance plan.
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3. Steam trap steam leakage calculation
Knowing the diameter of the orifice of the steam trap, it is possible to calculate the steam leakage that will happen in case the steam trap remains stuck open.
It must be noted however that the steam trap may not fail 100% open, but rather somewhere in between close and fully open. To take this into consideration, it is recommended to apply a factor 0.67 to the values calculated below.
Before estimating the steam loss, it is necessary to know the pressure of the condensate system at the discharge of the steam trap, as well as the pressure of the steam upstream the steam trap.
3.1 Discharge pressure of steam trap < 58% of absolute steam pressure
The steam flow through the open steam trap can be calculated thanks to :
W = 0.413*P*d2
With
W = steam flow (kg/h)
P = steam pressure (kPa abs)
d = orifice diameter of the steam trap (cm)
Pay attention to the unit as the coefficient 0.413 is valid only with the units above.
In imperial units : W = 40.4*P*d2
With
W = steam flow (lb/h)
P = steam pressure (Psi abs)
d = orifice diameter of the steam trap (in)
3.2 Discharge pressure of steam trap > 58% of absolute steam pressure
The steam flow through the open steam trap can be calculated thanks to :
W = 0.576*d2*((P-ΔP)*ΔP)1/2
With
W = steam flow (kg/h)
P = steam pressure (kPa abs)
ΔP = pressure difference inlet - discharge (kPa)
d = orifice diameter of the steam trap (cm)
Pay attention to the unit as the coefficient 0.413 is valid only with the units above.
In imperial units : W = 56.3*d2*((P-ΔP)*ΔP)1/2With
W = steam flow (lb/h)
P = steam pressure (Psi abs)
ΔP = pressure difference inlet - discharge (Psi)
d = orifice diameter of the steam trap (in)
3.3 Correct the value
As explained above, it is necessary to multiply the result of the calculation by 0.67 to get a more realistic value of the steam flow through the steam trap.
4. Orders of magnitude of steam loss values
This table is due to the Electric Power Research Institute, it gives order of magnitude of steam losses through steam traps that have failed open :
| Type of steam trap | Pressure range (Psi) | Steam loss (lb/h) |
| Steam drip traps | 600-1000 | 100-150 |
| Trace line traps | 50-150 | 15-45 |
| Liquid heater traps | 50-150 | 130-330 |
| Air heater traps | 40-150 | 110-330 |
| Other traps | All pressures | 90-100 |
5. Steam traps : steam loss estimation Excel calculator
You can access to an Excel calculation tool in order to estimate the steam loss that will happen through a steam trap that is stuck open : Steam traps : Steam loss Excel calculation tool (click here)
Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.
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