Steam trap: what it does and where it sits

01 / THREE FAMILIES

Telling condensate from steam

Boilers/Bosch UL-S/Steam valves/This page

Mechanical. A float or an inverted bucket rides on the liquid level and opens a valve when condensate collects. Float traps discharge continuously and suit process plant with a varying load.

Thermostatic. A bellows or bimetal element responds to the temperature difference between steam and condensate that has cooled a few degrees below saturation, holding condensate back until it has given up that heat.

Thermodynamic. A single disc that closes on the velocity of flash steam under it and opens when the pressure in the chamber above falls. One moving part, and it tolerates superheat and freezing.

02 / THE DIFFERENCE

Thermodynamic or thermostatic

	Thermodynamic	Thermostatic
What operates it	Velocity of flash steam under a disc	Temperature of condensate against saturation
Moving parts	One disc	Bellows or bimetal element
Discharge	Intermittent, in bursts	Intermittent, after sub-cooling
Suits	Steam mains, tracing, superheat	Air venting and light loads
Condensate held back	Little	Until it cools below saturation

03 / MEASURED

The valve set it sits on

Element	Temp °C	Area m²	Bare W	Panelled W	Surface °C
Steam valves and piping	190	0.45	1,430	70	36
Total		0.45	1,430	70	≤45

Temperatures are FLIR readings taken on a running boiler. Areas come from the CAD model of the same machine, uplifted for bolts and irregular geometry. Losses are ISO 12241:2022 steady state. The panelled column is capped at the 45 °C touch-safe design target, assessed to EN ISO 13732-1.

04 / COMBINATION

Watts per square metre

At 190 °C a bare valve body sheds 3,178 W per square metre, the highest figure in our survey set.

Surface °C	W / m²	Area surveyed m²	Where it was read
96	1086	2.33	Bosch UL-S front door
146	2094	0.85	Bosch burner flange
190	3178	0.45	Bosch steam valves

Each row is one of our own surveys: the measured loss divided by the CAD area of the same surface. Nothing is interpolated. ISO 12241:2022 steady state, still air, emissivity from the FLIR survey.

05 / PROCEDURE

How a trap set is surveyed

Surveying a trap set, the order that finds the failures

A trap fails in one of two directions, and the two cost differently. A trap blowing through wastes live steam continuously. A trap blocked shut floods the line and stalls the plant it drains. A survey has to separate the two, trap by trap.

Tag and list every trap. A survey without a list repeats itself and misses the traps nobody walks past. Each gets a tag, a location, a type and a service pressure.
Read the temperature either side. Inlet close to saturation and outlet well below it is a trap doing its work. Both sides at saturation points to a trap passing steam; both sides cold points to one blocked.
Listen with an ultrasonic probe. Temperature alone cannot separate a thermodynamic trap cycling normally from one blowing through, because both are hot. The discharge sound pattern can.
Match the pattern to the type. A float trap discharges continuously and a blast is wrong for it. A thermodynamic trap discharges in bursts and a continuous flow is wrong for it. The fault is a departure from the pattern of that type.
Classify and record. Passing, blocked, or working. A survey that records only the failures cannot be compared with the next one.
Re-survey on a cycle. Traps fail steadily rather than all at once, so the value of a survey is in the second one and the third.

Inzonex did not meter the traps on the machine surveyed. The 190 °C and the 0.45 m² on this page are the steam valves and piping on that view, 1,430 W bare. Loss through a passing trap is a separate quantity and we publish no figure for it.

06 / FAQ

Asked about this

What does a steam trap do?

It lets condensate and air out of a steam line and holds steam back, automatically and without an operator.

What are the types of steam trap?

Three families: mechanical (float, inverted bucket), thermostatic (bellows, bimetal) and thermodynamic (disc).

What is the difference between a thermodynamic and a thermostatic steam trap?

A thermodynamic trap is operated by the velocity of flash steam under a disc and discharges in bursts. A thermostatic trap is operated by temperature and holds condensate until it has cooled below saturation.

How hot is the line a steam trap sits on?

The steam valve set we measured on a Bosch UL-S read 190 °C on the FLIR, over 0.45 m²: 1,430 W bare.

Did you measure the trap itself?

No. The 190 °C reading and the 0.45 m² cover the steam valves and piping on that view. The trap set was not separately metered and we quote no figure for it.

07 / WHERE THIS SITS

Up and across

/ 01

Steam valves

Up

The surveyed view this page came from, with every element on it.Open

→/ 02

Boiler mountings and accessories

Mountings

The valve set on the other machine, measured item by item at 180 °C.Open

→/ 03

Fire tube against water tube

Comparison

Both machines in this section are fire tube. What that changes about where the heat is lost.Open

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08 / EVIDENCE

Where each number comes from

Survey	FLIR S62 Pro, emissivity 0.90, reflected 25 °C, 3 m, full radiometric correction. UK commercial boiler house.
Geometry	Rebuilt in CAD from the surveyed machine. Area allowances are stated on the page they are used on and are allowances, not measured areas.
Calculation	ISO 12241 steady state. Heat loss is calculated from the measured surface temperature and the modelled area — it is not metered. Full method, basis and limits.

09 / ACROSS THE SECTION

The questions behind the numbers

/ 01

Boiler efficiency: two methods and the radiation loss

Boiler efficiency

Where surface loss sits inside the indirect method, and what 49.8 kW is against the fuel input.Open

→/ 02

Boiler inspection and the written scheme

Inspection

What has to be opened, how often, and who signs it off under PSSR 2000.Open

→/ 03

Fire tube against water tube

Fire tube or water tube

Which machine this is, and why the losing surfaces differ between the two.Open

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