Mechanical steam traps operate by considering the difference in density between steam and condensate. They will pass through large volumes of condensate continuously and are suitable for a wide range of process applications. Types include float and inverted bucket steam traps.

Ball Float Steam Traps (Mechanical Steam Traps)

Float traps operate by sensing the difference in density between steam and condensate. In the case of the trap shown in the image to the right (a float trap with an air valve), condensate reaching the trap causes the float to rise, lifting the valve off its seat and causing deflation.

Modern traps use regulator vents, as shown in the photo to the right (Float Traps with Regulator Vents). This allows initial air to pass while the trap also handles condensate.

The automatic vent uses a balanced pressure bladder assembly similar to a regulator steam trap, located in the steam area above the condensate level.

When the initial air is released, it remains closed until air or other non-condensable gases accumulate during conventional operation and are opened by lowering the temperature of the air/steam mixture.

The regulator vent provides the added benefit of significantly improving condensation capacity during cold starts.

In the past, if there was water hammer in the system, the regulator vent had some degree of weakness. If the water hammer is severe, even the ball may break. However, in modern float traps, the vent can be a compact, very strong all stainless steel capsule, and modern welding techniques used on the ball make the entire float very strong and reliable in water hammer situations.

In some respects, the float thermostatic trap is the closest thing to a perfect steam trap. No matter how the steam pressure changes, it will be discharged as soon as possible after the condensate is produced.

Advantages of Float Thermostatic Steam Traps

The trap continuously discharges condensate at steam temperature. This makes it the prime choice for applications where the heat transfer rate of the provided heated surface area is high.

It handles large or light condensate loads equally well and is unaffected by wide and unexpected fluctuations in pressure or flow.

As long as an automatic vent is installed, the trap is free to vent air.

For its size, that’s an outsized capability.

The version with a steam lock release valve is the only trap fully suitable for any steam lock that is resistant to water hammer.

Disadvantages of Float Thermostatic Steam Traps

Although not as susceptible as inverted bucket traps, float traps can be damaged by violent phase changes, and if to be installed in an exposed location the main body should lag, and/or be supplemented with a small secondary adjustment drain trap.

Like all mechanical traps, a completely different internal structure is required to operate over a variable pressure range. Traps designed to operate at higher differential pressures have smaller orifices to balance the buoyancy of the float. If the trap is subjected to a higher differential pressure than expected, it will close and not pass condensate.

Inverted Bucket Steam Traps (Mechanical Steam Traps)

(i) The barrel sags, pulling the valve off its seat. Condensate flows under the bottom of the bucket, fills the bucket, and drains away through the outlet.

(ii) The arrival of steam floats the barrel, which then rises and closes the outlet.

(iii) The trap remains closed until the steam in the bucket condenses or bubbles through the vent hole to the top of the trap body. It then sinks, pulling most of the valve off its seat. Accumulated condensate is drained and the cycle is continuous.

In (ii), air reaching the trap at start-up will provide bucket buoyancy and close the valve. The bucket vent is important to allow air to escape to the top of the trap for eventual discharge through most of the valve seats. With small holes and small pressure differentials, traps are relatively slow in venting air. At the same time, it should pass through (and thus waste) a certain amount of steam for the trap to work after the air is cleared. Parallel vents installed outside the trap reduce start-up time.

Advantages of Inverted Bucket Steam Traps

The inverted bucket steam trap was created to resist high pressure.

Kind of like a floating thermostatic steam bait, it is very tolerant of water hammer conditions.

It can be used on the superheated steam line, adding a check valve on the groove.

The failure mode is sometimes open, so it is safer for applications that require this functionality, such as turbine drainage.

Disadvantages of Inverted Bucket Steam Traps

The small size of the opening at the top of the bucket means that this trap will only vent air very slowly. The opening cannot be enlarged as steam will pass through too quickly during normal operation.

There should be enough water in the body of the trap to act as a seal around the rim of the bucket. If the trap loses its water seal, steam is wasted through the outlet valve. This can often occur in applications where there is a sudden drop in steam pressure, causing some of the condensate in the trap body to “flash” into steam. The barrel loses buoyancy and sinks, allowing fresh steam to pass through the weep holes. Only when enough condensate reaches the steam trap can it be water sealed again to prevent steam waste.

If an inverted bucket trap is used in an application where plant pressure fluctuations are expected, a check valve should be installed in the inlet line prior to the trap. Steam and water can flow freely in the direction indicated, while reverse flow is impossible because the check valve is pressed against its seat.

The high temperature of superheated steam can cause an inverted bucket trap to lose its water seal. In such cases, a check valve preceding the trap should be considered essential. Very few inverted bucket traps are manufactured with an integrated “check valve” as standard.

If an inverted bucket trap is left exposed close to sub-zero, it can be damaged by a phase change. As with the different kinds of mechanical traps, proper insulation will overcome this shortcoming if conditions are not too harsh. If the expected environmental conditions are well below zero, then there are many powerful traps that should be carefully considered to do the job. In the case of a main drain, a thermos dynamic trap would be the primary choice.

Like the float trap, the opening of the inverted bucket trap is designed to accommodate the maximum pressure differential. If the trap is subjected to a higher differential pressure than expected, it will close and not pass condensate. Available in a range of orifice sizes to cover a wide range of pressures.