Inverted Bucket Steam Traps

Mechanical robustness, high-pressure capability (PN 63) and in-line maintainability: the choice for drain pockets, main distribution headers and demanding industrial environments where reliability is non-negotiable.

The inverted bucket mechanism

The inverted bucket is a small metal cup, open at the bottom, connected via a lever to the discharge valve at the top of the trap body. The body is always full of condensate, which acts as the hydraulic seal. When steam enters the bucket it floats upward, closing the valve. A small bleed hole at the bucket top allows trapped steam to escape gradually — when enough has bled off, the bucket sinks and the valve opens to discharge condensate. The cycle repeats in a characteristic rhythmic pattern.

The solid heavy-walled body construction withstands water hammer — the violent hydraulic shock caused by high-velocity steam driving condensate slugs in main distribution lines. This makes the inverted bucket the preferred trap type for drain pockets, where impact loads are frequent and severe.

The mechanism has no elastomeric seals exposed to live steam — the hydraulic seal is the body of condensate itself. This gives excellent performance with steam-treated condensate containing chemicals that would attack rubber seats.

Inverted bucket steam trap Spirax Sarco — Roffia srl

Spirax Sarco Series S

  • Max pressure: 14–21 bar
  • Max temperature: 220 °C
  • Connections: screwed ½"–1½", flanged DN 15–50
  • Body: cast iron or carbon steel
  • Applications: drain pockets, low-to-medium pressure distribution lines

Spirax Sarco Series SF

  • Max pressure: 32–40 bar
  • Max temperature: 250 °C
  • Connections: screwed ½"–1½", flanged DN 15–50
  • Body: forged carbon steel
  • Internals: stainless steel (stainless trim)
  • Applications: process steam, chemically treated condensate

Spirax Sarco Series SCA

  • Max pressure: 63 bar
  • Max temperature: 300 °C
  • Connections: screwed ½"–1", flanged DN 15–50
  • Body: ASTM A105 forged steel
  • Applications: power stations, petrochemical, high-pressure critical service

Typical applications

Drain pockets on main steam lines

Drain pockets (steam pockets) are installed every 30–50 m on horizontal main lines and before every isolating valve. The inverted bucket is the standard trap for these locations: resistant to water hammer, compact body, minimal maintenance. Properly maintained drain pocket traps can recover 5–10% of boiler energy output that would otherwise be lost to condensate flooding the steam space.

Steam headers and separators

Centrifugal separators and steam headers accumulate condensate intermittently — with high peaks at plant startup and reduced rates at steady state. Inverted bucket traps' intermittent discharge behaviour naturally matches this variable condensate pattern. In CHP and turbine plants, header traps are typically SCA series for high-pressure reliability and absolute trustworthiness.

High-pressure steam tracing

In oil & gas applications, steam tracing lines on pipelines (15–40 bar steam) maintain process fluid temperature. Inverted bucket traps are preferred here for their ability to operate at elevated pressures with minimal maintenance, tolerating the superheated steam common in high-pressure tracing networks.

Chemical and petrochemical plants

Industry standards (ASME B31.3, EN 13480) require certified, traceable components. SCA forged steel traps meet these requirements and carry ATEX-compatible installation options for classified zones. In-line maintenance — replacing internals without body removal — is especially valued in ATEX environments where preparing pipe sections for hot-work requires extensive safety procedures.

Domande frequenti

How does the inverted bucket steam trap work?
The inverted bucket is a small cylindrical metal cup open at the bottom, connected by a lever to the discharge valve at the top of the trap body. The body is always full of liquid (condensate), which acts as a hydraulic seal against live steam passing through. When live steam or non-condensable gas enters the bucket, it floats upward — lifting the bucket and closing the discharge valve. A small calibrated hole at the top of the bucket allows the trapped steam to bleed off gradually; as the steam dissipates, the bucket sinks, the lever pulls the valve open, and condensate is discharged. The cycle repeats — giving the intermittent discharge that is normal and expected behaviour for this trap type (not a fault).
Why is the inverted bucket trap suited to main steam distribution lines?
Main steam lines present more demanding conditions than equipment drain points: high pressure (often 15–40 bar in industrial plants), elevated temperatures, water hammer from condensate accumulating in low points of horizontal lines, and high-velocity flow that can drive condensate slugs into drain pockets with significant force. The inverted bucket handles these conditions through its robust solid-body construction (heavy-walled carbon steel or cast iron, PN 63) — far more resistant to water hammer impact than disc or float traps. Its simple internals (bucket, lever, seat) have no elastomeric seals exposed to live steam. Even when condensate metallurgy carries iron oxide particles at high velocity, the bucket mechanism withstands impact damage that would destroy a disc trap seat.
Does the inverted bucket trap waste live steam through the bleed hole?
The bleed hole is a controlled, intentional small steam loss: it allows trapped steam in the bucket to escape so the bucket sinks and opens the discharge valve. In quality construction the hole diameter is minimised — typically 0.5–1.5 mm — resulting in live steam loss of approximately 0.5–2% of nominal capacity. This loss is higher at elevated back-pressure (condensate return pressure). It is not comparable to a failed-open trap where the loss equals 100% of steam flow through the seat. On Spirax Sarco SCA series traps (for high-pressure service), the optimised bucket geometry and precision seat keep the bleed loss to a minimum while maintaining reliable cycling.
What is the difference between the S, SF and SCA series from Spirax Sarco?
Series S: basic inverted bucket, cast iron or carbon steel body, maximum 14–21 bar depending on size. Screwed and flanged connections. Suited to low-to-medium pressure drain pockets on main distribution lines. Series SF: stainless trim (seat and bucket in stainless steel) for improved corrosion resistance. Maximum 32–40 bar. For clean steam, process steam and condensate lines with aggressive chemical treatment. Series SCA: heavy-duty forged steel body for the highest pressures, up to 63 bar maximum. Used in steam generating plants, petrochemical installations and high-pressure process lines where other trap types do not provide adequate safety margins. All three series support in-line maintenance (internal assembly replacement) without removing the body.
How is an inverted bucket trap maintained?
Inverted bucket traps are designed for in-line maintenance: the complete internal mechanism (bucket, lever, seat assembly) is replaceable by removing the top cover, with the body remaining in the pipeline (laterally isolated). This is particularly important in plants where production downtime is costly. Maintenance frequency depends on steam quality: with clean saturated steam and well-treated boiler feed water, a stainless-trim trap may operate for 5–10 years without intervention. Poor water quality (iron oxide, scale, hardness) dramatically shortens seat life. Ultrasonic diagnosis is the primary condition monitoring method: a healthy inverted bucket produce a rhythmic clicking; a failed-closed trap is silent; a failed-open trap produces a continuous hiss. Infrared thermography can also detect abnormal downstream temperatures.
Can an inverted bucket trap handle superheated steam?
The inverted bucket trap can operate with superheated steam, with a caveat: superheated steam entering the bucket causes it to rise (steam in the bucket = floating), closing the discharge valve. If the superheat is moderate (< 30–50 °C), natural heat loss from the trap body allows the steam to condense into condensate within the bleed hole cycle time, and the trap cycles normally. With high superheat (> 50 °C above saturation), the bucket tends to stay raised for extended periods, slowing condensate discharge. The recommendation is to install the trap after a sufficient length of pipe to reduce superheat, or to use a thermodynamic disc trap which is inherently better suited to superheated steam service.

Inverted bucket steam trap selection

Tell us the operating pressure, back-pressure on the condensate return, estimated condensate load and application type. Roffia selects the correct series and size with a dedicated quotation.

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