O&M

Steam vent silencers: An important but overlooked boiler component

Safety vent silencers are an important component of steam generators and will fail without regular inspection and maintenance.
Steam vent silencers: An important but overlooked boiler component
(Figure 2. Several diffuser basket designs.)

By Brad Buecker – Buecker & Associates, LLC

Combined-cycle power plants are frequently located in or near residential or commercial areas, with many people residing or working near the plant. The high energy required for power production generates much noise, which, without abatement controls, would be intolerable to the public and would present a safety hazard. One potential source of intense noise are the steam vents on heat recovery steam generators (HRSGs). Design, inspection and maintenance of steam vent silencers (SVS) are critical items of a plant’s noise abatement plan.

SVS Design

A heat recovery steam generator has several steam vents including:

  • Drum relief (most HRSGs are multi-pressure units)
  • Safety relief
  • Blowdown tank
  • Deaerator
  • Startup

These vents are located at or extend to the top of the unit for safety reasons. Even at a high elevation, when a safety valve lifts the noise can be intense. Accordingly, the vents on many HRSG units are equipped with vent silencers to dampen the noise. The basic diagram of a steam vent silencer (SVS) is shown in Figure 1.

Figure 1. Basic schematic of a steam vent silencer.

Vent silencers must be designed to handle a variety of steam pressures with high velocity inlet flow. The silencers manufactured by SVI Bremco have three major components, an inlet radial diffuser, lower plenum section and an upper absorptive (passive) silencer. Let’s examine how these components work together to dampen noise.

Inlet Radial Diffuser

This is shown as the floating diffuser in Figure 1. The diffusers can be designed in a one-, two-, or three-wall arrangement based on system conditions. Each layer has a specifically-designed perforation pattern that allows the steam to expand through mesh material located between the wall stages and the core of the diffuser. The floating basket design allows for thermal expansion in both the axial and transverse directions.

Figure 2. Several diffuser basket designs.

This stage helps to dissipate the incoming acoustic energy of the stream and splits the single stream into hundreds of smaller streams at each wall stage. This begins the noise attenuation process and decelerates the flow for further attenuation in the upper absorptive silencer.

Lower Plenum Section

The lower plenum section serves as an expansion chamber for radial dispersion of the steam. This arrangement provides for uniform flow to the absorptive silencer upper stage.

Upper Absorptive (Passive) Silencer

Several designs are possible for this final noise silencing stage:

  • Concentric baffle
  • Tubular array
  • Bar array
  • Parallel baffle

General illustrations of these designs are shown below.

Figure 3. General arrangements of the upper passive silencer.

These baffles provide the final noise attenuation.

Silencer stress and failure mechanisms

As can readily be surmised, silencers, and especially the inlet radial diffuser, are subject to large mechanical and thermal stress. Material degradation over time is the result. SVI Bremco can, to some extent, proactively address these issues by replacing older pressure safety valves (PSV) with modern designs that reduce steam-flow mechanical stress on silencer components. Even so, wear and tear on silencer components is still of primary concern. The photos below illustrate several of the most important issues.

Figure 4. Mechanical degradation of the perforated liner and loss of acoustical insulation.
Figure 5. Cracked silencer center body support.

Figure 6. Corrosion from water accumulation at the bottom of the diffuser.

Not only do these corrosion mechanisms affect silencer performance, but failed components can blow out of the silencer, presenting a potential safety hazard.

Of course, thermal stress issues in combined cycle units and HRSGs are usually quite substantial because of the regular load cycling of most units. However, numerous proactive techniques are available to improve performance and longevity of silencers. Primary concepts include:

  1. Design and fabrication

a. Materials metallurgical composition. Higher grade alloys than plain carbon steel, although adding expense, may pay for themselves in increased durability. 

b. Materials thickness and well-designed support components. Techniques such as computational fluid dynamics (CFD) can help determine the stresses on components and provide maximized structural design. CFD is also a critical technique in evaluating the aerodynamics of silencers and optimizing designs to reduce high steam velocity and back pressure.

c. Welding techniques. Proper welding techniques and selection of weld filler material are critical throughout steam generating systems including silencers. Welding induces localized stresses that can become rapid failure points if the welding is not planned and performed properly.

2. Excessive Water-Induced Corrosion. To the greatest extent possible, silencers and their discharge vent should be designed to minimize water collection from rain and condensation. Given the cycling nature of combined cycle units, control of water accumulation can be a challenging task.

3. Inspections. Silencers, like some other power plant components, often fit in the “out of sight, out of mind” category; that is until a major failure brings the equipment to everyone’s attention. The next section outlines important items for silencer inspections.

Silencer inspection details

A recommendation at any industrial plant is to develop written protocols for every process and inspection, and to strictly follow the guidelines at all times. However, in this era of minimal staffing at many plants (combined cycle facilities are prime examples) plant personnel may not have the expertise to evaluate all situations. A solution is to work with an industry expert or company to develop inspection guidelines and perhaps assist directly with the inspections. The following lists outline primary inspection parameters for SVI silencers.

Visual Inspection

  • Silencer support brackets and welds for degradation, corrosion, and cracking
  • Outer shells for corrosion, degradation, cracks, or thinning
  • Inlet pipe connection for missing or loose bolts, or corrosion
  • Inlet pipe welding for cracks
  • Drainage pipe for corrosion
  • Exterior paint integrity

Internal Video

  • Baffle frame for degradation or cracking
  • Baffle support for degradation or cracking
  • Baffle perforated sheets for degradation or cracking
  • Weld between the diffuser base plate and inlet pipe for cracking
  • Weld between the diffuser basket and base plate for cracking
  • Diffuser perforated plates for degradation, corrosion, or cracking
  • Diffuser cap condition for corrosion

Early detection of component degradation allows repairs before a major failure occurs.

Conclusion

Safety vent silencers are an important component of steam generators. These components operate under high mechanical and thermal stress, and will fail without regular inspection and maintenance. A failure may raise noise to unacceptable levels. Furthermore, pieces of failed components may discharge from the vent to produce a safety hazard. SVI Bremco provides the services and equipment to protect and maintain these vital power plant items.

Contributing editor for this SVI Bremco article is Brad Buecker.



About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station. His work also included 11 years with two engineering firms, Burns & McDonnell and Kiewit, and he also spent two years as acting water/wastewater supervisor at a chemical plant. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines and has written three books on power plant chemistry and air pollution control.  He may be reached at beakertoo@aol.com.