2022 AMPP Technical Paper: Developments in Abrasive Blast Nozzle Technology: Reducing Noise Exposure While Preserving Nozzle Performance and Usability

Developments in Abrasive Blast Nozzle Technology: Reducing Noise Exposure While Preserving Nozzle Performance and Usability

 

Submitted By:  
Matthew Sullivan, Director of Product, Oceanit Laboratories, Inc.
Oceanit Laboratories, Inc. 
Honolulu, Hawaii, USA 
 
Additional Presenters:  
Dr. Christopher Sullivan, Director of Acoustics Research & Development
Oceanit Laboratories, Inc.

 

PRIMARY TOPIC CATEGORY:  Surface Preparation

 

ABSTRACT:

The U.S. Centers for Disease Control (CDC) estimates that 1 in 3 adults suffer from hearing loss.  The U.S. National Institute for Occupational Safety and Health (NIOSH) further provides that 24% of U.S. workers hearing loss is caused by occupational exposure. To prevent hearing loss, the U.S. Occupational Safety and Health Administration (OSHA) recommends that workers not be exposed to sounds at or greater than 85 decibels (dBA) for 8 hours.  Abrasive blast nozzles, however, can produce noise levels upward of 115dBA, for which OSHA sets a max safe exposure time of just 15 minutes per day.  

Leveraging cutting edge advancements in jet engine technology, new abrasive nozzle designs can now reduce noise by up to 17dB, an approximate 50x reduction in sound power, when compared with conventional venturi nozzles of the same bore size. Quiet nozzles accomplish this by reducing the nozzle air exit velocity and sound pressure level, while preserving particle velocity to maintain equal abrasive power. The difference in noise exposure enables operators to be safer and more productive, diminishes operator fatigue, and minimizes employer liability.  Furthermore, whereas NIOSH estimates a $100 per dBA of savings when purchasing quieter products, such improvements in acoustical performance can also translate to employer cost savings. The efficacy of the new quiet nozzle technology was evaluated by Oceanit and 3rd party industry partners interested in use or distribution of the nozzles for the industry. 

This paper explores the efficacy of such nozzles and details their performance in different conditions, with different blasting setups, with different abrasives, and on different surfaces. 

 

BACKGROUND:

Abrasive blasting operations used for paint and surface coatings removal are essential for the maintenance of the ships, aircraft, and land vehicles of the United States Armed Forces as well as use industries such as oil & gas, power generation, construction, mining, and infrastructure, among others. Abrasive blasting nozzle design is rudimentary and noise levels produced during abrasive blasting operations in shipyards, maintenance facilities, and factories for removing paint and surface coatings often exceed exposure limits put in place by Occupational Safety and Health Administration (OSHA). Reducing a worker's occupational noise exposure is imperative from a safety and economics perspective.

The U.S. Centers for Disease Control (CDC) estimates that one in three adults suffer from hearing loss and The U.S. National Institute for Occupational Safety and Health (NIOSH) further provides that 24% of U.S. workers' hearing loss is caused by occupational exposure. Exposure to high levels of noise can lead to difficulty communicating and distraction, which can lead to accidents and decreased productivity. Over the past decade, a large number of hearing loss claims have been filed and millions of dollars have been compensated to workers due to Noise Induced Hearing Loss (NIHL).

Figure 1. Abrasive blast operator cleaning a steel surface

To prevent hearing loss OSHA regulates noise exposure above certain levels. OSHA Standard 1910.95 requires hearing protection to be worn by employees based on the noise levels and duration they are exposed to and recommends that workers not be exposed to sounds at or greater than 85 decibels (dBA) for 8 hours. Abrasive blast nozzles, however, can produce noise levels upwards of 115dBA, for which OSHA sets a maximum safe exposure time of just 15 minutes per day. Exposure of personnel to these levels, even with hearing protection, substantially increases the risk of NIHL. Furthermore, OSHA regulations dictate that improved hearing protection does not constitute a reduction in worker noise exposure and requires the elimination or reduction of the acoustic hazard through engineering controls prior to implementing administrative controls or relying on personal protective hearing protection.

According to OSHA guidance for engineering control of noise in blasting operations at Naval Shipyards (U.S. Department of Labor, OSHA, 2006), common sources of noise include:

  • Air discharge from blast nozzle: 112 to 119 dBA
  • Supply air inside operator's helmet: 94 to 102 dBA
  • Abrasive blasting cabinets: 90 to 101 dBA
  • Air compressors: 85 to 88 dBA
  • Maximum noise levels up to 145 dBA have been measured at the operator with an empty grit pot

In addition to reducing noise levels, the productivity and efficiency of abrasive blast nozzles are equally if not more important when it comes to the successful adoption of a new nozzle by the surface preparation community. “Productivity” or work output of a nozzle is expressed as area per unit time cleaned by abrasive blasting, while efficiency of a nozzle is quantified by the kinetic energy flux of the airflow and blasting particle stream at the nozzle exit. Both translate directly into how long a blast operator must spend cleaning a surface.

 

 ABRASIVE NOZZLE DESIGN DEVELOPMENT:

​​Abrasive blast nozzle technology prior to the 1950’s focused on straight bore nozzles characterized by a converging entrance, followed by a throat and outlet length which have the same constant cross-section, as shown in Figure 2.

Figure 2. Straight bore nozzle geometry

The next big step in abrasive blasting technology was the venturi nozzle patented in 1955 by G.D. Albert and W.H. Hall. It was developed based on the observation that straight bore nozzles tended to become more efficient as they began to wear. The wear tended to enlarge the outlet, leading to a shape similar to that shown below in Figure 3.

Figure 3. Venturi nozzle geometry

Additional nozzle designs were developed between the introduction of the venturi nozzle and the early 1990’s, including the laminar flow nozzle, the double venturi nozzle, and the “Bazooka” nozzle. However, there is no evidence that these nozzle designs had been developed through a scientific optimization process for efficiency, instead being based on observations of existing nozzle performance or trial and error.

Seeing this opportunity, the Penn State Gas Dynamics Laboratory designed a new nozzle, dubbed the “Penn State Nozzle”, using a scientific optimization focusing on efficiency, specifically the doubling of the kinetic energy of the particles leaving the blasting nozzle. Based on computer simulations and experimental testing, the Penn State group claimed to have met their goals of at least doubling the kinetic energy in their patent filing.

The next big development in abrasive blasting technology was the development of a quiet nozzle, an abrasive blast nozzle designed specifically with the goal of reducing operating noise while maintaining production rates.

 

QUIET NOZZLE DESIGN:

An area of nozzle technology which has received considerable work in the field of aeroacoustics is the field of jet aircraft engines. Whether civilian or military in application, there has been a keen interest in producing quieter jet engines for aircraft since at least the early 1950’s. For the military, reducing jet noise is important for reducing noise signatures of aircraft as well as reducing general noise exposure for those working near the aircraft.

The noise from a jet can be traced to three main sources: the surface noise caused by shear between the moving fluid and a stationary wall or object, the jet noise caused by turbulent mixing within the jet, and for supersonic jets, the noise from the formation of shockwaves. Several methods have been devised to achieve jet noise reduction, among them adding chevrons to a nozzle outlet to encourage mixing, adding secondary flows which mix with the main nozzle exhaust at the exit, and redesigning nozzles to operate at lower Mach numbers.

To design the Quiet nozzle, some of the same principles for reducing jet aircraft engine noise were leveraged and applied to the nozzle design. It was determined that a reduction of abrasive blasting noise without loss of productivity could be possible with nozzle designs based on compressible flow theory. Computational Fluid Dynamic (CFD) modeling and simulations in the laboratory were used to provide initial design concepts of an approach that could achieve the objective of minimizing the acoustic energy in blast nozzles while maintaining the effectiveness of the blasting operation. CFD modeling showed improved particle acceleration zone but did not fully model the reduced noise levels which need more detailed modeling with both air and abrasive particles included.

Figure 4. Velocity magnitude simulation for a No. 6 venturi nozzle focusing on the nozzle exit

 

INTERNAL TESTING:

The noise performance and productivity of the quiet nozzle was compared with a standard commercially available No. 6 Venturi nozzle. Initial testing was conducted on March 1, 2018 in a privately operated blast booth at Sunset Powder Coating in Honolulu, HI. Prior to testing, twenty 18-inch x 18-inch panels of 14 gauge steel were uniformly powder coated to be used to evaluate nozzle productivity (time required to clean the panel to a set level). A test panel is shown in Figure 6. All tests were conducted with new 30/40 garnet media at a nozzle pressure of 67 psi.

For each nozzle tested the sound level was measured using a sound level meter at the operator’s left shoulder (see Figure 5) while operating the nozzle into open air (to avoid the sound generated by sand hitting metal during actual blasting). The sound levels for the 1/3 octave bands were measured for a 10 second period and MIN, MAX and AVG sound levels were automatically calculated and stored. Background sound levels were also recorded to confirm that background noise did not contribute to the measured noise levels the nozzles. Next, video was recorded of each nozzle as it was used to blast one side of a powder coated test panel as shown in Figure 6. In addition to providing future reference, the video was used to quantify the productivity of each nozzle (determine the time required to clean the test panel to a desired finish). The blaster’s feedback after using each nozzle was also noted, including impressions of sound levels and productivity.

Figure 5. Sound level measurements during nozzle operation.

Figure 6. Video and audio being recorded of the blasting of a test panel.

 

Table 1 summarizes the key results of the testing along with operator comments. Preliminary results show that the quiet nozzle was 17 dBA quieter and cleaned a test panel 26% faster than a standard Long Venturi nozzle.

 

Nozzle

Sound Level (dB)

Time to clean panel (sec)

Operator Notes

Conventional Long Venturi Nozzle

111

109

69

41

Nozzle part number: 10TCW6BP

Quiet Nozzle

95

94

51

39

Noticeably lower sound with greatest productivity. Less kickback than the Venturi nozzle. Nozzle part number: SH-6-PRO

 

Table 1. Key results from internal testing

 

The average sound levels measured for the 1/3 octave bands are shown in Figure 7. These confirm that the sound level for the quiet nozzle was lower than the standard Long Venturi nozzle across the entire spectrum. Also worth noting is the spike centered on 4000 Hz for the standard Long Venturi nozzle.

Figure 7. Average sound levels measured for the 1/3 octave bands

 

Results of comparative testing in a commercial blasting facility between the No.6 Quiet nozzle achieved a 17dBA noise reduction over the No. 6 Long Venturi nozzle while showing improvement in productivity in tests with garnet. A follow-on evaluation at Pearl Harbor Naval Shipyard showed improved productivity and reduced noise with steel shot using a No. 6 Quiet nozzle versus a No.6 Long Venturi nozzle. User feedback was encouraging. Both commercial and Navy blasters noted improved productivity, reduced acoustic noise, and reduced handling fatigue.

 

EXTERNAL TEST & EVALUATION:

 

MD&A Turbines, St. Louis, MO

Test carried out December 19, 2019 at MD&A facility.

Experimental Setup:

  • Workpiece: High Pressure Steam turbine valves for power plant
  • Workpiece material: 1 1⁄4 Cr-1 Mo and 422 SS, removing a scale build up
  • Media: 220 Aluminum Oxide
  • Pressure: Shop pressure ranges from 108 to 123 PSI
  • Control nozzle part number: SSD-6
  • Setup: Blast Booth with a steel indoor sub-structure with inlet slots on the front door and the exhaust heading out the back to the blower. Hose is 50’ with 1/14” ID.

Nozzle

Test Nozzle

Control Nozzle

Nozzle Type

Quiet

Long Venturi

Model Number

SH-6-PRO (3/8”)

SSD-6 (3/8”)

Nozzle sound level (dBA)

128

143

 

Results:

  • Surface profile comparison: Rough grey (bare steel) appearance with texture typical of valve components after blasting with both nozzles.
  • Observations: The SH-6-PRO Quiet nozzle is noticeably quieter (15 dB) and worked as smoothly and as quickly as the standard SSD-6 nozzle. Other than sound, the two nozzles are very similar.

 

Bilfinger, Norway

Test carried out on January 28, 2020 at Bilfinger training site.

Experimental Setup:

  • Workpiece: Test panel for nozzle comparison
  • Workpiece material: Steel plate
  • Media: Indian Garnet Abrasive 20/40
  • Pressure: Pressure: 7.2 Bar (104 PSI). Sand load added: 25 kg per test.
  • Control nozzle: Conventional Venturi nozzle
  • Setup: Test chamber constructed for nozzle comparison. The nozzle(s) were mounted at a fixed distance away from test panel for a controlled test.
  • Measurement equipment: Norsonic. Nor 131. Class 1 sound meter, designed for occupational hygiene, general sound level measurements and noise assessments. Calibration levels were checked regularly with Calibrator Nor1255. All measurements were performed 3 times. Time of measurement 30 sec.

 

Nozzle

Test Nozzle

Control Nozzle

Nozzle Type

Quiet

Long Venturi

Model Number

SH-6-PRO (9.5mm)

Venturi #6 (9.5mm)

Nozzle sound level (dBA)

106

118

 

Results:

  • Observations: A 12 dB reduction in noise was observed between the SH-6-PRO Quiet nozzle and the conventional Long Venturi nozzle used as the control.

 

Vance Metal Fabricators, Geneva, NY

Test carried out on October 20, 2021 at Vance Metal Fabricators site. 

Experimental Setup:

  • Workpiece: Metal tanks for the wine, cider, spirits and beer industry
  • Workpiece material: Carbon steel and stainless steel
  • Media: Aluminum Oxide R20
  • Pressure: Pressure: 70 PSI
  • Control nozzle: SN156-5P
  • Setup: 10’x15’ Blast Booth

Nozzle

Test Nozzle

Control Nozzle

Nozzle Type

Quiet

Long Venturi

Model Number

SH-5-PRO (5/16”)

SN156-5P (5/16”)

Nozzle sound level (dBA)

92

105

 

Results:

  • Observations: A 13 dB reduction in noise was observed between the SH-5-PRO Quiet nozzle and the conventional SN156-5P Long Venturi nozzle.
  • With the Quiet nozzle the blaster can stand further back.

 

CONCLUSION:

The U.S. National Institute for Occupational Safety and Health (NIOSH) estimates that 24% of U.S. workers hearing loss is caused by occupational exposure. To prevent hearing loss, the U.S. Occupational Safety and Health Administration (OSHA) recommends that workers not be exposed to sounds at or greater than 85 decibels (dBA) for 8 hours.  Abrasive blast nozzles, however, can produce noise levels upward of 115dBA, for which OSHA sets a max safe exposure time of just 15 minutes per day.  

Quiet nozzles have the promise to become a new industry standard for the surface preparation industry that can minimize the problem of high levels of noise exposure to blast operators via abrasive blasting. The Quiet nozzle was rigorously tested and evaluated both internally and through 3rd party testing to reduce noise levels at the nozzle exit while preserving nozzle performance and usability. The technology has been awarded one US patent with additional patents pending.

Quiet nozzles are currently manufactured in the USA with an industry leader in carbide and wear products, and the nozzles are commercially distributed and sold in multiple countries including the United States, Australia, New Zealand, the United Kingdom, and Norway. Large commercial customers in the energy services, manufacturing and construction industries currently use Quiet nozzles as a way to protect their workers, improve operational efficiency, and reduce liability.

For more information please contact hello@blastninja.com