Noise Control of Marine Gas Turbines in Propulsion and Auxiliary Power Applications

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1 74-GT-110 Copyright 1974 by ASME $3.00 PER COPY The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation. Full credit should be given to ASME, the Professional Division, and the author (s). $1.00 TO ASME MEMBERS Noise Control of Marine Gas Turbines in Propulsion and Auxiliary Power Applications M. I. SCHIFF Vice President, Special Products Department, Industrial Acoustics Co., Inc., Bronx, N. Y. Mem. ASME While techniques for silencing gas turbines have been applied with great success to numerous land systems, only limited experience has been gained aboard ship. The silencing of ship board gas turbines is made more complex due to the unique problems imposed by the effects of a sea air environment as well as the necessity for minimization of dead load and limitations on available space. The purpose of this paper is to review the state-of-the-art in gas turbine silencing specifically in relation to marine applications, Experiences in silencing the main propulsion system aboard the Adm. Wm. M. Callaghan, Military Sea Transport Service Roll- On/Roll-Off Cargo Ship and the Auxiliary Power units aboard the Royal Canadian DDH-280 Destroyers are described. Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for presentation at the Gas Turbine Conference & Products Show, Zurich, Switzerland, March 30-April 4, Manuscript received at ASME Headquarters November 21, Copies will be available until December 1, THE AMERICAN SOCIETY OF MECHANICAL. ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y

2 Noise Control of Marine Gas Turbines in Propulsion and Auxiliary Power Applications M. I. SCHIFF INTRODUCTION The increased utilization of gas turbines as a means of propulsion and auxiliary power for marine applications has created a need to control the noise generated by this equipment. It is the purpose of this paper to illustrate areas of experience in the silencing of gas turbines aboard ship, and to outline the procedures to follow in order to satisfy the stringent sound requirements that are being imposed for shipboard spaces. Specific references will be made to silencing achievements and applications aboard the "Admiral Wm. M. Callaghan" Military Sea Transport Service (Roll-On/Roll-Off) Cargo Ship, the U. S. Coast Guard Ice Breaker "Polar Star," the Canadian Navy DDH Class Helicopter Destroyers, and the U. S. Navy DD963 "Spruance" Class Destroyers. Whether for a land base or shipboard application, there are three major areas that must be considered in the silencing of gas turbines. Specifically, one must be concerned with the noise associated with the gas turbine intake, exhaust and casing. Gas turbine intake noise is predominantly high frequency in nature and is created by high velocity air passing over the blades in the compressor sections. The gas turbine exhaust noise is essentially broad band in spectrum, and is generated by the combustion process together with the airflow noise created in the power turbine. Casing radiated noise is the combination of all noise generated in the gas turbine. Due to losses through the turbine structure, this noise is usually decibels lower in sound power output to that of the gas turbine inlet and exhaust noise. with them individually or as a group. Usually, the engine enclosure and any related ventilation silencing are designed for the combined effects of intake, exhaust and casing radiated noise, while the intake and exhaust silencers are designed primarily for the noise generated by those respective components. Since in most ships the propulsion and auxiliary power systems are located at the lower deck levels, noise control considerations must also be applied to the duct systems that supply and discharge engine air. Furthermore, while most land base gas turbines have been designed for far field noise control requirements, (that is, for situations where the potential noise receiver is more than 100 ft away from the gas turbine), marine gas turbine noise control measures are generally required to satisfy near field criterion. Unlike the land base gas turbine, the noise generator and the receiver are in close proximity. As a result, means of natural attenuation such as directivity and divergence provide a very small component of the total noise reduction that may be required. The approach to follow in most shipboard systems can be briefly outlined as follows: 1 Select applicable criterion for shipboard space being considered. 2 Estimate unsilenced sound levels in the space in question. 3 If unsilenced levels exceed applicable criterion considered, take necessary steps to silence the component or components contributing to the excessive noise levels. SELECTION OF SILENCING EQUIPMENT FOR MARINE GAS ACCEPTABLE NOISE CONTROL CRITERIA ABOARD SHIP TURBINES Marine gas turbine silencing systems as In the silencing of gas turbine noise aboard well as land based gas turbines silepcing systems ship, one must consider each of these three are designed to satisfy comfort and hearing noise sources and attack the problems associated damage criteria. In addition, as aural communi- 1

3 SHIP SPACE CATEGORY *-- Table 1 Noise Level Criteria for Ship Spaces in Decibels Octave Band Center Frequencies, Hz SIL VALUE A SIL Value Requirement B C D E SIL Value Requirement F SIL Value Requirement Table 2 Approximate Sound Power Level of Marine Gas Turbine in Decibels, Re: Watts Octave Band Center Frequencies, Hz o o,-.1 m a,-1 '--j rp4 o o el 0 S-1 o a. m m z 0 cs,..-1 m Inlet Exhaust Casing i.: Z o 0 m. m a 8 a r.., i H a.4 A S p m a 0 Inlet Exhaust X Casing cation is essential for efficient ship operation, permissible sound levels for shipboard spaces are also dependent upon speech intelligibility requirements. While numerous studies have been conducted pertaining to the physiology and psychology of speech, hearing and comfort, it is difficult to provide a general criterion that will satisfy all requirements and a compromise must be reached. An acceptable compromise for airborne noise levels aboard ships is indicated in "General Specification for Ships of the U. S. Navy Department of Navy" dated January This specification is a workable one, as the operations that are carried out aboard ship have been carefully considered and the criterion established with ship spaces in mind. The type of spaces aboard ship are classified as follows: Category A: Spaces, other than category E spaces, where intelligible speech communication is necessary. Category B: Spaces where comfort of personnel in 2

4 1. POWER-FLOW INLET SILENCER 2. POWER-FLOW EXHAUST SILENCER 3. VENTILATION SILENCER 4. ENGINE ENCLOSURE 5. REDUCTION GEAR ENCLOSURE 6. ACOUSTICALLY TREATED INLET DUCT 7. ACOUSTICALLY TREATED EXHAUST DUCT Fig. 1 General arrangement of noise control system for typical gas turbine powered ship Category C: Category D: Category E: Category F: their quarters is normally considered to be an important factor. Spaces where it is essential to maintain especially quiet conditions. Spaces or areas where a higher noise level is expected and where deafness avoidance is a greater consideration than intelligible speech communication. High noise level areas where intelligible speech communication is necessary. Topside operating stations on weather decks where intelligible speech communication is necessary. The governing factor in Categories A, E and F is speech intelligibility. For those spaces, Speech Interference Levels (SILO limits are used to specify acceptable levels. Speech Interference Level, SIL is a measure of the effect of background noise on intelligible speech communication and numerically is the arithmetic average of the sound pressure level in decibels of the octave bands with center frequencies at 500, 1000 and 2000 Hz. For all other categories the permissible airborne noise levels are specified in each octave band in decibels. The noise level criteria for categories A to F including the SIL requirements are shown in Table 1. UNSILENCED SOUND LEVELS OF MARINE GAS TURBINES The accuracy of any acoustical analysis is 1. NARROW THROAT 4. EVASE EXIT 7. HEAVY PUNCHED FLANGES 2. STRAIGHT ACOUSTICAL 5. RUGGED BRACED HOUSING &SOLID ROUNDED NOSE PASSAGE 6. COMPRESSED LONG FIBER 3. CONTROLLED ANGLE BLANKET S. BELL MOUTH ENTRY Fig. 2 Cut-away view of a typical rectangular power-flow silencer limited by the reliability of the sound level data of the noise source. While it is possible to theoretically estimate the noise levels generated by a gas turbine and, in some case, this approach provides usable data, there is no substitute for actual Sound Power Levels of operating equipment. Whenever possible, this information should be obtained and guaranteed by the engine manufacturer. If the analysis is a preliminary one and the engines have not been selected, the Sound Power Level (PWL) data presented in Table 2 for 3

5 POWER-FLOW INLET SILENCERS MODU LINE NOISHIELD ENCLOSURE POWER-FLOW EXHAUST SILENCERS Fig. 3 Admiral Wm. M. Callaghan main propulsion gas turbine silencing system 25,000 H-P Main Propulsion Gas Turbine engines and 3500 H-P Auxiliary Power Gas Turbines may be used. The results obtained from using these data will generally prove to be conservative. NOISE CONTROL OF MARINE GAS TURBINES Once the noise levels have been determined and the selection of acceptable levels has been established, it is possible to select the appropriate noise control equipment required. A noise control system as indicated in Fig. 1 will, in most instances, assure satisfactory acoustic and aerodynamic operation of a gas turbine engine. Such a system will consist of the following silencing components for main propulsion or auxiliary power gas turbines: 1 Air Inlet Silencer 2 Exhaust Gas Silencer 3 Engine Enclosure 4 Ventilation Air Inlet Silencer 5 Air Inlet Duct Lagging 6 Exhaust Gas Duct Lagging 7 Reduction Gear/Generator Enclosure This silencing equipment should be designed for a 20-yr useful life, with due consideration given to the deterious effects that a marine environment will have on most materials of construction. The conditions of high temperature and high velocity that land gas turbine exhaust silencers are normally subjected to must also be considered. To ensure maximum effectiveness of the space available for silencing, experience dictates the utilization of splitter type silencers for the inlet, exhaust, and ventilation units. This type of silencer enables the noise control engineer to obtain an optimum balance between aerodynamic pressure loss and noise reduction since it is possible to minimize the aerodynamic losses across the silencer while developing the maximum noise reduction in the available space envelope. Fig. 2 illustrates a typical silencer element. The maximum effectiveness of a silencer is obtained by locating it as close as possible to the engine itself. This means placement at the base of the inlet and exhaust ducts. The location on deck where the inlet and exhaust openings are ducted are generally the area of acoustic concern and control the engine inlet and exhaust designs. The engine enclosure is a prefabricated structure that serves as a housing for the engine. It is constructed of sound absorbing panels designed to minimize build up of noise within the enclosure as well as to provide the required transmission loss across the walls. Doors, hatches, and piping penetrations must be designed to be compatible with the noise reduction provided by the basic panel enclosure. The reduction gear or generator enclosure is usually fabricated of the same type of acoustic structure as that of the engine housing. In both cases, these enclosures must provide accessibility to equipment, as well as to allow for removability of all components when required. Allowable levels in the engine room control the noise control design of the engine enclosure. It has generally been found that the exhaust duct design that offers the required heat loss across its walls, provides the necessary noise reduction required for satisfactory operation. In any event, both the inlet and exhaust ducting systems must be designed to ensure acceptable noise levels in the varied compartments adjacent to the ducting system path. This is accomplished by means of a double walled acoustical panel forming the basic structural element of the duct system. 4

6 Table 3 Dynamic Insertion Loss of Typical Ship Board Main Propulsion Gas Turbine Silencers, Decibels SHIP COMPONENT SILENCED Octave Band Center Frequencies, Hz Callaghan Inlet Callaghan Exhaust Polar Star Inlet DD963 Inlet DD963 Exhaust SILENCING THE ADMIRAL WM. M. CALLAGHAN MAIN PROPULSION GAS TURBINE The Admiral William M. Callaghan "Roll-On/ Roll-Off" Cargo ship, when commissioned in October 1967, was the worldls fastest and largest gas turbine powered merchant vessel. This 24,000 ton ship which is 696 ft in length is capable of speeds of 25 knots. The Callaghan was originally powered by two 25,000 H-P Pratt & Whitney FT 4A-2 gas turbine engines. These engines were subsequently replaced by General Electric Company Model LM-2500 Gas Turbines. However, the silencing system which was designed for operation with the Pratt & Whitney engines also proved aerodynamically and acoustically acceptable for the G.E. engines. Minor modifications were made to the engine enclosure to adapt to the physical difference of the silencers located in the engine inlet duct. The silencers were split into two modules to make removal between deck levels possible. The silencer was an all-welded fabrication with all steel surfaces galvanized to minimize corrosion. Parallel baffle type silencers were also employed to minimize engine exhaust noise. All steel components in the exhaust silencer were fabricated from hot rolled steel. The inlet and exhaust silencers were each designed to satisfy deck side requirements while the engine enclosure was designed to satisfy requirements in the engine room. The inlet system design was controlled by the requirement at the deck house for a sound pressure level not to exceed the sound pressure levels established by Category "B" of the U. S. Navy specification for noise and vibration aboard ship. Octave Band Center Frequency, Hz Allowable Sound Pressure Level, db, Re: Dynes/cm o o two engines. The Callaghan noise control system consists of inlet silencer, exhaust silencer, and engine enclosure for each of the engines. The system is shown schematically in Fig. 3. Silencing of the engine inlet noise is achieved by two stages of straight splitter The exhaust silencer was designed so that sound levels at topside operational stations were limited to a Speech Interference Level of 55. The exhaust system ducted 30 ft above the deck helped to minimize the length of exhaust treatment required. Dynamic Insertion Loss of the inlet and exhaust silencers is tabulated in 5

7 Table 4 Noise Reduction of Typical Marine Gas Turbine Engine Enclosures, Decibels SHIP Octave Band Center Frequencies, Hz Callaghan DDH DD963 Main Propulsion DD963 Auxiliary Power ENGINE INLET SILENCER 2. VENTILATION INLET SILENCER 3. ENGINE EXHAUST SILENCER 4. VENTILATION EXHAUST SILENCER 5. MODULINE ENGINE ENCLOSURE Fig. 4 Silencing system for the DDH-280 ship's service gas turbines Table 3. The engine enclosure was designed to satisfy Category "B" of specification in the engine room. Table 4 illustrates the noise reduction levels necessary to maintain this requirement. SILENCING THE DDH-280 SHIP'S SERVICE GAS TURBINES The four Canadian Armed Forces DDH-280 "Tribal Class" helicopter destroyers are considered the finest anti-submarine destroyers presently afloat. Displacing 4500 tons, these ships are equipped with the most sophisticated underwater surveillance and communications systems. Ship's service aboard this vessel are provided by four Solar, Division of International Harvester Company, Model T "Saturn" gas turbine engines. These generators provide enough power to supply a small city with all of its electrical power requirements. The DDH-280 class destroyers were designed to be among the quietest ships afloat, an obvious tactical advantage during anti-submarine missions. These gas turbines were designed in accordance with the Royal Canadian Navy's Specification for 6

8 Table 5 Dynamic Insertion Loss of Typical Ship Board Auxiliary Power Gas Turbine Silencers, Decibels SHIP COMPONENT SILENCED Octave Band Center Frequencies, Hz DDH-280 Inlet DDH-280 Exhaust DD963 Inlet DD963 Exhaust "Air Borne Noise Criteria for Shipboard Spaces." The specification for individual silencing components is tabulated below: thickness. This 11,000 ton CODOG-electric vessel is 400 ft in length and is the first icebreaker to be built in the United States since Sound Pressure Level, db Octave Band Center Frequency, Hz ft from inlet '3 ft from exhaust o eu3 ft from enclosure The complete noise control system for one of the ship's service gas turbines is illustrated in Fig. 4. The system consists of inlet silencer, engine exhaust silencer, ventilation air exhaust silencer and enclosurê. The intake silencer is somewhat unusual in that it is divided to draw air into the engine as well as cooling air into the engine enclosure. All steel surfaces in the air or gas stream are fabricated from type 304 stainless steel. Tables 4 and 5 summarize the acoustical performance provided by each of the silencing components. SILENCING THE ICEBREAKER "POLAR STAR" BOOST GAS TURBINES The U. S. Coast Guard "Polar Star" is intended to be the world's most powerful icebreaker having the capacity to ram through ice 21 ft in Boost power for breaking through ice is provided by three 25,000 H-P Pratt & Whitney FT 4A-12 marine gas turbines. The gas turbines are geared to independent shafts and are used for limited periods in thick ice fields when more than 18,000 H-P per shaft is required. The silencing provided for each gas turbine is as illustrated in Fig. 5. Only inlet silencing is required to meet the U. S. Coast Guard's noise criterion for this vessel. With all engines in operation, the noise levels at the inlet duct entrance will not exceed 90 dba. Table 3 indicates the silencer Dynamic Insertion Loss necessary to satisfy that specification. Due to space limitations on this vessel, the inlet silencers were designed to conform with the elliptical shape of the ship's inlet ducting. The straight splitter silencers employed were fabricated as three separate elements as shown. 7

9 POWER-FLOW INLET SILENCER POWER-FLOW INLET SILENCER VENTILATION SILENCER Fig. 5 Inlet silencer system for the U. S. Coast Guard icebreaker "Polar Star" boost gas turbines POWER-FLOW EXHAUST SILENCER Fig. 6 DD-963 main propulsion gas turbine silencing system SILENCER TURBINE SECTION ROOF PANEL GEARBOX SECTION ROOF PANELS TURBINE SECTION END WALL OVER PRESSURE RELIEF PANEL GEARBOX SECTION END WALL BLOW IN PANEL PANEL ACCESS PLUG TURBINE SECTION SIDE WALL CI I MAIN COUPLING REMOVAL ACCESS PLUG ACCESS PLUG GEARBOX SECTION SIDEWALL 8 Fig. 7 Enclosure for the DD-963 auxiliary power gas turbine engines