GTB Gas Turbine Basalt
The Proprietary Proven Solution for Gas Turbine Exhaust Silencing
Concern about the lack of stability of refractory ceramic fibre used in dynamic silencing applications and the consequential expulsion of RCF material into the environment have been key drivers in the selection of basalt fibrous material as the preferred replacement for RCF materials. In late 2002, following a thorough test programme to establish suitability for purpose, General Electric Power Systems specified Lancaster GTB Gas Turbine Basalt as the successor to RCF in gas turbine exhaust silencers. Approved and used by the leading turbine manufactures, GTB has become the industry benchmark for "fit-for-purpose" GT exhaust silencer infill. Industry-wide awareness of the acceptability of GTB as the successor to RCF has created an increase in demand for "Gas Turbine Basalt" material, providing some motivation to the perception that fibrous materials produced from basalt rock will be generically equivalent to Lancaster's GTB Gas Turbine Basalt. This is not the case, as indicated below.
The Gas Turbine Exhaust Silencer Environment
The gas turbine exhaust environment places a unique combination of stresses upon fibrous materials used in silencers and ducts. The hot exhaust efflux has an air-rich composition, creating a highly oxidative environment within ducts, silencers and stacks. This causes mineral fibre containing oxidisable iron to transform into a red/brown powder at temperatures as low as 208'C. This does not happen with GTB Gas Turbine Basalt.
The highly dynamic gas flow produces thermal imbalance, turbulence and pronounced vibrations at discrete frequencies. Locational stability under such conditions can only be maintained by using high-diameter fibres at relatively high density. GTB Gas Turbine Basalt is specifically manufactured to achieve this requirement.
The combination of the above effects will results in the rapid failure of unsuitable materials.
Basalt Deposits Are Not All The Same
Basalt deposits occur naturally as a result of volcanic activity. Differences in compositions, together with the effects of weathering over many years, mean that many basalt deposits are not chemically suitable for production of "Gas Turbine Basalt" materials. Thorough research has been undertaken by Lancaster to identify the most suitable basalt source rock. Following the analysis of many source rock samples from the U.K., South America, Europe, and the Middle East, a deposit in Scotland was clearly identifiable as being the most suitable for the production of Lancaster's GTB Gas Turbine Basalt.
Importance of the Manufacturing Process
The oxidising nature of the melting process operated by Lancaster is essential in maintaining the favourable chemistry of the source rock for GTB Gas Turbine Basalt. Similarly, the process parameters ensure the production of high-diameter fibres that not only provide the correct flow resistivity at a density that ensures locational stability, but also enable the GTB material to remain unclassified under the EU Directive on Man Made Vitreous Fibres (EU DG 97/69EC, 67/548/EEC MMVF AMENDMENT).
The Proven Solution
- Unique level of experience - 25 years of field operations.
- Unique endorsement - Specified by GE, approved by all turbine manufactures.
- Unique track record - No failures due to infill loss.
- Unique portfolio of machine types and applications - "Frame" machines, aero-derivatives and aviation test cell silencing.
GTB Basalt mineral fibre Cotton stitched mattress in rolls Wired mattress (stainless or galvanised) Engineered pillows - made to fit original equipment Engineered systems for cost effective refurbishment Tested fire barrier materials, Materials for brake lining Specialist fabrics, facing and needlemats Continuous filament glass fibres.
Quality System approved to ISO 9001
Responsive to customer's needs All our clients benefit from our free technical advice, developed over 30 years of experience of high temperature insulation materials
Gas Turbine and Aviation
An unrivalled applications knowledge base and the benefit of more than twenty years of relevant experience combine to make Lancaster GTB Systems Ltd the benchmark choice in the provision of acoustic treatment for industrial gas turbine installations and aviation test cells.
In applications ranging from gas compression (GC) and co-generation (CHP) plants to the largest combined cycle gas turbine (CCGT) power generating stations, the Company designs and supplies products that consistently achieve long service life and sustain high levels of acoustic and thermal performance in the most arduous of operating environments. A focussed "hands-on" approach, both at Corporate and Project level, ensures the realisation of client needs and contract requirements.
Whether the requirement is for fully tailored modular infill systems or for bulk materials supplied in roll form, the Company's products and services are approved by major turbine manufacturers General Electric, Siemens Westinghouse, Alstom, Pratt & Whitney and by airlines, defence organisations and leading silencer designers / fabricators.
The most widely specified product is Lancaster GTBŪ (Gas Turbine Basalt) fibre, having been developed by the Company as a non-carcinogenic, environmentally acceptable alternative to refractory ceramic fibre. GTBŪ enables top acoustic performance with high reliability and is in service in 100's of projects world-wide involving exhaust ducts, silencers, stacks and HRSG linings.
SCOPE for LANCASTER GAS TURBINE BASALT (GTBŪ)
MATERIALS in HRSG BOILERS and ASSOCIATED
With 15 years field experience, Lancaster GTBŪ thermal / acoustic fibrous basalt materials are the benchmark choice for highly dynamic hot gas silencing applications. Possessing the reputation of being "fit for purpose" in large gas turbine silencers, the high fibre diameter of these materials ensures low flow resistivity, promoting freedom of silencer design and providing exoneration from classification under the European Directive on categorisation of MMVF's, based on their potential for carcinogenicity.
Scope for the success of Lancaster Gas Turbine Basalt materials in boiler and duct lining applications can be explained by reference to three key drivers, as described below.
EU Directive 97/69EC (67/548/EEC Amendment) classifies refractory ceramic fibre as a Category 2 Carcinogen. In contrast, Lancaster GTBŪ materials are unclassified under the Directive (by the nature of fibre size).
Proven capability within gas turbine silencer baffles, internally lined diffusers, transition ducts and silencer casings has visibly demonstrated the capability of Lancaster GTBŪ materials. Deployment into HRSG applications allows for a comfortable safety margin, as indicated by the comparison of conditions given below.
Hot Gas Silencing Environment
- High gas velocity, significant impingement
- Very severe flow dynamics
- Perforated s/steel sheet, low thermal mass
- Rapid heat-up, especially if close to the turbine
- Direct impingement from turbine washing solutions
Boiler Lining Environment
- Low gas velocity, no direct impingement
- Mild flow dynamics
- Solid s/steel sheet, higher thermal mass
- Slow heat-up of structure and insulation.
- Little direct impingement from turbine solutions washing solutions.
Lancaster GTBŪ materials can reliably sustain working temperatures of 750'C and cope with transient conditions to 800'C.
GTBŪ materials in roll form can offer significant commercial advantage when compared with ceramic fibre materials.
More recently, Lancaster GTBŪ materials have quickly established themselves as a preferable alternative to the use of refractory ceramic fibres, soluble ceramic substitutes, and combination make-ups (ceramic substitutes plus bonded mineral fibre batts) within internally lined HRSG boilers and associated ductwork.
When supplied in pillow form, which on installation requires no cutting and generates no loose fibres nor waste, there is still an important cost saving compared with using ceramic fibre. Such materials significantly reduce installation time and hence reduce post-supply costs, providing a net saving on contract costs.back to the top
Lancaster Fireshield Total Comfort
Comprises an unbonded inorganic fleece of basalt mineral fibre, surfaced with facing tissue and reinforced with hexagonal galvanised steel netting, further consolidated with steelwire stitching. Principal applications are as fire barriers in roof or other voids to provide half or one hour fire rating in accordance with BS476, Parts 4, 6, 7 and 22.
Lancaster Wired Mattress
Manufactured from basalt mineral fibre with multiple stitching across its width and hexagonal steel mesh on one or both sides, or with stainless steel mesh for special applications where corrosion resistance is required. Offered in standard densities and a choice of thicknesses and roll sizes, Wired Mattress can also be made to size for specific applications.
Principal applications are for high temperature thermal, acoustic and fire safe insulation around boilers, tanks, process pipes, vessels, ducts, steam and gas turbines and turbo diesel applications.back to the top
Projects / News
Technical Aspects - Questions & Answers
HOW CAN HIGH DURABILITY WITH TOP ACOUSTIC PERFORMANCE BE ACHIEVED?
Durability is achieved by ensuring that the packed density of a silencer baffle is high enough to ensure the locational stability of the infill. Acoustic performance is achieved by the infill having the correct permeability (flow resistivity) to allow resistive passage of the noise energy into the absorbing core. The use of fine fibrous materials, such as ceramic fibres, required low density packing, so acoustic performance could only be achieved at the expense of durability. In contrast, the high fibre diameter of Lancaster GTB basalt fibres allows the packing density necessary to ensure locational stability and still achieves the level of permeability necessary for the best acoustic performance.
WHY IS HIGH PACKING DENSITY IMPORTANT IN GT EXHAUST SILENCERS?
High density is essential to maintain the locational stability of the silencer infill when subjected to highly turbulent gas flow. If the silencer infill undergoes repetitive movement, fibres will be broken down and volumetric packing capability will be lost, even if the infill itself remains. Loss of volume will allow the baffles to resonate, leading to fractured welds and possible break up of the baffles.
WHAT IS THE SIGNIFICANCE OF THE DIFFERENT UNITS OF MEASUREMENT RELATING TO ACOUSTIC PERFORMANCE?
Three different units are frequently referred to when discussing acoustic performance. These are described briefly as follows:
IINSERTION LOSS (dB) - Refers to the reduction in the measured downstream noise level which occurs when a silencing element is placed within a duct subjected to an upstream noise source. Insertion loss is generally used by a silencer designer to establish the required baffle arrangement and passage length. It can be theoretically modelled using library data obtained from reference baffles.
ACOUSTIC ABSORPTION COEFFICIENT (á) - Refers to the sound absorption capability of a porous material on a frequency-specific basis. Usually displayed as an acoustic spectrum showing a plot of alpha value (a) vs. frequency (Hz), acoustic absorption coefficients are material-specific. Alpha values assist the suppliers of acoustic materials in assessing the suitability of fibrous infill make-ups for particular applications, provided the predominant frequencies are known or can be reliably predicted.
FLOW RESISTIVITY (Rayls/m) - Flow resistivity is the single most important factor towards achieving optimum acoustic performance from bulk fibrous materials with favourable sound absorption characteristics. Flow resistivity is typically quoted in MKS Rayls/m and refers to the resistance offered to the passage of airborne pressure waves (sound energy) through the bulk fibrous material (acoustic absorber). The correct flow resistivity will cause the sound pressure waves to "do work" in passing through the bulk fibrous absorber, converting kinetic energy into thermal energy during the process. If the flow resistivity value is too low, the pressure waves will pass through the fibrous material with very little effort, which will barely reduce their kinetic content. If the flow resistivity value is too high, the sound pressure waves will be reflected rather than pass into the fibrous material, again failing to give up their kinetic content. For a gas turbine exhaust silencing application, the flow resistivity value for the core absorber should typically be between 14,000 and 18,000 MKS Rayls/m using a baffle thickness of around 450mm (18").
HOW DOES SILENCER BAFFLE THICKNESS INFLUENCE ACOUSTIC PERFORMANCE?
Thick baffles will absorb lower frequency (longer wavelength) noise more effectively, whilst thin baffles are more effective with higher frequency (shorter wavelength) noise. In GT exhaust silencers the baffles will generally be between 400 and 600mm thick, whilst in intake silencers, where high frequency compressor noise is evident, the baffles will typically be less than half this thickness. Aircraft test cells running very large fan engines may have baffles up to around 1m (40") thick. The drawback with thick baffles is that, unless there is a very long effective path length, they cause re-generated noise due to turbulent flow.
ARE PILLOW MODULES PREFERABLE TO BULK INFILL MATERIALS?
Pre-weighed, pre-sized pillow modules provide a "measured system" approach, offering fast fibre-free installation with no shop-floor waste. Although more expensive, this approach can significantly reduce labour time, especially if there is little previous experience of packing silencers. Bulk materials can be preferable in cases where hands-on supervision is present and the workforce is experienced in packing silencers using bulk materials and capable of achieving uniformity of the packing throughout the silencer.
DO PILLOW MODULES ENSURE HIGHER DURABILITY THAN MANUALLY PACKED INFILL?
Not necessarily. However, it is much easier to be confident of the durability of a pillow-based infill system than is the case with a manually packed silencer. This is because the pillow modules are constructed to a pre-determined specification, based upon the operating conditions. In manual packing, although the materials may be the same, operational durability will be influenced by the consistency of the packing process and, in particular, by the diligence which has been applied in correctly locating the facing materials overlaying the core absorber material.
WHAT CAUSES POOR INFILL DURABILITY IN GT EXHAUST SILENCERS?
There are three frequently encountered causes of poor durability: -
- Insufficient packing density, resulting in a lack of locational stability.
- Use of inappropriate thermal insulation, containing organic thermally degradable binders.
- Inadequate protection of fibrous core material, allowing fibre loss in turbulent in gas flow.
Note: Durability can also be affected by damage arising from non-specified operational circumstances.
DO PILLOW-BASED INFILL SYSTEMS GIVE BETTER ACOUSTIC PERFORMANCE THAN MANUALLY PACKED INFILL?
Provided that the flow resistivity is the same, there should be no difference in acoustic performance between pillow-based infill and manually-packed infill.
WHAT IS THE MOST COMMON CAUSE OF INFILL-INDUCED ACOUSTIC PERFORMANCE SHORTFALL?
The "choking" effect of resistive facing materials (acting upon the flow resistivity of the infill as a whole) is frequently underestimated, leading to a shortfall in the acoustic performance. This is most likely to happen when the core absorbing material and the facing material(s) are ordered from different suppliers. The situation can be prevented by ascertaining that the flow resistivity of the facing materials is equal to (or less than) that of the core absorber. A key attribute of pillow modules is that these are supplied with the flow resistivity of the outer envelope "in balance" with that of the core absorber.
HOW DOES THE SUPPLY OF A "ROLL BASED INFILL SYSTEM" DIFFER FROM THE PURCHASE OF "BULK MATERIALS"?
A roll-based system comprises a "kit" of the same materials as those used to construct pillow modules of the required specification. The core absorber is supplied together with overlaying felts and/or fabrics in the appropriate width and system quantities, all in roll form. Bulk materials are generally supplied on an individual basis, irrespective of the supply of any other materials.
WHAT IS THE SIGNIFICANCE OF THE "ZONING" OF SILENCER INFILL MATERIALS?
Zoning refers to selectively packing specific regions within the silencer. It is generally applied using pillow modules, but is also applicable by using a mixture of pillow modules and bulk materials. Zoning demands a thorough profile of the operating conditions within the silencer (usually provided by CFD data) in order to locate higher and lower specification infill in appropriate "zones" of the silencer. The object of zoning is to provide cost saving without compromising reliability. It can be considered as a fine tuning of the "technical/commercial window" (see above).
WHAT ARE THE MAXIMUM TEMPERASTURE AND VELOCITY CAPABILITIES FOR A BASALT-BASED INFILL?
The generic limiting temperature of basalt fibre is 820'C (1508'F). However, in order to allow an adequate margin of safety in large thermal systems, the maximum operating temperature is normally considered as being 775'C (1427'F). Silica, stainless steel and HT felts and fabrics ensure the retention of fibres at these temperatures. Although maximum velocity capability is influenced by operating temperature, Basalt-based infill systems can be supplied to cope with passage velocities in excess of 100m/s (325ft/sec) at operating temperatures up to 700'C (1290'F).
WHAT IS MEANT BY THE OXIDATION OF BASALT FIBRE IN GT EXHAUST DUCTS?
Not all basalt source rock deposits are suitable for production of fibre for gas turbine
exhaust applications. The air-rich exhaust flow will cause oxidation of the ferrous iron
content in unsuitable basalt fibre types at temperatures as low as 250-350'C (480-660'F),
converting the fibres to red powder. LANCASTER GTBŪ (GAS TURBINE BASALT) IS A PROVEN MATERIAL
IN AIR-RICH GT EXHAUST ENVIRONMENTS AND WILL NOT OXIDISE.
ARE THE FIBROUS MATERIALS SUPPLIED BY LANCASTER ENVIRONMENTALLY SAFE?
All fibrous materials supplied by Lancaster are unclassified under established regulatory procedures and are therefore considered to be environmentally safe on respect of handling, installation, operation and eventual disposal.back to the top