s90mec7

Exhaust gas silencer

The typical octave band sound pressure levels from the diesel engine’s exhaust gas system – at a distance of one meter from the top of the exhaust gas uptake – are shown in Fig.15.04.02.

The need for an exhaust gas silencer can be decided based on the requirement of a maximum permissible noise level at a specific position.

The exhaust gas noise data is valid for an exhaust gas system without boiler and silencer, etc.

The noise level is at nominal MCR at a distance of one metre from the exhaust gas pipe outlet edge at an angle of 30° to the gas flow direction.

For each doubling of the distance, the noise level will be reduced by about 6 dB (far field law).

When the noise level at the exhaust gas outlet to the atmosphere needs to be silenced, a silencer can be placed in the exhaust gas piping system after the exhaust gas boiler.

The exhaust gas silencer is usually of the absorption type and is dimensioned for a gas velocity of approximately 35 m/s through the central tube of the silencer.

An exhaust gas silencer can be designed based on the required damping of noise from the exhaust gas given on the graph.

In the event that an exhaust gas silencer is required – this depends on the actual noise level requirement on the bridge wing, which is normally maximum 60 70 dB(A) – a simple flow silencer of the absorption type is recommended. Depending on the manufacturer, this type of silencer normally has a pressure loss of around 20 mm WC at specified MCR.

MAN B&W S90MC-C/ME-C7/8

The exhaust gas back pressure after the turbo charger(s) depends on the total pressure drop in the exhaust gas piping system.

The components, exhaust gas boiler, silencer, and spark arrester, if fitted, usually contribute with a major part of the dynamic pressure drop through the entire exhaust gas piping system.

The components mentioned are to be specified so that the sum of the dynamic pressure drop through the different components should, if possible, approach 200 mm WC at an exhaust gas flow volume corresponding to the specified MCR at tropical ambient conditions. Then there will be a pressure drop of 100 mm WC for distribution among the remaining piping system.

Fig. 15.05.01 shows some guidelines regarding resistance coefficients and back pressure loss calculations which can be used, if the maker’s data for back pressure is not available at an early stage of the project.

The pressure loss calculations have to be based on the actual exhaust gas amount and temperature valid for specified MCR. Some general formulas and definitions are given in the following.

Exhaust gas data

M: exhaust gas amount at specified MCR in kg/sec. T: exhaust gas temperature at specified MCR in °C

Please note that the actual exhaust gas temperature is different before and after the boiler. The exhaust gas data valid after the turbocharger may be found in Chapter 6.

Exhaust gas velocity (v)

In a pipe with diameter D the exhaust gas velocity is:

v = Mρ ​x π x4D2​in m/s

Pressure losses in pipes (∆p)

For a pipe element, like a bend etc., with the resistance coefficient ζ, the corresponding pressure loss is:

∆p = ζ x ½ ρ v2 x 9.811 ​in mm WC

where the expression after ζ is the dynamic pressure of the flow in the pipe.

The friction losses in the straight pipes may, as a guidance, be estimated as :

1 mm WC per 1 diameter length

whereas the positive influence of the up draught in the vertical pipe is normally negligible.

Pressure losses across components (∆p)

The pressure loss ∆p across silencer, exhaust gas boiler, spark arrester, rain water trap, etc., to be measured/ stated as shown in Fig. 15.05.01 (at specified MCR) is normally given by the relevant manufacturer.

Total back pressure (∆pM)

The total back pressure, measured/stated as the static pressure in the pipe after the turbocharger, is then:

∆pM = Σ ∆p

Mass density of exhaust gas (ρ)

ρ 1.293 x 273273+ T x 1.015 in kg/m3

The factor 1.015 refers to the average back pressure of 150 mm WC (0.015 bar) in the exhaust gas system.

where ∆p incorporates all pipe elements and components etc. as described:

∆pM has to be lower than 350 mm WC.

(At design stage it is recommended to use max. 300 mm WC in order to have some margin for fouling).

MAN B&W MC/MC C, ME/ME C/ME GI/ME-B engines

MAN B&W MC/MC C, ME/ME C/ME GI/ME-B engines

Page of 3

ζ = 0.05

ζ = 0.14

178 06 85 3.0

MAN B&W MC/MC C, ME/ME C/ME GI/ME-B engines

MAN B&W S90MC-C7/8, S90ME-C7/8

MAN B&W S90MC-C7/8, S90ME-C7/8

Diameter of Exhaust Gas Pipes

The exhaust gas pipe diameters listed in Table 15.07.02 are based on the exhaust gas flow capacity according to ISO ambient conditions and an exhaust gas temperature of 250 ºC.

Page of 1

The exhaust gas velocities and mass flow listed apply to collector pipe D4. The table also lists the diameters of the corresponding exhaust gas pipes D0 for various numbers of turbochargers installed.

MAN B&W S90MC-C7, S90ME-C7