Man B&W S50MC-C8-TII - Power Related Unbalance; Calculation of External Moments; Power Related Unbalance (PRU) Values in Nm;Kw

MAN B&W 17.04
Page  of 
MAN Diesel
MAN B&W S50MC-C8, S50ME-C/ME-B8 198 69 88-8 .0
Power Related Unbalance
To evaluate if there is a risk that st and 2nd or-
der external moments will excite disturbing hull
vibrations, the concept Power Related Unbal-
ance (PRU) can be used as a guidance, see Table
7.04.0 below.
PRU =
External moment
___________
Engine power
Nm/kW
With the PRUvalue, stating the external moment
relative to the engine power, it is possible to give
an estimate of the risk of hull vibrations for a spe-
cic engine.
Based on service experience from a great number
of large ships with engines of different types and
cylinder numbers, the PRUvalues have been
classied in four groups as follows:
Table 17.04.01: Power Related Unbalance (PRU) values in Nm/kW
Calculation of External Moments
In the table at the end of this chapter, the exter-
nal moments (M

) are stated at the speed (n

) and
MCR rating in point L

of the layout diagram. For
other speeds (n
A
), the corresponding external mo-
ments (M
A
) are calculated by means of the formula:
M
A
= M

x
{
n
A
__
n

}
2
kNm
(The tolerance on the calculated values is 2.5%).
PRU Nm/kW Need for compensator
0 - 60 Not relevant
60 - 20 Unlikely
20
- 220 Likely
220 - Most likely
S50MC-C8/ME-C8/ME-B8 – 1,660 kW/cyl at 127 r/min
5 cyl. 6 cyl. 7 cyl. 8 cyl. 9 cyl. 10 cyl. 11 cyl. 12 cyl. 14 cyl.
PRU acc. to st order, Nm/kW . 0.0 4.7 3.8 2.6 N.a. N.a. N.a. N.a.
PRU acc. to 2nd order, Nm/kW 27.5 74.0 8.4 0.0 6. N.a. N.a. N.a. N.a.
Based on external moments in layout point L

N.a. Not applicable