Forces On Large Steam Turbine Blades: RWE Npower
Forces On Large Steam Turbine Blades: RWE Npower
Forces On Large Steam Turbine Blades: RWE Npower
RWE npower
Mechanical and Electrical Engineering
Power Industry
INTRODUCTION
RWE npower is a leading integrated UK energy
company and is part of the RWE Group, one of
Europe's leading utilities. We own and operate a
diverse portfolio of power plant, including gas-
fired combined cycle gas turbine, oil, and coal
fired power stations, along with Combined Heat
and Power plants on industrial site that supply
both electrical power and heat. RWE npower also
has a strong in-house operations and engineering
capability that supports our existing assets and
develops new power plant. Our retail business,
npower, is one of the UK's largest suppliers of
electricity and gas.
In the UK RWE is also at the forefront of
producing energy through renewable resources.
npower renewables leads the UK wind power
market and is a leader in hydroelectric
generation. It developed the UK's first major off-
shore wind farm, North Hoyle, off the North Wales Figure 1: Detailed view of turbine blades
coast, which began operation in 2003.
With blades (see Figure 1) rotating at such
Through the RWE Power International brand, speeds, it is important that the fleet of steam
RWE npower sells specialist services that cover turbines is managed to ensure safety and
every aspect of owning and operating a power continued operation. If a blade were to fail in-
plant, from construction, commissioning, service, this could result in safety risks and can
operations and maintenance to eventual cost £millions to repair and, whilst the machine is
decommissioning. not generating electricity, it can cost £hundreds of
SCENARIO thousands per day in lost revenue.
In thermal power plants, energy is extracted from It is therefore important to understand the forces
steam under high pressure and at a high and resultant stresses acting on a blade and its
temperature. The steam is produced in a boiler or root (where it is connected to the rotor disc) due
heat recovery steam generator and is routed to a to its rotational speed. Turbine blades are
steam turbine where it is partly expanded in a designed with margins of safety and the
high pressure stage, extracting energy from the calculation of blade stresses gives an
steam as it passes through turbine blades. The understanding of how they behave in-service
steam is then returned to the boiler for reheating under a variety of operating conditions.
to improve efficiency, after which it is returned to Knowledge of the forces and stresses is used
the turbine to continue expanding and extracting when assessing the damage to a blade or
energy in a number of further stages. The steam assessing the effects of using different materials.
turbine shaft rotates at 3000 revolutions per PROBLEM STATEMENT
minute (rpm) (50 revolutions per second).
With the knowledge that an understanding of the
forces and stresses acting on the turbine blades
is of vital importance, in this exemplar we will
calculate such a force acting on a last stage Low
Pressure (LP) blade and root of a large steam
turbine rotating at 3000 rpm in order to estimate
the material stresses at the blade root. One such
LP steam turbine rotor is shown in Figure 2
below:
Consider a small segment of mass δm , of length
having width δr at a distance r from the centre.
Then the equation for the centripetal force δF on
this small segment is given by:
δF = δmω 2 r … (2)
In practice, a blade tapers in thickness towards its
tip; but, for simplicity, assuming the blade to have
a constant cross sectional area A (m2) and
material density ρ (kg/m3), we can write:
δm = ρAδr
and equation (2) becomes:
δF = ρAω 2 rδr
or formally: