Hydraulic design outline
Introduction.
Alab is a commercially available program for design, operation simulation and power production calculation for hydropower plants.
The Hydraulic design outline is compiled mainly from the Alab program help text for the turbine designer module.
This document is general, not related to a specific project.
Output from Alab defines all turbine parts geometrically. Performance data necessary for operation simulation will automatically be available.
Summary
Alab is designed for:
Projecting new plants,
Evaluating old plants,
Modification of plants,
Operating support,
Operation simulation,
Production estimating,
Education, training.
Including turbine designer for:
Francis,
Mini Francis,
RPT,
Pelton,
Twin Pelton,
Kaplan,
Bulb.
Francis designer includes reversible pump turbines of Francis type.
The program does a practically complete hydraulic dimensioning of turbines and waterway. The turbine parts surrounded by water e.g. stay vane, guide vane and runner blades are also mechanically dimensioned, although by simple methods. The program presents the turbine losses in details. Losses due to guide vane clearances are, for example, calculated based on deformation of cover and ring, elongation of stay vane and clearance in "dry" state combined with actual roughness in the clearance. The program deals with design- and as built versions of the turbines.
The waterways with dams, tunnels, power station etc. are created by the aid of a drag-drop technique.
An editing program assists in generator and transformer definition.
The program is a normal Windows-program. Source code is C++.
The practical approach
As a comprehensive tool, Alab software offers many features for the design and evaluation of hydro power units and entire plants. In the following the normal design process for turbines is outlined.
Considering that plant levels are defined the minimum of input data for turbine designer will be nominal head, nominal power output and turbine level. If speed is undefined Alab will propose a default speed related to the actual grid frequency and head/power.
To meet the defined plant properties as suction head, dynamic head rise and energy production etc. in an optimal way a design trial and error process is normal. The interactive menus allow designer to alter parameters of the design, which might impact the geometry as well as the performance shown in diagrams or tables.
This trial and error loop must also take the waterway and generator/transformer into consideration. But as the waterway is a part of Alab turbine data are compatible and directly accessible from the waterway.
Based on the hydraulic design tables defining the turbine parts geometrically are created. These tables may be input for CAD programs.
Operation simulation
All Alab elements are principally defined as Δh = f (flow, speed, level, p1, p2, p3…). The parameters p1, p2… are related to geometry. Roughness is also considered as a geometrical parameter. Speed is for runner. Alab solves these normally nonlinear equations and the result is presented as different reports available for the user or as input for other program modules.
Simulation is carried out by controlling turbines servo stroke. Simulation can be performed as steady state- or dynamic operation. Load rejection, load acceptance or reduction gives information about speed and pressures related to the turbines but also levels, pressures and flow variations throughout the whole plant. In case of steady state operation with waterway included losses and local pressures for all plant items are calculated in addition to turbine flow, power output, efficiencies including plant efficiency.
Calculation of losses
An important part of the program is loss calculation.
The algorithm for flow calculation is based on the equations of Euler, Bernoulli and Colebrook.
Friction losses depending of local geometry, velocity, roughness and Reynolds number is the base for global loss mechanisms. This means that the loss mechanisms are valid for all types and sizes of turbines. This goes also for tunnels and tubes. Wetted perimeter is calculated from the section geometry, roughness, actual flow velocity and Reynolds number as input in Colebrook's equation gives losses. Observe that the friction losses are not exactly proportional to the square of flow. Friction losses are the dominating losses around the turbine design point. The design point is in the program defined as the operating point where all attack losses is zero and the flow rotation in the draft tube is minimum. This point is for normal Francis turbines close to best point, but for reversible pump turbines and Kaplan turbines the difference between design point and best point may be considerable. The scale effect on efficiency related to efficiency differences for model and prototype is automatically taken care of due to the generic properties of loss mechanisms.
The Reynolds number effect on attack- and trailing losses
or generally whirl losses is not well known. But it is not demanded the same exactness for loss calculations far from normal operating point. During runaway are whirl losses the essential losses. Calculation of runaway speed may therefore suffer due to that. Whirl losses are in the program based on empirical data which is "globalised", meaning that the same loss mechanism is used for all turbines.
Report examples.
Mussel diagram.
Loss distribution.
Turbine performance.
Turbine elements.
Spiral case.
Design of spiral case is based on parameters defined below. Spiral case is divided in a number of sections defined by the section angle.
Spiral case dimensions are referred to the inner wet surface of the shell.
Friction losses are the sum of losses for each section. Absolute roughness is considered to be constant.
Flow velocity and Reynolds’s number will vary from section to section.
Parameters.
Spin factor.
Spin is here angular momentum (radius x velocity, m^2/sec.).
Due to the spiral curvature velocities will vary over cross sections.
It takes a certain distance from inlet cross section to establish the velocity profile. Velocity profile will also be influenced of cross sections area variation,
velocity acceleration.
Setting a “Spin factor" give a possibility to take these effects into consideration
when calculating flow angle for stay vane inlet. See Design section angle below.
"Spin factor" varies between 1.0 and 0.0. For 1.0 spin is constant and calculated based on mean radius and velocity,
for Spin factor = 0.0 velocity is constant equal to mean velocity.
Values between 1.0 and 0.0 give velocities accordingly.
Value is 0.9.
Section angle.
Angle, degrees, between cross sections.
Value is 15.0.
Velocity acceleration.
Rate of change of mean velocity in flow direction.
Velocity acceleration = 1.0 means constant velocity, if it less than one mean velocity will decrease.
Value is 1.05.
Stay vane.
The stay ring consists of upper and lower discs with the stay vanes in between.
By default stay vanes are considered as pure structural items, hydraulically neutral with spin variation 0.0.
Spin variation is variation in spin-energy from inlet to outlet related to total energy. A negative value indicates spin-energy reduction.
Stay vanes designed to be hydraulically active will be checked for hydraulic properties related to the stay vane cascade.
A simple mean stress calculation determines the thickness and the elastic deformation which will influence the clearance between guide vanes and upper and lower cover.
The deformation is exported to "Guide vane" and listed together with other deformations influencing the clearance.
The stay vane nose height is considered to be a "Spiral case" property
Stay vane tail height is a "Guide vane" property.
In addition to the mechanical calculations friction losses and inlet/outlet losses are calculated.
The losses is calculated according to Alab principles taking into consideration Reynolds’s number, roughness and velocity gradients normal to the mean velocity.
By default the disc thickness is set to two times stay vane maximum thickness. The disc inner- and outer diameters are modified by half of the disc thickness.
The outer edge is rounded by a radius equal to 0.5 * disc thickness.
The spiral case shell plates are defined taking into consideration the stay vane height and discs thickness.
Inlet edge of stay vane number one must be modified to match inlet cone.
Ref: Spiral case.
Stay vane format.
The draft drawing shows the local co-ordinate system with terms and data involved in the definition of the stay vane.
Guide vane.
The guide vane will match the outlet angle from stay vane and runner inlet angle according to the conditions defined by Design point.
A simple stress-calculation for the vane is done and thickness set according to the defined stress-level. The stress is checked for two planes.
Guide vane moment is calculated outgoing from the force created by guide vane tangential acceleration. This force is anticipated to be a component of a total force normal to vane camber-line attacking at a distance 37.5 % from inlet. Force due to flow-friction is not taken into consideration. Friction forces will be less than calculation tolerances for this method. The moment for closed vanes and standstill is calculated for a pressure difference equal to nominal head.
Deformation of turbine covers.
To get the total clearance between guide vane ends and turbine-covers the turbine-covers angular flexibility is included as a guide vane parameter, because covers are not a separate element.
Forces are calculated and give, combined with cover geometry and flexibility a contribution to guide vane clearance gap.
The gap in a "dry condition" and stay vane extension is added.
The total clearance is equally shared between the two guide vane ends.
In addition to losses due to the clearance gap also inlet/outlet losses and friction losses is taken into consideration.
The friction losses is calculated according to Alab principles taking into consideration Reynolds’s number, roughness and velocity gradients normal to the mean velocity.
Definition of format.
The guide vane draft shows the local co-ordinate system, terms and dimensions involved in the definition of the guide vane.
Opening.
Pre stress.
Alab uses factor <Pre-stress> to, at least principally, make up for deformation and hysteresis in the servo-system.
If this factor is set to 5% servomotor has to move 5% of nominal stroke before guide-vane starts moving.
Runner design.
Alab will make a runner for a new turbine based on default parameters related to specific speed and head.
Changing axial section will start redesign of turbine-parts.
Runner blade design.
The runner blade is designed based on mean streamline properties combined with parameters set by the user.
The velocities and streamlines in the axial view of the runner are dependent on the radius of curvature for the streamlines and the rate of change of angular momentum.
The equations involved are solved in an iteration cycle taking all turbine parts into consideration.
Outline of method.
Blade is considered as a “stream surface” controlling flow through the turbine runner.
One of the important parameters derived from blade shape is the distribution of angular momentum.
In Alab distribution of angular momentum is an input and a stream surface is calculated.
This stream surface is slightly modified to get the geometrical blade definition.
Tools are available to display the geometrical- and hydraulic details for the blade.
The routine "Blade forge" creates by iteration a blade surface according to the defined angular momentum-field and calculates axial flow streamlines.
The blade displacement is related to the actual blade thickness set by user.
Below find the blade for a Francis turbine with low specific speed. The axial section geometry has great influence on blade shape. Alab has several parameters accessible for users to modify axial section.
Draft tube.
The draft-tube is considered to consist of three parts, Cone, Bend and Diffuser. Ref. draft drawing.
The "Draft-tube" methods calculate friction losses, losses due to boundary layer separation and outlet losses.
Both cone and diffuser are characterized by an angle. The cone-angle is defined on the draft drawing. The diffuser-angle is a virtual angle giving same rate of change of cross-sections (and velocities) as for a cone.
The draft-tube is not able to convert spin energy leaving runner to pressure energy. Spin will give rise to low pressure at cone center and increased pressure at cone wall. This will in turn increase axial velocity at wall and result in back-flow at center, a flow-situation with poor stability that may result in pulsations and cavitation noise.
Spin energy losses is considered as a draft-tube property.
Definition of format.
Draft drawing shows terms and figures involved in draft-tube definition.
Alab considers theoretically Outlet loss as a draft-tube property. But IEC-code rules that this loss is a waterway property.
This is taken care of in a proper way in the methods where the problem may come up.
Shaft sealing.
The shaft sealing system recommended in Alab combines shaft labyrinths and a pump integrated in runner hub.
Air admission.
Air admission is necessary to stabilize draft tube flow and reduce surges. Amount of air should be kept as small as possible to maintain draft tube efficiency.
Due to the low density of air compared with water shaft labyrinths will have a minor effect on air flow.
Therefore an adjustable air orifice should be mounted at runner center.
Shaft labyrinth.
For a submerged turbine during standstill with draft tube gate open and during starting the labyrinth seal shall collect leakage water and lead it through a separate pipe to the station drainage sump.
In case of a turbine with positive suction head there will be a temporary shaft leakage during starting. The labyrinth seal’s only task is to lead this leakage away with grace.
Integrated pump.
At normal speed the integrated pump shall pump water leaking trough the clearance at the pump disk inner radius back to the space above the disk.
It is important to pump water away from shaft to avoid choking of air borings in the shaft to secure air admission to runner and draft tube center.
Power consumed by this pump is small. Runner labyrinth leakage is normally about 0.3 %, but only a small part of it will leak trough the clearance at the pump disk
inner radius.
If pump head is set to 10.0 m., pump efficiency to 50 % and pump flow is 0.1 % of nominal flow ΔP ≈ 2.0 /Hn %.
For nominal head Hn = 100 m ΔP ≈ 0.02 %.
Servomechanism.
The servomechanism does not influence turbine losses, but turbine performance as a function of servo-stroke is normally registered for turbines in operation and will give important information.
The servomechanism is exposed to great forces depending on guide vane/turbine design and mode of operation.
Deformation due to these forces is not calculated, but a pre-stress related to stroke is taken into consideration. This will result in a constant stroke-displacement instead of a variable depending on forces. Guide vanes will stay closed for stroke less than the set pre-stress.
Closing and opening time.
Closing and opening time is by default set to 5 sec. giving a linear opening- and closing law.
Modification is done by entering new values in the table for stroking law.