Release Date --April 24, 2002
There have been a number of basic steam turbine design configurations that have been used over the past century. In this new century, most of the oddities are merely a part of history and are no longer used. Currently, worldwide, the vast majority of steam turbines are of horizontal-rotor-shaft configuration, most often with two-radial-bearing, simply-supported rotors (rotor wheel or wheels between the two bearings), whether they are small single-stage turbines or multi-stage turbines of any size. Further, contemporary single stage steam turbines may be found with principal pressure casing steam joints that are either radial (perpendicular or transverse to the rotor axis) or axial (parallel to the rotor axis)
This writing is focused on outlining and explaining the advantages and disadvantages of the two principal contemporary single-stage-turbine pressure casing constructions…principal pressure casing steam joints radial (radial-split) vs. axial (axial-split).
RADIAL-SPLIT Advantages
One major advantage of radial-split turbines is that they are much more economical to manufacture and generally less expensive to purchase.
All radial-split turbines are more symmetric than any axial-split turbine. This means that all radial-split turbines are inherently less susceptible to problematic pressure and temperature deflections than are axial-split turbines. For this reason radial-split turbines generally exhibit better steam sealing (Gland) leakage control, and longer steam seal and bearing life.
This is particularly the case with the NESTCO-17 radial-split turbine which has been designed using high performance computers with the latest cutting-edge Solid-Modeling design and Finite-Element programs to minimize and favorably control pressure casing and bearing support systems service deflections, to unprecedented minimal levels.
Radial-split turbines do not require the disturbance of any principal pressure casing steam joint nor rigging in order to access shaft seals (Glands) for carbon ring replacement, unlike many axial-split single-stage steam turbines. Access to the NESTCO-17 turbine steam seals also does not require disturbing any casing pressure-joint, but unlike many other contemporary single-stage turbines of any construction, all “wetted” metal components in the NESTCO steam seals are made entirely of 300-series SS or Inconel 600, and there are no axial-split steam joints in the NESTCO steam seals that require special sealants, three-way or four-way steam joints, or tricky assembly techniques, which are all difficult to perform, time-consuming and prone to leakage.
The position the rotor support bearings much farther away from the turbine hot-zones. This results in a much lower heat influx into the lubricating oil and generally requires less cooling and permits longer oil life than any axially-split, oil-ring-lubricated single-stage turbine. Unlike any turbine in its class, the NESTCO-17 turbine incorporates a shaft-mounted dynamic air-cooled heat extractor, and a multi-phase self-circulating lube oil system that does not use pumps, slingers or oil rings.
The basic symmetry of radial-split turbine casings and the simplicity of the single, radial, pressure casing steam joint, tend to minimize thermal stresses and make thermal stress distribution more uniform about the casing than in any axial-split turbine. This minimizes harmful deflections and dynamic misalignment with the driven equipment, making a radial-split turbine superior to any axially-split single-stage turbine, for rapid-start service.
Radial-split construction permits more than 50% nozzle arc-of-admission. This permits shorter blades with lower operating stress and lower bearing loads. Unlike any other contemporary radial-split mechanical-drive turbine, the NESTCO-17 goes further in that it’s nozzles are in a field-replaceable nozzle block, requiring no machining or hand fitting.
The NESTCO-17 has 12o nozzles, vs. the typical 20o, for higher efficiency and lower blade loads. They are constructed tightly adjacent to one-another, further enhancing efficiency and minimizing discreet impulses on, and vibratory excitation of, the turbine blading. The net result is lower blade stress levels. For any given steam condition, rotor speed, and power output the NESTCO-17 rotor blades have the lowest operating blade stress levels of any radial-split or axially-split single-stage turbine in the world.
An advantage unique to the NESTCO-17
radially-split
turbine is its ease of internal inspection. Unlike most turbines in its' class, and
for that matter, any axial-split turbine in the world, internal inspection is
easily accomplished in-place, without disturbing the turbine, steam piping or
alignment. An inspection plug located low in the inlet casing, and an off-side
exhaust blind flange (of the two exhaust connections always provided on every
NESTCO-17 turbine) may be removed to permit full visual inspection of the
turbine rotor blading. No bolted pressure-casing steam joints have to be
disturbed and no rigging is required to lift any cover.
RADIAL-SPLIT Disadvantages
The principal disadvantage of radial-split steam turbines has been that they must be removed from their installation in order to disassemble them for an internal inspection or overhaul. This usually requires the disturbance of the steam pipe connections and the alignment between the turbine and the driven equipment. As stated above, internal inspection of the NESTCO-17 can easily be performed in-place without disturbing piping connections or alignment.
As for overhaul, this disadvantage is often more “perceived” than truly significant. Many contemporary single-stage turbine sites, with today’s typically lean staffing, find it more expeditious and with better results to simply remove a single-stage steam turbine to a well-equipped indoor shop to perform overhauls. Today, the work is often performed by regional, off-site, expert mechanical contractors, who take the turbines to their own shops, regardless of construction. Breaking and making flange connections is done very quickly, and modern gaskets preclude bothersome leaks. Properly hot-aligned turbines with suitably dowelled turbine feet insure quick re-mounting of a shop-overhauled turbine, with effortless and accurate alignment to the driven equipment assured.
AXIAL-SPLIT
The principal advantage of the axial-split configuration, is that the upper half of the principal pressure casing can be removed without disturbing the lower half (or rotor), for internal inspections and overhauls. This design is intended to minimize downtime during internal inspections and overhauls. It permits direct access to the turbine internals in-place, and also avoids disturbing steam pipe connections thus eliminating the time required to break them and later to remake them, together with prevention of disturbing the alignment of the turbine to the driven equipment.
AXIAL-SPLIT Disadvantages
Obviously large multi-stage turbines will always be inspected and overhauled in-place. However, in the case of single-stage steam turbines, some of the advantages of axial-split construction are often more “perceived” than actual or practical. Depending on turbine size, teardown at the installation site although possible, may in fact be very difficult or even impractical. There are a number of factors that can influence this decision, such as installation “clutter” and steam pipe arrangements, whether the installation is indoors or outdoors, and in the latter case, what the environment and weather may be at the time, as well as what rigging equipment, transport and tools may be available at the site. As stated above, many contemporary single-stage turbine sites, are manned with minimal crews and fewer skilled mechanics. This often leads to a decision to remove the entire turbine from its installation, and relocate it to a heated/air conditioned, well-equipped shop for more expeditious and more reliable inspections and teardowns, thereby maximizing the effectiveness of smaller maintenance crews. Therefore, the perceived advantages of axially-split single-stage turbines are circumvented by the practical considerations of contemporary installations, environment and weather, limited equipment and personnel.
Axial-split constructions require extensive machining operations which in turn, requires large, expensive machinery, and therefore are necessarily much more expensive to manufacture. Which consequently, are more expensive to buy than an equal-performing radial-split turbine.
Axial-Split construction by design are more complex and difficult to seal the steam joints. There are often involving three-way (Tee) and four-way (Cross) joints which are both difficult to make up properly, and much more prone to develop leaks in service as movement occurs due to pressure and temperature.
Turbine deflections/disturbances in the alignment between turbine components are a weakness of all turbines. Axial-split turbines are by design not symmetrical in any direction, and tend to go out of alignment internally even at modest steam temperatures and pressures. These internal misalignments can, and often do, disturb the rotor steam seals (Glands) and bearings, shortening their lives markedly. In some extreme cases, requiring elaborate and costly lubrication systems to allow journal bearings to last for a reasonable time period.
In contemporary axial-split single-stage turbines, the majority have the steam inlet in the lower half of the unit. In higher capacity turbines of this type, the maximum available nozzle distribution is limited to only half (180o) of the full arc (360o). This restricted arc imposes elevated thermodynamic losses, those associated with the end of the active nozzle arc and those associated with greater inactive-arc rotor blade drag and turbulence (Windage). Further, in higher capacity turbines of this type, the arc of nozzles can, and often does, create a steam force vector that tends to lift the rotor and induce rotor vibration instabilities. Also, the restricted arc-of-admission often requires longer rotor blades. These will have an increased bending moment, lower natural frequency of vibration, will be more susceptible to vibration problems and fatigue, and will necessarily have much higher operating stresses.
New England Steam Turbine Corp.
NESTCO
322 Main Street
Spencer, MA 01562 USA
Tel: 508-885-7950
FAX: 508-885-7951
Email: Info@nestco1.com