Design Of A Reheat Turbine
The Required Design Features Of A Reheat Turbine Are Well Within The State-Of-The-Art Existing Today This paper presents the design of a marine reheat main propulsion turbine. A cross-compound unit is employed, utilizing a high pressure and intermediate pressure unit on one shaft and in one casing. It is this turbine that is most affected by the thermal transients of maneuvering due to variations in the inlet steam temperature to the IP section of the turbine when the reheater is secured and when it is returned to service. A rationale is provided for a reheat turbine design, pointing out the significant considerations that are involved.
The HP-IP turbine described is derived partly from land reheat designs, but primarily it is an evolution of the HP-IP turbine of the Navy series-parallel design.
The major design considerations required for a successful HP-IP reheat turbine evolved from the test data of the series-parallel unit and the problems brought to light on this type of turbine.
These lessons provided information sufficient to design reliable marine reheat HP-IP turbines, which will function in their thermal transient environment. The design requirements are well within the present state-of-the-art.
Cycles The design of a reheat turbine is directly related to the steam conditions and cycles of the plants it must accommodate. The first part, the steam conditions, have been stated partly by industry standards, partly by ANSI piping standards, and the economic consideration to fit these reheat HPIP turbines into an existing line of marine propulsion turbines.
Steam conditions used are 850 psig, 950°F inlet and 950° reheat with 1.5-inch HgAbs back pressure.
These choices accommodate existing piping standards while utilizing to the maximum extent the non-reheat components of existing marine turbines.
The cycle considerations should approach the ideal reheat cycle while maintaining the power dis- tribution of a cross-compound marine turbine (50/50 power split at maximum power between the HPIP and L-P turbines). The ideal reheat cycle is one in which there is continuous reheating of the steam to inlet conditions. For practical marine design, this translates to one step of reheat back to inlet temperature. The minimum reheat pressure is selected to give best efficiency while avoiding superheat in the L-P turbine exhaust at partial load conditions.
The optimum reheat pressure is a function of initial steam conditions, primarily inlet pressure.
The maximum gain for a variablepressure reheat cycle occurs when the reheat pressure is 15 to 19 percent of the initial absolute pressure.
It is good practice to choose slightly higher reheat pressure than the optimum.
Reheat cycles previously established in the industry meet the requirements of the reheat turbine design which this paper presents.
This approach allows a continuation of present industry practice of the shipyard and design agents maintaining cycle responsibility and allowing competitive turbine machinery manufacturers to supply turbines. Following this thinking, there is also the advantage of allowing boiler manufacturers to frame size their boilers more readily.
Frame Considerations Based on the size and speed of present ships and ships of the immediate future, reheat units of from 20,000 shp to 70,000 shp should cover the needs of the industry.
Also, the primary steam conditions for covering these powers could be 1,450 psig, 950°F (reheat) and 1.5-inches HgAbs exhaust. Economically this allows two HP-IP turbines to be designed to these conditions, in combination with three non-reheat L-P turbines, having annuli exhaust of approximately 18 square feet, 25 square feet, and 38 square feet.
The non-reheat L-P turbines require new astern elements to accommodate the 1,450 psig inlet pressure in lieu of the 850 psig astern turbines incorporated in existing designs. It should be noted the largest L-P frame could, with an added HP-IP turbine and some redesign, increase the maximum rated output to 100,000 shp.
With this basic approach, the two HP-IP turbines designed for 1,450 psig inlet conditions can be used with slight modifications for 850 psig. This, of course, limits the range of powers that can be accommodated at 850 psig. The first HP-IP turbine, for example, can be used from 18,000 shp to 50,000 shp with 1,450 psig but is limited to 32,000 shp using 850 psig.
Figure 1 is a diagrammatic of the maximum shp frame sizes possible with two HP-IP turbine designs.
Why Reheat Cycles?
The design of a reheat turbine emphasizes today's requirements that all new designs provide low fuel consumption to the operators.
The capital cost for these plants can be justified and the present designs of both reheat turbines and boilers strongly indicate that risk and maintenance costs are not prohibitive. While this paper is concerned with turbines, there are also new boiler innovations that support this premise.
The typical two heater nonreheat cycle with a steam air heater is no longer viable in today's economics. The industry must consider all variations of sophisticated non-reheat and reheat cycles. These can include four and five heater cycles with boilers using either regenerative air heaters, stack cooler with fluid air heaters, or stack coolers in combination with steam air heaters.
Boilers must be designed for low excess air operation for high efficiency with the stack temperature determined by the fuel to be burned and the type of operation the ship will be subjected to.
The use of improved non-reheat cycles will gain three to five percent in lower fuel rates over a two heater cycle. Compared to the same datum, reheat cycles with four and five heaters can realize eight to 13 percent reduction in fuel rates. The cycles compared in this paper are conservative, with steam air ejectors, and use a regenerative air heater with stack temperatures of 275 °F.
Application of Reheat Turbines In order to emphasize the practical application of reheat with its attendant gains a comparison to an existing commercial marine powerplant is included.
A present day U.S.-built vessel utilizing a typical cargo-liner cycle of two heaters and a steam air heater has a guaranteed fuel rate, as designed, of 0.478 pounds/shp hour at maximum ABS rating of 32,000 shp. Two reheat cycles are considered using one of the reheat turbines presented in this paper.
The cycles utilize 850 psig, 950°F with 950 °F reheat with one cycle using a regenerative air heater and the other a feedwater (fluid) regenerative air heater.
The most efficient cycle utilizes a regenerative gas air heater and both cycles have five feed heaters. The overall fuel rates are 0.424 pounds/shp-hour and 0.432 pounds/shp-hour. Compared to the existing cycle, the most efficient method shows 11.3 percent fuel savings and the other shows 9.3 percent fuel savings.
Reheat plants without complex attached auxiliaries and without other features that could increase maintenance can be supplied for present designs of U.S. flagships.
The overall specific fuel rates are competitive to any main propulsion type, with definite advantages to U.S. flag operators who are completely familiar with steam powerplants.
Other stories from June 1977 issue
- BSRA Publishes Code Of Procedure For Marine Equipment page: 4
- Sixth LNG/LPG Meeting Gastech 78 To Be Held Nov. 7-10 In Monte Car page: 4
- Nichols/Pittman/Choate Purchase Cox Interest In Mississippi Marine page: 6
- Japanese Shipbuilders Association Elects Dr. Shinto President page: 6
- Lockheed Shipbuilding Launches First Of Two Sub Tenders Building At Total Cost Of $253 Million page: 7
- Harland And Wolff Building LPG Carriers For Service To U.S. page: 8
- Bethlehem To Reconstruct Four PFEL Ships At Cost Of $5 Million Each page: 8
- Federal Barge Lines Names David Ruffner Manager Eastern Region page: 8
- Newly Formed ASNE Puget Sound Section Holds First Meeting page: 9
- McAllister Adds 4,290-HP Tug To Hampton Roads Fleet page: 9
- Western Gear Corp. Receives $8-Million Contract From Navy page: 10
- Union Mechling Corp. Elects Smail President —Mechling Chairman page: 10
- Model Of National Monument Honoring American Seamen Unveiled On Maritime Day page: 11
- New GE MST-21 9,000-19,000 SHP Steam Turbine Available page: 11
- Research Contracts To Combustion Engineering And Mortada International page: 11
- Hongkong United Dockyards Ltd. Sign Contracts For New Ship Repair Complex page: 12
- LNG-5 . . . Dusseldorf page: 12
- American Bureau Forms Republic Of China Technical Committee page: 13
- Zapata And Dutch Firm To Build And Operate U.S.-Flag Dredges page: 14
- Todd Seattle Lays Keel For Guided Missile Frigate —FFG-10 Is First In A Multi-Ship Navy Program page: 15
- Franz Krautkremer, President Of Schottel, Awarded Medal Of Merit page: 16
- $394,000 Contract To Study Inland Waterways And Gulf Coast Area page: 17
- Sembawang Shipyard Modifies Barges Converted From Tankers page: 18
- N.Y. Propeller Club Elects Hart President page: 19
- $28-Million Contract Awarded To Equitable page: 22
- Louisiana Names Amoss Maritime Man Of The Year page: 23
- Uniflite Receives $1,440,000 Contract For Utility Boats page: 23
- Marland Expanding— Manufacturing Facility Moved To Wisconsin page: 24
- Gulf Mississippi Marine Corp. Awarded Nine-Vessel Contract page: 25
- 260-Foot SEACON Is New Navy Offshore Construction Vessel page: 26
- A.C. Sargent And A.J. Herkes Form Naval Architectural Firm page: 26
- Pacific N.W. Section Hears Paper On Marine Cathodic Protection page: 28
- National Cargo Bureau Reports On '76 Activities page: 29
- Union Mechling Forms Water Analytics Division page: 30
- Design Of A Reheat Turbine page: 31
- BP And Stolt-Nielsen In Shipping Agreement page: 32
- U.S. Steel Orders Colt-Pielstick Engines For Huge Ore Carrier page: 33
- International Paint Announces Promotions And Appointments page: 38
- Raytheon Adopts New Warranty Program page: 38
- Lloyd's Register Pioneers Vibration Monitoring Techniques page: 40
- ABS Forms Republic Of China Technical Committee page: 42
- Modern Wooden Ship — Design And Construction Subject Of SNAME Philadelphia Section Meeting page: 43
- Lockheed Designs Oil Skimmer For Installation In Craft Used In Offshore Cleanup page: 44
- Marathon Introduces Jumbo Bottom-Supported Offshore Platform page: 44
- Seatrain Using Unique Shipbuilding Technique page: 44
- Propulsion Systems, Inc. To Supply Bow Thrusters For Two Ro/Ro Vessels page: 44
- Chairperson Named For Kings Point Museum page: 46
- Dixie Dredge Corp. Appoints James Bishop page: 47
- NYSA And CONASA Reelect James Dickman President page: 47
- Dry Dock Association Elects Massa President page: 50
- Hansen And Tidemann, Inc. Names Mitchell To Head First West Coast Office page: 50
- Delaware Marine Launches Pilot Boat For Port Of Monrovia page: 51
- New Register Shows Ro-Ro Fleet Much Larger page: 51
- Advanced Marine Vehicles Conference Issues Call For Papers page: 52
- ITT Decca Marine Introduces Two New Navigation Aids page: 53
- Singapore Shipbuilders Elect Chua Chor Teck page: 54
- Don McGee Joins Austin Marine, Inc page: 54
- Propulsion Systems, Inc. Appoints John Phinney Engineering Manager page: 55
- World Shipping Statistics 1976 page: 55
- Misener Brochure Describes Facilities At Tampa, Fla. Yard page: 56
- Electro-Nav Names Woolf General Manager San Francisco Office page: 56
- SNAME Los Angeles Section Discusses Use Of Concrete For Marine LNG Tankage page: 56
- San Diego Section Of SNAME Hears Paper On Epoxy Resin Machinery Chocking page: 57
- Raytheon Marine Adds New Series Of Weather Receivers page: 57
- Bethlehem Beaumont To Build Self-Setting Production-Storage Platform For Phillips Petroleum page: 57
- Safety In Marine Applications Course Sponsored By ISA —Washington, D.C., June 27-28 page: 58
- Prudential Installs Fleetwide Preventive Maintenance Program page: 58
- National Cargo Bureau Appoints Captain Gates page: 59
- Maryland Police And United Nations Order Boats From Uniflite page: 60
- Mitsubishi To Convert Sea-Land Containerships page: 61
- Marine Electronics Dealers Establish Reciprocal Service page: 61