Design Of A Reheat Turbine

U. Niatas And F. Conlow *

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


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