ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG SHIPS

Transcription

1 Jne 018 ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG SHIPS ABSTRACT Martina Živković, B.Sc. Antna Barca 9D, Zadar, Croatia Josip Orović, D.Sc. Igor Poljak, M.Sc. University of Zadar, Maritime Department Mihovila Pavlinovića 1, Zadar, Croatia Steam proplsion plants for LNG ships have main feed water pmps that are driven with steam trbines. Those steam driven feed water pmps are one of the most important components in the plant. Their fnction is to dose the feed water for main steam boilers timely and exactly in order to maintain normal fnctioning of the steam proplsion plant. In this paper the mostly sed steam driven feed water pmps are presented. Their constrction details, steam temperatres, pressres, velocities, efficiencies and losses over the steam trbine stages and wide range of steam trbine operating loads are analyzed. Keywords: Steam trbine, feed water pmp, working parameters, efficiency 1 INTRODUCTION This paper presents two types, Coffin and Shinko, feed water pmps that are driven with steam trbines and sed to deliver the feed water to main steam boilers of LNG ships. Coffin main feed water pmp is constrcted as a two staged centrifgal pmp [1] and Shinko main feed water pmp is of three stages centrifgal type pmp []. Both feed water pmps are driven by the short and light weight Crtis wheel steam trbine [1, ]. Feed water pmps take water sction from the highest point in the engine room, from the deaerator which acts as a header tank, and deliver the feed water with high pressre, over 60 bar, into the main steam boilers. Before it enters the main steam boilers the feed water temperatre is sally raised passing throgh the high pressre feed water heater and economizer in order to redce the fel oil consmption in the boilers and to increase the overall efficiency of the steam plant. The main steam boilers level control and ths the capacity control of main feed water pmp is often a three term control. This control measres water level, steam and water flow and controls the otpt to the boiler feed water control valves. The steam flow to the steam trbine and the rnning speed of feed water pmp is controlled by the differential discharge pressre control. This control compares the feed water discharge pressre with boiler steam drm pressre and adjsts the feed water pmp speed by reglating the steam flow across the steam trbine in order to maintain the constant difference across the feed water reglator []. Coffin feed water pmp, shown in Figre 1, consists of for sections: trbine assembly, bearing hosing assembly, governor gear and oil pmp assembly and pmp assembly. At the one end, the driving steam enters the single stage, implse, two bcket row, velocity componded, axial flow trbine and expands throgh the trbine wheel and then exhasts in an axial direction directly above the trbine end cover. A single shaft, sed for driving the two stage centrifgal pmp at the other end, passes throgh the trbine hosing, bearing hosing, and pmp hosing. This shaft is spported by two roller bearings located in the bearing casing and the thrst bearing located on the otboard end of the pmp hosing. The bearing casing additionally serves as an oil reservoir and for providing the power to the horizontal governor drive shaft throgh a worm and a worm gear. Feed water pmp control is accomplished by the pmp discharge pressre via constant pressre reglator, throgh centrifgal speed governor when rated speed has been exceeded and throgh excess exhast back pressre trip and low lbe oil pressre trip protection [1]. Oter design of one Shinko pmp is shown in Figre. It consists of feed water pmp assembly and steam trbine assembly interconnected by the gear copling. Feed water pmp is also mlti stage centrifgal pmp (in this case three stages) where the first stage impeller is of the doble sction type and the rest are of the single sction type. It has horizontally split casing in order to have easier maintenance. Rotor is spported by plain forced lbricated bearings and tilting pad type thrst bearing. Feed water pmp is directly connected to the driving steam trbine bt in this case throgh a forced lbricated gear copling. Steam trbine is of the horizontal single stage speed compond implse type. The trbine casing is also horizontally split into two parts. Bearing hosings are at the both ends of the trbine shaft. The governor end bearing hosing is provided with radial bearing and thrst bearing. The bearing hosing near the feed water pmp has radial bearing only. For the control of the steam trbine 441

2 Jne 018 operation the Woodward governor is sed. Its speed setting mechanism and the pressre controller nit are interconnected to effect constant discharge pressre control []. from inlet pressre to the exhast pressre and a kinetic energy of the steam is increased. This expansion shold be adiabatic as it gives the greatest possible heat drop bt de to friction losses it is polytropic (Figre ). After the first stator nozzles the steam enters the first row of moving or rotor blades where the kinetic energy and ths the absolte velocity of the steam is redced. This redction of kinetic energy and velocity prodces a force which acts on the trbine rotor blades and trns the rotor of the steam trbine. From there, steam flows throgh second stage of stator blades or gide blades where steam is proceeded to the second row of moving or rotor blades. There the kinetic energy and ths the absolte velocity of the steam is again redced in order to prodce a rotational force. Figre 4 shows the behavior of the steam pressre and absolte velocity over the stages in one Crtis wheel steam trbine. Sorce: Driven-Boiler-Feed-Pmp-Page-.jpg [4] Figre 1: Constrction details of Coffin feed water pmp Sorce: Athors Figre : Main feed water steam trbine isentropic and polytropic expansion 1st stage nd stage Sorce: [5] Figre : Shinko feed water pmp STEAM TURBINE FOR FEED WATER PUMPS Before steam enters in the steam trbine, the pressre is redced from boiler pressre to the steam chest pressre. This is de to steam passing over the trbine governing valve. There, the steam pressre and temperatre is redced bt the steam specific enthalpy remains constant. After passing the governing valve, steam enters the steam chest from where it flows to the first stage nozzles of the steam trbine and expands to exhast pressre (Figre ). Crtis wheel steam trbine, mostly sed for feed water pmps drive, comprises of first row of stationary or stator nozzles, two rows of moving or rotor (bcket) blades and one stator row of gide blades sitated between two rotor stages. In the first row of stator nozzles the steam expands P1 C0=0 c1 1st stage rotor blades nd stage stator blades c c nd stage rotor blades 1 4 Sorce: Athors Figre 4: Steam pressre and absolte velocity diagram in Crtis wheel steam trbine Becase of the steam trbine rotor rotation the absolte steam velocity has to be divided into relative steam velocity w and rotational velocity. Figre 5 shows c4 P 44

3 Jne 018 diagram of absolte steam velocities c, relative steam velocities w, rotor rotational velocity, axial forces F x, and thrst forces F y over the stages in one Crtis wheel steam trbine. c 1 w1 α 1 β 1 c 1st stage stationary or stator nozzle nd stage stationary or stator nozzle α β F y1 c 4 w F y Sorce: Athors F x1 β 4 w 4 F x 1st stage rotor or bcket blade c α β α 1 w nd stage rotor or bcket blade Figre 5: Absolte and relative steam velocity diagram at Crtis wheel steam trbine Each vale can be calclated sing following eqations [6]: c 1 = φ (h 1 h ) [ m s ] (1) α 4 nd stage 1st stage F y = m (w 4 sinβ 4 w sinβ ) [N] (15) P = F x [W] (16) η T = P (h 1 h ) m (17) Where φ is friction loss in stator nozzle, h 1 and h are steam specific enthalpies, D is rotor diameter, n is nmber of trbine revoltions, ψ is friction loss in rotor blades, m is steam mass flow in kg/h, P is steam trbine power and η T is steam trbine efficiency. Angles α and β can be calclated sing laws of cosine or sines. In an implse steam trbine, which Crtis wheel is, the β 1=β and β =β 4 becase of the same inlet and otlet geometry of the rotor blades [6]. Manfactring defects, carry-over deposits, corrosion and erosion in stator and rotor blades increase the friction and redce steam velocity. Some of the kinetic energy converts back into heat energy, re-heating the steam and representing losses in the trbine [7]. Those steam velocity losses are presented as a velocity losses in stator nozzles φ and are from and velocity losses in rotor blades ψ (from ) [6]. Also, Crtis wheel has high exhast velocity (which is like a available heat drop) and this represents a considerable loss of energy that is not sed in other stages [7]. 1 1st stage stator inlet = D π n [ m s ] () w 1 = + c 1 c 1 cosα 1 [ m s ] () w = ψ w 1 [ m s ] (4) c = + w w cosβ [ m s ] (5) enthalpy h (J/kg) c = φ c [ m ] s (6) w = + c c cos α [ m ] s (7) w 4 = ψ w [ m ] s (8) c 4 = + w 4 w 4 cosβ 4 [ m ] s (9) expansion with losses nd stage rotor otlet nd stage stator otlet 1st stage rotor otlet 5 1st stage stator otlet 4 hexit 1st stage stator otlet withot losses 6 hloss1 hloss F x = F x1 + F x [N] (10) F x1 = m (w 1 cosβ 1 + w cosβ ) [N] (11) F x = m (w cosβ + w 4 cosβ 4 ) [N] (1) F y = F y1 + F y [N] (1) F y1 = m (w sinβ w 1 sinβ 1 ) [N] (14) Sorce: Athors entropy s (J/kg K) Figre 6: Losses in the stator and rotor blades of Crtis wheel steam trbine Becase of the all mentioned losses, steam trbine efficiency decreases. Figre 6 shows how steam velocity 44

4 Jne 018 redction and high exhast velocity inflence the redction of heat drop and ths on steam trbine efficiency. This velocity losses and conseqently heat drop losses can be calclated sing following eqations [6]: h loss1 = c 1t c 1 + c c kg ] (18) c 1t = (h 1 h ) [ m s ] (19) h loss = w 1 h exh = c 4 w + w w 4 kg ] (0) kg ]. (1) Other losses in steam trbine like windage, gland and blade tip leakage can be neglected becase of its small inflence on overall efficiency and in this paper it will not be taken into consideration. STEAM TURBINE ANALYSIS In this paper the steam trbine for feed water pmp is analyzed sing the data from the performance test on one LNG ship. Dring the test, steam flow to the trbine was changed by opening and closing the governing valve. This change affects the trbine revoltion, steam chest pressre and exhast temperatre. Available steam trbine data from the performance test is presented in Table 1. Steam specific enthalpies and entropies necessary for the calclation were taken from steam tables [8]. Table 1: Data from the performance test of steam trbine for Shinko feed water pmp Steam inlet pressre bar Steam inlet temperatre C Steam chest pressre bar Exhast pressre bar C Exhast temperatre C Steam flow kg/h Revoltion min Sorce: Shinko Ind. Ltd. (006): Final drawing for Main Feed Pmp & Trbine, internal ship docmentation. Hiroshima, Japan. Steam trbine parameters for calclating all other data presented in Table are as follows: Rotor diameter D=800 mm, Steam inlet angle α 1=18, Stator losses φ=0.94, Rotor losses ψ= Rotor diameter and steam inlet angle sed in this analysis are vales taken jst for the calclation and in reality depend on steam trbine prodction model or type. Stator and rotor losses, as already mentioned, depend on varios factors. In this example vale for stator losses was assmed to be 0.94 in order to simplify the calclation. Vales for rotor losses were modified from depending on the initial steam and temperatre sitation in order to obtain certain steam trbine power. In reality, steam trbine power depends on varios combinations of stator and rotor losses, rotor diameter and steam inlet angle. Major inflence on this analysis have steam inlet and exhast pressres and temperatres. Rotor diameter and steam inlet angle are fixed for any analyzed trbine and losses depend on the actal stats of the steam trbine and will not change end parameters a lot. Table : Calclated data for steam trbine sed for driving the Shinko feed water pmp t1 C psc bar t C m kg/h min φ ψ c w α β w c α β c w α β w c α β Fx Fy hl hl hexh ηt % P kw Sorce: Athors As it can be seen from the analysis, steam trbine efficiency depends on many parameters and varies from 51. to 58.8%. Enthalpy of the inlet and otlet steam, steam losses in stator and rotor blades, vale of exhast velocity and steam mass flow are one of the crcial parameters that affect the steam trbine efficiency. Steam velocities across the trbine depend on the operational and manfactring parameters and that affects φ and ψ parameters and stator 444

5 Jne 018 and rotor enthalpy losses. Steam trbine power varies from 19 p to 49 kw. It depends on steam mass flow, revoltion and converted kinetic energy in rotor blades into the axial force for trning the rotor. The higher the vale of these parameters is, the higher will be the vale of steam trbine power. Analysis performed in this paper shows an example of how steam trbine can be modeled and presented in a way that end sers can easily nderstand all processes in the trbine and throgh available data/parameters in the engine room calclate all other specific parameters of the steam trbine. Every engineer shold follow instrction manal recommendation for operation and maintenance of the steam trbine in order to timely recognize any falt or failre of the feed water pmp. Additional information throgh parameter analysis can help in better nderstanding the processes and recognizing any problem dring normal operation of the steam trbine. 4 CONCLUSION This paper presents types of feed water pmps and driving steam trbine sed for delivering the feed water into the main boilers of the LNG ships. Coffin and Shinko feed water pmps are mostly sed types. They are centrifgal pmps with two or three stages. Both feed water pmps are driven with Crtis wheel steam trbine which comprises of first row of stationary or stator nozzles, two rows of moving or rotor blades and one stator row of gide blades sitated between two rotor stages. The expansion in the steam trbine stator nozzles is polytropic where increased steam kinetic energy in the stator is converted into force which acts on the trbine rotor blades and trns the rotor of the steam trbine. Steam velocities and losses in the stator and rotor blades directly inflence the steam trbine efficiency. Analysis performed sing the performance test data provides the end ser all necessary data and enables him a better nderstanding of the processes and to recognize the problem if any exists. Steam trbine efficiency in analyzed example depends on many parameters and varies from 51. to 58.8%. Some of the crcial parameters that affect the steam trbine efficiency are: enthalpy of the inlet and otlet steam, steam losses in stator and rotor blades, vale of exhast velocity and steam mass flow. Steam velocities across the trbine depend on manfactring defects, carry-over deposits, corrosion and erosion in stator and rotor blades. Other losses that can affect the steam trbine efficiency are windage, gland and blade tip leakage. Steam trbine power in analyzed example varies from 19 p to 49 kw. It depends on steam mass flow, revoltion and converted kinetic energy in rotor blades into the axial force for trning the rotor. REFERENCES [1] Coffin trbo pmp, Inc. (006). Operation Maintenance Manal for Coffin trbo pmp, internal ship docmentation. Englewood, USA [] Shinko Ind. Ltd. (006): Final drawing for Main Feed Pmp & Trbine, internal ship docmentation. Hiroshima, Japan. [] LNGC Maersk Qatar (006). Machinery operation Manal. Soth Korea: Samsng Heavy Indstries [4] content/ploads/01/07/trbine-driven-boiler-feed- Pmp-Page-.jpg [5] [6] Tireli, E., Martinović, D. (001) Brodske toplinske trbine. Rijeka: Visoka pomorska škola Rijeci. [7] Roy, G. J. (1975). Steam Trbines and Gearing. London: Stanford Maritime Limited. [8] Ražnjević, K. (1989). Termodinamičke tablice. Zagreb: Narodna tehnika Hrvatske. Sarajevo "Svjetlost". 445