A METHOD IN OR RELATING TO THE START OF A POWER TURBINE AND ARRANGEMENT IN POWER TURBINE IN ORDER TO AVOID START DAMAGE ON TURBINE WHEEL/HOUSING

Abstract: Upon start up of a power turbine shortly after stoppage, the turbine housing (30) would have been cooled quicker than the turbine wheel (14); as a result of material shrinkages, the intermediate clearance will be too small, such that the turbine wheel scratches in too large leakage grooves in the adjacent housing or seal portions. Therefore, it is suggested to heat the power turbine housing (30, 36) before the start of the power turbine, especially upon start shortly after last stoppage. In a power turbine, where start damage should be prevented/avoided, comprising a rotor (14) having turbine blades (14a) surrounded by a stator-forming housing (30), preferably equipped with a seal (36) opposed to the radially outer tips of the turbine blades (14a), and where the turbine housing (30, 36) is, preferably, assigned a cooling jacket, it is suggested that the power turbine housing (30, 36) is assigned a heating jacket (32), e.g. in the form of a thermal manifold.

Description: A METHOD IN OR RELATING TO THE START OF A POWER TURBINE AND ARRANGEMENT IN POWER TURBINE IN ORDER TO AVOID START DAMAGE ON TURBINE WHEEL/HOUSING The invention relates to a method in or relating to the start of a power turbine in order to avoid start damage on the turbine wheel and/or housing. Likewise, the invention relates to an arrangements in a power turbine which, unitary to the purpose of the method aspect, is intended to prevent damage on the turbine wheel and/or housing, especially in connection with start up. A power turbine is a separate expansion turbine driven by the combustion gas from a gas generator. When a power turbine stops, the turbine housing is more rapidly cooled than the turbine wheel, so that the clearance between adjacent portions turbine wheel and turbine housing becomes small. When the turbine wheel rotates, heat and centrifugal forces cause the turbine wheel diameter to increase. Upon start up of a power turbine it is, therefore, necessary that a certain clearance exists between turbine housing and wheel. In principle, this presupposes that the power turbine is allowed to settle down after a stop until material expansions due to heat have contracted themselves so much through shrinkage that the clearance conditions between turbine housing and wheel again are satisfactory for start up without any risk of being capable of damaging each other. However, there often exists a need for starting up a power turbine very shortly after a stop, in other words: too early. Then, it not seldom happens that the turbine wheel makes a groove-like scratch in the turbine housing. Then, when the temperature of the turbine housing reaches working temperature, the groove will give rise to leakage past the turbine wheel, inevitably resulting in a reduced efficiency. It represents prior art technique to mount a cooling jacket outside the turbine housing in order to cool it and shrink a possible leakage clearance due to a previous start damage. If it is cooled too much, damage may arise on account thereof. A cooling jacket cannot prevent a start damage and one has, therefore, tried to solve the problem through monitoring the power turbine's rotational speed. If the turbine does not start or accelerates when admission is given, this is considered as indicative of a non-satisfactory clearance between turbine housing and wheel, and the start trial is interrupted. Before the next trial, one has to wait a while. A substantial disadvantage of such a monitoring of rotational speed is that the turbine wheel may rotate a number of times before the problem is discovered and, then, damage will already have arisen. The main object of the invention is to provide a method and a device to prevent damage upon start of a power turbine. According to a subordinate aspect of the invention, one has aimed at enabling pointing out indications of possible damage risks at such an early point of time that damage would be avoidable. According to the invention, said object is achieved by proceeding in accordance with the method as set forth in claim 1, in which it is proposed to heat the turbine housing before start, respectively by equipping a power turbine housing with a heating jacket disposed outside the housing, generally in connection to a cooling jacket as known per se. Upon a start after a non-planned stop, the turbine housing is first heated such that it becomes approximately as hot as the remainder of the turbine, whereafter the turbine is started. When the turbine has reached the working temperature, the housing may, possibly, be cooled in order to reduce the clearance to the turbine wheel, achieving optimal efficiency. In order to detect risk for damage at a point of time as early as possible, one may, according to the invention, use a vibration measuring system working in accordance with so- called envelope technique and which is adapted to react on vibrations due to the fact that the turbine wheel has come into contact with the turbine housing. Upon pointing out such vibration phenomena, the device is adapted such that a starting trial can be interrupted in time, simultaneously as too much cooling is avoided if a cooling jacket is used, as known per se. Vibration and acoustic signals not due to unbalance, are usually to weak to be used for analysis or for influencing shut down systems. The reason is that such signals become "drowned" by the gas turbine after which the power turbine is coupled, a special technology being required to separate out the signals from the background noise. To-day's methods for protecting aeroplane-based gas generators with a power turbine connected thereto are so defective that shut-down systems with the task of stopping the turbines/ machines before damage occurs due to technical deficiency, are coupled in at a too late state. This means that when a machine is subjected to malfunctions and is switched off due to the protection systems, an unforeseen shut-down is already accrued, which, in its turn, will require considerable repair costs. Additionally, expenses due to equipment stop and replacement works would occur. The "envelope" or "enveloping technique" used in connection with the above mentioned vibration measurements, has for its purpose to amplify weak signals from e.g. an accelerometer. Through this method, higher frequencies are first separated from low-frequent machine vibrations by excluding unnecessary frequency ranges. The problem is to identify the weak signals. A signal from e.g. a bearing ball rolling in the ball race is very weak, and is diffused across a wide frequency range. Therefore, such a signal is well "hidden" in the remainder of the machine noise. The "envelope circuit" amplifies the filtered signal approximately corresponding to the square thereof. Since the sound of members rotating within the machine is repeatable, they may be simulated by harmonic series of sinus waves corresponding to integer multiples of repeatable signals. The drawings show a non-restricting exemplary embodiment, wherein: Figure 1 shows a side elevational view/axial section of a gas turbine with a power turbine shaped and designed in accordance with the present invention; Figure 2 shows a rear partial view of the power turbine; Figure 3 shows a detail view III stippled encircled in figure 2, on a larger scale; and Figure 4 shows a sectional view corresponding to the section line IV-IV in figure 3. In figure 1, reference numerals 10 and 12, respectively, denote a compressor rotor and a gas turbine rotor, 14 indicating the rotor of the power turbine. A compressor shaft 16 is suspended in a front frame bearing 18, an intermediate frame bearing 20 and a rear frame bearing 22, the shaft 16' of the power turbine being suspended in a front bearing 22' and a power turbine bearing 24. In the compressor-gas turbine-power turbine combination, the inlet 26 to the compressor portion is trumpet-shaped, the exhaust channel being denoted at 28. The housing 30 of the power turbine is, according to the invention, surrounded by a heating jacket 32 which, within the range of subjects, is mentioned as a "thermal manifold"; 34 denoting a heating coil for supplying heat energy to the heating jacket 32. In figures 2 - 4, 14a denotes turbine blades. 36 denotes a seal. The letter S denotes a slot between the seal 36 (included as a part of the housing 30 when mentioning a clearance between the turbine wheel and housing 30) and the outer tips of a turbine blade 14a. An air stream across this seal design will pass along the inverted V-shaped edge cavities of the seal 36. In combination with the heating jacket 32 (stippled in figures 2 - 4) , a cooling jacket, known per se, not shown, may be disposed. Upon start up such a gas generator assigned a power turbine, the turbine housing 30 is first heated when one desires start up a short time after stoppage, so that the turbine housing becomes approximately as hot as the remainder of the turbine, whereafter the turbine is started. Then, the seal 36 and the housing 30 will have expanded such that a clearance exists to the turbine blades 14a. A brake (not shown) can be used on the shaft 16' of the power turbine. Subsequently to a stoppage not planned, the brake is activated so that the rotor 14 of the power turbine cannot rotate. Thereafter, compressor and gas turbine are started and, then, hot gas from the gas turbine will heat the housing 30 of the power turbine. When the housing 30 of the power turbine has reached a satisfactory temperature, the brake is released.

Claims: C l a i m s 1. A method in or relating to the start of a power turbine, especially when a start is to take place shortly after stoppage, c h a r a c t e r i z e d i n that the housing (30,36) of the power turbine is heated prior to said start. 2. A method as set forth in claim 1, c h a r a c t e r i z e d i n that the housing (30, 36) of the power turbine upon start up is heated until it is approximately as hot as the turbine wheel (14, 14a) which has not been sufficiently cooled after stoppage, whereafter the turbine is started. 3. Arrangement in a power turbine in order to prevent/ avoid start damage, comprising a rotor (14) having turbine blades (14a) surrounded by a stator-forming housing (30) , preferably provided with a seal (36) opposed to the radially outer tips of the turbine blades (14a) , c h a r a c t e r i z e d i n that the housing (30,36) of the power turbine is assigned a heating jacket (32) , e.g. in the form of a thermal manifold. 4. A method for pointing out risks for start damage on a power turbine, c h a r a c t e r i z e d i n that the power turbine is monitored by means of a vibration system working according to the envelope/enveloping principle and reacting on vibrations caused by contact between the turbine wheel (14,14a) and the turbine housing (30), possibly the seal (36) of the latter opposed to the turbine wheel (14,14a), pointing out of such vibrations during start up giving rise to an interruption of the start up trial. 5. A method in or relating to the start of a power turbine, especially when a start is to take place shortly after stoppage, c h a r a c t e r i z e d i n that the shaft (16') of the power turbine is locked, whereafter compressor and gas turbine are started, so that hot gas heats the housing (30) of the power turbine, said locking being nullified when the housing (30) has been sufficiently heated. 6. Arrangements as set forth in claim 5, c h a r a c t e r i z e d i n that the shaft (16') of the power turbine is assigned a brake.

Inventors: MOEN LYDER, MOEN, LYDER
Applicants: DYNATREND AS, MOEN LYDER, MOEN, LYDER
Application year: 1998
Main classification: F01D19
IPC codes: F01D11, F01D19, F01D25, F02C7