EXPERIENCE IN OPERATING HYDRO DEVELOPMENTS
INSTALLATION OF SERVICE FLOWMETERS ON HYDRAULIC TURBINES UDC 621.224:681.121.8
V. A. Linyuchev
A large number of service instruments measuring active and reactive power, head, temperature of components, water and oil levels, rotational speed, and currents and voltages are used for controlling and optimizing the operating regime of hydraulic turbine-generator units. At present diagnostic systems measuring and analyzing a number of other characteristics of the unit are beginning to be introduced. And heretofore due attention has not been devoted only to the measurement of one of the most important parameters of a turbine, the discharge. The main cause of this is the low, unfortunately, reliability of the measuring equipment, exacerbated by its careless operation owing to the presently existing opinion that these devices are not necessary at a hydrostation. This opinion is due to a simple cause: there is no material incentive of the hydrostation operating staff to produce electricity, only the readiness of the units to take on a maximum load is encouraged. The installation of flowmeters on all turbines is nevertheless necessary for performing two important functions: optimization of turbine operation for the purpose of reducing the unit discharge of water per i kWh of energy produced and for an accurate accounting of the flow rate and discharge of water through the hydrostation structures (as a stream-gauging site). The current tendencies toward conservation of natural resources will inevitably lead to the installation and reliable operation of turbine flowmeters. Flowmeters of turbines using an indirect method of measuring the flow rate are presently used for service measurements in hydrostation designs. These are usually differential manometer of various types measuring the difference of pressures h at two points of the spiral casing of an operating turbine. This widely known method of measuring the relative flow rate of water (Winter-Kennedy method) makes it possible to measure the flow rate through a turbine with an accuracy sufficient for the needs of operation. The flow rate of water in this measurement method is determined by the simple formula
(1 )
Qt= Kao,~,
where h is the difference of pressures at two points of the spiral; K is an experimental constant discharge coefficient of the flow rate determined either from the results of measuring the "absolute" flow rate of a turbine or from the results of model tests. In the latter case it is necessary to maintain a strict geometrical similarity of the spiral casing and pressure tap points for the model and prototype turbine. Comparative measurements of the relative and absolute flow rates on prototype ~urbines showed that the variations of the coefficient K are within 1%, and the exponent 0.5 for the pressure difference has deviations within 1-2% for different spiral casings [i]. Such deviations in the values of the coefficients permit measuring the flow rate with an error of about 2.5%, which is completely sufficient for the needs of optimizing the operation of turbines and hydrological measurements of the discharge rate through a hydrostation. Calculation of the flow rate through a hydrostation by the operating personnel from the power characteristic of the turbine on the basis of the average power and average head of the hydrostation has a considerably smaller accuracy. The introduction of accurate ultrasonic flometers of turbines has recently begun both abroad and in the USSR. But these flometers are presently too expensive and require quite skilled care. Therefore, they are now being used only for conducting tests of units. As was already indicated, the discharge coefficient in formula (i) should be determined during model tests. For those turbines where this coefficient for various reasons was not determined or complete similarity of the measuring sites to those of the model was not maintained, it can be recommended to determine this coefficient either from the results of prototype measurements of the flow rate by the "absolute" method or by calculation by the method recommended by I. A. Chernyatin (B. E. Vedeneev All-Union Scientific-Research Institute of Translated from Gidrotekhnicheskoe
Stroitel'stvo,
0018-8220/89/2309-0527512.50
No. 9, pp. 28-31, September,
9 1990 Plenum Publishing Corporation
1989.
527
To central control p ~ e l | ~ l <~"----~----~----~----~
~
~
~
~
Cn]
~
LzJ L e J > ~ ~
# Co~ressed air
~,A
Fig. i. Schematic diagram of measuring the flow rate of a turbine by the relative method: a) nipple of the "minus" pressure; b) nipple of the "plus" pressure; i) membrane differential manometer; 2) secondary indicating instrument; 3) secondary integrating instrument; 4) secondary instrument; 5) choke. Hydraulic Engineering). According to this method, the discharge coefficient is determined for the zone of optimal efficiency from the plant characteristics of the turbine. Under the assumption that the plant's guarantee with respect to efficiency cannot have deviations of more than • then with consideration of the error of the measurement method the error of the absolute flow rate in the optimal zone of the characteristic will not exceed 22%. This is no worse than the error of measuring the absolute flow rate by means of current meters. The discharge coefficient for this method will be equal to
K=Qt/M,S=Nt/9,81HTloh~
(2)
where N t is the shaft power of the turbine in the optimal zone; H is the net head; D0 is the efficiency in the optimal zone; h is the difference at the measuring site for given N and H. The value of the discharge coefficient can be assumed constant in the entire operating range of the turbine characteristic. For those cases when there is no reliable plant 9perating characteristic of the turbine or this characteristic changed as a result of wear and reoonstruction, the discharge coefficient can be determined by calculation [2]. The calculated value of K is found from a formula determining the dependence of the difference h on the fluid flow rate in a smooth bend: q= ~
~/(R~ + RO2Eh/4(R= -- R,),
(3)
where q is the flow rate of water at the measuring site; ~ is the discharge coefficient which can be taken within 1.03-1.00; R z and R 2 are the radii of the pressure tap points (Fig. i); is the cross-sectional area of the spiral at the measuring site (without consideration of the area of the turbine stator). When ~ = 1.0 the coefficient K for the total flow rate of the turbine will be equal to K = 2,22~ l/'(R2 + R,)/(R= - - RO
.360/~,
(~)
where 8 is the angle between the tooth of the spiral and measuring site, and the total flow rate of the turbine:
528
i,
i
I
J~ [
~e
23 ~9
=
,
Ltll
',
"~i
I
i//
$Z
I\ik
9
I
I
, I.,=,o~
o
"'~ 91
i
k~
:;:.: :k..<.-
-~.:.-
o:?':.
80
//f / ol
,o,',
".>,...
k
.:;l
..0.~,
"
~
es
87
1.~:
I
I 711 '{ I
08
~
71
. Optimal zone 4 ~" -
I
,
I
,
1 I
.o.>,.
},
500 F00 Turbine discharge, mS /sec
JO0
Fig. 2
400
Fig. 3
Fig. 2. Passage of the flowmeter tube through the elastic liner of the metal spiral. Fig. 3.
Characteristic of the RO 697-VM 750 mixed-flow turbine. Qt = q (360/~).
for
(5)
Differential m a n o m e t e r s o f v a r i o u s t y p e s c a n be recommended as t h e p r i m a r y i n s t r u m e n t s flowmeters of turbines. The main r e q u i r e m e n t s imposed on t h e s e i n s t r u m e n t s a r e :
The o p e r a t i n g r a n g e o f t h e p r e s s u r e d i f f e r e n c e h b e i n g m e a s u r e d s h o u l d u s e t h e maximum scale of the instrument. The p r e s s u r e d i f f e r e n c e s f o r e x i s t i n g t u r b i n e s a r e w i t h i n 0 . 3 - 2 m f o r l o w - h e a d t u r b i n e s w i t h c o n c r e t e s p i r a l s and up t o 6 m f o r l a r g e h i g h - h e a d t u r b i n e s ; The o u t p u t e l e c t r i c s i g n a l from t h e d i f f e r e n t i a l manometer s h o u l d c o r r e s p o n d t o t h e s y s tem o f s i g n a l s u s e d by c o m m e r c i a l s e c o n d a r y i n s t r u m e n t s and c o n t r o l s y s t e m s o f t h e h y d r o s t a tion; The instrument should be made for operating under conditions of a humid room with minimum requirements with respect to maintenance; The instrument should be supplied with a voltage of 220 V of direct or alternating current; The accuracy class of the instruments should be not below I. Of the instruments presently being manufactured, we can recommend the DMER-M membrane differential manometers for differences up to 2.5 m and KSD-2-0.68 secondary instruments with a flow-rate integration unit. The secondary instruments should be placed on the control panel of the unit. They should include an indicating instrument and integrator for metering the discharge through the turbine. Metering of the discharge of each turbine jointly with an electric meter makes it possible to determine the efficiency of using each unit with respect to the unit flow rate of water [(m3/(kWh)] and to use these indices for the diagnostic system. An indicator of the total discharge of the hydrostation and an integrator for calculating the total discharge through the units are installed at the main control desk. The most frequent failures of turbine flowmeters occur due to unsatisfactory operation of the spiral pressure tap system: corrosion of the piezometric tubes, plugging by sediments, irregular blowing of the tubes, etc. Therefore, increased attention should be devoted to this system during its design and assembly. The following are the main requirements imposed on this piezometric system: The receiving nipples and piplines should be made of rustproof materials. not allowed for galvanized pipe;
Welding is
The thread connections should be sealed without the use of fibrous materials sealants or Bakelite-phenolic adhesives);
(with liquid
529
[!Y
I, I
Ii
= sJ .~
I
o
1
"~
gg
"
o Z
% "~I
~
L fl=28m
H=22m j I l
es
-
L 400 500 ggO 70U Turbine discharge, m~/sec
600
Fig. 4. Characteristic of the PL 587-V6 930 adjustable-blade turbine. P
Fig. 5. Scheme of regulating an adjustable-blade turbine with the use of a computer. The diameter of the embedded pipes should be at least 50 mm to preclude damages during concreting; To increase the accuracy of the measurements and to eliminate clogging of the tubes by sediments, it is recom~nended to use the pneumohydraulic method of measurement, in which an insignificant amount of compressed air constantly passes through these tubes. The scheme of such measurement is shown in Fig. 2. When using the pneumohydraulic method of transmitting pressure, one important condition should be fulfilled: the outlet holes of the nipples of the "plus" and "minus" tubes of the differential manometer should be located strictly at the same elevation. This will make it possible to begin using the scale of the differential manometer from zero. With the location of the outlet of the "plus" tube on the axis of the spiral (which is some-times done to increase the pressure difference), the "0" of the scale in the differential manometer will be offset by the amount of the difference in the elevations of the "plus" and "minus" nipples. And for large turbines this offset can be commensurate with the pressure difference being measured; In conformity with the Code of the International Electrotechnical Committee (IEC) [6], the diameter of the hole in the nipple should be 3-6 mm with rounding of the edge with a radius of 1 mm. For installing the piezometers in concrete spirals, the nipple should be placed on a smooth metal surface with a length along the flow of at least 250 mm before the nipple and I00 mm after it. In the case of leading the piezometric tubes from the upper part of metal spiral chambers having an elastic liner, there arises the problem of an expansion piece in the joint between the embedded piezometric tuba and shell of the spiral (owing to deformation of the shell during changes in the pressure). The Leningrad Metals Plant in this joint used a complex stuffing box, which did not guarantee the absence of leakage of water into the elastic liner and concrete. The effect of self-compensation of the free part of the piezometric tube is used in the latest designs of hydrostations. The free part of the tube is covered with
530
an elastic liner (for example, tarred cord on a bitumen layer) to create a gap between the tube and concrete, as is shown in Fig. 2. The length of the free part of the tube is determined by a simple formula derived from t h e c o n d l t l d n of equality of deformations of the tube and shell of the spiral: l = O, 86 f O s ~ t u ~ / ~ ~,
(6)
where Dsp is the cross-sectional diameter of the spiral in the cross section of the measuring site; dtu is the outside diameter of the tube; a I and a 2 are the allowable stresses in the wall of the spiral and tube, respectively. If we assume the allowable stresses in the walls of the spiral and tube are approximately equal and the usual outside diameter of the tube is 57 mm, then the formula is simplified to
t =2,05K
v
(6a)
With an accuracy of measuring the flow rate by the relative method of • flowmeters can be used effectively for purposes of optimizing the operating regimes of a turbine. For this purpose the known feature of the energy characteristic of a turbine can be used: the optimal efficiency at different heads rather exactly corresponds to the same (optimal) value of the discharge. Typical operating characteristics of mixedpflow and adjustable-blade turbines as a function of the discharge are shown in Fig. 4. This circumstance makes it possible to determine the optimal zone of turbine operation of the basis of one parameter, the discharge, instead of the usual use of two parameters, the head and power. After recording the operating characteristics of each turbine as a function of the discharge (relative method), the load can be distributed among the units with consideration of the individual characteristics of the turbine. In this case, having assigned the economically allowable level of decreases of efficiency during regulation (for example, 0.5%), we easily determine the zone of optimal operation of the turbine for use in the group active power control system of the hydrostation (Fig. 4). For adjustable-blade turbines the use of a flowmeter makes it possible to efficiently solve the scheme of control of the governor relation between opening of the gate apparatus and blade angle. If a flowmeter is used for control of the governor relation, then there is no need to measure the head and the problem reduces to finding the minimum discharge corresponding to the given power. The basic scheme with the use of a microprocessor proposed by "Asea" (Sweden), shown in Fig. 5, can serve as one of the examples of such governor schemes. The turbine governor, controlling the opening of the gate apparatus, maintains a power prescribed by the control system and the optimizer performs independent control of t u r n i n g the runner blades until the discharge of the turbine becomes minimum. With consideration of the broad possibilities of using flowmeters of turbines in modern control systems of hydrostations, requirements of high relaibility should be imposed both on the instruments oand on the entire measuring system. CONCLUSIONS i. The relative method of measuring the flow rates of water through a turbine is realized by simple means and provides a sufficient accuracy for the needs of hydrostation operation. 2. Further works of design organizations, operating services, and manufacturing plants is necessary for increasing the reliability of the entire flow-rate measuring system. 3. The operating staffs of hydrostations need to be materially encouraged to use the discharge efficiently for producing electricity. LITERATL~E CITED I. 2. 3.
N . M . Shchapov, Hydrometry of Hydraulic Structures and Machines [in Russian], Gosenergoizdat, Moscow (1957). N . M . Shchapov, Turbine Equipment of Hydrostations [in Russian], Gosenergoizdat, Moscow (1955). I . A . Chernyatin, Apparatus for Measuring the Characteristics of the Water Flow at Hydroelectric Stations and Its Operation [in Russian], Moscow, Gosenergoizdat, Moscow (1956).
531
4. 5.
6.
P . G . Kiselev, "Determination of the discharge of turbines from the pressure difference in the cross section of the spiral casing," Gidrotekh. Stroit., No. ii (1949). International Code of Model Acceptance Tests of Hydraulic Turbines, Con~nittee on Participation of the USSR in International Power Unions, Recommendations of the IPC [in Russian], No. 193 (1967). International Code of Prototype Acceptance Tests of Hydraulic Turbines. Committee on Participation of the USSR in International Power Unions, Recommendations othe IPC [in Russian], No. 41 (1966).
OPERATING EXPERIENCE AND RELIABILITY ASSESSMENT OF ELEMENTS OF PUMPING STATIONS UDC 621.671.004.6
O. Ya. Glovatskii
The development of water management in southern regions of the country has led to the construction of a large number of pumping stations (PSs). Pumping units (PUs) whose parameters are close to those of hydroelectric stations have been installed on the Kakhovka, Karshi, and other systems. Therefore, it is necessary to use the operating expereience of such PUs for increasing the reliability of functioning of individual structures of hydropower and reclamation systems. The majority of reliability indices are determined as the ratio of the actual and ideal (in the absence of failures) values. As a generalized index for a hydroelectric station it is customary to take the availability factor, equal to the ratio of the time the units are in operation and in standby to the selected period of operation. However, this index does not take into account the operating conditions of PUs. Thus, at the head PS of the Amu Darya-Bukhara canal the downtime due to emergency repairs of the first pumping unit exceeded the average value by 2.9 times and of the other extreme PU (9) by 3.8 times. The index also does not take into account the causes of failures, without which it is impossible to develop specific directions of technological progress. The Delphi method determined the main tasks of increasing the operating efficiency of hydrostations: an increase of the quality of the equipment and structures, improvement of maintenance, and incentive of the personnel [3]. The suggestion was realized: on the basis of expert evaluations, determine the probability of no-failure operation in fractions of unity. For the Kakhovka system such coefficients were obtained in the range 0.85-0.99, and after multiplication gave the result 0.71 [7] [sic]. In our opinion, calculation of the efficiency factors should be determined by the relationship
Keffi (1--KQ) (1--K~),
( 1)
where K = (Qc - Q)/Q is the delivery factor; 0 E KQ < i; Qc and Q are the calculated and actual deliveries of the PS; K~ = (~c - ~)/~ is the coefficient of disturbance of the on time; 0 E K~ < i; ~c and 9 are the calculated and actual on time of the PU. To determine the operating reliability index, a "tree of failures" with respect to three enlarged groups of elements of PSs was compiled as a result of analyzing statistical data on failures of the PSs of the Karshi canal and others [i]. As is seen from the data presented, the frequency of failures is stabilized by years. However, if these values are added, the probability of no-failure operation does not exceed 12%. The indicated distribution enables investigating the causes of failures when: as a result of failure of an element the conm~anded part is disconnected; the probability of failures of elements of the same type is equal; failure of an element occurs during its operation. Events, consisting in that a failure will occur during the course of time, can each be divided into a certain group of joint events:
Translated from Gidrotekhnicheskoe Stroitel'stvo, No. 9, pp. 32-35, September, 1989.
532
0018-8220/89/2309-0532512.50
9 1990 Plenum Publishing Corporation