- Email: [email protected]
Developments in geothermal energy in Mexico—part twenty-five: Operating experience with a geothermal wellhead turbine
Heat Recovery Systems & ClIP Vol. 9, No. 6, pp. 541-546, 1989 Printed in Great Britain
0890-4332/89 $3.00 + .00 Pergamon Press plc
DEVELOPMENTS IN GEOTHERMAL ENERGY IN MEXICO--PART TWENTY-FIVE: OPERATING EXPERIENCE WITH A GEOTHERMAL WELLHEAD TURBINE J. A. KUBIAK and J. F. I~R~Z Divisi6n de Equipos, Institutode Investigaciones El6ctricas,Cuernavaca, M6xico
(Received 21 April 1989) Abatract--After several months of operation of a wellhead 5 MW geothermal turbine, an incident occurred due to the failure of the governor oil pipe joint. The oil tank drained out causing damage to the turbine bearings. Heavy scaling of the first blading stage of the turbine was observed, particularly in the nozzles. Recommendations for the design and remote control operation of the wellhead unit are discussed with reference to the faults found in this case.
INTRODUCTION
Geothermal energy is exploited for electrical power generation in a number of ways. In liquid dominated reservoirs, the most common method is the use of a flash-steam cycle. The liquid from the reservoir undergoes depressurization during its rise up the wellbore and a proportion flashes to steam. The mixture which flows from the wellhead is usually further reduced in pressure. The steam is separated from the liquid phase for use in power generation turbines. The liquid phase, which at this stage is normally supersaturated in dissolved solids, is disposed of. The separated steam is used in two types of power plants, central and wellhead. In a central plant system, steam mains collect the output of a number of wells and supply a relatively larger (50 to 200 MWc) condensing turbo-generator plant. In a wellhead system, small (5 MW,) units are usually installed in the immediate vicinity of a wellhead. Such units are usually transportable back-pressure remotely operated turbo-generator plants. A 5 MW¢ portable wellhead turbine (5000 rpm) driving a generator through a gearbox was installed in a geothermal field as a base load unit without direct supervision (remote control). Most of the time, the unit has been operated at partial load (3 MW~). After six months of operation a joint of an oil pipe which supplied oil to the servomotor failed. The turbine was tripped but during the deceleration from the nominal speed to standstill (the normal time is about 12 rain) the oil tank drained out due to leakage from the broken pipe. It is worth noting that the governor oil system was not separated from the lubricating system by means of a check valve. The joint failure caused a decrease in oil pressure and then a deficiency in the lubrication of the turbine bearings. Finally, all the turbine bearings were damaged including severe damage to the auxiliary oil pump driven by a small steam turbine. It appeared during inspection of the turbine that the valves, main steam valve chest, main steam chamber and the ill'st stage nozzle were severely affected by scale and mineral deposits, as shown in Fig. 1. The causes of the oil pipe joint failure and scaling are analysed and the faults are identified. The remedies are discussed and recommendations on how to avoid similar problems in the future are made. General recommendations for the design and operation of wellhead turbines for remote control operation are discussed.
WELLHEAD
GEOTHERMAL
POWER
PLANT
Hiriart [1] pointed out that in certain circumstances wellhead power plants with a typical layout as shown in Fig. 2 are a very attractive option from an economic point of view. Wellhead units also have some technical advantages which were discussed by Kubiak and Guti6rrez [2]. However, due to the short distance of piping between the well and the turbine, contaminated steam may cause some difficulties. Thus adequate anti-corrosion and anti-fouling precautions must be included in Has 9/6--o
541
542
J.A. KUmAK and J. F. PE~z
Fig. 1. Scale deposits on first stage nozzles.
the design of the turbine [3, 4]. As is shown in Fig. 2, the principal components of the wellhead power plant are: the production well, main valve, separator, float valve, dehumidifier, pipeline, condensate traps and the turboset with its drains. The piping system must be provided with an adequate moisture separator and a sufficient number of condensate traps and drains to avoid the carryover of water, which can bring about various problems such as water hammer or deposits and scaling of the turbine components. Water, if not drained away, can destroy the blading system and/or bend the turbine shaft. The deposits can block passages, nozzles, vanes and in extreme cases the control and/or emergency valves and cause runaway of the rotor, which leads to its total destruction. Also, the deposits can cause corrosion, pitting corrosion and crevice corrosion of various components of the turbine [3]. CAUSES OF THE OIL PIPE F A I L U R E The turbine was mounted on a steel foundation frame in order to provide portability. It is known that the vibration damping factor of a steel foundation is much less than that of a reinforced concrete foundation. Therefore, preventative measures must be applied to keep vibration excitation forces, for example unbalance of the rotors (turbine, gear wheels and generator) or
~'~ t JJ trap J
dehumidifier separator
[~ _:~
condensate
drains brine disposal
W
production well Fig. 2. Well-head power plant.
exhaust to atmosphere
Developments in geothermal energy in Mexico---part twenty-five
s
s~
543
s
/ heavy rubbing / . ~ _J' II I/~ ~
brokenoil pipe orifice
pipe supports
Fig. 3. Control valve oil piping failure.
misalignment, as low as possible. Also, the steam and oil piping systems should be carefully considered and generally the oil pipes should be specially supported with clamps equipped with vibration insulators (dampers). The inspection of the turbine revealed that the failure of the governor oil pipe joint was caused by excessive vibration of the piping and a wrongly designed joint between the pipe and the servomotor (Fig. 3). Some of the pipe clamps and pipes at the place of fixing were worn out by the rubbing action caused by vibrations (Fig. 4). The piping vibrations were excited by a moderately unbalanced turbine rotor (although the bea~ing vibrations did not exceed the permissible limit) and by wrongly supported oil filters (Fig. 5), which in turn excited vibrations in the whole of the piping system. j
turbine center
•J
~
front
standard J J
I 2 /
/ 7 -strainer inlet
u-bolt
panel detail section
"-~
i ~ ~ i
u - b o l t ~ oil pipe thickness decreased by heavy rubbing Fig. 4. Heavy rubbing on the oil piping system.
~Ort
544
J.A. KUBIAK and J. F. PEg~z
liter
Fig. 5. Wrongly supported oil filters.
SCALING AND DEPOSITS During inspection of the turbine, the upper casing of the turbine was dismantled. The visual inspection of the inner parts of the turbine was carried out. It was found that the inlet steam casing chamber drain hole was not located in the lowest part of the casing (Fig. 6). Even during inspection, sludge and water were still present in the casing. The first stage nozzles had heavy deposits of scale as shown in Fig. 7. The blade exit area section was considerably diminished, by up to 40%. The scale was deposited on the outlet side (suction side) of the nozzle airfoils of the first stage. The scale was deposited in the zones where turbulence may occur. This is normally due to off design operation (partial load) or it may indicate a deficiency in the design of the steam path of the blading system. The inspection also revealed insufficient thermal insulation of the valves and piping and lack of adequate draining of the main steam valve chest. Some of the drains of the turbine were blocked with deposits. Also, there were an insufficient number of condensing traps between the separator and turbine, on the main steam line (Fig. 8). ANALYSIS OF THE FAULTS AND REMEDIES The analysis of the information gathered during inspection of the turbine allowed the identification of three primary causes of the faults which contributed to the failure of the pipe joint: (i) incorrect design and manufacture of the pipe joint connecting the oil pipe to the governor valve servomotor, (ii) a moderately unbalanced turbine rotor, (iii) inadequately supported oil filters, which excited vibration in the oil piping system. nozzle partition
"~
throat
deposits = drain
Fig. 6. Wrong location of the drain pipe.
Fig. 7. Scale deposits on the outlet side of the nozzles.
545
Developments in geothermal energy in Mexico--part twenty-five
:C
relief line
ahauet silencer
main steam line
f
I
dryer
i
!
f ~ cyclone ~ [ j l sep orator ! I
,11
I
i ~(~
geothermal
I
~
,v!
L ............
n,ll'I
,
;
L-~--".IJ
, ~ well
missin~ drain
machine house
~
11 silencer
Fig. 8. Condensate traps missing along the steam piping,
The remedies of the faults included: (i) balancing of the turbine rotor (ii) correction of the pipe joint design, and (iii) adequate support for the oil filters' mounting plate (Fig. 9). Through analysis of the log book it was found that the turbine was operated at part load (60%) and without an operator. It was recommended that the turbine be cleaned and that the damaged parts be replaced. The position of the main steam casing chamber drain hole must be put in the lowest point of the casing. The thermal insulation of the casing, valve chest and steam pipes should be corrected. It was pointed out that more attention should be given to the heating-up of the macl~ne as much as possible in the shortest period of time practicable during start-up of the turbine.
Fig. 9. Modification of oil filter support.
546
J.A. KUalAKand J. F. PEREZ G E N E R A L R E C O M M E N D A T I O N S FOR NEWLY INSTALLED WELLHEAD TURBINES
Normally, before putting the geothermal turbine into operation it is not possible to estimate the degree of accumulation of the scale deposits on the turbine parts in the inital period of operation. Therefore, this period is the most important in the operating life of the unit and requires greater attention to detei'mine the effect of mineral impurities in the geothermal steam on the safety and reliability of the turbine. First of all it is necessary to monitor the build-up of the deposits on the blading system of the turbine. This can be done by measuring the steam pressure before and after the governing stage. These data, compared with the values recorded during the commissioning test, will indicate the degree of deposits on the blading system. This should be carried out periodically. The governing valves should be closed and opened every 8 hr to avoid sticking of the valve stems. The operation of the turbine should be supervised, at least for the first six months of operation. All irregularities should be recorded and then analysed. The first inspection of the turbine should be carried out after three to six months of operation and then after one year to establish intervals between overhauls. The deposits on the rotor blades can be detected by vibration analysis (increased amplitude of vibration). During the overhaul it is essential to determine the effect of corrosive substances carried in the geothermal steam on the turbine components. Nondestructive tests such as ultrasonic, liquid penetrant, magnetic particles etc., can be used to detect cracks initiated by corrosion or stress corrosion. The progress of corrosion and/or erosion should be observed. A wellhead turbine is normally placed on ~i steel frame, which means that the damping factor is low. Therefore, the turbine/generator rotors must be perfectly balanced. The oil pipes should be fixed to their supports using clamps with rubber vibration insulators to avoid friction between metal parts of the joint. It is recommended that the governing and lubricating oil systems be separated by means of a check valve which is activated by the turbine trip. The most important factor in the design of a geothermal turbine is reliability rather than efficiency, in other words the turbine should be a robust one. By measuring steam pressure in various points of the steam system it is possible to determine the degree of scale deposits in the system. Applying simple methods of checking the functioning of all mechanisms and auxiliary equipment will save a lot of time and trouble. Only after analysing all the information obtained during the initial period of operation of the unit is it possible to decide if and when the unit can be operated with the remote control system. CONCLUSIONS Two simple errors in the installation of the oil piping system could have been avoided if the turbine had been properly checked during the commissioning test. Wellhead geothermal turbines for remote control must be exposed to more severe tests and requirement commencing at the design stage. This is because of the need for high reliability and robustness under a wide range of operating conditions and steam purities, coupled with the transportability of the unit. REFERENCES 1. Hiriart G. Developments in geothermal energy in Mexico--Part three: Economics of wellhead versus central power plants J. Heat Recovery Systems 6, 191-200 (1986). 2. Guti6rrez A. and Kubiak J. A. Developments in geothermal energy in Mexico--Part fifteen: Differencesbetween geothermal and conventional steam turbines Heat Recovery Systems & ClIP 7, 453-462 (1987). 3. Kubiak J. A., Guti6rrez A. and Per6z J. Developmentsin geothermal energy in Mexico--Part twenty: Technical advantages and disadvantages of geothermal wellhead units Heat Recovery Systems & ClIP 8, 529-536 (1988). 4. Bialy S. C. Steam turbine for geothermal application Power Engng May, 86-88 (1980).
0890-4332/89 $3.00 + .00 Pergamon Press plc
DEVELOPMENTS IN GEOTHERMAL ENERGY IN MEXICO--PART TWENTY-FIVE: OPERATING EXPERIENCE WITH A GEOTHERMAL WELLHEAD TURBINE J. A. KUBIAK and J. F. I~R~Z Divisi6n de Equipos, Institutode Investigaciones El6ctricas,Cuernavaca, M6xico
(Received 21 April 1989) Abatract--After several months of operation of a wellhead 5 MW geothermal turbine, an incident occurred due to the failure of the governor oil pipe joint. The oil tank drained out causing damage to the turbine bearings. Heavy scaling of the first blading stage of the turbine was observed, particularly in the nozzles. Recommendations for the design and remote control operation of the wellhead unit are discussed with reference to the faults found in this case.
INTRODUCTION
Geothermal energy is exploited for electrical power generation in a number of ways. In liquid dominated reservoirs, the most common method is the use of a flash-steam cycle. The liquid from the reservoir undergoes depressurization during its rise up the wellbore and a proportion flashes to steam. The mixture which flows from the wellhead is usually further reduced in pressure. The steam is separated from the liquid phase for use in power generation turbines. The liquid phase, which at this stage is normally supersaturated in dissolved solids, is disposed of. The separated steam is used in two types of power plants, central and wellhead. In a central plant system, steam mains collect the output of a number of wells and supply a relatively larger (50 to 200 MWc) condensing turbo-generator plant. In a wellhead system, small (5 MW,) units are usually installed in the immediate vicinity of a wellhead. Such units are usually transportable back-pressure remotely operated turbo-generator plants. A 5 MW¢ portable wellhead turbine (5000 rpm) driving a generator through a gearbox was installed in a geothermal field as a base load unit without direct supervision (remote control). Most of the time, the unit has been operated at partial load (3 MW~). After six months of operation a joint of an oil pipe which supplied oil to the servomotor failed. The turbine was tripped but during the deceleration from the nominal speed to standstill (the normal time is about 12 rain) the oil tank drained out due to leakage from the broken pipe. It is worth noting that the governor oil system was not separated from the lubricating system by means of a check valve. The joint failure caused a decrease in oil pressure and then a deficiency in the lubrication of the turbine bearings. Finally, all the turbine bearings were damaged including severe damage to the auxiliary oil pump driven by a small steam turbine. It appeared during inspection of the turbine that the valves, main steam valve chest, main steam chamber and the ill'st stage nozzle were severely affected by scale and mineral deposits, as shown in Fig. 1. The causes of the oil pipe joint failure and scaling are analysed and the faults are identified. The remedies are discussed and recommendations on how to avoid similar problems in the future are made. General recommendations for the design and operation of wellhead turbines for remote control operation are discussed.
WELLHEAD
GEOTHERMAL
POWER
PLANT
Hiriart [1] pointed out that in certain circumstances wellhead power plants with a typical layout as shown in Fig. 2 are a very attractive option from an economic point of view. Wellhead units also have some technical advantages which were discussed by Kubiak and Guti6rrez [2]. However, due to the short distance of piping between the well and the turbine, contaminated steam may cause some difficulties. Thus adequate anti-corrosion and anti-fouling precautions must be included in Has 9/6--o
541
542
J.A. KUmAK and J. F. PE~z
Fig. 1. Scale deposits on first stage nozzles.
the design of the turbine [3, 4]. As is shown in Fig. 2, the principal components of the wellhead power plant are: the production well, main valve, separator, float valve, dehumidifier, pipeline, condensate traps and the turboset with its drains. The piping system must be provided with an adequate moisture separator and a sufficient number of condensate traps and drains to avoid the carryover of water, which can bring about various problems such as water hammer or deposits and scaling of the turbine components. Water, if not drained away, can destroy the blading system and/or bend the turbine shaft. The deposits can block passages, nozzles, vanes and in extreme cases the control and/or emergency valves and cause runaway of the rotor, which leads to its total destruction. Also, the deposits can cause corrosion, pitting corrosion and crevice corrosion of various components of the turbine [3]. CAUSES OF THE OIL PIPE F A I L U R E The turbine was mounted on a steel foundation frame in order to provide portability. It is known that the vibration damping factor of a steel foundation is much less than that of a reinforced concrete foundation. Therefore, preventative measures must be applied to keep vibration excitation forces, for example unbalance of the rotors (turbine, gear wheels and generator) or
~'~ t JJ trap J
dehumidifier separator
[~ _:~
condensate
drains brine disposal
W
production well Fig. 2. Well-head power plant.
exhaust to atmosphere
Developments in geothermal energy in Mexico---part twenty-five
s
s~
543
s
/ heavy rubbing / . ~ _J' II I/~ ~
brokenoil pipe orifice
pipe supports
Fig. 3. Control valve oil piping failure.
misalignment, as low as possible. Also, the steam and oil piping systems should be carefully considered and generally the oil pipes should be specially supported with clamps equipped with vibration insulators (dampers). The inspection of the turbine revealed that the failure of the governor oil pipe joint was caused by excessive vibration of the piping and a wrongly designed joint between the pipe and the servomotor (Fig. 3). Some of the pipe clamps and pipes at the place of fixing were worn out by the rubbing action caused by vibrations (Fig. 4). The piping vibrations were excited by a moderately unbalanced turbine rotor (although the bea~ing vibrations did not exceed the permissible limit) and by wrongly supported oil filters (Fig. 5), which in turn excited vibrations in the whole of the piping system. j
turbine center
•J
~
front
standard J J
I 2 /
/ 7 -strainer inlet
u-bolt
panel detail section
"-~
i ~ ~ i
u - b o l t ~ oil pipe thickness decreased by heavy rubbing Fig. 4. Heavy rubbing on the oil piping system.
~Ort
544
J.A. KUBIAK and J. F. PEg~z
liter
Fig. 5. Wrongly supported oil filters.
SCALING AND DEPOSITS During inspection of the turbine, the upper casing of the turbine was dismantled. The visual inspection of the inner parts of the turbine was carried out. It was found that the inlet steam casing chamber drain hole was not located in the lowest part of the casing (Fig. 6). Even during inspection, sludge and water were still present in the casing. The first stage nozzles had heavy deposits of scale as shown in Fig. 7. The blade exit area section was considerably diminished, by up to 40%. The scale was deposited on the outlet side (suction side) of the nozzle airfoils of the first stage. The scale was deposited in the zones where turbulence may occur. This is normally due to off design operation (partial load) or it may indicate a deficiency in the design of the steam path of the blading system. The inspection also revealed insufficient thermal insulation of the valves and piping and lack of adequate draining of the main steam valve chest. Some of the drains of the turbine were blocked with deposits. Also, there were an insufficient number of condensing traps between the separator and turbine, on the main steam line (Fig. 8). ANALYSIS OF THE FAULTS AND REMEDIES The analysis of the information gathered during inspection of the turbine allowed the identification of three primary causes of the faults which contributed to the failure of the pipe joint: (i) incorrect design and manufacture of the pipe joint connecting the oil pipe to the governor valve servomotor, (ii) a moderately unbalanced turbine rotor, (iii) inadequately supported oil filters, which excited vibration in the oil piping system. nozzle partition
"~
throat
deposits = drain
Fig. 6. Wrong location of the drain pipe.
Fig. 7. Scale deposits on the outlet side of the nozzles.
545
Developments in geothermal energy in Mexico--part twenty-five
:C
relief line
ahauet silencer
main steam line
f
I
dryer
i
!
f ~ cyclone ~ [ j l sep orator ! I
,11
I
i ~(~
geothermal
I
~
,v!
L ............
n,ll'I
,
;
L-~--".IJ
, ~ well
missin~ drain
machine house
~
11 silencer
Fig. 8. Condensate traps missing along the steam piping,
The remedies of the faults included: (i) balancing of the turbine rotor (ii) correction of the pipe joint design, and (iii) adequate support for the oil filters' mounting plate (Fig. 9). Through analysis of the log book it was found that the turbine was operated at part load (60%) and without an operator. It was recommended that the turbine be cleaned and that the damaged parts be replaced. The position of the main steam casing chamber drain hole must be put in the lowest point of the casing. The thermal insulation of the casing, valve chest and steam pipes should be corrected. It was pointed out that more attention should be given to the heating-up of the macl~ne as much as possible in the shortest period of time practicable during start-up of the turbine.
Fig. 9. Modification of oil filter support.
546
J.A. KUalAKand J. F. PEREZ G E N E R A L R E C O M M E N D A T I O N S FOR NEWLY INSTALLED WELLHEAD TURBINES
Normally, before putting the geothermal turbine into operation it is not possible to estimate the degree of accumulation of the scale deposits on the turbine parts in the inital period of operation. Therefore, this period is the most important in the operating life of the unit and requires greater attention to detei'mine the effect of mineral impurities in the geothermal steam on the safety and reliability of the turbine. First of all it is necessary to monitor the build-up of the deposits on the blading system of the turbine. This can be done by measuring the steam pressure before and after the governing stage. These data, compared with the values recorded during the commissioning test, will indicate the degree of deposits on the blading system. This should be carried out periodically. The governing valves should be closed and opened every 8 hr to avoid sticking of the valve stems. The operation of the turbine should be supervised, at least for the first six months of operation. All irregularities should be recorded and then analysed. The first inspection of the turbine should be carried out after three to six months of operation and then after one year to establish intervals between overhauls. The deposits on the rotor blades can be detected by vibration analysis (increased amplitude of vibration). During the overhaul it is essential to determine the effect of corrosive substances carried in the geothermal steam on the turbine components. Nondestructive tests such as ultrasonic, liquid penetrant, magnetic particles etc., can be used to detect cracks initiated by corrosion or stress corrosion. The progress of corrosion and/or erosion should be observed. A wellhead turbine is normally placed on ~i steel frame, which means that the damping factor is low. Therefore, the turbine/generator rotors must be perfectly balanced. The oil pipes should be fixed to their supports using clamps with rubber vibration insulators to avoid friction between metal parts of the joint. It is recommended that the governing and lubricating oil systems be separated by means of a check valve which is activated by the turbine trip. The most important factor in the design of a geothermal turbine is reliability rather than efficiency, in other words the turbine should be a robust one. By measuring steam pressure in various points of the steam system it is possible to determine the degree of scale deposits in the system. Applying simple methods of checking the functioning of all mechanisms and auxiliary equipment will save a lot of time and trouble. Only after analysing all the information obtained during the initial period of operation of the unit is it possible to decide if and when the unit can be operated with the remote control system. CONCLUSIONS Two simple errors in the installation of the oil piping system could have been avoided if the turbine had been properly checked during the commissioning test. Wellhead geothermal turbines for remote control must be exposed to more severe tests and requirement commencing at the design stage. This is because of the need for high reliability and robustness under a wide range of operating conditions and steam purities, coupled with the transportability of the unit. REFERENCES 1. Hiriart G. Developments in geothermal energy in Mexico--Part three: Economics of wellhead versus central power plants J. Heat Recovery Systems 6, 191-200 (1986). 2. Guti6rrez A. and Kubiak J. A. Developments in geothermal energy in Mexico--Part fifteen: Differencesbetween geothermal and conventional steam turbines Heat Recovery Systems & ClIP 7, 453-462 (1987). 3. Kubiak J. A., Guti6rrez A. and Per6z J. Developmentsin geothermal energy in Mexico--Part twenty: Technical advantages and disadvantages of geothermal wellhead units Heat Recovery Systems & ClIP 8, 529-536 (1988). 4. Bialy S. C. Steam turbine for geothermal application Power Engng May, 86-88 (1980).