A solution to ensure penstocks safety
Despite their mechanical harmfulness for hydro components, transient behaviors are not under permanent surveillance. MOTRHYS fills this gap with its transient pressure monitoring solutions.
Thanks to HYDROSURGE software developed by MOTRHYS, it is now possible to process the continuous stream of pressure data measured during each hydraulic events occurring in a hydraulic power plant. From each detected pressure transient, HYDROSURGE’s algorithms perform a precise diagnosis of the hydraulic behaviour of the pipe system. As the pressure is the main cause of the stress applied on the penstock’s structure, this diagnosis allows extracting the essential information for safety-decision-making: Are all indicators green? Or, would it be necessary to have a closer look to a particular detected issue?
The great flexibility of HYDROSURGE enables MOTRHYS to propose a simple and efficient solution to be implemented by hydro power plant operators. This allows them to easily control dangerous phenomena that had never been continuously monitored before. This continuous monitoring requires specific skills and is very time-consuming if an automatic analysis tool is not available. With HYDROSURGE, it becomes easy and cheap to monitor transient pressure in all small hydro and large-scale hydroelectric power stations.
HYDROSURGE software offers two solutions for penstocks pressure monitoring
Audit of transient pressure phenomena
MOTRHYS uses his software HYDROSURGE for carrying out an audit of pressure transients experienced by a hydro facility.
This audit is a measurement step performed as a background task during few months of the plant operation. This transient pressure measurement needs a pressure sensor to measure pressure fluctuations in the penstock and a data disturbance recorder. The measurement period is defined in order to capture a sufficient number of events to get a global overview on the transient pressure phenomena of the hydro plant.
The audit is particularly useful in situations when the real penstock loading is not well-known, for example in the following situations:
Spot-checked of pressure in operation
What is the real operational maximal pressure in the penstock? That is the key question, asked especially before and after a power increase of the turbine. The goal of this audit step is to measure the current level of pressure, then to detect possible unexpected over pressures due to the refurbishment.
Long term safety in operation
The audit steps can also be achieved annually in order to thoroughly analyze the pressure surges and detect any behavior change. Thanks to these data, it becomes possible to accurately identify the early faults. And thus to sustainably control the operating risk factor of the hydro facility.
Thanks to data analysis performed by HYDROSURGE, the surveillance report delivered by MOTRHYS includes information such as:
Pressure profile estimation : (Maximum pressure level reached in each section of the penstock). see Overload Detection Module
Surge statistics: This concerns the classification of the observed transient events in the following classes: Mass water hammering, propagating water hammering, resonance and water column separation.
Hydro Diagnosis: Analysis of the severity level of the extreme behaviors in each transient class > see Overload Detection Module
Hydro Consulting: Proposition of corrective actions allowing to reduce the severity level of the extreme behaviors encountered> see Root Cause Module
Continuous monitoring of transient phenomena
MOTRHYS provides HYDROSURGE as a permanently installed software in order to ensure real-time safety
The modular structure of HYDROSURGE enables a flexible deployment according to the requirements and customer specifications.
The HYDROSURGE software solution provides real-time key figures for assessing the penstock structure overload risk over its whole length. This information can be easily integrated with the plant SCADA system.
For hydro facilities without SCADA system, HYDROSURGE can be a stand-alone solution. This option features a root cause module (RCM) which helps to find the equipment requiring maintenance or re-tuning.
With HYDROSURGE, don’t risk anymore to lose control of penstock transient regimes
The admissible penstock pressure is calculated during the design stage in order to determine the safety margin between the over pressure level in operation and the risk of structural damage. The only way to control if this margin has not been reduced over time is to record the pressure measured during a full scale trip test on the real machine.
If an overrun of the admissible threshold is detected, additional tests have to be performed to diagnose and to correct the problem. But these tests reproduce only a small part of what may happen in operation, and they do not guarantee operating safety. Moreover, these actions are very costly as they imply the stop of production.
How does HYDROSURGE work to provide a barrier against rupture risks of penstocks ?
HYDROSURGE software uses innovative algorithms to perform signal processing and carry out a real-time hydro diagnosis from each captured pressure transient in operation. Thus allowing the detection of the risks and bringing them under control.
To get hydraulic information from pressure measurements, HYDROSURGE software comprises a set of four modules which each deals with specific aspects
Signal Processing Module SPM
Overload Detection Module ODM
Root Cause Module RCM
Data Conversion Module DCM
pressure transient measurement
Captured pressure transient signals are imported in HYDROSURGE any time a new significant hydro event happens by using the Data Conversion Module.
Pressure transient capture is ensured by disturbance recorders who isolate the transient signal in the continuous flow of information from the pressure sensors.
Disturbance recorder captures any event leading to a significant change in pressure fluctuations such as:
- Absolute threshold overcoming,
- Relative amplitude changes,
- Self-sustained cyclical pressure fluctuations.
Pre-trigger enables the pressure recording just before threshold overcoming for a better hydro diagnosis.
MOTRHYS interfaces with 3rd party disturbance recorders, like YOKOGAWA or TIEPIE data acquisition systems to ensure effective transient signal measurement and recording.
Signal Processing Module SPM
Diagnosis-driven data processing of transient pressure-time signal
The Signal Processing Module (SPM) processes the signal and extracts, according to hydro transients type, up to 23 different hydraulic scalar parameters embedded in the waveform (see the red parts in the figure)
The main input parameters used by the ODM allow detecting any risk symptoms as :
- Maximum pressure reached,
- Wave water hammer propagation along the penstock,
- Water column separation caused by a pressure drop propagation and cavitation along the penstock
- Abnormal cyclical nature of the pressure fluctuations and penstock resonance.
These parameters are used partly by the Overload Detection Module (ODM) to detect any overrun of the admissible pressure along the entire water line.
All other parameters are used by the Root Cause Module (RCM) to perform the hydro diagnosis.
Thanks to MOTRHYS’s innovative approach of using Phase Diagram Analysis, the robustness of the SPM allows real-time parameter extraction of any signal independently of the penstock or hydraulic equipment.
Analysis of complex pressure transient recordings including several events is carried out thanks to the repetition of the extraction process. Thus, SPM detects each significant event embedded in the same captured pressure wave signal.
For each hydraulic events, SPM processes the continuous stream of pressure data measured.
The SPM algorithms come out of GIPSA Lab exclusive research, while ODM and RCM algorithms are based on the field experience gained for many years of test and monitoring of pressure transient in penstocks.
Data processing of transient is a vectorization of the pressure-time signal captured for each water hammer or cyclical phenomenon. Signal wave forms are analyzed automatically in real-time independently of the penstock configuration. Exhaustive hydraulic parameters are extracted from the signal, allowing automatic analysis & diagnosis carried out by other modules of HYDROSURGE (ODM and RCM).
Overload Detection Module ODM
The continuous diagnostic of over pressure is performed by the overload detection module, ODM. The ODM uses proprietary algorithms based on the behaviour of a simple hydro acoustic system. This allows to continuously diagnose the class of any pressure fluctuation phenomena. And finally to detect the risk of exceeding the admissible maximum pressure over each section of the penstock.
ODM assesses the impact of the transient on each point along the penstock. This assessment is carried out in real-time by using the pressure measured on the downstream section of the penstock. Thus allowing the detection of potential security risks in each section.
The diagnostic of penstock over pressure is carried out for all potentially damaging phenomena
ODM detects any Mass Water Hammer phenomena to determine if the measured pressure on the penstock does not exceed the admissible maximum pressure profile,
The mass water hammer phenomenon is only related to the inertia of the water column located upstream of the component which cuts the flow. Closure time is longer than the return time of the reflected wave from the top of the penstock.
The affected area is meanly the downstream part of the water system: penstock sections, manifolds, unit casings.
ODM detects any Wave Water Hammer phenomena (or rapid water hammer) to determine if the speed of the transient flow drop will not create a propagating pressure surge. This wave could exceed the admissible maximum pressure level over upstream sections of the penstock and damaging its structure.
The wave Water Hammer phenomenon results from a quick drop in the flow. The closure time is shorter than the return time of the reflected wave. Because of the wave propagation, the affected area is meanly the upstream sections of the penstock. This phenomenon is related to the overall compressibility of the water in the penstock. Thus, safety requires to quantify the maximum pressure profile reached along the penstock for each transient event.
ODM detects any risks of Water Column Separation phenomena to determine if the speed of a sudden increase in flow rate leads to a propagating pressure drop wave. The depressurization could lead to cavitation over the upstream sections of the penstock and finally a large void cavity. This cavity that collapses when the pressure recovers cause strong over pressure in upstream sections of the penstock.
Water Column Separation phenomenon results from a rapid increase in the flow. The opening time is shorter than the return time of the reflected wave. The affected area is meanly the upstream sections of penstocks. Thus, hydro safety requires to quantify the minimum pressure profile reached along the penstock for each transient event.
ODM detects any risks of resonance by checking if the cyclical levels of the pressure are properly amortized. If not, these cycles drive to self-sustained pressure fluctuations and dangerous structural fatigue damage.
Self-sustained cyclical pressure fluctuations may cause low-cycle fatigue damages of penstocks. The whole sections of the penstock are affected.
Case of resonance due to spherical valve seal ring instability
Root Cause Module RCM
Finding causes of penstock overload
Root Cause Module algorithms apply expert rules to detect plant operation process errors and equipment malfunctions
The trend analysis of historized data is conducted by RCM to assess the evolution of the risk and impact of warning signs detected. But also, to quantify corrective actions effectiveness after their implementation.
A synthesis of the safety diagnosis is then issued to help Operators to ensure optimal security, this security diagnosis includes the following recommendations :
Condition-based safety maintenance: initiate a test to validate new settings for inlet stroke valves, or for speed governor tuning.
Operating support: stop-gap measures as for example the rejection of operating ranges for critical machines, lowering of the head, limitation of the full wickets gate opening.
Questioning the design of certain key components, as for example: modifying the stroke law of closure of inlet valves, the guard valve bypass implementation.
Why HYDROSURGE allows to stay in control of over-pressure risk
Lack of monitoring of the transients causes poor control of internal pressure fluctuations. However, such fluctuations are involved in over 40% of the ruptures in hydro power plants. This impacts the penstock, manifold and turbine casing.
Causes are multiple but always associated with the hydraulic functioning:
Drifts of the settings managing the turbine start-up and shut down operations, defaults of key components, are commonly involved in these events. In the absence of monitoring, these risky trends go unnoticed until they turn into causes of rupture.
Despite their mechanical harmfulness, transient behaviors are not monitored continuously. Only a few pressure transient measurements are made during the life of the equipment. But these tests only make it possible to reproduce one single hydraulic phenomenon: the hammering of mass. Thereby one forgets to quantify all the other types of hydraulic phenomena frequently involved in penstock failures. Although post-rupture structural analyses are achieved, these analyses are only carried out after the penstock’s damage. So, it is often too late to have a thorough understanding of the hydraulic phenomena involved and related loading on the damaged penstock.
Finally, it is often difficult to take from these ruptures a reusable experience to improve the operation safety of penstocks.
HYDROSURGE also improves low head plants safety
Monitoring of transients is a key tool to control reverse pressure surges in the turbine draft tube.
Tripping of reaction hydraulic turbines may lead to a sudden increase of the draft tube rope vortex volume, as well as the separation of the draft tube water column. Reverse water hammers may thus happen when the pressure recovers, the turbine casing, and runner exhibiting high-pressure surges.
It is possible to detect, quantify and predict all the dangerous hydraulic phenomena affecting penstock and turbine structures.
That is what continuous monitoring over the entire pipe system length brings.
By embedding operation processes expert rules, advanced signal processing methods, HYDROSURGE provides key insights on the root cause of the phenomena. MOTRHYS suggests corrective or preventive actions. So allowing to improve equipment lifetime, reducing production downtime and ultimately improving the operator bottom line.
These well-known phenomena are taken into account during the turbine design stage. Mitigation measures are developed such as the adapted setting of the speed governor, venting valve implementation to prevent the occurrence of void cavities in the turbine casing, etc. However, drifts in speed governor settings or a malfunction of the venting device are common. These faults can slowly but surely worsen the transient behavior in the turbine draft tube. Without continuous monitoring, thorough understanding and estimation of these phenomena are not managed. Major damages such as blade failures of axial runners or shaft line lifting with thrust bearing damage are thus encountered in low head plants.
MOTRHYS field experience derives from over three decades of measurement and diagnose of faulty hydraulic behaviours. This experience has been confirmed by the monitoring of transients of a fleet of penstocks. We thus showed that the use of a simple hydro acoustic model is relevant in order to analyze a pressure transient signal. Especially for early detecting and diagnosing transient problems affecting a penstock. This approach shows that any pressure overloads can be broken down into one or more of the following phenomena impacting the whole length of the piping system
HYDROSURGE solution leads to a deep understanding of any pressure overloads due to the transient phenomenon. It will be possible to clearly identify any faulty component acting on the hydro transients during the running of the plant.
HYDROSURGE brings operating support to set up work around measures to keep the turbine operating after transient faults detection while awaiting curative maintenance.
HYDROSURGE suggests solutions to correct design aspect of keys components involved in critical transient phenomena in order to eliminate faulty behavior.