Modeling water hammer protection with a synchronous bypass valve.

Hello Arti,

Can you please confirm if you are talking about the bypass valve, like spherical valve as shown in below picture? If not then please attached a screenshot of the valve as per the arrangement in model. 

Here is technote about modeling turbines in Hammer, for your reference. 

Modeling reference - Turbines 

Hello Sushma,

Thank you for your reply. Actually I think I'm not talking about that bypass line. I think the one in your picture is used for equaling the pressure between and after the main inlet valve. 

I'm talking about a bypass valve that's usually built in the spiral itself which is connected to the turbine gates. Here's a description of the valve from a book on water hammer protection: 

A bypass valve, as the name implies, can be used to divert flow past passed the
turbine. The bypass valve is designed with a linkage to the turbine operating ring
in such a way that the bypass valve opens synchronously as the wicket gates
close. This allows the turbine to be closed quickly while diverting flow through
the bypass valve thereby avoiding excessive waterhammer and generator speed
rises. In effect the turbine wicket gate closure curve has two portions an initial
fast closure rate, until the bypass valve is fully opened and a slower rate governed
by closure of the bypass valve to the new operating position. A bypass valve
having a capacity of 33% - 60% of turbine flow capacity is usually satisfactory.
This approach provides good responses for loss off load situations, but load
addition characteristics are less satisfactory as the turbine would only be able to
take on load at a reduced (slow) rate. This type of bypass valve is sometimes
referred to as a synchronous bypass valve as it operates in unison with the turbine
wicket gates. 

https://imgur.com/a/rJAas9J this is a usual schematic design. 

Appreciate if you could guide me of how to input this in the hammer software. 

Thank you.

Hello Arti,

If you need to have a separate bypass valve that opens when the turbine wicket gates close, you could model that with the Throttle Control Valve (TCV) element. You would need to manually control the open and closure of the bypass valve using the TCV's operating rule pattern. Here is a screenshot of a modified version of the Turbine example model included with HAMMER:

The TCV's operating rule would be configured with a pattern that opens at the same time the turbine's wicket gates "Pattern (gate opening)" close. 

Although this may help keep the water column moving to avoid damaging transient pressures, you would still want to analyze this in HAMMER to confirm. You may also want to consider modeling the case of the bypass valve being stuck, to see what would happen if it did not open as the wicket gates close.

I have updated our turbine modeling reference wiki article with this information and some additional details including a reference to a separate wiki article on TCV modeling.

Please note that a subsequent reply to this thread was split to a separate thread since it is a separate question: Turbine results truncation and inertia of entrained water

Hello Jesse,

I followed your instructions about the bypass synchronous valve. But I have several questions I want to clarify before trusting the results. Below is the scheme I use for water hammer protection design on a small hydropower plant for 2 Francis turbines:

https://imgur.com/a/52UWtrI

Note the small bypass valve properties. I play with the discharge coefficient to allow different percentages of the turbine flow. In this example I set it up to allow 80% of the flow of the small turbine when it's fully open. I put the same coefficient at the transient (physical) setting, is that okay? And status (initial) is closed as the opening rule starts from 100% closure shown here:

https://imgur.com/a/M16O5NL

Is this way of setting up the valve to act as a bypass synchronous valve correct? Meaning it starts to  open as soon as the turbine starts to close. 

I get a warning error saying "The valve's initial closure percent does not match the initial closure percent in the valve's referenced Operating rule. The Operating Rule will be used as specified, but should be modified to get the expected results". Should i ignore this message?

The resulting flows appear to be correct as I have it in my mind. This is the flow time history of the small bypass valve: https://imgur.com/a/JUIwMvf It goes up to 80% of the turbine flow (0.8 m3/s). The same is true for the big bypass valve.

Another big doubt I have is related to the moment of inertia. I've read on a topic on this forum that Bentley uses WR2, not GD2 which is technically 4 times larger. 
I use technical data from a generator supplier who gives me WR2 (in their definition J). For example, for my case, I have a 800 kVA generator with a J=WR2=74, plus the turbine moment of inertia so all in all I input in the Bentley 96 kgm2. For the big turbine (1500 kVA) I input 440 kgm2. Are these values too small? Because they seem to me like they are. Should I multiply these values by 4? 

This is what happens when I do that (input GD2 instead of J=WR2), regarding overspeed: 

https://imgur.com/a/xGaWGdm

You can see that the blue line (4 times WR2) for the small turbine gets me almost the same overspeed number, 1075 rpm instead of 1115. 

Is this a sign that I'm using small values for the moment of inertia? Or should I trust the red line in the picture. Does the red line seem weird to you, with the oscillations around 1100 rpm?. Remember the bypass opens as the turbine is closing for the same time.

Regarding the time step warning I get (since now I'm using very long penstock length and very short bypass length) I get the same results with 10x smaller timestep and the current one (0.006s). 

Last question, I compare all the results with a manual calculation I do using the Allievi method, and Hammer always overshoots my predictions about pressure rise by about 10mH20. Regarding this parameter, should I stick to Allievi's method or be on the safe side with Hammer's predictions? I know it's a silly questions but I want to know what you think.

I apologize for the long thread, and the long list of questions, but I really need some clarification and most importantly I need to trust the results I get with the software to continue using it in the future as a go-to. So far I've done most of my calculations manually using monographs and empirical evidence, but I think Bentley along with your support can make my life much easier.

Thank you,
Best regards,
Arti Qormemeti  

Arti Qormemeti said:

Note the small bypass valve properties. I play with the discharge coefficient to allow different percentages of the turbine flow. In this example I set it up to allow 80% of the flow of the small turbine when it's fully open. I put the same coefficient at the transient (physical) setting, is that okay?

The "Discharge Coefficient (Transient)" is only exposed if you have the calculation option "Specify Initial Conditions" set to "true", which should only be done for very specific purposes as it means that all initial conditions must be manually entered (instead of being calculated by the initial conditions solver). I would suggest setting this calculation option to the default of "false" to avoid confusion. With the setting of false, you will not see that field in the TCV properties, and HAMMER will use the "Discharge Coefficient (fully Open) when the valve is at the 100% open position. 

Arti Qormemeti said:

Is this way of setting up the valve to act as a bypass synchronous valve correct? Meaning it starts to  open as soon as the turbine starts to close. 

Yes, aside from the fix required above (specify initial conditions calculation option) and assuming the fully open discharge coefficient value and valve type (needle) are appropriate, then you have modeled a valve that will open between 0.25 seconds and 8.05 seconds after the simulation starts. If this is when you want the valve to open, then the setup looks reasonable to me.

Arti Qormemeti said:

I get a warning error saying "The valve's initial closure percent does not match the initial closure percent in the valve's referenced Operating rule. The Operating Rule will be used as specified, but should be modified to get the expected results". Should i ignore this message?

This is probably related to the "specify initial conditions" setting. The "relative closure (initial transient)" field must be manually entered when that calculation option is set to "true", and the value seen in the first screenshot is 0%. So, HAMMER thinks that the valve is fully open in the first transient timestep, causing the user notification in question. Once you correct the calculation option, compute initial conditions and confirm that the "Relative closure (calculated)" field in the "Results" section of the TCV properties shows as 100%. You should then no longer see that user notification after computing the transient simulation.

Arti Qormemeti said:

Another big doubt I have is related to the moment of inertia. I've read on a topic on this forum that Bentley uses WR2, not GD2 which is technically 4 times larger. 
I use technical data from a generator supplier who gives me WR2 (in their definition J). For example, for my case, I have a 800 kVA generator with a J=WR2=74, plus the turbine moment of inertia so all in all I input in the Bentley 96 kgm2. For the big turbine (1500 kVA) I input 440 kgm2. Are these values too small? Because they seem to me like they are. Should I multiply these values by 4? 

Without in-depth knowledge about the turbine specifications, I cannot comment whether the values you have selected are correct, but I can confirm that HAMMER uses WR^2 (weight, which may include the weight of entrained water, times radius of gyration squared). Turbines also greatly range in size, and turbine models I have received from other users do not necessarily include any information about the size or power. There are many different types and sizes of turbines for different applications - low head vs. high head, large hydroelectric projects, or small turbines to recover pump energy spent to lift water over a hill. That said I took a look at some user turbine models and have listed below some values of moment of inertia (in kg-m^2)

4,796,746 (6 m)
1,900,000 (3 m)
690,595 (9 m)
6,500 (2 m)
4,600 (1 m)

So, your value of 96 and 440 kg-m^2 do seem small, but maybe your turbines are much smaller scale than the models I checked.

You can read more details about the HAMMER turbine moment of inertia here: Modeling reference - Turbines

Arti Qormemeti said:

You can see that the blue line (4 times WR2) for the small turbine gets me almost the same overspeed number, 1075 rpm instead of 1115. 

Is this a sign that I'm using small values for the moment of inertia?

It could be that the values are too small, or maybe the system is not sensitive to the inertia, or maybe there is another problem (for example due to the Specify Initial Conditions option).

Arti Qormemeti said:

Does the red line seem weird to you, with the oscillations around 1100 rpm?. Remember the bypass opens as the turbine is closing for the same time.

Without seeing the model it is hard to say if there is a problem with the red line - it could be due to other things happening in the system (take a look at an animation of the system along with that graph). Or, it could be due to the Specify Initial Conditions issue.

Arti Qormemeti said:

Last question, I compare all the results with a manual calculation I do using the Allievi method, and Hammer always overshoots my predictions about pressure rise by about 10mH20. Regarding this parameter, should I stick to Allievi's method or be on the safe side with Hammer's predictions?

Without a deep understanding of the physical system you are trying to simulate, versus what you have input in HAMMER, it is hard to say if there is some data entry issue causing the results to be different than the "Allievi method", or if it is simply a difference in the methodology. I would suggest first ensuring you have a sound understanding of HAMMER's assumptions and that the model is configured correctly (again as an example, fix the Specify Initial Conditions issue).

Arti Qormemeti said:

Note the small bypass valve properties. I play with the discharge coefficient to allow different percentages of the turbine flow. In this example I set it up to allow 80% of the flow of the small turbine when it's fully open. I put the same coefficient at the transient (physical) setting, is that okay?

The "Discharge Coefficient (Transient)" is only exposed if you have the calculation option "Specify Initial Conditions" set to "true", which should only be done for very specific purposes as it means that all initial conditions must be manually entered (instead of being calculated by the initial conditions solver). I would suggest setting this calculation option to the default of "false" to avoid confusion. With the setting of false, you will not see that field in the TCV properties, and HAMMER will use the "Discharge Coefficient (fully Open) when the valve is at the 100% open position. 

Arti Qormemeti said:

Is this way of setting up the valve to act as a bypass synchronous valve correct? Meaning it starts to  open as soon as the turbine starts to close. 

Yes, aside from the fix required above (specify initial conditions calculation option) and assuming the fully open discharge coefficient value and valve type (needle) are appropriate, then you have modeled a valve that will open between 0.25 seconds and 8.05 seconds after the simulation starts. If this is when you want the valve to open, then the setup looks reasonable to me.

Arti Qormemeti said:

I get a warning error saying "The valve's initial closure percent does not match the initial closure percent in the valve's referenced Operating rule. The Operating Rule will be used as specified, but should be modified to get the expected results". Should i ignore this message?

This is probably related to the "specify initial conditions" setting. The "relative closure (initial transient)" field must be manually entered when that calculation option is set to "true", and the value seen in the first screenshot is 0%. So, HAMMER thinks that the valve is fully open in the first transient timestep, causing the user notification in question. Once you correct the calculation option, compute initial conditions and confirm that the "Relative closure (calculated)" field in the "Results" section of the TCV properties shows as 100%. You should then no longer see that user notification after computing the transient simulation.

Arti Qormemeti said:

Another big doubt I have is related to the moment of inertia. I've read on a topic on this forum that Bentley uses WR2, not GD2 which is technically 4 times larger. 
I use technical data from a generator supplier who gives me WR2 (in their definition J). For example, for my case, I have a 800 kVA generator with a J=WR2=74, plus the turbine moment of inertia so all in all I input in the Bentley 96 kgm2. For the big turbine (1500 kVA) I input 440 kgm2. Are these values too small? Because they seem to me like they are. Should I multiply these values by 4? 

Without in-depth knowledge about the turbine specifications, I cannot comment whether the values you have selected are correct, but I can confirm that HAMMER uses WR^2 (weight, which may include the weight of entrained water, times radius of gyration squared). Turbines also greatly range in size, and turbine models I have received from other users do not necessarily include any information about the size or power. There are many different types and sizes of turbines for different applications - low head vs. high head, large hydroelectric projects, or small turbines to recover pump energy spent to lift water over a hill. That said I took a look at some user turbine models and have listed below some values of moment of inertia (in kg-m^2)

4,796,746 (6 m)
1,900,000 (3 m)
690,595 (9 m)
6,500 (2 m)
4,600 (1 m)

So, your value of 96 and 440 kg-m^2 do seem small, but maybe your turbines are much smaller scale than the models I checked.

You can read more details about the HAMMER turbine moment of inertia here: Modeling reference - Turbines

Arti Qormemeti said:

You can see that the blue line (4 times WR2) for the small turbine gets me almost the same overspeed number, 1075 rpm instead of 1115. 

Is this a sign that I'm using small values for the moment of inertia?

It could be that the values are too small, or maybe the system is not sensitive to the inertia, or maybe there is another problem (for example due to the Specify Initial Conditions option).

Arti Qormemeti said:

Does the red line seem weird to you, with the oscillations around 1100 rpm?. Remember the bypass opens as the turbine is closing for the same time.

Without seeing the model it is hard to say if there is a problem with the red line - it could be due to other things happening in the system (take a look at an animation of the system along with that graph). Or, it could be due to the Specify Initial Conditions issue.

Arti Qormemeti said:

Last question, I compare all the results with a manual calculation I do using the Allievi method, and Hammer always overshoots my predictions about pressure rise by about 10mH20. Regarding this parameter, should I stick to Allievi's method or be on the safe side with Hammer's predictions?

Without a deep understanding of the physical system you are trying to simulate, versus what you have input in HAMMER, it is hard to say if there is some data entry issue causing the results to be different than the "Allievi method", or if it is simply a difference in the methodology. I would suggest first ensuring you have a sound understanding of HAMMER's assumptions and that the model is configured correctly (again as an example, fix the Specify Initial Conditions issue).

Hello Jesse,

I followed your instructions regarding the Special Initial Conditions and now I don't receive the warnings anymore. I will continue to play with the model to understand better the pressure increase discrepancies.

Jesse Dringoli said:

There are many different types and sizes of turbines for different applications - low head vs. high head, large hydroelectric projects, or small turbines to recover pump energy spent to lift water over a hill. That said I took a look at some user turbine models and have listed below some values of moment of inertia (in kg-m^2)

4,796,746 (6 m)
1,900,000 (3 m)
690,595 (9 m)
6,500 (2 m)
4,600 (1 m)

Can you clarify what is the unit in the parentheses? Is that the diameter of the runner? Or the head? 

The turbines in my model use very small francis turbines, (runner D2=0.5 m), used on small hydropower plants with relatively long penstocks (L=3600 m, H=90m, Q=3m3/s, in this case) so a good transient protection is vital since the generators and turbines are small in relative to the water inertia. 

Thank you,
Best,
Arti

Arti Qormemeti said:

Can you clarify what is the unit in the parentheses? Is that the diameter of the runner? Or the head? 

My apologies - I meant to mention that the number in parenthesis is the penstock diameter (upstream pipe). I could go back and look at the head and penstock length, etc if needed but since they are user models, there is always the chance that any of the data input could have a mistake. I think the numbers I showed in my previous message show that most moment of inertia values are much, much higher than what you used, but again, it could be due to the small scale.

Hello Jesse,

I want to follow on this particular issue.

I got an information from the generator supplier that the values they provide in the catalog (96 kgm2 and 440 kgm2) are J = mr2. Is it possible that they give me the mass moment of inertia (while in the Bentley Hammer description it says: "The (weight) moment of inertia accounts for the turbine, generator and entrained water". Should I just multiply the values by g=9.81 ?

Thank you very much!
Best regards,

Arti Qormemeti

Hi Arti, I am not sure about the J=MR^2. You may be right about mass vs. weight but you may want to check with the manufacturer to confirm. You could also do the multiplication, run the transient analysis and see if that even makes a difference on the overall results (sensitivity analysis).