6 ms·

> Kurdish officials intended to shut down the turbines, but American officials told them that this would add more water to the reservoir, making the dam more li

by chris_7 10y ago

> Kurdish officials intended to shut down the turbines, but American officials told them that this would add more water to the reservoir, making the dam more likely to burst. So isis continued to profit from the dam. “We wanted to strangle them, but we weren’t allowed,” a Kurdish official told me. Not that destroying infrastructure and killing the power to a city of civilians, despite ISIS control, is necessarily a good idea, but could they not just cut the lines?

subway 10y ago
Probably lack of sufficient dump load.
- snowwindwaves 10y ago
  A dump load is a sign of a poor civil and mechanical engineering design of a power plant. It means the mechanical system can't change the shaft power as fast as required.
Tuna-Fish 10y ago
> Not that destroying infrastructure and killing the power to a city of civilians, despite ISIS control, is necessarily a good idea, but could they not just cut the lines? Do you know what happens when you run a turbine with nowhere to put the load? 750MW is enough to boil something like a kiloton of water every hour, that energy needs to go somewhere.
- stale2002 10y ago
  Open a valve that DOESNT go through a turbine? A dam should have something like that....
  - foota 10y ago
    I think the only equivalent is the floodgates, but, they only work when the water level is at a certain height, and I think I've read that if you open them to a true flood point you damage the dam. Drilling a second tunnel just to let water out is probably not considered economical.
    - elihu 10y ago
      According to the article there are two control gates and a spillway. The control gates are used to regulate the water level and the spillway is used in an emergency. I don't know if the control gates can drain the reservoir completely or they have some minimum level, but the article implies that there is some action that the Iraqi government could take: > Last spring, the Iraqi government prepared by lowering the maximum water level in the reservoir, to ease severe pressure on the dam wall. This year, such a precaution could dramatically lessen the number of people at risk—to about three hundred and sixty-four thousand. What isn't clear is whether the water level right now could be lowered immediately or the water level now is at a seasonal low point and the Iraqi government hasn't announced whether they'll spill water in the spring to keep it low.
    - xg15 10y ago
      > Drilling a second tunnel just to let water out is probably not considered economical. Economical relative to what? The projected losses if half the populated area of the country is flooded?
- saulrh 10y ago
  I wonder how much it'd cost to make a heating coil big enough to dump the energy. You've got all that water right there...
- viraptor 10y ago
  Honestly - no, I don't know what happens. Can you tell me? I mean without any modifications that would specifically covert that power into heat, what would happen with the lines cut? Something like the turbines melting, or welding themselves to the structure?
  - lisper 10y ago
    That's exactly right. Either that or a catastrophic mechanical failure. It's really not an experiment you want to do. One way or another, it would be both ugly and irreversible.
  - snowwindwaves 10y ago
    The turbines are Francis type which means they keep spinning until the water flow is stopped. When a generator is disconnected from the power system without first removing the power from the shaft then power in is greater than power out and the result is an acceleration of the generator. Generators are built to be able to withstand a certain maximum rotational speed without flying apart for a certain duration of time before the bearings get too hot. Of course it can operate at the normal speed forever, but the rated over speed might be twice as fast for 10 minutes. So when the power lines are cut the turbines and generators speed up until the flow of water is stopped. Pictures of the Mosul dam show four surge tanks, one for each turbine, so they can probably turn off the water in 5-10s. Pelton turbines are used for higher head and the flow can be maintained without turning the turbine by deflecting the water away from the runner. I've commissioned about a dozen hydro electric power plants and opening the breaker to simulate cutting the transmission line while making 10, 25, 50,75,100% of the power is standard procedure.
    - xg15 10y ago
      Thank you very much for the expert information. but that still doesn't make sense to me. The article states they have two control gates and a spillway. (one gate previously defunct but now repaired) So, shouldn't it be possible to block off the turbines and open the control gates instead, so that the total flow stays (roughly) the same? Moreover, if the dam is indeed that easy to destabilize, letting ISIS control the load seems equivalent to handing them a big red "trigger flood" button for use at their discretion. Sure, right now it's in their interest to keep the dam intact - because they still control many parts of Mossul and need the electricity. But "scorched earth" is not exactly a new concept and if the operation in Mossul continues as planned and ISIS is driven out, they could decide to cut the lines on their end to deliberately cause mechanical failures.
- hvidgaard 10y ago
  You could use it to boil some of the water away? A win/win if you're not a civilian relying on the power that is.
  - Filligree 10y ago
    Well, on the assumption that it'd work... Now you've got a dam full of boiling water. And as you'll know from cleaning dishes, boiling water is far better at dissolving minerals than is cold water.
    - VLM 10y ago
      I'm curious why you guys want to boil the upstream water instead of the downstream water.
      - hvidgaard 10y ago
        To reduce the amount of water behind the dam. I'm sure using the power to power pumps would be a better at that task though.
    - avar 10y ago
      How are you calculating that entire reservoir would boil? It's filled with 8B m^3 of water. I couldn't find the formulas to calculate this myself, but given Tuna-Fish's comment above that it's enough to boil a kiloton of water (1000 m^3) ever hour that's 1K/8B =~ 0.00001%. So it's going to be more on the order of dropping a single drop of boiling water into a backyard pool every hour, is it not? I.e. something that wouldn't change the overall temperature of the reservoir to any significant degree. This is all assuming that you'd have to use the energy to heat the reservoir itself, you could just as easily use it to pump out some water and boil it until it evaporates, not impacting the temperature of the reservoir at all, just emitting a lot of steam into the air.
    - hvidgaard 10y ago
      You pump water into a boiler, not just submerging a giant heater into the dam.
- chris_7 10y ago
  My vague understanding of a turbine was that it was a spinning magnet, so if you killed the circuit it would just... spin, I guess. If this is a concern, why isn't there a worry that ISIS or just a stray bomb would kill the power lines when retreating from Mosul? Though I suppose they could just target the dam itself...
  - cesarb 10y ago
    > My vague understanding of a turbine was that it was a spinning magnet, so if you killed the circuit it would just... spin, I guess. The load in the circuit acts as a brake. If you kill the circuit, the turbine spins faster and faster until it breaks into pieces. To prevent that, the control systems stop the water from flowing into the turbine, but stopping the water from draining is the opposite of what they want.
    - PietdeVries 10y ago
      The story suggests there is a 300 foot wide canal that can be used to let water go in casse of an emergency. Well, how about doing so now? Rather than using the turbines, use the canal to let the water out, but without it generating electricity...
      - takk309 10y ago
        The emergency spill way is in case of a large rain or melt event that brings the water level up quickly. The spill way is used to ensure that the dam is not over topped, which is usually catastrophic. As such, the spill way is usually placed at or near the maximum safe free board (distance from the top of the dam down to the water level). What this all means is that the spill way can only be used when the water level in the reservoir is allowed to get to high and possibly dangerous levels. The article stated the the water level has already been decreased by 10 meters which means that the spill way is going to remain dry for a long while. Next time you are near a dam look around and you will find that most have three paths for the water to leave the reservoir. The first is through the power generating turbines, second is the sluice gates which control water level, and third is the emergency spill way which is designed to fend off catastrophic failure.
- deleted 10y ago
  [deleted]

by chris_7 10y ago

subway 10y ago

Probably lack of sufficient dump load.

snowwindwaves 10y ago
A dump load is a sign of a poor civil and mechanical engineering design of a power plant. It means the mechanical system can't change the shaft power as fast as required.

Tuna-Fish 10y ago

> Not that destroying infrastructure and killing the power to a city of civilians, despite ISIS control, is necessarily a good idea, but could they not just cut the lines? Do you know what happens when you run a turbine with nowhere to put the load? 750MW is enough to boil something like a kiloton of water every hour, that energy needs to go somewhere.

stale2002 10y ago
Open a valve that DOESNT go through a turbine? A dam should have something like that....
- foota 10y ago
  I think the only equivalent is the floodgates, but, they only work when the water level is at a certain height, and I think I've read that if you open them to a true flood point you damage the dam. Drilling a second tunnel just to let water out is probably not considered economical.
  - elihu 10y ago
    According to the article there are two control gates and a spillway. The control gates are used to regulate the water level and the spillway is used in an emergency. I don't know if the control gates can drain the reservoir completely or they have some minimum level, but the article implies that there is some action that the Iraqi government could take: > Last spring, the Iraqi government prepared by lowering the maximum water level in the reservoir, to ease severe pressure on the dam wall. This year, such a precaution could dramatically lessen the number of people at risk—to about three hundred and sixty-four thousand. What isn't clear is whether the water level right now could be lowered immediately or the water level now is at a seasonal low point and the Iraqi government hasn't announced whether they'll spill water in the spring to keep it low.
  - xg15 10y ago
    > Drilling a second tunnel just to let water out is probably not considered economical. Economical relative to what? The projected losses if half the populated area of the country is flooded?
saulrh 10y ago
I wonder how much it'd cost to make a heating coil big enough to dump the energy. You've got all that water right there...
viraptor 10y ago
Honestly - no, I don't know what happens. Can you tell me? I mean without any modifications that would specifically covert that power into heat, what would happen with the lines cut? Something like the turbines melting, or welding themselves to the structure?
- lisper 10y ago
  That's exactly right. Either that or a catastrophic mechanical failure. It's really not an experiment you want to do. One way or another, it would be both ugly and irreversible.
- snowwindwaves 10y ago
  The turbines are Francis type which means they keep spinning until the water flow is stopped. When a generator is disconnected from the power system without first removing the power from the shaft then power in is greater than power out and the result is an acceleration of the generator. Generators are built to be able to withstand a certain maximum rotational speed without flying apart for a certain duration of time before the bearings get too hot. Of course it can operate at the normal speed forever, but the rated over speed might be twice as fast for 10 minutes. So when the power lines are cut the turbines and generators speed up until the flow of water is stopped. Pictures of the Mosul dam show four surge tanks, one for each turbine, so they can probably turn off the water in 5-10s. Pelton turbines are used for higher head and the flow can be maintained without turning the turbine by deflecting the water away from the runner. I've commissioned about a dozen hydro electric power plants and opening the breaker to simulate cutting the transmission line while making 10, 25, 50,75,100% of the power is standard procedure.
  - xg15 10y ago
    Thank you very much for the expert information. but that still doesn't make sense to me. The article states they have two control gates and a spillway. (one gate previously defunct but now repaired) So, shouldn't it be possible to block off the turbines and open the control gates instead, so that the total flow stays (roughly) the same? Moreover, if the dam is indeed that easy to destabilize, letting ISIS control the load seems equivalent to handing them a big red "trigger flood" button for use at their discretion. Sure, right now it's in their interest to keep the dam intact - because they still control many parts of Mossul and need the electricity. But "scorched earth" is not exactly a new concept and if the operation in Mossul continues as planned and ISIS is driven out, they could decide to cut the lines on their end to deliberately cause mechanical failures.
hvidgaard 10y ago
You could use it to boil some of the water away? A win/win if you're not a civilian relying on the power that is.
- Filligree 10y ago
  Well, on the assumption that it'd work... Now you've got a dam full of boiling water. And as you'll know from cleaning dishes, boiling water is far better at dissolving minerals than is cold water.
  - VLM 10y ago
    I'm curious why you guys want to boil the upstream water instead of the downstream water.
    - hvidgaard 10y ago
      To reduce the amount of water behind the dam. I'm sure using the power to power pumps would be a better at that task though.
  - avar 10y ago
    How are you calculating that entire reservoir would boil? It's filled with 8B m^3 of water. I couldn't find the formulas to calculate this myself, but given Tuna-Fish's comment above that it's enough to boil a kiloton of water (1000 m^3) ever hour that's 1K/8B =~ 0.00001%. So it's going to be more on the order of dropping a single drop of boiling water into a backyard pool every hour, is it not? I.e. something that wouldn't change the overall temperature of the reservoir to any significant degree. This is all assuming that you'd have to use the energy to heat the reservoir itself, you could just as easily use it to pump out some water and boil it until it evaporates, not impacting the temperature of the reservoir at all, just emitting a lot of steam into the air.
  - hvidgaard 10y ago
    You pump water into a boiler, not just submerging a giant heater into the dam.
chris_7 10y ago
My vague understanding of a turbine was that it was a spinning magnet, so if you killed the circuit it would just... spin, I guess. If this is a concern, why isn't there a worry that ISIS or just a stray bomb would kill the power lines when retreating from Mosul? Though I suppose they could just target the dam itself...
- cesarb 10y ago
  > My vague understanding of a turbine was that it was a spinning magnet, so if you killed the circuit it would just... spin, I guess. The load in the circuit acts as a brake. If you kill the circuit, the turbine spins faster and faster until it breaks into pieces. To prevent that, the control systems stop the water from flowing into the turbine, but stopping the water from draining is the opposite of what they want.
  - PietdeVries 10y ago
    The story suggests there is a 300 foot wide canal that can be used to let water go in casse of an emergency. Well, how about doing so now? Rather than using the turbines, use the canal to let the water out, but without it generating electricity...
    - takk309 10y ago
      The emergency spill way is in case of a large rain or melt event that brings the water level up quickly. The spill way is used to ensure that the dam is not over topped, which is usually catastrophic. As such, the spill way is usually placed at or near the maximum safe free board (distance from the top of the dam down to the water level). What this all means is that the spill way can only be used when the water level in the reservoir is allowed to get to high and possibly dangerous levels. The article stated the the water level has already been decreased by 10 meters which means that the spill way is going to remain dry for a long while. Next time you are near a dam look around and you will find that most have three paths for the water to leave the reservoir. The first is through the power generating turbines, second is the sluice gates which control water level, and third is the emergency spill way which is designed to fend off catastrophic failure.
deleted 10y ago
[deleted]