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Reverse pumps as turbines [See also Chapter 14]
APPENDIX G
Reverse pumps as turbines [See also Chapter 14] Externally, the differences between a centrifugal pump and turbine are not great. A non-engineer may not even notice the difference. Technically, the obvious difference is that pumps absorb power and do work on the liquid. Turbines do the reverse; they absorb power from the liquid and deliver it as useful work. In principle, a turbine is just a pump in reverse so it is reasonable to query if pumps can be used as makeshift turbines. Conversely, can turbines operate as pumps? In rare cases dual-purpose pump-turbines are designed but they are beyond the scope of this discussion. It is in the detail that pumps and turbines differ significantly. The vanes on a pump impeller are usually few e somewhere between 5 and 8 in the main. The passage areas between them will change rather gently. A turbine will have many more vanes and the passage areas between them will change more severely. The reason for this crucial difference is simple. - In a pump impeller, the passage flow is generally decelerating and diffusing. It is difficult to decelerate efficiently and great care is needed. That is why the process is performed very gently, using long lightly loaded passages. - In a turbine runner, the flow is accelerating which is inherently a more efficient and, importantly, more stable process. This allows more liberties to be taken in terms of passage loading. These facts help us answer the initial question. A pump impeller can operate effectively as a turbine runner because the internal flow patterns become more favourable. Instead of having to carefully diffuse the flow, it now has to moderately accelerate the liquid and, at least in turbine terms, tolerate only a mild level of passage loading. On the other hand, using a turbine runner as a pump impeller is not so favourable. Generally, the passage areas increase too rapidly to allow efficiently controlled diffusion. The passages are overloaded. Briefly, pumps can be run in reverse as passable turbines. Experience says that their efficiency may at worst be only 4 or 5% lower than a purpose made turbine. On the other hand, turbines operated as pumps show poor efficiency. Furthermore, the characteristic curves are not very useful.
447
448
Appendix G: Reverse pumps as turbines [See also Chapter 14]
Just for clarification; in a pump, the flow is outwards through the impeller. Using the same machine as a turbine, the flow is reversed and now passes inward across the runner. The direction of rotation is also reversed. [Just to confuse matters further, a pump can operate as an outward flow turbine, though not as effectively. This is called the ‘Windmill’ mode].
DELIBERATE OPERATION AS A TURBINE Deliberate use is generally restricted to situations where the machine is being used chiefly for its quiet head breakdown properties. Any energy recovered in the process is a welcome bonus [If energy recoveries were the only concern, a true turbine might be used to extract perhaps 5% more power and be physically smaller.] Typical head breakdown applications would be where pressure has been bestowed upon liquid due to [say] a chemical reaction. This pressure has to be dissipated to a lower pressure before further processing can take place. Examples would be: - Hydrocracking of hydrocarbons in refineries - Rich solutions in Gas sweetening plants [Gas Scrubbers] - In the Purification of water by the Reverse Osmosis process the high pressure embodied in the membrane reject needs to be dissipated before discharge to waste - On the downhill leg of pipelines, the static head can be dissipated to protect a downstream pumping station In every case the reverse running pump is being used in situations where a throttle orifice, or even pressure control valve may have been previously used.
ACCIDENTAL OPERATION AS A TURBINE Pumps can accidentally get into the turbine mode if not installed correctly. If a stationary pump can be exposed to a reverse differential pressure, and there is no impediment to reverse flow, then the pump will turbine. Examples would be: - Pumping against a large static head without a non-return valve, or with a stuck open valve. When the pump stops, the discharge line empties back through the pump. This may happen in mines and quarries. - In a parallel pump installation, the non-return valve may stay stuck open when one pump is shut down.
Appendix G: Reverse pumps as turbines [See also Chapter 14]
449
In either case, reverse rotation may be a serious problem. When turbines are deliberately installed as part of an energy recovery system, the power is fed back into the motor in order to reduce its power consumption. An electric motor forms an excellent governor and so speed is controlled. In the two examples just described, the electric motor would be de-energised so unable to exert any speed control. In the turbine mode, the shaft speed may significantly exceed the design speed of the motor. This may cause problems. Another possible risk is present when a pump enters the outward flow turbine mode. This could occur if a booster pump continues to run after the main pump has shut down. In this situation, the main pump may begin to ‘windmill’ as mentioned earlier. The wind milling speed will be quite low since this is not an effective turbine mode. If the main pump has ringoiled bearings, then the speed may be insufficient to allow oil pickup. Bearing failure will follow. In practice it would be very rare for a turbine to be used as a pump, either deliberately or accidentally. On the other hand, pumps can and frequently do function as turbines, both deliberately and accidentally. When a pump operates as a turbine, its best efficiency flow and head increases by roughly 40% e depending on size.
Reverse pumps as turbines [See also Chapter 14] Externally, the differences between a centrifugal pump and turbine are not great. A non-engineer may not even notice the difference. Technically, the obvious difference is that pumps absorb power and do work on the liquid. Turbines do the reverse; they absorb power from the liquid and deliver it as useful work. In principle, a turbine is just a pump in reverse so it is reasonable to query if pumps can be used as makeshift turbines. Conversely, can turbines operate as pumps? In rare cases dual-purpose pump-turbines are designed but they are beyond the scope of this discussion. It is in the detail that pumps and turbines differ significantly. The vanes on a pump impeller are usually few e somewhere between 5 and 8 in the main. The passage areas between them will change rather gently. A turbine will have many more vanes and the passage areas between them will change more severely. The reason for this crucial difference is simple. - In a pump impeller, the passage flow is generally decelerating and diffusing. It is difficult to decelerate efficiently and great care is needed. That is why the process is performed very gently, using long lightly loaded passages. - In a turbine runner, the flow is accelerating which is inherently a more efficient and, importantly, more stable process. This allows more liberties to be taken in terms of passage loading. These facts help us answer the initial question. A pump impeller can operate effectively as a turbine runner because the internal flow patterns become more favourable. Instead of having to carefully diffuse the flow, it now has to moderately accelerate the liquid and, at least in turbine terms, tolerate only a mild level of passage loading. On the other hand, using a turbine runner as a pump impeller is not so favourable. Generally, the passage areas increase too rapidly to allow efficiently controlled diffusion. The passages are overloaded. Briefly, pumps can be run in reverse as passable turbines. Experience says that their efficiency may at worst be only 4 or 5% lower than a purpose made turbine. On the other hand, turbines operated as pumps show poor efficiency. Furthermore, the characteristic curves are not very useful.
447
448
Appendix G: Reverse pumps as turbines [See also Chapter 14]
Just for clarification; in a pump, the flow is outwards through the impeller. Using the same machine as a turbine, the flow is reversed and now passes inward across the runner. The direction of rotation is also reversed. [Just to confuse matters further, a pump can operate as an outward flow turbine, though not as effectively. This is called the ‘Windmill’ mode].
DELIBERATE OPERATION AS A TURBINE Deliberate use is generally restricted to situations where the machine is being used chiefly for its quiet head breakdown properties. Any energy recovered in the process is a welcome bonus [If energy recoveries were the only concern, a true turbine might be used to extract perhaps 5% more power and be physically smaller.] Typical head breakdown applications would be where pressure has been bestowed upon liquid due to [say] a chemical reaction. This pressure has to be dissipated to a lower pressure before further processing can take place. Examples would be: - Hydrocracking of hydrocarbons in refineries - Rich solutions in Gas sweetening plants [Gas Scrubbers] - In the Purification of water by the Reverse Osmosis process the high pressure embodied in the membrane reject needs to be dissipated before discharge to waste - On the downhill leg of pipelines, the static head can be dissipated to protect a downstream pumping station In every case the reverse running pump is being used in situations where a throttle orifice, or even pressure control valve may have been previously used.
ACCIDENTAL OPERATION AS A TURBINE Pumps can accidentally get into the turbine mode if not installed correctly. If a stationary pump can be exposed to a reverse differential pressure, and there is no impediment to reverse flow, then the pump will turbine. Examples would be: - Pumping against a large static head without a non-return valve, or with a stuck open valve. When the pump stops, the discharge line empties back through the pump. This may happen in mines and quarries. - In a parallel pump installation, the non-return valve may stay stuck open when one pump is shut down.
Appendix G: Reverse pumps as turbines [See also Chapter 14]
449
In either case, reverse rotation may be a serious problem. When turbines are deliberately installed as part of an energy recovery system, the power is fed back into the motor in order to reduce its power consumption. An electric motor forms an excellent governor and so speed is controlled. In the two examples just described, the electric motor would be de-energised so unable to exert any speed control. In the turbine mode, the shaft speed may significantly exceed the design speed of the motor. This may cause problems. Another possible risk is present when a pump enters the outward flow turbine mode. This could occur if a booster pump continues to run after the main pump has shut down. In this situation, the main pump may begin to ‘windmill’ as mentioned earlier. The wind milling speed will be quite low since this is not an effective turbine mode. If the main pump has ringoiled bearings, then the speed may be insufficient to allow oil pickup. Bearing failure will follow. In practice it would be very rare for a turbine to be used as a pump, either deliberately or accidentally. On the other hand, pumps can and frequently do function as turbines, both deliberately and accidentally. When a pump operates as a turbine, its best efficiency flow and head increases by roughly 40% e depending on size.