JZinCO
New member
I want to address some misconceptions about return pumps. I think they stem from our knowledge about positive displacement pumps (you know, the kinds in your heart and car). But, to my knowledge, we are all dealing with centrifugal pumps.
I'm not going to go into detail about how pumps work. I'm only a novice applicator of fluid dynamics in my job. But I will give it a shot. So here are my rules:
1) There is nothing wrong with reducing outflow with a control valve. This will reduce the work that a pump has to do. In fact, my Eheim pump has a built in gate to reduce the outflow surface area. The work a pump does is a simple function:
Horsepower= (Flow Rate x Head Height x Specific Gravity) / Pump Efficiency
Cutting the flow rate will reduce horsepower. The reason this works is due to something known as Bernoulli's equation. You can look that up if you want to know more.
2) Reducing outflow will save money by reducing work. We can modify the above equation:
Cost Per Year ($) = (0.000189 x Flow (GPM) x Height height (ft) x $Kwh x specific gravity x 8,760)/ (Pump efficiency x Motor efficiency )
Note that efficiency is a non-linear response to head height and flow. Thus, a 1/2 reduction in flow rate doesn't mean a 1/2 reduction in cost.
3) Turning your pump on or off at full throttle should be avoided if possible. It creates a shaft deflection and will wear down your pump. How many times can you do it before something breaks? No idea. But in industrial settings with big pumps, it can be a huge issue (not to mention huge changes in electrical demand loading when pumps are suddenly turned on or off while at full throttle). Back when I was a firefighter, I never understood why but they hammered into us that you never start or kill a pump at full throttle.
4) Wasteful energy use increases heat. This will wear down the life of your pump. Though, to be realistic most pumps fail because foreign material erodes the impellers.
5) Turbulence wears down pump parts. This can be reduce by avoiding elbows at both inlet and outlet sides and reducing flow.
6) Introducing air kills pumps. This is caused by vorticity of the water when the water line is low. Having a higher water line, employing a vortex breaker, or lowering flow rates can help.
7) Above all, don't constrict the outflow to the point of cavitation.
8) Never ever restrict the inlet. Nuff said.
9) Having a bypass line is great but inefficient if it isn't feeding a fuge or reactors.
10) Select the right pump Use pump flow charts to your advantage and you can forget about most of these rules. My rule is to pick the lowest flow pump that can handle the head height you are giving it (Thus I run a return turnover of 1x but that's a discussion for another time).
If I haven't convinced you, I pulled a great excerpt:
"Controlling a centrifugal pump by throttling the pump discharge is an energy wasteful practice. However, throttle control of a centrifugal pump is generally less energy wasteful than two other widely used pump control alternatives: no control and bypass control. As such, throttle control can represent a means to save pump energy. Also, throttle control is the most widely used and is often the lowest investment cost method to control the output of a centrifugal pump.
Throttling the discharge is a simple, effective method of controlling the output of a centrifugal pump. Since a centrifugal pump is a variable capacity device, it will operate at the intersection of the pump curve and the system curve. If the pump discharge is throttled by closing a valve, the pressure drop across the valve increases and causes the pump to operate back on the pump curve, thereby reducing the pump output. The throttling can be controlled manually or by an automatically actuated control valve.
From an energy conservation viewpoint, throttle control should generally be used in preference to no control or bypass control. With no pump control, the pump will run out on the pump curve. Any excess flow represents wasted energy. By throttling the pump discharge, the pump will operate further back on the system curve and will use less energy. A bypass control system consumes energy like a pump system with no control; the pump always operates out on the pump curve at maximum flow. As a control system, bypass control generally does not save energy."
Additional reading:
http://www.cyclestopvalves.com/csvtechinfo_21.html
http://www.pumpfundamentals.com/centrifugal-pump-tips.htm
https://www.dultmeier.com/technical-library/how-does-a-centrifugal-pump-work.php
http://turbolab.tamu.edu/proc/turboproc/T9/T983-101.pdf
I'm not going to go into detail about how pumps work. I'm only a novice applicator of fluid dynamics in my job. But I will give it a shot. So here are my rules:
1) There is nothing wrong with reducing outflow with a control valve. This will reduce the work that a pump has to do. In fact, my Eheim pump has a built in gate to reduce the outflow surface area. The work a pump does is a simple function:
Horsepower= (Flow Rate x Head Height x Specific Gravity) / Pump Efficiency
Cutting the flow rate will reduce horsepower. The reason this works is due to something known as Bernoulli's equation. You can look that up if you want to know more.
2) Reducing outflow will save money by reducing work. We can modify the above equation:
Cost Per Year ($) = (0.000189 x Flow (GPM) x Height height (ft) x $Kwh x specific gravity x 8,760)/ (Pump efficiency x Motor efficiency )
Note that efficiency is a non-linear response to head height and flow. Thus, a 1/2 reduction in flow rate doesn't mean a 1/2 reduction in cost.
3) Turning your pump on or off at full throttle should be avoided if possible. It creates a shaft deflection and will wear down your pump. How many times can you do it before something breaks? No idea. But in industrial settings with big pumps, it can be a huge issue (not to mention huge changes in electrical demand loading when pumps are suddenly turned on or off while at full throttle). Back when I was a firefighter, I never understood why but they hammered into us that you never start or kill a pump at full throttle.
4) Wasteful energy use increases heat. This will wear down the life of your pump. Though, to be realistic most pumps fail because foreign material erodes the impellers.
5) Turbulence wears down pump parts. This can be reduce by avoiding elbows at both inlet and outlet sides and reducing flow.
6) Introducing air kills pumps. This is caused by vorticity of the water when the water line is low. Having a higher water line, employing a vortex breaker, or lowering flow rates can help.
7) Above all, don't constrict the outflow to the point of cavitation.
8) Never ever restrict the inlet. Nuff said.
9) Having a bypass line is great but inefficient if it isn't feeding a fuge or reactors.
10) Select the right pump Use pump flow charts to your advantage and you can forget about most of these rules. My rule is to pick the lowest flow pump that can handle the head height you are giving it (Thus I run a return turnover of 1x but that's a discussion for another time).
If I haven't convinced you, I pulled a great excerpt:
"Controlling a centrifugal pump by throttling the pump discharge is an energy wasteful practice. However, throttle control of a centrifugal pump is generally less energy wasteful than two other widely used pump control alternatives: no control and bypass control. As such, throttle control can represent a means to save pump energy. Also, throttle control is the most widely used and is often the lowest investment cost method to control the output of a centrifugal pump.
Throttling the discharge is a simple, effective method of controlling the output of a centrifugal pump. Since a centrifugal pump is a variable capacity device, it will operate at the intersection of the pump curve and the system curve. If the pump discharge is throttled by closing a valve, the pressure drop across the valve increases and causes the pump to operate back on the pump curve, thereby reducing the pump output. The throttling can be controlled manually or by an automatically actuated control valve.
From an energy conservation viewpoint, throttle control should generally be used in preference to no control or bypass control. With no pump control, the pump will run out on the pump curve. Any excess flow represents wasted energy. By throttling the pump discharge, the pump will operate further back on the system curve and will use less energy. A bypass control system consumes energy like a pump system with no control; the pump always operates out on the pump curve at maximum flow. As a control system, bypass control generally does not save energy."
Additional reading:
http://www.cyclestopvalves.com/csvtechinfo_21.html
http://www.pumpfundamentals.com/centrifugal-pump-tips.htm
https://www.dultmeier.com/technical-library/how-does-a-centrifugal-pump-work.php
http://turbolab.tamu.edu/proc/turboproc/T9/T983-101.pdf
