parallel operation, hi capacity-low head pump operation

[pmover]

situation is as follows:
have several hi-capacity, low head pumps operating in parallel configuration. anybody have comments/suggestions regarding piping system configuration (valves and or min flow control?) to smoothly start/stop pumps without shutting down other pumps (i.e. slamming check valves, etc.)? If additional info is needed, will provide upon request.

 

[dcasto]

This would be fun project. If these are really large pumps and huge motors, a PLC could be used to control the pumps, staging in pumps as needed, ect.. Ther lots of things that would work. These are some ideas to get you thinking on optimizing controls.

I would use either a pinched discharge valve or a discharge to suction to control valve to control the flow. A meter in the recycle could be used to stage pumps on and off or if the discharge pressure up stream of the control valve is much greater than downstream. A Variable Frequency drive (VFD) could be added to minimize recycle and power.

On the cheaper end, a fixed restctive orifice (RO) could be added to the system to keep each pump above minimum flow. Each pump would have its own. If the fluid is not being pumped quick enough, then a pressure or level sensor would activate a switch to turn the pumps on and off.

To keep from slamming valves, the discharge valve on each pump could be closed before starting or stopping a pump so you start againt a closed valve, or a suction to discharge valve would be opened to bring the pump on of off. Each valve would be set up to open and close slowly.

Have Fun

 

[pmover]

dcasto,
yes, this is a fun project (1800 hp motors) and alternatives are being investigated to existing system. alternatives being investigated are: 1) individual pump recycle for start/stop conditions, 2) pump recycle piping back to reservoir (two pumps upstream of these pumps in process operation), and 3) replace existing control valve on pump discharge (current one is butterfly type-not good for flow/pressure control on pump discharge). Items #3 and #2 are preferred choices, with #2 being really beneficial from a process/operation perspective, even though more $. Your suggestion for a restrictive orifice is a good one and will definitely be considered, but will add recycle block valve to shut-off recycle flow. I like the idea of vfd's, but suspect this choice to be costly; however, it will be added to the list. Of course, these options do not address the sudden shutdown of a pump, thereby causing a check valve to slam, thus waterhammer. Non-slamming type check valves can be installed, but will need to consider an accumulator or pulsation damper of some type. The situation really depends upon what the client wants after all is said and done. Thanks for your response and feel free to add/comment.

 

[electricpete]

We have huge power plant with 4 circ water pumps in parallel adding to 1 million gpm total.

The control system is very complex and I don't know all the details. Maybe if I describe it there may be some useful ideas you can carry away.

Uses anti-reverse rotation ratchets built into the motor instead of fluid system check valves. That eliminate slamming, also eliminates pressure drop which would be associated with the check valves during normal forward flow. Attractive option for new installation but probably not a cheap option as a backfit for existing installation.

An interlock prevents starting a pump when another pump has been started within last 5 minutes (ensures system is stabilized).

An interlock valve requires discharge valve closed during pump start to prevent watter hammer. Also interlock checks that pipe is full on the system side of the valve based on water pressure. Then after time delay to allow pump to draw water into it's column, the discharge valve automaticaly cycles open (to prevent motor overload). If valve is not fully open within certain time then pump trips.

All pumps trip when the system is sensed to be in an unstable configuration.