Vertical Turbine Pump and VFD 1

[ozmosis]

A query came through our USA sales channel asking about the use of a VFD on a Vertical Turbine Pump, 100Hp rated (little else at this stage).
I recalled an application I did with one about 8 years or so ago and one of the main issues with using one with a VFD was:
1) The acceleration time had to be very quick (this was a 160kW rated pump and we needed an accel time of about 10 secs) to ensure the bearings were quickly lubricated. The resulting starting duty demanded a VFD with a high overload.
2) Stopping the pump resulted in the 'backflow' of the pumped medium causing a certain level of regeneration (brake resistors fitting).

The question is, am I generalising with Vertical Turbine pumps in thinking they will all have similar performance requirements or is this about right? If the feeling is that all VTP's are different, then I will go back and get a lot more details from them.
They wanted to use one of our low dynamic HVAC drives, but I've initially said no, I think they should be looking at a more higher performance 'industrial' drive.

 

[DickDV]

You do need to get the pump up to speed fairly quickly but, in my experience, if you choose the VFD based on service factor amps being the continuous duty amps of the drive and the drive is rated for 10% short-term overload (commonly called variable torque or normal duty rated), you should be ok.

As for brake resistors, I've never encountered that. If you set the drive up for ramp to stop, you can set the decel ramp long enough to get the motor to drive the pump all the way down to stop. This need be only slightly longer than the coast-to-stop time since you don't want the bearings to be stressed on stopping any more than they are on starting.

 

[ozmosis]

Thanks DickDV,
I've a feeling the brake resistors were exceptional to this particular application and due to the remoteness of the site: a certain degree of "belts and braces", as we say.

 

[waross]

Re; Braking,
If you have any screwed together couplings you will want to avoid braking. Negative torque may unscrew your shaft connections.

As for brake resistors, I've never encountered that. If you set the drive up for ramp to stop, you can set the decel ramp long enough to get the motor to drive the pump all the way down to stop. This need be only slightly longer than the coast-to-stop time since you don't want the bearings to be stressed on stopping any more than they are on starting.

Once the motor is stopped you may be able to apply holding torque to prevent backspin. I prefer a check valve or a backspin preventor.
respectfully

 

[thewellguy]

I have several thoughts. First there are two different typed of tubbine pumps with VHS motors, water and oil lube. There are also submersible motors. From your message I am assuming that you are ot talking about a sub. The first this that you need to determine if you are working with a water or oil lubercated pump. If it is water the type of bearings and if there is a prelube line on the system needs to be determined. This info will dictate the ramp time on start up. There are several other factors that need to be determined also.

In reqards to braking on a vtp. DO NOT DO IT. There is no reason to. I would reccommend ramping done to the minimun speed and then coast to a stop.

Also you will need a delay timer so the pump does not try and start while the pump is in backspin.

 

[ozmosis]

Gents
Thanks for the info. You've told me enough that the first thing is to wait for infom on the type of pump before he does anything else.
It's got me thinking about the job we did all those years ago where we controlled the stop of that VTP. I guess it was ok and didn't unscrew anything otherwise I'm sure I would have heard about it.
Regards

 

[jraef]

Not to make your situation worse, but it is not just the pump, it is the entire pump system. As thewellguy implied, backspin is a question of system design. Many VTPs do NOT use check valves or backspin ratchets for a variety of mechanical reasons. They simply allow the water left in the pipe to fall back down, reversing the motor spin. In those cases the backspin timer lockout described by thewellguy is essential, even critical.

Other systems, sewage for one, cannot allow backflow and valves may take too long to operate, so they use a backspin ratchet on the top of the motor. I have seen some engineers insist on dynamic braking in order to get the motor slow enough for the centrifugal clutch of the backspin ratchet to engage as fast as possible. I have yet to see one of those designs be practical in the long run and sooner or later the braking is disabled. It appears to me as one of those solutions that comes with a whole new set of problems.

Braking is used exensively on screw pumps (PD progressive cavity) because the screw shaft twists, and in untwisting at stop will reverse flow. Then the wieght of fluid in the cavity can sometimes apply enough negative torque to keep the revers flow going. They brake the motor shaft and hold it until after the screw untwists so that flow stops before the motor shaft is released. That way the weight of the fluid cannot create enough negative torque to overcome friction and inertia. That is the only legit application for braking that I know of fr pumps and it's not VTPs.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ozmosis]

thanks jraef. Useful info.
How popular are VTP's? The project I was involved in all those years ago was in Canada, this latest one is in the US and I must say I never hear the term VTP in Europe (unless they have a different name). Speaking to a few drive aplication guys about them here in the UK, they are not too familiar either.

 

[macmil]

There is a huge population of Vertical Turbine Pumps. Most municipal water supply and many large industrial plants use them. Typically they are very reliable and long lasting, which is perhaps why you don't hear much about them !