Submersible pump torque

Hi Elnav,

Wow. You'd need to know a lot about the pump to calculate the torque. Start up torque would no doubt be highest, with the casing tending to move in one direction and the rotor plus some fluid in the other. That start up torque would only last while the pump accelerates to speed. (It would be the same effect that you'd see if you put an electric motor on the floor and energized it -- the rotor goes one way, the housing the other.) A soft start would minimize this. By pure guess, the instantaneous torque of the motor at start up might be 10-15 times its running torque... but that is a pure guess.

Once the pump is up to speed, the torque is probably minimal, although if the flow comes in straight and goes out spiraling, then there is some torque that would not be reacted inside the pump itself.

I'd be inclined to go with the manufacturer's recommendation.

The manufacturer recommends installing the pump in a well. However . . . we were planning to install it in the lake.

The Bible Study camp has a water system for flushing toilets, showers, sinks, etc that is supplied from the lake. Not to mention lawn sprinklers and outdoor faucets. Potable drinking water comes from a separate 300 foot deep well. The drilled well cannot supply enough water for washing, flushing etc. whe there are 50 or 100 people in camp. Hence the dual system. The lake water pump died. It was suggested we use a submersible pump instead of the a jet pump. The camp grounds has a considerable gradient from down by the lake up to the upper reaches where the chapel, dining hall, toilets and dormitories are. We suspect the old pump burned out because it was over loaded to begin with. Hence the search for something more durable. I have visions of the pump doing pirouettes out in the lake as it starts up. Just how big a support structure do I need to build to hold the pump? Normally in a well casing, you do not have such a situation. The Jet pump was 1-1/2 HP; but well drillers and water service people suggest we can get the same volume of water from a 3/4 HP submerged pump. After all, its easier to push water than suck it up to where its needed.

You guessed it. I'm now the designated technical support specialist for the church and camp. Up til now they relied on volunteers and you normally don't find many engineers or technicians among loggers and ranchers. Getting a service call from any town is a minimum of $100 per hour and they charge for the several hours driving to and from their shop.

Hi Elnav,

As Ken states the torque reaction will be at start up. Once running there will be no further torque on your pipes. We run submersibles in two artisanal (built by artisans, not artesian) and in a 200m (600ft) borehole. None have any form of torque mitigation. The shallow, eight-metre pumps can be seen to jerk at switch on but this causes no problems.

Remember that these pumps generally need to be suspended off the bottom. Failure to do so will result in it picking up too much detritus and abrasion damage to the casing from the vibrations and the torque reaction.

Happy pumping

Chas

Ask the supplier for the start torque curve. It is typically 20% of full load running torque. You can Google how to calculate full load torque for pumping water (specific gravity of 1.0) if the supplier can't help. In an X-Y graph the torque curve looks like a beginning point of about 20% at zero speed, then it immediately goes down to about 2-3% then climbs up almost linear to the max torque. Looks like a "Check Mark" on the graph without a sharp corner at the bottom. i.e a lazy check mark.

There is a post here suggesting there is no more torque once the pump is running, but this is not exactly accurate. If there was no torque reaction, you won't need a motor to overcome the resistance to flow. A pump is a torque reactive device. If there was no resistance to the torque moment, you wouldn't pump anything. It's like stirring a jar of peanut butter with a tongue depressor. If you don't hold the jar, the whole thing turns and you are aren't doing any work. Resist the torque reaction by holding the jar (pump support) and the stick (pump impeller) can react against a fixed object (jar / pump diffuser housing mounted to pump casing) and pump water. Water is just very thin peanut butter in this example, but it does take power and speed (rotation) to pump it. If you have attempted movement, and resistance to that movement, you have a torque reaction. The resistance to flow in a pump is the density of the fluid (water is harder to pump than oil for example because it is heavier) and the weight of the column of water above the pump (head).

There will be no further torque shock once you started, but there will be running torque. And no coupling can save you from running torque reaction to the supports, only torque shock and peak stresses. Ask the supplier to provide references and call those references and see if people using this product will claim "My old pump coupling / support broke every 5 years . . .. but with this new Commander 12 torque absorber, I've not broken one coupling or support at the same duty cycle." [same flow and pressure rating]. Otherwise it may just be a sales gimmick to be different as there is no real technology changes in ESPs in 30 years.

Let's keep this is in perspective here. This is a 3/4 HP pump if I read this right. Your vacuum cleaner may have more power ! In my world of high energy pumps up to 25,000 HP we would look at this quite hard and do FEAs and Pipe Stress models, etc. If it only adds $50 go for it. If is adds $300 then study what it is they are trying to solve. Call references. Call references. Call references. If it is an elastomeric device, it will need to be maintained every few years. May actually cause you suffering to have it if you need to check it every 5 years ! Get the part number of the 'device' and call around and see how many of that component they sell, or stock. If they never sell that part, or stock it, it never needs replacing. If they stock 6 and sell 10 each month, there is a reason.

Good luck

Petropower,

Although I suppose your experience in pumping is greater than mine I would still argue about the torque on the suspension. The submersibles we normally see have their delivery port coaxial to the pump body so the necessary reaction is taking place with the pump assembly and thus transmits no force to the tube or suspension cord. If this were not so then every well pump would have a nicely tangled riser and suspension cord. (Assuming riser is hose not steel pipe or similar).

Even if the delivery port exited tangentially and then went through an elbow to get the product moving vertically (that is the object I believe) the force would only be seen as a longitudinal force trying to "stretch" the section of pipe between the port and the elbow.

Shoot me down if I'm wrong.

Incidentally you have also made an assumption regarding the pump, that it is centrifugal. "A pump is a torque reactive device." I had a similar size and duty pump operating for years that had no rotary parts whatsoever. It was a vibratory magnetic unit with a diaphragm. It worked fine but the inductance damaged the mosfets of my inverter if it got left on after the generator was stopped!

Kind regards

Chas

Capblanc and Petropower; one big difference between a normal drilled well setup and this lake immersion arrangement is the fact the pump will be suspended out in the water and the supply pipe to shore will be going from the pump at right angle (using an elbow fitting). If the pump was suspended from a float you can imagine the gyrations the pump would execute at every start up. I envisaged some sort of light weight support structure to restrain the pump from torquing around each time it started and also to position the pump above the bottom silt but below the ice that forms every winter. Leaving the pump in place during the winter would provide valuable fire fighting service during the heating season and would also mean the camp could be used year round as a full service facility. We plan to install heat tracers at all vulnerable points and the main underground piping is going to be below the frost line.

Hi Capblanc

I am assuming this post is for a centrifugal; yes. Regardless of discharge location (co-axial), there is twisting torque on the pump case. The conductor pipe, casing or wires or any other resistance keeps it from rotating. Impellers are a dynamic device, and they are throwing fluid in a circular motion. The fluid is 'stripped off' by diffusers or volute tongues and due to some physics of lowering the velocity as the fluid comes off the impeller exit vanes, pressure builds. Since the diffusers or volutes are part of the pump case (fixed) energy is transfered to those fixed parts, radially initially.

I am working on some concepts for New Orleans and those pumps are vertical (in my concept), 6m (20 feet) diameter impeller, 2m (6 foot) input shaft and each one pumps 15,000 USG/second. Lots of twist at 13000 bhp at only 100 RPM.

If there is a momentary twist at startup (break away torque), there is twist while running.

Cheers

can you tell me what that reaction would be in terms of input efforts.. is there any relation between the amount of input and reaction torque(both are equal or some percentage etc..).

After the initial start-up 'jerk' the torque will fall to the running level. The pipe that mounts the pump will easily handle this. I do it all the time.

You have overlooked another important concern: Motor cooling. These pumps require flow across the motor to cool it. Put the pump in a shroud so that all water flows over the motor, from the bottom, or you will burn it up. With a 3/4 hp pump, I use 4" pvc. This must be suspended off bottom to keep from pumping mud.

We use submersible or deep well pumps all the time in salt water feed and fire protection in the gulf.

Although much larger than yours, we use a single wire reinforced hose to connect them with a little slack and suspend the pump by rigging cables.

There is no science involved in this, only practical experience. It would make sense to me to incorporate a small section of hose (discharge size x a couple meters in length) to help absorb the start up torque and make assembly/dis-assembly much easier. Take care to make sure your electrical supply is attached securely and that there is enough slack to accommodate the movement.

The biggest number of failures I see is due to men not protecting the electrical supply connections and shortly there after having ground faults. 3-M makes flexible rubber pourable casts to protect the connections.

Good Luck

Hi,

Don't worry about torque. Make sure the pump is installed vertically, and include a check valve. If in a lake, install a flow inducer, and at least 4-5' above bottom, as deep as possible, if algae is present, install a self cleaning filter. A good pump, well installed will give you years of reliable service.

Wangito.

By being submirged the check valve is of little importance and the pump will start with less torque if you do not have all the head pressue that will be created by installing a check valve in this set up.

Thank you one and all for some really good advice. One thing I had overlooked was the need for a "flow inducer" I had assumed that since the pump was submerged, heat would be conducted away from the pump body in any case. Evidently this is not the case.

Someone ( not a forum poster) had previously suggested holding the pump in a section of 4" pipe but had not elaborated on the details. Given that some of his other comments were of questionable veracity, I didn't know how much stock to place on that particular bit of knowledge. Now it makes sense. If I understand it correctly; the pump sits in a section of pipe with the bottom closed to prevent the pump from falling through. The top of this pipe section is open and allows entry of water. The pump inlet port is at its bottom. This will have the effect of drawing water past the pump housing before entering the pump impellers. I have learned the cost of adding a torque absorber fitting is approximately $7 so that's a no brainer. Anchoring the support structure to the lake bottom is going to be accomplished by forcing the 1.5" legs into the soft mud. The support structure itself is going to be built from schedule 40 PVC pipe and fittings. The reason being, the camp caretaker has plenty of experience assembling irrigation systems made from PVC and feel confident working with that. If we need more strength, we could use Schedule 80 pipe. There are no welders available in reasonable proximity to the camp and given the present cost of any steel work, it would likely cost more than the pump. Weight is also a consideration. Placement of the structure will have to be done from canoes or a swimming raft. No motorized craft are allowed on that lake and there are no facilities for launching such a boat.

Therefore light weight is important. I want to include provisions for retrieving the pump from time to time if the trash screens get clogged. Aqautic weeds do grow right to the surface in some places during August. The small lake is only 15' - 20' deep in most places. I would expect these weeds might clog the trash screen on occasion.

I don't expect problems with the electrical supply. In a previous job I was the underground splicing specialist so have plenty of experience making good water proof connections and sealing it.

Something that is not clear, is how to provide the 230V power. Although the local electrical code does not require a GFCI on well pumps, I plan to install a GFCI double pole breaker. I anticipate there may be swimmers in the lake during the warmest part of summer. The motor shop sales guy said not to bother since we would be plagued with nuisance trips. He based this advice on his experience with using submerged pumps for cattle watering stations using ponds. My own experience with yachts and ship-board wiring is that most electrical insulation starts to show deterioration after 18 months of steady immersion in salt water. After 3 years they may not even pass the normal Coast Guard megger test required every four years on passenger carrying or commercial vessels. I have no experience with, or data on fresh water immersion. Milli-amp leakage in a closed well is not a good guideline. In any case the leakage current field would be contained within the casing which is grounded. Nobody goes swimming inside a drilled well.

In any case it looks like the torque of the pump can be restrained by a PVC pipe structure stuck into the muddy botom and perhaps augumented by some cement blocks tied to the structure. I wanted to keep the weight light enough to enable someone to retrieve the pump using a canoe or the swim raft. We are 500 miles from the coast, so finding a scuba diver is not very likely. It has to be done from the surface. To get the pump into clear water away from surface ice (12" - 14" ) and bottom silt means placing it about 100 feet from shore.

I think you have it almost complete now. The only thing I want to add is the 'flow inducer' should be open both ends. When such a pump is suspended in a well, most flow will be up from the well screen. A length of flexible discharge hose is a good idea for this rig. Cost is maybe $2/ft. Use double clamps and then coat them with waterproof grease to prevent corrosion. Cold water holds lots of air, so much more corrosion than warm water surprisingly.

A double pole GFCI is highly desirable. You want to know early if electrical leakage starts to happen. Be sure and run a separate earth ground wire in a 3 wire cable--2 'hot' 1 earth ground, connected to a grounding rod at shore.

regarding the "flow inducer": How much longer than the pump body should it be? This hasn't been discussed so far, but it looks like it may be a significant factor. For obvious reasons too long will make it unwieldy, but too short will not produce the desired flow characteristics. Where do you mean for the flexible discharge hose to go on input or outlet side? We will have semi flexible PVC pipe going from pump to shore and a torque absorber fitting on the discharge side of the pump. If in the discharge side, do you mean for this to serve as a torque absorber? Or did you have something else in mind.

GFCI: This raises some interesting questions. The leakage current detector will of necessity be located at the pump house right on the shore. Leakage will be detected by measuring the diferential between supply and return lines. The pipe from pump to shore is non conductive. Assuming the point of insulation failure is exactly midway between the pump and shore grounding point (ground rod). Leakage current path to pump shell and then via the grounding wire back to shore is going to be slightly higher resistance than the path from point of leakage directly to the ground rod ashore. The difference is going to be small but can be determined by calculation. Does it make a difference if the pump housing is grounded or not in this situation? In fact, the resistance of the leakage path through fresh water may be enough to limit the current to a point below the trip point of the protective device.

A friend ( marine electrician) who has served as "expert witness" in coroner's inquests where drowning was linked to stray current fields in the water, tell me there is more danger of muscular paralysis in fresh water than in salt water. You get a "standing field" due to the higher resistance of fresh water compared to salt water. Sort of like Step Potential near a big power station or switch gear yard.

Salt water has sufficient conductivity to guarantee enough fault current to trip a GFCI device. Not so in fresh water. Two of the drownings he is aware of involved a running genset and leakage current alongside the exterior hull surface from one metal hull fitting to another. The current leak was sufficient to cause muscular paralysis but not cardiac arrest. He said it was not clear if the paralysis stopped the breathing muscles or just the limb muscles involved in swimming. Either way, the victim sank and eventually became unconscious and then died from lack of oxygen. At present there is insufficient data to formulate any better regulations or protective devices until more details become available. The code simply assumes a GFCI is going to be enough protection in all situations.

There should only be an "output side" avalible for connection if this pump is the type that everyone is talking about that would require an inducer or shuck depending on what part of the world your in.

Typicaly, vertical, motor mounted on the bottom of the pump, suction between pump and motor and discharge on top.

If this is the case, the shuck only needs to be designed so that water is forced to flow past the motor before entering the pump suction. Be careful not to restrict the flow to the pump when doing this.

Wow You guys are really ridiculous, but AtLeast consistent. It doesn't take a rocket scientist to figure this out.

Answer: Up grade your pipe to an 80 gauge rather then a 40 gauge pipe or go to a metallic type of pipe in shorter lengths.

Truth is; it really doesn't matter one way or the other, the pipe taking the torque will out last the pump. Keep in mind the pipe was engineered for these typical applications.

For the rest of you Guru's stop asking so many ridiculous questions and get down to the solution. This is a world where there needs to be progress in a forward motion and not being bogged down by someone trying to sound intelligent.

We dont need to try to sound inteligent as long as there are people like you out there doing it for us.

Just a follow up on my query about adapting a well pump to free standing out in middle of lake.

At the time I posted my question I had not seen a submerged well pump up close. Nor had I experienced how much or little torque the motor had.

To avoid picking up bottom silt or surface floating debris it was necessary to suspend the pump half way between the surface and the bottom. A vertical orientation would have been cumbersome so we slanted the pump at 45 degrees.

In the absence of a well casing we built a PVC frame to support the pump. The legs were long enough to imbed in the silt on the lake bottom while holding the pump up high enough. The pump was also submerged far enough that canoes and other small craft would not snag the pump piping or wiring. Remember this is a camp and such traffic and swimmers are unavoidable. And it is below the lower limit of the 24 inch thick ice formed during winter.

The torque is noticable but not excessive. The support frame is 30 inches wide and long thus providing a stable platform of support. It also provides a useful grab handle in the event the pump has to be pulled to the surface for service.

Someone commented that this type pump would give better service and performance than the older style suction jet pump. Quite so! Lower power consumption and better flow. We are in the process of installing a new pipe main deep enough to be below the frost line. This will give us some form of fire main protection even during winter. Closest fire station is 20 miles away. Self reliance is important.

Thanks for the useful advice provided by forum members

elnav