Well pump & pressure tank for fire hall 3

[DomDoucet]

I am helping to design the plumbing systems for a new municipal building in Eastern Canada (New Brunswick) that will house a volunteer Fire Department and the municipal offices. The Fire Department requires roughly 200GPM of water flow from the well in order to fill their trucks. The building includes washrooms, showers, janitor closet and a kitchenette. The building is occupied by a max of two persons, Monday-Friday, 8-5. The building may be used for other activities on weeknights. We are concerned about sizing the pressure tank for the domestic water system based on the pump, which is 20HP. That would be a very large pressure tank. The reason for this is we are worried that with so little demand on a typical day (assume 16 gals a day max), it will take quite a long time for the pressure tank to deplete and the pump to kick in. We don't want stagnant water. We want to specify a VFD well pump with an 80 gal pressure tank. I am looking for suggestions on how to design this or opinions on our preliminary design.
thank you very much

 

[Pumpsonly]

Check with your electrical friend what is motor starting current. The motor will have same starting current aka locked rotor current even with no load. Starting current is a function of the rotor design and the voltage applied at the motor terminals.
Check out a current / speed/ torque curve of a motor. It has no relationship to the driven load. The starting current is expressed as xxx% of the full load current. Not the operating current. Operating current depends on the load.

 

[bimr]

Agree with pumpsonly on this. You are confused. Get yourself a copy of: Sulzer Centrifugal Pump Handbook

http://books.google.ru/books?id=Gjp...current centrifugal pump closed valve&f=false

"Starting a pump against a closed valve is only recommended for pumps with low specific speed, where the power input and speed is less than the pump duty point."

"Starting the centrifugal pump with a shut discharge valve is standard practice with many operation departments. The concern is to save power without realizing the damage that is being done to the mechanical seal, impeller, wear rings and bearings. "

http://www.mcnallyinstitute.com/06-html/6-7.html

 

[jonr12]

See the chart on page 2 of this link.

http://www.rmhindustries.com/wp-con...tive-accouplements-magnetiques-magnadrive.pdf

This shows when the motor is disconnected from the load there is about 15% less peak load at start up and a big difference in time. I find submersibles to do much better. I guess that is because of what bimr says and I quote here.

“Submersible well pumps have slightly different operating characteristics than the common centrifugal pumps. The reason no soft start is required for a submersible pump is that the rotational inertia of the pump is low. The impeller diameter is much smaller than the common centrifugal pump impeller. That is why the well pump starting amps are 4:1 ratio of operating amps. As a result of the low inertia design, the pump starting time is also low, less than a second.” bimr

My peak amp meter shows a submersible starting when the pipe is empty to be almost 6 times the running amps. I believe my meter is correct because this is the same as it says in the motor specs from Franklin between SFA and Locked Rotor Amps. But when starting with the pipe full and against a closed valve I only see about 2 times the running amps. This is about the same as I see when ramping up with a soft starter. My amp meter is telling me that starting against a closed valve greatly reduces the amp draw, and I have yet to see any of the failures mentioned for doing so. I have tried different meters and they all say the same thing. So you are right, I am confused, because I believe my meter.

 

[bimr]

Most sources will tell you the following. Centrifugal pumps should be started against a throttled valve, not a closed valve:

"A. Starting Centrifugal. The almost closed discharge valve creates a minimum load on the driver when the pump is started. ... Assuming that the motor inrush current allows and that the motor will not kick off, the discharge valve may be just "cracked" — about 1/8 open — before the pump is started."

http://books.google.ru/books?id=cHI...a=X&ei=xdibUPjPEqXd4QSQsoHIDw&ved=0CDQQ6AEwAg

There is a difference between a closed valve and an "almost closed", "cracked" or "throttled" valve.

One final comment is that I have been referencing large pumps (450 HP} where you are referencing small HP pump. Most likely the motor characteristics are slightly different.

 

[jonr12]

Thanks for that. I have been saying “closed” valve, but the valve I use is a non-closing type. It is open just a crack when the pump starts. And yes I know larger pumps and pumps with axial or mixed flow impellers are different. The largest pump I have done is 250HP, and the ones I was talking about checking with my meter were 10HP and 50HP submersibles.

 

[KllrWolf]

I figured I would chime in here on the subject of the starting of a submersible motor. A submersible motor will be up to full speed in less than 1/2 a second if started DOL and full starting power is available. Due to the type of bearings used in the motors, any start that takes more than 4 seconds can cause damage to the bearings. The main reason we do not want the pump started against a closed valve is that the motors generate a massive amount of heat when starting, and need the flow of water past the motor to cool it immediately. The starting characteristics of the motor themselves are the same from 5 Hp to 950 Hp motors (the sizes we build as standard motors), unless requested to be different.

The starting current is a tricky subject. With the fast start, it is not an easy measurement to take. For the briefest fraction of a second locked rotor amps is seen until the rotor actually starts turning. It then decreases as the motor speeds up until it reaches the operating speed, at which the current should be steady. A soft starter lengthens the time it takes to get from the initial start to operating speed, thus reducing the amperage draw at any moment of the start, but with the converse of actually generating more heat from the motor over the length of the start (but not enough to make a difference if the motor has water flowing past it, i.e. not against a closed valve).

 

[jonr12]

It takes a little time for the heat generated at start up to be transferred to the motor housing. Water flow at the moment of pump start has no heat to work on. Within a minute or so the housing starts to warm and then the water flow can start cooling the motor housing. The motor company tells me I can start and run the pump/motor at deadhead or no flow for one minute. After one minute we need some flow for cooling.

This is also evident in the fact that the motor must stay off for a minimum of one minute, to allow the internal motor heat to dissipate. It takes the same minute of time for the heat to be transferred to the motor housing after starting a cool motor.

The following is a letter from Sun Star, who is the Hitachi motor company in the US, which is also the same motor as Centri-pro from Goulds and Pentek from Pentair or Sta-Rite. It seems that if the motor is de-rated enough, (pulling reduced amps), that it will even function properly with no flow past the motor.


Dear Customer
All submersible motors require cooling flow past the motor to dissipate heat generated within the motor. Hitachi has a standard for ambient temperature and required flow dependent on load. And although the specifications of your application are not covered on this chart we have performed extensive testing covering ranges outside the chart.

In your application (5 GPM, 7" shroud, 6" motor with an actual O.D. of 5.5") the flow velocity past the motor will be .109 feet per second. At this flow velocity at an ambient temperature of 95°F the service factor of the of the motor will be .82 and the motor must be de-rated from 10 HP to 8.2 HP with no service factor. Your application of 6 HP at 60-77°F at this same flow is well within the allowable limits of the motor rating at this reduced flow and temperature. We would expect lower internal motor temperatures, which will actually increase your service factor in this operating temperature range (60-77°F). We have performed research at temperatures above and below 77°F with "No-Flow" and the motor has performed satisfactorily. We anticipate no problem with your operating range.

Thanks you for this opportunity to be of service. If you have any questions or desire additional information, please do not hesitate to contact this office.

Sincerely,

Kevin P. Price
Vice President Sales & Service
Sun-Star Electric, Inc.

 

[bimr]

What is the point?

The motor will start from dead stop to full speed in a time period of approximately half a second. Any benefit of throttling the valve on startup for the time period of half second will be slight. However, the downside of throttling the valve longer than the half second startup will be probably be much greater than any possible benefit.

At best this seems to be an academic exercise as nobody will be interested in absorbing the costs to throttle a valve for such a small motor.

 

[KllrWolf]

Sorry for another long post, but I hate to see motors get damaged from misapplication and misinformation when it could have been prevented, no matter the manufacturer. People take information from one vendor and apply it to all others, so erroneous information seems to circulate, and much faster and longer than the correct information.

This is also evident in the fact that the motor must stay off for a minimum of one minute, to allow the internal motor heat to dissipate. It takes the same minute of time for the heat to be transferred to the motor housing after starting a cool motor.

The cool down period is actually 15 minutes between shutdown and restart (check the Installation and Operation manual that comes with the motor). This is because the cooling of the motor is reduced when the water around it is not flowing and the heat generated at startup is added to any existing heat in the motor. Many motors are killed by ignoring this simple rule. The motor does not take a full minute for the heat to get to the motor housing. The flow past the motor keeps the external temperature of the motor down, which aids in the heat transfer process. You may only measure a 10°C rise of the motor externally, but the internal temperature is much higher.

The letter you reference is for a specific motor in a specific instance. The information is only really applicable to certain motors in certain circumstances. You are referencing a small 2 Pole, 6” Hitachi motor. The construction of a 6” Hitachi is different than any of their other motor sizes. The heat handling capability of the 6” motors depends on the horsepower. What works for a 10 Hp does not necessarily apply to a 25 Hp. The motor used was probably one of their Hi-Temp motors, which but rated for more than double the ambient temperature of their standard motor. Heck, I am surprised they did not just reduce the size of the shroud to get a higher flow rate and avoid the problem completely.

To de-rate a motor so you can run it at a no flow condition makes no sense. You would have to buy a larger motor (probably much larger) and then run it at a lower load point, where the motor efficiency is much poorer, thus increasing the cost to run the motor. It would be much more cost effective to operate the motor properly, with the correct flow, than to force a no flow condition on the motor (which would probably reduce the life of the motor). Heck, it would be cheaper to have a motor custom built to handle the conditions than to operate a motor that far from its load rating.

 

[jonr12]

You are right, one minute is for 4” motors. The fact that it takes 15 minutes for larger motors further proves that it takes considerable time for the internal heat to transfer to the housing where it can be cooled down. Which should also mean that cooling flow is not required at the instant of start up. Flow past the motor is not going to cool anything until the internal heat from the motor transfers to the motor case, which takes a little time.

The OP is asking about a 20 HP, 6”, 2 pole motor. Below is another letter from Sun Star being ask about a 30 HP motor. This 30 HP motor is being de-rated to a 15 HP load by simply restricting the flow. You don’t need to put on a larger motor to de-rate, simply restricting the flow with a valve will do that. The letter states that a 30 HP motor only needs to be de-rated to a 24.6 HP load to work safely at 5 GPM flow. This particular 30 HP will drop to a 15 HP load when restricted to 5 GPM flow, which is de-rated even more, and requires even less flow for cooling. These are standard motors, no special high temp design.

Operating a pump at 200 GPM all the time would take a very large pressure tank. Then at 16 gpd use, the water in this tank will be sitting idle for many days getting contaminated. It would be much better to fill a small pressure tank at reduced flow, as long as the amps decrease (de-rating the motor). Starting against an almost closed valve or running at reduced flow with reduced amperage will not hurt this pump or motor.

You can’t reduce the size of the shroud without starving the pump at high flow rates. Throttling with a valve will be much less expensive than a large pressure tank or any other way to do this job. The inefficiency of running a 20 HP motor that is pulling about a 12 HP load while pumping 5 GPM for only a couple of minutes per day doesn’t add up to much. The effect of exercising the pump/motor every day or two and not having a lot of stored water to worry about more than makes up for any inefficiency. After all, one of the most important things is that the pump will be in working condition and able to supply 200 GPM when needed for an occasional fire. Letting it sit idle for days or weeks at the time may mean that the pump will not start when there is a fire and it is really needed.


Dear Customer
In your application (5 GPM, 7" shroud, 6" motor with an actual O.D. of 5.5") the flow velocity past the 30 HP motor will be .109 feet per second. At this flow velocity at an ambient temperature of 95°F the service factor of the motor will be .82 and the motor must be de-rated to 24.6 HP with no service factor. Your application of 15 HP at 40°F at this same flow is well within the allowable limits of the motor rating at this reduced flow and temperature. We would expect lower internal motor temperatures, which will actually increase your service factor in this operating temperature range (40°F). We have performed research at temperatures above and below 40°F with "No-Flow" and the motor has performed satisfactory. We anticipate no problem with your operating range.

Sincerely,

Kevin P. Price
Sales Manager
Sun-Star Electric, Inc.

 

[KllrWolf]

I am not addressing system design or anything besides the motor as I am sure my knowledge on that part of the subject is nowhere near that of the rest of you taking part in the discussion. I am just concerned about the information on the motor and submersible motors in general.

The fact that it takes 15 minutes for larger motors further proves that it takes considerable time for the internal heat to transfer to the housing where it can be cooled down. Which should also mean that cooling flow is not required at the instant of start up. Flow past the motor is not going to cool anything until the internal heat from the motor transfers to the motor case, which takes a little time.

You are misinterpreting the data on the times for cooling. There are two factors that come into play for the cooling, ambient water and surface area. These motors heat up rapidly due to the amount of surface area available for cooling. Once they shut off, it takes time for the outer surface to transfer the built up heat out into the surround water. It is not because of the slow change inside the motor, but the limits of available cooling surface area. Flow at startup helps keep ahead of the heat, and actually does help slow the rise of the internal temperature to a degree (no pun intended).

When the ambient water is not flowing, the water in contact with the motor quickly rises toward the temperature of the motor. As the temperature of the water rises, the heat transfer from the motor to the surrounding water slows, thus slowing the motor cooling down.

The heat generated by a motor starting is always the same (assuming load, power, and other variables do not vary). Thus a motor at ambient will rise X degrees at start. If the internal temperature of the motor is higher than ambient, then it will be temperature over ambient plus X degrees at start.

Between those two items, you can not judge startup temperature rise and cooling needs based on the shutdown cooling rate. The heat transfer profiles are different between the two situations

The 6” motors from Hitachi have a different construction than rewindable larger sized motors available from Hitachi and other manufacturers that allows a greater internal heat rise, thus the letter that Sun Star sent you addressing a specific motor in specific circumstance.

 

[jonr12]

As you say, rewindable or "wet winding" motors allow an even greater internal heat rise. So the canned stators of the Hitachi motors in question are a worst-case scenario for heat transfer. I can lay a 30 HP Hitachi motor on a workbench, (not even in water), turn it on and put my hand or a thermometer on the housing skin, and it takes about 60 seconds before I can measure a temperature increase. So if the temperature of the motor skin doesn’t increase for 60 seconds, the temperature of the water surrounding it doesn’t increase either, and therefore doesn’t need to be flowing for at least 60 seconds.

In the water well industry, these are standard motors in standard conditions and there is nothing specific about the circumstances.

 

[dicksewerrat]

Is this motor/pump submerged? Well pumps seem to stay cooler if they are below static water level. Or maybe I'm spoiled. My well pump is about 45 feet below water level in the well pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[KllrWolf]

jonr12:

It is the 6" that have the higher allowable internal heat rise, hence the higher ambient temperature rating for them.

 

[KllrWolf]

Correction to myself: the 6" has better internal tolerance to heat, thus the higher ambient rating

 

[jonr12]

How deep the pump is under water has nothing to do with motor cooling. If there is not sufficient flow going past the motor, the water around the motor can boil, no matter how deep the pump/motor is set underwater.

If you overheat a motor with a canned stator, the stator will swell and grip the rotor, and the motor will never work again.

With a wet winding, when the motor cools back down, the stator will still be good and will work fine as long as the thrust bearing is not damaged.

 

[KllrWolf]

With a wet winding, when the motor cools back down, the stator will still be good and will work fine as long as the thrust bearing is not damaged.

If you overheat a wet winding, there is a good chance you have done some damage, if not melted, the insulation of the winding wire. The temperature at which the standard wet winding wire starts to breakdown and melt is lower than the temperature at which the thrust bearing will usually lose its water layer due to the higher temps (depends on thrust loading). Heck, in a normal wet winding motor application, monitoring temperature is not really needed. Monitoring your voltage and current in all phases give you a quicker notification of when something changes. They can even be used over the life of the install to determine when the pump is wearing out.

 

[jonr12]

Agreed. But if the windings are not shorted, the wet winding motor will still run where the canned stator is done. I just hope this poster will come back and tell us what they did and how it is working.

 

[KllrWolf]

I will agree with you there. Once a canned motor has a problem, it is now scrap metal at best. I too am interested in the final solution, as this thread has got me back into learning more about the various pump types and systems, not just the basics for analysing motor failures.