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Well pump & pressure tank for fire hall
3

Well pump & pressure tank for fire hall

Well pump & pressure tank for fire hall

(OP)
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

RE: Well pump & pressure tank for fire hall

Using a VFD on a well pump flow of 16 gal/day to 200 GPM is not reasonable.
Even if turned down to 100 gpm will get you only 1/4 of the way up to the surface.
The VFD turned down to 20 gpm will hardly get the water out of the pump.
Use the well pump only to fill the fire truck tanks.
Get a small separate water tank and have a commercial water service truck bring potable water once a month.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

Two pumps? one for trucks, one for domestic use.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Well pump & pressure tank for fire hall

(OP)
Thanks for the feedback on the VFD. Two pumps would be ideal. Not sure we would be able to fit two in the well, casing is 8".

RE: Well pump & pressure tank for fire hall

You should put in separate wells. You can use a 4-Inch well pump. A separate well installation may cost $3-10K.

RE: Well pump & pressure tank for fire hall

(OP)
What about having the 200GPM 20HP pump serving a water storage tank (open to atmosphere)? The storage tank could have a small pump to maintain pressure in the building domestic water piping. When the tank is down to say 25%, the well pump fills it up. When the Fire Department needs to fill their trucks, a separate line from the well pump to the fill station feeds their connection directly. This would avoid having to have another well drilled.

RE: Well pump & pressure tank for fire hall

Not too open if it is to be used as potable water bigsmile.

A storage tank makes sense with a level control system to keep the tank filled for the domestic service, float isolating valve to shut off the storage tank when the tankers are being filled, connect a small domestic pressure pump / system to the storage tank for domestic service.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

I think he said that such a system would not guarantee that the water would remain as fresh as he liked to have.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

Note on "two pumps in a well":

When I worked for the water well industry, we visited a guy in Oregon that made an apparatus so that you could put two pumps in a well; One on top of the other.

let me know if you want information on something like this. If it's a one time thing, I remember enough about it to explain the design. It was easy enough to manufacture.

Charlie
www.facsco.com

RE: Well pump & pressure tank for fire hall

How large is the pipe from the well for the Fire Dept.? How far into the water table is the pump? You could probably get both pumps into the 8 inch casing. Use a 2-3 inch pump for domestic and a 4-5 inch pump and pipe for the Fire Dept. Just make sure the Fire Dept. pump is lower than the domestic pump. Need offset pump dischage connections. I know my well has about 130 feet of water in it and my pump is down into the water 50 feet.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Well pump & pressure tank for fire hall

"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."

How long do you envisage that it will take the water to become stagnant - and is the water from the well drinkable anyway?

Have you considered of a deep well ejector pump surface mounted with a pressure tank. The drive pipe and return pipe require very little space in a well casing. Should be available off the shelf from a number of supplier.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

I think the solution is to have a tank with a booster pump to use for filling the trucks, and use a relatively normal sized pressure tank for the potable water. That way, you'll have relatively fresh water for drinking, and still have the capacity for filling the trucks.

RE: Well pump & pressure tank for fire hall

That's 3 pumps when you can get by with 2 units.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

One pump, constant pressure valve, 80 gallon diaphragm tank, all you need.

RE: Well pump & pressure tank for fire hall

So then, that's back to running 20 HP pump to fill a diaphram tank.

We've gone full circle.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

(OP)
Happy to see so many suggestions. Keep them coming! Here are my thoughts:
- The storage tank would not actually be "open to atmosphere"; just a standard water storage tank, same as a cistern. It would be fairly small, say 80 gals. This would eliminate our worry for freshness of water for domestic purposes.
- A separate well is still our worst case scenario option. We were given a quote of roughly $9,500 for the well and pump.
- Regarding the idea of two pumps in the well, the well driller told us it would be very difficult and did not recommend it. He was concerned over having to remove the top pump in order to service the bottom pump. The well casing is 8", the fire department line is 4" and the domestic water system requires 1-1/2" to 2" max. The well is 200 feet deep with the casing down to 130 feet.
FACS: if you could forward me that information, that would be great!
- Yes the well water is drinkable; further to this, we are specifying carbon filters at the faucets.
- TenPenny: I agree with what you are suggesting, but the large capacity of the pump required for the fire hall vs the small pressure tank is what is concerning us at this point. Essentially, even with a soft start drive on the 20 HP pump, it will only take 30 seconds - 45 seconds for the pump to fill the 80 gal tank. Based on our calculations, this would happen 2-3 times a week, so we aren't terribly concerned about short cycling the pump. Just trying to get a feel for how this surge will affect the system in whole.
- jonr12: How would the constant pressure valve protect the domestic system from the surge of water from the 20HP pump? I assume the valve would release when pressure of 60-70psi is reached?
- We are also considering a small expansion tank to help with water hammer.

RE: Well pump & pressure tank for fire hall

In my opinion, you only need one pump in the well, to supply both potable water and the trucks, and one booster/transfer pump to fill the trucks from the storage tank.

The storage tank needs to be sized appropriately considering what capacity you need to fill the trucks, and at what rate you can refill this tank.

Out of curiousity, where in NB is this located?



RE: Well pump & pressure tank for fire hall

A constant pressure valve or CPV will start and stop the pump at 5 GPM, no water hammer at all. 80 gallon diaphragm tank holds 20 gallons of water. The CPV is set at 50 PSI with a 40/60 pressure switch attached to the tank. When using water domestically, the CPV makes the pump work like a 5 GPM pump. It starts and stops the pump at 5 GPM, and refills the tank at 5 GPM, which takes about 2 minutes at those pressure settings. Not very efficient to run a 300 GPM pump at 5 GPM, but you only use 16 gallons per day, so who cares. Because the amps are reduced the motor gets plenty of cooling at 5 GPM, and won’t be damaged. When you need to fill a truck, the CPV will open up and supply as much water as the pump can produce while holding 50 PSI. In this way one pump can do both jobs.

RE: Well pump & pressure tank for fire hall

I agree with TenPenny.

If you pump from the well to a booster tank, Potable water will be renewed every time a truck is filled.

If there is enough space, you can pump from the well to an elevated tank, and fill the trucks by gravity. This way you will save a pump.

If the elevated tank is big enough, you even can use an smaller well pump. (But will be pumping more time).

RE: Well pump & pressure tank for fire hall

I would imagine that he knows the frequency of fire truck refills and that's exactly upon what he based his stale water opinion and that small demand for water just won't justify a two pump solution.

The aquarium in my living room has half a month's supply.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

Your best option is a deep-well jet pump as suggested earlier.
No need for any above ground tank with water available on-demand for your building.
Link below describes the operation etc of a deep-well jet pump - costs probably a few hundred's, easy to install and maintain.

http://inspectapedia.com/water/JetPumps2Line.htm

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

Deep Well jet pumps are extremely common in rural New Brunswick, I grew up with one, and had one at my previous house until the foot valve died one January day, we replaced it with a submersible.

The issue I see is that he needs to have fresh water for potable use, and a reservoir tank for filling the trucks. You could do this with one pump, all you have to do is separate the plumbing above the well head, so that the potable water comes directly from the well. Why not just use check valves, so that the pump kicks on when there is demand either from the potable side or the truck supply side, but that neither downstream use will cross feed the other? I don't see why two pumps are needed.

RE: Well pump & pressure tank for fire hall

If there is a reservoir tank for both requirements, unless it is well elevated you will need a second pump to pressurise the building supply and the truck fill operation (a 200GPM requirement) - back to 2 pumps.

To use the well pump for the building supply is over-kill, open a tap for a drink of water and you bring a 20HP pump on-line for 5 - 10 seconds.

I see only one reasonable solution, the well pump dedicated to filling the tanker, and a seperate deep well pump with pressure tank to service the building.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

Can the well even supply 200 gpm, and for how long? It seems wasteful to use a 20 hp pump when a 3/4 hp pump and an elevated plastic storage tank will suffice. All that horsepower is required because you are trying to lift water from depth at high rate.

RE: Well pump & pressure tank for fire hall

Ya. I think that was at least half the reason for the OP's post. After all, it is somewhat obvious that two pumps could be used, or a small tank off the large tank, etc. etc., all seemingly extreme overkill for providing only 500 gal/month.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

I agree with jonr12. You only need the 80 gallon pressure tank, the 20 HP well, and a control valve. Sure it's not efficient, but we're talking about wasting about $10/year on electricity by using the large pump for domestic water. With a bladder pressure tank, there's no worry about stagnant water. (How old is the groundwater before you pump it?)

RE: Well pump & pressure tank for fire hall

How old is the groundwater, probably many billions of years old.
Anyway seems the OP has given up.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

(OP)
- Didn't give up, just got sidetracked! glasses Thanks for all comments up to date! Very informative.
- Yes, the well can provide 200 gpm. Well is 300' deep.
- We are looking into the deep well pump option. Do I understand correctly that a submersible pump could be installed in the well in conjunction with a deep well pump? We are back to two pumps on a single well; other than capital cost, what are the cons to this setup?
- We can't count on potable water being replenished when the trucks are filled; the fire department does not get that many calls.
- Trucks are 1250 gals (2), 2800 gals (1) and a future 4000 gals tanker (1). The client wishes to have capacity to fill one 2800 gal truck at this well.
- What is the advantage of using a storage tank for the truck fill? Why not pump directly from the well?
- Here is our current design, comments are appreciated:
20 HP submersible pump to 4" line into building, 4" line ran directly to fire truck fill stations, 1-1/2" line tee-ed off from 4" main to 80 gal pressure tank for domestic use, control valve located on 1-1/2" line prior to pressure tank. Pump complete with soft start drive. Pump rep is not concerned about burning out the pump, as it will only turn on every few days to replenish the pressure tank. For the amount of time is will run for domestic use, we are not overly concerned about operating costs either.

RE: Well pump & pressure tank for fire hall

Sounds good to me. I'd add backflow prevention to the truck fill as well, ideally as an air gap, but sometimes the truck guys prefer a direct connection. With a direct connection, our regulators would require a reduced pressure principle backflow device.

RE: Well pump & pressure tank for fire hall

(OP)
Good point, we have specified a double check backflow preventer, which suits local regulations here.

RE: Well pump & pressure tank for fire hall

Hey, why not just install a tap on the truck.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

(OP)
A tap?

RE: Well pump & pressure tank for fire hall

Your making this harder than it needs to be. You don't need a soft start panel, and I am guessing that 1.5" control valve you mention is a pressure relief valve, that won't help. Also the truck fill guys will need to open the line all the way or the pump will cycle rapidly. I have done several of these and it works great.

A constant pressure valve or CPV will start and stop the pump at 5 GPM, no water hammer at all. 80 gallon diaphragm tank holds 20 gallons of water. The CPV is set at 50 PSI with a 40/60 pressure switch attached to the tank. When using water domestically, the CPV makes the pump work like a 5 GPM pump. It starts and stops the pump at 5 GPM, and refills the tank at 5 GPM, which takes about 2 minutes at those pressure settings. Not very efficient to run a 300 GPM pump at 5 GPM, but you only use 16 gallons per day, so who cares. Because the amps are reduced the motor gets plenty of cooling at 5 GPM, and won’t be damaged. When you need to fill a truck, the CPV will open up and supply as much water as the pump can produce while holding 50 PSI. In this way one pump can do both jobs.

RE: Well pump & pressure tank for fire hall

(OP)
Sorry to be a pain, I haven't used a constant pressure valve before. It does seem adequate for what we need. But the pump has no VFD, how will it work like a 5GPM pump? If the pump is only pushing 5 GPM, it will not even get the water out of the well.

RE: Well pump & pressure tank for fire hall

(OP)
Wow thanks for the link! That really makes it easy to understand. I think this is our solution.

RE: Well pump & pressure tank for fire hall

Those 16 gpd are going to be produced using starting current amperage.
Starts and stops of the pump are the worst duty cycle possible.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

An 80 gallon pressure tank will hold about 20 gallons of water. With 16gpd the pump will come on once a day, and will run about two minutes to refill the tank. So it won’t short cycle. I have also found that starting a pump against a closed valve will make the motor start under a no load condition, which reduces the starting current equal to or even less than a soft start panel.

RE: Well pump & pressure tank for fire hall

Seems the problem has been solved so lets see it put into practice.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

(OP)
BigInch: short of putting in two pumps, do you have any other suggestions?

RE: Well pump & pressure tank for fire hall

(OP)
Thanks again for all the help guys. Now on to selecting the equipment and drafting up the detail! pc3

RE: Well pump & pressure tank for fire hall

See above. My aquarium is 1/2 of your volume per month. I just can't believe you want to use any pump for that. I'm in the UAE and there are loads of places that don't have water. Even more so in Spain, since there is a much greater population, a water delivery is an easy solution. How much would that cost where you live? In each of those countries and many others, I'd just buy 1, one m3 tank and have somebody come buy with a water truck and fill it one time a month. Or maybe you need to fill your fire truck once a month. It hardly sounds like it, but if you do, fill your tank when you fill your truck. I was just thinking that surely you have better things to do with your money than buy pumps for that quantity of water. Maybe all you need is a hose and a tank. Anyway, be thankful that you have a well and so many options.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

Not so fast. You have received many answers but not many correct answers to your query; "We don't want stagnant water".

A 4-Inch well pipe has a water capacity of 0.65 gals/ft. If you use 16 gals per day, it will take 2 weeks for the water to move through the well pipe, and an addtional week or so to go through the pressure tank. If you consider that the office is probably closed on weekends, you have one month of travel time for the water in the system from the bottom of the well to the tap. One would think that would meet the definition of stale water.

Using a single pump for both applications may be the cheapest solution, but it is not the best solution. If you are going to all of the trouble to build a new municipal building, why chintz out for just a few thousand on the water supply. Remember the phrase, "The pain of poor quality lasts far longer than the joy of a low price".

jonr12:

Where do you come up with this concept:

"I have also found that starting a pump against a closed valve will make the motor start under a no load condition, which reduces the starting current equal to or even less than a soft start panel."

That is not correct.

In actual practice, submersible well pumps actually have 4:1 starting current:operating current ratio (instead of the typical electrical pump motor's 6:1 ratio) because of the small diameter of the pump impeller (as compared to a centrifugal pump). That is the reason that soft starts are not typically used on submersible well pumps.

RE: Well pump & pressure tank for fire hall

The water delivery tank solution is the only one that avoids stagnant water as far as I can see.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

In the building code there is usually a requirement that a water service be installed in a new building.

RE: Well pump & pressure tank for fire hall

bimr, the 0.65 gals / ft seems a bit arbitary to me, well capacity is based on permeability, screen length and well screen open area (slot size). However, I must agree on the tight a_rse approach to a successfull installation.

Just in case the OP missed my earlier suggestion, simplest method for a good permanent easy to operate / maintain installation is a down-the-well 2 pipe jet pump with above ground diaphram pressure tank pump, seems the well casing is 8" (OP comment) so should be enough room for the jet pump and the riser pipe from the pump.

http://inspectapedia.com/water/JetPumps2Line.htm shows a typical arrangement.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

A water service can't be supplied by a tank?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Well pump & pressure tank for fire hall

Sure an elevated tank is one solution, positioned high enough to give reasonable pressure for the showers etc, but then, is there sufficient head available from the well pump to reach the tank. Not forgetting the tank will require a ball-cock, float level control system to start / stop the pump and able to be over-ridden automatically when the pump is required to fill the tanker/s.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

Artisi, don't understand the "the 0.65 gals / ft seems a bit arbitary to me"

A 4-Inch well pipe holds .65 gals of water per foot of length. The OP says that the well is 300 feet deep. That means he will have 195 gals sitting in the well pipe. What is arbitrary about that?

On another tack, perhaps the OP has underestimated the water usage. We would generally use 15 gpd per employee. With the 2 employees, that equates to 30 gpd.

http://www.ilga.gov/commission/jcar/admincode/035/...

In support of a higher estimated water demand, the OP has also stated "The building includes washrooms, showers, janitor closet and a kitchenette."

A modern building would also include a 1 to 1.5-Inch water service as well.

RE: Well pump & pressure tank for fire hall

bimr, sorry about that blush, I read it as inflow well capacity not storage capacity within the well casing.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Well pump & pressure tank for fire hall

"I have also found that starting a pump against a closed valve will make the motor start under a no load condition, which reduces the starting current equal to or even less than a soft start panel."

This is true. Starting a submersible with an open discharge line will usually causes the starting amps to be about 6 times the running amps. But have you ever checked the starting amps of a submersible motor with no pump attached? The starting amps peak is only 1 to 2 times the running amps. It is the same with centrifugal pumps and motors. I learned this using one of those magnetic couplings between the pump and motor. With no load on the motor, the starting amps are not any higher than with a soft start panel.

So when the system is pressured up and the pump starts against a closed valve, the starting amps are the same as if the motor was started without a pump attached. The only time you see the 6 times running amps is the very first time the pump is started when the pipe is empty and there is no pressure to hold the valve closed. After the first start and the system is pressured up, the starting current is the same or even less than when using a soft start panel.

It is just as important for the pump to get exercised as it is for the water to stay fresh. You want to make sure the pump hasn’t rusted up and locked down, so it will work when you have a fire. So it would be a good thing for the pump to start and run a couple of minutes everyday to keep it in ready condition. Then if you want the water to stay fresh, just use a little more than 16gpd. Maybe hook up a sprinkler with a timer and water the grass for a few minute every couple of days.

RE: Well pump & pressure tank for fire hall

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.

Submersible well pumps are also always designed to drain back into the well on pump shutdown so that you are not restarting against a full water column. In summary, for a submersible well pump, it does not make any difference if you are starting against a closed valve or open valve. The submersible well pump is designed to start against an empty column pipe.


DomDoucet: The fire water system design should be reviewed by a fire protection specialist with experience in rural fire systems. There may be additional requirements such as an automatic stand-by generator, or at minimum, an ability should be provided to rapidly connect a mobile generator in the event of a long term power failure. There may be minimum insurance requirements such as FM approval of components. Specific requirements and technical details should be based on fire department and health department requirements and local engineering standards.

RE: Well pump & pressure tank for fire hall

If you leave the check valve out of a submersible pump installation, the water will drain back down the well when the pump shuts off. However, this is not good for the pump. The pump can spin backwards at uncontrolled speed. This is not good and can even break a shaft if the pump tries to restart while spinning backwards. Starting a submersible when the pipe is empty also causes up-thrust damage and high starting amperage. Most submersible pump or motor companies recommend a check valve every 200’. While I don’t agree with multiple check valves because of other problems they cause, it does prove that sub pump manufacturers want a check valve in the line so as not to start the pump in an up-thrust condition.

When using a soft starter and starting a pump against a closed valve, you have to turn the torque setting way down. Otherwise the starting load on the pump/motor is so low that it will go to full speed almost instantly, no matter how long you have the soft starter set to ramp up.

RE: Well pump & pressure tank for fire hall

(OP)
Great discussion. Definitely giving us loads to think about!

RE: Well pump & pressure tank for fire hall

Don't remember anybody mentioning leaving check valves out of a well pump installation but you. Typically the well driller will install a small orifice on the check valve flapper so that the check valve allows water to slowly drain back down the well.

I constructed a 1000-feet deep well recently. Are you stating that you would try to start a well pump against 1000-ft of head?

That is exactly the reason that the check valves are allowed to leak water back down into the well. It is a fact that the check valves allowed the water to drain back into the well. It is also a fact that there was no soft start. It is also a fact that the well pump does not start against a closed valve. Reread the other details above.

RE: Well pump & pressure tank for fire hall

Yeah you can drill a hole in a check valve to let the water drain back slowly. That is much better than letting the pump spin backwards. But it is still much harder for a pump to start with empty pipe than with full pipe. It is even easier on the pump/motor to start with the pipe full and against a closed valve. I worked for a pump manufacturer for a while. It was SOP to start the pumps against a closed valve in the test pit.

Yes I would rather see the pump start against 1000’ of head than with no head. I have done some that where twice that depth, and I would never let them drain back. The deeper the well, the harder it is for the pump to start when the pipe is empty.

RE: Well pump & pressure tank for fire hall

If you have a check valve, the pump won't take very long to develop 1000 ft of head and open the check valve, as the volume needed to fill the space between the check valve and the impeller will be quite small.

If the well pipe is empty, the pump will have to run that much longer to fill the discharge pump and start working against the higher head.

Not explained particularly well, but I'd much prefer having a check valve.

RE: Well pump & pressure tank for fire hall

Starting current or locked rotor current is the design of the motor. Nothing to do with the driven load type.
The driven load type / inertial will have effect on the starting time.
Starting a centrifugal pump with closed valve or static head will take shorter time than starting with a open valve or zero/low static head.

RE: Well pump & pressure tank for fire hall

A person with well experience will know that there is never no head. The minimum water level is the difference between the water level in the well and the pump setting depth.

RE: Well pump & pressure tank for fire hall

A person with well experience knows that water under water doesn’t weigh anything to the pump. When there is no check valve and the water in the discharge pipe is the same as the static water level, there is the same head on the inlet of the pump as there is on the outlet. So the pump starts against zero head causing up-thrust. Only when the water in the discharge pipe becomes higher than the water level in the well does the pump start to see head.

The driven load does have a lot to do with starting current. The starting current for a motor with no pump attached or pumping against a closed valve is only about the same as the running current. With a pump attached and starting at zero head the current can be six times the running current.

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

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

http://books.google.ru/books?id=GjpZFOQeH3cC&p...

"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

RE: Well pump & pressure tank for fire hall

See the chart on page 2 of this link.

http://www.rmhindustries.com/wp-content/uploads/20...

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.

RE: Well pump & pressure tank for fire hall

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=cHI17qlMXpwC&p...

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.

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

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).

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

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.

Quote (jonr12)

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.

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

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.

Quote (jonr12)

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.

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

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.
WWW.amlinereast.com

RE: Well pump & pressure tank for fire hall

jonr12:

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

RE: Well pump & pressure tank for fire hall

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

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

Quote (jonr12)

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.

RE: Well pump & pressure tank for fire hall

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.

RE: Well pump & pressure tank for fire hall

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.

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