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

 

[DomDoucet]

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

 

[BigInch]

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

 

[bimr]

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 currentperating 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.

 

[BigInch]

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

 

[bimr]

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

 

[Artisi]

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.)

 

[BigInch]

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

 

[Artisi]

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.)

 

[bimr]

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/035/03500370ZZ9996bR.html

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.

 

[Artisi]

bimr, sorry about that , 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.)

 

[jonr12]

"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.

 

[bimr]

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.

 

[jonr12]

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.

 

[DomDoucet]

Great discussion. Definitely giving us loads to think about!

 

[bimr]

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.

 

[jonr12]

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.

 

[TenPenny]

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.

 

[Pumpsonly]

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.

 

[bimr]

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.

 

[jonr12]

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.