Extremely unusual distribution system pressure drop 6

[chileheadcraig]

I work for a water utility and we have a conundrum that none of us can figure out. One of our zones is a pump station/water tank fed setup where water going to the distribution system is controlled by tank level. While a typical setup would consist of a pump station being tripped on and off by tank level, our setup is a bit different and I'll try to explain as best as I can as follows:

We have a VFD pump station which maintains a constant pressure our water plant that provides both plant water and serves a portion of our system by filling a reservoir. The pump station needs to run 100% of the time to provide plant water, so delivery to the distribution system is controlled by a valve. When the tank reaches 'low', the valve at the plant opens and allows the tank to fill. When its full, the valve closes and the zone is fed by the tank. Similar setup as a standard pump on/off scenario but instead its a valve open/close.

The problem is, when the valve closes (and is no longer fed from the PS), the pressure drops from about 90 psi to 20 psi (or less) and takes about 30 seconds to recover to normal pressures (being totally tank fed). The tank is located pretty central in the zone and the pump station is at the southern end. The pressure drop is only seen in the area between the pump station and tank. All areas north of the tank are isolated from the phenomena and continue to get fed from the tank, as expected. The valve closes from 75% open to closed over a period of 2 minutes, so its not slamming shut, so transients do not seem to be the culprit.

Any idea whats causing this pressure drop and slow recovery? Why would the reservoir not simply take over and feed the area to the south? We have checked the check and altitude valves at the reservoir and all isolation valves on the line from the tank back to the plant and have ruled out issues there.

Attached are several events from our Telog pressure recorder, which samples at high frequency. The pattern appear like clockwork every time. Thanks for any feedback or ideas.

 

[Valvecrazy]

Oh I believe in ground storage and generators! You could have a backup generator for the backup several times over for much less than a water tower cost. Then you have emergency water as long as you have fuel, not just until the tower is empty. I am sure Los Alamos now has generators and is no longer counting on a water tower to save their butts, because it was almost a catastrophe.

 

[bimr]

If your pumps are not big enough to handle peak demands and you don't have generators to keep those pumps running, you could end up in the same boat as Los Alamos and not have enough water to fight a fire.

Not sure what you are referencing here. If you have an elevated water tower, the pumps are sized closer to the average daily demand to fill the tower, not the peak system demand.

 

[Valvecrazy]

The cities I work with are usually the smaller ones. I would never count on a single water tower to supply peak and/or fire flow demands. Ground storage with adequate booster pumps to supply max demands needed and backed up with a generator or two is the only sure way to have water in an emergency.

During a power outage a few years ago the city of Lubbock, where I live, which has several water towers was out of water in about 2 hours. And that was just a normal day with no emergency or fire. Now the city has at least one generator system to keep the pumps running, as they discovered water towers by themselves are not adequate.

 

[bimr]

During a power outage a few years ago the city of Lubbock, where I live, which has several water towers was out of water in about 2 hours. And that was just a normal day with no emergency or fire. Now the city has at least one generator system to keep the pumps running, as they discovered water towers by themselves are not adequate.

Based on the news reports, the water outage had nothing to do with water towers. In fact, Lubbock would have been better served with water towers than ground storage tanks. It would appear that Lubbock lacked standby power or backup power for the water distribution pumps on the ground storage tanks. Every modern water distribution system should have backup power for water distribution pumps.

"The City of Lubbock has an extensive distribution system which includes 11 primary pump stations, 4 elevated towers and 13 ground tanks, and nearly 1,800 miles of underground pipes we call water mains - all designed to deliver water to our more than 83,000 service connections."

https://www.mylubbock.us/docs/defau...ter-quality-report-2015.pdf?sfvrsn=41da4cc9_4

July 21, 2012

Lubbock, TX
Power Outage leads City Officials to issue boil water order. Loss of power crippled water distribution pumps, resulting in low pressure and potential for backflow of contamination or pollution into the drinking water supply

 

[Valvecrazy]

It would appear that Lubbock lacked standby power or backup power for the water distribution pumps on the ground storage tanks. Every modern water distribution system should have backup power for water distribution pumps.

I believe that is exactly what I said. But I don't see how you could say the problems was not caused by the water towers lack of ability to handle an emergency? Afterwards Lubbock didn't add anymore towers, only generators.

My point is that even with multiple elevated towers, even a short power outage was enough to stop the water from flowing. Now with enough backup power, why are the towers even needed? Seems as in most cases the towers where just a false sense of security anyway.

Not to mention just how useless those towers are when there is a power outage for an extended period of time. Having enough generators with enough fuel is the only way to maintain the water supply when an ice storm or something keeps the power off for days or weeks at a time. For these reasons and many others I believe the 3000 year old technology of elevated towers is obsolete and a complete waste of taxpayer money.

But there is so much money spent purchasing and maintaining water towers that it is a completely political issue. I always get blasted just for mentioning it. Years ago after explaining this to a small city water manager, he looked at me and said, "boy don't you know there won't be any pressure without a water tower?" I tried to explain to him that the pump builds the pressure and supplies the water, not the water tower. But the "deer in the headlights" look he had meant he was never going to understand, which I am afraid is the case most of the time.

 

[bimr]

The selection of water storage facilities is not as simple as elevated versus ground storage.

There are a number of factors to consider when developing a water system including cost, population of municipality, topography, area of municipality, fire scenarios, water source, etc.

 

[Valvecrazy]

In this case, as in most of the cases I am tasked to help with, the water tower is the problem. The pump station runs all the time maintaining a constant pressure. The tower is not being used so the water quality becomes an issue. I know of many systems like this where at least once per day the pumps must be shut off or a valve to the tower closed, so the tower can drain and then be refilled with fresh water. As in this case, shutting off the pumps or closing the valve to the tower is causing the transients. Without the tower, the pump station would just maintain constant pressure to the system, the quality of the water in the tower would not be an issue, valves to the tower would not need to be opened or closed, and there would be no transients. Problem solved.

I am sure there could be backup pumps and generators for the backup pumps and generators several times over for much less than the cost of a tower, or even just maintaining the tower. As long as emergency water supply can be maintained in other ways, the tower would not be needed. Plus as stated earlier, these other ways to maintain emergency water supply can be much more effective than a tower when needed for long periods of time.

The calls I typically get are when the tower has a hole in it, needs to be inspected and painted, or the legs are too short to supply the pressure needed to distribution. I typically set the pump system up to maintain constant pressure, so the tower can be shut off for maintenance. During the month or three it takes to repair the tower I usually get a call from the water operator. He will say, "you know the pumps are not cycling at all so I am sure they will last longer this way". "The system pressure is so good the subdivision up on the hill has not called to complain about the pressure since the tower has been out of service." "We are no longer seeing transients or water hammer in the system, and line breaks have almost completely disappeared." "Not having line breaks also means we are wasting much less water in the distribution system." The conversation always ends up with the question, "So why are we spending so much time and money repairing the water tower"?

There are many better ways to supply water during a power outage. Plus towers cause so many problems with transients, valve speed, water quality, and other issues that my answer is always, "I don't know why you are spending so much repairing that tower".

 

[chileheadcraig]

So we tried a scenario where we slowed the valve closure down to take about 8 minutes to close from 30% to closed and there was no transient. Pressure dipped slightly to about 55 psi before bouncing back to 65 psi when feeding from the tank. I say its successful! Thanks again for the tips!

Valvecrazy, I think you hijacked my thread To speak to your proposed solution, most systems I work with benefit from having either ground storage or elevated storage (depending on topography), with an exception to standpipes, which tend to lead to more dead water. If you are sizing pump stations to handle peak hour and worst case fire flows, then you are over-sizing the pump station. "Textbook" design sizes pump stations for Max Day demand while tanks are used for peak hour, emergency and fire suppression and it has worked well in most all cases that I've looked at. It especially helps during peak hour for customers at the extreme ends of your system. If you are relying on pumps to push water to the end ends of your system and maintain adequate pressure, then you also have to oversize pipes or deal with pressure swings and major pressure fall off. Maybe small systems can get by without storage, but its a must for most any larger utility. If your customers are having issues with dead water or transients, then the design and operations need tweaking (like in our case) or the design was flawed (possibly bad tank sizing or placement). Now that this is behind us, we are back on track. No issues.

 

[Valvecrazy]

Wasn't hijacking anything, still on the same subject. Glad you were able to slow it down enough to reduce the transients. When you tell the doctor "it hurts when I do this", the doctor will say, "then don't do that". When you tell me closing a valve is causing transients and water hammer, I am going to say, "then don't close the valve". May not be what you want to hear as the tower would be useless. But when you have to make accommodations to regularly change the water in the tower for quality issues, then it sounds like the tower is useless anyway. You still have a transient causing a 10 PSI bounce even after taking 8 minutes to close the valve. You wouldn't see any of that if you didn't have to close the valve in the first place.

 

[chileheadcraig]

Whats the largest water system, in MGD, that you have personally either designed or done an extensive hydraulic evaluation on that does not need elevated or ground storage?

 

[Valvecrazy]

I mostly work on systems that are 1MGD or less. But I see towers as even less useful in larger cities. For instance what good is 4M in elevated storage to a city like Lubbock that uses 38MGD? Just like last time, we will be out of water in a couple hours at best if they don't get some generators fired off. But people still think their water comes from those towers. LOL

BUT I never said ground storage was not important. That is where the REAL water comes from once they get the generators fired up.