Orifice Plate for Flow Control

[Clmsn08]

Fellas,

I have a project that I have been working on for about two weeks that I could use some advise/suggestions on.

We have an area of our distribution system that is above our normal HGL. The system was built by a developer in the 60's and we "inherited" it shortly after. The current system has two small pumps that feed a 70,000 gallon tank which in turn provides water to about 25 homes and 4 fire hydrants. Due to the high elevation difference between the tank and hydrants, they can currently flow 1,000+ GPM.

We want to remove the existing set-up and replace it with a constant pressure VFD pumping system (due to poor water quality from lack of turnover in the system). The new pump station will be able to provide approximately 600 GPM.

My problem comes into play when the fire hydrants are opened. The available head can produce a flow of 1,000+ GPM while the pump station can only produce 600 GPM. We can not justify a larger pump station for 25 homes so I am wanting to throttle the flow at the hydrants.

To reduce the flow, I looked at PRVs/Pressure Sustaining Valves first. However these valves are expensive, will require maintenance, and we will need multiple ones due to loops in the system.

The second option is to install an orifice plate in the hydrant legs to control the amount of flow that the hydrant can produce. This flow would be limited to a little over 500 gpm @ 20 psi (Local Regulation minimum).

Have ya'll used anything like this or seen a similar set-up/scenario before? Do you know of a good orifice plate manufacturer?

Any advice is greatly appreciated.

Thanks

Tyler

 

[bimr]

The ISO NFF requirement for residences with sprinklers is 500 gpm. It does not appear that you will have adequate fire flow. Have you run this past the fire department?

http://www.isomitigation.com/downloads/ppc3001.pdf

Orifice plates can not protect your system from flow fluctuations. If your system is sensitive to flow fluctuations an orifice plate isn't a good choice. The orifice plate will allow different flow capacities depending on the applied water pressure.

Why not just put a larger pump in? The pump will only operate at high capacity during fire flow scenarios.

Without additional looping, adding a VFD pumping system will also not remedy the problem with stale water.

 

[Clmsn08]

Bimr,

Thanks for the response.

I should have been more clear. The 70,000 gallon tank will be removed from service and the system fed solely off of the VFD pumps which will ramp up and down to maintain a fixed pressure within the system. This will insure that water will be pumped in only when there is a demand, thus preventing water from becoming stagnant.

Since the majority of the flow is <10 GPM, I have designed three small pumps that are each capable of reaching this low flow requirement and one large 30 hp pump that can produce almost 400 GPM. With all pumps running, the system will produce 640 GPM, 140 GPM larger than the 500 GPM requirement.

I am thinking that the system wont be subject to flow fluctuations if the orifice plate is installed on the hydrant leg and is sized to provide a certain flow at the fireflow pressure.

I have not discussed with the fire department yet since I am still in the preliminary/feasibility stage. Just trying to get some feedback from other people in the business.

Thanks

Tyler

 

[77JQX]

Our system has several constant pumping zones, so I have some opinions on this type of setup. I agree that using a tank to store water for such a small demand can lead to quality problems like lack of disinfectant residual or production of disinfection byproducts.

There are several ways to address this situation. Maybe the simplest is to install a control valve to cycle the tank back to the lower zone from time to time. Yes, this wastes energy, but you can buy lots of energy for the price of a new pump station.

If you elect to go the constant pumping route, you'll need at least two pumps for redundancy, and to allow for maintenance of one. You'll also need a genset backup for when the power fails - both things your tank does now.

I think VFDs are a poor technology for booster pump stations with a high static head requirement. We have a few, and they're not worth the trouble. If you search these forums, you'll find lots of discussion about them, but basically, they don't save money and don't do a better job of pressure control than a properly sized pump with a pressure regulator. We'd set this up as a three pump station - a small jockey pump sized to handle winter demands, a medium pump for irrigation season peaking (our irrigation peak is about 8 times our winter demand), and a large fire pump (our minimum fire flow requirement is 1,500 gpm). You may be able to get by with two pumps depending on how large your seasonal peaks are. But I wouldn't spin a 600 gpm 24/7 just to supply 25 houses. Even with a VFD, your annual energy efficiency won't be above 5% with that setup, and you'll tend to eat up the impeller due to recirculation cavitation.

I don't know what your proposed pump station looks like, but the cheapest ones we've installed have been using submersible pumps in a pitless adapter, with an above ground control panel. This avoids the cost of a pump building. The submersible pump motors have relatively poor efficiency, but again, electricity is pretty cheap, especially when we're only talking about 25 homes.

Good luck, and let us know what you decide.

 

[Clmsn08]

77JQX,

Very helpful post, thanks for the feedback!

I believe with my current sizing that I have two redundant pumps for about 80% of the flow scenarios, with one additional pump we will be able to handle 99% of the scenarios out there, the last 1% being fire flow.

Generator back up is on my list as something that needs to be installed.

I am leaning towards a Grundfos BoosterpaQ type booster pump skid. They use multi-stage vertical turbine pumps that create a lot of head using relatively small (5 hp) motors. We will have the 30 hp for the very high demands. If the project continues, we will install the new pump skid in the existing pump house. My main concern is that the flow created from the available head on the hydrants will be higher than the pump station capacity running wide open.

I assume your 1,500 gpm fire flow requirement is because your system serves a much larger population?

Again, thanks for the feedback. Great info!

Tyler

 

[77JQX]

Our 1,500 gpm fire flow is because the local fire authority uses the UFC, not ISO. It's not population dependent.

You have a sticky problem with that fire flow and the pump runout. I haven't ever throttled hydrants. Can you color code the hydrants for the maximum draw?

 

[bimr]

You need to think through the alternatives.

You most likely will be required to install a spare for the largest pump in the system. If you have the spare pump, you do not need to worry about whether you need to control the flow with orifice plates.

You will also need backup power facilities.

As I posted earlier, you are not eliminating the problem with stagnant water since you have only the normal 10 gpm flow. You still need to loop the system back to ensure that the water will not go stale. It is probably the lease expensive to loop it back as suggested by 77JQX without the pump station.

Most of the modern water systems have a minimum 1,500 gpm fire flow capacity. I assume that you are located in a rural area.

I have never heard of using orifice plates on fire hydrants either.

 

[CivilEngAus]

"I believe with my current sizing that I have two redundant pumps for about 80% of the flow scenarios, with one additional pump we will be able to handle 99% of the scenarios out there, the last 1% being fire flow."

Think of the worst scenario possibly where more than one structure is engulfed in fire during a power outage. How would you system work then?

An orifice plate on a fire hydrant is unlikely to gain approval.

As mentioned above, fix the problem by looping your water main back to the tank.