Flow Switch Won't Activate (Stay Engaged)

1/22/2019

Have a paddle flow switch for a wet system that is located on the first story riser. The riser serves the entire 3-story building.

When flowing an inspector's test on the first floor or even the third floor, the flow switch paddle only stays engaged for 10-12 seconds before cycling back and forth. Without keeping the engaged position, the alarm won't activate.

Has anyone come across this before? What remedies could help the flow switch operate properly?

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12 Comments

Bob Jeske

1/22/2019 10:14:58 am

I have seen this occur with backflow preventers installed back in the 1990's. The issue was that the backflow preventer's checks had too stiff springs installed on the backflow preventer. The manufacturer of the backflow sent us new check valves with softer springs in order for the flow switch to not cycle.

PETE

1/22/2019 11:18:13 am

Is there a pump with a pressure reducing valve on the discharge that is feeding this wet system?

JOE

1/22/2019 11:23:39 am

Nope, I was told it's just a city feed wet system.

PETE

1/22/2019 11:29:02 am

Here is what I am thinking. It ma be useful to read the Principles of Operation section on this cut sheet. https://www.systemsensor.com/en-us/Documents/WFD_Series_Manual_I56-0459.pdf

The pressure reducing valves that are commonly put on pump discharges have a "break-in" flow rate, beneath which they do not operate. That is why for a typical 8" pump discharge, a parallel flow path has to be made with 2" pipe and a 2" PRV with control valves for maintenance in order to accommodate a low flow rate scenario and still achieve the pressure reducing function. That's why it's an optimal scenario to just have the pump properly sized from the beginning, because this takes up a lot of real estate and it's a pain in the butt. But, this is the proper solution.

Another solution that is not proper, but more of a work-around... shorten the time delay on the flow switch. As you can in the linked document, the delay is not additive (cumulative). It resets when the flow drops. But, understand that the delay is designed to minimize false trips, and by lowering the time delay... what happens is the valve seats on the system valves wear out over time. Then flowing a hydrant or some other system will create a pressure differential across that valve and water will flow causing false trips.

PETE

1/22/2019 11:31:17 am

With no pump, it sounds like a partially closed valve on the system is causing friction loss. A damaged valve stem will do the same thing.

Kent Wise

1/22/2019 11:54:33 am

What you are experiencing is caused by too much trapped air in your system. When you flow an inspectors test the pressure drops low enough for the clapper to open on the alarm valve. Once the clapper opens a column of water the diameter of your main riser pipe starts to flow up into the riser. The mass in motion of the water column causes it to compress the air in your riser. That compression pressure eventually exceeds the city water pressure causing the clapper to be forced closed. The pressure has to drop below the city water pressure by additional flow out the inspectors test till the whole process starts over again.

We have seen this happen a lot on large risers, particularly in storage facilities. Your solution is to remove the trapped air by either manual or automatic air vents on the upper parts of the riser.

Mike

1/22/2019 12:42:03 pm

These are all great comments and I have learned some valuable insights.

I'd go with the easiest option to lower the retard feature if water surge isn't a problem.

Doug

1/22/2019 07:52:02 pm

I would generally recommend the same thing except he stated the paddle only stayed down for about 12 seconds.
If he adjusts the retard down to that level the will probably get a lot of false alarms.

Doug Funk

1/22/2019 05:19:59 pm

Back in the mid to late 90's we had a similar problem on many systems. Mainly it was the Ames 2000 backflow preventers. The spring on one of the checks was to stiff. The paddle on the flow switch would cycle exactly like you described.
You can call the backflow maker and they can help you with it.

James Armentor

1/24/2019 06:56:28 am

I've run into this same issue. I ended up loosening the tension on the return spring to get the switch to work. There is a small flat head screw at the top of the switch that holds the spring.

PETE

1/24/2019 08:29:25 am

Bear in mind that modifying the switch in a way that is inconsistent with the O&M sheet will void the listing. If the switch you're using doesn't describe this procedure and you are going to do it anyway, you might consider using Locktite or super glue on the screw threads so it doesn't fall out completely.

Randall link

6/12/2020 07:08:11 am

This issue is NOT caused by air trapped in the system!
That is something the manufacturer of the backflow told me when I ran into this problem. They claimed it WAS not the backflow issue. They were wrong.

It was caused by the springs being too strong for the amount of city pressure supplied.

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Flow Switch Won't Activate (Stay Engaged)

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