Short Cycling on a Bladder Tank Delivery Setup

Short Cycling on a Bladder Tank Delivery Setup

Posted by: Steve Samudio on Jul 23, 2020 10:01:04 AM

Short Cycling

Sometimes you may come across your delivery pump with bladder tank short cycling.  What does this mean?  Short cycling is when your pump is coming on and off in very short intervals rather than a normal cycle (default is 2 second on delay followed with a 5 second off delay).  A bladder tank will help reduce the on/off cycling of the pump. 

Bladder Tank

Inside the tank is a rubber bladder mostly shaped like the metal tank itself.  This bladder is charged or filled with ambient air, much like a bicycle tube, and is set at a pressure according to the tank size and specs.  As the delivery pump runs, water is pushed through the plumbing and delivered to the applications, and at the same time, fills water into the tank.  Once the applications close, the pump still pushes water into the bladder tank and does so until the desired pressure (typically about 30-40psi) is achieved.  This pressure is determined by the pressure switch, located on the side of the bladder tank.  Once an application opens down the line, the water from the bladder tank is pushed out via the bladder that is charged with air, until the pressure switch activates to run the pump.  We also have on and off delays built into our system to minimize short cycling even further.

 
Here are some possible causes of short cycling:

Leaking pipes or applications

If the applications we are feeding with the bladder tank delivery system have a leak, the pressure will continue to drop and trigger the pressure switch to run the pump.  Depending on the leak, the pump could cycle many times in a minute, or maybe once or twice in a minute.  With all applications off and the delivery pump off, the pressure gauge on the system should hold pressure.  If you see it is dropping off, it is possible there is a leak down the line on the applications or on the suction line that is described below. 

Bladder Tank

Bladder is popped or damaged within the tank.  If the bladder (rubber type balloon bag) is popped or does not hold the air charged in it, the bladder tank is now just a large metal pipe in the water flow.  Instead of having the few seconds of water being sent from the bladder, the pump will kick on immediately (after the built-in delay) to deliver water. 

Check valve failure

If the check valve on the suction line has failed, the pump will never be able to build up pressure to satisfy the pressure switch. This is because the water will be pushed backwards into the holding tank instead of being checked or stopped by the check valve. 

Check valve location

The check valve on the suction line should be located at the inlet of the pump.  While most applications use a suction rated hose, the pressure from the pump can make the hose bulge or possibly leak which can lead to short cycling from pressure drop.  If your suction line is hard plumbed, you may not have this issue, but we still recommend the check valve at the inlet to minimize any possible pressure drop. 

Delay settings

Although our systems are default of a 2 second on delay and a 5 second off delay, changing these seconds can have a dramatic effect on the performance or cycle duty.  Having to little of off delay may result in premature pressure switch signals or having no on delay can have a same effect.  Contact PurClean to verify you have the default and correct settings on your PLC. 

Pump Impeller

It is possible that the pump impeller is damaged and may not have the capability of building pressure to meet the needs of the pressure switch.  There are many different pump models with different curves, but the simplest of test would be a visual test or inspection on the applications it is feeding.  Do the applications look weak or do not spray like they used to?

Pressure Switch

First off, the pressure switch is a normally closed switch.  This means the contacts are closed with no pressure applied.  While there is no pressure in the line, the signal from the pressure switch would tell the PLC to run the pump.  Once pressure is applied or built up to the desired pressure, the pressure switch contacts will open and the lack of signal from the pressure switch will tell the PLC to stop the pump. 

If the pressure switch has failed or does not correspond with the proper pressure settings, this can result in short cycling.  If your desired pressure is to feed applications at 35 psi, the “on” pressure for the pressure switch should be somewhere around 30 psi.  With that set, you will want to have your “off” pressure around 40 to maybe 45 psi.  This allows the pressure switch to start the pump before all the water has left the bladder, and also allows the pressure to build up a little higher than operating pressure to prevent a premature “off” signal (lack of signal) from the pressure switch.

Although these are some of the most common causes of bladder tank pumps short cycling, there are possibilities of other reasons.  If you need further assistance, please contact our Technical Support Department by calling 800.882.8854 or emailing techsupport@purclean.com.

Steve Samudio

Steve Samudio

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