CONTROLS FOR WATER SUPPLY SYSTEM 2

[CJACKC]

I HAVE A PROJECT IS A WATER SUPPLY SYSTEM FOR A SCHOOL. I BELIEVE IT SUPPLIES IRRIGATION AND "OTHER?". IT CONSISTS OF A WELL PUMP THAT SUPPLIES 2 VERY BIG TANKS ABOUT 30 FEET IN DIAMETER. 3 PUMPS PUMP WATER OUT OF THE BIG TANKS INTO A SMALLER PRESURIZED TANK ( WITH AIR COMPRESSOR) THE SCHOOL WATER SYSTEM IS SUPPLIED FROM THE PRESSURIZED TANK. I HAVE PHOTO'S OF THE SYSTEM, BUT THE EXISTING CONTROLS ARE NOT CLEAR. THERE APPEARS TO BE A PRESSURE SWITCH AND A WARRICK CONTROL SYSTEM. TWO OF THE PUMPS ARE 15HP, ONE PUMP IS A 75 HP FIRE PUMP, ALL HAVE CHECK VALVES. THE CONTROL IS AN OLD RELAY SYSTEM. THE OWNER WANTS A NEW CONTROL SYSTEM WITH PLC. CAN ANYONE TELL ME HOW IT IS LIKELY THAT THIS SYSTEM WORKS, AND WHAT CONTROL ITEMS ARE NEEDED?

 

[byrdj]

Google found this

http://www.stacon.com/products/hydropneumatic_tank_control_system.htm

 

[JJPellin]

It sounds a little bit like the systems we would use for fire water. A typical installation has a smaller pump called a jockey pump that runs regularly. There is a second larger pump that only comes on at high demand. I don't know the exact set points, but I think the basic logic works like this. There are pressure switches that start and stop the pumps. If the system pressure drops below 80 psi, the small pumps kicks on. If the pressure increases above 100 psi, the smaller pump kicks off. This results in the smaller pump getting most of the starts and stops during normal low usage times. However, if the pressure continues to drop with the smaller pump on line, at perhaps 60 psi, the larger pump kicks on. The switch that starts the larger pump stops the smaller one so they never run in parallel. You have the added complexity of the pressure tank and air compressor. I am not sure how to set the pressure switches that start and stop the air compressor without conflicting with the logic that starts and stops the pumps. I don't know what a Warrick control is. But I would control your system as follows: The compressor would start and stop based pressure in the small tank. It might kick on at 60 psi and kick off at 80 psi. The pumps would be controlled on level in the tank. In the same manner as our fire water system, you use the small pump as much as possible to limit starts of the large one. At 50 percent in the tank, the small pump kicks on and runs until the level reaches 60 percent. If the small pump is running but the level continues to drop, the large pump will start at a tank level of 40 percent. It might be possible to match the curves well enough to allow both large and small pumps to run at the same time, but I would prefer to kick the small pump off when the large one starts. If the level increases to 50 percent, the large pump kicks off and the small one kicks on again. The water pressure delivered is held fairly constant and level controls determine usage and run the smallest pump that can meet the demand. You could add a second step in the level controls to go from one small pump to two small pumps to one large pump, if this made sense.