Dosing Tank Piping: How to Connect Make-up Water, Pump Suction, Overflow and Drain

Dosing tank piping comes down to five connections on the tank: make-up water enters from above, chemical leaves near the bottom toward the metering pump, the overflow sits high on the tank wall and drains freely, the drain is at the lowest point, and the relief valve return comes back into the tank. Four rules cover almost every case: no backflow into the water supply, no sludge into the pump, no valve on the overflow, and a drain that empties the tank completely. This guide covers the tank side only, for installers and operators taking over a system. The discharge line from pump to injection point is a separate topic.

Identify every connection before laying dosing tank piping

Before any pipe is cut, take the supplier's piping drawing and label every nozzle on the tank. If you need help reading the drawing itself, see dosing system piping connection. This article is about getting the dosing tank piping right on site.

The usual nozzles are make-up water inlet, pump suction outlet, overflow, drain and relief valve return. Polymer and PAC units that dissolve chemical on site also have a powder inlet and a mixer mounting. Packaged units arrive with all of these in place. If you are designing dosing tank piping around a bare tank, plan the nozzle positions before drilling.

Dosing tank piping for the make-up water inlet

Of all the dosing tank piping, this line carries the biggest safety risk. Bring potable or recycled water in through the top of the tank and keep the outlet above the highest liquid level, leaving an air gap. If the inlet pipe dips below the liquid surface, a drop in mains pressure can siphon chemical back into the water supply. That is the one mistake to avoid at all costs.

A typical inlet line has, in order, a manual valve, a strainer and a solenoid valve. Automatic make-up units add a flowmeter to set the dilution ratio. The solenoid valve follows the level switch: open on low level, close on high. If you use recycled water, clean the strainer often, because whatever passes it ends up on the pump check valve balls.

What if the supply pressure is high?

High pressure gives water hammer when the solenoid closes and splashes chemical around the tank. Fit a pressure reducing valve, or turn the outlet down with an elbow so the stream does not hit the surface hard.

Metering pump suction piping: the dosing tank piping that feeds the pump

Put the suction outlet on the lower side wall, a little above the tank floor, so sediment stays on the bottom instead of entering the pump. This matters most with PAC, lime slurry and anything else that settles.

Keep the line from tank to pump inlet short and straight with as few elbows as possible. Ideally the pump is flooded, meaning its inlet sits below the tank level so liquid flows into the head by gravity. Fit an isolation valve and Y-strainer for pump maintenance. If you have a calibration column, its three-way valve goes on this section.

Never make the suction pipe smaller than the pump inlet. A narrow line adds resistance, and viscous chemicals then under-deliver. Use unions so a pump can be swapped without cutting pipe.

Dosing tank piping for overflow and drain

The overflow sits on the upper tank wall, above the normal high level. Its job is to carry away water if the solenoid sticks open, so the tank never floods over the lid. Size it no smaller than the make-up line, and never put a valve on the dosing tank overflow, because a closed overflow is no overflow. Let it discharge openly into a drain channel instead of plugging straight into a sewer pipe, which brings odors and debris back up.

The drain is at the lowest point of the tank with a ball valve, normally closed, used for cleaning or changing chemical. Overflow and drain can share one drain header, as long as they join downstream of the drain valve.

Where should the relief valve return go?

Returning the relief valve line to the tank is the safest choice, ideally entering above the liquid so you can see when it is relieving. Returning to the pump suction line also works, but make sure the suction side can take that flow without drawing gas into the head.

Common dosing tank piping mistakes

Most dosing tank piping problems on site trace back to the same few errors:

• Make-up inlet below the surface, so chemical siphons into the water supply during an outage.
• Suction outlet at the very bottom, so sludge reaches the pump and the check valves stick.
• A valve on the overflow that someone closes, and the tank overflows through the lid.
• Rigid pipe forced against a PE tank wall. The tank expands and contracts, and the nozzle cracks.
• Pipe material not checked against the chemical. Sodium hypochlorite and sulfuric acid need PVC, PVDF or other materials confirmed for that service.

Distance from tank to injection point limits how far the lines can run, so plan the layout with where to install a dosing system. For the full make-up and storage process, read how a PAC dosing system works. Tank level signals come from level switches, and packaged units with all nozzles pre-fitted are under dosing skids.

Safety first: stop the pump, isolate power, empty and flush the tank before modifying any dosing tank piping, and wear gloves and goggles when opening acid or caustic lines.

Conclusion

Get the five tank connections right and dosing tank piping stays simple: make-up water above the surface with an air gap, suction outlet above the floor and short and straight to the pump, overflow high and never valved, drain at the lowest point, relief return back to the tank. Do not reduce pipe sizes, match materials to the chemical, and use unions for service. To size a tank and pump or check a site piping plan, send us the chemical, tank volume and injection pressure, and contact our engineers.

FAQ

Can overflow and drain share one pipe in dosing tank piping?

They can join a common drain header, but the overflow itself must have no valve and the drain valve must sit upstream of the junction.

Does the make-up line need a solenoid valve?

Not for manual batching. For automatic make-up it is required and should be switched by the level switch.

How high above the floor should the suction outlet be?

It depends on how much the chemical settles and on tank size. Leave room for sediment without letting the pump draw air. Supplier tanks usually come with the nozzle already placed.

Rigid or flexible hose for chemical dosing tank connections?

Rigid pipe for fixed runs, with a short flexible section or union at the PE tank and the pump head to absorb vibration and movement.

The drawing does not match the tank on site. What now?

Ask the supplier before connecting anything. Follow the nozzle labels from the supplier, not their position, because inlet and overflow are easy to swap by mistake.