Dosing System Piping: Reading the Four Lines on a Skid Drawing

Dosing system piping looks busy on a drawing, but it breaks down into four lines: water inlet, chemical discharge, overflow and drain. The inlet fills and dilutes the tank. The discharge runs from the tank bottom through the metering pump to the injection point. The overflow sits near the top and carries away excess. The drain sits at the lowest point for cleaning and emptying. Find those four, then read the valves and accessories on each, and very little gets missed. This guide is for installers and plant staff checking work against drawings.

The four lines on a dosing system piping drawing

A typical packaged dosing skid has a tank, mixer, metering pumps, relief valve, back pressure valve, pulsation dampener, level instrument and control panel on one base frame. Connection points are usually at the tank and frame edges, marked with size, material and purpose.

1. Water inlet: from potable or process water into the top of the tank, for filling and dilution.
2. Chemical discharge: from the tank bottom through a strainer and suction valve to the pump inlet; from the pump outlet through the dampener, relief valve and back pressure valve to the injection point.
3. Overflow: a side outlet just below the tank top, piped to a drain.
4. Drain: at the lowest point of the tank, with a valve, for cleaning and emptying.

Water inlet dosing system piping: valve and backflow protection

The inlet carries a manual valve or a solenoid valve controlled by tank level. Terminate it above the maximum liquid level so chemical cannot be drawn back into the water supply if pressure drops. Many drawings show a check valve or air gap here; do not leave it out.

Discharge: read suction and outlet separately

Keep the suction side of the dosing system piping short, large enough and with few bends, and make every joint airtight, because a small air leak stops the pump priming. Fit the strainer before the suction valve so it can be cleaned. On the outlet side, the usual order is pump outlet, dampener, relief valve, gauge, back pressure valve, injection point. If the order on site differs from the drawing, pressure relief and back pressure will not work as designed.

Dosing system piping: installation points

Keep overflow separate. Combining overflow and drain with other process lines to save pipe can cause backflow into the tank. Run overflow on its own to a drain, with a continuous fall and no low points that hold liquid.

Put the drain at the true low point. If not, a layer of sediment stays in the tank after every clean. PAC and polymer make this obvious quickly.

Return relief valve discharge to the tank. The relief valve return should go back to the tank or the suction side, with no isolating valve in that line.

Match dosing system piping material to chemical. Hypochlorite, acids, caustic and PAC need different materials. Choose UPVC, PE, PVDF or stainless steel per chemical rather than one material for the whole skid.

Protect outdoor lines. Insulate against frost and shade against sunlight where needed.

Water testing the dosing system piping

Once installed, test with clean water, line by line:

1. Inlet: open the valve, fill, and confirm the level instrument and solenoid valve work together.
2. Overflow: keep filling until it overflows, and check it drains freely with no backflow.
3. Discharge: start the pump, prime it, and check every suction and outlet joint for leaks.
4. Relief valve: briefly close the discharge isolation valve and confirm the relief valve opens at its set point, then reopen straight away.
5. Drain: open the drain and confirm the tank empties fully.

Safety: test with water first. Once chemical is in the system, stop the pump and relieve pressure before opening any joint, and wear gloves and eye protection.

After this, every line on the dosing system piping drawing has been proven, and later faults are easier to locate. For problems after start-up, such as no dosing or level interlocks failing, see integrated dosing system troubleshooting.

What people miss on a dosing system piping drawing

Connection orientation. The drawing shows which way each nozzle faces and its height. Piping brought in from the other side can block access to the level instrument or maintenance space.

Mixer removal height. Leave headroom above the tank to lift the mixer shaft out.

Drain location. Overflow and drain both need a floor drain or trench, planned at the civil works stage. Overlay the dosing system piping drawing on the civil drawing to catch clashes with trenches, sleeves and cable trays early.

Polymer make-up units add powder feed and ageing compartments, and the inlet water flow must match the powder feed rate; see PAM integrated dosing system selection. For matching tank, mixer and pump, read how to select a dosing system, and browse our dosing skids.

Conclusion

Read any dosing system piping drawing by finding the inlet, discharge, overflow and drain, then checking the valves and accessory order on each. Install overflow separately, put the drain at the low point, keep the relief return unisolated, and match pipe material to each chemical. Prove every line with a water test before chemical goes in. Send us your chemicals, flow and site layout, and contact our engineers for a connection layout and configuration advice.

FAQ

Can overflow and drain share one pipe?

They can join outside the tank and run together to a drain, provided the junction is below both outlets and the line does not connect to other process piping.

Why must the water inlet sit above the liquid level?

To stop chemical being drawn back into the water supply if pressure drops. This matters most on drinking water systems.

How big should the suction line be in dosing system piping?

Usually no smaller than the pump inlet connection, and larger for viscous chemicals. Short, large and straight makes priming easier.

Can the relief valve discharge go to drain?

It is not recommended. Chemical is wasted and may damage the drainage system. Return it to the tank or the suction side.

How should dosing system piping be laid out for easy maintenance?

Fit isolation valves before and after each pump, unions around strainers and back pressure valves, and route pipes clear of the mixer lifting space and level instrument.