How to Read a Dosing System P&ID: Lines, Valves and Instrument Symbols

Reading a dosing system P&ID gets much easier if you do it in three passes. First check the legend so you know which symbols this drawing uses. Then follow the chemical the way it actually flows, from the dilution water inlet through the mixing tank and the metering pump to the injection point. Finally read the instrument bubbles and dashed signal lines to see what controls what. This guide is for operators reviewing their first skid drawing, process engineers coordinating with a design firm, and buyers who want to check scope against a quote.

What a dosing system P&ID shows, and what it does not

P&ID stands for piping and instrumentation diagram. A dosing system P&ID shows equipment, piping, valves, instruments and control relationships, but it is not drawn to scale and does not show real positions. Some vendors also issue a simplified process flow diagram that shows only the main path and how the pump is controlled.

A general arrangement or nozzle drawing is a different document. It shows connection sizes, locations and orientation for installation. If you are working out where the water inlet, chemical outlet, overflow and drain lines go, see our guide to reading the four lines on a skid piping drawing.

Use the dosing system P&ID for process and control review, and the piping drawing for installation. Mixing them up causes most of the confusion on site.

Reading a dosing system P&ID: legend first, then flow

Symbol conventions vary between vendors and engineering firms, and the same valve can be drawn two or three ways. The legend sheet, usually the first page or a box on the right side, removes the guesswork.

Then find the starting point. On most skids it is the dilution water line feeding the tank. A mixer sits on the tank, the solution leaves through a bottom outlet to the pump suction, and the pump discharge runs to the injection point. Walk that line one symbol at a time and ask what each item is there to do.

Process lines vs signal lines

Thick solid lines are usually main process piping, thinner solid lines are secondary lines such as drains, overflows and relief returns. Dashed lines, or lines with short hatch marks, normally carry electrical or instrument signals, not liquid. Line labels next to the pipe often give size and material, so PVC, PVDF or stainless steel is visible right on the dosing system P&ID.

Valve symbols on a dosing system P&ID

Valves are the most common symbols on the sheet. Typical conventions:

1. Isolation valves such as ball and gate valves are drawn as two triangles meeting at their tips, with a dot or bar to show the valve type.
2. Check valves add a bar or arrow to that symbol to show the permitted flow direction. Install one backward and the pump will not deliver.
3. Pressure relief valves are usually an angle valve symbol with a spring, with an outlet line returning to the tank or the pump suction.
4. Back pressure valves look similar but sit on the discharge line, holding steady pressure on the pump and preventing siphoning.

Relief and back pressure valves look alike but do different jobs. Tell them apart by location and tag, not by shape alone.

Instrument tags in a dosing system P&ID

Instruments appear as small circles, called bubbles, containing letters and a loop number. The first letter is the measured variable, the following letters describe the function:

• L is level: LT is a level transmitter, LS a level switch, LSL a low level switch.
• P is pressure: PI is a local gauge, PT a pressure transmitter.
• F is flow: FI is flow indication, FT a flow transmitter.
• Analyzers for pH, ORP or conductivity often start with A and carry a text note.

A bubble with no line through the middle is typically field mounted; a line through it means the reading is shown on a panel or in the control room. The legend on your dosing system P&ID has the final word.

Signal lines tell you who controls whom

Follow the dashed lines. The level switch reports to the control panel, and the panel connects to the metering pump and mixer. Once those few lines are clear, you know which pump stops on low level, whether the mixer stops too, and when the fill solenoid opens.

What reviewers most often miss on a dosing system P&ID

• Where the relief valve return goes. Back to the tank is safest; a relief line dumped to a floor drain means chemical can end up on the floor.
• Isolation valves on both pumps in a duty and standby pair. Miss one, and servicing one pump means shutting down dosing.
• A calibration column on the pump suction with a changeover valve, which you need to verify pump output.
• The order of the pulsation dampener and back pressure valve on long discharge lines. Our metering pump installation diagram guide shows the usual accessory sequence.

For gassing chemicals such as hypochlorite, also look for degassing and vent lines; our chlorine dosing system diagram walkthrough covers that case. Valves, dampeners and back pressure valves shown on the drawing can be matched in our dosing accessories range, and complete packages are listed under chemical dosing skids.

Safety note: before opening piping or replacing a valve shown on the drawing, stop and isolate the pump, close upstream and downstream valves, relieve pressure, and wear the right protection for acids and caustics.

Conclusion

Read a dosing system P&ID in a fixed order: legend, flow path, instruments and signal lines. Use the P&ID for process and control, and the piping drawing for installation. During review, focus on check valve direction, relief valve return, standby pump isolation, the calibration column and back pressure valve location, because mistakes there are the hardest to fix after installation. If you want a second pair of eyes on your dosing system P&ID, or need pumps and accessories matched to it, send us the drawing with the chemical, flow and pressure data and contact our engineers.

FAQ

Is a dosing system P&ID the same as a process flow diagram?

Not quite. A process flow diagram shows only the main path and major equipment. The P&ID adds every valve, instrument and control link.

How are instrument tag numbers assigned?

Letters describe what is measured and what the device does. Numbers usually follow area or loop numbering, and instruments in the same loop share a number. The project's tagging rules decide the details.

Can I install from the vendor's simplified schematic?

It is not a good idea. A simplified schematic does not show connection sizes or orientation, so installation should follow the piping drawing and site dimensions.

The drawing shows a back pressure valve. Can we leave it out?

It depends on injection pressure. If the pressure differential across the pump is low or the injection point sits below the pump, leaving it out invites siphoning and overfeeding, so install it.

Can I sketch a simple dosing P&ID myself?

Yes. Use standard valve and instrument symbols, draw the equipment, valves, instruments and signal lines along the flow path, and add a small legend.