Small Flow Meter for DEF Dosing Systems

Background

A DEF (Diesel Exhaust Fluid) dosing equipment builder in Ontario, Canada reached out to Silver Automation Instruments for a small flow meter to monitor dosing rate on a SCR (selective catalytic reduction) skid. Their engineer had a tight spec sheet ready, which made the selection process quick.

DEF, also sold as AUS32, is a 32.5% aqueous urea solution used to reduce NOx emissions in diesel exhaust after-treatment. It is not aggressive like an acid, but it is corrosive to several common metals. That single fact drives most of the material decisions in a DEF flow meter, more than the flow rate itself.

Customer Requirements

The inquiry came with a full requirement list, which we have reproduced below without the customer's contact details.

Why an Oval Gear Low Flow Flow Meter

Small oval gear flow meter for DEF Dosing

At 2.0 L/min or below, most flow technologies stop working well. Electromagnetic meters need a minimum velocity and a conductive fluid; DEF is conductive enough, but a DN15 mag meter at 2 L/min gives almost no signal to work with. Vortex meters need turbulence, which a flow this small will not generate. Coriolis meters can do it, but the price jumps sharply for a duty this light, and a full mass flow measurement is more than a dosing skid usually needs.

A positive displacement (oval gear) meter is built for exactly this range. The LC-M series low flow liquid meter measures down to 0.5 ml/min at plus or minus 0.5% of full scale, so a 2.0 L/min duty point sits comfortably inside its working band rather than at the ragged edge of it.

Model Selection: LC-M9-SS

Two models in the LC-M line could technically cover 2.0 L/min. The LC-M6 tops out at 100 L/H, which is 1.67 L/min, too close to the customer's stated maximum to leave any margin. The LC-M9 covers 6 to 600 L/H, or roughly 0.1 to 10 L/min, which puts 2.0 L/min well inside the range rather than near the top.

We recommended LC-M9-SS, the all stainless steel version, over the aluminum-body LC-M9-AL. Here's the thing: DEF attacks aluminum, zinc, and copper alloys over time, forming deposits and eventually pitting the metal. A stainless body and stainless gear set sidestep that problem entirely, which is why we steered the customer away from the AL and PPS-gear options and toward full 316L wetted parts.

Where the Standard Product Does Not Match the Request

Good technical selling means saying what does not fit, not just what does. Three points needed a straight answer instead of a workaround.

• Process connection: the LC-M9 ships with a G1/4" BSPP thread, not 1/4" NPT. A customer wanting NPT needs either a BSPP-to-NPT adapter fitting or a custom-machined connection, and we asked the customer to confirm which route works for their skid design before ordering.
• Seal material: EPDM is not listed in the standard LC-M datasheet. Seal compound needs direct confirmation from our engineering team against the exact DEF concentration and operating temperature before this can be called out on a quote.
• PVDF, PEEK, or ceramic internals: the LC-M line's standard wetted internal options are 316L stainless steel or PPS. Neither PVDF nor PEEK nor ceramic rotors are standard on this model. For this duty, 316L gears already handle DEF well, so this did not block the sale, but we were direct with the customer that we were not simply relabeling their spec sheet back to them.

We also do not hold CE  certification for the LC-M series. DEF compatibility here is based on the well-established industry practice of using 316L stainless and PPS in AUS32 service, not on a formal certificate. Customers running certified after-treatment systems should confirm this against their own qualification requirements.

Recommended Configuration

Based on the requirements, the configuration below covers the application. Model code follows Silver Instruments' standard build: series and shell, gear material, pressure class, temperature class, explosion-proof code, and options.

Application Notes for DEF Dosing

DEF dosing loops are usually low-flow and precision-sensitive, since dosing too little urea under-treats NOx and dosing too much wastes fluid and can leave deposits downstream. An oval gear meter's pulse output feeds directly into a PLC or dosing controller as a totalized or rate signal, which is why the Modbus RTU and 4-20 mA combination worked for this application. We keep seeing this pairing on emissions after-treatment skids across North America and Europe, not just for DEF but for ammonia water dosing on industrial SCR systems too.

One practical point worth flagging for anyone speccing a similar system: DEF crystallizes below about minus 11°C and degrades above roughly 30°C over extended exposure. The LC-M9's minus 40 to 80°C rating covers both ends comfortably, but heat tracing or an insulated enclosure is still worth considering if the skid sits outdoors through a Canadian winter.

Frequently Asked Questions