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Flow Meter Liquid Control Strategies: Automated Batching Systems for Chemical Skids
Quick Answer: Use a Coriolis mass flow meter for chemical skid batching when recipe accuracy is tight or viscosity changes often. For conductive chemicals under 5 cP, an electromagnetic flow meter works well. For oils and solvents with stable viscosity, an oval gear meter gives a lower cost option.
Chemical skids are compact process units. They blend, dose or transfer acids, caustics, solvents, resins and surfactants. A batching system on a skid usually has a storage tank, a pump or gravity feed, a flow meter, a shutoff valve and a batch controller. The flow meter is the core feedback device. If the meter drifts, the whole batch drifts. That is why skid builders in Vietnam, Thailand, Indonesia and Saudi Arabia spend more time on flow meter selection than any other sensor.
Silver Automation Instruments has supplied flow meters to chemical skid builders for over ten years. We sell direct to OEMs and distributors. Our main products include Coriolis mass flow meters, electromagnetic flow meters, ultrasonic flow meters, vortex flow meters, oval gear flow meters, thermal mass flow meters, pressure transmitters and paperless recorders. This article explains liquid control strategies for automated batching on chemical skids.
Match the Flow Meter to the Chemical and the Batch Size
Here is the thing. No single meter fits every chemical. The first inputs are viscosity, conductivity, temperature, pressure and flow range. For acids and caustics with conductivity above 5 µS/cm, an electromagnetic flow meter with PTFE or PFA liner is a workhorse. It has no moving parts and handles aggressive fluids. For solvents, alcohols and light oils, conductivity may be below 0.1 µS/cm. An electromagnetic meter will struggle. An oval gear meter or a Coriolis meter is better.
Batch size decides line size. A small paint skid in Malaysia may use a DN15 meter. A resin transfer skid in the Middle East may use a DN50 meter. A large acid dilution skid in Chile may need DN80. We ask for pipe size DN, flow rate in kg/h or litres per hour, viscosity in cP and chemical name. With those four values we can recommend a meter that will hold repeatability.
Coriolis Mass Flow Meters for Batch Accuracy
Coriolis meters measure mass directly. They do not need density or temperature compensation for mass batching. They handle viscosity from 1 cP to more than 10,000 cP. They also report density and temperature. This helps the batch controller detect wrong raw material or air in the line. We have seen this on customer sites many times. A solvent batch pulls air because the drum emptied. The density reading drops. The controller stops the pump before a bad batch leaves the skid.
A typical chemical skid uses a DN15 to DN80 Coriolis meter from Silver Instruments. Accuracy is plus or minus 0.2 percent of rate or better. Repeatability is often 0.05 percent to 0.1 percent. The output can be pulse, 4-20 mA HART or RS485. Many batch controllers accept high frequency pulse for fast cutoff. HART is useful for setup and diagnostics. If the skid sits in a hazardous area, choose ATEX Zone 1 certification for the meter and the PT100 temperature sensor if one is used.
Electromagnetic Flow Meters for Conductive Chemical Batching
Electromagnetic meters are simple. The flow tube is empty except for the liner and electrodes. A magnetic field reads velocity. The meter converts velocity to volume. They are good for caustic soda, hydrochloric acid, ferric chloride and wastewater treatment chemicals. The chemistry must have a minimum conductivity. Most Silver Instruments electromagnetic meters need at least 5 µS/cm. Some special versions work down to 0.5 µS/cm but check with our engineers.
Volume batching with electromagnetic meters needs temperature correction. A 10 °C change in a solvent can shift volume by more than 1 percent. For water based chemicals the shift is smaller but not zero. Add a PT100 or use a flow meter with built in temperature measurement. The batch controller can then compensate volume to a reference temperature. We see this on acid dilution skids in Africa and Southeast Asia where daytime tank temperature varies by 15 °C.
Oval Gear Meters for Solvents, Oils and Resins
Oval gear meters are positive displacement devices. They measure volume by counting small chambers between two rotating gears. They work well for oils, solvents, resins
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Oval gear meters output pulse signals. Each pulse represents a fixed volume like 0.01 litres. A batch controller counts pulses and closes a valve at the exact fill volume. For a 200 litre solvent batch, a DN25 oval gear meter with 0.5 percent accuracy can fill within plus or minus 1 litre. A Coriolis meter can bring that down to plus or minus 0.4 litre but costs more. If your solvent price is high, the Coriolis upgrade often pays for itself.
Batch Controller Wiring and Cutoff Logic
Most engineers skip this part. The flow meter is only half the system. The batch controller must stop the valve at the right time. There are two common methods. The first is full speed fill then slow fill. The controller opens the main valve for 90 percent of the batch. Then it closes the main valve and opens a small trim valve for the last 10 percent. This gives high speed and high accuracy. The second method is valve throttling with a control valve. It is more expensive but needed for foaming chemicals like surfactants.
For pulse output meters, the controller counts pulses and stops instantly. For 4-20 mA HART meters, the controller reads flow rate and integrates volume. Pulse output is better for batching because it has less lag. If you use 4-20 mA HART for diagnostics, bring the pulse signal to the batch controller and the HART signal to a local display or handheld communicator.
A typical wiring setup from Silver Instruments is simple. Flow meter pulse out connects to batch controller input. Valve relay output connects to a solenoid valve. A PT100 connects to the temperature input if volume compensation is needed. Power is 24 V DC. For hazardous areas, use intrinsically safe barriers or ATEX Zone 1 rated instruments.
Calibration and Verification on Site
Some skid builders skip calibration after installation. That is a risk. The meter calibration from the factory is on water or a reference oil. Your chemical may have different viscosity, density or coating behaviour. Run a water calibration on site before chemical startup. Fill a known volume or weigh a known mass. Check the batch controller total against the reference. Adjust the K factor on the flow meter or the batch controller. We recommend a three point check at 20 percent, 50 percent and 100 percent of the batch size. We have seen a paint manufacturer in Thailand cut batch error from 1.5 percent to 0.25 percent just by this on site check.
Five FAQ for Chemical Skid Batching
What flow meter is best for a chemical skid that batches multiple products? A Coriolis mass flow meter is usually best. It handles wide viscosity and density changes. It also gives density data so the controller can flag a wrong product.
Can an electromagnetic flow meter batch solvents? No, unless the solvent has enough conductivity. Most solvents are non conductive. Use an oval gear or Coriolis meter instead.
Do I need a PT100 for batching? If you batch by volume and the temperature changes more than 5 °C, a PT100 helps correct volume. For mass batching with a Coriolis meter, you do not need it for correction.
What output is better for batching, pulse or 4-20 mA HART? Pulse is better for fast cutoff and totalizer counting. 4-20 mA HART is better for diagnostics and setup. Many systems use pulse for control and HART for maintenance.
What line size should I choose? Use DN15 to DN25 for batches under 100 litres. Use DN25 to DN50 for 100 to 1000 litres. Use DN50 to DN80 for larger skids. Flow range in kg/h matters more than pipe size alone.
Request a Quote from Silver Instruments
Tell us your pressure in bar, temperature in °C, pipe size DN, flow range kg/h, viscosity cP and conductivity µS/cm. We will recommend a meter for your chemical skid batching system. Contact Silver Automation Instruments at Tel: +86-25-68650347, Whatsapp: +86-25-52155837, WeChat: +86 15365082610. You can also check product pages at flow-meter.com.au. Our team serves skid builders and end users in Southeast Asia, Oceania, Latin America, Africa and the Middle East.
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