Chemical dosing pump skid (packaged duty/standby dosing system for water treatment)

01

Work out the pump rate from flow, dose and solution strength

Florida DEP's operator formula sheet gives the chain a dosing skid has to deliver: feed rate in lbs/day = dosage (mg/L) × flow (MGD) × 8.34 lbs/gal; for sodium hypochlorite, gallons of solution = pure chlorine required (lbs/day) ÷ (purity as a decimal × 8.34 lbs/gal); and chemical feed pump setting (% stroke) = desired feed flow ÷ maximum feed flow × 100%. It also gives chemical flow (gpm) = volume pumped ÷ (pumping time × 60), the basis of a drawdown check. [1]

Editorial calculation (metric form of the same arithmetic, with an assumed solution strength): 50 m³/h of water at a 2 mg/L dose needs 50 × 2 = 100 g/h of active chemical; with a solution assumed to contain 120 g/L of active chemical, the pump must deliver 100 ÷ 120 ≈ 0.83 L/h. Use the strength stated on your supplier's certificate, not the assumption. [1]

Batch dosing works the same way. The City of Winnipeg's specification for a metering pump skid at its North End Water Pollution Control Centre requires each pump to discharge 11 to 27 mL of product per batch, which it states corresponds to 3 to 7 mg/L in a 3,800 L batch; the plant control system starts the pump at the start of the batch sequence and the pump injects the volume needed, then shuts off. [2] Editorial calculation: 3 mg/L × 3,800 L = 11.4 g and 7 mg/L × 3,800 L = 26.6 g, matching the 11–27 mL range at about 1 g of product per mL. [2]

Trility's dosing standard skids are selected by pump size (listed as DDA7.5, DDA12, DDA30, DME375 or DME940), by capacity from virtually zero up to 940 L/h for duty/standby or 300 L/h for duty/duty, and by chemical group — group 1, for example, covers sodium hypochlorite, all acids to 96% solution and general chemicals. [3]

Give the supplier the chemical and its strength, specific gravity and viscosity, water flow range and dose range (or batch volume and dose), injection-point pressure, suction arrangement, duty/standby or duty/duty, control signals and site ambient range.

02

Lay out the skid: duty/standby pumps, protection and calibration

Trility lists the key components of its two-pump standard skids: a fully enclosed cabinet as a chemical barrier, duty/standby dosing pumps, a rigid PVC suction manifold with in-line strainers, a pressure relief valve on each pump, a PVC pressure loading valve that keeps the dose rate accurate and prevents siphoning, a drain/flush system, a calibration cylinder to test pump settings, a pressure gauge with protector, and a pre-wired termination panel for pump input and output signals. Dampeners, check valves, a flow meter and a dilution line are options. [3]

The City of Winnipeg specification spells out the same items as purchase requirements: each pumping system includes the metering pump, a pressure relief valve, a pulsation damper if the pump is a diaphragm type, pressure and back-pressure valves, a drip tray and the piping; each pump discharge gets a relief valve and a back-pressure regulating valve sized for maximum pump flow with an adjustable range. It sets the duty points — maximum back-pressure 100 kPa before the back-pressure valve, maximum suction lift 2.5 m with the drum empty, and maximum differential pressure 135 kPa — and a pumping accuracy of ±5%. [2]

Site conditions enter the specification too: an ambient range of −40 to 35 °C, heat tracing with a thermostat on the enclosure if needed, 120 V motors, and a gearbox with manual speed adjustment. The spare parts list names two diaphragms (diaphragm pump) or four hoses (peristaltic pump), check balls and seats, gaskets and O-rings, and a diaphragm each for the back-pressure and relief valves. [2]

Editorial calculation: the Winnipeg duty points leave 135 − 100 = 35 kPa between the stated maximum back-pressure and the maximum differential pressure across the pump. [2]

03

Duty/standby, duty/duty and drum-fed batch dosing skids

Ask each supplier for a P&ID of the skid, the list of valves and instruments on each pump line, and the materials of every wetted part for your chemical.

04

Pumps, calibration cylinder, relief and back-pressure valves on the skid

05

Dosing rate, pressure rating and wetted parts of the skid

The pump rate comes from flow, dose and solution strength; the chemical decides the wetted materials.

06

Send the chemical, dose and skid data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Chemical: [name; strength; SG; viscosity]
Dose: [mg/L range; water flow or batch volume]
Pumps: [duty/standby or duty/duty; type; rate]
Pressure: [injection point; suction lift]
Skid: [relief; back-pressure; damper; calibration cylinder; strainers; cabinet]
Control: [signals; flow-paced or batch]
Site: [voltage; ambient; heat tracing]

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