How to Specify a Chemical Injection Skid: Chemicals, Rates, Pressures and Pumps

8 hours ago

3 min read

At a glance. Specify a chemical injection skid one chemical and one injection point at a time. For each, give the dose rate range, the maximum pressure at the injection point and the chemical's properties. Those three decide the pump type (diaphragm or plunger metering pumps to API 675), the tank and wetted materials, and whether one multipoint skid can replace several single-point skids.

Which chemicals will the skid inject?

Chemical	Purpose	Dosing note
Corrosion inhibitor	Protects pipelines, tubing and vessels	Continuous, in ppm of the liquid or gas flow
Scale inhibitor	Prevents carbonate and sulphate scale	Continuous; sometimes downhole through a capillary line
Methanol or MEG	Prevents hydrates	High rates; often the largest skid on site
Demulsifier	Breaks oil–water emulsions	Upstream of separators and desalters
Biocide	Controls bacteria and reservoir souring	Batch dosing on a timer
Oxygen scavenger	Removes oxygen from injection water	Continuous, paced from the water flow
Drag-reducing agent (DRA)	Raises pipeline throughput	ppm dosing downstream of the pumps, with low-shear pumps

Give each chemical's safety data sheet to the vendor. Viscosity, freezing point, vapour pressure and compatibility decide pump heads, tank design, heat tracing and seal materials.

How are dose rate and pressure turned into a pump?

Convert each dose into litres per hour at minimum, normal and maximum flow of the stream being treated. The pump must deliver the maximum with margin and still meter accurately at the minimum, so the turndown matters as much as the top rate.

For pressure, take the highest pressure at the injection point, add line losses and a margin, and protect each discharge with a relief valve set below the rating of the pump and tubing. Plunger pumps reach the highest pressures, up to 10,000 psi. Hydraulically actuated diaphragm pumps are leak-free and preferred for toxic or hazardous chemicals. Pneumatic pumps suit sites without electrical power.

One skid per point, or a multipoint skid?

A pump head per injection point gives the most independent control. A multipoint skid uses one pump and a pressure-compensated flow controller on each line, holding every point at its own rate against its own back-pressure. Where several wells or tie-ins need the same chemical, a multipoint skid cuts footprint, weight and maintenance. Compartmented tanks let one skid carry several chemicals.

Tank, materials and controls

Tank – SS 316L as standard, sized for the refill interval, with level gauge and transmitter, calibration column and fill and drain connections.
Blanketing – nitrogen blanketing for oxygen scavenger and flammable chemicals.
Materials – SS 316L wetted parts, with alloys for aggressive chemicals and NACE MR0175 / ISO 15156 for sour service. Check elastomers against every chemical.
Heat tracing – for wax inhibitors, pour-point depressants and cold sites.
Controls – local panel or PLC, flow-paced dosing from a 4–20 mA signal, and remote monitoring of level, pressure and pump status.

Standards that normally apply

Metering pumps to API 675, piping to ASME B31.3, pressure vessels to ASME Section VIII where applicable, sour-service materials to NACE MR0175 / ISO 15156 and electrical equipment to IEC 60079 for the hazardous zone.

Specification checklist

Each chemical, with its safety data sheet
Dose rate at minimum, normal and maximum flow for each point
Maximum injection pressure at each point
Number of injection points, now and planned
Power available: electric, pneumatic or solar
Tank capacity and refill interval
Hazardous-area zone and ambient temperature range
Control, telemetry and dose-verification requirements

Tech Inc. chemical injection skids

Tech Inc. builds single and multi-pump skids, multipoint skids, multi-compartment skids, solar-powered skids, portable units and odorant dosing skids, with water-injection and DRA versions, and supplies the injection quills and access fittings that go with them. Send your chemical list and injection data to mail@techincresearch.com.

Frequently asked questions

What does API 675 require?

API 675 covers controlled-volume (metering) pumps. It calls for steady-state accuracy within ±1% of rated flow over a 10:1 turndown, which is why it is specified for chemical injection.

How can the dose rate be checked on site?

With a calibration column on the pump suction: the operator closes the tank valve, runs the pump from the column and times the fall in level. Flowmeters on each line give continuous verification.

Why use an injection quill?

A quill releases the chemical into the centre of the flow, where it mixes quickly, rather than onto the pipe wall, where concentrated chemical can attack the metal.