API 570 Injection Point Circuits: TML Placement and Frequency

Short answer: API 570 treats each injection point as its own inspection circuit. The circuit runs a short distance upstream of the injection nozzle and downstream to a defined limit past the flow changes. Thickness measurement locations (now called CMLs) go on the fittings, at the expected impingement point of the injected fluid, along long straight runs and at both circuit limits. The code's recommended maximum thickness interval for injection points is 3 years. That is tighter than the class-based intervals for the rest of the piping, and it is still capped by half the remaining life.

This guide explains how that works in practice: how to set the circuit boundaries, where to put the TMLs, which NDE methods find the localized attack typical of injection points, how the frequency relates to the rest of the circuit, and what the record has to show. It is written in our own words from API 570 and its companion documents. The circuit-limit and placement details below are taken from the 4th edition (2016) and earlier editions. API 570 is now in its 5th edition (February 2024, with Addendum 1 in 2025). Confirm every number against your licensed current copy before building it into an inspection plan. Intervals are always the owner's and the authorized piping inspector's decision under the code and the jurisdiction.

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What API 570 counts as an injection point

Direct answer: an injection point is where a relatively small quantity of material is injected into a process stream to control chemistry or another process variable. Examples are corrosion inhibitor, neutralizer, wash water, antifoulant, caustic or oxygen scavenger. A mixing tee where two process streams join is not an injection point under the code's definition, though it may deserve similar attention.

The definition matters because it decides which lines get the special circuit and the tighter interval. Typical refinery and petrochemical examples include wash water upstream of a hydrotreater effluent air cooler, neutralizing amine or filming inhibitor injection into a crude unit overhead, caustic injection into crude charge, and chemical injection into gas plant streams. Upstream and midstream facilities have their own versions, such as methanol or glycol injection for hydrate control and inhibitor injection into gathering lines. Whether those lines fall under API 570 depends on the code boundary at your facility. Our guide to the API 570 and pipeline code boundary covers that question.

Process mix points sit just outside the definition. When two process streams of different temperature, composition or phase meet, the mixing zone can corrode, crack or suffer thermal fatigue just as badly as an injection point. API 570 leaves mix points to the owner's judgment. NACE (now AMPP) Standard Practice SP0114, Refinery Injection and Process Mix Points, covers both. It replaced the older NACE technical report 34101. Many owners therefore circuit significant mix points the same way as injection points. That is good practice, not a code mandate.

Why injection points get their own circuit

Direct answer: injection points concentrate damage in a small zone. The injected fluid can be corrosive before it fully mixes, can hit the opposite wall, can flash or condense, and can cause temperature swings. The rest of the circuit may corrode slowly while the metal just downstream of the nozzle thins fast. Averaging those two behaviours hides the worst point.

The damage mechanisms that drive this are described in API RP 571. Our API 571 damage mechanisms guide covers them in depth. The common ones at injection points are:

• Localized corrosion from incomplete mixing. Concentrated chemical, or water that has not yet dispersed, attacks the wall near the nozzle and along the bottom of the line.
• Impingement and erosion-corrosion. A jet from a poorly designed or damaged quill can strike the far wall or the first elbow.
• Under-deposit and salt corrosion. Wash water that is too little or poorly distributed can leave ammonium chloride or bisulfide salts, which are wet and corrosive.
• Thermal fatigue. A cold injection into a hot line, or intermittent injection, cycles the wall and nozzle area and can crack it.
• Environmental cracking. Caustic injection, for example, raises the risk of caustic stress corrosion cracking where temperature and stress allow.

Most of these are localized. That is the key to both TML placement and method selection. A handful of spot readings can miss the damage entirely, so API 570 asks for more coverage at injection points than on an ordinary run of pipe.

Setting the circuit limits

Direct answer: in the 4th edition (2016), and in earlier editions, the recommended upstream limit is the greater of 12 inches (300 mm) or three pipe diameters upstream of the injection point. The downstream limit is the lesser of the second change in flow direction past the injection point, or 25 feet (7.6 m) beyond the first change in flow direction. Confirm these limits in the current edition.

A few practical points make the limits work on real isometrics:

• Upstream is short on purpose. The injected fluid mainly travels downstream. The upstream allowance catches back-mixing and the nozzle area itself.
• Downstream follows the flow changes. Elbows and tees are where poorly mixed fluid impinges. The limit is set by counting changes in direction, then applying the 25-foot cap, so the circuit length varies with the routing.
• The circuit can extend further. Earlier editions note that the circuit may need to extend to the next piece of pressure equipment where damage is known to continue, for example into an exchanger inlet. Owners with history of damage beyond the standard limits often extend the circuit on that basis.
• Each injection point gets its own circuit. Two injection points close together may overlap. Draw them as separate circuits so their rates are not blended, or justify the combination in writing.

Mark the circuit on the inspection isometric with the injection nozzle, the quill, the limits and every CML. Without that drawing, the next crew cannot find the same points, and the corrosion rate becomes guesswork. Our page on piping circuit inspection and CMLs explains why repeatability is what turns readings into a trend.

Where to place TMLs on an injection point circuit

Direct answer: put TMLs (CMLs) on the appropriate fittings within the circuit, at the expected impingement point of the injected fluid, at intermediate points along longer straight runs, and at both the upstream and downstream circuit limits. At each location, cover the full circumference and the areas where flow turns, not one spot.

API 570 changed its terminology from TML (thickness measurement location) to CML (condition monitoring location) in later editions. The idea is the same, but a CML can also cover other damage, such as cracking or CUI. In practice, injection-point CMLs are placed like this:

Two habits improve the data. First, record the orientation of each reading against a fixed reference, such as top-of-pipe or flow direction, so the same quadrant is measured next time. Second, record what was measured at each CML: the minimum from a scan, an average of a small grid, or a single reading. API 570 accepts the minimum or an average at an examination point. But mixing those approaches between campaigns invents corrosion rates that are not real.

If the quill is removable, many owners inspect it whenever it is pulled. A worn or broken quill changes the jet pattern and can move the impingement point. The CMLs then need checking against the new pattern.

How often: injection points vs the rest of the circuit

Direct answer: API 570's table of recommended maximum intervals lists injection points at 3 years for thickness measurement. External visual inspection follows the piping class. The rest of the piping uses class-based thickness intervals, which are longer. The governing interval is still the lesser of the table value and half the remaining life calculated from the measured corrosion rate.

These are the values stated in API 570's interval table and used on our Class 1 interval guide. Confirm them, and any footnotes, in the current edition. Several points govern how the 3-year figure is applied:

• It is a maximum, not a target. If the injection circuit's corrosion rate gives a half-remaining-life shorter than 3 years, the shorter interval governs.
• The circuit can be inspected more often than the line it sits in. A Class 2 line may have a 10-year thickness interval while its injection circuit is measured every 3 years or less. Keep the injection circuit separate in the plan, so its interval does not get dragged out to match the parent line.
• Extent can be wider at the periodic inspection. Earlier editions called for a more extensive examination of the area immediately upstream and downstream of the injection nozzle during each periodic inspection, beyond the routine CML readings. The required extent has been worded differently across editions and secondary sources disagree. Check the current edition for the distance required.
• RBI can change the interval. API 570 lets owners set intervals through a risk-based inspection assessment under API RP 580 and 581. Injection points are usually high-likelihood items, so an RBI programme rarely extends them. RBI is an engineering assessment the owner commissions, separate from NDE.
• Process changes reset the clock. A change in injection chemical, rate, temperature or nozzle design is a management-of-change item. It should trigger a review of the circuit and possibly an early inspection.

NDE methods that find injection point damage

Direct answer: API 570 prefers radiography and ultrasonic methods at injection points, including UT scanning or closely spaced UT grids. Spot UT alone is risky because the damage is localized. Profile RT, grid or scanned UT, and phased-array corrosion mapping give the coverage needed to find the minimum.

For most injection circuits, a sound approach is a mapping method at the nozzle zone, impingement point and first fitting, with spot or banded UT at the other CMLs. Add profile RT where insulation, small bore or geometry makes UT impractical. Measurement should follow a written procedure; ASME Section V and ASTM E797 cover manual UT thickness practice. Our article on UT thickness measurement requirements covers calibration and recording. For mapping, see corrosion mapping and phased array inspection services.

A worked example, described qualitatively

Take a wash water injection into a reactor effluent line upstream of an air cooler. The line is Class 2 on the plant's register. The authorized piping inspector sets up the circuit as follows:

1. Define the limits. Upstream, the greater of 12 inches or three diameters before the nozzle. Downstream, the circuit runs to the second elbow, which comes before the 25-foot point. History of fouling at the cooler inlet header leads the inspector to extend the circuit to the header, recording the reason.
2. Place CMLs. One at the nozzle zone, one at the predicted impingement area opposite the quill, one at each elbow (outside radius and cheeks), one on the straight run between, and one at each limit. Each is numbered on the isometric with orientation marked.
3. Choose methods. PAUT corrosion mapping on the nozzle zone, impingement area and first elbow. Grid UT on the other CMLs. Profile RT on a small-bore drain inside the circuit.
4. Set the interval. The table maximum is 3 years. The first campaign shows a short-term rate at the first elbow that gives a half-remaining-life of about two years. The inspector sets a two-year interval for the circuit, while the parent line stays on its class interval.
5. Follow up. The inspector raises a recommendation to review wash water rate and distribution with process engineering. A management-of-change trigger is added so any change to the injection prompts a circuit review.

The numbers will differ at your site. The method of working is what carries over: separate circuit, mechanism-led CMLs, coverage that can find localized damage, and an interval driven by the measured rate.

Records, mistakes and the regulatory overlay

Direct answer: the piping record should identify the circuit as an injection point, show its limits and CMLs on the isometric, and hold readings, rates, remaining life and the interval. The most common mistakes are spot readings in place of coverage, letting the injection circuit inherit the parent line's interval, and losing CML positions between campaigns.

API 570 lists what to record for each circuit. That includes whether it is a dead leg, an injection point or intermittent service, its corrosion rate and remaining life at the limiting point, and the maximum intervals that apply. Common mistakes we see in inherited data:

• Injection circuits never created, so the area is buried in a long Class 2 or 3 circuit.
• CMLs placed at convenient spots rather than at the impingement point and fittings.
• One reading per CML in a zone known for pitting.
• Readings without orientation, so the same quadrant is not re-measured.
• Quill changes or chemical changes not linked to an inspection review.
• Nominal thickness used as the baseline, which corrupts the long-term rate.

At OSHA PSM-covered facilities, 29 CFR 1910.119(j) requires inspection and testing following recognized and generally accepted good engineering practice, with frequency consistent with that practice and with operating experience. API 570 is widely used as that practice for piping. Some states add their own rules, and Canadian provinces regulate piping through bodies such as ABSA in Alberta and TSSA in Ontario, with CSA B51 as the base pressure-equipment code. Confirm what applies with your jurisdiction.

How Atlantis supports this

Atlantis performs the NDE that injection-point circuits need. Our ASNT-certified technicians, under ASNT Level III oversight, carry out grid and scanned UT thickness surveys, PAUT corrosion mapping, profile RT (through radiography crews licensed where the work is), guided-wave screening and surface methods at nozzles. We work to your circuit definitions and CML numbering and deliver data in your format. Results go to your API 570 authorized piping inspector, who remains inspector of record and sets the circuit, the CMLs and the interval. See piping circuit inspection and our API 570 inspection support. Ask for an injection circuit mapping quote; we reply within 24 hours.

FAQ

How does API 570 want me to select thickness measurement locations on injection point circuits?

Place TMLs (CMLs) on the fittings in the circuit, at the expected impingement point of the injected fluid, at intermediate points on longer straight runs, and at both circuit limits. Cover each location well enough to find localized attack, using grid UT, scanning or RT rather than single spots.

What is the recommended frequency for injection point TMLs compared to the rest of the circuit?

API 570's recommended maximum thickness interval for injection points is 3 years, against 5 years for Class 1 and 10 years for Class 2 and 3 piping. Half the remaining life still governs if it is shorter. Confirm the current table in your licensed copy.

What are the upstream and downstream limits of an injection point circuit?

In the 2016 edition and earlier editions: upstream, the greater of 12 inches (300 mm) or three pipe diameters. Downstream, the lesser of the second change in flow direction or 25 feet (7.6 m) past the first change. Extend further where damage is known to continue, and check the current edition.

Is a mixing tee an injection point under API 570?

Not by the code's definition, which excludes points where two process streams join. Many owners still circuit significant mix points the same way, following AMPP (NACE) SP0114.

Which NDE methods does API 570 prefer for injection points?

Radiography and ultrasonic methods, including UT scanning or closely spaced UT grids. PAUT corrosion mapping and profile RT are common choices because the damage is usually localized.

Can RBI extend the injection point interval?

API 570 allows intervals to be set by an RBI assessment under API RP 580/581. Because injection points tend to have high likelihood of damage, RBI rarely lengthens them much. The assessment is an engineering study the owner commissions.

Do injection quills need inspecting?

The quill controls where the fluid goes. Many owners inspect quills whenever they are removed and re-check CML positions if the quill was damaged or redesigned.

What is the difference between a TML and a CML?

TML stands for thickness measurement location. Later API 570 editions use condition monitoring location (CML), which covers thickness and other damage monitoring. On injection circuits the placement logic is the same.

Who decides the injection circuit limits and interval?

The owner's API 570 authorized piping inspector, with a corrosion specialist where the code calls for one. NDE contractors supply the data.

Planning the next round of injection-point readings? Talk to our Level III team about method selection and coverage.

Related reading: API 570 piping inspection code requirements.