Sodium Hypochlorite Dosing System: No Flow, No Protection

The field problem: blocked hypochlorite dosing lines

The call comes from the field: the hypochlorite dosing lines are blocked. Something white and chalky has built up inside them. Flow to one or more users has dropped or stopped entirely. The electrochlorination system is running, but some users are not getting any flow.

This is one of the most common operational problems on offshore EC systems, and it is almost always preventable. The white material is not scale, it is not corrosion, and it is not a sign that something has gone wrong with the electrochlorination chemistry. It is a predictable byproduct of the electrolysis process itself: calcium carbonate and magnesium hydroxide particles that form during normal EC operation and settle out of suspension whenever flow velocity in the dosing line is insufficient to keep them moving.

H2O's PEPCON Systems Division has supplied electrochlorination packages since 1968, with installations in every major offshore oil and gas producing region worldwide. Over the last decade we have carried out more than 20 offshore EC refurbishment and replacement projects. Blocked dosing lines are a consistent finding, and in nearly every case the root cause is the same: dosing headers and branch lines operating below the velocity needed to keep hydroxide particles in suspension.

The physics are well understood, the velocity threshold is defined, and the problem is entirely preventable at the design stage. For systems already in service, effective remediation options exist that do not require replacing a single foot of pipe.

What the white buildup actually is

When direct current passes through seawater in an electrolyzer cell, the intended product is sodium hypochlorite. The electrochemical reactions at the cathode also generate hydroxide ions (OH⁻) as an unavoidable byproduct of the hydrogen evolution reaction.

At the cathode surface, the local accumulation of hydroxide ions drives the boundary layer pH up to around 9.5 to 10. This is the range at which calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂) begin to precipitate from ions that are naturally abundant in seawater.

Both compounds are insoluble in seawater. They form as fine solid particles in the hypochlorite stream leaving the electrolyzer. They cannot be redissolved under normal operating conditions. They are not symptoms of a process upset or a chemistry problem. They form every time the EC system runs, in every seawater EC system, without exception.

Under adequate flow conditions these particles stay in suspension and travel harmlessly through the dosing lines to the injection points, where they discharge with the hypochlorite into the receiving seawater. The problem begins when flow velocity falls below the threshold needed to keep them moving. When that happens, they settle to the bottom of whatever low-velocity zone they encounter and accumulate there over time. The lower the velocity, the faster the buildup.

The amount of CaCO₃ and Mg(OH)₂ formed scales linearly with electrolyzer current, and therefore with the amount of hypochlorite the system is producing. But production rate is set by the chlorine demand of the users, so a facility cannot reduce fouling by deliberately underproducing without also failing to deliver the protection the platform requires. A line at adequate velocity stays clear no matter how much hypochlorite passes through it. Velocity, not production rate, is the variable that determines whether deposits form.

The velocity rule and its basis

Stokes' Law defines the terminal settling velocity of an individual particle in a fluid. For the fine CaCO₃ and Mg(OH)₂ particles that form at the cathode surface, individual Stokes' settling velocities are very low, in the range of fractions of a millimeter to a few millimeters per second for particles in the 10 to 50 micron size range.

The required pipe velocity is substantially higher because the relevant failure mode is not individual particle settling. It is settled bed formation. Once particles begin accumulating at a low-velocity point they aggregate and consolidate into a bed that is significantly harder to re-entrain than individual free particles. The appropriate framework for sizing EC dosing lines is therefore slurry transport theory, specifically the Durand critical deposit velocity: the minimum pipe velocity required to prevent a settled bed from forming in a horizontal pipe carrying a particle-laden fluid.

V_c is the critical deposit velocity in m/s; F_L is a dimensionless factor in the range of 0.9 to 1.2, with 1.0 applied as a conservative value for the fine dilute slurry conditions typical of EC dosing systems; g is gravitational acceleration (9.81 m/s²); D is the internal pipe diameter in meters; and S_s is the specific gravity of the solid particles relative to the carrier fluid (2.64 for CaCO₃ and 2.31 for Mg(OH)₂ relative to seawater).

• Below this velocity, particles settle and form a bed that grows over time.
• At or above this velocity, particles remain in suspension and are transported with the flow.
• It sets the design target used to size headers and branches to prevent white buildup.

What the numbers show

Design target: 1.5 m/s. For the pipe sizes most commonly used as EC dosing headers, 1.5-inch to 3-inch, the Durand critical deposit velocity for CaCO₃ falls between 1.30 and 1.59 m/s. A design target of 1.5 m/s is therefore appropriate. For pipe sizes below 1 inch, the critical deposit velocity falls below 0.75 m/s, which is why 0.75 m/s is used as the active settling threshold.

Pipe diameter matters. Smaller pipes have lower critical deposit velocities. For half inch and three quarter inch pipe, the Durand V_c for CaCO₃ is 0.71 and 0.82 m/s respectively, and for Mg(OH)₂ it falls to 0.64 and 0.73 m/s. A short terminal branch line in small pipe is therefore more tolerant of reduced flow than a 2-inch or 3-inch distribution header at the same velocity.

Flow requirement increases with pipe size. A 2-inch header needs nearly 12 m³/hr to reach 1.5 m/s. A 3-inch header needs more than 25 m³/hr. Many offshore EC systems produce total flows well below these values. A modest EC system producing 4 to 6 m³/hr routed through a 2-inch header is operating at roughly one third of the design target velocity, and deposits are forming.

The PDF includes the full Durand chart by pipe size and a minimum flow rate table for Schedule 40 and Schedule 80 PVC.

Why this problem is so common: a design coordination gap

On most offshore projects the electrochlorination system is specified and procured as a package unit. The EC vendor supplies the electrolyzer, power supply, degassing tank, controls, and hypochlorite dosing pump. The dosing distribution system, meaning the headers, branch lines, valves, and injection points that carry hypochlorite from the EC package to each user, is typically designed and sized by the EPC contractor or the platform engineering team as part of the wider piping scope.

Process piping design for most offshore systems is optimized to keep velocity low. That is the correct approach for the vast majority of offshore process lines. EC hypochlorite dosing lines are the exception. They have a velocity minimum, not just a velocity maximum. When conventional low-velocity piping logic is applied to an EC dosing header, the result is a pipe that meets every normal design criterion and will still accumulate hydroxide deposits in service.

This is not a question of fault. It is a scoping gap that can be closed by including velocity verification as a standard step in both the original design review and any replacement or refurbishment assessment. The EC system vendor is in the best position to provide that input.

Where fouling occurs in practice

Electrolyzer cells

CaCO₃ and Mg(OH)₂ deposit directly on cathode surfaces during electrolysis. This is unavoidable and is managed by periodic cleaning with EC cell cleaner on a defined maintenance schedule.

Degassing tank floors

In residence-time degassing tanks, hydroxide particles settle to the tank floor if velocity through the tank is low. A common and effective remedy is to ensure the hypochlorite inlet enters the tank below the waterline and tangentially to the tank wall. This induces a swirl that keeps the tank well mixed. A top-entry inlet that drops flow straight into the tank does not produce this mixing effect, and settlement is the predictable result.

Dosing line headers and sub-headers

This is the primary fouling location in offshore EC systems. As discussed in our companion paper, The Like-for-Like Trap, the EC system must maintain mass balance at all times. The total flow through the main header always equals the total flow produced by the EC unit. What changes between operating scenarios is how that flow is distributed across the active branch lines.

When fewer users are active, the flow in sub-headers drops while the pipe diameter stays the same. Velocity falls proportionally. Consider a sub-header serving five users at 2.0 m/s. With only two users active, velocity drops below 1.0 m/s. With one more user offline it may fall below the 0.75 m/s settling threshold.

Branch lines and injection points

Terminal fouling at injection nozzles and strainers is often the first visible symptom, but the injection point is usually not the source. Particles that accumulated upstream are dislodged by a flow change and concentrate at the first restriction they meet. Clearing the nozzle without addressing the upstream velocity condition will result in the nozzle blocking again.

Shock-dosed lines: a special case

Lines that serve users who are only shock dosed are stagnant between dosing events. When the dosing valve closes, suspended particles settle, and over repeated cycles the settled layer builds. The correct mitigation is a seawater flush before the dosing valve closes at the end of each event.

Consequences on an offshore platform

• At the injection point: flow to one or more users is restricted or lost, and biofouling protection for those users is compromised.
• At the platform level: in warm offshore waters, marine growth establishes rapidly. Protection loss measured in days to weeks is enough for organisms to colonize cooling water circuits, firewater systems, and lift pump caissons. Once established, marine growth cannot be cleared by restoring dosing. It requires mechanical intervention. This is why a working marine growth prevention system depends on dosing lines that stay clear.
• At the maintenance level: in long offshore dosing runs there is no practical way to recirculate cleaner. Once lines reach full blockage, mechanical clearing is typically the only realistic option.

Remedies

At the design stage or during replacement

• Size all dosing headers and branch lines to maintain 1.5 m/s under all anticipated operating scenarios, including minimum flow cases. Any scenario producing velocity below 0.75 m/s in a 1-inch or larger pipe identifies a line that needs to be resized.
• Include seawater flush capability on all shock-dosed-only lines.
• Engage the EC system vendor in the dosing distribution design review, not just the package scope.

Seawater dilution: the most versatile field remedy

A seawater dilution valve introduces additional seawater flow into the hypochlorite stream downstream of the EC unit. This raises velocity through the headers and branch lines without touching the pipe.

Dilution does not compromise dosing. What determines whether a dosing target is met is the mass flow of active chlorine delivered, in kg/hr. Doubling the flow and halving the concentration delivers exactly the same mass of hypochlorite. The EC current setpoint does not change. Seawater dilution raises line velocity, eliminates the settling condition, and has no effect on the hypochlorite delivered to users.

Other options

• Pipe diameter reduction: the next smaller pipe size in a low-velocity section increases velocity at the existing flow rate.
• Flow consolidation: isolating spare or redundant branch lines concentrates flow into fewer pipes, a zero-cost operational adjustment where the layout allows it.
• Adding seawater flush to shock-dosed lines: recommended regardless of other remediation.

Use the EC Dosing System Simulator

A real offshore platform has multiple headers, sub-headers, branch lines, users and operating scenarios, each producing different velocities in different parts of the distribution system. H2O's EC Dosing System Simulator was built for this assessment. Sign up is free using SSO or an email address, and you can request a review with an H2O engineer from within the tool.

Before specifying a dosing line repair, run the velocity assessment. A line that is repaired without correcting the underlying velocity condition will accumulate deposits again.

Learn more about our electrochlorination systems for offshore biofouling control.

About H2O, Inc. H2O designs and manufactures industrial water treatment systems for offshore oil and gas and commercial marine applications. The company's PEPCON Systems Division has supplied electrochlorination packages since 1968, with installations in every major offshore oil and gas producing region worldwide.