13.5 System Flushing, Chemical Cleaning, Boil-Out, Steam Blows & Service Leak Testing

System Flushing, Chemical Cleaning, Steam Blows & Service Leak Testing

Core Trade Concept: A hydrostatic test proves the pressure boundary holds (Section 13.4). It proves nothing about what is inside the pipe. Module 34308, Testing Piping Systems and Equipment, also covers the cleaning steps that come between fabrication and commissioning — and on a new steam plant those steps exist because a single piece of weld slag driven into a turbine at operating velocity can destroy the machine.

---

1. The Commissioning Sequence

   HYDROSTATIC TEST  ->  DRAIN + DRY
            |
   MECHANICAL CLEANING  -> flushing, air blowing, pigging
            |
   CHEMICAL CLEANING    -> degrease, acid, rinse, neutralize, passivate
            |
   BOIL-OUT (new boilers)
            |
   STEAM BLOW           -> main steam, reheat, auxiliary lines
            |
   INITIAL SERVICE / TIGHTNESS CHECK  -> hot walkdown for leaks

Each step targets a different contaminant: flushing removes loose debris, chemical cleaning removes mill scale and oil, boil-out removes fabrication oils from a new boiler's water side, and the steam blow removes anything that survived everything before it.

---

2. Flushing and Mechanical Cleaning

• Flushing velocity is the whole mechanism. Flushing at a trickle just moves debris to a low point. The flush must reach a velocity at least equal to — preferably above — the design operating velocity so that particles that would move in service move now.
• Temporary strainers are installed at pump suctions and at critical equipment inlets, then removed after a defined clean period. The strainer is the evidence: what comes out of it tells you when the system is clean and, on a new system, tells you what the fabricator left behind.
• Air blowing is used where filling with water is impractical, though it recovers far less debris than a liquid flush at velocity.
• Never flush through equipment you are trying to protect. Pumps, control valves, turbines, orifice plates and flow elements are removed or bypassed with temporary spools during the flush and reinstalled afterward.

---

3. Chemical Cleaning

Chemical cleaning dissolves mill scale, rust and oil that mechanical means cannot remove. It is performed by a specialist contractor to a written procedure, and boilermakers install and remove the temporary circuits, blinds and connections that make it possible.

The safety envelope is the real hazard. Chemical cleaning fills a large system with hot inhibited acid, generates hydrogen gas from the reaction with steel, and creates a spent-solution disposal problem. Controls are the same discipline as any hazardous work: exclusion zones, continuous hydrogen and pH monitoring, vent management to prevent a hydrogen accumulation, full chemical PPE, emergency showers and eyewash stations at the circuit, and a written spill and disposal plan.

Boil-out is the water-side equivalent for a new boiler: an alkaline solution (commonly soda ash and trisodium phosphate, sometimes with a detergent) is circulated in the boiler at low fire for a period of hours to days to saponify and remove fabrication oils and preservatives before the unit is fired for real.

---

4. Steam Blows

A steam blow cleans the main steam, hot reheat and auxiliary steam lines by driving steam through them at a velocity higher than they will ever see in service, so that any scale or debris that would break loose in operation breaks loose now, into the atmosphere instead of into a turbine.

The cleaning force ratio

The mechanism is momentum. The blow must generate more disturbing force than normal operation:

$\text{CFR} = \frac{(\rho V^2)_{\text{blow}}}{(\rho V^2)_{\text{operating}}}$

A cleaning force ratio of 1.0 is the minimum, and blows are commonly designed to exceed that with margin. Because blow pressure is much lower than operating pressure, the density term drops sharply — so the velocity has to rise dramatically to compensate, which is why steam blows are so violent.

Target plates

Cleanliness is judged with a polished target plate (typically aluminum or brass) inserted in the steam path for a measured blow. Particles striking it leave visible impact marks, and the count and size of those marks are compared against the project's acceptance criteria. Acceptance normally requires consecutive blows with no indication above the specified size, so that a single lucky blow cannot pass the system.

Blow methods

• Puffing (intermittent). Pressure is built up, then released rapidly in a burst. The thermal shock of repeated heating and cooling helps break scale loose, and it is the classic method.
• Continuous. Steam flows steadily at the design blow condition, using less total energy and less thermal cycling of the piping.

Safety — this is the loudest and most dangerous commissioning activity on a plant

• Noise. An unsilenced steam blow can exceed levels at which hearing damage is immediate. Hearing protection, a silencer where practical, exclusion zones and advance notification to the site and to neighbours are all standard.
• Reaction force. The discharge exerts a large thrust. Temporary blow piping must be engineered and anchored, not braced with scaffold tube. Blowpipes have moved and killed people.
• Discharge path. The blow ejects steam and hot debris; the discharge is directed away from occupied areas, elevated, and the zone below and downwind is cleared.
• Thermal cycling of permanent piping. Hangers and springs must be set correctly for the blow condition, and spring travel is checked afterward; the blow is a genuine thermal event for the pipe.

---

5. Service and Sensitive Leak Testing

Not every test is hydrostatic:

• Initial service leak test. Certain systems are permitted by ASME B31.1 to be examined for leaks during their initial operation, with the system at operating pressure and temperature and a documented visual walkdown. This is the standard approach for systems where hydrostatic filling is impractical.
• Sensitive leak test (bubble test). Low-pressure gas with a bubble-forming solution, used for tightness rather than strength — for example on casing, ductwork and vacuum boundaries.
• The hot walkdown. Flanged joints relax as the system heats. Many specifications require a hot bolting pass after the system reaches operating temperature, performed by a qualified crew with the line at pressure, following a written procedure — because retightening a hot flange is itself a hazardous operation.

---

6. Realistic Trade Scenario: A Failed Target Plate Traced Back to Fabrication

On a new cogeneration unit, the first three main steam blows all foul the target plate, and one recovered fragment is identified as a piece of weld backing ring.

Working backwards:

1. Backing rings were used on several shop butt welds where the drawing permitted them but the commissioning specification for turbine-connected piping did not.
2. One ring was not fully fused and broke free under the first blow's thermal cycling.
3. The line had passed hydrostatic testing without a hint of the problem — because a hydrostatic test loads the boundary and says nothing about the bore.

Consequences and correction. Additional blows are run until two consecutive plates are clean, which costs schedule and fuel; the fabrication records are reviewed for every joint where a backing ring was permitted; and the specification is corrected to prohibit backing rings on any line that discharges to a turbine. The general lesson is the one that opens this section: a clean hydrostatic test is not a clean system, and the bore is a separate quality problem with its own tests.