Table of Contents

  1. Vessels
  2. Equipment
  3. Inspection Windows
  4. References

1. Purpose

The purpose of this section is to give further guidelines to the Company Construction Specialists, for the procedure to be followed for flushing operations prior to, and or, in preparation for hydrotesting. In the event of a conflict with specific project requirements, the latter shall prevail.

The purpose of the activity, flushing, is to remove loose rust, mill scale and construction debris from piping systems before commissioning.

2. Definition

As a pre-commissioning activity, flushing can be defined as the act of pumping water, in sufficient quantity and velocity, through a given section of pipe or piping, completely filling that section of pipe or piping, such that the flow of water through the system will forcibly remove any loose rust, mill scale or construction debris from the system.

3. Scope

The scope of work for flushing shall be limited to the identification of, and the definition of the system to be flushed, the preparation of detailed flushing plans for each system, supervision of the execution of the activity, followed by inspection and acceptance of the completed work.

Flushing is already mentioned elsewhere within the overall context of piping works (see Reference 1613.1).

4. Guidelines

There are some basic guidelines which should always be followed when flushing lines with water:

  • Detailed flushing plans shall be prepared for each system to be flushed, these plans shall include information on the time of flushing, type of flushing and advice on how the activity should be performed.

  • Flushing shall be carried-out under the supervision of the commissioning engineer, who will inspect the cleaned system and sign acceptance for such systems.

  • Flushing shall be performed through fully open flanges and/or open pipe ends.

  • Flushing through smaller openings, such as drains or vents, is not acceptable.

  • Flush from the highest elevation to the lowest, where possible following the normal flow direction of the system.

  • When flushing is completed, de-water the system and dry as soon as possible.

5. Safety

At the time when flushing of piping would normally begin there will still be a relatively large number of construction personnel working on the plant or unit. It is absolutely imperative that consideration of these people, or anyone else working in the vicinity, is planned.

Although the lines are being flushed with water, the purpose of the exercise is to remove solids from inside the piping system, it is these solids which can cause serious injury. An area around the exhaust of the system being flushed must be kept free. One person from the crew executing the flushing activities must be present at the discharge end at all times during the flushing exercise.

If flushing is being executed in winter time there will be a serious possibility of freezing, not only within the piping itself but also freezing of discharged water onto the ground. This is a possible danger to people and must be remembered when planning the exercise.

6. Flushing Criteria

The following information shall be contained in the detailed flushing plans prepared, per system, for each system to be flushed. The following general guidelines will apply.

  • Medium/Quality

Stainless steel lines - potable/demineralized water, with chloride content < 20 ppm.

Carbon steel and alloy lines - potable water/plant water, or any other prior approved flushing medium.

  • Duration

The flushing duration for an "open ended" system is such that for a period of > 5 minutes "clear water" (no debris or other foreign materials) must be discharged.

Closed circuit loop flushing until the circulating pump strainer is free of foreign materials (debris) and the circulating water is clear.

  • Quantity (volume) and Pressure

Flushing of piping systems/lines should only then be executed when a sufficient quantity (volume) of water is available, at a suitable pressure.

The following table provides a guide for piping systems/line flushing, using standard 2½" fire hoses with 3" fire hose couplings, at approximately 3 to 7 bar pressure.

Pipe System/line size

2½" Fire Hose No. Required

up to 6”

6” to 10”

2 - 3

3 - 4

The practicality of flushing large diameter lines with water reduces as the line size increases, in reality lines over 250mm. or 10" diameter, should not be designated as requiring full bore flushing, these lines can usually be vacuum pigged, hydro-jetted or visually inspected.

Safety Note: If a plant fire water distribution system is to be used for the supply of testing and/or flushing water, the system shall be physically separated from the remaining, and still live, fire protection systems

7. Exceptions

The following systems and/or lines shall not be flushed:

  • Process lines in which the presence of water would cause operational difficulties.

  • Instrument air lines: these lines will be service tested and blown with air. The air used must be free from oil, use existing instrument air supplies where possible or mobile compressors fitted with oil filtering devices.

  • Nitrogen systems: these will be service tested after blowing with instrument air.

  • Steam supply systems: after hydrotest the water shall be drained and the system blown with steam, when available.

  • Condensate systems: after hydrotest the water shall be drained and the system blown with steam, when available.

  • Steam tracing: steam tracing supplies will be service tested with steam (remove or bypass the steamtraps).

8. General Requirements

Ensure that all piping systems/lines are thoroughly flushed with clear water or other approved medium and are free of rust, sand, construction debris, obstructions, etc.

Flushing operations, shall be executed as defined in the detailed flushing plan.

Ensure that a detailed flushing plan has been prepared, and that it contains all the required information and drawings.

Verify that sufficient flushing or circulating water is available, tankers, hydrants, storage tanks or pond etc.

Maintain a pre-commissioning log for all lines and systems flushed, blown, drained and dried, where necessary, and that an acceptance sheet has been signed.

Ensure that sufficient pump capacity (volume and pressure) is available for flushing.

Ensure that the required temporary piping, hose connections, blind flanges, temporary gaskets, temporary strainers, dummy spools, etc., for flushing, and blowing are installed.

Ensure that all temporary fittings are removed after completion and acceptance of the flushing/testing operation.

Check that the piping systems/lines have been re-installed with the correct materials and equipment, as required by the specifications.

The flushing/testing water can be discharged to the sewer systems only after agreement with the relevant parties.

When the piping system/lines discharge point for flushing or testing water would be into a closed sewer system, or below paving, a temporary drainline shall be installed to allow discharge into an open sewer or onto a paved area sloping towards an open sewer system.

Verify that any "damage" caused by the flushing/testing operation has been rectified.

9. Piping

Verify the type of mesh and cleanliness of temporary strainers prior to and during the flushing/testing operation.

Verify that, in-line components such as control valves, orifice plates, positive displacement meters, turbine meters, rotameters, etc., are isolated or removed prior to flushing.

Verify that the instruments isolated or removed from the piping system/line are stored and or protected properly.

  • Control Valves with Bypass Line

Remove the control valve, flush first through the bypass line and then flush through the opened block valves.

  • Control Valves without Bypass

Remove the control valve. Install temporary spool piece leaving the piping system/line unimpaired, or flush through the open ended block valves.

  • Instrument connections (transmitters, gauges, local indicators, analyzer traps, etc.)

Remove the analyser probes, close the line block valve(s) and disconnect the instrument leads at the block valve(s). Never fill the instruments with water.

  • Positive Displacement Meters

Remove the meter and strainer prior to flushing and hydrostatic testing. (The PD meter bodies should be filled with light oil to prevent corrosion.) Install temporary spool piece or flush through the opened block valves.

  • Turbine Meters

Remove the turbine meters and strainers prior to flushing and hydrostatic testing. Install temporary spool piece in place of turbine meter, or flush through the open ended block valves.

  • Orifice plates and other flow elements such as flow nozzles, pitot tubes, magnetic flow meters, etc.

Verify that said instruments and components are removed or isolated prior to flushing/testing.

  • Safety Valves

All safety valves must be spaded off or removed prior to flushing and/or testing.

Verify, prior to the flushing operation, that the flushing plans, schematic, procedures, etc. do not endanger the installed instruments, equipment, etc.

Verify that the instruments are properly removed, disconnected, or isolated and stored.

Verify that temporary spool pieces are adequately fabricated and installed.

10. Vessels

Verify that lines do not flush through exchangers, flame arrestors or rotating equipment.

Flushing into tanks is allowed providing the tank is thoroughly cleaned after flushing.

11. Equipment

Verify that lines do not flush through exchangers, flame arrestors or rotating equipment.

12. Inspection Windows

Verify that flow inspection windows are marked on the drawings for insulating purposes (vessels, piping, etc.).

13. References


Document Number





Piping Systems Inspection and Testing