Turbine Hydraulic System Flushing Procedure

Turbine Hydraulic System Flushing Procedure

1. Piping distribution flushing procedure
Prior to the connection of the piping distribution to hydraulic equipment the assembly organization is obliged to flush the piping in order to clear the pipe walls of any impurities. Individual construction modal points must be disconnected, hydraulic cylinders shortcircuited (bypass protection). During the flushing procedure the piping distribution must be interconnected to ensure maximum flow (turbulent flow) in every branch. Mineral oils HLP ISO VG22 are used as medium. The flow in the system being flushed must be so that turbulent flow is achieved (Re minimum 3000). During the flushing procedure all regulations stated in the documentation must be followed, as well as specific regulations applicable at the place of construction. The organization charged with the flushing has to carefully prepare the flushing procedure and
make sure to provide all preparations, tools and equipment needed for the flushing procedure in the scope and quality necessary for the operation of hydraulic equipment.
The hydraulic circuit will be flushed by an external source provided by the assembly organization. Usually a mobile unit with several (two to three) consecutive filters, the
filtration ability of which gradually elevates, is used as the external source. This unit is temporarily connected to the flushing circuit. It is not admissible to flush hydraulic circuit by the supplied hydraulic unit.

1.1 Cleanliness
In performing the work absolute cleanliness must be maintained since impurities may lead to a failure and thus limit the safe function of the components. Prior to releasing the piping connections and installed elements, the surrounding area needs to be cleaned. All open spots should be blanked off to prevent impurities from penetration into the system.
1.2 Material needed
- Material for setting up flushing points - see Bill of material 6-H-56 188
- Flushing liquid
- External flushing unit
- Flushing filtration inserts (by thy type of the unit used)
- Material for taking oil samples to carry out purity analysis
- Instrument to count particles for oil purity assessment, or a microscope with adequate equipment
- Nitrogen for blowing out residual flushing oil from the manifold Parameters of external flushing unit 2x7,5kW – see scheme, drawing No. 3-H-45 711:
- Total flow 92 l/min (2 pumps with flow 46 l/min)
- Max working pressure 10 MPa (100 bar)
- Pressure filter on the output – filtration capacity 5 μm or 10μm (can be exchanged)
- Waste filter on the entry – filtration capacity 5 μm or 10μm (can be exchanged)
- Flow meter at the entry to check the flow in the flushed circuit
- Thermometer for flushing oil temperature monitoring
- Heating element, approx. 1 kW
- Tank capacity 100 l

Parameters of external flushing unit 1x7,5kW – see scheme, drawing No. 3-H-56 189:
- Flow 87 l/min
- Max working pressure 4,5 MPa (45 bar)
- Return filter – filtration capacity 5 μm
- Hand pump 29cm3, max. 32 MPa (320bar)
- Thermometer for flushing oil temperature monitoring - can be used from HPU TG
- Tank capacity 100 l

1.3 Flushing liquid
The flushing liquid used must be miscible with the working pressure liquid, with the materials used in the hydraulic plant, without disturbing the seals. We recommend that mineral oil HLP ISO VG22 is used as the flushing medium. To fill the external flushing unit and the piping itself about 150 litres off flushing mineral oil will be needed (the anticipated unit tank capacity is 100 litres).

1.4 Flushing temperature
Heating of flushing oil shall provide more thorough removal of impurities after the assembly; these impurities may get stuck on the inner surface of the piping walls. Besides that, the viscosity will decrease which will simplify the occurrence of turbulent flow needed to ensure a good quality flushing procedure. The temperature of the flushing oil is associated with turbulent flow, see par.

1.6. The temperature of the flushing should be kept within +50°C až +60°C. Any temperature above +60°C causes oil oxidation and, on the other hand, any temperature below +50°C increases viscosity and higher flow of the flushing unit pump is needed to achieve the required turbulent flow. Rapid heating of the flushing oil can be achieved by means of a relief valve (safety valve) of the external flushing unit. Subsequent adjustment of the right flow during the flushing procedure usually facilitates the prescribed oil temperature. In case the ambient temperature is low, the flushing unit tank has to be heated by the heating element.

1.5 Flushing time
The actual degree of purity inside the system is determined not only by the time of flushing needed to achieve a certain degree of purity but also by how much the purity
degree decreases when the flushing unit disconnects from the circuit. No generally applicable information can be provided regarding the time of flushing. During
the flushing procedure it is recommended to take oil samples regularly at least every 4 hours, and let them analyse for purity. A suitable sampling point is e.g. the return tube before the mounted back filter. Based on the result it has to be decided whether the flushing procedure must be terminated (once the target purity is achieved), or whether it should continue. The minimum flushing time of every branch is 4 hours. The flushing procedure is completed once the liquid purity class 4 is achieved according to NAS 1638 or 15/13/10 according to ISO4406.

1.6 Flow for flushing – turbulent flow
Most efficient flushing is achieved in turbulent flow in the piping system. Turbulent flow is achieved if the velocity of the liquid flow is relatively high, and/or the viscosity of the liquid is relatively low. The flow in the system that is being flushed must be so that turbulent flow is achieved – Reynolds number is at least 3000. Reynolds equation:
Re= w x d x 1000 => 3000
v
w = flow velocity (m/s)
d = inner tube diameter (mm)
v = kinematic viscosity (cSt)
For mineral oil HLP ISO VG22 the below table applies, stating the minimum flows for achieving turbulent flow, Re=3000: Tube 33,4x3,38 26,7x2,87 21,3x3,73 17,1x3,2 Viscosity
Inside diameter (mm) 26,64 20,96 13,84 10,70 cSt
Flow at 60°C (l/min) 42 33 22 17 11
Flow at 50°C (l/min) 57 45 29 23 15
Flow at 40°C (l/min) 83 65 43 33 22
Flow at 20°C (l/min) 208 164 108 84 55
The dimensions of the piping correspond with the actual execution.

1.7 Flushing procedure
Interconnection of individual flushing circuits is shown in the scheme, drawing number 3- H-56 187. Upon the successful performance of the pressure test according to the Pressure test program 6-H-56 184 the first flushing circuit is connected as per the scheme of drawing number 3-H-56 187 page 1. Once this circuit is flushed and based on the flushing liquid purity evaluation (see par..8 and 1.9)., the flushing circuit switches accordingly with the scheme in drawing number 3-H-56 187 page 2 and the flushing continues; then the circuit according to page 3 is interconnected up to the circuit on page 8. Flushing of pipes ø12x2mm (leakage piping) will be not done. Those pipes will be mechanically cleaned and blown out with compressed nitrogen.

1.8 Purity tests
In order to determine the purity of the service liquid the following methods can be applied: - Counting and separation of impurity particles using an electronic instrument, e.g. ARGO-HYTOS OPCom, OPCount, HYDAC FCU... This method allows online connection of the instrument to the flushed circuit and continuous monitoring of oil
purity. - Microscopic tests. This method allows evaluation of division of solid substances by size and rough determination of their composition. (e.g. metal particles, abrasion from sealing, fibres, etc.). This method can be applied to perform fast examination at the place but requires that the surroundings is cleaned when the analysis is performed. It applies generally that proper sampling is essential for the oil analysis result.

How to correctly take an oil sample:
- Take an oil sample during the flushing procedure when the circuit has a prescribed temperature.
- Clean carefully the minimess hose with a textile that does not discharge fibres.
- Screw down completely totally the hose to the minimess pipe adapter.
- Drain at least 200 ml of oil before taking the sample.
- Take the required amount of oil without handling the minimess pipe adapter. The amount of takes sample should be at least 50% and maximum 80% of the volume of
clean bottle for the sample.
- When taking the sample tilt the bottle in the angle of 28° to 30° in order to reduce the risk of penetration of contamination from ambient air. By tilting the bottle the inlet hole gets smaller.
- One sufficient amount is taken, put the bottle aside whilst the oil flows out permanently.
- Without waiting close the bottle with the sample.

1.9 Successfulness criteria
Hydraulic systems of turbine regulation require purity class at least 5 according to NAS 1638. The flushing procedure has to ensure (for longer life) purity at least one degree better, i.e. class 4 according to NAS1638, i.e. 15/13/10 according to ISO4406.

1.10 After the flushing procedure
When the flushing procedure is completed the oil residues from the piping system has to be removed. First drain it at lowest points. Then push out the residues by means of inert gas – Technical nitrogen – class 4.0 ( 99.99 vol. %). Special attention has to be paid to maintain cleanliness when dismantling the flushing circuit and subsequent completion of the hydraulic system. After that overall control has to be carried out, i.e. the quality of assembly and correctness of circuit connection.

2. Staffing requirements
The flushing procedure shall be carried out and managed only by workers who are familiarized in detail with this program, and who are the holders of valid HS (Health&Safety), EP (Environment Protection) and FP (Fire Prevention) training certificates. Anticipated staffing:

- 2 workers (attendance of auxiliary units, assembly of temporaries, supervision over the flushing procedure, purity evaluation). Customer's workers can anytime perform the inspection.

3. Specific safety procedures Main principles of safe operation of hydraulic plant:

- do not overload hydraulic elements by higher pressure than those to which they are designed for
- the electrical parts of the hydraulic plant has to be protected from acting of the service liquid and radiant heat
- ensure perfect tightness of the hydraulic distribution system, particularly in hot plants
- if the service liquid flows out of the system clean the area immediately and provide for safe disposal of the leaked liquid in order to prevent consequent ecological accidents (adequate cleaning agents and special container for temporary storage of contaminated material have to be present at the workplace)
- hydraulic drive, hydraulic elements and piping have to be protected from acting of radiant heat and from mechanical damage
- in case of any modifications and repairs, always disconnect the drive of hydrogenerators and discharge the pressure from the system Even at high pressures hydraulic circuits are reliable and safe as long as all elements are suitably sized. Due to human error, material defect or fatigue a failure may however occur which could jeopardize the safety of workers, or damage the ambient area by oil. We can name examples of failures that are apparent.
- dripping oil
- destruction by parts of circuit loaded with pressure The oil dripping from an unscrewed joint or another spot that accumulates on the ground might cause a worker slip and fall, or damage the floor and, if penetrates in surroundings, it might contaminate big amount of water. If the parts loaded with pressure are destroyed there will be no explosion. If a small crack occurs the oil disperses into the neighbourhood in form of mist and if there is a bigger crack, usually the entire tank drains. Destruction of pressure hoses becomes evident by oil infiltration and often even by ripping the hose out of the tailpiece. In this case, the hose may bounce very abruptly and cause a severe mechanical injury. Leak of the service liquid may cause a large fire. As is clear from the aforementioned, the below principles have to be adhered to:
- In plants with high temperature sources a hardly incendiary liquid has to be used instead of oil.
- The machine room needs to be kept clean, and any spilled oil has to be filled in with sawdust or VAPEX absorbent, then sweep the floor and wipe up dry with a suitable
solvent.
- During the disassembly suitable containers have to be used so that oil does not run on the floor.
- Units and any hydraulic elements, including hoses and piping, have to be protected from any external mechanical damage and heat sources.
- If circuit is under pressure it is not advisable to approximate especially with one's face, to the proximity of the piping and the hoses.
- If oil penetrates the eye, the eyes must be flushed straight away with Ophtal or a similar suitable agent. If a non-flammable liquid penetrates the eye the person must
seek an eye specialist.
- In the area of the storage and in the proximity of units and actuators smoking and using open fire is not allowed.
- In case of any modifications or repairs, always disconnect the drive of pump, or at least relieve and disconnect relevant circuit.
- Electrical equipment must comply with the regulations.
- All hydraulic systems and circuits must be protected from overload by a pressure valve.
- A person must be appointed to be in charge of maintenance and adjustment of the unit.

4. Waste disposal
Residues of flushing oil shall be disposed of according to the instructions stated in the safety data sheet. Flushing oil shall be taken by the contractor for further use.

5. Conclusion
Flushes of turbine regulation high pressure hydraulic piping system need to be paid special attention. It was demonstrated that clean hydraulic systems are able to operate much more efficiently and longer without any failures than contaminated systems. One thing is common to all hydraulic systems; no failure has ever been caused by oil that would be too pure.