How should the hydraulic system be cleaned?
1. General requirements for flushing
1.1. Achieve and maintain satisfactory initial cleanliness levels
a. Chemical cleaning and treatment of internal surfaces of the system (components, steel pipes and hoses).
b. Perform hot oil flushing to achieve the target cleanliness of the system.
c. Check and evaluate whether the cleanliness meets the requirements.
d. Disassemble the flushing circuit according to the following steps to prevent pollutants from entering the cleaned system.
Seal all element joints and flanges.
e. Implement daily maintenance to ensure cleanliness is within the control target range.
1.2. Maintain cleanliness after flushing
A. prevent new pollutants from entering the system.
b. Select the appropriate system filter. The new oil must be filtered before it can be added to the oil tank.
All new components and valve blocks connected to the system must meet the previous requirements. After component replacement, disassembly, or similar actions, fresh cleaning and hot oil flushing must be carried out after component replacement and disassembly.
1.3. Key points of flushing
a. Connect the flushing circuit in sequence.
b. Components that will be damaged by high flow rate, moisture, particles, or chemical flushing materials must be short-circuited (not connected to the flushing circuit) and cleaned separately.
c. The flow regulating element will increase the system’s pressure drop, so it should be short-circuited from the flushing circuit and cleaned separately.
d. Block, valve block, pump station, motor, oil tank, and assembly shall be flushed according to specific procedures.
1.4. Cleanliness requirements for hydraulic components
Some components and assemblies will be connected to the cleaned primary system, and the cleanliness of these components should be at least better than the cleanliness requirements of the system. The supplier of hydraulic components shall provide the cleanliness certificate of parts. If the component cleanliness certificate cannot be obtained from the component supplier, the system installation unit must clean these components according to specific procedures. However, if the supplier’s cleaning does not meet the requirements, the cleanliness certificate cannot be considered adequate.
Remarks: If anti-corrosion additives are incompatible with the hydraulic medium inside the components, 5% – 10% degreasing additives shall be added to the flushing medium to flush these components. Degreasing additives that do not damage the element seals must be selected.
2. System flushing preparation
2.1. Pipeline mechanical cleaning
Sizing of steel pipes – cutting, bending, welding and removal of scale and corrosion – chemical cleaning
Washing and hot oil flushing. The welded pipes shall be mechanically cleaned internally with plastic pigs. A plastic pig is a plug with a brush, scraper and roller on its outer surface. It moves in the pipeline under oil pressure to achieve the cleaning effect. This ensures that the pipe and pipe flange are smooth and removes welding slag and external particles.
All steel pipes and hoses must be carefully inspected and purged with highly filtered industrial compressed air. In this way, large particles generated during steel pipe cutting and hose joint installation can be removed.
2.2. Short circuit of essential components
To effectively clean all components in the system and avoid damage to sensitive parts, relevant parts must be short-circuited or removed from the flushing circuit. Each element or subsystem must be thoroughly cleaned to meet the cleanliness requirements.
When flushing the piping system, disconnect all components and systems to limit flow and details that may be damaged during flushing.
3. Chemical cleaning and hot oil flushing
Each flushing circuit must be connected to obtain a specific flow rate and Reynolds number. At the same time, all elements, pipes, and joints have sufficient fluid pressure and avoid particles remaining in prohibited areas and dead corners. The flow and pressure of flushing must also be taken into account.
3.1. chemical cleaning
According to the dewa DPI system, chemical cleaning consists of some special chemicals, which will be put into the same acid-base washing tank in sequence. Dewa is Greek for “strong and vigorous.” DPI means degreasing, Pickling and inhibiting. The DPI chemical industry company developed it in Norway. All chemicals are water-soluble, environmentally friendly and inorganic. The cleaning process consists of the following five stages.
a. Degreasing and alkaline washing
Fill the oil tank with clean water, heat it to 50c (up to 80C), and add chemical substance A to pH 14. Circulate at the maximum flow rate for 30 minutes, and any grease and oil film should have been removed. PH value and temperature shall be controlled during operation.
b. Pickling
Add chemical B to reduce the pH value to 5.5, and then add 10% (volume ratio) of chemical C
Cycle at maximum flow for 60 minutes. Pay attention to controlling pH value and temperature. Add chemical D, continue to cycle to pH 7.5, keep the temperature at the temperature of procedure a, and cycle at the maximum flow rate for 30 minutes. Pay attention to controlling pH value and temperature.
d. Chemical Anticorrosion
No preservative shall be added if the interval between chemical cleaning and hot oil flushing is less than 24 hours. If the interval is more than 24 hours, 2% (volume ratio) of chemical substance e must be added, and the circulation shall be continued for 30 minutes without heating. These liquids must be diluted by adding 4-5% water before discharge, and the pH value shall be controlled.
e. Dryness
After neutralization, dry the pipe with dry, hot air for 30 minutes. Use oil-free and anhydrous compressed air or pure nitrogen after effective filtration.
f. Flushing equipment
·Acid alkali washing pump station: oil tank, oil pump, filter and heating equipment. The best flushing flow is 3m / s. The filter shall be the same as that during hot oil cleaning.
Equipment for supplying dry and clean hot air or nitrogen. The air must be oil-free.
Special flanges, valve blocks and joints shall be cleaned in advance.
g. Control during cleaning
To identify whether the chemical cleaning is correct, the following test data must be recorded:
PH value and temperature
·Amount and flow of chemicals added in each step
3.2. Hot oil flushing
Generally, the target cleanliness of system flushing should be better than that during regular operation. If the cleanliness requirement of the system during the normal process is ISO 15 / 13 / 11, the system shall at least reach ISO 14 / 12 / 10 during flushing. The flushing must meet the cleanliness requirements of solid particles and moisture simultaneously.
3.2.1. Flushing medium
The flushing medium must have good compatibility with the medium in the system’s normal operation, and the viscosity at different temperatures shall be given. If the viscosity of flushing oil is 10-15cst at 40C, the standard flushing station can provide sufficient turbulent flow. Theoretically, as long as the temperature exceeds 70c, the flushing oil can reach this viscosity. The working medium is usually selected as the flushing medium in actual operation.
3.2.2. Turbulence, velocity, temperature and pressure turbulence and Reynolds number when the Reynolds number is greater than or equal to 4000, the motion of the fluid will naturally be turbulent. Turbulence can effectively wash away particles in the pipeline and avoid pollutants suspended in the oil during operation. During flushing, the Reynolds number must be greater than or equal to 1.2 times the Reynolds number during the normal process, but it shall be at least 4000.
For example, the Reynolds number of a hydraulic system during regular operation is 3400, and the Reynolds number during flushing shall be at least 4080.
Reynolds number NR is used to determine whether the flow state of the fluid in the pipeline is laminar or turbulent. The calculation formula is:
Nr = ρν D/ η
ρ = Fluid density
ν = Average velocity
D = pipe inner diameter
η = Dynamic viscosity
η= ρμ ( μ Is the kinematic viscosity of the fluid), so the following formula can also calculate NR:
Nr= ν D/ μ
For example, a hydraulic oil with a viscosity of 46cst flows at a flow rate of 4m / s in a pipe with an inner diameter of 800px, and its Reynolds number is: NR = (10000px / s) (800px) / (11.5px2 / s) = 2000
Empirical surface, laminar flow when NR < 2000; Turbulence when NR > 3000; NR is an unstable flow between 2000 and 3000, and changes between the two modes.
The flow rate shall not be less than 2-3m / s in any flushing circuit, which can prevent particles from remaining in the pipeline· The temperature of the coldest part of the flushing circuit shall be higher than 50c, which can reach the pressure by providing 60C flushing oil. The pressure shall be more significant than 3-5bar measured from the downstream oil return filter and sampling port of the flushing circuit. The hydraulic components should be fully open to ensure that all areas are effectively cleaned.
3.2.3. cleaning of the oil tank, filter, oil cylinder, accumulator, oil pump and motor
These elements should be cleaned in different circuits.
Oil tank: This is one of the most challenging components to clean. The oil tank shall be cleaned manually first, and then flushing oil. A flushing oil pump and filter form a circuit for flushing.
· filter can be connected to the flushing circuit or a separate tank flushing circuit.
Oil cylinder, animal energy device, motor and oil pump: clean separately.
3.2.4. Minimum flushing time once the flushing oil sample shows that the cleanliness has met the requirements, flush it for 30 minutes in the form of turbulence to increase the reliability of flushing.
3.2.5. Each flushing circuit’s evaluation of flushing results shall be unique and traceable. Establish the actual intention of each flushing circuit and corresponding pipelines, and mark the temperature measurement points, flow detection points and sampling points.
Record all parameters in documents, such as start flushing time, temperature, flow, particle pollution degree, moisture and end time, etc.
If possible, ask a third party to evaluate the cleanliness of system flushing.
4. Flushing equipment
4.1. Filtration system
The filtration system shall have sufficient capacity and high performance to filter solid particles and moisture reasonably and meet the cleanliness requirements.
The original filter in the system shall not be used as a flushing filter.
The latest practice shows that with a pressure difference transmitting device and a large enough pollution holding cavity, the filter of 100 is very effective. The sending device shall give an alarm before the filter is bypassed (the filter does not take the role of filtration).
There are many ways to remove water: filter elements made of water-absorbent material, combined filter, oil purifier (such as vacuum distillation), or change oil. Generally, water-absorbent filters are effective.
Note: do not replace the filter for filtering solid particles with a water removal filter!
4.2. pumping station
The pump station shall provide sufficient flow, flow rate, viscosity and pressure to clean the system.
4.3. The temperature regulating system’s temperature shall be frequently detected and adjusted to ensure that the flushing oil has appropriate viscosity to form turbulence in the whole flushing circuit and meet the lubrication requirements of the flushing oil pump.
5. Detection of system cleanliness during flushing
5.1. Cleanliness of the system during regular operation (NAS or ISO 4406): determined according to the equipment manufacturer’s requirements. If the equipment manufacturer has no specific needs, it is recommended to be no less than ISO4406 17 / 14 or NAS 8.
5.2. Target cleanliness of system flushing: two orders of magnitude lower than regular operation. If it is NAS grade 8 in normal operation, the target cleanliness of flushing shall be NAS grade 6.
5.3. Cleanliness detection: it can be carried out in the laboratory, or a portable cleanliness detector can be used.
On-site inspection. The following figure shows pall portable Contamination Tester, which Mobil engineers can test on-site. Cleanliness test cycle: it shall be formulated according to the site conditions. Sampling: it is best to take samples at the oil return header and tank (sampling valve shall be installed on the oil return header). Sampling container: clean sampling bottle.
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