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As centrifugal machines, run with very close clearances at very high RPM, even a small foreign particle can have devastating effect. It is always recommended to ensure cleanliness of piping before connecting it with machine.
This article contains an excerpt from the paper, “Save your Centrifugal Machinery during Commissioning” by Arun Kumar and Mohit Sabharwal of HPCL- Mittal Energy Ltd at the 2018 Asia Turbomachinery & Pump Symposia.
There are many methods of pipeline cleaning like: 1. Cleaning by water 2. Air Flushing 3. Steam Flushing 4. Chemical cleaning 5. Oil flushing 6. Hydro jetting 7. Rotomol (Rotary Head Device) cleaning Deciding on the suitable cleaning approach is always a critical decision of for the project manager and commissioning team. The decision depends on the type of fluid handled, pipe material and internal wall conditions. Below is a steam cleaning procedure:
Reove the piping spool between the turbine trip valve and the isolation block for the turbine inlet. If there is a steam strainer downstream of the block valve, the strainer must be removed. Support inlet piping to withstand the reactive force from the steam blow.
Place a covering over the turbine inlet flange. The cover protects the turbine from particles entering during the steam blow, and acts as a device to hold the target.
Target mounting methods must ensure that the targets will remain safely attached during the steam blowing process. Actual target material shall be polished SS304 or SS316.
Close the inlet valve at the header and open the inlet valve at the turbine.
Start the steam flow and allow the piping to warm. After the piping is warm, increase the steam flow to 50% of the desired load. If everything is normal without significant leakages from temporary joints, continue ramping the steam to reach the desired flow rate for the designed blow conditions.
It is preferred to use low pressure high velocity steam blowing due to lower noise steam consumption. Quench water is added to control the noise level of exhaust steam thereby making the vicinity area workable. A comparison of low pressure high velocity continuous blowing and puff blowing is shown in Annexure-F.
The blow conditions must be maintained so that a Cleaning Force Ratio (CFR) of 1.0 is achieved throughout the targeted steam blow path. CFR=m 2 SBVSB / m 2 maxVmax Where, m 2 SB = Mass Flow Rate during Steam Blow m 2 SB = Mass Flow Rate at Max Operating Conditions VSB = Specific Volume at Steam Blow Conditions Vmax = Specific Volume at Max Operating Condition
Required flow rate, pressure and temperature to achieve CFR are to be calculated for each section prior to the start of steam blowing.
Blow steam through the system without any backpressure at flow rates as close to maximum until no particles can be observed from the line. Several cycles of blowing may be required to remove the particles. Thermal Shocks during steam blowing can assist the process.
In thermal shocks, steam temperature is reduced to 30Deg C above saturation temperature (for example if Saturation Temperature is 100DegC, Steam temperature is to be reduced up to 130DegC). Cooling is achieved by adding BFW into the steam header using a temporary attemperator. The temperature is again increased to highest limit and maintained for at least 1 hour. A minimum of three thermal shock cycles is to be performed.
Close the header valve once no particles are seen and securely attach the polished target on the target support.
Open the steam header valve and blow for at least 15 minutes. Close the header valve and inspect the target.
Acceptance criteria for piping cleanliness is based on the following: a. An acceptance target will have no raised pits. b. An acceptable target will have less than three pits in any square centimeter of the target, and no pit shall be larger than 1 millimeter. Steam blowing shall be repeated until the acceptance criteria have been met.