Off-line ultrasonic cleaning – restoring as-built performance during Turnarounds
COST OF FOULING
After some operational period, surfaces of heat exchangers are no longer in the pristine condition found
upon installation. The surfaces can become coated with deposits from the process or cooling streams,
corrosion products or microbiological films. This additional structure on the surface creates a
supplementary resistance to heat transfer, a decrease in performance, and an increase in energy
consumption, fouling rate and corrosion problems.
Fouling affects both capital and operating costs of heat exchangers:
1. Higher capital expenditures for oversized plants which includes excess surface area (10-50%), costs for
extra space, increased transport and installation costs.
2. Energy losses due to the decrease in thermal efficiency and increase in the pressure drop.
3. Production losses during planned and unplanned plant shutdowns for fouling cleaning.
4. Maintenance including cleaning of heat transfer equipment and use of antifoulants.
The loss of heat transfer efficiency usually means that somewhere else in the system, additional energy
is required to make up for the short fall. The increased pressure drop through a heat exchanger
represents an increase in the pumping energy required to maintain the same flow rate. Any extensive
fouling means that the heat exchanger will have to be cleaned on a regular basis to restore the loss of its
heat transfer capacity. While this already is a challenge during regular maintenance, with turnarounds a
strict time limitation is added: a large number of equipment has to be cleaned in a very short period of
time, while still maintaining a first time right, high level of cleaning quality, allowing us to go from a 4
year TA period to a 6 year one.
CLEANING TECHNIQUES
There are several cleaning techniques that can be used to restore the thermal and hydraulic
performance of a heat exchanger. The conventional method used is High Pressure Water Jetting. This
technique however, is time consuming and not very effective. HPWJ only cleans the zone of direct
impact and the equipment is eroded by the force of the HP water. The shell side of heat exchangers between the tubes - cannot be cleaned and the metal surface is scraped instead of removing 100% of
the deposits, leaving a lot of fouling and an uneven and rough surface. While precisely the first microns
of fouling are the most important to remove: 0,5 mm of fouling can cause heat transfer losses of 4099,7%. The impact of the fouling above the 0,5 mm often is relatively insignificant (fig. 1).
Heat transfer loss
(%)
Fouling thickness (m)
Fig. 1 Heat transfer loss of a Stainless Steel tube (10 mm diam., 1 mm thick) in relation to the thickness of
a ceramic type of fouling (k = 1)
This is why eliminating 100% of the fouling is an important goal when cleaning heat exchangers.
ULTRASONIC CLEANING
In search of a cleaning technique able to restore the as-built performance in a limited amount of time, a
pilot test was performed in 2012 with ultrasonic cleaning. The results were very promising and the
method was applied during the TA 2013 of Moerdijk; the biggest TA ever on this site.
Ultrasonic cleaning involves the use of high-frequency sound waves (25 kHz; above the upper range of
human hearing of about 18 kHz) to remove a variety of contaminants from parts immersed in aqueous
media. The contaminants can be dirt, oil, grease, buffing/polishing compounds, and mold release
agents, just to name a few. Materials that can be cleaned include metals, glass, ceramics, and so
on. Ultrasonic cleaning is powerful enough to remove tough contaminants, yet gentle enough not to
damage the substrate. It provides excellent penetration and cleaning in the smallest crevices and
between tightly spaced parts of the equipment.
In a process named cavitation, micron-size bubbles form and grow due to alternating positive and
negative pressure waves in a solution. The bubbles subjected to these alternating pressure waves
continue to grow until they reach resonant size. Just prior to the bubble implosion (Fig. 2), there is a
tremendous amount of energy stored inside the
bubble itself.
Temperature inside a cavitating bubble can be
extremely high, with pressures up to 500 atm. The
implosion event, when it occurs near a hard
surface, changes the bubble into a jet about onetenth the bubble size, which travels at speeds up
to 400 km/hr toward the hard surface. With the
combination of pressure, temperature, and
velocity, the jet frees contaminants from their
bonds with the substrate. Because of the
inherently small size of the jet and the relatively
large energy, ultrasonic cleaning has the ability to
reach into small crevices and remove entrapped
deposits very effectively.
Fig. 2 Illustration of imploding cavity in a liquid
irradiated with ultrasound
An excellent demonstration of this phenomenon is to take two flat glass microscope slides, put lipstick
on a side of one, place the other slide over top, and wrap the slides with a rubber band. When the slides
are placed into an ultrasonic bath with nothing more than a mild detergent and hot water, within a few
minutes the process of cavitation will work the lipstick out from between the slide assembly. It is the
powerful scrubbing action and the extremely small size of the jet action that enable this to happen.
The solution used in ultrasonic cleaning is a very important consideration and should be suitable for the
fouling to be removed and the material of the equipment.
The solution temperature has a profound effect on ultrasonic cleaning effectiveness. In general, higher
temperatures will result in higher cavitation intensity and better cleaning. However, if the temperature
approaches the boiling point of the solution too closely, the liquid will boil in the negative pressure areas
of the sound waves, reducing or eliminating cavitation. Water cavitates most effectively at about 70ºC
(160ºF); a caustic/water solution, on the other hand, cleans most effectively at about 82ºC (180ºF)
because of the increased effectiveness of the chemicals at the higher temperature. Solvents should be
used at temperatures at least 6ºC (10ºF) below their boiling points.
Totally blocked tubes have to be unblocked before going into the vessel. Oily heat exchangers typically
take 3 hours per rinse, so 3-4 heat exchangers can be cleaned in 24 hours per ultrasonic vessel. The
cleaning process continues with or without people and the use of HPWJ is significantly reduced.
The application on SNC Moerdijk during TA 2013
LAYOUT OF THE WASH PAD
350 pk
US containers
500 bar ww unit
350 pk
Not restricted
may 2013
3
Scope for the wash pad during the TA of 16 days was 51 heat exchangers and 700 other pieces of
equipment. Two ultrasonic vessels were used:
- 9,75 m long with a soap solution for oily fouling
- 5,9 m long with a acid solution for lime scale and polymer fouling.
The acid solution was saturated after the TA, the soap solution was still good for further use. The
reduction of waste water was 86% compared to HPWJ.
This technique is not only suitable for cleaning heat exchangers, but for any item that can get wet:
scaffolding, compressor casings and diaphragms, filters, screens, demisters, pipes, plates, heads, rotors,
(motor operated) valves, packings, vessels, rashing rings, etc.