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There are many methods of
continuous direct liquid level measurement in a vessel. These
methods range from the very simple to quite complex. One of the
simplest methods is using a float. Widely accepted where it applies,
the float method is simple, accurate, and easy to operate. The
operating principle of the float is based on Archimedes’ buoyancy
principle which states that a body that is wholly or partly
submerged in a liquid experiences an upward force which is equal in
magnitude and opposite in direction to the downward force of the
liquid displaced by the body. The primary device is a float which by
virtue of buoyancy, will follow the changing level of the liquid,
and a mechanism that will transmit the motion of the float to a
pointer usually placed outside the vessel, to indicate the level of
liquid inside. |
Figure 1. A
typical tank float configuration in a pulp operation |
| CKF Inc., a North
American manufacturer of disposable paper plates using virgin and/or
secondary fibers, depending on the product being made, is one of the
millions of users of the float. A specific application is in one of
their recovery tanks. The float is basically made of rolled sheet
metal 8” in diameter. The seam, top and bottom is silver solder
welded to make a leak proof canister or float. On the top of the
tank, a ¼” pipe nipple is welded on to allow a stainless chain to be
attached to this float. The other end of the chain is connected to a
mechanism, which is connected to an electrical switch that gets
activated when the tank float rises with the tank level contents to
a set point, which will then start a pump that will drain the
recovery tank into a larger tank used to make the product. When the
recovery tank is drained, the float lowers to the bottom of the tank
turning the pump off until the float starts rising again.
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Figure 2. CKF, Hantsport, Nova Scotia |
| “Recently we had a float
fail in one of our recovery tanks. The float had several holes in
the welded joints, which allowed it to fill with water and pulp. The
float could not rise with the tank level which made the pump
required to drain the recovery tank contents back into the system,
not start. As a result, the recovery tank overflowed,” said Randy
Warner. “I removed the float, cleaned it, and had our plant welder
re-weld all of the seams and joints. I then hooked up a small
compressed air line to the float while I had it on the bench to leak
test the welded joints. I filled the float with compressed air and
all seemed okay. I ran my hand over the float to feel for any air
leaks and felt nothing. I then used the SDT Ultrawave unit and found
three rather small leaks that I did not find earlier just running my
hand over the seams feeling for air leaks. |
Figure 3. Using compressed air and ultrasonic
inspection to pinpoint weld leaks on tank floats |
| I had the earphones on
as well and I was able to pinpoint the location of the air leaks
quickly and easily. I then sprayed the areas with a little water and
the leaks were accented even more on the earphones and the SDT
Ultrawave unit. A loud popping noise could be heard through the
earphones connected to the SDT Ultrawave ultrasonic detector. I was
really impressed. We re-welded the leaking spots and retested for
leaks using the SDT Ultrawave. No leaks this time. Very little air
was required inside the float for it to be detected through the leak
paths. It shows how sensitive the SDT170 Ultrawave can be. If I had
not used the SDT Ultrawave unit to leak test this float, I would
have reinstalled it after welding and it definitely would have
filled with water and slurry again and obviously failed,” recounted
Randy Warner. |
Figure 4. Leaking weld seams found
quickly using ultrasound inspection |
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“Thanks to the SDT
Ultrawave unit, I only had to do the job once and I am confident the
float will work as intended without failure. This makes the SDT 170MD my
choice for leak detection from now on along with the many other uses for
this unit,” he stated.
Leaks will sink a float,
indeed. When the float fills up with water, the downward force becomes
greater than the upward force. The resulting effect of the sunk float
for this particular situation, is wasted pulp. For other situations, it
could be catastrophic. Proper re-testing after repair should be done as
CKF Inc. does, to ensure that the problem has been corrected. |
Figure 5. Handheld inspection is fast and
reliable |
|
The Ultrawave 170 is a
hand held ultrasonic detector. Ultrasonic detectors are tuned to sense
high frequency sound wave pressure produced by various events. These
include turbulence from compressed air leaks and friction from
mechanical movement. Other uses of the SDT Ultrawave includes Bearing
Condition Monitoring using AVM principles, Valves Inspections to check
for flow or no flow conditions, Bearing Lubrication, Electrical
Inspections to detect and locate corona, tracking, and arcing discharge,
and Steam Traps inspections. The SDT Ultrawave only detects high
frequency sound or ultrasound which is above human hearing capability.
If you hear the sounds in your facility, the SDT Ultrawave will not
detect it. A digital display on the screen and 60,000 points memory
capability with true RMS data logging makes the SDT Ultrawave a valuable
tool for PDM inspections, energy management applications, and quality
control. And the best part? It’s easy to use and it works!
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Figure 6. The Ultrawave 170M Leak
Surveyor's Kit |
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For additional information
on the SDT Ultrawave, call 1(800) 667-5325. |
Randy Warner
is a practicing Level 1 Certified Ultrasonic Inspector. He has worked
for CKF Inc. for over 15 years as industrial millwright. He is very
pleased with the proactive spirit at CKF Inc. The benefit of their
collective efforts is showing up in their increased operational uptime
and knowledge of their equipments.
Liane Harris,
BSChE, MSE, is a Chemical Engineer by profession and has worked as a
Process and Energy Engineer in Food and Chemical Industries in the
Philippines and in Pulp and Paper Mills in the U.S. and Canada. She has
helped hundreds of companies worldwide become successful in the
applications of Airborne and Structure-borne Ultrasound Technology since
1996. She is SDT North America’s Corporate Trainer and Ontario’s
District Manager. Call (905) 785-1474 or e-mail at
Lianesdt@sympatico.ca to contact her directly. |
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