Post on 21-Feb-2017
transcript
Introduction Project title: The diagnose of fracture mechanism of valve seat
Date and duration: March-May 2014
Location of the project: Xi’an, China
Name of the organization: DASUO Corporation
My status: Intern engineer assistant
Background The nature of the project
the valve pump is a critical component in slurry (alumina ore mixed with water)
transportation as it provides a driving force of flowing. A valve pump consists of two
major parts, the valve cone, and the valve seat (showed in figure 1). The driving force
is produced by pulling and pushing the valve cone to generate the pressure difference,
by which the slurry can flow. Due to the close contact with slurry which contains hard
particles, alumina, the valve seat is expected to work for at least 27 days before it is
replaced. By the time when I was an intern in DASUO, I came across the problematic
situation where a valve seat cracked only on the 15th day (showed in figure 2).
Although this was not a frequent situation, DASUO wanted to figure out what
happened so that the same problem will not appear again.
Figure 1. Valve pump Figure 2. Cracked valve seat
The description of the pump mechanism As mentioned before, the valve pump is a device to control the flow in the pipe in order
to transport the slurry to the smelter. By moving forward and backward, the difference
in pressure would drive the slurry flow through the valve (showed in figure 4). Apart
from the function of a pump, it can also be used as the valve to control the velocity of
the slurry. If the velocity is required to be low, the range of movement of the valve cone
would be shortened so that the gap between valve cone and valve seat is decreased.
This would allow less volume of the slurry flow through the valve.
Figure 4. The mechanism of valve pump
Failure observation
As showed in figure 5 which is the overview of the value seat. The white curve marked
on the 35 degree-plane indicates the crack, and it almost forms a complete circle.
From the side view, which is showed in figure 6, the external end of the crack connects
with O-shape groove. Also, from the close view, the crack edge is not sharp but blunt.
Figure 5. Overview of the faulty the seat Figure 6. Side view of the seat
Assumptions of the crack initiation Since the crack connects the external surface with the internal surface of the valve
seat, it can either start from both sides. Therefore, two assumptions can be proposed.
The first assumption was that the crack initiated from the inside whereas the second
one was from the external surface.
The rationale behind the first assumption was due to the impact from reciprocating
movement of the valve cone. When the valve is controlled to close, the value cone
would be pushed to touch the 35-degree plane so that the flow of slurry can be blocked.
After the repeated impact day after day, the crack was finally created on the 35-degree
plane and the crack grows through of wall of the seat to the O-shape groove as a
result. However, why does the crack connect with O- groove instead of other part of
the external surface? I assumed that this was due to the rough surface in the groove
which caused the stress concentration. However, after checking out the O-shape
groove that have not cracked, the surface of the groove is relatively smooth which did
not support the view that the rough surface caused the “connection”. In order to further
prove there were micro cracks, the red ink was poured in the groove and wiped off
after a while. Then a piece of white paper is appressed on the groove surface for a
few minutes. As a result, the paper can be marked by the ink if there is any micro crack.
It turned out that there were many micro marks on the surface of groove as there were
considerable amount of the red marks on the paper.
One reason causes this would be the blunt cutting tools that caused the strong
shearing stress on surfaces for which numerous micro cracks are created. Since
stress concentration is prone to happen in sharp corner such as the crack tip; and
cracks tend to grow towards the stress-concentrated region, I supposed these are the
reasons that the crack grew to the O-shape groove. Therefore, in order to justify the
assumption, two workers were asked to do the same groove-making work and one of
them to was asked to sharpen the cutting tool in a frequent manner while the other
didn’t. The result, as predicted, was that the grooves made by frequently sharpened
cutting tool was clean and tidy with few micro cracks.
As for the second assumption that the crack initiated from the O-shape groove, it has
some rationale as well and this is also due to the rough surface in grooves. This time,
however, the reason for initiating from O-grooves is because the effect of stress
concentration in the external surface is greater than the one of impact on internal
surface. For this reason, the crack initiated in O-shape groove in the first place ahead
of the one in internal surface. With the time goes by, the crack grows and connect with
stress concentrated area at the other side, which is the area impacted by the cone.
Recommendation Although both assumptions are equally reasonable, the fundamental reason for the
crack initiation is due to the micro cracks on the grooves surface. Based on the
experiment of comparing the groove of not sharpened and sharpened regularly, rules
should be made to regulate the procedure.
Firstly, the cutting tools must be sharpened in a timely manner, and from the
experiment, the best frequency for sharpening is about every 25 groves. Apart from
this, DASUO should adopt a sophisticated flaw detection approach such as the
ultrasonic flaw detector to detect micro cracks in the valve seat especially after
machining grooves. If there are substantial amount of micro cracks, the valve seat has
to be re-processed. Finally, the broken valve seat should be repaired rather than
melting to make the new product. This is because of the low mechanical property
requirement for the valve seat, compared with valve cone which is required to have
high strength, toughness. For this reason, the broken part should be welded and heat
treated in order to continue its working life.