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.