Journal of Power and Energy Engineering, 2013, 1, 40-44
http://dx.doi.org/10.4236/jpee.2013.17007 Published Online December 2013 (http://www.scirp.org/journal/jpee)
Study of Seals on Long Stroke and Reciprocating Motion
Condition
Zhaoxia Wang, Yujie Zhi, Chengyong Hu, Jinlu Xi, Jing Hu, Jie An
China Petroleum Engineering Co., Ltd., Beijing, China.
Received October 2013
ABSTRACT
The equipment has a high and strict requirement of dynamic seals, especially working with Long stroke reciprocating
motion under a sandy condition and with a great deal of gas. The seal form of power cylinder on the underground part
of hydrau lic pow er rod less type oil extraction equipment is studied. We design three seal structures, do the performance
testing and the life testing w ith related equipment. It turn ed out that the seal fo rm that combines gap seal with sa nd pr e-
vention te chniques has hig h pe rform ance, longer life. The power cylinder works stably and reliably.
Keywords: Dynamic Seals; Long Stroke; Reciprocating Motion; Gap Seal; Composite Seal
1. Introduction
Dynamic seal is an important part of the machinery, it
plays an important role on security, reliability and equip-
ment life and operation reliability [1].
Dynamic seal also directly affect the economy of ener-
gy and material. According to the survey, friction bring a
very big damage on the surface of sealing, for example,
in centrifugal impeller seal consumed power take up
more than 20% of the total shaft power, for example [2].
Dynamic seal has a high and strict requirement for the
power cylinder of oil extraction equipment which works
with long stroke reciprocating motion under a sandy
condition and with a great deal of gas. The cylinder of
hydraulic power rodless type oil extraction equipment
works in the liquid, which may contain oil, water, gas
and sand, the axial movement of the dynamic seal is very
difficult, the aggravation of abrasion will cause serious
loss. The working stroke of hydraulic power rodless type
oil extraction equipment is 3 m - 5 m, much longer than
conventional hydraulic equipment, so the life of dynamic
seal have a great influence on the life of the whole sys-
tem. The dynamic seal problem is a bottleneck problem.
We designed three combined seal forms, such as soft
seal, piston seal ring and gap seal, aiming at a better seal-
ing of power cylinder underground, and studied the life
test technology.
2. Dynamic Sealing Structure Design
In many important machines, usually we combined many
sealing forms for a reliable sealing effect. [3] We do
many researches and analysis of soft seal, piston seal ring
and gap seal on the basis of conventional sealing forms,
we also design many seal forms to find a combined dy-
namic sealing form suits for hydraulic power rodless type
oil extraction equipment.
2.1. Soft Seal
When soft seal work is under low pre ssure condition, it is
sealed by elastic of the seal ring; when under high pres-
sure condition, it is sealed by the elastic and the pressure
of the fluid. The higher the pressure, the more reliable of
the seal, the less of the leakage, even no leakage. The
section of the seal ring has 3 forms: Y-shape, V-shape,
and O-shape. But it needs fluid to be very clean, because
solid impurities can lead to a quicker damage to the seal,
and result in the failure of the seal for excessive wear.
We take the conventional soft seal form and better
combined sealing material in our system. We design sand
control structure with a soft seal, the structure of com-
bined seal shown in Figure 1.
2.2. Unnotched Piston Seal Ring
Designed an unnotched piston seal ring specific for the
request and feature of combined seal structure, as shown
in Figure 2. Piston ring is made by Tungsten copper
chromium, CrMoCu, vanadium and titanium alloy, etc.
and its surface is hardened (higher than HRC60). Piston
ring has wear-resisting, high temperature resistance, good
bending strength , strong sealing, etc. The piston seal r ing
structure combined with sand control is shown in Figure 2.

Study of Seals on Long Stroke and Reciprocating Motion Condition
41
Figure 1. Structure diagram of the combined seal.
Figure 2. Picture of the unnotched piston seal ri ng.
2.3. Gap Seal
Gap seal is an non-contact form seal. Gap between shaft
and the hole of bearing cover is the simplest stru ctur e . By
analyzing the operation of the power cylinder process,
we find two key positions of seal: position between pow-
er pipe and cylinder, and position between piston and
power cylinder, they all do reciprocating motion with
each other. We combine sand control structure with gap
seal in this dynamic seal.
3. Special Test Equipment for Seal
To test the practical effect of our design proposal, a life
test equipment of seal is specially designed and manu-
factured. Then laboratory simulation test is done to ob-
serve the life, performance, friction, leakage.
3.1. Friction Testing Machine
Friction testing machine is driven by electricity, which
can realize reciprocating motion rapidly about 50 times
per minute. So it can test the life of many seals quickly,
and provide an effective means for preliminary selection
of sealing method, as shown in Figure 3.
3.2. Dynamic Seal Simulator Test
To test dynamic seal simulated condition, a dynamic seal
condition simulator is designed to test the life of seal.
The simulator consists of hydraulic station, test bed, test
cylinder, the structure and picture of seal shown in Fig-
ure 4.
Figure 3. Friction testing machine.
Figure 4. The structure of life test equipment.

Study of Seals on Long Stroke and Reciprocating Motion Condition
42
The device is driven by hydraulic station, make the
piston rod do the reciprocate movement. The seal be-
tween piston rod and cylinder is to be tested. Test me-
dium is water, and the leakage can be detected. We h ave
PCV on the device to suit for the different condition.
We control the direction of relay by electronic timer,
and adjust the reversing time freely. The testing simu-
lates the working condition of hydraulic pump basically.
3.3. Field Test Equipment of Hydraulic Pump
With a test well the system simula tion experiment is car-
ried to observe the life of dynamic seal.
The power cylinder is mounted with specific seal on
the test well. The test well is 50 meters depth and the
internal diameter of dr ivepipe is 159 millimeter. Increase
the wellhead pressure to simulate the depth of power
cylinder under the well. Then run the device all day and
record the change of leakage to test the life of seal.
4. Laboratory Test and Field Test
Many different simulation tests are carried to the sealing
forms designed. The procedure as follow: Firstly, use
pump simulation to test the specific sealing form with
fiction testing machine and life testing equipment, compare
the life of different sea l forms through the change of lea-
kage. Secondly, run the device on the well continually to
test their performance.
Three seal forms pr eliminary tes t data are shown in Ta-
ble 1. Partial laboratory sim ula tion da ta as Figures 5-8.
Result of laboratory Test
1) Life of standard soft seal is 7 days in underground
condition.
2) Unnotched piston seal ring has better performance
than soft seal and works well in a certain range of leakage.
But it has a much big leakage that more than 60 L at 5
MPa after 20 days.
3) Leakage of gap seal ring with 300 mm gap length
arrives 50 L after 20 days.
4) Service life of gap seal ring with 300 mm g a p length
is more than 180 days, when it works in underground con-
dition. And its leakage (20 L/d @5MPa) can meet the
requirement of hydraulic well pump.
5. Conclusion
According to the theoretical analysis and experimental
Table 1. Three seal forms from preliminary test data.
NO. STRUCTURE MATERIAL LENGTH OF SEAL DIFFERENTIAL
PRESSURE (MPa)
1 Soft seal polyurethane -
2
3
4
5
6
2 unnotched pis ton seal ring 35 CrMo 3 - 5 groups
3/group
3 gap seal Piston rod: The thickness of the spray welding is 0.4 - 0.5 mm Pump cylinder: 35CrMo, carbonitriding
150 mm
300 mm
400 mm
500 mm
5
10
15
20
25
30
35
40
45
50
55
60
65
70
010203040 T（D）
Q（L）
Figure 5. Leakage change curve over time-soft seal at 2 Mpa differential pressure.

Study of Seals on Long Stroke and Reciprocating Motion Condition
43
5
10
15
20
25
30
35
40
45
50
55
60
65
70
010203040T（D）
Q（L）
Figure 6. Leakage change curve over time-combined piston seal ring at 5Mpa differential pressure.
5
10
15
20
25
30
35
40
45
50
55
60
65
70
01020 30 40 T（D）
Q（L）
Figure 7. Leakage change curve over time-gap gap seal ring at 5 Mpa differential pressure (length of gap is 150 mm).
5
10
15
20
25
30
35
40
45
50
55
60
65
70
01020 30 40
T（D）
Q（L）
Figure 8. Leakage change curve over time-gap gap seal ring at 5 Mpa differential pressure (length of gap is 300 mm).

Study of Seals on Long Stroke and Reciprocating Motion Condition
44
study of these sealing forms above, piston seal ring has a
better performance than soft seal, but neither of them
suits our condition, because they all have big leakage and
short life. Gap seal ring has a well sealing effect, longer
life, less leakage (in t he allowed range) a nd running steadily.
Combination seal form with gap seal ring and sand control
struc ture is suitable for the operating condition of hydraulic
pump, and it also meets the requirement of sand control
and the re quirement of power cylinder meanwhile.
6. Acknowledgements
First I appreciate my company who provide a comforta-
ble rearching atmosphere, second I want to thank Wan
zhong engineer, Sun xiaohui engineer, Ma zhaoyan en-
gineer for their help and Luo wensheng engineer during
the experimentation. To my colleague, I want to let them
know that: being with them is the happiest time I ever
had. Also I have a word to my Family: thanks for your
support and share with me all the happy time.
REFERENCES
[1] Y. Q. Gu, “Fluid Dy namic Seal ,” China University of Pe-
troleum Press, 1990.
[2] Symposium of Liquid Dynamic Seal.
http://bbs.chinalubricant.com/dispbbs.asp?boardid=12&Id
=843
[3] T. J. Lei, “Hydraulic Engineering Manual,” Beijing Insti-
tute of Technology Press, 1998.