Study of Seals on Long Stroke and Reciprocating Motion Condition

doi:10.4236/jpee.2013.17007

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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.

Email: [email protected]

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

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

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 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

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

010203040T（D）

Q（L）

Figure 6. Leakage change curve over time-combined piston seal ring at 5Mpa differential pressure.

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).

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

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.