Sand-proof sealing structure and sealing method for submersible electric pump piston

Abstract

The present application belongs to the technical field of submersible electric pumps, and discloses a sand-proof sealing structure and sealing method for a piston of a submersible electric pump. The sand-proof sealing structure comprises three sealing structures arranged on the piston; the first sealing structure is an upper seal formed by the upper sand-proof sealing ring and the cylinder body, the second sealing structure is a main seal formed by the piston ring and the cylinder body, and the third sealing structure is a lower seal formed by the lower sand-proof sealing ring and the cylinder body. The present application effectively prevents sand particles in oil from entering the sealing surface, avoids mechanical wear and seal failure caused by sand particles, significantly improves the sealing reliability of the submersible electric pump in sand-containing oil wells, prolongs the service life of the piston ring and the cylinder body, and reduces the risk of equipment failure caused by sand particles.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electric submersible pumps, and particularly relates to a sand-proof sealing structure and sealing method for a piston of an electric submersible pump. BACKGROUND

[0002] In oil production operations, the electric submersible pump is an important downhole lifting device, and the sealing performance of the core component piston directly affects the pump efficiency and equipment life. The existing technology relies on the radial compression force of the piston ring to achieve sealing. When sand particles invade, a leakage channel is formed at the inclined opening of the piston ring, uniform wear of multiple piston rings leads to "series failure", and there is a lack of active sand flow guiding mechanism, and abrasive particles continuously damage the sealing surface. In addition, there is still the problem that the axial size of the piston is too large and cannot effectively intercept sand particles, and the problem of sand particle deposition in the sealing area has not been solved. SUMMARY

[0003] To overcome the problems in the related art, the application discloses a sand-proof sealing structure and sealing method for a piston of an electric submersible pump,

[0004] The technical solution is as follows: a sand-proof sealing structure for a piston of an electric submersible pump, which comprises three sealing structures arranged on the piston; the first sealing structure is an upper sand-proof sealing ring and a cylinder body, the second sealing structure is a piston ring and a cylinder body, and the third sealing structure is a lower sand-proof sealing ring and a cylinder body.

[0005] The piston is matched with the piston ring through the annular installation groove, the piston ring is tightly attached to the cylinder body, and a sealing surface is formed to prevent oil leakage from the sealing surface.

[0006] The piston is provided with an annular groove at the upper end and the lower end to install the upper sand-proof sealing ring and the lower sand-proof sealing ring, and the upper sand-proof sealing ring and the lower sand-proof sealing ring are symmetrically installed at the upper end and the lower end of the piston.

[0007] The inner side inclined surface of the upper sand-proof sealing ring near the axis is provided with a spiral flow channel, so that the sand particles on the upper part of the piston enter the central axis position along the spiral flow channel and flow back to the working cavity, and the spiral flow channel is equally deep or gradually deep from the outside to the inside.

[0008] The number of the piston rings is not less than three, and the piston rings are provided with inclined openings, and the multiple piston rings are uniformly and staggeredly arranged in the circumferential direction of the piston to form multiple levels of sealing defense lines.

[0009] The upper sand-proof sealing ring and the lower sand-proof sealing ring are formed by filling polytetrafluoroethylene material and molding.

[0010] The upper sand-proof sealing check ring and the lower sand-proof sealing check ring adopt a floating installation mode, and the core of the floating installation is that a micro movement gap in the axial and radial directions is reserved between the check ring and the piston annular groove, when the cylinder body is deformed due to temperature change or pressure fluctuation, the check ring can freely move in the groove, and the gap change between the cylinder body and the piston is compensated by the displacement of the check ring, so that hard jamming is avoided. For example, when the inner diameter of the cylinder body is reduced due to thermal expansion, the check ring is extruded and shrinks into the piston groove; on the contrary, the check ring expands outward to maintain contact with the cylinder body to automatically compensate the gap change caused by the deformation or eccentricity of the cylinder body.

[0011] Another purpose of the present application is to provide a sand-proof sealing method for the sand-proof sealing structure of the submersible electric pump piston, and the method comprises the following steps:

[0012] The designed triple sealing structure forms a stepped sand-proof system to form a sand-proof barrier, so that the sand particles flow back to the working cavity under the action of the first sealing structure, and part of the leaked oil is further subjected to main sealing through the second sealing structure, and zero leakage is completed through the third sealing structure.

[0013] The method for making the sand particles flow back to the working cavity under the action of the first sealing structure comprises the following steps:

[0014] Step one, in the liquid-solid two-phase system, the sliding angle of the sand particles on the upper surface of the upper sand-proof sealing check ring is set in combination with the particle-liquid interaction, the paper resistance and the particle characteristics;

[0015] The sliding angle is expressed as:

[0016]

[0017] In the formula, is the effective friction coefficient, including dry friction and liquid viscosity effect; is a constant; is the effective cohesion, including liquid bridge force and van der Waals force:

[0018]

[0019] In the formula, is the dry state internal friction angle of the sand particles, is the dynamic viscosity of the crude oil, is the density of the sand particles, is the diameter of the sand particles, is the shear rate, is the viscosity correction coefficient;

[0020] Step two, the sliding angle of the sand particles on the inclined surface of the upper sand-proof sealing check ring is set according to the sand-containing working condition and is calculated to obtain the actual working condition sliding angle.

[0021] Step three, according to the obtained actual working condition sliding angle, the upper surface of the upper sand prevention sealing check ring is set at an angle with the upper end surface of the piston, and the inner side slope of the upper sand prevention sealing check ring close to the piston axis is obtained according to the set angle;

[0022] Step four, based on the obtained inner side slope of the upper sand prevention sealing check ring close to the piston axis, the resultant force direction of the sand particle oil impact force and the reaction force of the upper sand prevention sealing check ring points to the working chamber, and the sand particle flow is guided back into the working chamber.

[0023] The actual working condition sliding angle of step two is 60°;

[0024] The set angle of step three and the inner side slope of the upper sand prevention sealing check ring close to the piston axis are 60°-90°.

[0025] In combination with all the above technical solutions, the beneficial effects possessed by the present application are:

[0026] Firstly, the present application provides a sand prevention sealing structure for a submersible electric pump piston ring, which comprises a cylinder body, an upper sand prevention sealing check ring, a piston, a piston ring, a lower sand prevention sealing check ring and a piston rod.

[0027] Secondly, the triple sand prevention sealing structure of the present application can significantly improve the running life of the submersible electric pump in the sand-containing oil well, reduce the failure rate caused by sand particle jamming, and reduce the maintenance cost of the submersible electric pump in the later period. BRIEF DESCRIPTION OF DRAWINGS

[0028] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure and serve to explain the principles of the present disclosure together with the specification;

[0029] Figure 1 is a schematic diagram of the sand prevention sealing structure for a submersible electric pump piston provided by the embodiments of the present application;

[0030] Figure 2 is a three-dimensional model effect diagram of the split type sand prevention sealing check ring provided by the embodiments of the present application;

[0031] Figure 3 is a top view of the split type sand prevention sealing check ring provided by the embodiments of the present application;

[0032] Figure 4 is the front view of the split sand-proof sealing check ring provided by the embodiment of the present application;

[0033] Figure 5 is the back view of the split sand-proof sealing check ring provided by the embodiment of the present application;

[0034] The figure label: 1, cylinder; 2, upper sand-proof sealing check ring; 3, piston; 4, piston ring; 5, lower sand-proof sealing check ring; 6, piston rod; 7, annular groove; DETAILED DESCRIPTION

[0035] In order to make the above-mentioned objectives, features and advantages of the present application more apparent and easy to understand, the specific embodiments of the present application are described in detail below with reference to the accompanying drawings. In the following description, a large number of specific details are set forth in order to fully understand the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the concept of the present application, so the present application is not limited to the specific implementation disclosed below.

[0036] The innovation of the present application lies in that: the present application adopts a triple-step sealing system, the first step is formed by the contact between the upper sealing check ring and the cylinder 1, forming a dynamic seal, the inclined spiral flow channel design realizes the self-guided flow of sand particles, the second step is formed by the staggered combination arrangement of the three inclined opening piston rings 4, constituting the main seal, and the third step is the lower sealing check ring, which ensures zero leakage. Through the floating installation and the pressure self-tightening effect of the piston ring 4, under the condition that the length of the piston 3 is shortened by 40%-50%, high-efficiency sand particle interception and reliable sealing are realized.

[0037] In the embodiment of the present application, the piston assembly of the sand-proof sealing structure for the submersible electric pump piston includes a piston body and a piston rod 6, wherein a plurality of annular mounting grooves are uniformly arranged on the circumference of the piston 3 for mounting a plurality of piston rings 4. Split sand-proof sealing check rings (including upper sand-proof sealing check rings 2 and lower sand-proof sealing check rings 5) are arranged at the upper and lower ends of the piston 3 respectively, which together constitute a three-step sealing structure with the piston ring 4.

[0038] Specifically, as shown in Figures 1-5 The sand-proof sealing structure for the submersible electric pump piston provided by the embodiment of the present application includes three sealing structures arranged on the piston 3; the first sealing structure is the upper seal formed by the contact between the upper sand-proof sealing check ring 2 and the cylinder 1, the second sealing structure is the main seal formed by the contact between the piston ring 4 and the cylinder 1, and the third sealing structure is the lower seal formed by the contact between the lower sand-proof sealing check ring 5 and the cylinder 1. It can effectively block sand particles larger than 50 μm from entering;

[0039] Exemplary, the piston 3 is matched with the piston ring 4 through the annular installation slot opened in the radial direction, the piston ring 4 is closely attached to the cylinder body 1 to form a sealing surface, and the oil liquid is prevented from leaking from the sealing surface;

[0040] The piston 3 is provided with the annular groove 7 at the upper and lower ends to respectively install the upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5, and the upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 are symmetrically installed at the upper and lower ends of the piston 3 to prevent the sand particles from entering and damaging the sealing.

[0041] The inner side inclined surface of the upper sand-proof sealing check ring 2 close to the axis is provided with a spiral flow channel, so that the sand particles at the upper part of the piston 3 enter the central axis position along the spiral flow channel and flow back to the working cavity, the depth of the spiral flow channel is 0.5mm-5mm, the width is 0.5mm-1mm, and the spiral flow channel is opened in equal depth or gradually deep from the outer side to the inner side, which can effectively guide the sand particles with a particle size of more than 50um to flow back.

[0042] The number of the piston ring 4 is not less than 3, and the piston ring 4 is provided with an inclined opening to produce elastic deformation and closely attach to the cylinder body 1.

[0043] The arrangement mode of the piston ring 4 depends on the number, and the multiple piston rings 4 are uniformly and staggeredly arranged in the circumferential direction of the piston 3 to form a multi-stage sealing defense line.

[0044] The upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 are of a split structure and are nested and installed in the annular grooves 7 at the upper and lower ends of the piston 3. The upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 form a split sand-proof sealing check ring.

[0045] The upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 are formed by filling polytetrafluoroethylene material and are molded. The upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 are nested and installed in the annular grooves 7 at the two ends of the piston 3. The inclination angle between the upper sand-proof sealing check ring 2, the lower sand-proof sealing check ring 5 and the inner wall of the cylinder body 1 is controlled to be between 120°-150° to form a sand particle flow guiding channel.

[0046] Working principle: the piston body is provided with three sealing structures: the first sealing structure is formed by the upper sand-proof sealing check ring 2 closely attaching to the cylinder body 1; the second sealing structure is formed by the piston ring 4 closely attaching to the cylinder body 1; and the third sealing structure is formed by the lower sand-proof sealing check ring 5 closely attaching to the cylinder body 1. The piston 3 is provided with the annular installation slot opened in the radial direction to match with the piston ring 4, the piston ring 4 is closely attached to the cylinder body 1 to form a main sealing surface; the piston 3 is provided with the annular groove 7 at the upper and lower ends to respectively install the upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 to form a symmetric sand-proof barrier.

[0047] The piston ring 4 in the application is made of a steel material after surface treatment to ensure the wear resistance under high sand content working conditions. The upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 are added with wear-resistant fillers to improve the sand particle impact resistance.

[0048] The triple sealing structure of the present application forms a stepped sand prevention system: the first sand prevention sealing check ring 2 intercepts large particle sand; the second piston ring 4 realizes main sealing; and the third lower sand prevention sealing check ring 5 ensures zero leakage. Each sealing element works independently and cooperatively.

[0049] The upper sand prevention sealing check ring 2 and the lower sand prevention sealing check ring 5 in the present application adopt a floating installation mode, which can automatically compensate for the gap change caused by the deformation or eccentricity of the cylinder body 1, and maintain stable sand prevention effect.

[0050] The overall length of the piston 3 in the present application is shortened by 40%-50% compared with the conventional design, compact design is realized through optimization of sealing layout, and the motion inertia and energy consumption are reduced.

[0051] Embodiment 2, a sand prevention sealing method for the sand prevention sealing structure of the submersible electric pump piston, comprising:

[0052] The triple sealing structure is used to form a stepped sand prevention system to form a sand prevention barrier, so that the sand particles flow back to the working cavity under the action of the first sealing structure, and part of the leaked oil is further subjected to main sealing through the second sealing structure, and zero leakage is completed through the third sealing structure.

[0053] Illustratively, the first sealing structure is formed by the upper sand prevention sealing check ring 2 and the cylinder body 1; the second sealing structure is formed by the piston ring 4 and the cylinder body 1; and the third sealing structure is formed by the lower sand prevention sealing check ring 5 and the cylinder body 1.

[0054] The piston 3 is provided with a ring-shaped installation groove in the radial direction and cooperates with the piston ring 4, the piston ring 4 is tightly attached to the cylinder body 1 to form a main sealing surface; and the piston 3 is provided with ring-shaped grooves 7 at the upper and lower ends to respectively install the upper sand prevention sealing check ring 2 and the lower sand prevention sealing check ring 5, thereby forming a symmetrical sand prevention barrier.

[0055] Illustratively, the first upper sand prevention sealing check ring 2 intercepts large particle sand.

[0056] Illustratively, the method for making the sand particles flow back to the working cavity under the action of the first sealing structure comprises:

[0057] Step one, in the liquid-solid two-phase system, the sliding angle of the sand particles on the inclined surface of the upper sand prevention sealing check ring 2 is set in combination with the particle-liquid interaction, the paper resistance and the particle characteristics;

[0058] The sliding angle is expressed as:

[0059]

[0060] In the formula, is the effective friction coefficient, including dry friction and liquid viscous effect; is a constant; is the effective cohesion force, including liquid bridge force and Van der Waals force:

[0061]

[0062] in the formula, is the dry internal friction angle of the sand particles, is the dynamic viscosity of the crude oil, is the density of the sand particles, is the diameter of the sand particles, is the shear rate (depending on the flow conditions), is the viscosity correction coefficient;

[0063] Step two, according to the sand-containing working condition, the sliding angle of the sand particles on the inclined surface of the upper sand-proof sealing retainer 2 is set and calculated, and the actual working condition sliding angle is obtained, which is preferably about 60°;

[0064] Step three, according to the obtained actual working condition sliding angle, the upper surface of the upper sand-proof sealing retainer 2 and the upper end surface of the piston 3 are set at an angle, and the inner inclined surface of the upper sand-proof sealing retainer 2 close to the axis of the piston 3 is obtained according to the set angle, and the set angle is 60°-90°;

[0065] Step four, based on the obtained inner inclined surface of the upper sand-proof sealing retainer 2 close to the axis of the piston 3, the resultant force direction of the oil liquid impact force and the reaction force of the upper sand-proof sealing retainer 2 points to the working chamber, so that the sand particles are guided to flow back to the working chamber.

[0066] Preferably, the inner inclined surface of the upper sand-proof sealing retainer 2 close to the axis is provided with a spiral flow channel, so that the sand particles in the upper part of the piston 3 enter the central axis position along the spiral flow channel and flow back to the working chamber, the depth of the spiral flow channel is 0.5mm-5mm, the width is 0.5mm-1mm, and the spiral flow channel is opened with equal depth or gradually deepening from the outside to the inside, which can effectively guide the sand particles with a particle size of 50μm or more to flow back. The inner inclined surface of the upper sand-proof sealing retainer 2 close to the axis of the piston 3 and the upper end surface of the piston 3 are 60°-90°,

[0067] Illustratively, the inner inclined surface of the upper sand-proof sealing retainer 2 close to the axis of the piston 3 and the inclination angle of the inner wall of the cylinder body 1 are controlled between 120°-150°.

[0068] As known from the above embodiments, the sand-proof sealing structure of the submersible electric pump piston provided in the present application adopts a triple sealing structure composed of the upper sand-proof sealing retainer 2, the piston ring 4 and the lower sand-proof sealing retainer 5, forms a stepped sand-proof sealing system, the first sand-proof retainer can intercept the main sand particles, the second piston ring realizes the main sealing function and works in a clean environment, and the third sand-proof retainer serves as the final barrier to ensure zero sand particle leakage;

[0069] The sand-proof sealing structure of the submersible electric pump piston provided in the application shortens the piston length by 40-50% while ensuring the sealing effect, reduces the overall weight, and significantly reduces the moving inertia and energy consumption through the collaborative arrangement of the split type check ring and the multiple piston rings;

[0070] The sand-proof sealing structure of the submersible electric pump piston provided in the application forms a sand particle flow guide channel through the inclination design of the sand-proof sealing check ring, and can guide the invading sand particles back to the working cavity, avoiding accumulation.

[0071] The sand-proof sealing structure of the submersible electric pump piston provided in the application is designed as a split type check ring, which facilitates quick replacement of damaged components without the need to disassemble the entire piston, thereby shortening the maintenance time.

[0072] The sand-proof sealing structure of the submersible electric pump piston provided in the application is designed as a split type check ring, which facilitates quick replacement of damaged components without the need to disassemble the entire piston, thereby shortening the maintenance time.

[0073] Application example: The piston 3 is in precise fit with the piston ring 4 through the circumferential annular mounting groove, the piston ring 4 is in oblique opening in the free state, the opening gap is less than 1mm after installation, the piston ring 4 is tightly attached to the inner wall of the cylinder body 1 under the action of its own elastic force, and the gap between the piston 3 and the cylinder body 1 is blocked, and the oil is sealed. In addition, considering that there is still a small amount of oil leakage from the opening of the piston ring 4 and the periodic tight attachment of the two end surfaces of the piston ring 4 to the groove side walls in the annular mounting groove, multiple piston rings 4 are arranged in staggered phase to achieve throttling and pressure reduction. At the same time, the piston 3 is embedded in the upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 in an interference fit manner at the upper and lower ends, which blocks the sand particles from entering the sealing ring, accelerates the wear of the piston ring 4, and reduces the service life of the sealing structure. When the submersible electric pump is working, the sand particles are first intercepted by the upper sand-proof sealing check ring 2, are guided back to the working cavity along the inclined surface of the upper sand-proof sealing check ring 2, and part of the oil leaks into the piston ring 4 area. The multiple piston rings 4 are tightly attached to the cylinder body 1, avoiding oil leakage.

[0074] Figure 2 For Figure 1 The sand-proof sealing check ring is split type, and the material is filled with polytetrafluoroethylene. The upper sand-proof sealing check ring 2 and the cylinder body 1 form a flow guide inclination angle after installation, so that the sand particles flow back and no longer enter the piston ring 4 sealing structure. The upper sand-proof sealing check ring 2 and the lower sand-proof sealing check ring 5 are the same in structure, except that the lower sand-proof sealing check ring 5 is installed at the lower part of the piston 3, and the installation form is opposite to that of the upper sand-proof sealing check ring 2.

[0075] The above merely describes the preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any modification, equivalent replacement and improvement within the technical range disclosed by the present application and within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A sand seal structure for a submersible electric pump piston, characterized by, The structure comprises a piston (3) and three sealing structures arranged on the piston (3); the first sealing structure is an upper sand-proof sealing ring (2) and a cylinder body (1) to form an upper seal, the second sealing structure is a piston ring (4) and the cylinder body (1) to form a main seal, and the third sealing structure is a lower sand-proof sealing ring (5) and the cylinder body (1) to form a lower seal; The piston (3) is provided with an annular groove (7) at the upper and lower ends to install the upper sand-proof sealing ring (2) and the lower sand-proof sealing ring (5), and the upper sand-proof sealing ring (2) and the lower sand-proof sealing ring (5) are symmetrically installed at the upper and lower ends of the piston (3). The inner side slope of the upper sand-proof sealing ring (2) close to the axis is provided with a spiral flow channel, so that the sand particles on the upper part of the piston (3) enter the central axis position through the spiral flow channel and flow back to the working cavity, and the spiral flow channel is opened from the outside to the inside with equal depth or gradually deepening. The sand-proof sealing method for the sand-proof sealing structure of the submersible electric pump piston comprises the following steps: The three sealing structures are designed to form a stepped sand-proof system to form a sand-proof barrier, so that the sand particles flow back to the working cavity under the action of the first sealing structure, and part of the leaked oil is further subjected to the main sealing through the second sealing structure, and the zero leakage is completed through the third sealing structure; The method for making the sand particles flow back to the working cavity under the action of the first sealing structure comprises the following steps: Step one: in the liquid-solid two-phase system, the sliding angle of the sand particles on the upper surface of the upper sand-proof sealing ring (2) is set according to the particle-liquid interaction, the paper resistance and the particle characteristics; The sliding angle θ is expressed as: In the formula, μ eff The effective coefficient of friction includes dry friction and the viscosity effect of the liquid; σ is a constant; C eff For effective cohesion, including liquid bridging forces and van der Waals forces: wherein is the dry internal friction angle of the sand, η is the kinematic viscosity of the crude oil, ρ p is the sand density, d is the sand diameter, is the shear rate, k η is the viscosity correction factor; Step two: the sliding angle of the sand particles on the slope of the upper sand-proof sealing ring (2) is set according to the sand-containing working condition and calculated to obtain the actual working condition sliding angle; Step three: according to the obtained actual working condition sliding angle, the upper surface of the upper sand-proof sealing ring (2) and the upper end surface of the piston (3) are set at an angle, and the inner side slope of the upper sand-proof sealing ring (2) close to the axis of the piston (3) is obtained according to the set angle; Step four: based on the obtained inner side slope of the upper sand-proof sealing ring (2) close to the axis of the piston (3), the resultant force direction of the impact force of the sand particles on the oil and the reaction force of the upper sand-proof sealing ring (2) points to the working cavity, so that the sand particles are guided to flow back to the working cavity; The actual working condition sliding angle of step two is 60°. The set angle of step three and the inner side slope of the upper sand-proof sealing ring (2) close to the axis of the piston (3) are 60°-90°.

2. The sand seal structure for the electrical submersible pump piston according to claim 1, characterized in that, The piston (3) is matched with the piston ring (4) through the annular installation groove in the radial direction, the piston ring (4) is closely matched with the cylinder body (1) to form a sealing surface, and the sealing surface is used to block the leakage of oil.

3. The sand seal structure for electrical submersible pump piston of claim 1, wherein, The number of the piston ring (4) is not less than three, and the piston ring (4) is provided with a slant opening, and the multiple piston rings (4) are evenly staggered in the circumferential direction of the piston (3) to form multiple levels of sealing defense lines.

4. The sand prevention seal structure for the electrical submersible pump piston according to claim 1, characterized by, The upper sand-proof sealing ring (2) and the lower sand-proof sealing ring (5) are molded by filling polytetrafluoroethylene material.

5. The sand prevention seal structure for electrical submersible pump piston of claim 1, wherein, The upper sand-proof sealing check ring (2) and the lower sand-proof sealing check ring (5) are installed in a floating mode, and an axial and radial clearance is reserved between the check ring and the annular groove (7) of the piston (3). When the cylinder body (1) is deformed due to temperature change or pressure fluctuation, the check ring is free to move in the groove, and the clearance change between the cylinder body (1) and the piston (3) is compensated by the displacement of the check ring, so that hard jamming is avoided.

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