壓裂泵柱塞密封副摩擦學(xué)性能分析與研究

1、 壓裂泵柱塞密封副摩擦學(xué)性能分析與研究 研究生 朱維兵 指導(dǎo)教師 周錫容 楊啟明 目前，壓裂或酸化工藝所增產(chǎn)的油氣量在整個(gè)油氣產(chǎn)量中的比重越來越大。從壓裂、酸化的工作原理看，大排量、高壓力注入是該工藝的必要前提，而壓裂泵就是完成該工作的核心設(shè)備。在長期的實(shí)踐和研究中，壓裂及酸化工藝日趨完善，相比之下，壓裂及酸化設(shè)備與之很不相適應(yīng)，嚴(yán)重影響了壓裂及酸化工藝的正常實(shí)施。 壓裂泵中的柱塞、柱塞密封的工作條件最惡劣，是泵的最薄弱環(huán)節(jié)，壓裂泵工作的可靠程度主要取決于它們的可靠程度。 從目前我國各油田所使用的柱塞及密封圈來看，其壽命相當(dāng)?shù)?，每年消耗量很大。調(diào)查表明，在四川地區(qū)，酸化施工中，柱塞壽命為100

2、多個(gè)小時(shí)，密封圈的壽命一般為3040小時(shí)，有時(shí)只有1020小時(shí)。隨著壓裂、酸化施工的頻繁化，柱塞與密封圈的壽命問題將顯得越來越突出，現(xiàn)場迫切要求進(jìn)行壓裂泵柱塞密封摩擦副的系統(tǒng)研究，提高其壽命，保證施工順利進(jìn)行。 為考查該項(xiàng)研究的現(xiàn)狀，我們查閱了近十年國內(nèi)、國外有關(guān)文獻(xiàn)，并檢索有關(guān)壓裂泵柱塞與密封的研究。調(diào)研表明，該內(nèi)容的研究是很不夠的。因此本文對壓裂泵柱塞密封副進(jìn)行了實(shí)驗(yàn)研究和理論分析，主要包括以下四個(gè)方面的內(nèi)容： 1 柱塞密封圈的有限元分析 為了把流體動力潤滑逆理論應(yīng)用到V型橡膠密封圈的實(shí)際性能上去，必須把V型橡膠密封圈作用于柱塞表面的接觸應(yīng)力計(jì)算出來，特別是最大壓力梯度應(yīng)該算出。本文采用有

3、限單元法計(jì)算了橡膠密封圈與柱塞間的接觸應(yīng)力，獲得了接觸應(yīng)力的分布規(guī)律以及其與工作介質(zhì)壓力的關(guān)系，同時(shí)對密封圈的重要結(jié)構(gòu)參數(shù)進(jìn)行了優(yōu)化。 2 柱塞密封摩擦副潤滑狀態(tài)的分析 現(xiàn)用的壓裂泵柱塞密封副都采用強(qiáng)制性的全損式潤滑，然而人們對這種潤滑方式缺乏理性認(rèn)識。本章的分析試圖利用工作介質(zhì)(如酸化液體)來潤滑柱塞與密封這對摩擦副是否存在可行性。分析表明，寄希望于工作介質(zhì)來潤滑這對密封副是不可行的，為此，我們又重點(diǎn)討論了強(qiáng)制性潤滑的影響。 3 柱塞及密封圈的失效分析 為了弄清柱塞及密封的失效機(jī)理，本章對現(xiàn)場使用的美國柱塞和國產(chǎn)柱塞及密封進(jìn)行了初步的失效分析，探究了失效機(jī)理，從而為柱塞及密封材料的選擇、柱塞

4、表面強(qiáng)化方式的確定以及密封結(jié)構(gòu)的設(shè)計(jì)提供了依據(jù)。 4 壓裂泵柱塞密封副的改進(jìn)設(shè)計(jì) 通過前幾章的分析和研究，我們對壓裂泵柱塞密封摩擦副有了比較全面的了解，所有這些工作的落腳點(diǎn)應(yīng)該體現(xiàn)在柱塞與密封的改進(jìn)設(shè)計(jì)上。本章對壓裂泵柱塞密封圈的結(jié)構(gòu)、材料以及柱塞的結(jié)構(gòu)、表面強(qiáng)化方式等進(jìn)行了詳細(xì)的分析和討論，最后設(shè)計(jì)了一套壽命和可靠性都較高的柱塞密封副結(jié)構(gòu)。 結(jié)論 通過以上各章的理論分析和實(shí)驗(yàn)研究，可以得出如下主要結(jié)論： 1 通過有限元計(jì)算，獲得了密封圈接觸應(yīng)力的分布規(guī)律。接觸應(yīng)力在接觸寬度上的分布是不均勻的，呈一個(gè)壓力峰的形式；同時(shí)，隨工作介質(zhì)壓力的增加，只增加接觸應(yīng)力的大小，并不改變應(yīng)力分布的形態(tài)，即在不

5、同壓力下的應(yīng)力分布曲線是相似的。 2 通過有限元計(jì)算，證明了壓裂泵柱塞密封在工作介質(zhì)壓力的作用下，有“向心效應(yīng)”的存在。橡膠密封圈在工作時(shí)，由于工作介質(zhì)壓力的作用，使得唇部受到拉伸作用，離開柱塞表面翹起，即所謂的“向心效應(yīng)”。這種“向心效應(yīng)”存在還說明因唇部過盈的初始接觸密封力，在載荷情況下是可以忽略的。還應(yīng)該看到，“向心效應(yīng)”的出現(xiàn)，使得磨礪性顆粒有可能進(jìn)入對磨界面，從而加劇密封與柱塞的磨料磨損。 3 通過有限元計(jì)算，獲得了三道密封圈接觸應(yīng)力的分布規(guī)律。即從第一道密封到第三道密封呈遞減趨勢，并且第一道密封圈的接觸應(yīng)力遠(yuǎn)大于后兩道密封圈的接觸應(yīng)力，這為后面的失效分析提供了依據(jù)；同時(shí)也表明：第一

6、道密封圈在工作的過程中起著關(guān)鍵性的作用，當(dāng)其破壞失效后，后面的密封圈又處于第一道密封圈的工況，由于多個(gè)密封圈的重疊使用，從而達(dá)到密封的效果。 4 通過對壓裂泵柱塞密封副的理論分析，獲得了強(qiáng)制潤滑時(shí)油膜的形成機(jī)理：在壓裂泵的排出行程，潤滑油由于油壓和柱塞運(yùn)動的聯(lián)合作用下，被帶入密封界面內(nèi)，密封背部端口設(shè)計(jì)成具有半徑為的圓角(一般地說這一圓角半徑在受壓下還要變大)或者因變形形成一個(gè)圓角，這樣自然就構(gòu)成了一收斂形油楔，同時(shí)因潤滑油具有一定的粘度，在柱塞速度為時(shí)，就能產(chǎn)生流體動壓。收斂楔出口端(=0處)的油膜厚度為,這樣就可以認(rèn)為沿密封寬度上能保持一厚度為的平行油膜潤滑密封。在吸入行程時(shí)，密封背部不產(chǎn)

7、生流體壓力，但原有的潤滑油仍能保持在界面上(因?yàn)榇藭r(shí)接觸應(yīng)力小，不會把潤滑油完全擠掉，柱塞攜帶著油向后運(yùn)行)，這就是壓裂泵強(qiáng)制性潤滑的機(jī)理。 5 通過對密封圈的宏觀和微觀失效分析，發(fā)現(xiàn)密封圈的失效形式為永久變形、偏磨、燒傷、根部損傷及磨料磨損、疲勞磨損和卷筒形成磨損等多種破壞形式。針對這些情況應(yīng)采取多種措施如合理選擇密封材料，加強(qiáng)潤滑冷卻，改善安裝條件來改善密封圈的工作情況，達(dá)到減少磨損，提高壽命的目的。除燒傷外，各失效形式造成密封能力喪失均是一個(gè)積累過程。而燒傷則使密封馬上喪失工作能力，因此應(yīng)引起足夠的重視。 6 通過對柱塞進(jìn)行宏觀和微觀的分析表明，柱塞失效的主要形式：GC-1柱塞，在磨蝕條

8、件下的磨料磨損，伴隨輕度腐蝕和疲勞；GC-2柱塞，表面嚴(yán)重擦傷磨損和表面薄層剝落；MC柱塞，表面嚴(yán)重腐蝕、疲勞和化合物剝落。造成失效的主要原因：壓裂液中含砂量大，并且還含有較多的石英砂等硬質(zhì)相；壓裂液中含有酸等腐蝕性介質(zhì)；以上兩種原因造成柱塞的磨料磨損和腐蝕破壞。在工作過程中，工作壓力大，由于長期往復(fù)載荷(壓應(yīng)力)作用，硬質(zhì)相顆粒嵌入密封圈，并在高密封壓力下對柱塞往復(fù)作用，造成疲勞，產(chǎn)生疲勞裂紋和球形屑。 針對壓裂泵的工況，在使用柱塞時(shí)，可采用以下一些方法： 對于加砂壓裂，柱塞采用噴鎳的表面強(qiáng)化方式，噴焊材料為鎳基F11-60；對于酸化壓裂，柱塞也采用噴鎳的表面強(qiáng)化方式，噴焊材料為鎳基F11-

9、55。此外，在柱塞表面采用化學(xué)熱處理如滲硼等后，其抗腐蝕和磨料磨損的性能得到較好的改善，也能滿足柱塞的工作要求，因而可代替耐磨合金的噴焊工藝。 TRIBOLOGY ANALYSIS AND RESEARCH OF THE SYSTEM OF PLUNGER AND SEAL IN FRACTURING PUMPS At present,the proportion of output of of oil and gas increased by fracture acidizing to the whole output of oil and gas is getting larger and

10、 larger.Judged by its working priciple,the essential preregaisite of fracturing technology is large displacement and high pressure,and the key equipment for this task is fracturing pump.Trough long practise and research, the fracturing technology is getting perfecter and perfecter,but in contrast, t

11、he fracturing equipment is unsuitable,so the fracturing technology cannot be put into effect normally. Working at adverse circumstances,the plunger and seal in fracturing pumps are weak links,and their reliabilities decide the reliability of pump. The lifetime of plunger and seal which are serving i

12、n all fields of our country is very short,every year large amounts of them are needed.Take sichuan district as an example,the lifetime of plunger is about 100 hours,but the seal is about 3040 hours,sometimes only 1020 hours.With its frequently used,the problem of lifetime of plunger and seal is stic

13、king out.In order to raise their lifetime and put the fracturing technology into use normally ,the research for this system is urgent. With investigation,the research for this system of plunger and seal is very scarce.This paper aims at experimental studies and theoretic analysis of this system,and

14、there are four parts included: 1 finite element analysis of sealing ring In order to put converse theory of hydrodynamic lubrication into use,the stress distribution which acts on the surface of plunger must be obtained,especially the maxium pressure gradient.With finite element method the laws of s

15、tress distribution in seal are obtained,at the same time, the most important structural factor of sealing ring is optimized. 2 lubricating analysis of the system of plunger and seal Fracturing pumps which are in use adopt forced lubrication for the system of plunger and seal,but man has little knowl

16、edge about this kind of lubricating pattern.In this chapter,we try to use actuating medium(for example, acidic medium) to lubricate this system ,but we failed,so we discussed the affection of forced lubrication emphatically. 3 failure analysis of plunger and seal In this chapter,in order to understa

17、nd failure mechanism of plunger and seal,we have conducted failure analysis of plunger made by America or China and sealing rings which are in use,and we have obtained their failure mechanism.So we can provide basis for material selection of plunger and sealing ring、selection of heat treatment patte

18、rn of plunger、the design of structure of sealing ring. 4 improved design of plunger and seal in fracturing pumps Through analysis and research the above-mentioned,we have understood the system of plunger and seal in fracturing pumps comprehensively.All this work must be boiled down to the improved d

19、esign of plunger and seal.In this chapter, we have analysed and discussed in detail the structure and material of sealing ring、the structure and heatment pattern of plunger,and so on.At last,we have designed a system of plunger and seal with higher lifetime and more reliability. conclusions Through

20、experimental studies and theoretic analysis the above mentioned,we can get some key conclusions as followed: 1 We have obtained the laws of stress distribution in seal by finite element analysis,that is, the stress is not well-distributed,having a pressure peak. At the same time,with increasing pres

21、sure in actuating medium the contacted stress only increased in size and the appearance of stress distribution has the same appearance at different pressures. 2 Under the action of actuating medium pressure, the centripetal effect in seal is proved by finite element analysis. That is,working at the

22、action of actuating medium pressure,the lip of sealing ring made by rubber bears a tensile force,so it departs from the surface of plunger,It is called "centripetal effect",it gives a chance for the abrasive to get into the system of plunger and seal,so it aggravates their abrasive wear. 3

23、 We have obtained the laws of contact stress distribution in three sealing rings by finite element analysis.That is, the contact stress from the first sealing ring to the third one decreases progressively,so it serves as a basis for failure analysis,it also proved that the first sealing ring plays a

24、n important role in sealing,when it failures ,the later sealing ring works at the same conditions as the first one.Because many sealing rings are used,their effect is perfectly well. 4 Under forced lubricating conditions,the formation of film is brough into light by theoretic analysis of the system

25、of plunger and seal.That is,at the out-stoke of fracturing pumps,because of the action of oil pressure and plunger's movement,lubricating oil is drawed into the system of plunger and seal,there is a converge edge(formed by deformation or designed)at the back of sealing ring,and the lubricating o

26、il has certain viscosity,with the speed of plunger,so hydrodynamic lubrication can be formed.The film thickness at the exit of the converge edge(z=0) is .So we can think there is aparallelling oil film which thickness is lubricating the surface of plunger.At the in-stroke of fracturing pumps,hydrody

27、namic lubrication cannot be formed at the back of sealing ring,but the oil remains in the system of plunger and seal,this is the formation mechanism of film which is under forced lubricating conditions. 5 With macroscopic and microcosmic failure analysis of sealing ring,we find its failure forms are

28、 permanental deformation、slanting wear、burns、root damages and abrasive wear、fatigue wear、rolling wear,ect.So we should adopt suitable measures to improve the working conditions of sealing ring,such as choosing suitable material for seal、reinforcing lubrication and cooling、improving installing condit

29、ions,so to raise its lifetime and reduce its wear.All these faliure forms except burns are a accumulating process,but burns will cause a sudden failure.So we should pay special attention to it. 6 With macroscopic and microcosmic falure analysis of plunger,we find their failure forms are,GC-1 plunger:abrasive wear、light corrosion and fatigue wear,GC-2 plunger:surface serious rubbing and surface thin section peeling,MC plunger:surface seri