資源目錄里展示的全都有，所見即所得。下載后全都有,請放心下載。原稿可自行編輯修改=【QQ：401339828 或11970985 有疑問可加】 附錄 Pressure transient theory Before embarking on the analysis of pressure transient phenomena and the derivation of the appropriate wave equations,it will be usefull to describe the general mechanism of pressure propagation by reference to the events fllowing the instantaneous closure of a value postioned at the med-length point of a frictionless pipeline carrying fluid between two reservoirs.The two pipeline sections upstream and downstream of the value are identical in all respects.Transient pressure waves will be propagated in both pipes by valve operation and it will be assumed that rate of value closure precludes the use of rigid column theory. As the valve is closed,so the fluide approaching its upstream face is retarded with a consequent compression of the flude and an expansion od the pipe cross-section.The increase in pressure at the valve results in a pressure wave being propagated upstream which conveys the retardation of flow to the column of fluid approaching the valve along the upstream pipeline.This pressure wave travels through the fluid at the appropriate sonic velocity,which will be shown to depend on the properties of the fluid and the pipe material. Similarly,on the downstream side of the valve the retardation of flow results in a reduction in pressure at the valve,with the result that a negative pressure waves is propagated along the downstream pipe which,in turn,retards the fluid flow.It will be assumed that this pressure drop in the downstream pipe is insufficient to reduce the fluid pressure to either its vapour pressure or its dissolved gas release pressure,which may be considerable different. Thus,closure of the valve results in propagation of pressure waves along both pipes and,although these waves are of different sign relative to the steady pressure in the pipe prior to valve operation,the effect is to retard the flow in both pipe sections.The pipe itself is affected by the wave propagation as the upstream pipe swells as the pressure rise wave passes along it,while the downstream pipe contracts due to the passage of the pressure reducting wave.The magnitude of the deformation of the pipe cross-section depends on the pipe material and can be well demonstrated if,for example,thin-walled rubber tubing is employed.The passage of the pressure wave through the fluid is preceded,in practice,by a strain wave propagating along the pipe wall at a velocity close to the sonic velocity in the pipe material.However,this is a secondary effect and,while knowledge of its existence can explain some parts of a pressure-time trace following valve closure,it has little effect on the pressure levels generated in practical transient situations. Following valve closure,the subsequent pressure-time history will depend on the conditions prevailing at the boundaries of the system.In order to describe the events following valve closure in the simple pipe system outlined above,it will be easier to refer to a series of diagrams illustrating conditions in the pipe at a number of time steps. Assuming that valve closure was instantaneous,the fluid adjacent to the valve in each pipe would have been brought to rest and pressure waves conveying this information would have been propagated at each pipe at the appropriate sonic velocity c.At a later time t,the situation is as shown in fig.The wavefronts having moved a distance 1=ct,in each pipe,the deformation of the pipe cross-section will also have traveled a distancel as shown. The pressure waves reach the reservoirs terminating the pipes at a time t=1/c.at this instant,an unbalanced situation arises at the pipe-reservior junction,as it is clearly impossible for the layer of fluid adjacent to the reservoir inlet to maintain a pressure different to that prevailing at that depth in the reservoir.Hence,a restoring pressure wave having a magnitude suffcient to bring the pipeline pressure back to its value prior to valve closure is transmitted from each reservoit at a time 1/c.For the upstream pipe,this means that a pressure wave is propagated towards the closed valve,reducing the pipe pressure to its original value and restoring the pipe cross-section.The propagation of this wave also preduces a fluid flow from the pipe into the reservoir as the pipe ahead of the moving wave is at a higher pressure than the reservoir.Now,as the system is assumed to be frictionless,the magnitude of this reversed flow will be the exact opposite of the original flow velocity,as shown in fig. At the downstream reservoir,the converse occurs,resulting in the propagation of a pressure rise wave towards the valve and the establishment of a flow from the downstream reservoir towards the valve. For the simple pipe considered here,the restoring pressure waves in both pipes reach the valve at a time 21/c.The whole of the upstream pipe has,thus,been returned to its original pressure and a flow has been established out of the pipe.At time 21/c,as the wave has reached the valve,there remains no fluid ahead of the wave to support the reversed flow.A low pressure region,therefore,forms at the valve,destroying the flow and giving rise to a pressure reducing wave which is transmitted upstream from the valve,once again bringing the flow to rest along the pipe and reducing the pressure within the pipe .It is assumed that the pressure drop at the valve is insufficient to reduce the pressure to the fluid vapour pressure.As the system has been assumed to be frictionless,all the waves will have the same absolute magnitude and will be equal to the pressure increment,above steady running pressure,generated by the closure of the valve.If this pressure increment is h,then all the waves propagating will be±h,Thus,the wave propagation upstream from the valve at time 21/c has a value-h,and reduces all points along the pipe to –h below the initial pressure by the time it reachs the upstream reservoir at time 31/c. Similarly,the restoring wave from the downstream reservoir that reached the valve at time 21/c had established a reversed flow along the downstream pipe towards the closed valve .This is brought to rest at the valve,with a consequent rise in pressure which is transmitted.downstream as a +h wave arriving at the downstream reservoir at 31/c,at which time the whole of the downstream pipe is at pressure +h above the initial pressure whth the fuid at rest. Thus,at time 31/c an unbalanced situation similar to the situation at t=1/c again arises at the reservoir –pipe junctions with the difference that it is the upstream pipe which is at a pressure below the reservoir pressure and the downstream pipe that is above reservoir pressure .However,the mechanism of restoring wave propagation is identical with that at t=1/c,resulting in a-h wave being transmitted from the upstream reservior,which effectively restores conditions along the pipe to their initial state,and a+h wave being propagated upstream from the downstream reservoir,which establishes a flow out of the downstream pipe.Thus,at time t=41/c when these waves reach the closed valve,the conditions along both pipes are identical to the conditions at t=0,i.e.the instant of valve closure.However ,as the valve is still shut,the established flow cannot be maintained and the cycle described above repeats. The pipe system chosen to illustrate the cycle of transient propagation was a special case as,for convenience,the pipes upstream and downstream of the valve were identical.In practice,this would be unusual.However,the cycle described would still apply,except that the pressure variations in the two pipes would no longer show the same phase relationship.The period of each individual pressure cycle would be 41/c,where I and c took the appropriate values for each pipe.It is important to note that once the valve is closed the two pipes will respond separately to any further transient propagation. The period of the pressure cycle described is 41/c.However,a term ofen met in transient analysis is pipe period,this is defined as the time taken for a restoring reflection to arrive at the source of the initial transient propagation and,thus,has a value 21/c.In the case described,the pipe period for both pipes was the same and was the time taken for the reflection of the transient wave propagated by valve from the reservoirs. From the description of the transient cycle above,it is possible to draw the pressure-time records at points along the pipeline.These variations are arrived at simply by calculating the time at which any one of the±h waves reaches a point in the system assuming a constant propagation velocity c.The major interest in pressure transients lies in methods of limiting excessive pressure rises and one obcious method is to reduce valve speeds.However,reference to fig.illustrates an important point no reduction in generated pressure will occur until the valve closing time exceeds one pipe period.The reduction in peak pressure achieved by slowing the valve before a time 21/c from the start of valve closure and,as no beneficial pressure relief can be achieved if the valve is not open beyond this time.Generally,valve closures in less than a pipe period are referred to as rapid and those taking longer than 21/c are slow. In the absence of friction,the cycle would continue indefinitely.However ,in practice, friction damps the pressure oscillations within a short period of time .In system where the frictional losses are high,the neglect of frictional effects can result in a serious underestimate of the pressure rise following valve closure.In these case,the head at the valve is considerably lower than the reservoir head.However,as the flow is retarded,so the frictional head loss is reduced along the pipe and the head at the valve increase towards the reservoir value.As each layer of fluid between the valve and the reservoir is brought to rest by the passage of the initial +h wave so a series of secondary positive waves each of a magnitude corresponding to the friction head recoverd is transmitted toward the valve,resulting in the full effect being felt at time 21/c.As the flow reverses in the pipe during time 21/c to 41/c,the opposite effect is recorded at the valve because of the re-establishment of a high friction loss,these variations being shown by lines AB and CD.In certain cases,such as long distance oilpipelines,this effect may contribute the larger part of the pressure rise following valve closure. In addition to the assumptions made with regard to friction in the cycle description,mention was also made of the condition that the pressure drop waves at no time reduced the pressure in the system to the fluid vapour pressure.If this had occurred,then the fluid column would have separated and the simple cycle described would have been disrupted by the formation of a vapour cavity at the position where the pressure was reduced to vapour level.In the system described,this could happen on the valve’s downstream face at time 0 or on the upstream face at time 21/c.The formation of such a cavity is followed by a period of time when the fluid column moves under the influence of the pressure gradients between the cavity and the system boundaries.The period is normally terminated by the generation of excessive pressure on the final collapse of the cavity.This phenomena is generally referred to as column separation and is frequently made more complex by the release of dissolved gas in the vicinity of the cavity. Pressure transient propagation may be defined in any closed pipe application by two basic equations,namely the equations of motion and continuity applied to a short segment of the fluid column.The dependent variables are the fluid’s average pressure and velocity at any pipe cross section and the independent variables are time and distance,normally considered positive in the steady flow direction.Friction will be assumed proportional to velicity squared and steady flow friction relationships will be assumed to apply to the unsteady flow cases considered. 壓力沖擊現(xiàn)象 在著手分析壓力沖擊現(xiàn)象和化分合理的流體方程之前，去描繪一般的關(guān)于壓力傳遞的機械理論。通過參與這個關(guān)于閥門定位在一個較長點幾乎沒有摩擦的管道傳輸液體于兩個蓄能源之間的結(jié)果之后是必要的。這個閥門連接的順流管道截面和逆流管道截面考慮是一樣的。壓力沖擊流將通過閥門操作傳遞在兩個管道之間，并且假設(shè)閥門的關(guān)閉速度不應(yīng)用于堅固圓管理論。 如果閥門是關(guān)閉的，而液體的流向是逆方向的，緩慢前進，結(jié)果導(dǎo)致液體被壓縮和管道的橫截面膨脹。閥門的壓力增加導(dǎo)致高壓液體逆向流動，延長了液體流過圓管通向閥這段管道的時間。這種高壓液體的流動類似聲音的傳播，是依靠液體和管道材料作為介質(zhì)的。 同樣，閥的順流面流動的延遲，將導(dǎo)致減小壓力在閥門處。這個結(jié)果否定了高壓液體的流動是沿著順流管道的，阻止液體流動，假設(shè)流體壓力在順流管道是不能減小液體壓力的或者蒸汽壓力或者溶解氣體釋放的壓力，各種愿意的考慮是不同的。 這樣，關(guān)閉著的閥門導(dǎo)致高壓液體的流動是沿著管道的，盡管那些流動有著各種不同的征兆。相對于穩(wěn)定的壓力流經(jīng)閥門開啟的管道。這種影響是關(guān)于液體流動的延遲在兩種管道截面之間，管道自身受到影響由于液體逆向產(chǎn)生高壓，管壁膨脹。同時，順流管道縮短，由于流經(jīng)液體的壓力降低，這種管道橫截面的巨大變形是由于管道材料的，并且能夠被證明。例如，使用薄壁型橡膠管材。高壓液體沿著液流前進。實踐證明，由于液體的張力流向沿著管壁，它的速度接近于聲速。在這種管道材料中，然而，這是一種次要作用，當(dāng)認(rèn)識到它的存在，能夠解釋一部分壓力的傳遞時間隨著閥門關(guān)閉特點，它幾乎沒有影響到壓力標(biāo)準(zhǔn)應(yīng)用在壓力沖擊現(xiàn)象。 在閥門關(guān)閉之后，這時是受壓時間將主要依靠系統(tǒng)的邊界條件，為了描繪閥門關(guān)閉的結(jié)果在同一個系統(tǒng)上，它將很容易說明在大量的圖表上面，管道在每個時間段的情形。 由于閥門的關(guān)閉是瞬時的，液體接近每一段管道的閥門會帶來停止，并且高壓液體流動情況可能已經(jīng)流過每一段管道。在適應(yīng)的流速c和一段時間t，這時液體已經(jīng)流過了一段距離1=ct，在每一段管道內(nèi)，這時管道的橫截面是變形也有一段距離1。 高壓液體到達蓄能站通過管道的時間為t=1/c，在這段距離中出現(xiàn)了一個不穩(wěn)定的位置，是在管道與蓄能站連接處。由于是不可能出現(xiàn)層流在蓄能站連接處，而保持壓力不同及其它的值在閥門關(guān)閉之前，流過每一個蓄能站的時間為1/c，在逆向管道這邊是高壓液體的流動朝向閥門的關(guān)閉。減小管壁的壓力到其原值，并且恢復(fù)管壁的橫截面積。這時液體的流動需要產(chǎn)生差值。從管道流向蓄能站，在管道的前段的液體流動有比較高的壓力比蓄能站?，F(xiàn)在，由于系統(tǒng)假設(shè)沒有摩擦，這種巨大的逆向流動會有精確的對比和最初的流動速度。 在順流蓄能站，存在相反的情況，導(dǎo)致液體壓力上升流向和確定的順流流向從蓄能站到閥門。 由于這里考慮的是簡單的管道，恢復(fù)高壓液體在管道和閥門之間的時間為21/c。整個逆流管道也是同樣，在返回最初的壓力和流向在管道外也被確定時間為21/c，由于液體已經(jīng)到達閥門，意味著沒有液體提前在提供的逆向一個低的壓力區(qū)域形成在閥門外，破壞了流向和給上升的壓力減小流動流向逆方向的閥門。再一次，帶來流動的停止沿著管道且減小壓力在管道中。它已經(jīng)被假設(shè)在閥門處壓力下降，減小蒸發(fā)壓力。由于系統(tǒng)已經(jīng)假設(shè)沒有摩擦，所有的液面會有相同，絕對的，巨大的壓力增加。在穩(wěn)定的運動壓力下，會通過閥門的關(guān)閉產(chǎn)生。如果壓力增長是h，這時所有的液面是h，因此，液體逆流經(jīng)過閥門的時間為21/c，存在一個值-h,同時，減少所有沿著管道的點從h降到最初的壓力時間逆向流動到蓄能站的時間為31/c。 類似的，恢復(fù)液體最初的順流到閥門的時間為21/c，并且流向從順流管道流向閥門關(guān)閉，這會在閥門處帶來流動停止，導(dǎo)致壓力上升。在整個順流管道的每一段時間內(nèi)壓力h上升到最初的壓力在流動停止時。 因此，在31/c時是一種不穩(wěn)定的情形類似于在t=1/c的情形，出現(xiàn)在蓄能站和管道的連接處存在著不同。即是逆流管道壓力下降到最初壓力和順流管道上升到最初壓力，然而，這種液體流動恢復(fù)機構(gòu)所用時間是相同的t=1/c。結(jié)果是逆流流向蓄能站，它有效地恢復(fù)環(huán)境沿著管道到它的最初值。當(dāng)液體到達關(guān)閉的閥門時，沿著每一段管壁都是相同的時間t=0，然而，由于閥門一直是關(guān)閉的，這種情形不能保持循環(huán)流動周期。 管道系統(tǒng)采用循環(huán)流動周期，瞬時選擇一種專門的機械情形，管道的順流和逆流對于閥是一樣的。實際 ，這是不同的。因而，所描繪的周期將一直被使用，除了壓力變化在兩管道之間不再表示相同相位關(guān)系，每一個壓力周期的變化將是41/c，那里1和c代表著每一段管道適應(yīng)的時期，這是重要的標(biāo)記，一旦閥門是關(guān)閉的，這兩個管道將做出相應(yīng)的流動到任何一段距離。 通過上述沖擊周期的描繪，可以劃分壓力-時間關(guān)系，在某一點沿管道上，這些變化的出現(xiàn)是類似的。通過時間在任何一點h，液體到達某一點，系統(tǒng)假設(shè)流動速度為一個常數(shù)c，這主要集中在壓力沖擊依靠的方法是限制壓力的升高和減小閥的啟閉速度。然而，存在著很重要的一點，沒有減小開啟壓力，將發(fā)生直到閥的關(guān)閉時間先于另一個管道。減小壓力達到出現(xiàn)閥門慢速關(guān)閉的結(jié)果先于忽略液體逆流到閥門關(guān)閉。由于沒有影響，返回到閥門時間21/c前，從閥門開始運動沒有壓力減小能夠到達如果閥門沒有打開超過了時間。一般來說，閥門的關(guān)閉小于管道涉及的速度并且它將比21/c短。 在沒有摩擦的情況下，周期的繼續(xù)是不確定的。然而，實際中，摩擦力是壓力損失在很短的時間內(nèi)，系統(tǒng)的摩擦損失越高，忽略摩擦力的影響導(dǎo)致結(jié)果越嚴(yán)重。事實上，閥門的頂點低相對于蓄能站頂點。然而，由于緩慢的流動，摩擦點的損失減少。沿著管壁并且這個點向著蓄能站的方向增長。由于液體的每一層，在閥門和蓄能站中會帶來停止，通過流動最初的液面，所以大多在第二個液面位置相應(yīng)的摩擦點恢復(fù)流向。閥門導(dǎo)致影響整個時間21/c。由于流動是相反的在管道中時間為21/c和41/c。這個位置影響主要在閥門，由于重新建立一個新的摩擦損失，在確切的事例中，例如，長距離油管，在閥門關(guān)閉之前，它將上升一部分壓力。 隨著假設(shè)條件對摩擦周期的描繪，提及到使壓力下降的條件，如果這些情況發(fā)生，這時流向圓管已經(jīng)分離出類似的周期描繪，可能中斷通過形成蒸氣壓力減小的位置有蒸氣生成。因此，系統(tǒng)描述可能發(fā)生在閥門的順流時間0或者逆流時間21/c形成一個腔。由于一段時間液體沿管壁流動在一個壓力梯度下，在這個腔和系統(tǒng)邊界之間。這種方法是通常由于產(chǎn)生額外壓力在最后的腔中。這種現(xiàn)象一般涉及到像圓管的分離和通常的制作更多的錯綜復(fù)雜的由于釋放溶解的氣體在附近的腔中。 沖擊壓力也許被定義為在一些封閉的管道中應(yīng)用，通過兩個基本的方程，分別是運動平衡方程和連續(xù)應(yīng)用在一個短的流體圓管。它依靠可變的流體平均壓力和速度在任何一段管道的橫截面，且不依靠可變的時間和距離。通常考慮實際的穩(wěn)流方向。摩擦力將被假設(shè)與速度平方成比例，并且穩(wěn)流摩擦關(guān)系將被假設(shè)應(yīng)用在非穩(wěn)定事例中。 10