plc四柱液壓機控制系統設計畢業論文-資料下載頁

　　

【正文】 been established out of the time 21/c,as the wave has reached the valve,there remains no fluid ahead of the wave to support the reversed 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 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 this pressure increment is h,then all the waves propagating will be177。h,Thus,the wave propagation upstream from the valve at time 21/c has a valueh,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 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 ah 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 ,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, instant of valve ,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 practice,this would be ,the cycle described would still apply,except that the pressure variations in the two pipes would no longer show the same phase period of each individual pressure cycle would be 41/c,where I and c took the appropriate values for each 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/,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/ 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 pressuretime records at points along the variations are arrived at simply by calculating the time at which any one of the177。h waves reaches a point in the system assuming a constant propagation velocity major interest in pressure transients lies in methods of limiting excessive pressure rises and one obcious method is to reduce valve ,reference to an important point no reduction in generated pressure will occur until the valve closing time exceeds one pipe 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 ,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 these case,the head at the valve is considerably lower than the reservoir ,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 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/ the flow reverses in the pipe during time 21/c to 41/c,the opposite effect is recorded at the valve because of the reestablishment of a high friction loss,these variations being shown by lines AB and 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 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 the system described,this could happen on the valve’s downstream face at time 0 or on the upstream face at time 21/ 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 cavit