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Abstract
This work aims to the designing of snubber test rig which is used to check the shock absorbing capability of snubber, and also to verify the further application of snubber in nuclear power plant. A brief introduction about the snubber is given followed by different types of snubbers. The operation of mechanical and hydraulic snubber is discussed briefly.Initialy a test rig with a rotating fly wheel was suggested to apply the compressive load on the snubber but due to the load limitation and misalignment problem the test rig was remodeled and a hydraulic system is now been designed. The hydraulic system is used to apply 3500 N force on the snubber. The mechanism involved in its operation is a gear oil pump of 1.5 Horse Power is used to suck the oil from the lower portion of the cylinder and bring it to the upper portion ,so due to pressure difference the piston rod moves in downward direction and apply load on the snubber. When the snubber reaches to its lower position the valve in the piping system is opened and due to the pressure difference the oil comes from the upper portion of the cylinder to the lower and the snubber brings the piston to its original position due to the stored potential energy in it. The cylinder of outer diameter 0. m and wall thickness of 0.0152 m due to internal pressure of 1.31 MPa is having 3. MPa,-1.24 MPa, 1.03 MPa and 2.4 MPa of radial, tangential, longitudinal stress and fracture pressure respectively is designed. The cylinder cover plate of 35 mm is designed for the internal pressure in the cylinder. The gland which stops the oil leakage from the cylinder is having a depth of 7.59 mm into the cylinder. Similarly the due to 1.31 MPa pressure the pipes of outer diameter of 21.34 mm and wall thickness of 2.108 mm is having 5.97 MPa,-1.31 MPa and 2.33 MPa of radial ,tangential and longitudinal stresses respectively. The bolt used to connect the snubber and test rig is having the diameter of 25.4 mm. the critical load for piston rod, gear oil pump and stopper are 134.27 kN, 28.42 kN and 3.6 kN which is greater than the applied load so the design is safe and will not buckle. The failure theories have been applied to every component which verifies their safe operation and the manufacturing drawings have also been made for each component.
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Table of Contents
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1 Introduction
A mechanical seismic shock-absorbing snubber adapted to accept extremely high impact loads that may be imposed upon it by reason of its interposition between heavy relatively moveable structures. While permitting essentially unrestricted, slow, relative motion between the structures, such as may be caused by thermal effects (e.g., the expansion or contraction of a high pressure steam pipe yieldably attached by way of hangers or supports to a building structure as in a nuclear power plant), the snubber limits both the velocity and acceleration of relative motion between the structures such as might result from violent seismic shock or vibration. Snubbers find important application in industry and are widely employed in electric power generating plants to protect fluid carrying pipes from seismic shock or from load transients introduced by flow variations resulting from turbine valve trip, water hammer, relief valve blow down or pump operation [1]. The snubbers are linear supports designed to limit unwanted sudden movements of components such as Pipes Tanks Control valves Steam generators Safety valves Pumps, motors etc. The snubbers enable slow movements due to thermal dilatation in both directions and oppose only low resistance to such movements. But when the movement speed leads to dynamic displacement that are harmful to the installation, the snubbers are immediately locked at the desired speed and support the dynamic force by transferring it to the fixed structure. Once the disturbance has passed, the device returns to its initial state and enables slow movements once again. In this way snubbers provide temporary additional support to the installation in order to prevent if from entering into resonance, thereby minimizing the risk of breaking due to vibrations. The snubbers prevent damage resulted from the following forces [2].
Earthquakes Water hammer effects Violent thrusts due to safety valve discharge of breaks in piping Wind Other similar disturbances The snubbers are used in all power plants and especially extensively used in Nuclear power plants. The main function of the snubbers in nuclear power plants is to allow the motion of pipes and other supporting structure with in acceptable limit and damp out unwanted motions. The snubbers in nuclear power plant are used to allow the normal thermal expansion and contraction of the pipes during normal operation of the power plant and damp out the abnormal expansion .they also limit the high impact loads occurring during the earth quack. So due its tremendous importance in nuclear industry snubbers are used extensively. For the safe operation of the snubbers they are tested regularly. There are 114 snubbers in Chashma Nuclear power plant and at least 10- snubbers are tested at each refueling. So snubber test rig is designed to test the function of these snubbers.
1.1 Background
Snubbers traces its history back to 1955-1960 when focus were given to the fact that to utilize the huge amount of energy with in fissile elements for peaceful purposes. So scientist started work on the development of nuclear power plants. Since there is a tremendous amount of heat with in the core of nuclear power plant so to remove this heat and make possible the safe operation of the plant pumps having 96000 gallons per minute discharge are used. This produces heavy vibrations in the primary loop. So in order to overcome these vibrations and heavy shock loads due to earth quack snubbers were developed [2].
1.2 Scope and Objective
The main objective of this project is design the snubber testing facility in Pakistan for testing the snubbers in Nuclear power plants in Pakistan. First part of thesis is specified for the study of the snubbers, types of snubbers, testing techniques and snubber testing criteria are discussed. In the second part of the thesis designing of all the components of the snubber test rig has been carried out and the manufacturing drawing are also made.
devices of this mechanical type the inertia effects of the inertial mass have been insufficient to provide the accurate damping or restraint desired and it has been necessary to include auxiliary damping such as might be provided by an associated friction braking device. Often such friction braking devices are actuated through other mechanisms including one or more inertial masses and involving still other moving parts such as springs and friction clutches. In addition to adding to snubber complexity and cost of manufacture the effectiveness of friction brakes, springs and friction clutches, tends to vary with wear and environmental conditions, particularly temperature and radiation [4].
2.1 Types of Snubbers
Snubbers used in nuclear industry are mainly divided in the following to types
Mechanical Snubber Hydraulic Snubber
2.2 Mechanical Snubber
The snubber which involves the application of gears, springs, and inertial masses are further divided into the following major types
2.2.1 Mechanical Snubber with Capstan Spring
Mechanical Snubbers have two modes of operation. Passive mode Active mode In passive mode motion is caused by thermal loads and the resisting mechanism is disengaged and the snubber "free wheels" with very low resistance. In active mode the mechanism is engaged, and the snubber limits the acceleration to a low threshold value. The snubber operates on the principle of limiting the acceleration of any pipe movement to a threshold level of 0.02 g's. This is the maximum acceleration that the snubber will permit the piping to see. When a disturbance attempt to accelerate the pipe in either direction, a braking force will be applied within the snubber of whatever magnitude required limiting the acceleration to a value less than 0.02 g's. At the same time thermal expansion and contraction being a gradual movement is not restricted.
When a sudden acceleration occur and sustain continuously in one direction the snubber will apply whatever force is necessary to limit the pipe movement to its present threshold value.
2.2.2 Principle of Operation
The principle of operation can best be seen in the cross section view below. Two structural telescoping members are connected between the pipe and fixed structure. Within these telescoping tubes is a ball screw and nut which serve to convert the linear telescoping motion which would occur during a seismic disturbance or thermal changes to rotational motion of the blue ball screw and drum assembly. This rotational motion is coupled to inertia mass. The coupling consists of a resilient capstan spring.
When a disturbance occurs that exceeds the threshold "g" level ( 0 .02 g's) the ball screw and drum attempts to angularly accelerate the inertia mass. The inertial resistance of the mass causes the resilient capstan spring to tighten around a hardened mandrel which is part of the structural tube. In this manner, a restraining force is applied against rotation of the ball screw and in turn linear telescoping of the members [4].
Figure 2-1 Mechanical Snubber
varying environmental conditions that conforms very closely to the design specifications.
2.3.1 Principle of Operation
This kind of Snubber has two main sections. One sections being attached to a support structure and the other section being secured to the pipe or other structure whose motion relative to the support structure must be controlled, limited or damped. There is secured to the first of the snubber sections a linear rack member which in response to telescoping movements of the snubber sections slides telescopically with respect to the second snubber section. Journalled in the second snubber section is an oscillating engagement means of a predetermined effective mass. The engagement means is preferably provided by a pallet member having a pair of engaging members or pallets which alternately engage teeth on the rack as the rack moves axially out of or into the second snubber section driving the pallet member into oscillation.
Alternatively the rack may drive through an intervening gear train a toothed escapement wheel which in turn drives the pallet member into oscillation. The alternating acceleration, deceleration, and reversal i.e. oscillation of the pallet member absorbs energy from the system and hence acts to damp or limit the forces applied to the snubber and thereby restrain undesired rapid movement between the support structures to which the ends of the snubber are attached. Moreover the oscillatory effect of the pallet member mass limits acceleration and acts to place an upper limit on the velocity with which one section of the snubber moves toward or away from the other. This action results from the use of an escapement mechanism in which the effective mass of the oscillating member serves to limit the speed of the mechanism.
2.3.2 Detail Description of the drawing
Consider the figure 2 - 2 which illustrates the snubber when viewed a basically two sections telescoping Snubber geometry. The first section of the snubber comprises a tubular member. The second section of the snubber comprises a housing portion having an end wall which incorporates a tongue, lug or other suitable attachment means which may be secured to another external structure .snubber is attached at side to pipes. When
the motion of the pipes occurs then it transfers its motion to the tabular member. It moves linearly into the second portion of the snubber with the help of rack. The liner motion of the rack is converted into rotary motion by rack and pinion arrangement in the liner to rotary convertor. This rotary motion puts the gears into rotation as well. These gears are attached to the pallet members. When the gear rotates it also put the pallet member into oscillation. One pallet member rotates in clockwise direction and other rotates in counter clock wise direction. Thus it absorbs the disturbing forces from the system
Figure 2-2 Mechanical Snubber with Liner to Rotary Motion Convertor
Figure 2-3 Pallets and Gear Arrangement of Mechanical Snubber
maximum clock wise displacement and so on. Thus the pallet member rotates in opposite direction and absorbs energy from the system. So it is observed from both curves that they are 180 degree out of phase [1].
2.3.4 Material of the Snubber
Since most snubbers are used in heavy duty applications where the actuation may be severe snubbers which are subjected to shock, vibration and wear normally made of hardened steels preferably hardened stainless steel. Applicant has used the "Electrolyzing Process" a well known proprietary process that yields Rockwell C hardness value of 70-
- Such surface treatment offers high wear resistance very low coefficient of friction excellent anti seizure properties and high corrosion resistance.
Figure 2-5 Exploded View of Mechanical Snubber
2.4 Rotary Type of Mechanical Snubber
All of the snubbers described above may be referred to generally as being of the telescoping type in that one member of the snubber telescopes into or with respect to the other. However there is also snubber adapted for use with the rotary type of snubber. Such a snubber is illustrated in the following Figure 2 - 6 [2]. As the pipe moves in vertical direction the connecting link which is attached to the pipe also moves in vertical direction. Due to the vertical motion of the connecting link the lever arm rotates. Since this lever arm is keyed to the shaft so the shaft is put into rotation. The gear mounted on
shaft also rotates. The gear rotates the pinion gear and escapement wheel shaft is put into rotation and as a result of its rotation the pallet member is put into rotation and thus absorbs energy from system, thus brings the system into a stable position.
Figure 2-6 Rotary Mechanical Snubber
2.5 Damping Effect Mechanical Snubber
The damping effect of mechanical snubber is increased by
The magnitude of the damping, limiting or restraining effect of the snubber is proportional to the magnitude of the effective mass of the engagement means or pallet member or members employed. It is also a function of the eccentricity of the mass of the pallet members. The damping effect of the pallet members is, of course, increased by employing pallet members whose center of mass is spaced substantially from the axis of rotation of the pallet member Also the damping effect of the pallet member may be increased by increasing its polar moment of inertia. Also the damping effect of the pallet member may be further increased by in- creasing its rate of oscillation which may be accomplished through a step-up gear ratio [1].
