Shock and Vibration Testof Valves Cheng Da-Yun¡BChiu Liang-Ning ABSTRACT¡G The shock load inflicted by hostile fire and the vibration induced by the ship itself should be taken into account during the design and con struction stages of warships. Therefore these two factors will not bec ome the causes of failure of equipments and systems and subsequently r esult in the loss of warfare performance and survivability.There have been many researchers devoting their efforts to the design theory and calculation in this field. However, in the present paper, the shock an d vibration tests of a shipboard valve, which needs to resist shock an d vibration load will be introduced after a brief theoretical instruct ion. We hope the design and test concepts inspired by this case can be widely promoted in navy so as to make our warships gain substantial a nti-shock capability. 1.Introduction A sudden, fierce, instantaneously short-period, and explosive force in flicting on ships is called a shock. The shock load mainly acts horizo ntally when the explosion happens in the air. While for the underwater explosion,the main load comes in the vertical direction. Under the c ondition of same distance,the shipboard equipments are more sensitive to shock load than a ship hull is. Similarly, the shock response of li ght items is larger than the one of heavy equipments. In order to red uce the damage, shock levels are required for the equipments of vital systems, weapon systems, and the interface systems. Only qualified ite ms can be installed onboard. The vibration caused by revolutionary machinery will not be discussed in this paper. The forced vibration of valves is induced by environmen tal vibration. If the valve is not endurable to the external force at certain frequency, or any resonance is induced because of the natural frequency of the valve coincides with the force frequency, failure or damage of the valve may be caused and consequently results in malfunct ion of the system it belongs to. Therefore the resistance to environme ntal vibration is as well one of the necessary characteristics of syst ems onboard warships. After the Kidds-class destroyers were delivered to Taiwan, a pressure reducing station needed to be installed onboard since the sea water pr essure of their main gearbox lubrication oil cooler was too high. This alteration was expected to make the sea water pressure closer to the lubrication oil pressure. However, the valves of the pressure reducing station should meet the requirements of MIL-STD-167-1 (environmental vibration) and MIL-S-901, Grade A (shock) since the resistance to shoc k and vibration was demanded for the sea water cooling system. Accordi ng to the alteration technical document,DDG993-353, MIL-V-24624 for bu tterfly valves, and MIL-V-2042 for pressure reducing valves, the resis tance to shock and environmental vibration was required for the possib le candidates of the valves, which would be installed onboard only aft er being qualified by associated tests. From this alteration we could understand that to warships how important the shock and vibration resistance was. As shown in figure 1. the FFG-7and 59 had ever performed the whole-shi p full-scale shock tests. Although our self-built Cheng-Kong class fri gates did not perform this kind of test, associated shock and environm ental tests had been carried out for our self-developed military valve s. Except the valves used in the tests (both tests were destructive te sts), other items of the same lot had been received and installed onbo ard. The experimental process of this self-developed military valves u sed onboard Cheng-Kong class frigates is introduced thereinafter. 2.Shock test equipment (MIL-S-901) The underwater explosives, which do not directly contact the ship hull , such as torpedo, missile and mine can instantaneously (less than 0.5 second ) generate shock wave that will cause fierce vibration of bott om, shell, and decks. The shock wave propagating in the structure will induce huge acceleration of shipboard equipments and systems and cons equently cause the failure of them. The purpose of shock test is to verify the anti-shock capability of eq uipments onboard warships. The shock intensity (energy) can not be des cribed with linear relations between associated parameters. The formul a is derived by experiments and theory, from which the shock factor (Q ) can be calculated by considering the distance (D) between the ship a nd the explosive, angle (¡@), and equivalent weight of TNT (W) (NAVSEA 0908-LP-000-3010 US Navy Shock Design Criteria). For example, the kee l shock factor of surface ship is 0.3 (0.6 for submarine), which is eq uivalent to the shock load imposed by the explosion of 250kg TNT with distance of 50 meters away from the ship. The shock-wave-induced accel eration of the object located on the keel will be 1000 to 10000 times the acceleration of gravity (The acceleration of gravity will be descr ibed as "G" hereinafter). USN has taken account of the shock resistanc e capability into the ship design,in which the shock factors for struc ture, personnel, military equipments, and commercial equipments are as signed as 0.8, 0.6, 0.2, and 0.1 or lesser respectively. Rather than 0 .3, the keel shock factor of the whole-ship full-scale shock test perf ormed by USN was 0.2. (The first ship of each class would perform four shock tests with keel shock factor of 0.06, 0.1, 0.15 and 0.2 in orde r.) Because of the instantaneously accelerating effect caused by the shock wave, the acceleration of the equipment with weight of 25 pounds will be increased to 75G to 250G. While for the equipment with weight of 5000 pounds, the acceleration will be 50G to 120G. Under the act ion of shock wave, the acceleration of equipments is inversely proport ional to their weights and also related to the direction of acting for ce. The movement of equipments fitted with shock mounts is about 1 to 2 inches in vertical direction and 6 inches in horizontal direction. The equipments with shock resistance can be classified into two grades . Grade A equipments are those such as systems of steering, propulsi on, navigation, damage control, communication, fire control, and comma nding, which will affect constant warfare performance and survivabilit y. Although the Grade B items do not directly relate to warfare perfor mance and survivability their damage will still cause the danger to pe rsonnel, Grade A items, or the ship itself. The tests can be classifie d into three types. Type A test is for the principal units directly a ttached to the ship structure, such as piping systems, generators, air conditioning facilities, and valves. Type B test is for the subsidiar y components of the principal units, such as the engine of generator, and the motor of air conditioning facilities. Type C test is for the s ubassemblies of subsidiary components or principal units, like gauges and resistors. The equipments to be tested are also classified into th ree classes. Class I equipment is the one directly installed without r esilient mounts fitted between the ship structure and the equipment. C lass II equipment is equipped with resilient mounts. Class III equipme nt has to meet both class I and II requirements no matter it is equipp ed with resilient mounts or not. The test facilities, shock platforms, are categorized into types of lightweight, medium weight, and heavywe ight. The maximum weight carried by the Lightweight shock platform is 550 pounds (including test subject and associated fixtures), and its m aximum deflection capacity shall not exceed 1-1/2 inches. For the medi um weight shock platform, the maximum weight to be carried is 7400 pou nds with deflection capacity of 3 inches. The heavyweight shock platfo rm is a large floating platform used to test the shell-mounted equipme nts below waterline or items heavier than the capacity of medium weight platform. According to the definition mentioned above, valves should meet the re quirements of Grade A (B), Type A, Class I, and its testing facility i s a lightweight shock platform since its weight is light. The lightwei ght shock test machine adopts hammer-striking method, in which the str iking force is controlled by adjusting the height of the hammer. The e quipment to be tested is fixed on the platform that can rotate to spec ified test direction. The hammer weighs 400 pounds, and the vertical distance between the hammer and the equipment to be tested is 1, 3, a nd 5 feet in order. For the blows in X, and Y directions figure 2. the hammer will move along a round trajectory behind the platform, while for the test in Z direction figure 3. the hammer will be dropped from a position vertically above the platform. Since every test subject nee ds to take three blows with different striking distances from each dir ection, there are totally nine blows in every shock test. Qualified pe rsonnel with associated certificates are required. Before performing t he shock test, the test subject should pass all the manufacturing insp ection and examination process, and its installation status should be similar to the real situation as much as possible. All the test instr uments and gauges should have been calibrated and their expiration dat es are not due. After shock test, the tested equipment should be subje cted into NDT (according to the specifications of the test subject) an d functional test. The design of the equipment should be reviewed and improved if any permanent deformation, misalignment, and malfunction w ere found. 3.Environmental vibration te-st equipment(MIL-STD-167-1) Any object with mass and elasticity can vibrate. The characteristics of vibration are commonly represented by its frequency and amplitude ( displacement of a harmonic movement). A valve will vibrate under the a ction of force from environment, so it can be categorized as Type I eq uipment with vibration resistance. The exciting force generated by the propeller blades and the unbalance of shafts is the main steady-state vibration source at frequency of 0 to 33Hz (2000RPM) or 50Hz (3000RPM ). The test machine should be able to simulate the possible shipboard vib ration environment, controlling vibration direction, adjusting frequen cy and amplitude, and performing the following three tests. The explor atory vibration test is used to search the resonant point by adjusting frequency from 4Hz to 50Hz. Each frequency will be maintained for 15 seconds. From 4Hz to 33 Hz, the vibration amplitude will be 0.010€±0.0 02 inch, while for 34Hz to 50Hz, the amplitude will be 0.003-0.001(+0) inch. The response prominences will constantly occur if the test freq uency coincides with the natural frequency and consequently causes res onance. In the variable frequency test, each test frequency will be maintained for five minutes, and the amplitudes will be changed as the following : 0.030€±0.006 inch for 4Hz to 15Hz, 0.020€±0.004 inch for 16Hz to 25H z, 0.010€±.0002 inch for 26Hz to 33Hz, 0.005€±0.001 inch for 34Hz to 4 0Hz, 0.003-0.001(+0) inch for 41Hz to 50Hz. Under the resonance frequency determined in the above two tests, which may cause most damage of the test subject, the endurance test will be performed for two hours with the corresponding amplitude used in the variable frequency test. If there is not any resonance frequency found in the exploratory vibration test and variable frequency test, 50Hz o r higher will be adopted in the endurance test.(Generally speaking, mo st of the ships are not very likely to encounter the vibration source with frequency of 50Hz or higher, the practical way is to use the high est frequency of exciting force occurring onboard. For the equipments of Cheng-Gong class frigates, the endurance test was performed with v ibration amplitude of 0.030€±0.006 inch, and its frequency is 15Hz, wh ich is calculated according to the maximum revolution of propulsion sh aft and number of propeller blades.) Among those three tests, the fir st two tests are used to determine the resonance frequency, and the ot her is executed to examine if the test subject can sustain the vibrati on without any damage, failure, malfunction, and deterioration of its major functions. If not, the design has to be reviewed or select oth er appropriate equipment. Equipped with qualified and well-calibrated instruments, any testing m achine which conforms to the previous test conditions can be adopted f or the above tests after appropriate installation and inspection. The manufacturer of the self-developed military valves for the Cheng-Gong class frigates introduced the environmental vibration testing machine figure 4. Its test platform (48"€×48"€×30")can carry test subject with weight of 1000 pounds (including fixtures and jigs) and its maximum a llowable acceleration is 4.5G while loaded. This platform can perform horizontal and vertical vibration with frequency range from 4Hz to 60H z so as to allow the vibration amplitude in X, Y, and Z directions to be measured figure 5¡B6. 4.The process inspection before test Most of the valve manufacturers point out that the process inspection of military valve is very rigorous. In comparison with big valves, it is difficult for small valves to be manufactured and pass various exam inations. The pressure gauge valves which are most used onboard the Ch eng-Kong class frigates can be a good example hereinafter because of i ts small size and high design operating pressure. According to the des ign by USN, this MIL-V-24578B, Type I, Class 1,6000psi,1/4" O.D. forge d stainless steel valve (A global valve with a union connection fitted on the valve stem through which associated test can be performed.) in cluded a steel stem and body which were made of ASTM A473, Type 316 or 316L forged stainless steel, and the material of its gasket was carbo n fluoride rubber conforming to MIL-R-83248. Firstly, the following matters should be confirmed before the test: th e manufacturing drawing adopted was the newest version, the material c ertification (including physical and chemical properties) could prove all the material requirements were satisfied, the dimensions (includin g the mark of the valve) were within the allowable tolerance, and the records of process inspections were well completed. Secondly, the stru ctural pressure endurance test of the valve would be performed. For th e test instruments, the expiration date of their calibration should no t be due. The operational range of associated gauges should be appropr iate, and the test system should possess pressure releasing circuit an d relief valves for safety reason. During the pressure test of the val ve body, the test cape at the top of stem should be half-opened. The p ressure was hydraulically increased to 9000psi and maintained for one minute. After the test the function of the valve should still be norma l, and it would be disqualified if there was any permanent deformation existent. Thirdly, the valve was manually closed in order to perform the valve seat test. The 6000psi pressure was hydraulically imposed on its inlet and outlet, one at a time, then the valve seat should withs tand the pressure for one minute, during which no leakage should be fo und, and its other functions should be normal as well. After this tes t, the same valve seat test will be repeated with 100psi compressed ai r. Fourthly, the valve stem back-out test would be performed. A torque three times the maximum allowable torque ( 62 in-lb) was applied to the valve stem to check if this load could pu ll the bonnet out of the valve body and damage the bonnet and body. Ju st like the shock and environmental vibration tests, this test was als o a destructive test which could be arranged in the test scope of the first item of a lot. The sampling of this valve was executed according to MIL-STD-105, general inspection II, AQL 2.5 percent. The process d efective rate records had not been included in the evaluation scope of the qualified domestic manufacturers before the contract of this proj ect was offered. If the process defective rate of the manufacturer was found up to 8% after the production of valves, then the whole lot wou ld be rejected without the need of sampling. The sampling process woul d only be performed only when the process defective rate decreased to 2.5%. Therefore, except the destructive tests, all the inspection item s were required to be performed on each valve. After the whole lot of valves passed the process inspections, the buyer would do the samplin g and other associated test of which the destructive ones would be per formed according to the military standard on the first item of each lo t. 5.Shock test The shock test would be executed after the valve was qualified for the process inspections. This test had to be performed under simulated op erational status. The test personnel would install the valve on the te st platform, and connect associated pipes, pump, pressure gauges, and pressure relieving facility. After the system was filled with clean wa ter and all the air had been expelled out, pressurized the system to t he designed operating value of 6000psi. Since the military standards did not define the valve position (fully open or close), this test was performed first at the fully opened posi tion and then at the fully closed position. The valve would take blows from X, Y, and Z directions. The hammer slid along the curved track b ehind the test machine or directly dropped down from the position abov e the platform. In each direction, three blows would be taken in which the vertical distance from the hammer to the striking point would be 1, 3, and 5 feet respectively. Therefore there would be 18 blows condu cted in this test. After each blow, the valve would be examined for de formation, loosened parts, cracks, leakage, and seepage; the jigs shou ld not be deformed and cracked; no bolt was loosened, deformed or unab le to be tightened. All the inspection results would be recorded in th e test report. A temporary malfunction and leakage instantly happened after the blow was allowable. However permanent deformation, misalignm ent, or malfunction would make it disqualified. For instance, the wate r seeped out of the valve while being struck, but it was still qualifi ed if no further leakage was found after the water was wiped out or th e associated bolts were secured again. It was disqualified if the foll owing situations occurred: the valve stem deformed, the valve was unab le to be closed and opened, and the valve tongue misaligned with the v alve seat and consequently damage the seal condition. Therefore the fu nction test after the shock test was very important, and the test resu lts should be compared with the data before the shock test. The functi on test items included pressure test, fully open/close operation, disa ssembly, and NDT. With the whole set of tests, the defects of the val ve would be found and then it would be judged as qualified, disqualifi ed or deserving to be tested again after repair. 6.Environmental vibration te st Generally, an item can easily pass the shock test if it is qualified f or the environmental vibration test. Thus the environmental vibration test is very important. However it is not necessary for every equipmen t to pass both of these tests. It depends on the requirement of its sp ecifications. Nevertheless, the gauge valve mentioned above was the ca se which needed to pass both of the tests. The valve needed to satisfy the requirements of process inspection, di mensions, configuration, and pressure test before being installed on t he environmental vibration test platform. Similar to the shock test, this test also needed to be conduct under simulated operational condit ions. The vibration parallel to the X, Y, and Z axes would be measured . Accelerometers would be used to obtain the vibration signals paralle l to one of the axes from the platform (near its center) and the equip ment. This process would be conducted in sequence, from one axis to a nother. The test would be immediately stopped once any damage occurred . After necessary repair and remedy, the test would be started again. The amplitude versus frequency diagram printed by the testing machine figure 7¡B8. had to be contained in the test report. Examinations such as pressure test, fully open/close operation test, and NDT should als o be conducted after the environmental vibration test. 7.Common mistakes made in the tests for the self-developed valves In the early stage, the manufacturers of these self-developed military valves had made many mistakes in those two tests.However, after some efforts of improvement, they have established the capability of conduc ting those tests. The common mistakes are shown as the following: 1.Incorrectly conduct the tests because of the misunderstanding of the military standard. 2.Instead of the simulated operating conditions, the valves were teste d under the pressure several times the pressure used for process inspe ction. 3.The environmental vibration test was tedious, and its frequency and amplitudes also needed to be varied frequently. However, very often, t he testers did not stay by the test platform all the time. From the te st records we could see the test had been interrupted or not been cond ucted according to the standard. As shown in figure 7 and 8, two mista kes had happened in the exploratory vibration test: The vibration ampl itude had been set at the same value for all the frequency from 4Hz to 50Hz, and the test name was wrongly typed as "endurance test". 4.During the two-hour-long endurance test at frequency of 15Hz, the am plitude should be the corresponding amplitude of 0.030€±0.006 inch def ined in the variable frequency test rather than the amplitude of 0.010 €±0.002 inch defined in the exploratory vibration test. 5.The test subject and the delivered items should belong to the same l ot of products. If the quality of a certain product has become stable, sampling can be done to reduce the quantity of test subject, and, dep ending on the quality, the following lots may be exempted from the des tructive tests 6.The fixing status of the test subject should be similar to the real situation as much as possible. For example, the instrument valve shoul d be installed on the test platform with the panel it belongs to. 8.Conclusion The price of a warship is higher than the price of a merchant ship. It is not because of the title. It results from some special requirement s they have to meet for executing their missions. For example, a milit ary equipment can sustain the shock load four times a commercial equip ment can do under the same shock condition. There would be no reason t o pay such a high price for those domestically built warships if they could not match the necessary shock and environmental vibration requir ements. In addition, if a warship lacks for the special design such as the shock and vibration resistance capability of systems, equipments, and damage control facilities, once it goes to the war, the shock loa d and serious vibration may damage its systems immediately, and conseq uently cause the loss of warfare performance and risk the lives of per sonnel. This kind of warship is just a merchant ship equipped with wea pon systems. In both the design and construction practice, the FFG-7 h ad been required by the USN to match every requirement a warship shoul d fulfill. Through the environmental vibration test and whole-ship f ull-scale shock test, defects had been constantly found and remedied. Even for the FFG-59, defects were still found through the shock tests . We can not imagine the result what if a domestically designed and bu ilt warship that was not required to meet the shock and vibration requ irements is subjected to a whole-ship full-scale shock test. In the development history of those domestically-built warships, at le ast four projects such as Jhung-Yi, Kuang-Hwa I, Kuang-Hwa III, and Ku ang-Hwa VI have been carried out. During Jhung-Yi project, ROCN had integrated the resource of the domestic design and construction instit utions, sending trainees to USN to participate the design of their war ships. The ships of the Kuang-Hwa I project had been built in Taiwan a ccording to the drawings modified for the requirements of ROCN by USN. Through years of practice of design and construction, certain capacit y of developing, maintenance, and alteration of warships has been esta blished. However, the realization of anti-shock concepts onboard our s hips, in both design and construction aspects, has not been thoroughly implemented and there is still some improvement to be done. As regards the instrument valve cited in this paper, since it is shock resistant, logically other gauges located on the same panel it is loc ated have to be shock resistant as well. For other pipe systems, besid es the valves directly installed on the pipes, their remote control co mponents also need to possess shock resistance capability. In additio n, the cut-off valves on the side shell, bulkhead valve, as well the d rains on the damage control deck should pass the shock test, too. Oth erwise, the requirements of V-line and single compartment damage stabi lity will not be fulfilled during the combat environment. Furthermore, the sprinklers of sprinkling system, strainers of pipe systems, found ations, first-aids boxes, or even the furniture should possess some le vel of shock resistance required by the system design. Practically, t he installation tolerance of equipments, sufficient gap reserved aroun d the equipments, and types of hangers should all be well considered s o as to prevent the damage caused by the tearing and collision occurri ng when the ship structure is fiercely shaking and deforming under sho ck. Figure 9¡ã12. Our predecessors had gained great achievement in the design and constr uction of warships. However, the superior knowledge and experience the y accumulated can not be well handed down and applied in the following projects of new ship construction or alteration of existent units. Th e shock resistant requirements cited in the alteration technical docum ent for the Kidds destroyers inspired that there is still a lot of job s we need to do. We have to take action to implement those theory and principles we wrote on the papers about this issue. Besides the shock and vibration tests mentioned above, other special requirements for w arships, such as the equipment selection, design concepts, test and ve rification during construction, repair and maintenance during the serv ice life, and the configuration management of alteration should all be fulfilled, and it is an imperative task to which the entire navy shou ld devote.