Valve Maintenance PDF

Chapter 04 Valve Maintenance MS4001 Mechanical Maintenance  Valve Maintenance 1. Introduction  Valves are mechanical devices specifically designed to direct, start, stop, mix, or regulate the flow, pressure, or temperature of a process fluid.  Berdasarkan atas fungsinya, aplikasinya...

Chapter 04 Valve Maintenance MS4001 Mechanical Maintenance  Valve Maintenance 1. Introduction  Valves are mechanical devices specifically designed to direct, start, stop, mix, or regulate the flow, pressure, or temperature of a process fluid.  Berdasarkan atas fungsinya, aplikasinya, dan rancangannya, valve dapat dibedakan ke dalam beragam kelas styles, sizes, dan ragam pressure.  Tipe yang umum ditemukan adalah plug, ball, globe, butterfly, dan gate valves.  Valve dapat dibuat dari berbagai material seperti: steel, iron, plastic, brass, atau sejumlah material paduan khusus lainnya. MS4001 Mechanical Maintenance - 2 Valve Maintenance The internal elements of a valve are collectively referred to as a valve's trim. The trim typically includes a disc, seat, stem, and sleeves needed to guide the stem. A valve's performa nce is determined by the disk and seat interface and the relation of the disk position to the seat. Gbr. 1 Basic components of a valve MS4001 Mechanical Maintenance - 3 Valve Maintenance Gbr. 2 Manual Valve Gbr.3 Control Valve MS4001 Mechanical Maintenance - 4 Valve Maintenance Gbr. 4 Ball Valve Gbr. 5 Gate Valve Gbr. 6 Plug Valve MS4001 Mechanical Maintenance - 5 Valve Maintenance Gbr. 7 Globe Valve Gbr. 8 Butterfly Valve MS4001 Mechanical Maintenance - 6  Valve Maintenance Gbr. 9 Pinch Valve open throttling closed Gbr. 10 Diaphragm Valve MS4001 Mechanical Maintenance - 7  Valve Maintenance 2 Klasifikasi 2.1 Menurut Fungsinya  Berdasarkan atas rancangan dan fungsi valve in handling the fluid, valve dapat dibagi ke dalam 3 kelompok: on-off valves, non-return valves, and throttling valves.  On-off valves: handle the function of blocking the flow or allowing it to pass.  Non-return valves: allow one direction flow only.  Throttling valve: allow for regulation of the flow at any point between fully open to fully closed.  One confusing aspect of defining valves by function and its specific valve-body design (plug, ball, globe, gate, butterfly, pinch, etc.) MS4001 Mechanical Maintenance - 8  Valve Maintenance 1. On-off Valves  Terkadang juga disebut sebagai block valve.  Kelompok valve ini yang umum dijumpai: gate, plug, ball, pressure relief, dan tank-bottom valves.  Jenis on-off valve umumnya hand-operated, walaupun dapat diotomasi dengan tambahan aktuator.  Pressure-relief valves adalah self-actuated on-off valves yang dapat terbuka hanya jika preset pressure dilampaui (mencegah over pressuration).  Pressure-relief valves dibagi menjadi 2: Relief valves, untuk aplikasi pada fluida cair dan safety valves, untuk aplikasi pada fluida gas. MS4001 Mechanical Maintenance - 9 Valve Maintenance Gbr. 11 Pengetesan pressure-relief valve MS4001 Mechanical Maintenance - 10  Valve Maintenance 2. Non-return Valves  Nonreturn valves memungkinkan fluida untuk mengalir ke suatu arah yang diinginkan (hanya satu arah saja).  Semua check valves termasuk ke dalam kelompok nonreturn valves.  Valve jenis ini digunakan untuk mencegah terjadinya backflow yang tidak diinginkan.  Contoh aplikasi: untuk melindungi instalasi pompa atau kompresor dari backflow saat shut down.  Nonreturn valves juga digunakan dalam sistem proses dengan variasi tekanan yang harus dijaga secara terpisah. MS4001 Mechanical Maintenance - 11 Valve Maintenance Gbr. 12 Piston check valve dalam bidang migas MS4001 Mechanical Maintenance - 12  Valve Maintenance 3. Throttling Valves  Valve jenis ini digunakan untuk mengatur aliran, temperatur, atau tekanan.  Operasi throttling valves: manual (handwheel or lever) atau dengan kontrol otomatis, Gbr. 13(a).  Pressure regulators adalah throttling valve yang memvariasikan posisi valve untuk menjaga tekanan di hilir valve tetap konstan, Gbr. 13(b)  Automatic control valves atau control valves is a term commonly used to describe valves that capable of varying flow conditions to match the process requirements  dilengkapi dengan actuator. MS4001 Mechanical Maintenance - 13 Valve Maintenance (b) Pressure regulator (a) Globe control valve Gbr. 13 Contoh throttling valves MS4001 Mechanical Maintenance - 14  Valve Maintenance 2.2 Klasifikasi Menurut Aplikasinya  Tiga klasifikasi valve menurut aplikasinya: general service valve, special service valves, dan severe service valves.  General service valves: valve yang dapat digunakan pada banyak aplikasi tanpa perlu modifikasi.  Special service valves: valve yang dirancang khusus untuk aplikasi tertentu/spesifik.  Severe service valves: valve yang dirancang khusus untuk mencegah efek samping pada aplikasi yang sulit. MS4001 Mechanical Maintenance - 15  Valve Maintenance 1. General Service Valves  Termasuk ke dalam kelompok ini adalah valve yang dirancang untuk mayoritas aplikasi yang umum: Lower-pressure: ANSI Class 150 s/d 600. Moderate-temperature rating: -50oF s/d 650oF. Non-corrosive fluid Common pressure drop yang tidak menyebabkan kavitasi atau flashing.  Dalam perancangannya, valve tipe ini dibuat dengan level interchangeability dan flexibility yang tinggi sehingga memiliki rentang aplikasi yang lebar.  Material body: baja karbon atau baja tahan karat. MS4001 Mechanical Maintenance - 16 Valve Maintenance Manual Otomatis Gbr. 14 Example of two general service valves MS4001 Mechanical Maintenance - 17  Valve Maintenance 2. Special Service Valves  Special service valves is a term used for custom- engineered valves that are designed for a single application that is outside normal process application.  Valve jenis ini biasanya untuk aplikasi pada temperatur tertentu, tekanan tinggi, atau medium yang korosif.  Gbr. 15 memperlihatkan contoh sebuah control valve yang dirancang dengan “sweep-style body dan trim dari keramik untuk aplikasi yang erosif dengan pasir dan tekanan yang tinggi. MS4001 Mechanical Maintenance - 18 Valve Maintenance Gbr. 15 Sweep-style globe valve used in erosive mining application MS4001 Mechanical Maintenance - 19  Valve Maintenance 3. Severe Service Valves  Severe service valves are valves equipped with special features to handle volatile applications, such as high pressure drops that result in severe cavitation, flashing, choking, or high noise levels.  Pada kondisi tertentu sering dibutuhkan special actuation untuk mengatasi gaya yang terjadi.  Gbr. 16 shows severe service valve to handle 1100oF (593oC) liquid-sodium application with multistage trim to handle a high pressure drop and a bonnet with special cooling fins.  The electro-hydraulic actuator is capable of producing almost 89 ton of thrust. MS4001 Mechanical Maintenance - 20  Valve Maintenance Gbr. 16 Severe service valve designed to handle high- pressure drop, high temperature liquid-sodium application MS4001 Mechanical Maintenance - 21  Valve Maintenance 2.3 Klasifikasi Menurut Gerakan  Some users classify valves according to the mechanical motion of the valve: linear or rotary.  Linear valves are known for their simple design, easy maintenance, and flexibility with more size, pressure class, and design options than other motion classification  the most common type of valves.  Linear-motion valves (linear valves) memiliki rancangan sliding-stem yang mendorong “closure element” saat valve membuka atau menutup.  Gate, globe, pinch, diaphragm, split-body, three-way, and angle valves semuanya masuk ke dalam kelompok linear valves. MS4001 Mechanical Maintenance - 22  Valve Maintenance  Rotary-motion valves (rotary valves) use a closure element that rotates-through a quarter-turn or 90o range-to open and close the flow.  Rotary valves are usually smaller in size and weigh less than comparable linear valves.  The applications is limited to certain pressure drops and are prone to cavitation and flashing problem.  As rotary-valve designs have matured, they have overcome these inherent limitations and now being used at an increasing rate.  Ball, plug, butterfly valves semuanya masuk ke dalam kategori rotary-motion valves. MS4001 Mechanical Maintenance - 23  Valve Maintenance 2.4 Klasifikasi Menurut Port Size 1. Full-Port Size  Valves with internal flow equal to the full area of the inlet port.  Jenis ini utamanya digunakan sebagai on-off and blocking service valves, dimana aliran harus distop atau dialihkan.  Full-port valves memungkinkan penggunaan pig dalam pipeline.  The pig is a self-driven (or flow-driven) mechanism designed to clean the inside of the pipeline and to remove any process buildup or scale. MS4001 Mechanical Maintenance - 24  Valve Maintenance 2. Reduced-Port Size  Reduced-port valves are those whose closure elements restrict the flow (area of closure element is less then the area of the inside diameter of pipeline.)  Pembatasan aliran menyebabkan terjadinya pressure drop saat aliran melewati closure element dan sebagian tekanan dapat direcovery setelahnya.  The primary purpose of reduced-port valves is to control the flow through reduced flow or throttling, which is defined as regulating the closure element to provide varying levels of flow at a certain opening of the valve. MS4001 Mechanical Maintenance - 25  Valve Maintenance 3. Common Piping Nomenclature  Valve and piping nomenclature has been heavily influenced by imperial system (ANSI standards).  The psi is used to refer to pressure and NPS to refer to valve and pipe size (in inches across the pipe’s inside diameter.  In many countries, outside of the USA, valve and piping nomenclature is based on the International System of Units (metric system) from ISO.  Typically metric valve measurement are called out in millimeter and pressure are noted in kilopascal (kPa) (or bar).  ISO standards refer to pipe diameter as nominal diameter (DN) and pressure rating as nominal pressure (PN).  Tables 1 and table 2 provide quick reference for both ANSI and ISO standards. MS4001 Mechanical Maintenance - 26 Valve Maintenance Table 1 NPS versus DN NPS (in.) DN (mm) NPS (in.) DN (mm) 0.25 6 8.0 200 0.5 15 10.0 250 0.75 20 12.0 300 1.0 25 14.0 350 1.25 32 16.0 400 1.5 40 18.0 450 2.0 50 20.0 500 2.5 65 24.0 600 3.0 80 36.0 900 4.0 100 42.0 1000 6.0 150 48.0 1200 MS4001 Mechanical Maintenance - 27  Valve Maintenance Table 2 ANSI Pressure Class vs. Nominal Pressure*) ANSI Pressure Class Nominal Pressure (PN) (pounds of force per square (allowable pressure in bar) inch of surface area) 150 16 300 40 600 100 900 160 1500 250 2500 400 4500 700 Note: PN is an approximation to the corresponding ANSI pressure class, and should not be used as an exact correlation between two standards. MS4001 Mechanical Maintenance - 28  Valve Maintenance 4. Common Valve Problems  The common problems discussed here include:  High pressure drops  Cavitation  Flashing  Choked flow  High velocity  Water-hammer effect  High noise level  Fugitive emissions MS4001 Mechanical Maintenance - 29  Valve Maintenance 4.1 High Pressure Drops  Flow moves through a valve due to a difference between the upstream and downstream pressures, which is called the pressure drop (P), that is caused by frictional losses between fluid and the wall.  By narrowing the cross section, the frictional losses is higher  restriction in the fluid system.  As the cross section gets smaller  velocity becomes higher  higher pressure drops. MS4001 Mechanical Maintenance -  Valve Maintenance  Permanent pressure drop – Pressure recovery Permanent Pressure pressure drop Pressure recovery Distance along flow Fig. Flow curve showing pressure recovery and permanent pressure drop. MS4001 Mechanical Maintenance -  Valve Maintenance  Immediate effects include:  Cavitation: Cavitation damage to the body and trim, coating, film or oxide then base material  Choked flow: Malfunction or poor performance of the valve  Vibration: Wandering calibration of attached instruments, piping fatigue  High noise level: hearing damage to nearby workers MS4001 Mechanical Maintenance -  Valve Maintenance  Next consequences:  Expensive trims requirement  More frequent maintenance Higher maintenance  Large spare part inventory and engineering costs  Piping supports  High pressure drop means loss of energy, therefore additional energy must be added to the process/system, by:  Heating  Larger pumps/compressors MS4001 Mechanical Maintenance -  Valve Maintenance 4.2 Cavitation  Cavitation occurs in 2 steps:  The formation of the bubble in the vena contracta and,  its subsequent implosion downstream  Bubble formation is caused by decreasing local static pressure in a fluid until it reaches a level below the vapor pressure of the liquid at the actual temperature.  This may happen when the fluid accelerates in a control valve or around a pump impeller. MS4001 Mechanical Maintenance -  Valve Maintenance  Flow Curve showing PVC below PV Fig. Flow curve showing pressure drop falling below the vapor pressure, which results in cavitation. MS4001 Mechanical Maintenance -  Valve Maintenance  Cavitation Process:  The liquid’s pressure drops below the vapor pressure as velocity increases through the valve’s restriction  The liquid expands into vapor around a nuclei host, which is either a particulate or an entrained gas  The bubble grows until the flow moves away from the vena contracta and the increasing pressure recovery inhibits the growth of the bubble  As the flow moves away from the vena contracta, the area expands – slowing velocity and increasing pressure. This increased pressure collapses or implodes the bubble vapor back to a liquid  If the bubble is near a valve surface, the force of the implosion is directed toward the surface wall, causing material fatigue. MS4001 Mechanical Maintenance -  Valve Maintenance  Cavitation Process (cotd.) Fig. 7.4 Implosion of cavitation bubbles by a valve-body wall MS4001 Mechanical Maintenance -  Valve Maintenance  Effects of cavitation:  Permanent damage in the deterioration of the interior of the valve, by the implosion up to 100 ksi (6900 bar).  Attack of the material’s coating, film, or oxide then base material  Distort the flow characteristic, decrease the valve performance and reduce the efficiency in the process system. MS4001 Mechanical Maintenance - Valve Maintenance Fig. Failure due to Cavitation MS4001 Mechanical Maintenance -  Valve Maintenance  Cavitation index, σ  This cavitation index, approved by the Instrument Society of America can be used as a guidance to determine the severity of cavitation that might occur in some valves, that is defined as where: P = Upstream pressure 1 P = Downstream pressure 2 P = Liquid vapor pressure at flowing temperature V MS4001 Mechanical Maintenance -  Valve Maintenance Tab. Cavitation Index criteria Valve type Flow direction Trim size Incipient Choked Globe Over the plug Full area 0.73 0.38 Over the plug Reduced 0.93 0.56 Under the plug Full/reduced 0.52 0.52 Butterfly 600 open Full 1.4 0.73 900 open Full 3.16 2.19 Ball 600 open Full 1.4 0.64 900 open Full 5.2 2.19 Globe with Under the plug Full/reduced 0.30 – 0.001 - special trim MS4001 Mechanical Maintenance -  Valve Maintenance  If the criteria falls between incipient and choked cavitation indices, action should be taken, e.g. using special trim, hard materials, or process modification) Fig. 7.6 Incipient cavitation vs choked cavitation MS4001 Mechanical Maintenance -  Valve Maintenance  Example  Fluid : water  Temperature : 800F  Vapor pressure, PV : 0.5 psia  Upstream pressure, P1 : 500 psia  Downstream pressure, P2 : 55 psia  Valve-type : single-seated globe valve, 100% open, flow- over-the plug  So, the cavitation index, σ =0.38  Referring to the table above, indicates that the valve is choked and cannot increase flow any further. MS4001 Mechanical Maintenance -  Valve Maintenance  Cavitation Effects Prevention  System modifications to prevent cavitation Using a special trim to avoid a single-large pressure drops, instead becomes a series of small pressure drops Air injection, to disrupt the explosive force of the imploding bubbles, therefore reduce the overall damage (for large valves dumping into a tank Downstream back pressure device  to lower the pressure drop by increasing the downstream pressure MS4001 Mechanical Maintenance - Valve Maintenance  Materials construction  Using valve material of Rockwell hardness 40 or higher, e.g. Chrome-Molybdenum and steel alloys (ASTM SA-217 Grade WC9 and C5) for body, and a solid alloy overlay with 316 or 416 stainless-steel for trim parts.  The use of a replaceable hardened seat ring, an extended Venturi seat ring, or body liner, for angle-style valves.  PTFE or other plastic materials are prone to cavities  Cavitation-control devices  Cavitation-elimination devices  Combination of three concepts: tortuise path, multistage pressure drop, expanded flow areas MS4001 Mechanical Maintenance - Valve Maintenance Fig. Multiple-small pressure Fig. Back-pressure device drops above vapor pressure MS4001 Mechanical Maintenance - Valve Maintenance Fig. Torquise-path trim for velocity reduction Fig. Anticavitation trim with multiple pressure reduction mechanism MS4001 Mechanical Maintenance -  Valve Maintenance 4.3 Flashing  Flashing occurs when the downstream pressure is equal to or less than the vapor pressure, so the vapor bubbles generated at the vena contracta stay intact and do not collapse.  This mixture of liquid and vapor travels in a very high velocity that results in erosion in the valve and in the downstream piping. MS4001 Mechanical Maintenance -  Valve Maintenance  Difference between Cavitation & Flashing Fig. Pressure curve showing outlet pressure below the vapor Pressure, resulting in flashing. MS4001 Mechanical Maintenance -  Valve Maintenance  Minimizing Flashing  Eliminating flashing completely by modifying the system itself, in particular the downstream pressure or the vapor pressure.  Considering the location of the valve, e.g. placing the valve closer to the larger vessel will allow the flow into the larger volume of the vessel and away from any critical surfaces  If not possible, the use of hardened trim materials is the only solution MS4001 Mechanical Maintenance -  Valve Maintenance 4.4 Choked Flow  Choked flow occurs in gases and vapors when the velocity of a process fluid achieves sonic speeds (mach 1) in the valve or the downstream piping and cannot be increased, even though the differential pressure is increased  This occurs for air flow when the absolute pressure ratio (P2/P1) is.528. MS4001 Mechanical Maintenance -  Valve Maintenance  In liquid application, choked flow occurs after cavitation or flashing until the amount of vapor that is generated reaches a certain specific volume that causes the velocity reach the sonic speed  The occurrence of choked flow limit the velocity of the gas/vapor in the vena contracta, that may affect the process MS4001 Mechanical Maintenance -  Valve Maintenance 4.5 High Velocities  In general, large pressure differentials create high velocities through a valve and in downstream piping.  This in turn, creates turbulence hence high level of vibration in liquid application, or high level of noise for gas application. Fig. Inverse-relationship between velocity and pressure MS4001 Mechanical Maintenance -  Valve Maintenance  Although depending on the valve size, as a guidance, the velocity limit recommended to avoid the effects of the high velocity:  Liquid: 50 ft/s (15.2 m/s) or 30 ft/s (9 m/s) if cavitation may occur  Gas: Mach 1 (300 m/s)  Mixture of gases and liquids (e.g. flashing): 500 ft/s (152 m/s)  Smaller-sized valves can normally handle higher velocities.  Generally, process liquids with temperatures close to the saturation points must keep the velocity at or under 30 ft/s (9 m/s). It also applies to cavitation services. MS4001 Mechanical Maintenance -  Valve Maintenance 4.6 Water-hammer Effects  Water hammer occurs when a transfer system is quickly started, stopped or is forced to make a rapid change in direction.  Any of these events can lead to catastrophic system component failure.  Without question, the primary cause of water hammer in process applications is the quick closing valve, whether manual or automatic.  A valve closing in 1.5 sec. or less depending upon valve size and system conditions, causes an abrupt stoppage of flow. The pressure spike (acoustic wave) created at rapid valve closure can be high as five (5) times the system working pressure. MS4001 Mechanical Maintenance -  Valve Maintenance  Causes:  Sudden pump shut-off or start up  Valve slamming shut (valve’s closure elements is suddenly sucked into the seat as the valve nears shut- off  actuator stiffness problem)  Valves with quick open  As a guidance, stroking duration of a valve (in seconds) should be:  (1 – 3) x diam. Valve (in), for valve opening  1 x diam. Valve (in), for valve closing (minimum) MS4001 Mechanical Maintenance -  Valve Maintenance  Effect of the water-hammer  Ruptured piping  Leaking connections  Weakened connections  Pipe vibration and noise  Damaged valves  Damaged check valves  Damaged water meters  Damaged pressure regulators and gauges  Damaged recording apparatus  Loosened pipe hangers and supports  Ruptured tanks and water heaters  Premature failure of other devices MS4001 Mechanical Maintenance -  Valve Maintenance  The best way to minimize water hammer in valves is to prevent any sudden pressure canges to the system, by:  Slowing the closure of the valve  Providing greater degree of stiffness of the valve actuator  Other defense involves:  Adding some surge protection in the process line, e.g. pressure relief valve  Injecting gas to increase fluid compressibility to handle unexpected surge. MS4001 Mechanical Maintenance -  Valve Maintenance 4.7 High Noise Levels  Noise is generated when vibration produces wide variations in atmospheric pressure, which are then transferred to the eardrums as noise  The most common cause is turbulence generated by the geometry of the valve, which is radiated by the downstream piping  Other causes include flashing and cavitation. MS4001 Mechanical Maintenance - Valve Maintenance Fig. 7.13 Downstream pipeline vibration caused by valve turbulence MS4001 Mechanical Maintenance -  Valve Maintenance  Effects of noise on humans  Noise with 1000 – 8000 Hz frequency is most sensitive to the human hearing  Gaseous noise above 100 dBA  Hearing damage occurs as cumulative and irreversible, beginning with the loss of high frequency sound  Lower frequency noise affects the performance of organs, e.g. heart, liver.  Noise and vibration also affects the valve’s performance and causes fatigue in valve, piping, and nearby process equipment MS4001 Mechanical Maintenance - Valve Maintenance Tab. Permissible noise levels (Walsh Healy Public Contracts Act) Duration dBA (hour per day) 0.25 or less 115 0.5 110 1 100 2 97 4 95 6 92 8 90 MS4001 Mechanical Maintenance -  Valve Maintenance 4.8 Fugitive Emissions/Leakage  Motivation:  Health and Safety (esp. for high temperature or poisonous agents)  More strict antipollution laws.  Higher efficiency (esp. in steam power-plant)  The improvements target all devices that penetrate a process line, such as valves, sensors, regulators, flow meters, etc.  SEALING…. MS4001 Mechanical Maintenance -  Valve Maintenance  Improvements of sealing:  Packing box upgrades from existing Generally, upgrades are more cost effective than purchasing a newer design, but if performance is affected it may not be the case. Moreover, maintenance costs may increase as continual monitoring is required during break-in.  Live-loading By applying constant packing load without requiring continual retightening.  Metal-bellows seal It provides zero-leakage packing to linear valves. MS4001 Mechanical Maintenance - Valve Maintenance Fig. 7.14 Two types of sealing improvement: a) live-loading, b) metal bellows MS4001 Mechanical Maintenance -  Valve Maintenance 5. Regular Maintenance 5.1 Manual Ball Valve  pemeriksaan visual, khususnya pada lokasi dimana mungkin terjadi kebocoran.  Jika terjadi kebocoran, lakukan pengencangan pada baut sesuai dengan prosedur.  Jika kebocoran tetap terjadi, kemungkinan gasket, packing sudah aus, kotor, atau rusak  valve perlu dibongkar dari instalasinya untuk dilakukan reparasi.  After each adjustment, valve harus dioperasikan secara berulang untuk memastikan “smooth quarter-turn action”. MS4001 Mechanical Maintenance - 66  Valve Maintenance Gbr. Side-entry, full-port quarter-turn manual ball valve Gbr. Top-entry, full-port single-seat with tilt-action quarter-turn manual ball valve MS4001 Mechanical Maintenance - 67  Valve Maintenance  Petunjuk dari pabrik harus diikuti sepenuhnya, khususnya jika masih dalam periode masa garansi.  Beberapa petunjuk umum:  Untuk membongkar dan merakit kembali valve, pastikan bahwa valve telah bebas dari tekanan dan fluida kerja.  Untuk kasus aplikasi yang korosif, beracun, caustic, dsb. valve harus didekontaminasi untuk mencegah bahaya pada personel atau peralatan lain di dekatnya.  Setelah dilakukan reparasi lakukan prosedur instalasi seperti yang sudah dijelaskan sebelumnya. MS4001 Mechanical Maintenance - 68  Valve Maintenance 5.2 Manual Globe Valve  Beberapa tindakan yang perlu diperhatikan saat melakukan troubleshooting: Lakukan pemeriksaan dan troubleshooting secara periodik. Lakukan pemeriksaan kemungkinan terjadinya kebocoran pada berbagai lokasi: end connection, daerah antara body dan bonnet, body plug, packing box, dsb.  Jika terjadi kebocoran, lakukan pengencangan pada baut ybs sesuai dengan prosedur.  Jika kebocoran tetap terjadi, kemungkinan gasket, packing sudah aus, kotor, atau rusak  valve perlu dibongkar dari instalasinya untuk dilakukan reparasi. MS4001 Mechanical Maintenance - 69 Valve Maintenance  After each adjustment, valve harus dioperasikan secara berulang untuk memastikan “full and smooth stroke”.  Jika terjadi kebocoran yang berlebih pada valve seat, sedangkan prosedur maintenance telah dilakukan sesuai manual instruction, maka besar kemungkinan sudah terjadi keausan yang berlebih pada seat ring- plug.  Berbagai kemungkinan penyebab keausan tersebut: erosion, corrosion, cavitation atau flashing.  Kasus kebocoran yang terjadi pada valve yang baru saja direparasi, pada umumnya disebabkan oleh misalignment antara plug dan seat. MS4001 Mechanical Maintenance - 70  Valve Maintenance  Beberapa petunjuk umum Service:  Untuk membongkar dan merakit kembali valve, pastikan bahwa valve telah bebas dari tekanan dan fluida kerja.  Untuk kasus aplikasi yang korosif, beracun, caustic, dsb. valve harus didekontaminasi untuk mencegah bahaya pada personel atau peralatan lain di dekatnya.  Setelah dilakukan reparasi lakukan prosedur instalasi seperti yang sudah dijelaskan sebelumnya. MS4001 Mechanical Maintenance - 71  Valve Maintenance 5.3 Manual Gate Valve  Beberapa tindakan yang perlu diperhatikan saat melakukan troubleshooting: Lakukan pemeriksaan dan troubleshooting secara periodik. Umumnya troubleshooting dapat dilakukan saat valve sedang beroperasi. Jika diperlukan pemeriksaan/perbaikan gate, seat, packing box, proses perlu dihentikan atau diblok di sekitar valve yang akan direparasi. Dalam setiap pemeriksaan, pastikan tidak terjadi kebocoran pada bagian-bagian yang penting. MS4001 Mechanical Maintenance - 72 Valve Maintenance  Jika terjadi kebocoran, lakukan pengencangan pada baut sesuai dengan prosedur.  Jika kebocoran tetap terjadi, kemungkinan gasket, packing sudah aus, kotor, atau rusak  valve perlu dibongkar dari instalasinya untuk dilakukan reparasi.  Dapat terjadi, tetapi sangat jarang, kerusakan pada body atau bonnet. Jika kerusakannya masih tergolong minor  lakukan perbaikan dengan pengelasan atau penembelan.  Kebocoran yang terjadi pada gate and seat umumnya disebabkan oleh aus, korosi, erosi, atau akibat benda asing yang memisahkan gate dan seat atau terjadinya proses galling, khususnya pada kasus metal seat. MS4001 Mechanical Maintenance - 73  Valve Maintenance  Beberapa petunjuk umum untuk Servising:  Untuk membongkar dan merakit kembali valve, pastikan bahwa valve telah bebas dari tekanan dan fluida kerja.  Untuk kasus aplikasi yang korosif, beracun, caustic, dsb. valve harus didekontaminasi untuk mencegah bahaya pada personel atau peralatan lain di dekatnya.  Setelah dilakukan reparasi lakukan prosedur instalasi seperti yang sudah dijelaskan sebelumnya. MS4001 Mechanical Maintenance - 74  Valve Maintenance 5.4 Manual Butterfly Valve  Valve’s life time can be extended by periodically troubleshooting the valves for proper operation and PM.  In nearly all case, manual butterfly valves can be checked for operation while in-line.  Primary inspection: check for process leakage and ensure smooth operation.  Common malfunction of the butterfly valve is a seat that leaks beyond the expected leakage rate and leakage through the packing box.  Probable cause of disk-seat leakage: erosion, mechanical failure of seat, frictional wear, damage from a foreign object, cavitation, etc.  The disk and seat may be misaligned if the shaft guides or bearing are worn. MS4001 Mechanical Maintenance - 75  Valve Maintenance  The MIs should be followed exactly as they are intended.  If the process must stay in operation during servicing, any existing bypass block valves should be used to channel the flow.  Before the line flange bolting is removed, the disk should be placed in the closed (or seated) position so that the valve-body subassembly will clear the piping.  Use mechanical spreaders between the flanges to spread the piping slightly to release the valve.  To ensure correct orientation of the operator during reassembly, alignment marks should be placed on the shaft and the operator. MS4001 Mechanical Maintenance - 76 Valve Maintenance MS4001 Mechanical Maintenance - 77  Valve Maintenance Reference  Skousen, P.L., Valve Handbook, McGraw-Hill, USA, 1998.  Fisher, Control Valve Handbook, Fisher Control International, USA, 2001. MS4001 Mechanical Maintenance - 78