1、 MSS SP-143-2012 Live-Loaded Valve Stem Packing Systems Standard Practice Developed and Approved by the Manufacturers Standardization Society of the Valve and Fittings Industry, Inc. 127 Park Street, NE Vienna, Virginia 22180-4602 Phone: (703) 281-6613 Fax: (703) 281-6671 E-mail: standardsmss-hq.org
2、 MSS www.mss-hq.org MSS STANDARD PRACTICE SP-143 i This MSS Standard Practice was developed under the consensus of the MSS Technical Committee 308 and the MSS Coordinating Committee. The content of this Standard Practice is the resulting efforts of competent and experienced volunteers to provide an
3、effective, clear, and non-exclusive standard that will benefit the industry as a whole. This MSS Standard Practice describes minimal requirements and is intended as a basis for common practice by the manufacturer, the user, and the general public. The existence of an MSS Standard Practice does not i
4、n itself preclude the manufacture, sale, or use of products not conforming to the Standard Practice. Mandatory conformance to this Standard Practice is established only by reference in other documents such as a code, specification, sales contract, or public law, as applicable. MSS has no power, nor
5、does it undertake, to enforce or certify compliance with this document. Any certification or other statement of compliance with the requirements of this Standard Practice shall not be attributable to MSS and is solely the responsibility of the certifier or maker of the statement. “Unless indicated o
6、therwise within this MSS Standard Practice, other standards documents referenced to herein are identified by the date of issue that was applicable to this Standard Practice at the date of approval of this MSS Standard Practice (see Annex A). This Standard Practice shall remain silent on the validity
7、 of those other standards of prior or subsequent dates of issue even though applicable provisions may not have changed.” By publication of this Standard Practice, no position is taken with respect to the validity of any potential claim(s) or of any patent rights in connection therewith. MSS shall no
8、t be held responsible for identifying any patent rights. Users are expressly advised that determination of patent rights and the risk of infringement of such rights are entirely their responsibility. In this Standard Practice, all text, notes, annexes, tables, figures, and references are construed t
9、o be essential to the understanding of the message of the standard, and are considered normative unless indicated as “supplemental”. All appendices, if included, that appear in this document are construed as “supplemental”. Note that supplemental information does not include mandatory requirements.
10、U.S. customary units in this Standard Practice are the standard; (SI) metric units are for reference only. Non-toleranced dimensions in this Standard Practice are nominal and, unless otherwise specified, shall be considered “for reference only”. Excerpts of this Standard Practice may be quoted with
11、permission. Credit lines should read Extracted from MSS SP-143-2012 with permission of the publisher, Manufacturers Standardization Society of the Valve and Fittings Industry. Reproduction and/or electronic transmission or dissemination is prohibited under copyright convention unless written permiss
12、ion is granted by the Manufacturers Standardization Society of the Valve and Fittings Industry Inc. All rights reserved. Originally Approved: February 2012 Originally Published: March 2012 MSS is a registered trademark of Manufacturers Standardization Society of the Valve and Fittings Industry, Inc.
13、 Copyright 2012 by Manufacturers Standardization Society of the Valve and Fittings Industry, Inc. Printed in U.S.A. MSS STANDARD PRACTICE SP-143 ii FOREWORD This Standard Practice was developed by a cooperative effort of representatives of valve and packing manufacturers. This Standard Practice is i
14、ntended primarily to be an aid in the design and application of live-loaded packing systems. This Standard Practice represents the consensus input from a broad spectrum of industry representatives. This Standard Practice should not be construed to be effective for all pressures and types of services
15、 expected of ASME B16.34 valves. MSS STANDARD PRACTICE SP-143 iii TABLE OF CONTENTS SECTION PAGE 1 SCOPE . 1 2 DEFINITIONS . 1 3 APPLICATION GUIDELINES . 2 4 DESIGN GUIDELINES 2 5 PACKING LOAD DETERMINATION 4 6 SPRING SELECTION . 6 7 PACKING INSTALLATION 9 8 PRE-LOADING LIVE-LOADED SYSTEMS 9 TABLE 1
16、 Packing Factor 5 2 Typical Spring Material Properties 8 FIGURE 1 General Arrangement for Live-Loaded Packing System Using Belleville Springs . 2 2 Packing Chamber Details . 4 3 Belleville Spring Arrangements . 6 4 Belleville Dimensional Details 7 ANNEX A Referenced Standards and Applicable Dates 10
17、 MSS STANDARD PRACTICE SP-143 1 1. SCOPE This Standard Practice establishes the minimum requirements for the application, design and installation of live-loaded stem packing systems for valves designed in accordance with ASME B16.34, having rising and/or rotating stems. It applies to valves with fle
18、xible graphite, polytetrafluoroethylene (PTFE), or other packing stem seal material, regardless of frequency of operation. This Standard Practice does not apply to valves that include a lantern ring(s) as part of its packing system or to self-energized packing. 2. DEFINITIONS 2.1 See MSS SP-96 for d
19、efinitions of terms used in this Standard Practice. 2.2 The following definitions are unique to this Standard Practice: a. Live-loading System A system designed to provide a continuous compression sealing load on a valves packing system and has a build-in deflection feature to maintain the packing s
20、ealing load to overcome negative packing volumetric changes (relaxation, consolidation, shrinkage or wear) that may occur during a valves service life. b. Coil Spring A spring made by winding a wire around a cylinder or conical shape (volute spring) that has a near linear force/defection curve. c. B
21、elleville Spring, Belleville Disc Spring, Disc Spring, or Conical Compression Washer A conical shaped disc that will deflect (flatten) at a given rate. The spring rate is usually very high, allowing the deflective spring to produce a very large compressive load while occupying a very small space. It
22、 can be dimensionally defined by its outside diameter (O.D.), inside diameter (I.D.), material thickness (t), overall height (H) and flat deflection (h) capability. d. Sleeve Tubular device that fits over either Belleville O.D. or I.D. packing gland bolting to prevent lateral movement for stack of B
23、elleville disc springs having a larger I.D. than gland bolt, thickness less than gland bolt pitch, or to limit torque that can be applied to the disc spring stack. e. Wave and Nested Wave Springs Produced from flat spring stock in a washer style but waves are pressed in the washer perimeter, which p
24、rovides a deflection mechanism. They can be series stacked or parallel stacked like disc springs to meet load and deflection requirements. f. Gland Plates Designed to act like cantilever springs, providing load characteristics across a narrow deflection range. g. Packing “K” Factor A multiplier of t
25、he force on the bottom ring of packing that will insure the gland face force will produce a radial load on the packing stack below the top packing ring capable of sealing the packing system from leakage and eliminating any packing consolidation of the bottom rings of packing due to service pressure
26、that could create a void in the packing at the bottom of the packing chamber. h. Consolidation Reduction in overall height of packing as a result of wear, thermal and pressure cycles, and time-dependent volumetric changes of packing while in-service. 2.3 Figure 1 illustrates a general arrangement fo
27、r a live-loading device that uses a Belleville washer stack design, as covered by this Standard Practice. LIVE-LOADED VALVE STEM PACKING SYSTEMS MSS STANDARD PRACTICE SP-143 2 FIGURE 1 General Arrangement for Live-Loaded Packing System Using Belleville Springs(a) Note: (a)Washer between nut and Bell
28、eville spring is not illustrated but is recommended if Belleville washer or nut material is subject to galling. Belleville Springs 3. APPLICATION GUIDELINES(1) 3.1 The purpose of live-loading a valve packing system is to extend the required time period for maintenance attention. Valves in service th
29、at are inaccessible for routine packing maintenance or for valves in service where the maintenance effort has proven historically ineffective in preventing packing leaks should be candidates for live-loading. Other candidates include frequently cycled valves, air and motor operated valves, and valve
30、s being monitored for fugitive emissions. Live-loading the valve packing can greatly extend the maintenance interval for servicing the valve packing. A live-loading device preloads the valves packing system with a sealing force and restoring deflection capability, which can overcome in-service packi
31、ng consolidation, as the valve packing undergoes changes due to valve cycling, thermal and pressure cycles, and time-dependent packing volumetric changes. The construction details of this Standard Practice are recommendations for the live-loading of valves. Note: (1)Fire-tested valve certification i
32、s outside the scope of this Standard Practice. A fire-tested valve certification is voided when the valve is modified to a live-loaded packing design. 3.2 The valve design and packing system used for the valve selected for live-loading shall be evaluated to determine the best type live-loading syste
33、m to be used by the valve manufacturer. 3.3 The selected packing live-loading system shall provide an as-shipped compression load and deflection capability for the packing system to maintain a sealing force on the packing for frequent and infrequent cycling service for an extended maintenance period
34、. 4. DESIGN GUIDELINES 4.1 Valve Packing Chamber Recommended Requirements 4.1.1 The live-loading design shall consider not only packing compression stress development but also the transfer of this compression stress to a packing radial sealing stress. Compressive and radial stresses are not necessar
35、ily distributed equally throughout the packing stack. Shaped packing designs may enhance radial sealing stress development by forced radial packing movement while under compression. MSS STANDARD PRACTICE SP-143 3 4.1.2 The packing chamber, packing gland, packing ring (if used), and stem dimensions s
36、hall be designed to minimize the possibility of packing being extruded from the packing chamber. The assembled diametrical clearance or “gap” between packing gland/stuffing box, packing gland /stem, and stem/packing ring interfaces shall be kept to a minimum to prevent loss of packing. 4.1.3 For rot
37、ating/rising and non-rotating/rising stem valves with flexible graphite packing, the packing chamber details should be in compliance with the requirements of MSS SP-120. For packing chamber depths greater than those recommend by MSS SP-120, consider adding a filler sleeve to the packing chamber so a
38、s to normalize the difference. An appropriate anti-extrusion device (top and bottom of packing set) should be employed and is particularly critical for graphitic packing sets for high-pressure service. 4.1.4 For solid flat and shaped PTFE packed valves, the surface finish for packing chamber bore an
39、d stem finish should be 32 RMS maximum. The diametrical clearances (gap) between the stuffing box and packing gland, and stem dimensions with the packing gland and packing ring (if used) dimensions, shall be kept minimal to minimize extrusion of packing from the packing chamber. 4.1.5 For other type
40、 packing systems, the stuffing box and stem finishes shall be as recommended by the packing manufacturer. The diametrical clearances between the stuffing box, packing ring (if used), and stem dimensions with the packing gland dimensions, shall be kept minimal to minimize extrusion of packing from th
41、e packing chamber. 4.1.6 The diametrical clearances (gap), between the packing chamber and stem-to-packing O.D. and I.D., shall be kept minimal to minimize packing radial compression requirements for sealing and to insure that additional rings do not have to be added to the valves stuffing box to me
42、et remaining gland adjustment life for as-shipped valves. 4.2 Packing Compressive Stress Level 4.2.1 Different packing materials require different stress levels to function properly. Equally important, the over-stressing of the packing shortens its design life and unnecessarily increases frictional
43、loads against the stem and increases packing wear. A selected packings compression stress capability, at a minimum, should exceed 1.5 times the valves service pressure or 1.5 times the valves 100 F rating to accommodate the valve manufacturers production shell test requirement with valve partially o
44、pen and exposing the valve packing to shell test pressure. 4.2.2 In all cases, the compressive stress(es) at the face of the gland (against packing) in the packing chamber must be greater than the intended working pressure. The resultant compressive stress at the bottom ring of packing should as a m
45、inimum equal or exceed the service pressure; otherwise, the packing may be compressed by the service pressure creating a consolidation void at the bottom of the packing stack. This void could lead to total loss of compressive load on packing. 4.2.3 The compression load applied at the face of the gla
46、nd decays as it is distributed to rings below the top ring of packing. This decay in compression load is due to the selected packing characteristics (number of rings, density, tolerances, friction factor, etc.) as well as the friction between the packing and the valves packing chamber and stem. The
47、compression load at the face of the valve gland shall be great enough to create a packing seal near the bottom of the packing set that will meet the performance standard. The valves packing system “no visible leak” performance standard is based on a pressure test against packing conducted at 1.1 tim
48、es the service pressure or 1.1 times the valves 100 F rating. Packing stress decay is the source of the packing factor “K” noted in Table 1. The greater the “K” the greater force the valve gland and live-load system must produce to bring the packing to a sealing and solid structure. MSS STANDARD PRA
49、CTICE SP-143 4 4.3 Deflection Determination 4.3.1 The range of deflection over which a live-loading system is to be effective while in service is affected by several factors: a. The packings susceptibility to packing consolidation. b. Upper and lower temperature limits of the intended application. For PTFE-based packing, in-service thermal cycling service shall be considered. c. The packings wear rate. 4.3.2 Pre-service packing gland remaining adjustment travel is typically chosen to be a fraction of the packing set height “H” (see Figure 2). Graphitic packing ma
