MSS SP-143-2018 Live-Loaded Valve Stem Packing Systems.pdf

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1、MSS SP-143-2018 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: standardsmsshq.org w

2、ww.msshq.org MSS STANDARD PRACTICE SP-143i 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 effectiv

3、e, 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 in itself

4、 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 does it

5、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 otherwise

6、 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 of thos

7、e 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 not be hel

8、d 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 to be ess

9、ential 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. Substant

10、ive changes in this 2018 edition are “flagged” by parallel bars as shown on the margins of this paragraph. The specific detail of the change may be determined by comparing the material flagged with that in the previous 2012 edition. Non-toleranced dimensions in this Standard Practice are nominal unl

11、ess otherwise specified. Excerpts of this Standard Practice may be quoted with permission. Credit lines should read Extracted from MSS SP-143-2018 with permission of the publisher, Manufacturers Standardization Society of the Valve and Fittings Industry. Reproduction and/or electronic transmission o

12、r dissemination is prohibited under copyright convention unless written permission 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 Current Edition Approved: Februa

13、ry 2018 Current Edition Published: March 2018 MSS is a registered trademark of Manufacturers Standardization Society of the Valve and Fittings Industry, Inc. Copyright 2018 by Manufacturers Standardization Society of the Valve and Fittings Industry, Inc. Printed in U.S.A. MSS STANDARD PRACTICE SP-14

14、3ii FOREWORD This Standard Practice was developed by a cooperative effort of representatives of valve and packing manufacturers. This Standard Practice is intended primarily to be an aid in the design and application of live-loaded packing systems. This Standard Practice represents the consensus inp

15、ut from a broad spectrum of industry representatives. This Standard Practice should not be construed to be effective for all pressures and types of services expected of ASME B16.34 valves. MSS STANDARD PRACTICE SP-143iii This Page Intentionally Left Blank Manufacturers Standardization Society of the

16、 Valve and Fittings Industry MSS STANDARD PRACTICE SP-143iv TABLE OF CONTENTS SECTION PAGE 1 SCOPE . 1 2 DEFINITIONS . 1 3 APPLICATION GUIDELINES . 2 4 DESIGN GUIDELINES 3 5 PACKING LOAD DETERMINATION 5 6 SPRING SELECTION . 7 7 PACKING INSTALLATION 11 8 PRE-LOADING LIVE-LOADED SYSTEMS 11 TABLE 1 Pac

17、king Factor 4 2 Typical Disc Spring Material Properties 8 FIGURE 1 General Arrangement for Live-Loaded Packing System Using Belleville Springs 2 2 Packing Chamber Details . 5 3 Belleville Spring Arrangements . 7 4 Belleville Dimensional Details . 9 ANNEX A Referenced Standards and Applicable Dates 1

18、2 MSS STANDARD PRACTICE SP-1431 LIVE-LOADED VALVE STEM PACKING SYSTEMS 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 rotati

19、ng stems. It applies to valves with flexible graphite, polytetrafluoroethylene (PTFE), expanded polytetrafluoroethylene (ePTFE), 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 pa

20、cking system or to self-energized packing. 2. DEFINITIONS 2.1 See MSS SP-96 for definitions of terms used in this Standard Practice. 2.2 The following terms and definitions are unique to this Standard Practice: As-Shipped The condition of the assembled product when it leaves the manufacturers shippi

21、ng dock. Belleville Spring, Belleville Disc Spring, Disc Spring, Belleville Spring Washer 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 whi

22、le occupying a very small space. It 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. Cantilever Spring A flat spring supported at one end and fastened to its load at the other end. Coil

23、 Spring A spring made by winding a wire around a cylinder or conical shape ( volute spring) that has a near linear force/deflection curve. Consolidation Reduction in overall height of packing as result of wear, thermal and pressure cycles, and time-dependent volumetric changes of packing while in-se

24、rvice. Gland Plates Designed to act like cantilever springs, providing load characteristics across a narrow deflection range. Live-loading System A system designed to provide a continuous compression sealing load on a valves packing system and has a built-in deflection feature to maintain the packin

25、g sealing load to overcome negative packing volumetric changes (relaxation, consolidation, shrinkage or wear) that may occur during a valves service life. Packing “K” Factor A multiplier of the force on the bottom ring of packing that will insure the gland face force will produce a radial load on th

26、e 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 that could create a void in the packing at the bottom of the packing chamber. Sleeve Tubular device that fits

27、 over either Belleville O.D. or I.D. packing gland bolting to prevent lateral movement for stack of Belleville 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. Snap Over Spring deflection directi

28、on reversal if the ratio of the conical height of the spring washer, prior to compression to the thickness of the spring washer, approaches or exceeds two (2). Wave and Nested Wave Springs Produced from flat spring stock in a washer style but waves are pressed in the washer perimeter, which provides

29、 a deflection mechanism. They can be series stacked or parallel stacked like disc springs to meet load and deflection requirements. MSS STANDARD PRACTICE SP-1432 2.3 Figure 1 illustrates a general arrangement for a live-loading device that uses a Belleville spring washer stack design, as covered by

30、this Standard Practice. FIGURE 1 General Arrangement for Live-Loaded Packing System Using Belleville Springs(a)NOTE: (a) A washer located between nut and Belleville spring is not illustrated, but is recommended if Belleville washer or nut material is subject to galling. 3. APPLICATION GUIDELINES(1)

31、3.1 The purpose of live-loading a valve packing system is to extend the required time period for maintenance attention. Valves in service that are inaccessible for routine packing maintenance or for valves in service where the maintenance effort has proven historically ineffective in preventing pack

32、ing leaks should be candidates for live-loading. Other candidates include frequently cycled valves, air and motor operated valves, and valves being monitored for fugitive emissions. Live-loading the valve packing can greatly extend the maintenance interval for servicing the valve packing. A live-loa

33、ding device preloads the valves packing system with a sealing force and restoring deflection capability, which can overcome in-service packing consolidation, as the valve packing undergoes changes due to valve cycling, thermal and pressure cycles, and time-dependent packing volumetric changes. The c

34、onstruction details of this Standard Practice are recommendations for the live-loading of valves. 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 system to be used by the valve manufacturer. 3.3 The selec

35、ted 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. NOTE: (1) Fire-tested valve certification is outsid

36、e the scope of this Standard Practice. Belleville Springs MSS STANDARD PRACTICE SP-1433 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 stres

37、s to a packing radial sealing stress. Compressive and radial stresses are not necessarily distributed equally throughout the packing stack. Shaped packing designs may enhance radial sealing stress development by forced radial packing movement while under compression. 4.1.2 The packing chamber, packi

38、ng gland, packing ring (if used), and stem dimensions shall 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

39、 to a minimum to prevent loss of packing. 4.1.3 For rotating/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, cons

40、ider adding a filler sleeve to the packing chamber so as 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

41、 valves, the surface finish for packing chamber bore and 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

42、 packing from the packing chamber. 4.1.5 For other type 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 b

43、e kept minimal to minimize extrusion of packing from the 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 ensure that additional rings d

44、o not have to be added to the valves stuffing box to meet 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

45、its design life and unnecessarily increases frictional 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 p

46、roduction shell test requirement with valve partially open 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 compres

47、sive stress at the bottom ring of packing should as a minimum 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.

48、2.3 The compression load applied at the face of the gland 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 th

49、e packing and the valves packing chamber and stem. The 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. MSS STANDARD PRACTICE SP-1434 The valves packing system “no visible leak” performance standard is based on a pressure test against packing conducted at 1.1 times 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