The materials for valve packing

Valve packing is a kind of dynamic sealing structure which is installed between the valve stem and the packing box of the valve cover to prevent outside leakage. Packing material, reasonable packing box structure and installation methods ensure the valve reliable sealing performance. There are various valve sealing packing materials are available and different packing suitable for different working conditions, including asbestos, graphite, PTFE, etc.

Flexible graphite packing

Flexible graphite packing is the most widely used material in the valve, which can be pressed molding, has been widely used in the field of petroleum, chemical industry, power generation, chemical fertilizer, medicine, paper, machinery, metallurgy, aerospace and atomic energy and other industries wherewith the nominal pressure ≤32MPa. It has the following excellent performance:

Good flexibility and resilience. The incision packing can be freely bent more than 90° in the axial direction and will leak-free due to the change of temperature/pressure/vibration, safe and reliable; Good temperature resistance. The wide range of -200℃-500℃ usages, even in non-oxidizing medium up to 2000℃ and maintain excellent sealing; Strong corrosion resistance. It has good corrosion resistance to acid, alkali, organic solvent, organic gas and steam. Low friction coefficient, good self – lubrication; Excellent impermeability to gases and liquids; Long service life, can be repeated use.

PTFE packing

Polytetrafluoroethylene packing has good lubrication, weaving polytetrafluoroethylene packing has excellent corrosion resistance, and can be used for cryogenic medium, but its temperature resistance is poor generally only used in the temperature below 200℃ conditions, while can not be used for melting alkali metal and high temperature of fluorine, hydrogen fluoride medium.

Vegetable fiber packing

Made of hemp or cotton impregnated oil, wax or other anti-seepage materials, used for low-pressure valves below 100℃ and media like water, ammonia and etc.

Asbestos packing

Asbestos fiber has better heat resistance, absorption and strength can withstand weak acid, strong alkali. Inked asbestos, rubber asbestos and oil-impregnated asbestos are suitable for valves with the steam temperature of 450℃.

Rubber filler

Rubber cloth, rubber rod, ring rubber packing for temperature ≤140℃ ammonia, concentrated sulfuric acid and other media.

Carbon fiber packing

The carbon fiber filler is made of polytetrafluoroethylene emulsion impregnated with carbon fiber is a woven rope. Carbon fiber packing has excellent elasticity, excellent self-moistening and high-temperature resistance. It can work stably in the air temperature range of -120 ~ 350℃, and the pressure resistance is less than 35MPa.

Metal + rubber packing

It can include metal-wrapped packing, metal laminated packing, metal corrugated packing, lead packing, etc. The metal wrapped packing and metal laminated packing is characterized by high-temperature resistance, erosion resistance, abrasion resistance, high strength, good thermal conductivity, but poor sealing performance must be used with plastic packing, its temperature, pressure, corrosion resistance that depends on metal material.

Stainless steel wire + flexible graphite woven packing

Generally, v-shaped packing is composed of upper packing, middle packing and lower packing. The upper and middle packing is made of PTFE or nylon, and the lower packing is made of 1Cr13, 1Cr18Ni9 and A3 steel. PTFE can withstand high temperature 232℃, nylon 93℃, general pressure 32MPa, often used in corrosive media.

Generally speaking, the valve packing materials are mainly PTFE and flexible graphite, be noted that the packing box dimension accuracy, roughness, stem surface dimension accuracy also affect packing seal performance.

What is a valve body?

The valve is a type of device used to control, change or stop the moving components of flow direction, pressure and discharge in the pipeline system. The valve body is a main part of valve. It is made by different manufacturing processes according to pressure class, such as casting, forging, etc. Valve body with low-pressure is usually cast while valve body with medium and high pressure is manufactured by the forging process.

The Materials for Valve Body
The commonly used materials of valve body are: cast iron, forged steel, carbon steel, stainless steel, Nickel-based alloy, copper, titanium, plastic, etc

Carbon steel
In the oil and gas industry, the most commonly used material for valve body is ASTM A216(for casting) and ASTM A105(forging). For low-temperature service, ASTM A352 LCB/LCB for cast and ASTM A350 LF2/LF3 for forged bodies are used.

Stainless steel
When there are more requirements for the temperature, pressure or corrosion increase, stainless steel bodies become necessary: ASTM A351 CF8 (SS304) and CF8M (SS316) for cast devices, and the various ASTM A182 F304, F316, F321, F347 for forged types. For specific applications, special material grades are used such as duplex and super steels (F51, F53, F55) and nickel alloys (Monel, Inconel, Incoloy, Hastelloy) for valve bodies.

Non-ferrous
For more severer applications, non-ferrous materials or alloys like, Aluminum, Copper, Titanium alloys and other plastic, ceramic material combining alloys can be used for body manufacturing.

The End Connections of Valve Body
Valve body can be connected to other mechanical devices and pipes in different ways. The main end types are flanged and buttweld (for devices above 2 inches) and socket weld or threaded/screwed (NPT or BSP) for small diameter devices.

Flanged End Valve
Flanged ends are the most frequently used form of connection between valves and piping or equipment. It is a detachable connection with flange, gasket, stud bolts and nuts as a group of sealing structure.

Indicated by the ASME B16.5 specification, flange connection can be applied to a variety of larger diameter valves, and nominal pressure valves, but there are certain restrictions on the use temperature, in high-temperature conditions, due to the flange connection bolts easy to creep phenomenon and cause leakage, generally speaking, flange connection is recommended to use at a temperature ≤350℃.

The flange face may be raised (RF), flat (FF), ring joint, tongue and groove and male & female and be finished in any of the available variants (stock, serrated or smooth).

Welding Ends Valve
Welding connection between valve and pipeline can be butt welding connection (BW) and socket welding connection (SW) used for high-pressure pipelines (socket weld for smaller sizes, below 2 inches, and buttweld for larger diameters). These welded connections are more expensive to execute than flanged joints, as they require more work, but are more reliable and less prone to leakages in the long run.

Valves with socket weld ASME B16.11 or buttweld ends ASME B16.25 are welded with the connecting pipe. Buttweld connections require full welding of the beveled ends of the two parts to be joined, whereas socket weld connections are made by fillet welds.

Threaded End Valve
This is a simple connection and often used for low-pressure or small valves below 2 inches. The valve is connected to the pipe by a tapered thread ends, which may be BSP or NPT. Threaded connections are cheaper and easier installation, as the pipe is simply screwed onto the valve, stud bolts or welding operations without the need for flanges.

The selection of valve operation modes

Depending on the mode of operation, the valve can be divided into the manual valve and an actuator driven valve. Valve actuators are devices that operate and are connected to the valve, controlled by hand (handwheel/spring lever), electric (solenoid /motor), pneumatic (diaphragm, cylinder, blade, air engine, film and ratchet combination), hydraulic (hydraulic cylinder/hydraulic motor) and combination (electro &hydraulic, pneumatic &hydraulic).

Valve drive device can be divided into straight stroke and angle stroke according to the motion modes. The straight stroke drive device is multi-turn drive, mainly suitable for various types of gate valves, globe valves and throttle valves; The angular stroke drive device is a partial rotary drive device that only needs a 90° angle. mainly applicable to various types of ball valves and butterfly valves. The selection of valve actuators should be based on a full understanding of the type and performance of the valve actuators, depending on the type of valve, the operating specification of the device and the position of the valve on the line or device.

 

Valve with self-acting by fluid

The automatic valve is to rely on the energy of the medium itself to open and close the valve does not need external force drive such as safety valve, pressure reducing valve, steam trap, check valve, automatic regulating valve.

 

Handwheel or lever valve

Manual operated valves are the most widely used type of valve, that are manually driven valves with handwheels, handles, levers, and chain wheels.  When the opening and closing torque of the valve is larger, this wheel or worm gear reducer can be set between the handwheel and the valve stem. Universal joint and driveshaft can also be used when remote operation is necessary.

Manually operated valves are usually equipped with a handwheel attached to the valve’s stem or Yoke nut which is rotated clockwise or counterclockwise to close or open a valve. Globe and gate valves are opened and closed in this way.

Hand-operated, quarter-turn valves, such as Ball valve, Plug valve or Butterfly valve, which need a lever to actuate the valve. While there are applications where it is not possible or desirable to actuate the valve manually by handwheel or lever. In these situations may be the actuators are needed.

 

Valve drove by actuators

An actuator is a driving device that provides linear or rotational motion, utilizing a certain source of power and operating under a certain control signal. Basic actuators are used to fully open or close a valve. Actuators for controlling or regulating valves are given a positioning signal to move to any intermediate position. There many different types of actuators, the commonly used valve actuators are showed below:

  • Gear Actuators
  • Electric Motor Actuators
  • Pneumatic Actuators
  • Hydraulic Actuators
  • Solenoid Actuators

Large valves must be operated against high hydrostatic pressure and they must be operated from a remote location. When the time for the opening, closing, throttle or manually controlling the valve is longer, than required by system-design standards. These valves are usually equipped with an actuator.

 

Generally speaking, selecting the actuators that depend on several factors like the valve type, operation intervals, torque, switch control, continuous control, external power availability, economy, maintenance and so on being these factors dependable on each situation.

The leak rates standards of industrial valve

Valves are one of the main leakage sources in the pipeline system of the petrochemical industry, so it’s critical to the leakage of valves. Valve leakage rates are actually the valve sealing level, valve sealing performance is referred to as the valve sealing parts to prevent the ability of media leakage.

The main sealing parts of the valve including: the contact surface between the opening and closing parts and the seat, the fitting of packing and stem and packing box, the connection between the valve body and the bonnets. The former belongs to internal leakage, which directly affects the ability of the valve to cut off the medium and the normal operation of the equipment. The last two are external leakage, that is, media leakage from the inner valve. The loss and environmental pollution caused by external leakage are often more serious than that caused by internal leakage. Valve leakage is not allowed especially for high temperature and pressure condition, flammable, explosive, toxic or corrosive media, so the valve must provide reliable sealing performance to meet the requirements of its use conditions on the leakage. At present, there are five kinds of valve seal classification standards commonly used in the world.

 

ISO 5208

International Organization for Standardization ISO 5208 specifies examinations and tests that a valve manufacturer needs to act upon in order to establish the integrity of the pressure boundary of an industrial metallic valve and to verify the degree of valve closure tightness and the structural adequacy of its closure mechanism.

There are 10 leakage rates specified in ISO 5208: A, AA, A, B, C, CC, D, E, EE, F, G and the rate A is the highest grades. There is a loosely defined correspondence between the leakage rate acceptance values of API 598 and leakage value rate A as applied to DN 50, rate CC-liquid for other than metal-seated check valves and for check valves rate EE-gas and rate G-liquid. Rate A, B, C, D, F and G correspond to values in EN 12266-1.

API 598

American petroleum institute standard API 598 is the most commonly used test standard for American standard valves. It is applicable to the following API standard valve sealing performance tests:

API 594 Flanged, lug, wafer and butt weld connection check valves

API 599 Flanged, threaded and butt-welded metal plug valves

API 602 Steel gate and check valves DN 00 and below for the oil and natural gas industry

API 603 Flanged and butt-welded corrosion-resistant bolted cover gate valves

API 608 Flanged, threaded and butt-welded metal ball valves

API 609 Double flanged, lug and wafer butterfly valves

MSS SP61

American Association for standardization of manufacturers of valves and fittings MSS SP61 pressure test for metallic valves specifies the allowable leakage requirements are as follows:

(1) In the case that one of the sealing surfaces of the valve sealing seat is made of plastic or rubber, no leakage shall be observed during the duration of the sealing test.

(2) The maximum allowable leakage on each side when closed shall be: the liquid shall be the nominal size (DN) 0 per mm, 0 per hour.4 ml; Gas is the nominal size (DN) per millimeter, 120 ml per hour.

(3) The leakage allowed by the check valve can be increased by 4 times.

It should be noted that MS SSP 61 is often used for inspection of “fully open” and “fully closed” steel valves, but not for control valves. MSS SP61 is not usually used for testing American standard valves.

ANSIFCI 70-2

American national standards/American instrument association standards ANSI/FCI 70-2(ASME B16).104) is applicable to the control valve seal grade requirements. Metal-elastic seal or metal seal should be selected in engineering design according to the characteristics of the medium and the opening frequency of the valve. Metal seated valve seal grades should be is stipulated in the order contract, the rates I, Ⅱ, Ⅲ is used less due to request a lower level, generally choose Ⅳat least and V or Ⅵ for higher requirements.

EN 12266—1

EN 12266-1, tests on industrial valves part l specifies the pressure tests, test methods and acceptance criteria – mandatory requirements. EN 12266-1 meets the requirements of ISO 5208 for seal classification but lacks AA, CC and EE ratings. The new edition of ISO 5208 adds six levels of AA, CC, E, EE, F, and G and gives comparisons with several seal levels of API 598 and EN 12266.

 

It should be noted in the engineering design that API 600-2001(ISO 10434–1998) specifies that the sealing performance of the valve is tested in accordance with ISO 5208, but the leakage in tables 17 and 18 is equivalent to API 598–1996, not ISO 5208. Therefore, when API 600 and its sealing performance test API 598 standard are selected for the engineering design, the version of the standard must be clarified to ensure the uniformity of the standard content.

The relevant guidelines of API 6D(ISO 14313) for valve leakage are: “soft seated valves and oil-seal plug valves shall not exceed ISO 5208 A (no visible leakage), metal seat valves shall not exceed ISO 5208(1993) D unless otherwise specified.” Note in the standard: “special applications may require leakage less than ISO 5208(1993) class D. Therefore, leakage requirements higher than the standard shall be given in the order contract.

 

Full port ball valve VS reduced port ball valve

As we all know that the ball valve can be divided into full port ball valve and reduced ball valves according to the flow passage form. A full port ball valve, commonly known as full bore ball valve has an over-sized ball so that the hole in the ball is the same size as the pipeline resulting without obvious limitations, is mainly used in switches and circuit applications. Reduced ball valves, also known as the standard port valve, are valves with the opening of the closing part to control flow, of which area is less than of the inner diameter of the pipeline.

There is no valve standards concept for full port ball valves and reduced ball valves. ASTM, GB only requires the ball valve to be tested for pressure drop while the Korean standard made provisions on their concept: valve ball diameter less than or equal to 85% of the ball valve port diameter is called reduced ball valve, ball valve diameter greater than 95% of the ball valve port diameter is called full diameter ball valve. Generally speaking, a full port ball valve is equal width channel, its size can not be less than the nominal size specified in the standard, such as DN50 full diameter ball valve channel diameter is about 50mm. The inlet of reduced diameter ball valve passage is larger than the diameter of the passage, and the actual diameter of the passage is probably smaller than this specification. For example, the diameter of the DN50 reduced diameter ball valve is about 38, roughly equivalent to DN40.

Medium:

The full port ball valve is mainly used for conveying viscous, easy slagging medium, regular cleaning convenient. The reduced port ball valve is mainly used for conveying gas or medium physical performance similar to water in the pipeline system, its weight about 30% lighter than the full port ball valve, and the flow resistance is only 1/7 of the same diameter of the globe valve.

Application:

The full port ball valve offers small flow resistance, especially suitable for demanding conditions. Fully welded full port ball valves are required for buried landowners in oil and gas pipelines. The reduced port ball valve is suitable for some low requirements, low convection resistance requirements and other conditions.

Pipeline circulation capacity:

Experimental tests have shown that when the valve inner diameter is greater than 80% of the pipe end inner diameter, it has little effect on the pipeline fluid flow capacity. On the one hand, the reduced diameter design reduces the flow capacity of the valve (Kv value), increases the pressure drop at both ends of the valve, and causes the loss of energy, which may not have a great impact on the pipeline but increases the erosion of the pipeline.

 

In general, reduced port ball valve has a smaller size, smaller installation space, about 30% than the full port of the ball valve weight, is conducive to reduce the pipe load and transportation costs, extends the service life of the valve, also cheaper. For full port ball valve, flow is unrestricted but the valve is larger and more expensive so this is only used where free flow is required, for example in pipelines which require pigging.

Valve pressure test of DBB and DIB ball valve

DBB (double block and bleed valve) and DIB (double isolation and bleed valve) are two kinds of commonly used seat sealing structures for trunnion mounted ball valves. According to API 6D, DBB ball valve is a single valve with two sealed auxiliary, the closed position of which provides the pressure seal at both ends of the valve by means of bleed of the body cavity between the two seal surfaces, if the first seal leaks, the second will not seal in the same direction. DIB ball valve is a single valve with two seating surfaces, each of these seal seat provides a single source of pressure seal in the closed position by discharging the valve chamber between the seal seats.

 

The pressure test of the DBB valve:

The valve is partially opened so that the experimental flow is fully injected into the valve chamber, and then the valve is closed so that the bleeding of the valve body is open and the excess medium is allowed to overflow from the valve chamber test junction. Pressure should be applied simultaneously from both ends of the valve to monitor seat tightness through overflow at the valve chamber test junction. The figure below shows a typical DBB ball valve configuration.

When the valve is closed and the valve chamber test port is opened and both ends of the valve are pressurized (or pressurized separately), the valve chamber port detects leakage from each end to the valve chamber. Theoretically, DBB valve can not provide positive double isolation when only one side is under pressure, the valve does not provide positive double isolation when only one side is under pressure.

 

The pressure test of DIB-1(Two bi-directional sealing seats)

Each seat shall be tested in both directions and the installed cavity pressure relief valve shall be removed. The valve shall be half-opened so that the valve and valve chamber shall be injected with the test medium until the test liquid spills through the test port of the valve chamber. Close the valve to prevent leakage of the chamber in the direction of the test seat, the test pressure shall be applied successively to each end of the valve to test the leakage of each seat upstream separately, and then to test each seat as the downstream seat. Open both ends of the valve to fill the cavity with media and then pressurize while observing leakage of each seat at both ends of the valve.

Because the pressure in the cavity of the DIB-1 valve cannot be released automatically, when the temperature of the valve is abnormally raised, the volume of the medium in the valve cavity increases accordingly, thus forcing the pressure in the cavity to increase automatically. When the pressure reaches a certain level, it will be very dangerous, so the cavity of the DIB-1 valve must be installed with a safety valve.

 

The pressure test of DIB-2(One bi-directional and one unidirectional sealing seat)

One of the seats of the DIB-2 valve can withstand pressure from either the chamber or the end of the valve in any direction without leakage. The other seat can only withstand pressure from the end of the valve. When the valve is closed and the valve chamber test interface is open and both ends of the valve are pressurized (or pressurized separately), the valve chamber test interface can detect whether there is leakage from each end to the valve chamber. Two-way seat test should be pressurized valve chamber and valve upstream observe whether the downstream valve leakage.

The advantage of the valve is tight protection for the valve, the valve closed after the medium will never enter the pipeline downstream, at the same time when the cavity pressure abnormal rise can automatically pressure relief to the upstream of the valve. Please note that the valve installation direction requirements, the opposite direction is the same as DBB.

 

Both DBB and DIB valves have their unique application and media, and various environmental challenges where critical isolation is needed to ensure that leakage does not occur such as LNG, petrochemical, transmission and storage, natural gas industrial processes, mainline and manifold valves in liquid pipelines, and refined products transmission lines.