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Trim

API 600 defines gate valve "trim" as being comprised of the stem,
body seat surface, gate seat surface, bushing or weld deposit for backseat and
stem hole guide, and small internal parts normally contacting the service fluid,
excluding the pin used in making a stem-to-gate connection
(the pin is specified to be an austenitic stainless steel).
Trim is therefore not a valve component as such, but a treatment of wear
surfaces exposed to the process fluid.
Trims are selected to prevent galling of metal wear surfaces in sliding contact
with each other, and to minimize corrosion and erosion effects.
API 600 lists 16 different nominal trims by composition of seating materials
and associated hardness.

Block valve trim requirements for a number of service applications, based on
the contained fluid and design pressure-temperature conditions.
Stellite 6 (TRIM No. 5) is specified for the more severe/critical service
applications, due to its superior anti-galling characteristics.
However, Stellite 6 and equivalent hard facing materials listed as
TRIM No. 5 in API 600 are cobalt based and have become increasingly
expensive and difficult to obtain. In recognition of this problem, API600 includes
an alternative TRIM No. 5A, a nickel-chromium hard facing material.
We do not have service experience with TRIM No. 5A to date, but it is not rated
fully equal to Stellite 6 by the API Standard and valve manufacturers.
Its use in services normally requiring TRIM No. 5 should be considered only
if cobalt based trims are unavailable, and then only after consultation with the
valve manufacturer and/or materials specialists on the specific service
conditions.
Other commercially available hardfaced trims per API 600 may be similarly
evaluated for use in specific service applications when TRIM No. 5 is either
unavailable or available only on a limited basis.
In addition to listed API 600 trims, Colmonoy 5, a nickel-boron based
hard facing material, is offered by some valve vendors and considered a close
equivalent to Stellite 6 except for sulfur bearing streams above 500°F (260°C).
Trim is discussed in more detail in the "Design and Materials" subsection
under "Hard Seats".

Stems

In a gate valve, the function of the stem is to raise and lower the gate.
The stem is designed to withstand the forces generated during seating of
the gate, which creates compression and tension, respectively, in the stem.
However, the stem is not designed to support the fluid forces acting on the
disc, and a loose stem to gate connection is provided to permit the fluid forces
acting on the gate to be transmitted to the body gate guides, which are provided
for that purpose.
The stem is provided with some foLm of power screw (stem thread) which,
upon manual or powered turning, causes it to drive the gate up or down.
It is very important that the strength in tension of the stem to gate connection
exceed that of the stem at the root of its operating thread, as stipulated in
API 600 and discussed further below.

End Connections

The valve end connections provide a means to connect the valve to
the associated piping. The connections must be pressure tight and capable
of withstanding internal and external loadings resulting from fluid pressure,
weight and thermal expansion. The specific type of end connection to be
employed is dictated by the service requirements and usually is the same type
employed for piping connections to other components of the system such as
vessels and pumps.
The three principal means of connecting a valve to the piping are flanges,
welding, and threading. These are discussed in the following paragraphs.

Flanged End Connections

Flanged ends are the most common method of connection for process
plant applications. Flanges provide a tight, reliable system and in addition,
facilitate removal of valves with minimum disturbance to the connecting
piping.
The valve flange design is, in most cases, identical to the mating pipe flange.
Specific flange design requirements are covered in the design portion of this
section, but are generally per ANSI B16.5 for sizes/ratings within the scope of
that standard.
The use of flanged gate valves in sizes greater than 2 inch (50 mm) is required
by most of the customers for all service applications.
This facilitates standardization and ease of reconditioning.
Flanged valves are available in sizes 2 inch and less, but the customers favor
threaded or socket weld ends for these smaller valves because they
are more economical and more readily handled with such end types in
sizes 2 inch and less.

Butt Welding End Connections

Butt welding ends are prepared in accordance with ANSI B16.25.
This type of end connection employs a full penetration girth weld between
the valve body and connecting pipe.
It therefore provides the ultimate in pressure/leak tightness and line strength,
being equivalent to pipe girth welds. However, this type connection
complicates valve inspection and maintenance by requiring the cutting and
making of two complete pipe welds if the valve is removed.
This is a significant consideration in low alloy or other piping systems which
require stress relieving.
In addition, precautions must be taken during welding and stress relieving
operations to avoid seat distortions which could cause valve leakage problems.
Butt welding end valves should be considered for high temperature or
high pressure services where serious flange leakage problems have been
experienced.
Powerformer reaction and regeneration, Hydrocracker, Residfiner, and
similar high pressure-temperature or cyclic services have been identified
as processes where butt welding end connections are advantageous.
The decision on whether to use weld end valves should be based on the
particular unit design conditions.
One attractive area is relatively clean service where valve seat maintenance
would not be anticipated for about 5 years.
Some customers requires Owner's Engineer approval for the use of this
type end connection.

Since valve body removal for maintenance and subsequent reinstallation
are costly and difficult, some customers include several buttwelding end
valve requirements intended to extend service life, minimize operating
problems, and facilitate in-situ reconditioning.

These include the following:
o The use of flex wedge, split wedge, or double disc gates.
o When hardness differential between seating surfaces is required,
the gate seats shall be the "softer" material.
o Provide a body bleed connection (position G or H per ANSI B16.34) to
permit in-situ leakage testing via the body cavity.
o Treat the machined weld bevels of buttwelding ends as critical areas
for all types of required non-destructive examination.

Buttwelding end valves can be specified to the "Special Class Buttwelding
End Category" per ANSI B16.34. Special Class valves have
pressure-temperature ratings that generally exceed those for Standard Class
valves, but require supplementary non-destruction examination of valve bodies
and bonnets to qualify for these higher ratings.

Socket Welding End Connections

The socket end is machined in accordance with API 602 and ANSI B16.11.
The valve is attached to the pipe by means of a strength welded socket joint,
producing a pressure tight joint. The limits use of this type end connection
to valve sizes 2 in. (50 mm) and less.
Removal of this type valve is accomplished by cutting the welds or pipe.
This has not proven to be a problem since valves in these smaller sizes do
not normally require frequent maintenance and, in addition, small valves are
usually discarded and replaced rather than rebuilt.
Where frequent valve removal is required, such as at troublesome drain or
clean-out locations, a flanged or threaded end valve should be specified.

The socket weld end valve has the additional feature of not requiring a thread
allowance in the connecting pipe, thereby permitting the use of a lower
schedule pipe and, in some instances, smaller diameter pipe.
(In small size high corrosion allowance lines, where nominal pipe sizes are
to standard outside diameters, a wall thickness sufficient to satisfy both
corrosion and thread allowance may result in an inside pipe diameter less
than that required for flow considerations.
In such cases, either a larger pipe or a thinner walled pipe without threads
would be necessary.)

Extended Body End Connections

An extended body valve type has been developed in recent years, and,
in accordance with API 606, has an extension at one end for threaded or
welded attachment and a threaded or socket welding connection at the
other end.
This valve is used in place of an API 602 valve with extension stub welded
into the valve socket, for vents, drains, and other takeoff connections.
It affords a higher strength connection with the elimination of one field joint.
The integrally reinforced extended body end type, designed for full penetration
welding to the mating component without additional reinforcement of
that component.

Two-plane gusseting requirements for valved piping connections
1 1/2 inch NPS (40mm) or less in vibrating services.
Some consider the inherent strength of the integrally reinforced body
extension. Sufficient to eliminate the need for gusseting.
However, worldwide experience to date has not been sufficient to justify
a waiver of the gusseting requirement for integrally reinforced extended
body valves.

Threaded End Connections

Threaded end gate valves are attached to the connecting pipe by means of
internal taper threads on the valve ends and external threads on the pipe ends.
The threading complies with API 602 and ANSI B2.1.
The limits used of this type end connection to valve sizes 2 inches (50 mm)
NPS and smaller.
Threaded end valves may be used in applications requiring seal welding.
Unfortunately, the required seal welding is occasionally overlooked.
In addition, pipe fitters usually make up threaded joints with either threading
compound or Teflon tape and a porous weld, prone to leakage, can result
when attempting to weld over such a threaded connection.
Usually, however, a second weld attempt will produce a sound weld.
The thread depth allowance requirement for piping may result in the need for
a higher schedule connecting pipe than would be required with
socket weld end valves.
Therefore, it is recommended that threaded end valves be employed in
services not requiring seal welding or where frequent removal is required.
The socket weld valve is otherwise preferred.

Seats

The basic function of a shut off valve is to prevent fluid flow into other portions
of a circuit.
The extent to which this is accomplished depends directly on the sealing
ability of the seating surfaces, consisting of body seats and gate seats.
The seats therefore can be considered to be the "heart" of a gate valve.
The ability of two contacting surfaces to form a seal is a function of the degree
of surface conformability (mating) between them.
Perfect contact, i.e., no voids, would give zero leakage.
But this cannot be reached in practice since an absolutely smooth surface
with 100% conformability cannot be practically achieved.
The problem therefore is to achieve sufficient conformability to keep the
leakage rate within system limits.
There are two general categories of valve seats; the hard seat with
metal-to-metal contact, and the soft seat with a metal-to-non-metal primary
contact interface.
In addition, gate valves can be obtained with body seat rings which are integral,
screwed-in, pressed-in, or welded in.
Seating material and hardness selection requirements are covered in the
Design and Materials subsection.
The following describe the principal type of seat rings and service related
selection considerations.

Integral Body Seats

Integral body seats are integral with the valve body material.
This seat design is usually employed in valves made of corrosion resistant
materials such as austenitic stainless steel or Monel.
Integral seats can also be obtained in carbon or low alloy steel valves
where a more suitable seating material is deposited directly on the valve
body to form the seat face.
Integral seats cannot be removed for maintenance or replacement and
therefore must be reconditioned in place.
This imposes practical and economic restrictions on the use of valves with
integral body seats.
As previously indicated, the use of integral body seats should generally be
limited to valves made of corrosion resistant materials.
However, even for these valves, it should be confirmed that they will be
employed in services that will not require body seat removal for maintenance
required as a result of corrosion, erosion, or galling.
Integral body seats are not employed in small size (less than 2 inch/50 mm)
valves of any type because of in-place seat machining and finishing difficulties,
and are not recommended for carbon steel or low alloy steel valves in process
plant services.
Therefore, the use of integral body seats is limited to large corrosion
resistant valves, in clean lubricating services.

Materials

Gate valves are usually classified by their materials of construction.
For example, there are steel gate valves, corrosion resistant gate valves,
cast iron gate valves, etc.
The intent is to identify the most common materials from which gate valves
are fabricated and to familiarize the guide user with the codes and standards
that cover these materials.
The following lists the materials covered in the codes and standards:

Valve Type
Steel Gate Valve
Steel Venturi Gate Valve
Compact Carbon Steel Gate Valves
Corrosion Resistant Gate Valves
Ductile Iron Gate Valves, Flanged Ends
Cast Iron Gate Valves, Flanged Ends
Bronze Gate Valves

Std/Code API 600 API 597 API 598 API 602 API 603 API 604 API 595
Manufacturers Std.

Materials Covered
Carbon Steel, Moly Steel, Chrome Steels, Austenitic, and Nickel Steels
(Forged & Cast) Carbon Steels (Forged & Cast)
Stainless Steels, 20-29 Cr-Ni, Nickel, Inconel, Monel, Hastelloy B & C. (Cast)
Ductile Iron (Cast)
Gray Iron (Cast)
Bronze

In order to select the proper material for a given service fluid,
the Customers Construction Materials Manual should be consulted.
It should be noted that the Customers Construction Materials Manual can
contain specific limitations regarding the use of some materials.

As examples, the extent to which cast iron and bronze valves can
be used is limited within the onsite process area; supplementary
non-destructive examination to specified acceptance criteria is required for
low alloy valves in hydrogen service and austenitic stainless steel valves in
high pressure service; hardness limits are stipulated for specific valve
materials/ components for valves in wet H2S service.
The valve selection and gate valve should be consulted for limitations
placed on various valve materials.
When a proposed gate valve material is not listed in the API standards or in
the Customers Construction Materials Manual , its evaluation should include
safety considerations, its ability to resist erosion/corrosion, and compatability
with the service fluid.
The Customers Construction Materials Manual can be of assistance in
this evaluation.

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