Author Archives: ctrvalve

Laminated and Filament Graphite

1.Suitable for high temperature nuclear service or where low chloride content is desirable

2.Provides leak-free operation, high thermal conductivity, and long service life, but produces high stem friction and resultant hysteresis.

3. Impervious to most hard-to-handle fluids and high radiation

4.Suitable temperature range: Cryogenic temperatures to 1200°F (649°C)

5. Lubrication not required, but an extension bonnet or steel yoke should be used when packing box temperature exceeds 800°F (427°C).




Valve Types and Characteristics

The control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by force from the actuator. To do this, the valve must:
Contain the fluid without external leakage. Have adequate capacity for the intended
service.
Be capable of withstanding the erosive,
corrosive, and temperature influences of the process.
Incorporate appropriate end connections to mate with adjacent pipelines and actuator
attachment means to permit transmission of actuator thrust to the valve plug stem or rotary
shaft.
Many styles of control valve bodies have been developed. Some can be used effectively in a
number of applications while others meet specific service demands or conditions and are used less frequently. The subsequent text describes popular
control valve body styles utilized today.

What Is A Control Valve?

Process plants consist of hundreds, or even thousands, of control loops all networked together
to produce a product to be offered for sale. Each of these control loops is designed to control a
critical process variable such as pressure, flow,level, temperature, etc., within a required operating range to ensure the quality of the end-product.

These loops receive, and internally create,disturbances that detrimentally affect the process
variable. Interaction from other loops in the network provides disturbances that influence the
process variable. To reduce the effect of these load disturbances, sensors and transmitters collect information regarding the process variable and its relationship to a desired set point. A controller then processes this information and decides what must occur in order to get the process variable back to where it should be after a load disturbance occurs.
When all measuring, comparing, and calculating are complete, the strategy selected by the
controller is implemented via some type of final control element. The most common final control element in the process control industries is the control valve.

A control valve manipulates a flowing fluid such as gas, steam, water, or chemical compounds to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.

Many people who speak of “control valves” are actually referring to “control valve assemblies.”
The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and a variety of additional valve accessories, which may include positioners, transducers, supply pressure regulators, manual operators, snubbers, or limit switches.


It is best to think of a control loop as an instrumentation chain. Like any other chain, the
entire chain is only as good as its weakest link. It is important to ensure that the control valve is not the weakest link.




Problem Valves

Often there are references to valve-caused problems or difficulties. The list of problems include valve erosion from process media,stickiness caused by excessive friction (stiction),excessive play in valve to actuator linkages (typically found in rotary valves) that causes deadband, excessive valve stem packing leakage, and valve materials that are incompatible with the flowing medium. Any one,or a combination of these difficulties, may affect process quality and throughput with a resulting negative impact on mill profitability.

Many of these problems can be avoided or minimized through proper valve selection. Consideration should be given to valve style and size, actuator capabilities, analog versus digital instrumentation, materials of construction, etc. Although not being all-inclusive, the information found in this sourcebook should facilitate the valve selection process.

General Selection Criteria for control valve

Most of the considerations that guide the selection of control valve type and brand are rather basic. However,there are some matters that may be overlooked by users whose familiarity is mainly limited to just one or a few valve types. Follow provides a checklist of important criteria.

Suggested General Criteria for Selecting Type and Brand of Control Valve

1.Body pressure rating

2.High and low temperature limits

3.Material compatibility and durability

4.Inherent flow characteristic and rangeability

5.Maximum pressure drop (shut off and flowing)

6.Noise and cavitations

7.Shut off leakage

8.Capacity versus cost

9.Nature of flowing media

10.Dynamic performance

11.End connections


Valve Plug Guiding

Accurate guiding of the valve plug is necessary for
proper alignment with the seat ring and efficient
control of the process fluid. The common methods
used are listed below.

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zzyp control downstream perssure valve

ZZYP self-operated control pressure valve

Feature: ZZYP self-operated pressure regulator adopts the external pressure extraction structure, which can automatically adjust and stabilize pressure depending on the pressure change of the medium itself. It has the characteristics of simple structure and reliable movement. Using balanced bellows valve core structure, has the unbalanced force is zero, high sensitivity, widely used in all kinds of low viscosity liquid, gas and steam, is a kind of applied in the use of energy-saving products without gas and electricity;

Normal Pressure: ANSI Class 150~900#
Control accuracy: 2%
Shut-off ability: 10%
Air 、gases、liquid
Operating temperature: -40~60℃
Leakage level: VI