|
Hakkında:
|
Control valves are used to manage the flow rate of a liquid
or a gas and in-turn control the temperature, pressure or
liquid level within a process. As such, they are defined by the
way in which they operate to control flow and include globe
valves, angle seat, diaphragm, quarter-turn, knife and needle
valves, to name a few. In most cases the valve bodies are made
from metal; either brass, forged steel or in hygienic
applications 316 stainless steel.
Actuators will use an on-board system to measure the position
of the valve with varying degrees of accuracy, depending on the
application. A contactless, digital encoder can place the valve
in any of a thousand positions, making it very accurate, while
more rudimentary measurements can be applied to less sensitive
designs.
One of the main areas of debate when specifying
[url=http://www.dingchengmachinery.com/control-valve/]control
valves[/url] is determining the size of the valve required.
Often process engineers will know the pipe diameter used in an
application and it is tempting to take that as the control
valve’s defining characteristic. Of greater importance are the
flow conditions within the system as these will dictate the
size of the orifice within the control valve. The pressure
either side of the valve and the expected flow rate are
essential pieces of information when deciding on the valve
design.
Inside the valve body, the actuator design is often either a
piston or a diaphragm design. The piston design typically
offers a smaller, more compact valve which is also lighter and
easier to handle than the diaphragm designs. Actuators are
usually made from stainless steel or polyphenolsulpide (PPS),
which is a chemically-resistant plastic. The actuator is topped
off by the control head or positioner.
Older, pneumatically operated positioners had a flapper/nozzle
arrangement and operated on 3-15psi, so no matter what the
state of the valve, open closed or somewhere in between, the
system was always expelling some compressed air to the
atmosphere.
Compressed air is an expensive commodity, requiring
considerable energy to generate and when a manufacturing line
is equipped with multiple
[url=http://www.dingchengmachinery.com/pressure-relief-
valve/pressure-relief-valves.html]pressure relief valve[/url]
all venting to the atmosphere, this can equate to a
considerable waste of energy. It is important to not only
establish the most appropriate valve design, but also a cost-
effective solution that takes account of annual running costs.
Modern, digital, electro-pneumatic valves that use micro-
solenoid valves to control the air in and out of the actuator
have introduced significant improvements for operators. This
design means that while the valve is fully open, fully closed
or in a steady state, it is not consuming any air. This, and
many other engineering improvements, have made substantial
advances in both economy and precision.
Valve seats can be interchangeable within a standard valve
body, which allows the valve to fit existing pipework and the
valve seat to the sized to the application more accurately. In
some cases, this can be achieved after the valve has been
installed, which would enable a process change to be
accommodated without replacing the complete valve assembly.
Selecting the most appropriate seal materials is also an
important step to ensure reliable operation; Steam processes
would normally use metal-to-metal seals, whereas a process that
included a sterilization stage may require chemically resistant
seals.
Setting up and installing a new valve is now comparatively easy
and much less time-consuming. In-built calibration procedures
should be able perform the initial setup procedures
automatically, measuring the air required to open and close the
valve, the resistance of the piston seals on the valve stem and
the response time of the valve itself.
Improving safety
Control valves should be specified so they operate in the 40-
85% range so if the valve is commanded to a 10% setting, it can
detect if something has potentially gone wrong with the control
system and the best course of action is to close the valve
completely. If the valve is commanded to a position of 10% or
less this can cause very high fluid or gas velocities, which
have damaging effects on the system and cause considerable
noise and damage to the valve itself.
Modern control functionality can offer a solution that acts as
a safety device to prevent damage to the process pipework and
components. By building in a fail-safe mechanism, any valve
position setting below a pre-set threshold will result in the
valve closing completely, preventing damage to the surrounding
system.
Control inputs can also include safety circuits to ensure safe
operating conditions within the process equipment. For example,
if an access panel on a vessel containing steam is opened, an
interlock switch will open and the valve controlling the steam
supply to the vessel can be automatically closed, helping
mitigate any risks.
Improving reliability
Many process control environments offer less than ideal
conditions for long-term reliability. Moisture-laden
atmospheres, corrosive chemicals and regular wash-downs all
have the capacity to shorten the service life of
[url=http://www.dingchengmachinery.com/control-valve/common-
rail-control-valve-f00vc01334.html]common rail control valve
F00vc01334.[/url] One of the potential weaknesses of the
actuator is the spring chamber where atmospheric air is drawn
in each time the valve operates.
One solution is to use clean, instrument air to replenish the
spring chamber, preventing any contamination from entering.
This offers a defense against the ingress of airborne
contaminants by diverting a small amount of clean control air
into the control head, maintaining a slight positive pressure,
thus achieving a simple, innovative solution. This prevents
corrosion of the internal elements and can make a significant
improvement to reliability and longevity in certain operating
conditions.
While choosing the most appropriate process control valve can
be a complex task, it is often best achieved with the
assistance of expert knowledge. Working directly with
manufacturers or knowledgeable distributors enables process
control systems to be optimized for long-term reliability as
well as precision and efficiency.
|