P01544 -Throttle Actuation Potentiometer Signal too High*************When selecting a valve actuator the correct type and size can be found utilizing the following selection criteria:
- Power source -The common sources of power for automated valves are
electricity or fluid power. If electric power is selected, a three-phase
supply is usually required for large valves; however, small valves can
be operated on a single-phase supply. Usually an electric valve actuator
can accommodate any of the common voltages. Sometimes a DC supply is
available. This is often an emergency back-up power supply.
of fluid power are much greater. First there is a variety of fluid
media such as compressed air, nitrogen, hydraulic fluid or natural gas.
Then, there are the variations in the available pressures of those
media. With a variety of cylinder sizes, most of the variations can be
accommodated for a particular valve size.
- The type of valve -Whenever sizing an actuator for a valve, the
type of valve has to be known, so that the correct type of actuator can
be selected. There are some valves that need multi-turn input, where as
others need quarter-turn. This has a great impact on the type of
actuator that is required. When combined with the available power
supply, then the size and type of actuator quickly comes into focus.
multi-turn fluid power actuators are more expensive than multi-turn
electric actuators. However, for rising non-rotating stem valves a
linear fluid power actuator may be less expensive. A definitive
selection cannot be made until the power requirements of the valve are
determined. After that decision has been made, then the torque
requirement of the valve is the next selection criterion.
- Calculating the torque required by the valve -For a
quarter-turn valve, the best way of determining the torque required is
by obtaining the valve maker's torque data. Most valve makers have
measured the torque required to operate their valves over the range of
operating line pressures. They make this information available for
The situation is different for multi-turn valves. These
can be subdivided into several groups: the rising rotating, rising
non-rotating, and non-rising rotating valves. In each of these cases the
measurement of the stem diameter together with the lead and pitch of
the valve stem thread is required in order to size the automation for
the valve. This information coupled with the size of the valve and the
differential pressure across the valve can be used to calculate torque
The type and size of the actuator can be determined after the
power supply, the type of valve, and the torque demand of that valve
have been defined.
- Sizing the actuator -Once the actuator type has been selected
and the torque requirement of the valve has been determined, then the
actuator can be sized using one of the actuator manufacturer's sizing
programs or tables. A further consideration in sizing the actuator is
the required speed of operation of the valve. As speed has a direct
relationship to the power required from the actuator, more horsepower
would be needed to operate a valve at a faster speed.
actuators can adjust speed of operation using fluid control valves.
However, electric motor operators of the three-phase type have a fixed
speed of operation. Smaller, quarter-turn actuators utilize DC motors,
and may have adjustable speed of operation.
The great advantage of having an
automated valve is that it can be remotely controlled. This means that
operators can sit in a control room and control a process without having
to physically go to the valve and give it an open or close command, the
most basic type of control for an automated valve. The ability to
remotely control a valve is easily achieved by running a pair of wires
out to the actuator from the control room. Applying power across the
wires can energize a coil, initiating motion in an electric or fluid
power actuator. Positioning a valve in an intermediate position can be
done using this type of control. However, feedback would be needed to
verify the actuator is at the desired position. A more common method of
positioning an actuator is to feed a proportional signal to the actuator
such as 4-20 mA, so that the actuator, using a comparator device, can
position itself in direct portion to the received signal.
If an actuator is required to
control a level, flow or pressure in a system, then it may be required
to move frequently. Modulating or positioning control can be achieved
using the same 4-20 milliamps signal. However, the signal would change
as frequently as the process required. If very high rates of modulation
are required then special modulating control valve actuators are needed
that can accommodate the frequent starts required for such duty.