Electric Valve Actuators Set to Move into Control Applications

VALVE MAGAZINE: How much penetration have electric actuators made in the power industry?

WARNETT: Electric actuators have always had a place in the power industry on isolating valves. Main steam stop valves and other important valves have traditionally been electrically powered. However, until the advent of recent technological advances, the traditional actuator for modulating control valves has typically been the pneumatic spring diaphragm type.

In the past, there have been several attempts at designing an all-electrically actuated power plant, but the limitations of electric actuators in terms of constant modulation and fail-to-position capability have proved to be insurmountable hurdles to overcome.

However, there have been some applications where electric actuation has successfully replaced pneumatic actuators. For example, damper controls that typically used pneumatic-cylinder or hydraulic-cylinder actuators are now frequently automated with electric actuators capable of constant modulation.

VM: What have been the main impediments to their adoption?

WARNETT: Control valves constantly move, following the set point from the main controller. A typical electric actuator is not designed for constant modulation. Worn gears and over-heated motors would result. However, today, new technology in motors and motor control combined with carefully selected drive trains can overcome this.

In the past, another impediment to using electric actuators on control valves has been the lack of an elegant method of driving the control valve to the desired position on the loss of power. With a pneumatic spring diaphragm actuator, there is always a default fail position. Therefore, process engineers traditionally are accustomed to adopting that method into their designs.

Up until now the only way to drive an electric actuator to a fail position was either with a spring or a battery—neither of which are suitable solutions. Springs require the electric motor to be three times the size otherwise required, and batteries do not deliver power quickly enough to provide the rapid shutdown needed on an electric actuator to replicate spring closure.

VM: There are electric actuators available from several manufactures that are intended for modulating applications, but are these generally restricted to smaller sizes?

WARNETT: There are many small modulating electric actuators; however, the degree of modulating capability varies from manufacturer to manufacturer. Very few are capable of achieving IEC 34, S9, 100% modulating capability. The cost of buying an electric actuator is insignificant when compared to the cost of ownership. Perhaps, most relevant is the cost of lost production due to plant inefficiencies caused by the inaccuracies or even the failure of a misapplied electric actuator.

VM: What have actuator manufacturers done to enable electric actuators to be used in control valve applications in the power industry?

WARNETT: The innovations in electric control valve actuators have ramifications for all industrial applications and not just the power industry. Perhaps the most important three innovations are:

1. Actuator performance, levels of overall resolution, repeatability and speed of response have reached new standards of excellence. The elimination of dead time and overshoot (common with pneumatic actuators) is a result of utilizing a combination of advanced technology in highly accurate sensors, motors and controls to deliver performance that yields important benefits to process control.
2. Fail-to-position capability is now available in electric actuators due to advances in the reliability and application technology of super capacitors. Electric actuators that incorporate the advanced super capacitors can be easily configured to drive a valve to any position (open, close or any intermediate position) on loss of power or control signal.
3. User-friendly human machine interfaces (HMI) allow rapid and intuitive set up, configuration, adjustment and diagnostics. By using Bluetooth technology, the HMI can be non-intrusive to preclude a hostile environment from contaminating the actuator’s internal electronic components. Using an advanced HMI can dramatically speed up the commissioning process and improve subsequent reliability of the whole automated control valve assembly.

VM: Can electric control valve actuators provide the power (torque or thrust times speed) of a pneumatic unit?

WARNETT: Pneumatic control valve actuators are capable of significant thrust and speed.

Thrust is a function of the surface area of the diaphragm or piston employed. Provided there is sufficient flow into the actuator, then relatively fast speeds can be achieved.

More important for a control valve, however, is the speed of response to a change in set point. A pneumatic actuator often suffers from dead time. This is the time it takes to build up sufficient air pressure to overcome static friction in a valve.

Dead time becomes more significant as the changes in set point become smaller. The normal movement during the life of a control valve consists of millions of small changes in position. Because the new designs of electric control valve actuators have no dead time, the response to a change in set point is almost instantaneous. That leads to closer control of the process variable (e.g., temperature, pressure or flow), and, therefore, more efficient utilization of the plant.

VM: There are low-power electric actuators available, including ones that can run on solar power. But, it would seem that an electric actuator for a large control valve would use more power than a typical solar panel could supply. Is that true?

WARNETT: Traditional electric actuators that are powered from solar panels are often employed for isolating rather than modulating duty. A solar panel can store its energy in a battery bank and when the valve is required to open or close, power is drawn from the battery bank rather than directly from the solar panel. This means that quite large valves and actuators can be employed for isolating duty.

For modulating a control valve, continuous power is required to constantly move the motor to the required set point. Depending on the size of the valve, this would probably preclude solar powering for older-style electric control valve actuators. However, the new designs of high-efficiency electric control valve actuators would be capable of running comparatively large control valves. Their low power draw, due to their high efficiency, could allow a solar panel to provide the power required to constantly modulate a control valve.

VM: The amount of power needed by an electric actuator clearly keeps it from being intrinsically safe. Are electric actuators available for Class 1, Div I and II, and other hazardous locations?

WARNETT: There are certainly many electric actuators that are certified for hazardous areas including Class 1, Div I and II. Most major electric manufactures have such equipment available for use in refineries and other areas classified as hazardous. There are, however, only a few electric control valve actuators that are certified for hazardous areas. Not many manufacturers are able to supply electric control valve actuators into the typical applications satisfied by pneumatic actuators and intrinsically safe positioners.

VM: How does the purchase price of an intelligent electric actuator compare with that of an intelligent pneumatic unit?

WARNETT: From a user’s perspective, the functionalities of the electric and pneumatic units appear to be very close. They both have an easy setup capability, diagnostics and high performance.

A pneumatic actuator consists of two components, the diaphragm or piston unit and the intelligent positioner. These are two separate components, usually supplied by the control valve maker together with the valve. Quite often the intelligent positioner may be a “bought in” item for the valve manufacturer.

On the other hand, an intelligent electric control valve actuator is a single unit in a robust enclosure. This also can be a “bought in” item for the control valve maker and can be supplied as a package with the valve. Price is really a function of the thrust or torque demand of the valve, but usually the electric option, initially, is a little more costly. The big payoff in favor of the new electric control valve actuators is the money a plant can save in reducing process variability.

VM: How about long-term cost of ownership? Are figures available on that?

WARNETT: A major factor in cost of ownership is not the actuator itself, but the power used to drive it. A pneumatic supply requires a compressor, accumulator, driers, filters, piping and fittings to transmit the compressed air power from the compressor room to the actuator.

The associated losses in compression, transmission and expansion can waste up to 60% of the electric power supplied to the compressor. In comparison, the efficiency of an electric actuator is significantly higher, nearer to 80-85%.

This means that in terms of running cost, an electric actuator is significantly less expensive. When you add in the cost of maintenance to the compressor and its infrastructure, then the figure becomes even more significant. However, each individual case is different. There are interactive cost-of-ownership spreadsheets* available that enable cost comparisons to be done quickly and simply.

*Anyone wishing a copy of the spreadsheet can send an email request to chris.warnett@rotork.com.